PREPARATION OF JANUS NANOPARTICLES AND

ITS APPLICATION IN OIL INDUSTRY

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Wenhao Li

August 2019 PREPARATION OF JANUS NANOPARTICLES AND

ITS APPLICATION IN OIL INDUSTRY

Wenhao Li

Thesis

Approved: Accepted:

______Advisor Department Chair Dr. Younjin Min Dr. Mark D. Soucek

______Committee Member Interim Dean of the College Dr. Thein Kyu Dr. Ali Dhinojwala

______Committee Member Dean of the Graduate School Dr. Sadhan C. Jana Dr. Chand Midha

______Date

ii ABSTRACT

Janus nanoparticles, named after the two-faced Roman god, possess anisotropic interfacial, chemical, and physical properties at their two different “faces”. Recently,

Janus nanoparticles have received increasing attention in the literature owing to their unique individual and collective properties. Such colloids have been used or considered to be utilized in the areas of emulsion stabilizers, viscosity modifiers, microreactors, pho- tonic materials, and enhanced oil recovery. The main objective of this work is to develop new synthesis approaches for large-scale production of monodisperse Janus nanoparticles and to investigate the influence of hydrophilic-lipophilic balance, relative areas of anisotropic faces, and particle size on the phase behavior of oil-water-nanoparticle ternary mixtures as well as the colloidal stability of Janus particles themselves and their

Pickering emulsions (oil-in-water emulsions stabilized by Janus nano-particles).

This work has mainly focused on precisely fabricating surface modified Janus nanoparticles based on a controlled sinking process. For this purpose, first, highly monodisperse spherical silica particles with diameters of 50 and 400 nm were synthesized through the modified Stöber method. The morphology and size of silica nanoparticles were characterized using Scanning Electron Microscope (SEM) and Dynamic Light

Scattering (DLS) techniques. The zeta potential measurements revealed a surface charge of -40 to -45 mV for silica nanoparticles, indicating that the sufficient electrostatic surface charges were attained, giving rise to a fairly stable dispersion in aqueous solution.

iii Highly ordered two-dimensional colloidal monolayers were attempted to be fabricated onto poly (methyl methacrylate) (PMMA) coated substrates using the Langmuir-Blodgett

(LB) deposition technique. The pressure-area isotherms of silica nanoparticles at the air- water interface demonstrate a roughly linear-dependence between interfacial area (A) and surface pressure () up to  ≈ 25 mN/m. Above this set point, surface pressure stayed constant with decreasing A, indicative of collapsing colloidal monolayers from the interface to the subphase. The hysteresis between expansion and compression isotherm cycles was found as a clear sign of the irreversible directed assembly processes. A well- ordered, packed monolayer of silica nanoparticles could be obtained at a surface pressure of 10 mN/m as a result of attractive van der Waals interactions that could be promoted upon reaching below a critical interparticle separation. The exposed (unembedded) areas of the colloidal monolayers were carefully controlled by heating the PMMA matrix at pre-determined temperatures and subsequently, modified by interfacially active ligands

(e.g. chlorosilanes) through a chemical vapor deposition (CVD) method in order to achieve Janus characteristics. Contact angle measurements were also performed to qualify and quantify the wetting dynamics of Janus nanoparticles fabricated of which characteristics are found to be susceptible to ligand concentration as well as exposure time. Overall, it is anticipated that novel amphiphilic Janus nanoparticles fabricated through this study could significantly advance the current state-of-the-art in emulsion stability issues arising in many industrial processes including enhanced oil recovery.

iv ACKNOWLEDGMENTS

I would like to express my deep gratitude to Professor Younjin Min, my research advisor, for her patient guidance, enthusiastic encouragement and useful critiques of this research work. I would also like to thank Dr. Yuanzhong Zhang, for his advice and assistance in keeping my progress on schedule. My grateful thanks are also extended to

Dr. Alessandro Perego, Dr. Stephen Merriman, Mr. Yuchen Zuo, Mr. Shifeng Huang, Mr.

Wenhe Chen and Mr. Rundong Huang, my lab mates, for helping my research when I met problems.

I would also like to extend my thanks to the technicians of the laboratory for their help in offering me the resources in running the program.

Finally, I want to thank my parents for their support and encouragement throughout my study.

v TABLE OF CONTENTS

Page

LIST OF FIGURES ...... viii

LIST OF TABLES ...... ix

LIST OF EQUATIONS ...... x

LIST OF ILLUSTRATIONS ...... xi

CHAPTER

I. INTRODUCTION ...... 1

II. FOUNDATION THEORY OF STUDY ...... 4

2.1 Enhanced Oil Recovery ...... 4

2.2 Janus Particles ...... 5

2.3 Pickering Emulsion ...... 6

2.4 Janus Particles Stabilized Pickering Emulsion ...... 7

2.5 Synthesis of Monodispersed Silica Nanoparticles ...... 8

2.6 Langmuir-Blodgett Technique...... 11

2.7 Dip Coating ...... 14

2.8 Chemical Vapor Deposition Technique ...... 15

2.9 Dynamic Light Scattering ...... 16

2.10 Zeta Potential ...... 17

2.11 Scanning Electronic Microscope ...... 18

2.12 Goniometer Measured Sessile Drop Contact Angle ...... 19

vi III. EXPERIMENTAL SECTION ...... 21

3.1 Chemicals ...... 21

3.2 Synthesis of 400nm Silica Nanoparticles ...... 21

3.3 Synthesis of 50nm Silica Nanoparticles ...... 22

3.4 Purification of 400nm Particles ...... 23

3.5 Preparation of PMMA Film ...... 24

3.6 Preparation of Langmuir-Blodgett Film ...... 24

3.7 PMMA Sinking ...... 26

3.8 Grafting of Ligands using Chemical Vapor Deposition ...... 27

3.9 Characterization of Size and Morphology of Nanoparticles...... 27

IV. RESULT AND DISCUSSION ...... 29

4.1 Synthesis of Monodispersed Silica Nanoparticles ...... 29

4.2 Purification of Monodispersed Silica Nanoparticles ...... 32

4.3 Preparation of PMMA Film on Glass Substrates by Dip Coating ...... 36

4.4 Preparation of Colloid Monolayers with Langmuir-Blodgett Method ...... 38

4.5 PMMA Sinking ...... 44

4.6 Grafting of Ligands using Chemical Vapor Deposition ...... 46

V. SUMMARY ...... 50

BIBLIOGRAPHY ...... 52

vii LIST OF FIGURES

Figure Page

1.1 The photo of SEM samples during the particle synthesis ...... 29

1.2 The SEM images of the synthesized bare silica nanoparticles ...... 30

1.3 The DLS result of the 400nm particles ...... 31

1.4 The DLS result of the 50nm particles ...... 32

2.1 The plot of the result of conductivity during the purification ...... 33

2.2 The plot of the ion concentration during the purification ...... 34

2.3 The Zeta-potential result of the 400nm silica nanoparticles ...... 35

2.4 The plot of the Zeta-potential versus pH ...... 36

3.1 The plot of thickness versus withdraw speed ...... 37

4.1 The plot of isotherm curves of hysteresis experiment ...... 40

4.2 The SEM images of multiple places on the LB film in liquid expand state ...... 41

4.3 The SEM images of multiple places on the LB film in liquid condense state ...... 41

4.4 The plot of compression modulus versus surface pressure ...... 43

5.1 The plot of exposed height versus heating time...... 45

6.1 The plot of contact angle result of various positions in the CVD chamber I...... 47

6.2 The plot of contact angle result of various positions in the CVD chamber II ...... 48

viii LIST OF TABLES

Table Page

1. The arranged result of the compressibility experiment ...... 44

2. The list of the optimized CVD conditions and results ...... 49

ix LIST OF EQUATIONS

Equation Page

1. The Landau-Levich equation ...... 14

2. The transformed Landau-Levich equation I ...... 36

3. The transformed Landau-Levich equation II ...... 37

4. The density of mixture calculation ...... 37

5. Compression modulus calculation ...... 42

x LIST OF ILLUSTRATIONS

Illustration Page

1. The schematic of generating LB film through Langmuir-Blodgett method ...... 12

2. The inner structure of the CVD reaction chamber ...... 15

3. The way in which the hydrodynamic diameter data are collected ...... 16

4. The definition of zeta potential ...... 17

5. The inner structure of a scanning electronic microscope ...... 18

6. The Young’s Equation ...... 19

7. The way in which the contact angles are measured with a goniometer ...... 20

8. The experiment condition of PMMA sinking ...... 46

xi CHAPTER I

INTRODUCTION

The aim of enhanced oil recovery (EOR) is to extract as much of the residual oil as possible in the oil fields with the novel techniques and methods. However, current emulsion-based EOR approach faces some difficulties. For example, the applied chemicals and catalysts can have potential influences on the environment[1]. Addition- ally, the temperature of an oil reservoir can be as high as 250℃[2], which is a challenge to the stability of the surfactants applied.

An attempt was made to synthesize Janus nanoparticles that have potential applications in the EOR industry based on silica nanoparticles. One research reported that the silica particles are of natural composition to sandstone reservoir and can be easily surface-modified by grafting specific functional groups[3]. At the same time, the applied silica particles proved to retain their interfacial properties when the system was heated to as high as 650℃[4]. In other words, the application of silica nanoparticles in EOR would not put an extra burden on the environment while the particles could be easily modified to Janus particles with amphiphilic properties and these Janus particles can remain the interfacial properties at high temperature in the oil reservoir.

1 Despite many studies on Janus particles stabilized Pickering emulsions, especially in mechanism studies, limited open to date literature have reported or involved major parameters that influence the dynamics and stabilities of Janus nanoparticles in a

Pickering emulsion. Also, few studies have provided mass-production methods for Janus particles synthesis and some studies reported the hindrance of mass production of Janus particles with consistent feature during the experiments[5][6].

