Hydrophobization of Silica Nanoparticles in Water: Nanostructure and Response to Drying Stress Solenn Moro, Caroline Parneix, Bernard Cabane, Nicolas Sanson, Jean-Baptiste d’Espinose de Lacaillerie
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Solenn Moro, Caroline Parneix, Bernard Cabane, Nicolas Sanson, Jean-Baptiste d’Espinose de Lacaillerie. Hydrophobization of Silica Nanoparticles in Water: Nanostructure and Re- sponse to Drying Stress. Langmuir, American Chemical Society, 2017, 33 (19), pp.4709-4719. 10.1021/acs.langmuir.6b04505. hal-01520275
HAL Id: hal-01520275 https://hal.sorbonne-universite.fr/hal-01520275 Submitted on 10 May 2017
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1 2 3 4 1 Hydrophobization of Silica Nanoparticles in Water: 5 6 7 8 9 2 Nanostructure and Response to Drying Stress 10 11 12 13 †,‡,§ § † ,†,‡ 14 3 Solenn Moro, Caroline Parneix, Bernard Cabane, Nicolas Sanson,* and Jean Baptiste 15 16 4 d'Espinose de Lacaillerie*,† 17 18 19 5 † PSL Research University, ESPCI Paris, CNRS UMR 7615 and UMR 8231, 10 rue Vauquelin, F 75231 20 21 22 6 Paris cedex 05, France 23 24 25 7 ‡ Sorbonne Universités, UPMC Univ Paris 06, SIMM, 10 rue Vauquelin, F 75231 Paris cedex 05, 26 27 28 8 France 29 30 § 31 9 Saint Gobain Recherche, 39 quai Lucien Lefranc, 93303 Aubervilliers Cedex 32 33 34 10 35 36 37 11 38 39 40 12 41 42 13 43 44 14 45 46 47 15 48 49 16 50 51 52 17 53 54 18 55 56 19 57 58 59 60 ACS Paragon Plus Environment 1 Langmuir Page 2 of 34
20 ABSTRACT 1 2 3 4 21 In this paper, we investigated the impact of surface hydrophobization on the structure of aqueous 5 6 7 22 silica dispersions and how this structure resists drying stress. To achieve this, hydrophilic silica particles 8 9 23 were hydrophobized directly in water using a range of organosilane precursors, with a precise control of 10 11 24 the grafting density. The resulting nanostructure was precisely analyzed by a combination of small angle 12 13 14 25 X ray scattering (SAXS) and cryo microscopy (cryo TEM). Then, the dispersion was progressively 15 16 26 concentrated by drying and the evolution of the nanostructures as a function of the grafting density was 17 18 27 followed by SAXS. At the fundamental level, because the hydrophobic character of the silica surfaces 19 20 21 28 could be varied continuously through a precise control of the grafting density, we were able to observe 22 23 29 how the hydrophobic interactions change particles interactions and aggregates structures. Practically, 24 25 26 30 this opened a new route to tailor the final structure, the residual porosity and the damp proof properties 27 28 31 of the fully dried silica. For example, regardless of the nature of the hydrophobic precursor, a grafting 29 30 32 density of 1 grafter per nm 2 optimized the interparticle interactions in solution in view to maximize the 31 32 3 33 33 residual porosity in the dried material (0.9 cm /g) and reduced the water uptake to less than 4 % in 34 35 34 weight compared to the typical value of 13 % for hydrophilic particles (at T=25 °C and relative 36 37 35 humidity RH=80 %). 38 39 40 36 41 42 37 43 44 38 45 46 47 39 1. INTRODUCTION 48 49 40 When an aqueous colloidal dispersion is dried, the concentration of the particles results in a 50 51 52 41 stress build up. Depending on the interaction potential between the particles, this added stress can result 53 54 42 in various transitions through the colloidal phase diagram leading for examples to ordered solid states or 55 56 43 to the formation of aggregated structures. The control of these phenomena is essential in many 57 58 59 44 applications involving coating and drying. It is generally achieved by modulating the strength and the 60 ACS Paragon Plus Environment 2 Page 3 of 34 Langmuir