This research will provide a novel method to obtain the Janus particles with monodispersed sizes, similar hydrophilic-lipophilic balance (HLB) and similar relative areas of anisotropic faces through mass production in controlled experimental conditions.

At the same time, the multiple hydrophilic-lipophilic balances and various relative areas of anisotropic faces can be adjusted with controlled experiment conditions, while different monodispersed sizes were achieved with a controlled volume of reactants added during the reaction. Once the Janus particles of various sizes, hydrophilic-lipophilic balance, and relative areas of anisotropic faces were obtained through this method, the phase behavior of oil-water-nanoparticle ternary mixtures and the colloidal stability of

Janus particles and Janus particles stabilized Pickering emulsions would be investigated.

It is predicted that a solid theoretical foundation of EOR application could be provided through these investigations.

In the following chapters, the foundation theory of this study would be introduced first including some basic concepts, mechanisms, synthesis techniques, and characterization techniques. Next, the experimental section and result discussion would be arranged in the order of Janus particles synthesis process. The detailed synthesis

2 methods and characterization conditions would be introduced in the experimental section.

The optimization result and explanation would be presented after the experimental section. The conclusions were made in the final chapter.

3 CHAPTER II

FOUNDATION THEORY OF STUDY

2.1 Enhanced Oil Recovery

Currently, the crude oil produced from wells in oil fields generally undergoes three stages. In general, massive crude oil is initially pressed through the wells with a pressure difference between wells and reservoirs. Next, a waterflood is used to maintain the pressure difference (i.e., secondary recovery). Then, the floods with specific chemicals are applied to adjust the interfacial tension and to mobilize the oil drops, which is called enhanced oil recovery (EOR)[7]. In other words, residual oil is generally left after several extractions. As a result, oil recovery is usually called tertiary recovery after the primary and secondary recovery. Finding economically viable and environmentally friendly methods to extract the huge amount of residual oil after primary and secondary recovery remains challenging for the oil and gas industry and is also of significant importance to efforts to satisfy the world’s increasing energy demand. According to some data statistics, improving crude oil recovery by 1% worldwide would result in a huge amount of crude oil resources becoming available[7]. Moreover, from a long-term point of view, recovering more oil in the original oil wells becomes a more eco-friendly and

4 efficient choice in contrast to finding new oil fields, especially in the present situation of declining oil production in existing oil wells[8].

As mentioned in the introduction chapter, the severely high temperature in the oil reservoir and environment issues remain two big challenges in modern EOR industry.

Some previous studies have provided some ideas and solutions to these challenges.

Sharma et al.[9] proposed the application of a nanoparticle-surfactant-polymer system for a Pickering emulsion with improved thermal stability; however, the improvement in temperature is restricted to as high as 356K compared to a sole surfactant system. Yet, the idea of introducing nanoparticles working together with surfactant lead to the right research direction. Later, Wang et al.[4] proved that the applied silica particles were able to retain their interfacial properties when the system was heated to as high as 650℃; this provided a method to overcome the thermal issue of the oil reservoirs.

With the rapid development of nanotechnology, the application of nanomaterials in the oil recovery industry has become a trend due to their superior properties brought about from their unique structures[10][11][12][13], especially in a high surface area-to- volume ratio and active surface sites. The recycling of nanocatalysts, especially the environment effect, has been discussed[1], and the application of nanoparticles has proved to be a promising solution in the EOR industry.

2.2 Janus Particles

Janus particles, first coined by Pierre-Gilles de Gennes in his Nobel Lecture, are named after the two-faced Roman god to depict the particles with two different properties on each side[14]. According to his lecture, Janus particles were expected to function as

5 amphiphilic surfactants that can adsorb at the interface, while Janus particles can leave some interstices, which make the monolayers appear to be “breathing skin”[14]. Since the concept was presented, the studies following the concept have become popular, especially with the introduction of nanotechnology, which revealed other interesting properties of the particles. Recently, in biomedical fields, the carriers of multiple drugs and drugs along with a diagnostic agent utilizing the unique combination of physicochemical properties from Janus particles were further developed[6]. In addition, the formation and assembly of amphiphilic Janus nanoparticles, current challenges of

Janus particle synthesis, and applications of Janus particles were collectively discussed recently in one literature[5]. In the literature, the author indicated that the potential applications of Janus particles included displays[15], stabilizers[16], antireflection coatings[17], sensors[18], and biomedical materials[19]. In this study, the application as a stabilizer in a typical Pickering emulsion in the EOR industry is intensively discussed.

2.3 Pickering Emulsion

A Pickering emulsion is a type of emulsion that is stabilized by solid particles. It is named after Percival Spencer Umfreville Pickering, a British chemist and horticulturist, who independently observed and studied the solid particles situated at the oil-water interface, thus generating a stable emulsion in 1907[20][21][22]. In other words, the two insoluble phases are mixed with solids interacting on the interface and generating a relatively abnormal phenomenon[21]. In normal cases, when oil and water are mixed together with agitation, small droplets are formed and are dispersed throughout another phase depending on the specific property and the volume ratio initially.

6 Eventually, the droplets attempt to coalesce with each other to decrease the amount of energy in the system by decreasing the area per volume; however, when specific solid particles are added to the dispersion system, they could create a solid barrier that appears at the interface due to the aggregation effect at the surface of the interface, which makes the emulsion more stable because this solid barrier prevents the coalescence of oil and the aqueous phase and leads to the appearance of a massive amount of oil and aqueous phase separation cells. The solid particles are usually rigid structures, which effectively prevent the droplets from coalescing, thereby enhancing the colloidal stability of the emulsion.

Consequently, the Pickering emulsion is also known as a solid-stabilized emulsion.

Various kinds of Pickering emulsions were studied before. In some cases, the

Pickering emulsions were generated based on functionalized particles [23][24][25][26], while in other cases, the Pickering emulsions were generated by controlling of a specific system(magnetic field[27] or acidity-dependence system[28]).

2.4 Janus Particles Stabilized Pickering Emulsion

The functions of silica nanoparticles in Pickering emulsions generally lie in the mechanism of the wettability alteration and interfacial tension reduction in oil recovery[29]. The oil droplets are expected to move more freely as the capillary force is decreased with the alteration of wettability at the interface. Like the surfactant absorbing at the surfaces of rocks, these applied Janus nanoparticles likely have a similar mechanism in changing wettability and the ability to change wettability is responsive to the sizes of particles, the cosurfactants applied and pH of the system[30][31].

7 Through the contact angle measurement and interfacial experiments in oil recovery experiments (flooding tests in oil reservoirs), the amphiphilic silica nanoparticles were demonstrated to have better efficiency in interfacial tension reduction and wettability transformation[32][33].

In recent years, another specific mechanism of nanoparticles in Pickering emulsions was proposed, which was the correlation between disjoining pressure and mobility ratio[3][34]. The disjoining pressure is caused by the pressure difference from the forming of a wedge film when the nanoparticles adhere to the oil phase and the oil droplets disjoin the rock surface due to the effect of Brownian motion and electrostatic repulsion[34].

2.5 Synthesis of Monodispersed Silica Nanoparticles

After Stöber provided methods and standard procedures to synthesize silica spheres of a predetermined size in the range of 50nm to 2000nm in diameter in 1968[35], various studies began to focus on methods of monodispersed nanoparticle synthesis due to the specific properties of monodispersed nanoparticles having a potential application in colloid and interface studies and related industries. The method provided by Stöber was based on the hydrolysis and the condensation reaction of the tetraethyl orthosilicate

(TEOS) in the base reaction media (water, ethanol, and catalyst ammonium hydroxide).

Before a standard procedure was developed, the mechanism had already been discussed by V. K. LaMer et al.[36]. During hydrolysis and condensation reactions, the concentration of the intermediate [Si (OC2H5)4-x(OH)x] proceeded a rapid increment, and nucleation began at one critical point. The nucleation was eventually stopped as the

8 concentration decreased to another critical point, and thus they believed that the final nanoparticles were determined by the intermediates. V. Blaaderen et al.[37] analyzed the effect of a reactant concentration from a reaction mechanism based on the experimental result. After analyzing the growth process of the silica particles, they confirmed that the process was controlled by the condensation of hydrolyzed monomers, while the reaction was limited by the hydrolysis rate of alkoxide. The researchers have also reported that the process of particle nucleation is influenced by the concentration of ammonium hydroxide and reaction media that affected the ionic strength and surface properties of nanoparticles. The ionic strength and surface properties of nanoparticles are believed to be the factors that determine the colloid stability and the progress of the growth reaction.

Regarding the stability behavior of the reaction mechanism, the stability of the nanoparticles is affected by the concentration of the base in water as the dissociation of silanol groups; therefore, the ion concentration affects the zeta potential at the surface of the nanoparticles. In 1988, Bogush et al.[38] proposed a seeded growth technique, which was conducted in this study and was called the “single addition method” (during regrowth, the addition amount of TEOS and water remained the same). This method proved to be productive with up to 17wt% in the reaction, despite a long reaction time and multiple single additions. Due to the characteristics of the single addition, as the initial concentration of TEOS increased, the weight fraction increased with a better size distribution. Bogush et al. agreed with the theory that the initial particles determined the rate and that the surface charge determined the stability of nanoparticles. Later, the “seed regrowth” technique[39][40] was developed to control the monodispersed sizes by controlling new particle formation by adding a silica precursor, which means the final

9 sizes of the particles were controlled by the amount of the precursor (TEOS) and the sizes of the pre-formed seeds.

Based on the mechanism theories and synthesis methods developed, additional novel methods were developed for a wider application. A synthesis method in a batch and semi-batch process with the introduction of LiCl and KCl in a buffer solution was reported[41][42]. Apart from the factors of elementary reactions, reactant diffusion, and precipitated coagulations during hydrolysis and the condensation of a silicon precursor in a water-ethanol medium, other factors, such as solvent compositions, adsorption of ions, and ionic strength, were expected to further influence the formation of the particles. The surface potential of the silica particle was decreased and the rapidly changing ionic strength was suppressed by the introduced electrolytes in the batch and semi-batch processes. K. D. Hartlen et al.[43] reported a seeds regrowth method controlled by the concentration of the base. The nucleation process and seed regrowth were conducted in an emulsion system consisting of a water phase and a cyclohexane phase with arginine as the catalyst. The method was called the interfacial preparation method, and the base was substituted with arginine for eco-friendly concerns and concentration control. The ethanol was introduced into the reaction system only during the regrowth process for monodispersed 230nm particles. Later, R. Watanabe et al.[44] also reported a seed regrowth method in an emulsion system with particle sizes in the range from 8 to 550nm with the control of seed dispersion proportion and the amount of TEOS added. Compared to the previous method, the ethanol existed in the reaction system from start to the end and thus the reaction system remained the same for all particles sizes synthesized.

10 2.6 Langmuir-Blodgett Technique

Almost 100 years ago, I. Langmuir systematically conducted research on floating monolayers on the water surface and successfully transferred fatty acid molecules from the water surface to substrates. Years later, K. B. Blodgett first studied and depicted monolayers transferring work; however, only 40 years ago, the Langmuir-Blodgett method was intensively studied when the first conference on LB film was held.

Currently, this transferring method is named after them due to their pioneering work, and the mechanism of the LB technique is becoming clearer. Therefore, this technique is being widely used in producing monolayers on substrates.

The Langmuir-Blodgett technique is one of the fastest, most versatile, and scalable techniques to generate nanoparticle structures with control over thickness, the number of layers, and the composition of each layer. At the same time, the mechanism was studied to fully understand the factors that can affect the stability of the monolayer at the interface.

Monodispersed silica particles functionalized by the silane[45] or the surfactant[46] were used to generate monolayers using the LB technique. In both conditions, the self-assembly was witnessed with the well-organized structure of a close- packed 2D array at the air-water interface. The resulted morphology of the LB monolayers was attributed to sizes and functionality[45] and spreading agent[46]. The hexagonal close packing phenomenon was explained as a result of a long-range attraction between similarly charged particles in confined geometries because the symmetric counter ion fluctuation is believed to induce a long-range attraction[47].

11 B. Duffel et al.[48] discussed the factors that would result in the different morphologies of LB monolayers when preparing advanced optical materials. In their experiment condition, the well-ordered structure was witnessed only with the partially hydrophobic particles, while the fully hydrophobized particles formed weakly ordered structures and bare silica particles could not form films at the air-water interface. Based on these experimental results and the theory proposed by P. Pieranski and A. J.

Hurd[49][50], the patches of particles were oriented to minimize contact with a certain hydrophobicity of particles and the exposed area with the remaining hydrophilicity along with the hydrophobicity on the other side to create an electric double layer. These double layers at the particle surface (induced by the air-water interface) acted as repulsing dipoles that hindered the aggregation of particles at the interface and allowed for the formation of a hexagonal close packing structure within a defined area.

4 1

3

2 3. The barrier would move forward and backward under the control to maintain the surface pressure. 5 1. 2. The particles were compressed with 2 barriers at a slow constant speed 4. 5. The substrates coated with PMMA were lifted at a until they were closed packed. slow speed while the barrier kept compressing to maintain the surface pressure. Finally, the monolayers could be transferred from the interface to the substrates through this way.

Illustration 1. The schematic representation of the generation of LB film with the Langmuir-Blodgett method.

12 G. Tolnai et al.[51] also attributed the high interparticle repulsion to the dipole- dipole interactions; however, they indicated a weakly cohesive force between particles before compression, while the capillary and electrostatic forces were recognized as a long-range attraction during compression. R. Aveyard et al.[52] believed that silica particles could form flocculated-weakly cohesive domains while spreading, and the surface pressure could be considered a consequence of particles-particles repulsion. As a result, during the compression progress of the experiment, the dipole-dipole interparticle forces increased as the surface pressure increased.

The interaction of particles at the interface was summarized by S. Lugomer et al.[53] based on previous studies. The particles were separated into three levels based on the sizes ranging from 80 to 450nm. For particles ranging from 80 to 110nm, the LB films appeared as hydro clusters due to the strong Coulomb repulsion and significant particle motion. For particles ranging from 180 to 220nm, the LB films appeared as rhombic and hexagonal-crystal grains due to weak Coulomb repulsion and the relatively small motion of particles. For particles ranging from 350 to 450nm, the LB films formed

2D hexagonal crystals due to very weak Coulomb repulsion and were reorganized by the extremely small motion of the particles.

In conclusion, the particles spread at the interface of air-water are affected by various interactions subject to Brownian motion, electrostatic, and capillary forces.

During compression, some forces become dominant in the system, which is dependent on the size, wettability, and size dispersity of the particles.

13 2.7 Dip Coating

Dip coating is generally considered a simple and fast method used to create thin films on a solid substrate. The process can be separated into three steps. First, the substrate is immersed in the solution of coating material at a constant speed. Then, the substrate is kept still to allow the coating material molecules to adsorb on the surface.

Finally, the substrate is withdrawn from the solution at a constant rate to achieve a uniform thickness. Although various coating parameters need to be controlled to achieve a uniform and reproducible film, the simplicity of the dip coating method leads to the development of automation in various manufacture situations when either the production condition is controllable, or the standard of the products is not too strict. A coating that is not properly controlled may lead to nonuniform thickness, which could cause a wedge effect, fatty edges, and peeling off.

The dip coating conditions were defined to control the thickness and quality of the thin films. Under a proper condition when the withdraw speed is determined, which leads to a shear rate that keeps the system in a Newtonian regime, the thickness can be calculated by the Landau-Levich equation (see Equation 1.)[54].

(휂 ∙ v)2/3 Equation 1. h = 0.94 ∙ 1/6 1/2 훾LV ∙ (휌 ∙ 푔)

In the equation, the thickness is determined by several parameters, including viscosity, withdraw velocity, surface tension, and density. In this study, the parameter considered was simplified to withdraw the velocity and the dwelling time for the fixed coating material; however, for the actual experimental condition when the environmental

14 factors were considered, the relative humidity and solvent evaporation were also controlled, which is discussed in the description of the work.

2.8 Chemical Vapor Deposition Technique

The chemical vapor deposition (CVD) method has been commonly applied in modern manufacturing. From optical materials in cameras to food packages, this method has been proven to be reliable and suitable for mass production. On the other hand, this technique has provided material science with novel ideas, especially for cases when the precursors are insoluble, which means it is impossible to apply solution-based methods to generate complex materials with the property. Moreover, the CVD method makes it possible for the complex surfaces to be coated with a layer of uniform film. For example, it was used to uniformly coat carbon nanotubes that have complex structures[55].

In this study, hydrophobic grafting was conducted using this method to uniformly coat a layer of hydrophobic chemical groups while protecting the part of the particles beneath the PMMA under controlled conditions to achieve different levels of hydrophobicity.

Input flow

Substrates Teflon shelves Output flow

Illustration 2. The schematic representation of the inner structure of the self- designed CVD reaction chamber.

15 2.9 Dynamic Light Scattering

The dynamic light scattering (DLS) measurement is based on the theory that larger particles have slower Brownian motion, and thus a normal DLS is conducted in the system of particles suspended in a liquid. The hydrodynamic diameter is measured, and this diameter is related to medium property, surface structure, and the shape of the particles. The motion of the particles is measured based on the Rayleigh scattering and

Mie scattering theories as the fluctuation of the signals from the scattering light is recorded, and with the help of the correlator, the size and size distribution are obtained.

Illustration 3. The schematic representation of the way in which the hydrodynamic diameter data are collected (From Wikimedia Commons, the free media repository, the work from Mike Jones).

16 2.10 Zeta Potential

The concept of zeta potential is based on the presence of an electrical double layer when the charge at the surface of the particles affects the distribution of ions in the surrounding interfacial region. Generally, the concentration of counter ions increases in a specific region, and an electrical double layer is formed around each particle. The layer can be further divided into a Stern layer (the inner region) and a diffuse region (the outer region). The notional boundary is defined as the ions in the region inside the boundary move with the particles, while the ions outside the boundary are considered to be with the dispersant when particles move in the colloid. The potential at this boundary is defined as zeta potential. This potential is specifically defined because it is considered an indication of the potential stability of a colloid system. With a low value of zeta potential, the particles tend to flocculate with no force hindrance, while the particles with a high value of zeta potential tend to repel each other in the system. The boundary between stable and unstable is usually ±30mV.

Particles with negative surface charge

Stern Layer Diffuse Layer

mV Surface potential Zeta potential

0 distance

Illustration 4. The schematic representation of the definition of zeta potential.

17 2.11 Scanning Electronic Microscope

The scanning electronic microscope (SEM) was designed with the concept that the wavelength of electrons is much smaller than the wavelength of light and that the resolution of SEM should be much better than a light microscope. In fact, the scope of the

SEM can reach as far as the nanometer scale. The electrons were accelerated to generate an electron beam, and the beam was controlled with electromagnetic lenses to focus on the sample because electrons are sensitive to the magnetic fields. With the help of a specific detector, the backscattered and the secondary electrons were detected from the

Illustration 5. The schematic representation of the inner structure of a scanning electronic microscope (From Wikimedia Commons, the free media repository). sample. The information from the surface topography and composition was analyzed from the signals. The SEM chamber is usually vacuumed to clear the route of electrons

18 and to reduce energy loss to obtain a clearer view. In this study, the SEM was frequently used to obtain visualized information of the nanoparticles with sizes under one micron.

The voltage was controlled at 1kV in case the energy accumulated affected the result. For the samples on glass substrates, the silver ions were sputtered when preparing samples in case of the accumulation of the electrons at the surface of the samples.