45 range of attractive and repulsive interactions between nanoparticles. Silica particles constitute a 1 2 3 46 convenient model system for the investigation of such phenomena because their surface chemistry in 4 5 47 aqueous media is well known and make them good candidates for the study of aggregation processes. 6 7 48 Indeed, variations of pH result in a modulation of the number of negatively charged silanolate surface 8 9 10 49 groups while the range of the resulting repulsive electrostatic interactions can be controlled through the 11 12 50 ionic strength of the suspending solution. 1, 2 Furthermore, aggregation of silica particles in aqueous 13 14 3 4 7 15 51 solution can also be induced using multivalent ions or polymers leading to the formation of 16 17 52 aggregates in aqueous solution. 18 19 20 53 Hydrophobic interactions can also modify the colloidal state of silica particles in water. 21 22 54 However, those short range attractive forces are not as commonly used as the one described above to 23 24 25 55 induce the destabilization of silica particles in polar media. Generally, the main method reported in the 26 27 56 literature to modify a particle’s hydrophilic lipophilic balance is through the adsorption of amphiphilic 28 29 30 57 molecules, such as cationic surfactants or charged copolymers, whose hydrophilic part adsorbs onto the 31 32 58 silica surface and whose hydrophobic part points outwards. In these systems, a fast flocculation occurs 33 34 59 at low surfactant concentrations due to strong hydrophobic interactions between the modified silica 35 36 8, 9 37 60 particles. However, the amphiphilic molecules can easily be desorbed upon modification of the 38 39 61 surrounding media and thus the silica hydrophobization can be reversible. When an irreversible 40 41 62 hydrophobization is desired, hydrophobic species must be covalently grafted onto silica surface. This 42 43 44 63 hydrophobic grafting of silica nanoparticles is performed through an hydrolysis condensation 45 46 64 mechanism of alkylsilanes with the surface silanol groups. The reaction is generally carried out in 47 48 10 12 49 65 organic solvents to insure that hydrolysis only occurs with the residual adsorbed water on the silica 50 10, 51 66 surface and that the condensation is principally obtained between the precursor and the silica surface. 52 53 67 13 In this manner, a rather well defined monolayer coverage of the silica surface can be obtained but this 54 55 56 68 is at the inconvenience and cost of using an organic apolar solvent which must be further exchanged 57 58 69 with water. Thus, during the last 20 years, numerous investigations were conducted on the hydrophobic 59 60 ACS Paragon Plus Environment 3 Langmuir Page 4 of 34
70 grafting in water rich or biphasic media. De Monredon Serani et al. studied the grafting of precipitated 1 2 14 3 71 silica particles in a water / ethanol (25/75 v/v) mixture by alkylalcoxysilanes. Schewertfeger et al. 4 5 72 found that the hydrophobization of a xerogel with trimethylchlorosilane in water is possible by addition 6 7 73 of hexamethyldisiloxane. 15 The principal inconvenient of such water rich mixtures is the fact that the 8 9 10 74 precursor must be introduced in large excess with regards to the reactive silanol functions on the silica 11 12 75 surface. More recently, silica particles were chemically modified with high reaction yields thanks to 13 14 16, 17 15 76 multifunctional glycidoxysilanes or polyalkyleneoxysilanes in pure water. A good grafting 16 17 77 efficiency was achieved with the dropwise precursor addition in the silica dispersion so that it 18 19 78 preferentially reacts with the surface silica instead of inducing a self condensation. Inspiring ourselves 20 21 22 79 from this work, we have developed a protocol for the chemical modification of silica particles with pure 23 24 80 hydrophobic silanes directly in water. This original chemistry provided an opportunity to study in water, 25 26 81 without solvent exchange, the phase diagram of silica dispersions with respect to their hydrophobic 27 28 29 82 character and applied drying stress. 30 31 32 83 In the present work, we investigate the relation between the chemical hydrophobization of silica 33 34 84 nanoparticles in water and the resulting dispersion nanostructure, its drying behavior and the water 35 36 37 85 uptake of the hydrophobized silica particles. To achieve this, hydrophilic silica nanoparticles were first 38 39 86 hydrophobized in water using different hydrophobic organosilane precursors. The colloidal state of the 40 41 87 modified silica dispersions was studied as a function of both the organosilane precursor's nature and the 42 43 44 88 grafting density. Then, the resulting nanostructures were followed using cryo transmission electronic 45 46 89 microscopy (cryo TEM) and small angle X ray scattering (SAXS). As the drying state constitutes the 47 48 49 90 ultimate concentration process in which capillary pressures are known to collapse porous structures, the 50 51 91 impact of the silica hydrophobization in pure water on the structural changes upon drying has been 52 53 92 investigated by SAXS experiments. In parallel, mercury intrusion porosimetry on dried powders gave 54 55 56 93 complementary results on the aggregate's organization at larger scale upon drying in the final porous 57 58 94 material. Finally, the damp proof behavior of such hydrophobized silica particles was evaluated by 59 60 ACS Paragon Plus Environment 4 Page 5 of 34 Langmuir
95 means of contact angle measurements on dip coated colloidal films and water uptake on the formed 1 2 3 96 materials after drying. This set of analysis allowed us to respond efficiently to several questions: 4 5 6 97 (i) What is the impact of hydrophobization on the colloidal stability of the silica dispersion and 7 8 98 consequently on their resulting nanostructure? Indeed, the presence of hydrophobic groups on the 9 10 11 99 surfaces of neighbouring particles should change their interactions in solution. 12 13 14100 (ii) Compared to pure hydrophilic silica particles where dense structures are obtained under 15 16101 drying, how do the structural pathways of hydrophobized silica evolve under drying stress? In other 17 18 19102 words, how does a hydrophobized silica dispersion dry? 20 21 22103 (iii) Finally does this chemical hydrophobization induce damp proof properties to the final 23 24104 material, that is to the dried modified silica dispersion? This can be appreciated through the water 25 26 27105 uptake of the silica material formed from the drying of the hydrophobized silica particles solution. 28 29 30106 In short, by answering the questions listed above, we investigated in this paper the possibility and 31 32107 potential of ingeniously controlling in water nanoparticle interactions using hydrophobic grafters. 33 34 35108 36 37 38109 2. EXPERIMENTAL PART 39 40110 2.1. Materials. A commercial Ludox® TM 50 colloidal silica, with a surface area of 140 m²/g, was 41 42111 used in this study. The average radius of the particles (R of 13 nm) and the width of the distribution 43 p 44 45112 (σR/Rp=0.12) were determined by fitting the X ray scattering curve of a highly diluted dispersion 46 47113 (volume fraction Φv = 0.005) with a Schultz distribution of homogeneous spheres (see Figure S1 in 48 49 50114 Supporting information). The commercial dispersion was dialyzed (Spectra/por dialysis membrane 51 52115 MWCO: 12 14 kDa from SpectrumLaboratories, Inc.) against ultrapure water until its conductivity 53 54116 dropped below 150 µS/cm. The silica dispersion’s pH was readjusted to 9.0 with a few drops of a 55 56 57117 concentrated (1 M) sodium hydroxide solution. Different methoxy hydrophobic organosilane precursors 58 59118 were used in the hydrophobization reaction: propyl(trimethoxy)silane (PTMS), 60 ACS Paragon Plus Environment 5 Langmuir Page 6 of 34