2.12 Goniometer Measured Sessile Drop Contact Angle

When the liquid phase comes in contact with a solid and gas phase, the three phases form an equilibrium of tangential forces with the interfacial and surface tensions.

The contact angle forms at the contact line and can be described by Young’s equation.

When the contact angle is 0 degrees, the liquid is fully spread. When the contact angle is

180 degrees, the liquid drops form spheres and contact with the solid phase at one point.

The contact angles are dependent on the properties of the phases, so the properties can be characterized as either a solid phase or a liquid phase with the control of the other the

Illustration 6. The schematic representation of the sessile drop contact angle and the Young’s Equation (From the rame-hart instrument co.[56]).

19 variations. One common method used to characterize the static contact angle is the sessile drop method conducted on a goniometer, which is a type of optical determination. A drop

(a decided volume) of liquid sitting at a solid substrate is considered a sessile drop. With the help of a light source, the clear outline of the sessile drop is caught by the camera, and the software can automatically generate the contact angle data after setting the baseline and the range of the left and right contact angles.

Camera Light Source

Illustration 7. The schematic representation of the way in which the contact angles are measured with a goniometer.

20 CHAPTER III

EXPERIMENTAL SECTION

3.1 Chemicals

The tetraethyl orthosilicate (TEOS, Alfa Aesar 14082, 99+%) was acquired from

Alfa Aesar. The TEOS was filtered with a 0.2 μm PTFE filter before use. The ethanol

(Inc 190 Proof) was acquired from Decon Lab. The ammonium hydroxide (221228, 28%-

30%), Toluene (179418, >99.5%), L-arginine (A4474, >98.5%) and cyclohexane

(227048, 99.5%) were acquired from Sigma Aldrich. The Milli-Q water (18.2MΩ∙cm) was produced using Water Filtration Unit Direct-Q UV8. Polymethyl methacrylate

(PMMA) (M.W. 15000) and Chloroform (C298-4) were acquired from Fisher Scientific.

Trimethyl chlorosilane (SIT8510.0) and n-propyldimethylchlorosilane (SIP6910.0) were acquired from Gelest as a coupling agent for CVD grafting.

3.2 Synthesis of 400nm Silica Nanoparticles

The 400nm particles were synthesized based on the single addition method[38]. In one research[57], the TEOS was diluted in ethanol with a 1:4 volume ratio to generate particles with the narrowest size distribution compared to other volume ratios.

Consequently, 5.65ml filtered TEOS was diluted in 22.6ml ethanol. Then, (120-

21 5.65×4=97.4ml) 97.4ml of ethanol was added into a cleaned round bottom flask. After that, 14.06 ml filtered Milli-Q water and 10.30 ml ammonium hydroxide were added and stirred at 400rpm until homogeneous. 5.65ml TEOS diluted with 22.6ml ethanol was rapidly added into the round bottom flask and would react for roughly 8 h. After the 8 h reaction, 5.65ml TEOS and 0.92ml Milli-Q water (1:2 mol ratio according to the study of

Bogush[38]) were added separately into the reaction vessel and reacted for another 8 hours. The addition of TEOS and Milli-Q water was repeated 10 times until the silica nanoparticles reached around 400nm in diameter.

3.3 Synthesis of 50nm Silica Nanoparticles

The 50nm particles were synthesized based on the interfacial preparation method[43]. First, 46mg L-Arginine was dissolved in 34.5ml Milli-Q water in a round bottom flask, and the concentration was at around 7.5mM. Cyclohexane was added with a long needle on the surface of the L-arginine aqueous solution to create a layer of cyclohexane above the aqueous phase. The reaction occurred at 60℃[43], so the temperature of the system was increased to 60℃ with stirring at 300 RPM. Then, the filtered TEOS was added from the side of the flask to the cyclohexane phase without disrupting the layers. The stirring speed might be decreased in case the layers are disrupted. The reaction occurred at the interface of cyclohexane and the aqueous phase with the slow supplement of TEOS in cyclohexane. To regrow the seeds, the base environment should be further diluted to a 1-2mM L-arginine concentration according to the previous study result from Hartlen, et al.[43]. In our experiment condition, 15ml of seed suspension was transferred from the aqueous phase to another clean round bottom

22 flask with a stir bar in it. 54ml of Milli-Q water and 7.5ml cyclohexane were added separately. The system was heated to 60℃ again while stirring at a proper speed. 5.28ml filtered TEOS was added and allowed to react for 30h. The sizes of the nanoparticles did not reach 50nm according to the DLS result, so the aqueous phase of the reaction system was transferred to another cleaned round-bottom flask with a stir bar in it. To maintain the same concentration of L-arginine at around 1-2mM, only the 7.5ml of cyclohexane and 5.28ml TEOS were added respectively when the reaction flask was at 60℃. After 12 h of reaction, the particle suspension was taken to check with DLS every 2 h. After 16 h of reaction, the particles reached 53nm according to the DLS result. The aqueous phase was immediately separated from the reaction vessel and washed with Milli-Q water and was preserved in a refrigerator for later use.

3.4 Purification of 400nm Particles

The synthesized nanoparticle suspension was directly transferred from the round bottom flask into several 50ml centrifuge tubes (from VWR). The particles were centrifuged down in the Ultracentrifuge (Beckman Coulter Allegra 6R Refrigerated) at

3000 RPM for 15-20 min at 10℃. Then, the supernatant was substituted with the ethanol and was homogenized for several minutes with an Ultrasonic Homogenizer (MISONIX

Sonicator 3000) according to the homogenizing conditions. These steps were collectively referred to one complete washing. The synthesized particles suspension was first washed with ethanol for 3-4 times to remove the residual TEOS and ammonium hydroxide. After that, the suspension was washed with Milli-Q water multiple times to further remove the residual TEOS and ammonium hydroxide. The conductivity and pH value were measured

23 during purification. When the conductivity measured was lower than 10μS/cm, the zeta potential would be measured to obtain the particles surface properties. After that, the dispersant of the suspension was replaced with ethanol for several times, and then the suspension was preserved in the fridge at 4℃ for the Langmuir-Blodgett experiment.

3.5 Preparation of PMMA Film

A uniform layer of 400nm PMMA film was coated onto glass substrates

(24mm×60mm-1.5 from Leica) using the dip-coating technique. The glass substrates were first cleaned by blowing dry air to remove dust, and then was plasma cleaned in vacuum for 1 min to remove the organic contaminant. 10 wt% PMMA in toluene solution was prepared as a coating solution. One glass substrate was first immersed in the coating solution for 30 seconds, then it was lifted at a constant 1mm/s upstroke speed. Constant lifting speed can be provided with a syringe pump. The dip coating was conducted in a hood with 10% relative humidity. The coated substrates were also dried in this hood. The

PMMA in toluene solution was supplied from a 10wt% mother solution.

3.6 Preparation of Langmuir-Blodgett Film

The colloid suspension preserved in ethanol was prepared in a mixture of chloroform and ethanol at a specific ratio. Before the deposition process, the mixture of the suspension was fully homogenized with the Ultrasonic Homogenizer in the lab. The preparation of the colloid monolayer was conducted on a Langmuir Trough and

Deposition Unit (KSV NIMA) in the lab. The platinum built Wilhelmy plate balance was used to measure the surface pressure. The plate was cleaned with ethanol and water and blown with dry air. Before use, it was flame-cleaned to remove organic contaminants.

24 The LB trough was cleaned with ethanol and chloroform and was blown with dry air. Then, the trough was filled with a specific amount of Milli-Q water, creating a convex meniscus on a Teflon base. The water surface was separated by two moveable barriers on each side, creating a 2D area at the surface due to the convex meniscus in the trough. The flame-cleaned Wilhelmy plate was set in the tough with a balance. After that, the fully homogeneous colloid suspension was carefully dispersed with a Hamilton syringe drop by drop on the water surface, and enough time was given to allow the evaporation of the spreading solvent.

The surface area of the trough can vary due to the movable barrier composed of hydrophilic Delrin over the trough. The surface pressure and the mean molecular area were monitored during the experiment with the control and feedback system in the computer, and thus the surface condition was maintained when it was stable or when substrates were upstroke and downstroke to obtain colloid monolayers.

During the hysteresis experiment, the compression and expansion speed was set at

2mm/min for the movable barrier. The condition of the spreading suspension was set at 1,

2, 5, and 10 mg/ml for particle concentration in the colloid suspension. The spreading volume was set at 850μl. Fifteen min after spreading the suspension, the monolayer was compressed to a collapsed plateau and was then expanded until the surface pressure was stable at the baseline and the monolayer was in the gaseous state. The monolayer was compressed again to a collapsed plateau and then was expanded again to complete the hysteresis experiment.

25 During the compressibility experiment, the plasma-treated glass substrates were sunk in the trough beforehand. Fifteen min (sometimes 25 min) after spreading, to allow for the evaporation of the solvent, the compression progress began and stopped when the surface pressure reached 10mN/m. The glass substrates were lifted at 5mm/min with a controlled surface pressure at 10mN/m.

During the mass production of LB films for Janus particles synthesis, 2 pieces of glass substrates were set together to become a 48mm×60mm area. The compression speed was set up to 4mm/min. The lifting speed was set up to 7mm/min to increase efficiency while maintaining the quality of LB film.

3.7 PMMA Sinking

The heating plate was preheated to 120℃, which was around the glass transition temperature of the PMMA with a PID controller. The setting temperature was usually a little higher than the target temperature. The glass substrates with monolayers and

PMMA coating were arranged on a self-made shelf that supported both sides of the substrates. The sinking condition is affected by heating temperature and heating time, which were optimized beforehand by other group members. About 25% of the monolayer sunk into the PMMA was achieved at 120℃ heating for 110 s. About 50% of the monolayer sunk into the PMMA was achieved at 125℃ heating for 60 s. About 75% of the monolayer sunk into the PMMA was achieved at 125℃ heating for 110 s. The sinking condition can be adjusted according to the environment, so the sinking quality must be checked occasionally with SEM.

26 3.8 Grafting of Ligands using Chemical Vapor Deposition

The chemical vapor deposition experiments were conducted using a self-made

CVD chamber with a Teflon shelf that can support 24mm×60mm glass slides from four edges and that exposed most of the glass slides in the chamber. The glass slides coated with partially sunk silica monolayers in PMMA were plasma cleaned and water-vapor treated (gently stained the substrate with water vapor) to help to graft the reaction groups.

The chamber was first purged with N2 for 20 min to remove the other contents. Then, the chamber was sealed and vacuumed and heated to the reaction temperature. The coupling agent was set in the reservoir, which was sealed and connected to the reaction chamber.

The coupling agent vaporized due to low air pressure and heat. The gaseous coupling agent was sent to the chamber through flow control. In this study, the reaction time period ranged from 30 min to 60 min. The vacuum rate was controlled at -28 inHg.

Trimethylchlorosilane and n-propyldimethylchlorosilane were used as the coupling agents. After the reaction, the chamber was purged with N2 to remove the residual silane vapor. The samples were preserved in a petri dish for contact angle measurement.

3.9 Characterization of Size and Morphology of Nanoparticles

Scanning electron microscopy (SEM) was performed using JEOL Scanning

Electron Microscope in GDYR in the University of Akron. The 400nm particles suspension in the round bottom flask was directly dropped on a plasma-cleaned silicon wafer and was checked with SEM every 8h. The average size and standard deviation were determined from SEM images by averaging diameters of 50 nanoparticles. For LB

27 films on PMMA coated glass substrates, the samples were sputtered with silver ions before characterization.

Dynamic light scattering (DLS) and Zeta potential measurements were performed using the Zetasizer from Malvern. About 4ml of 50nm particles suspension was transferred into a disposable cuvette where the suspension was cooled down to room temperature. About 30-50 μl of 400nm particles suspension was diluted in 4ml Milli-Q water in a disposable cuvette. The sizes obtained by DLS was measured with Zetasizer for 3 groups and 10 times measurement in each group.

The pH and conductivity were measured using the Multiparameter

Meter (Orion™ 5-Star Plus pH/ORP/ISE/Cond/DO) to characterize the washing condition.

The contact angle was measured with the goniometer from Rame-Hart using the sessile drop technique to characterize the grafting condition.

28 CHAPTER IV

RESULT AND DISCUSSION

4.1 Synthesis of Monodispersed Silica Nanoparticles

The nanoparticles prepared by single addition method were characterized by

SEM. From the SEM images shown in Fig. 1.2, we can see the spherical particles with relatively monodispersed size distribution. The particles self-assembling to hexagonal close packing structure with little defects and large grains indicated a narrow size distribution. These little defects were attributed to the imperfect size distribution of the synthesized nanoparticles. Duffel et al.[48] believed that local imperfect hcp matches due

Fig. 1.1 The photo of the SEM samples during the particle synthesis. The nanoparticles suspension in the reaction beaker was dropped on a plasma cleaned silicon wafer and was checked with SEM every 8 hours. The changing iridescent phenomenon can be observed as the sizes of the particles increase due to light scattering.

29 to polydispersity accumulation as the particles aggregating into a 2D crystal would eventually cause a lattice defect because the mismatch is so large an hcp ordering is not possible. Comparing to their results, we believe that the fewer defects and more large grains observed in the SEM images of our results can be the proof of narrower size distribution.

After calculating the sizes of 50 particles randomly picked from the SEM images, we concluded that the particles reached 434nm±8.6nm in diameter after 10 times of single additions. The intention of synthesizing particles with narrow size distribution is to have a better utility of surface when generating colloid monolayers and to control the parameters of particles for PMMA sinking. When the particles appeared to be monodispersed in sizes, they tended to self-assemble to hexagonal close packing structure

휋 to achieve the highest space utilization ratio (74%). The value was calculated as ≈ 3√2

0.74 which was proved by Carl Friedrich Gauss to be the highest average density. The

PMMA sinking step was designed to sink the particles into the PMMA to a certain extent.

a b

Fig. 1.2 The SEM images (a) 10kX (b) 30kX of the synthesized bare silica nanoparticles. The nanoparticles reached a diameter of 434nm±8.6nm after 10 times of TEOS and water addition.

30 When the particles appeared to be monodispersed in sizes, we would get most particles at the same depth in PMMA or with same exposure area outside PMMA.

From the integrated DLS result (see Fig. 1.3), we can see that the measured sizes

(450nm) were a little higher than the calculated SEM result (434nm). This can be attributed to the hydrodynamic diameter measured by DLS. There was an electric dipole layer of solvent (water) at the surface of hydrophilic silica nanoparticles. This layer of solvent will influence the movement the particles and will result as a higher hydrodynamic diameter measured by DLS.

The nanoparticles prepared by interfacial preparation method were also characterized by DLS. From the Fig. 1.4, the integrated DLS intensity data indicated that the particles size at 53nm with PDI at 0.074 demonstrating a good monodispersed of the nanoparticles.

30 Intensity Volume 25 Number

20

15

Intensity (%) Intensity 10

5

0 10 100 1000 10000 Size (nm) Fig. 1.3 The image of integrated DLS result. The overlap area of highest intensity indicated the sizes of particles at around 450nm.

31 10 100 1000

25 Intensity 25 Volume Number 20 20

15 15

10 10 Intensity (%) Intensity

5 5

0 0 10 100 1000 Size (nm)

Fig. 1.4 The image of integrated DLS result. The overlap area of highest intensity indicated the sizes of particles at around 50 nm.

4.2 Purification of Monodispersed Silica Nanoparticles

The monodispersed silica nanoparticles need to be purified for the following two reasons. (1) The hydrophilic particles are still reactive in the vessel, especially for the

400nm particles (the reaction occurred at room temperature). The aggregation phenomenon and sintering phenomenon happened in the flask would lead to the quality reduction of nanoparticles. (2) The surface of the particles is hydrophilic after synthesis.

The homogeneous nanoparticles can barely be maintained at the air-water interface in the

Langmuir-Blodgett trough due to the density and hydrophilicity of the particles.

Based on the above two reasons, the monodispersed nanoparticles were washed several times while monitoring the pH and conductivity of the suspension. The zeta potential was measured before the particles were ready to be tested with the LB method.

32 The successful condition is illustrated in Fig. 2.1. The suspension from the reaction beaker was washed with ethanol 4 times and 12 times with Milli-Q water beforehand. Then, 1g of nanoparticles were diluted in 40ml Milli-Q water, and conductivity was monitored during purification. The figure presented the trend of conductivity as the particles were washed with Milli-Q water. After 26 times of washings, the conductivity of the supernatant reached 6.4 μS/cm and the rate of conductivity decreasing became slower because the subphase was approaching Milli-Q water. Once the conductivity was obtained, the concentration of salt was calculated by a given correlation of ions concentration and conductivity[58]. The plot of the concentration of ions versus the washing times was also presented here (see Fig. 2.2).

The concentration of ions decreased rapidly at first and then a plateau was reached where the concentration maintained at around 5×10-5 mol/L, indicating that when the water purification was finished, the residual ammonium salts were almost washed away. The

Conductivity 40

35

30

25

20

15 Conductivity (μS/cm) Conductivity 10

5

0 5 10 15 20 25 30 Washing times

Fig. 2.1 The plot of the conductivity of supernatant versus the washing times. As the washing times increased, the conductivity decreased rapidly at first and then a plateau was reached where the conductivity maintained at around 6.4 μS/cm.

33 0.0025 + NH4

0.0020

0.0015

0.0010

0.0005 Concentration of Ion (mol/L) Ion of Concentration

0.0000 0 5 10 15 20 25 30 Times Fig. 2.2 The plot of the concentration of ions versus the washing times. As the washing times increased, the concentration of ions decreased rapidly at first and then a plateau was reached where the concentration maintained at around 5×10-5 mol/L. calculation was based on the assumption that the conductivity was induced only by the ionization of the salt. In fact, the conductivity could, at least partially, be induced by the ionization happened near the hydrophilic surface of the nanoparticles.

Also, we should notice here that the Milli-Q water obtained from the Water

Filtration Unit is processed of conductivity as 0.055μS/cm at room temperature. The difference in conductivity could be explained by ionization of residual salts that were not washed and the hydrophilic nanoparticles with negative surface charge.

The final condition of the particles is characterized by a Multiparameter

Meter and a ZetaSizer, and based on the result (see Fig. 2.3), when the pH of the colloid suspension was around 7.4 at room temperature, the zeta potential of the particles was around -40 mV, indicating the stability of the colloidal suspension because of sufficient repulsive forces among nanoparticles. Measurement of zeta potential is depending on the

34 600000

500000

400000

300000

Total Counts Total 200000

100000

0 -100 -75 -50 -25 0 Apparent Zeta Potential (mV) Fig. 2.3 The integrated zeta-potential result of the 400nm silica nano particles with PH=7.4 at room temperature. The result of the overlap area located at around -40~-41 mV. The result indicated the stable dispersion of nanoparticles in the suspension. movement of nanoparticles under the influence of an applied electric field. This movement depends upon surface charge and the local environment of the particle.

Comparing the Zeta potential measured in a SiOH silica suspension (see Fig. 2.4) from other’s research[59], we can see my absolute Zeta-potential was lower than the value from the research, which I believe could be attributed to the remaining salts in the suspension that buffered the Zeta potential.

After finishing water wash and characterization, the suspension was washed with ethanol several times. The thin layer of water around hydrophilic particles was replaced with ethanol after several times of ethanol washing. This step was to prepare for spreading suspension with chloroform-ethanol dispersant.