119 isobutyl(trimethoxy)silane (iBTMS) and dimethoxydimethylsilane (DDMS). All products (reagent 1 2 3 120 grade) were purchased from Sigma Aldrich and used as received. Ultrapure deionized water with a 4 5 121 minimum resistivity of 18 M .cm (milliQ, Millipore, France) was used in the experiments. 6 7 122 2.2. Synthesis of Hydrophobized Silica Particles . After dialysis against ultrapure water, a 0.05 8 9 10123 volume fraction aqueous silica dispersion at pH = 9.0 was stirred at 60 °C. Then, the methoxy 11 12124 hydrophobic organosilane precursor was very slowly added into the silica dispersion under vigorous 13 14 15125 stirring during 8 hours with a syringe pump and the mixture was kept at 60 °C under stirring during 24 16 17126 hours. The amount of added organosilane precursors was varied to target a range of molar grafting ratio 18 19127 between 0 and 2 in order to reach different degrees of hydrophobicity. The molar grafting ratio is 20 21 22128 defined here as the ratio of organosilane precursors to the total surface SiOH groups assuming an 23 24129 hypothetical average surface density of 5 SiOH/nm 2.18 Note that this target value thus correspond to a 25 26130 working parameter different from the actual grafting ratio which will depend on the grafting reaction 27 28 29131 efficiency and on the true silanol surface density. Finally, the silica dispersion was dialyzed against 30 31132 water at pH = 9.0 to remove the non grafted precursors. At this stage, the volume fraction of silica in 32 33133 water was readjusted to 0.05. To assess the grafting efficiency, part of each sample, corresponding to 34 35 36134 about 1 g of modified silica was dried at 120 °C overnight and crushed with a pestle and mortar. The 37 38135 obtained powder was washed in a soxhlet device with a 1:1 dichloromethane / diethyl ether solvent to 39 40 41136 extract any adsorbed precursor left. Grafting efficiency measurements were performed on both the 42 43137 dialyzed modified silica and the particles washed with 1:1 dichloromethane / diethyl ether solvent. No 44 45138 significant difference was found between the two samples, indicating that dialysis is a sufficiently 46 47 48139 efficient purification step. 49 50140 2.3. Concentration and Drying of the Silica Dispersions . 15 g of the silica dispersion, grafted or 51 52141 not, were introduced into different polypropylene containers with an internal diameter of 30 mm. The 53 54 55142 samples were let to concentrate and dry at 90 °C in an oven during 3 to 30 hours. Before analysis, the 56 57143 obtained concentrated pastes were re homogenized by gentle manual stirring. The volume fraction, Φv, 58 59 60144 was determined by weighting dry extracts for each samples. ACS Paragon Plus Environment 6 Page 7 of 34 Langmuir
145 2.4. Characterization . Thermogravimetric Analysis . Thermogravimetric analysis (TGA) was 1 2 3 146 performed using an SDT Q600 analyser from TA Instruments equipped with a flow gas system. After an 4 5 147 isotherm at 110 °C during 20 min, the samples were heated up to 1150 °C with a heating rate of 10 6 7 148 °C/min in an air atmosphere. The weight loss of grafted silica can be attributed to (i) condensation of 8 9 10149 silanol groups (or dehydroxylation) and (ii) oxidation of the hydrophobic alkyl grafters (pretreatment at 11 12150 110 °C for 20 min insured the prior removal of physisorbed water molecules). While dehydroxylation 13 14 15151 takes place over a large temperature range, the oxidation of the alkyl grafters takes place around 450 °C 16 17152 with only slight variations depending on the hydrophobic organosilane precursor. Finally, at 1150 °C, 18 19153 the dehydroxylation is complete and the silica surface is purely composed of siloxane bonds. The 20 21 22154 grafter’s contribution and consequently the grafting density (number of grafters per nm²) was therefore 23 24155 obtained by subtracting the pure silica thermogram from the one of the modified samples and calculated 25 26156 by taking into account the alkyl chain’s length. Moreover, TGA measurements allows us to directly 27 28 29157 derive a grafting surface density without assumptions on the silanol density and/or the chemical 30 31158 reactivity of the hydrophobic grafter. TGA measurements were repeated twice on several samples and 32 33159 found very reproducible, within 0.1 % of weight loss variation. The grafting density could therefore be 34 35 36160 determined when the TGA weight loss value was at least 0.3 wt % higher than the one for unmodified 37 38161 silica. 