35 Fig. 2.4 This is the plot of relation between Zeta potential and pH in a SiOH silica suspension[58]. We can see the trend of increasing absolute Zeta potential as the pH value increasing from 2 to 10. The red cross was where my condition located. We can see the absolute Zeta-potential was lower than the value from the research, which could be attributed to the remaining salts in the suspension that buffered the Zeta potential.

4.3 Preparation of PMMA Film on Glass Substrates by Dip Coating

The thickness of the dip coating film is affected by several factors, including viscosity, withdraw velocity, surface tension, and density[54]. The thickness of the films can be calculated by the Landau-Levich equation (Equation 2.) when the withdraw speed is determined, which leads to a shear rate that keeps the system in a Newtonian regime. In this research, when the viscosity, surface tension, and density was defined, the thickness have an exponential relation with the withdraw velocity.

2/3 휂 2/3 h = 0.94 1/6 1/2 ∙ v Equation 2. 훾퐿푉 (휌 ∙ 푔)

36 700

600 h = 퐴 ∙ v2/3 500

400

300

Thickness h (nm) Thickness 200

100

0 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Withdraw speed v (mm/s)

Fig. 3.1 The plot of the exponential relationship with thickness and withdraw speed. When viscosity, surface tension and density was defined, the withdraw speed can be considered as an independent variable to the thickness with a constant coefficient A. The red spot was corresponding to the dip coating condition used for preparing PMMA film.

The original Landau-Levich equation was transformed to Equation 3 with the constant C including the viscosity of the PMMA (M.W. 15,000) in toluene solution and the surface pressure of the PMMA in toluene solution. The constant was optimized by

0.94 h = C ∙ ∙ v2/3 휌 ∙ 푔 1/2 Equation 3.

휂2/3 C = 1/6 훾퐿푉 other group members with the value at around 4×10-10.

The density of the solution was calculated with Equation 4. The density of 15k

PMMA was 1180 kg/m3 and the density of toluene is 867 kg/m3 in room temperature. As a result, the density of 10% PMMA solution was at 900 kg/m3. The coefficient A was calculated as 4×10-12 and the exponential relationship with thickness and withdraw speed

휌푆표푙 = 휌푃푀푀퐴 ∙ 푊푃푀푀퐴 + 휌푇표푙푢푒푛푒 ∙ 푊푇표푙푢푒푛푒 Equation 4. 37 was plotted. The dip coating condition was plotted on the figure based on this relationship. The withdraw speed of 1mm/s was used to prepare a 400nm PMMA film and the substrates were set still in the PMMA solution for 30 seconds before withdrawing.

4.4 Preparation of Colloid Monolayers with Langmuir-Blodgett Method

The colloid suspension preserved in Ethanol was prepared in a mixture of chloroform and ethanol at a specific ratio. The chloroform was solvent with properties including being water-insoluble and volatile, which could transport the silica nanoparticles to the surface. The process can be observed as a slight decrease in surface tension after colloid suspension deposition. Also, the introduction of the chloroform contributed to slightly modified silica nanoparticles as the wettability of the particles was changed by adsorption of chloroform molecules. In this study, the ratio of chloroform in the colloid suspension was 1:3 or 1:4 in a volume that was mostly used to deposit colloid monolayers, and sometimes the suspension was preserved overnight to obtain particles with relatively higher hydrophobicity. With the activation from ultrasonication, the chloroform molecules, ethanol molecules, and silica nanoparticles are fully mixed in the suspension. Before the particles are spread at the air-water surface, some of the chloroform molecules will be adsorbed onto the surface of hydrophilic nanoparticles.

Thus, the particles are partially hydrophobic. This is supported by the observation that without the introduction of chloroform into the spreading suspension, particulate films could not be formed. In some research[48], fully hydrophobized particles (by silylation) only formed weakly ordered structures rather than hexagonal close packing. The

38 researcher explained that, due to the hydrophobic effect, the adsorbed surfactant will aggregate and one or a few patches of surfactant domains will cover the surface of the sphere. These nanoparticles being able to maintain at the air-water interface were attributed to the fact that the patches of particles were oriented to minimize contact with a certain hydrophobicity of particles and the exposed area with the remaining hydrophilicity along with the hydrophobicity on the other side to create an electric double layer. These double layers at the particle surface (induced by the air-water interface) acted as repulsing dipoles that hindered the aggregation of particles at the interface and allowed for the formation of a hexagonal close packing structure within a defined area[49][50].

Before the deposition process, the mixture of the suspension was fully homogenized with the Ultrasonic Homogenizer in the lab. The preparation of the colloid monolayer was conducted on a Langmuir Trough and Deposition Unit (KSV NIMA) in the lab. The platinum built Wilhelmy plate balance was used to measure the surface pressure. The plate was cleaned with ethanol and water and blown with dry air. Before use, it was flame-cleaned, which made the surface hydrophilic. When the platinum plate was partially immersed in the water surface, the meniscus shifted the direction of the surface tension to a vertical direction, and the force was measured by an electric balance.

The surface tension can be calculated with the measured force and the parameters of the

Wilhelmy plate. If the value was zero when the Wilhelmy plate was hung in the air and the force was measured when the plate was partially immersed in the water surface, the value of the surface tension of the water could be directly obtained, which should be at around 73mN/m. The surface area of the trough can vary due to the movable barrier

39 composed of hydrophilic Delrin over the trough. The surface pressure and the mean molecular area were monitored during the experiment with the control and feedback system in the computer, and thus the surface condition was maintained when it was stable or when substrates were upstroke and downstroke to obtain colloid monolayers.

In this study, the hysteresis property and compressibility of the colloid monolayers were investigated using the isotherm generated during the Langmuir-

Blodgett experiment.

60

55

50

45

40

35

30 First compress 25

SP (mN/m) Second expand 20

15 First expand

Second compress 10

5

0 0.00E+000 5.60E+007 1.12E+008 1.68E+008 2.24E+008 MMA (A2/particle)

Fig. 4.1 This is the plot of isotherm curves of the hysteresis experiment. We can see that film underwent phase transitions from gaseous state to liquid extend state, to liquid condense state and finally to solid state. Also, we can see that the second isotherm is not identical to the first isotherm which means that the compression was irreversible.

40 2k resolution 1k resolution

Fig. 4.2 These are SEM images of multiple places on the LB film generated when the film was in liquid expand state. We can see that there were many large defects and gaps between patches, which indicated that the film was not closely packed and had space to be compressed.

Generally, the monolayer would exist as a gaseous state on a large surface area, which was reflected by the steady and low surface pressure. With a constant compression of barriers, the monolayer underwent a phase transition state, which was reflected by slowly increasing the surface pressure. Upon further compression, a steep increase in

2k resolution 1k resolution

Fig. 4.3 These are SEM images of multiple places on the LB film generated when the film was in liquid condense state. We can see that there were many small defects but there were no significant gaps between patches, which indicated that the film has already been closely packed and hardly had space to be compressed.

41 surface pressure was observed until either a tilted or a flat plateau was observed, which indicated the collapse of the monolayer (see Fig. 4.1).

From the isotherm curves of the hysteresis experiment (see Fig. 4.1), we can see that the second isotherm is not identical to the first isotherm which means that the compression was irreversible. This could be explained by the collapse or overlap happened during the compression. Collapse or overlap could happen when the compression speed was set too quick giving no sufficient time for particles reorganization, or the Langmuir film reached the solid state and there was no more space for compression, or the particles simply sink into the water phase because of density difference and hydrophilicity.

Comparing the SEM images of LB films obtained in both liquid expand and liquid condense state (See Figs. 4.2, 4.3), significant differences were observed. For a typical Langmuir film obtained at the interface of air and water, it underwent several phase transitions while the compressibility decreased as the increasing surface pressure.

This can be attributed to the reduction of free space at the interface during the compression. The compression modulus can be calculated from the isotherm by equation

5.[60],

−1 휕휋 퐶푠 = −퐴 ∙ Equation 5. 휕퐴 푇

-1 It is seen that Cs is proportional to the first derivative of the surface pressure () with respect to the mean molecular area (A), which makes this parameter sensitive to the isotherm slop changes. The compression modulus as the reciprocal of compressibility is strongly related to the molecular packing in the Langmuir films and hence provides

42 information about their thermodynamic state. In this case (see Fig. 4.4), the calculated

-1 compression modulus, Cs of surface pressure at 0-1mN/m was the lowest for gaseous

-1 -1 state; Cs of surface pressure at 1-30mN/m was still low for liquid expand state; Cs of

-1 surface pressure at 31-55mN/m was increasing rapidly for liquid condense state; Cs of surface pressure at >55 mN/m was the highest for solid state. We should notice that the calculated result of surface pressure over 55mN/m in the first compression was decreasing rapidly because of the isotherm plateau near solid state. As a result, the peak turning point was considered as the compression modulus of the solid state. The result of compressibility (reciprocal of compression modulus) coincided with the SEM result (see

Figs. 4.2, 4.3) with the highest compressibility at the gaseous state and lowest compressibility at solid state. Comparing the trend of compression modulus of the first and second compression, we noticed that the modulus of the first compression was

175 First Compression Second Compression 150

125

100

75

50

Compression modulus (mN/m) modulus Compression 25

0 0 10 20 30 40 50 60 SP (mN/m) Fig. 4.4 The plot of compression modulus-surface pressure formed from the isotherms of the hysteresis experiment. The plot clearly indicated that the compressibility of film remained same after -1 irreversible compression for the same CS at 134mN/m and the turning point of liquid expand to liquid condense for SP at 30mN/m.

43 Table 1. This is the arranged result of compressibility experiment. For a typical Langmuir film obtained at the interface of air and water, it underwent several phase transitions while the compressibility decreased as the increasing surface pressure. This can be attributed to the reduction of free space at the interface during the compression.

identical to the modulus of the second compression, indicating that the property of the

Langmuir film remained same after irreversible compression. The integrated results were presented in a table (see Table 1.).