39 40 41162 Microscopy . The colloidal dispersions were spread on ultrathin 300 mesh Formvar/carbon coated 42 43163 copper grids and maintained in a frozen hydrated state by quenching into liquid ethane cooled by liquid 44 45164 nitrogen. The cryofixed specimens were mounted into a Gatan cryoholder for direct observation at 180 46 47 48165 °C in a JEOL 2100HC cryo TEM operating at 200 kV with a LaB 6 filament. Images were recorded in 49 50166 zero loss mode with a Gif Tridiem energy filtered CCD camera equipped with a 2k x 2k pixel sized 51 52167 chip. Acquisition was accomplished with Digital Micrograph™ software. 53 54 55168 Small Angle X ray Scattering (SAXS) . SAXS experiments were carried out on the SWING beamline at 56 57169 the Soleil Synchrotron (Saclay, France). The detector, an AVIEX CCD camera, was placed at 8 m from 58 59170 the sample. In this configuration, the q range extended from 0.00107 to 0.152 Å 1. All measurements 60 ACS Paragon Plus Environment 7 Langmuir Page 8 of 34
171 were done under atmospheric pressure, at 22 °C and in ambient humidity (relative humidity of 50 % 1 2 3 172 measured with a humidity sensor). Diluted samples were studied in a fix capillary tube (diameter of 2.05 4 5 173 mm) whereas more concentrated dispersions were deposited in 1 mm gap hermetic cells with kapton 6 7 174 films. The assembly was hermetically sealed to prevent drying. The backgrounds scattering from the 8 9 10175 empty and water filled measurement cells were subtracted from the intensity curves. More details on 11 12176 SAXS data processing are given in Supporting Information. 13 14 15177 Mercury Intrusion Porosimetry . Mercury intrusion porosimetry measurements were performed with 16 17178 Pascal 140 and Pascal 240 porosimeters. The preliminary dried materials were maintained at 105 °C 18 19179 until the beginning of the measurement to prevent any re adsorption of water. Approximately 0.2 g of 20 21 22180 sample was introduced into the measurement cell where a vacuum of 10 Pa was reached. The cell was 23 24181 filled with mercury and an increased pressure was applied on the cell up to a maximum of 200 MPa. 25 26182 The pore size's distribution was obtained assuming cylindrical pores. 27 28 29183 Near Infrared Spectroscopy . The near infra red spectra were recorded using a Bruker NIR MPA at 30 31184 ambient temperature and humidity. 32 33185 Contact Angle Measurements . Contact angle measurements were carried out on dip coated silica 34 35 36186 dispersions. The thicknesses of the films were measured using a 3D optical profilometer (FOGALE 37 38187 nanotech). The thin films were maintained in a controlled relative humidity of 43 % thanks to a 39 40 41188 saturated aqueous solution of potassium carbonate K2CO 3. A 3 µL drop of ultrapure deionized water 42 43189 was deposited on the surface and the drop’s profile was recorded over time with a monochrome video 44 45190 camera. Contact angles were obtained from the drop’s profiles. 46 47 48191 Water Adsorption . Water adsorption measurements were realized on the modified silica. After a 48 h 49 50192 drying performed at 120 °C in an oven, the powders were introduced in an environmental test chamber 51 52193 Espec SH 641 at 25 °C and a relative humidity of 80 %. The powders weight increase was followed 53 54 55194 over time until equilibrium which can take from several days to several weeks depending on the 56 57195 hydrophilicity of the silica surface. 58 59196 60 ACS Paragon Plus Environment 8 Page 9 of 34 Langmuir
197 1 2 3 198 4 5 199 6 7 200 3. RESULTS AND DISCUSSION 8 9 10201 3.1. Hydrophobization reaction. The hydrophobization of silica particles was performed in water by 11 12202 grafting hydrophobized organosilane precursors on SiOH groups located at the silica particle surfaces. 13 14 15203 The introduced molar grafting ratio was defined as the number of organosilane precursors divided by 16 2 18 17204 the total number of SiOH groups assuming an average surface density of 5 SiOH/nm . The 18 19205 hydrophobized precursors were very slowly added to the silica dispersion in order to favor their grafting 20 21 22206 on silica surface whilst avoiding their reaction with each other (self condensation) in water. Methoxy 23 24207 organosilane precursors were preferred from halogenated precursors in order to avoid the formation of 25 26208 hydrochloric acid that modifies both the pH and the ionic strength of the solution and consequently can 27 28 29209 change the colloidal stability of the silica dispersion. Depending on both the molar grafting ratio and on 30 31210 the organosilane precursor, the silica dispersions went from slightly turbid to completely opaque and 32 33211 viscous for high grafting ratios (see Figure S2 in Supporting Information). The efficiency of the 34 35 36212 hydrophobization reaction was evaluated through thermogravimetric analysis (TGA) by measuring the 37 38213 evolution of the experimental grafting density as a function of the amount of grafters introduced in the 39 40 41214 systems for different hydrophobic organosilane precursors (see experimental part). The results are 42 43215 shown in Figure 1. At low grafting densities, for PTMS and iBTMS grafters, it appeared that the 44 45216 grafting densities were roughly linear with the amount of introduced grafters, regardless to the alkyl 46 47 48217 chain’s length and reached an efficiency of 20 30 % on average. In this range, the hydrophobization 49 50218 caused little changes of turbidity (see Figure S2 in Supporting information). Note that for the DDMS 51 52219 precursor, the TGA was insufficiently sensitive, as the weight loss was too small to provide accurate 53 54 55220 values for low grafting densities. For higher grafting densities and for all the studied precursors, the 56 57221 grafting efficiencies were not constant and increased to about 56 %. The boundary between high and 58 59222 low grafting densities matched to the point where the solution exhibited a significant macroscopic visual 60 ACS Paragon Plus Environment 9 Langmuir Page 10 of 34
223 aspect change from translucent to very turbid samples (see Figure S2 in Supporting information). It 1 2 3 224 could thus be assumed that, in our experimental conditions, at low grafting ratios, the precursor 4 5 225 condensation preferentially took place with the surface silanol groups. On the contrary, at higher ratios, 6 7 226 the self condensation dominated leading to an efficiency of 100 % as shown in Figure 1 . Such 8 9 10227 efficiency is possible only if one considers that for this amount of introduced grafters, all introduced 11 12228 organosilane precursors reacted with each other in solution. 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
38 39 Figure 1. Grafting efficiency evaluated by TGA for dimethoxydimethylsilane DDMS (), 40 41 trimethoxy(propyl)silane PTMS (▲), isobutyl(trimethoxy)silane iBTMS (▼). The solid line (green) represents a 42 43 theoretical 100 % grafting efficiency. 44229 45 46 47230 48 49231 50 51232 3.2. Nanostructure in Solution. In order to investigate the influence of the hydrophobization reaction 52 53 54233 on the colloidal state of the silica nanoparticles in aqueous solution, we performed a coupled cryo TEM 55 56234 / SAXS study on silica particles that had been hydrophobized with the dimethoxydimethylsilane 57 58 59235 precursor (DDMS). The results are presented in Figures 2 and 3 . Figure 2 shows cryo TEM images of 60 ACS Paragon Plus Environment 10 Page 11 of 34 Langmuir