In the hysteresis experiment, we found that the compression was an irreversible process once the Langmuir films reached solid state. However, after comparing the compression modulus, the property of the film remained the same after the compression and expansion process. As a result, we believed that this kind of compression process could be considered as a physical change on the Langmuir film, and the intrinsic property of the film remained unchanged. Also, a clear corresponding relation was found among the phase state of Langmuir film, surface pressure, and the compressibility.

4.5 PMMA Sinking

The PMMA sinking process was optimized by group members before. There was an apparent relationship between exposed height and heating time in the given temperature (see Fig. 5.1). We can see the particles were more deeply embedded in the

PMMA as increasing heating time in both 120 ℃ and 125 ℃. When the heating time was

44 settled, the higher temperature could lead to lower exposed height. Just above the glass

transition temperature of the 15k PMMA, the polymer chains in the amorphous region

began to slide past one another, which led to the sinking process of particles due to

gravity. With either longer heating time or higher heating temperature, more energy was

applied to mobilize the molecules and speed up the sinking process. Above all, three

conditions were mentioned in the graph with 25%, 50% and 75% exposed and were used

for preparing Janus particles.

The PMMA film was coated on the glass substrates with a 400nm thickness

previously; however, the actual condition was that when the particles were sunk into the

PMMA, the original space of the softened PMMA was captured by the particles, and thus

the thickness needed was lower than 400nm.

120℃ 500 125℃

400

300

200

Exposed height, h (nm) h height, Exposed 100

0 1 10 100 1000 Heating time, t (s)

Fig. 5.1 This is a plot of the relation between exposed height and heating time. This condition was optimized by group members before.

45 The PMMA sinking step is a critical step because the HLB of the Janus particles

is directly affected by the Janus balance. The parts protected by the PMMA remained

hydrophilic as silica particles, while the exposed parts were grafted alkyl groups or

fluoroalkyl groups that became hydrophobic during the CVD grafting step.

PID controller Heating plate

A self-made shelf consists of 4 A layer of silica pieces of thick nanoparticles on microscopy slides the PMMA coated glass substrate

Illustration 8. The schematic representation of PMMA sinking conducted on a heating plate with a self-made shelf and a PID controller.

4.6 Grafting of Ligands using Chemical Vapor Deposition

The chemical vapor deposition experiment was conducted using a self-made CVD

chamber with a Teflon shelf that can support 24mm×60mm glass slides from four edges

and that exposed most of the glass slides in the chamber. The coupling agent was set in

the reservoir, which was sealed and connected to the reaction chamber. The coupling

agent vaporized due to low air pressure and heat. The gaseous coupling agent was sent to

the chamber through flow control. The reaction occurred on the surface of the substrates

when the highly reactive chlorine group combined with the reactive hydroxyl group on

the particles and grafted the surface with either alkyl groups, which made the particles

hydrophobic. After the reaction, the chamber was purged with N2 to remove the residual

Silane vapor.

46 In this study, trimethylchlorosilane and n-propyldimethylchlorosilane were used as the coupling agents, and the contact angle was measured with a goniometer using the sessile drop technique to characterize the sample surface.

41.7 48.5 42.5 43.6 43.1 46.9 43.8 45.9

Fig. 6.1 This is the plot of contact angle results in various positions in the chamber. The trimethyl chlorosilane was used as coupling agent. No significant difference was observed in various positions in the chamber. The contact angle measured was lower than the theoretical value.

For the optimized condition of trimethylchlorosilane (see Fig. 6.1), the measured contact angle was 44.5±2.3 degrees. No significant difference was observed in various positions in the chamber. The contact angle of plasma treated glass substrates should be at around 7.5~10 degrees[61]. The contact angle can be as high as 70-80 degrees with the saturated TMS groups on the glass surface[62][63]. The difference between the actual contact angles and the other researcher’s result was attributed to the lack of reaction times

47 and reaction temperature. The close contact angle result could be expected with longer reaction temperature and improved heating control.

For the optimized condition of n-propyldimethylchlorosilane (see Fig. 6.2), the

64.7 62.1 60.3 62.8 59.7 57.7 56.9 60.4

Fig. 6.2 This is the plot of contact angle results in various positions in the chamber. The n-propyl- dimethylchlorosilane was used as coupling agent. No significant difference was observed in various positions in the chamber. After changing the coupling agent with longer alkyl group, the modified surface was becoming more hydrophobic. measured contact angle was 60.6±2.6 degrees. No significant difference was observed in various positions in the chamber. After changing the coupling agent with a longer alkyl group, a more hydrophobic surface was obtained after modification. The reaction time was extended to 60min from 30min for n-propyldimethylchlorosilane with the intention that the larger molecules needed more time to react at the surface.

48 In this research, the homogeneous results were obtained in the self-made CVD with the optimized CVD conditions (see Table 2.).

Table 2. The list of the optimized CVD conditions and results.

49 CHAPTER V

SUMMARY

To synthesize Janus particles, several experimental conditions before the PMMA sinking step were optimized in this study. For the bare silica nanoparticle synthesis, the single addition method and interfacial preparation method were proved to be the effective methods through experiments. Despite the long synthesis time period, the productivity and the quality of the particles desired were achieved. For silica nanoparticle purification, the washing criteria were settled to remove residual reactants and maintain the bare silica nanoparticles at the air-water interface. Langmuir-Blodgett films were obtained on the substrates coated with PMMA despite the appearance of lumps on the colloid monolayers. PMMA sinking and CVD modification conditions have been optimized before by group members and these conditions need to be further optimized to accommodate various sinking environment and modification situations.

This research provided a novel method to synthesize Janus particles that have potential application in Pickering emulsion stabilizers and in the EOR industry. The future of this research can be studying Janus particles in two ways. One is to synthesize monodispersed Janus particles with various sizes and study their efficiency on Pickering emulsion stabilization. The other is to synthesize Janus particles with different HLB

50 values and relative areas of anisotropic faces by using versatile coupling silanes or by changing modification area to study the phase behavior of oil-water-nanoparticle ternary mixtures and the colloidal stability of Janus particles and Janus particles stabilized

Pickering emulsions.

51 BIBLIOGRAPHY

[1] Hashemi, R., Nassar, N. N., & Almao, P. P. (2014). Nanoparticle technology for heavy oil in-situ upgrading and recovery enhancement: Opportunities and challenges. Applied Energy, 133, 374-387.

[2] Kumar, A., Park, B. J., Tu, F., & Lee, D. (2013). Amphiphilic Janus particles at fluid interfaces. Soft Matter, 9(29), 6604-6617.

[3] Youssif, M. I., El-Maghraby, R. M., Saleh, S. M., & Elgibaly, A. (2018). Silica nanofluid flooding for enhanced oil recovery in sandstone rocks. Egyptian Journal of Petroleum, 105-110. 27(1).

[4] Wang, L., Wang, Z., Yang, H., & Yang, G. (1999). The study of thermal stability of the SiO2 powders with high specific surface area. Materials chemistry and physics, 57(3), 260-263.

[5] Percebom, A. M., & Costa, L. H. M. (2019). Formation and assembly of amphiphilic Janus nanoparticles promoted by polymer interactions. Advances in colloid and interface science.

[6] Agrawal, G., & Agrawal, R. (2019). Janus nanoparticles: recent advances in their interfacial and biomedical applications. ACS Applied Nano Materials, 2(4), 1738-1757.

[7] Luo, D., Wang, F., Zhu, J., Cao, F., Liu, Y., Li, X., & Ren, Z. (2016). Nanofluid of graphene-based amphiphilic Janus nanosheets for tertiary or enhanced oil recovery: High performance at low concentration. Proceedings of the National Academy of Sciences, 113(28), 7711-7716.

[8] Dahle, G. S. (2014). Investigation of how hydrophilic silica nanoparticles affect oil recovery in Berea sandstone: an experimental study (Master's thesis, Institutt for petroleumsteknologi og anvendt geofysikk).

[9] Sharma, T., Kumar, G. S., Chon, B. H., & Sangwai, J. S. (2015). Thermal stability of oil- in-water Pickering emulsion in the presence of nanoparticle, surfactant, and polymer. Journal of Industrial and Engineering Chemistry, 22, 324-334.

[10] Kong, X., & Ohadi, M. (2010, January). Applications of micro and nano technologies in the oil and gas industry-overview of the recent progress. In Abu Dhabi international petroleum exhibition and conference. Society of Petroleum Engineers.

[11] Kapusta, S., Balzano, L., & Te Riele, P. M. (2011). Presented in part at the International petroleum technology conference. Bangkok, Thailand.

[12] Li, L., Yuan, X., Xu, X., Li, S., & Wang, L. (2013, March). Vital role of nanotechnology and nanomaterials in the field of oilfield chemistry. In IPTC 2013: International Petroleum Technology Conference. 52 [13] Ayatollahi, S., & Zerafat, M. M. (2012, January). Nanotechnology-assisted EOR techniques: New solutions to old challenges. In SPE international oilfield nanotechnology conference and exhibition. Society of Petroleum Engineers.

[14] de Gennes, P. G. (1992). Soft matter (Nobel lecture). Angewandte Chemie International Edition in English, 31(7), 842-845.

[15] Wang, H., Yang, S., Yin, S. N., Chen, L., & Chen, S. (2015). Janus suprabead displays derived from the modified photonic crystals toward temperature magnetism and optics multiple responses. ACS applied materials & interfaces, 7(16), 8827-8833.

[16] Bradley, L. C., Chen, W. H., Stebe, K. J., & Lee, D. (2017). Janus and patchy colloids at fluid interfaces. Current opinion in colloid & interface science, 30, 25-33.

[17] Yoshida, M., & Lahann, J. (2008). Smart nanomaterials. ACS nano, 2(6), 1101- 1107.