236 hydrophobized silica solutions at different grafting densities with dimethyldimethoxysilane (DDMS) as 1 2 3 237 organosilane precursor. SAXS measurements were performed on the same samples (0.05 volume 4 5 238 fraction) and the scattered intensity profiles of those modified silica dispersions were represented in 6 7 239 Figure 3 . The scattering profile of non grafted hydrophilic silica dispersion at high pH (pH = 9.0) and 8 9 10240 low ionic strength shows a strong depression at low q values followed by a scattering peak at qpeak = 11 12 1 241 0.011 Å matching approximately an average interparticle distance at this concentration d = 2π/qpeak of 13 14 15242 55 nm. This profile is characteristic of a concentrated dispersion of repelling particles. Indeed it can be 16 17243 well fitted using the Mean Spherical Approximation (MSA) which takes into account the repulsive 18 19244 interactions between charged colloids (see Figure S3 in Supporting information).19 At low q values, i.e., 20 21 22245 for values corresponding to distances higher than the mean interparticular distance, the scattered profiles 23 24246 exhibit a plateau which indicates that the system is homogeneous at these scales. The interparticle 25 26 27247 center to center distance determined from the peak position (d = 55 nm) is consistent with the average 28 29248 distance measured between the particles on cryo TEM image (d = 52 ± 5 nm) (see Figure 2a ). 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 11 Langmuir Page 12 of 34
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29249 30 31250 Figure 2. Cryo TEM images of modified silica dispersions with dimethoxydimethylsilane (DDMS) at 32 33251 different grafting ratios: (a) hydrophilic silica dispersion, (b) 0.7 grafters per nm², (c) 0.9 grafters per 34 35252 nm² and (d) 3.0 grafters per nm². For sample (b), due to the TGA detection limit, the exact grafter 36 37253 density is not known. The value of 0.7 grafters per nm² is assumed from the introduced amount taking 38254 the grafting efficiency of the 0.9 grafters per nm² sample, i.e. 18 %. (see Figure 1). 39 40255 41 42 43256 Under the same conditions, the slightly hydrophobized silica particles, i.e., with a low grafting 44 45257 density (0.7 grafters per nm²), remained also well dispersed in water as observed on the corresponding 46 47258 cryo TEM image ( Figure 2b ) where a similar organization was evidenced. Its scattered intensity profile 48 49 50259 slightly changes over the whole q range compared to the scattering profile of a dispersion of hydrophilic 51 52260 silica particles in similar conditions of pH and ionic strength (see Figure S4 in Supporting information). 53 54261 Consequently, a surface modification of 0.7 grafters per nm² produced no measurable differences in the 55 56 57262 organization of the particles in the dispersion. 58 59 60 ACS Paragon Plus Environment 12 Page 13 of 34 Langmuir
263 By increasing the grafting density to 0.9 grafters per nm², the SAXS profile and the colloidal 1 2 3 264 organization in the cryo TEM image were modified. On the one hand, as the grafting density increases, 4 5 265 the position of the structural peak shifted from 0.011 Å 1 in the case of pure hydrophilic silica particles 6 7 266 to 0.0092 Å 1 for hydrophobized silica particles indicating a higher average distance (d = 68 nm) 8 9 10267 between the particles. Also remarkable was the fact that the height of the constant intensity plateau at q 11 12268 values lower than 0.005 Å 1 was increased for this modified silica dispersion with regards to the purely 13 14 15269 hydrophilic dispersion. Accordingly, the spatial distribution of the particles was less homogeneous than 16 17270 in the two previous cases: this was also in line with the observed increase of the structural peak width. 18 19271 Since the scattering profiles I( q) were similar at high q values, corresponding to intraparticle distances, 20 21 1 22272 the increase of the intensity at low q values(q < 0.005 Å ) means that the hydrophobization reaction has 23 24273 perturbed the interparticle correlations, leading to a less organized system. This is characterized by a 25 26 27274 pair correlation g(r) or by its Fourier transform, the structure factor, S(q) with weaker oscillations (see 28 29275 Supporting Information). This is also supported by the cryo TEM observations showing a 30 31276 heterogeneous system in which smaller linear aggregates made of two to six silica particles coexisted 32 33 34277 with isolated silica particles (see Figure 2c ). Note that the aggregation observed by both experiments 35 36278 can also be correlated with the observed turbidity increase (Figure S2 in Supporting information). 