[18] Yue, S., Sun, X., Wang, N., Wang, Y., Wang, Y., Xu, Z., & Wang, J. (2017). SERS– fluorescence dual-mode pH-sensing method based on Janus microparticles. ACS applied materials & interfaces, 9(45), 39699-39707.

[19] Fan, Yan Liang, et al. "Mechanistic formation of drug-encapsulated Janus particles through emulsion solvent evaporation." RSC advances 8.29 (2018): 16032- 16042.

[20] Modern Aspects of Emulsion Science; Binks, B. P., Ed.; The Royal Society of Chemistry: Cambridge, 1998.

[21] Pickering, S. U. (1907). Cxcvi-emulsions. Journal of the Chemical Society, Transactions, 2001-2021.

[22] Aveyard, R., Binks, B. P., & Clint, J. H. (2003). Emulsions stabilised solely by colloidal particles. Advances in Colloid and Interface Science, 100, 503-546.

[23] Ikem, V. O., Menner, A., & Bismarck, A. (2008). High internal phase emulsions stabilized solely by functionalized silica particles. Angewandte Chemie International Edition, 47(43), 8277-8279.

[24] Saleh, N., Sarbu, T., Sirk, K., Lowry, G. V., Matyjaszewski, K., & Tilton, R. D. (2005). Oil-in- water emulsions stabilized by highly charged polyelectrolyte-grafted silica nanoparticles. Langmuir, 21(22), 9873-9878.

[25] Binks, B. P., & Horozov, T. S. (2005). Aqueous foams stabilized solely by silica nanoparticles. Angewandte Chemie International Edition, 44(24), 3722-3725.

[26] Faria, J., Ruiz, M. P., & Resasco, D. E. (2010). Phase‐selective catalysis in emulsions stabilized by Janus silica‐nanoparticles. Advanced Synthesis & Catalysis, 352(14‐15), 2359-2364. 53 [27] Melle, S., Lask, M., & Fuller, G. G. (2005). Pickering emulsions with controllable stability. Langmuir, 21(6), 2158-2162.

[28] Li, J., & Stö ver, H. D. (2008). Doubly pH-responsive pickering emulsion. 24(23), 13237-13240.

[29] Negin, C., Ali, S., & Xie, Q. (2016). Application of nanotechnology for enhancing oil recovery–A review. Petroleum, 2(4), 324-333.

[30] Nazari Moghaddam, R., Bahramian, A., Fakhroueian, Z., Karimi, A., & Arya, S. (2015). Comparative study of using nanoparticles for enhanced oil recovery: wettability alteration of carbonate rocks. Energy & Fuels, 29(4), 2111-2119.

[31] Cheraghian, G., & Hendraningrat, L. (2016). A review on applications of nanotechnology in the enhanced oil recovery part A: effects of nanoparticles on interfacial tension. International Nano Letters, 6(2), 129-138.

[32] Zargartalebi, M., Kharrat, R., & Barati, N. (2015). Enhancement of surfactant flooding performance by the use of silica nanoparticles. Fuel, 143, 21-27.

[33] Roustaei, A., Saffarzadeh, S., & Mohammadi, M. (2013). An evaluation of modified silica nanoparticles’ efficiency in enhancing oil recovery of light and intermediate oil reservoirs. Egyptian Journal of Petroleum, 22(3), 427-433.

[34] Ali, J. A., Kolo, K., Manshad, A. K., & Mohammadi, A. H. (2018). Recent advances in application of nanotechnology in chemical enhanced oil recovery: Effects of nanoparticles on wettability alteration, interfacial tension reduction, and flooding. Egyptian journal of petroleum.

[35] Stöber, W., Fink, A., & Bohn, E. (1968). Controlled growth of monodisperse silica spheres in the micron size range. Journal of colloid and interface science, 26(1), 62- 69.

[36] LaMer, V. K., & Dinegar, R. H. (1950). Theory, production and mechanism of formation of monodispersed hydrosols. Journal of the American Chemical Society, 72(11), 4847-4854.

[37] Van Blaaderen, A., Van Geest, J., & Vrij, A. (1992). Monodisperse colloidal silica spheres from tetraalkoxysilanes: particle formation and growth mechanism. Journal of colloid and interface science, 154(2), 481-501.

[38] Bogush, G. H., Tracy, M. A., & Zukoski Iv, C. F. (1988). Preparation of monodisperse silica particles: control of size and mass fraction. Journal of non-crystalline solids, and continuous production process. Journal of the European Ceramic 1S4o(c3ie),ty2,05-214.

[40] Chang, S. M., Lee, M., & Kim, W. S. (2005). Preparation of large monodispersed particles using seed particle growth. Journal of colloid and interface science, 286(2), 536- 54 542.

[41] Nagao, D., Nakabayashi, H., Ishii, H., & Konno, M. (2013). A unified mechanism to quantitatively understand silica particle formation from tetraethyl orthosilicate in batch and semi- batch processes. Journal of colloid and interface science, 394, 63-68.

[42] Nagao, D., Osuzu, H., Yamada, A., Mine, E., Kobayashi, Y., & Konno, M. (2004). Particle formation in the hydrolysis of tetraethyl orthosilicate in pH buffer solution. Journal of colloid and interface science, 279(1), 143-149.

[43] Hartlen, K. D., Athanasopoulos, A. P., & Kitaev, V. (2008). Facile preparation of highly monodisperse small silica spheres (15 to> 200 nm) suitable for colloidal templating and formation of ordered arrays. Langmuir, 24(5), 1714-1720.

[44] Watanabe, R., Yokoi, T., Kobayashi, E., Otsuka, Y., Shimojima, A., Okubo, T., & Tatsumi, T. (2011). Extension of size of monodisperse silica nanospheres and their well- ordered assembly. Journal of colloid and interface science, 360(1), 1-7.

[45] Reculusa, S., & Ravaine, S. (2003). Synthesis of colloidal crystals of controllable thickness through the Langmuir-Blodgett technique. Chemistry of materials, 15(2), 598- 605.

[46] Szekeres, M., Kamalin, O., Schoonheydt, R. A., Wostyn, K., Clays, K., Persoons, A., & Dékány,I. (2002). Ordering and optical properties of monolayers and multilayers of silica spheres deposited by the Langmuir–Blodgett method. Journal of Materials Chemistry, 3268- 3274.

[47] Gopinathan, A., Zhou, T., Coppersmith, S. N., Kadanoff, L. P., & Grier, D. G. (2002). Weak long-ranged Casimir attraction in colloidal crystals. EPL (Europhysics Letters), 57(3), 451.

[48] Duffel, B., Robin, H. A., & áDe Schryver, F. C. (2001). Langmuir –Blodgett deposition and optical diffraction of two-dimensional opal. Journal of Materials Chemistry, 3333-3336.

[49] Pieranski, P. (1980). Two-dimensional interfacial colloidal crystals. Physical Review Letters, 45(7), 569.

[50] Hurd, A. J. (1985). The electrostatic interaction between interfacial colloidal particles. Journal of Physics A: Mathematical and General, 18(16), L1055.

[51] Tolnai, G., Agod, A., Kabai-Faix, M., Kovacs, A. L., Ramsden, J. J., & Hórvölgyi, Z. (2003). Evidence for Secondary Minimum Flocculation of Stö ber Silica Nanoparticles at the Air− Water Interface: Film Balance Investigations and Computer Simulations. The Journal of Physical Chemistry B, 107(40), 11109-11116.

[52] Aveyard, R., Clint, J. H., Nees, D., & Paunov, V. N. (2000). Compression and structure of monolayers of charged latex particles at air/water and octane/water 55 interfaces. Langmuir, 16(4), 1969–1979.

[53] Lugomer, S., Zolnai, Z., Toth, A. L., Barsony, I., Maksimović, A., & Nagy, N. (2012). Reorganization of Langmuir–Blodgett layers of silica nanoparticles induced by the low energy, high fluence ion irradiation. Thin Solid Films, vol. 520, no. 11, 2012, pp. 4046–4056.

[54] L. D. Landau, B. G. Levich, Acta Physiochim, U.R.S.S., 17 (1942) 42-54

[55] David L. Chandler “Explained: chemical vapor deposition Technique enables production of pure, uniform coatings of metals or polymers, even on contoured surfaces.” 2015. MIT News

[56] “Information on Contact Angle” 2019. ramé-hart instrument co.

[57] Zhang, J. H., Zhan, P., Wang, Z. L., Zhang, W. Y., & Ming, N. B. (2003). Preparation of monodisperse silica particles with controllable size and shape. Journal of materials research, 18(3), 649-653.

[58] LaBelle, B. (2005) Sensing pH, controlling pH, Liquid analysis systems and sensors are cost effective tools against corrosion. Intech Magazine.

[59] Dyab, A. K. (2012). Destabilisation of Pickering emulsions using pH. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 402, 2-12.

[60] Yu, Z. W., Jin, J., & Cao, Y. (2002). Characterization of the liquid-expanded to liquid-condensed phase transition of monolayers by means of compressibility. Langmuir, 18(11), 4530-4531.

[61] Homola, T., Matoušek, J., Kormunda, M., Wu, L. Y., & Černák, M. (2013). Plasma treatment of glass surfaces using diffuse coplanar surface barrier discharge in ambient air. Plasma Chemistry and Plasma Processing, 33(5), 881-894.

[62] Fuji, Masayoshi & Fujimori, Hiroshi & Takei, Takashi & Watanabe, Tohru & Chikazawa, Masatoshi. (1998). Wettability of Glass-Bead Surface Modified by Trimethylchlorosilane. Journal of Physical Chemistry B - J PHYS CHEM B. 102. 10.1021/jp981983d.

[63] Brito e Abreu, Susana & Skinner, William. (2012). Determination of Contact Angles, Silane Coverage, and Hydrophobicity Heterogeneity of Methylated Quartz Surfaces Using ToF-SIMS. Langmuir: the ACS journal of surfaces and colloids. 28. 7360-7. 10.1021/la300352f.

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