37 38279 Finally, at a even higher grafting density (3.0 grafters per nm²), a white and viscous suspension 39 40 41280 was obtained (Figure S2 in Supporting information) and larger objects, extending over several hundreds 42 43281 of nanometers, were observed according to the cryo TEM image ( Figure 2d ). No residual isolated silica 44 45282 particles were detected. The formed aggregates were not fully dense and an intra aggregate porosity 46 47 48283 appeared in the electronic microscopy observations. The SAXS profile for the same silica particles 49 50284 hydrophobized with a grafting density of 3.0 grafters per nm 2 corresponds to an aggregated system 51 52 2 53285 exhibiting an intensity increasing at low q values where I( q) varies as q over more than one decade. 54 55286 We can also notice that no structural peak was observed for the aggregated system in the q region lower 56 57287 than 0.025 Å 1. This means that the number of neighbors constituting the coordination shell of one silica 58 59 60288 particle was low, thus indicating the formation of loose aggregates. The value of the exponent of the ACS Paragon Plus Environment 13 Langmuir Page 14 of 34
289 scaling law is close to what is expected from a reaction limited cluster aggregation (RLCA) process.20, 21 1 2 3 290 Together with the absence of a structure peak, this might be taken as a manifestation of aggregation in 4 5 291 presence of long range electrostatic repulsions preventing formation of dense aggregates.22 6 7 292 As for the other studied hydrophobic precursors (PTMS and iBTMS), similar trends in terms of 8 9 10293 scattering profiles and colloidal state were observed with the increase of the molar grafting ratio. Table 11 12294 S1 resumes the aggregation state, determined by cryo TEM observations (see Figure S5 in Supporting 13 14 15295 information), of the samples obtained with the range of hydrophobic organosilane precursors used in 16 17296 this study at various grafting densities. In all cases, modified silica nanoparticles remain isolated at low 18 19297 grafting density and then progressively aggregate in small linear aggregates followed by 3D fractal 20 21 22298 aggregates for higher grafting density (Figure 2 ). 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51299 52 53300 Figure 3. Scattered intensity I( q) of modified silica dispersions (Φv = 0.05) with 54 55301 dimethoxydimethylsilane (DDMS) at different grafting densities expressed as the number of grafters per 56 2 57302 nm (●) hydrophilic silica particles, ( ) 0.9 grafters per nm² and ( □) 3.0 grafters per nm². All spectra 58303 were normalized at high q values. The inserts display schematically the corresponding colloidal states. 59 60 ACS Paragon Plus Environment 14 Page 15 of 34 Langmuir
304 1 2 3 305 To summarize, the combined cryo TEM / SAXS study revealed that the increase in grafting ratio 4 5 306 was responsible for a controlled and progressive aggregation in water. For low grafting densities, the 6 7 307 colloidal stability was maintained as the long range electrostatic repulsions overpowered the short range 8 9 2 10308 Van der Waals and hydrophobic interactions. At some point, between 0.7 and 0.9 grafters per nm , the 11 12309 strength and range of the hydrophobic increased and started to counteract the electrostatic repulsion. 13 14 15310 The hydrophobized particles aggregated into doublets and then, progressively, into small linear chains. 16 17311 The aggregate structures were determined by the balance between the hydrophobic interactions and the 18 19312 still present longer range electrostatic repulsions that prevented the collapse of the aggregates in dense 20 21 22313 structures. This behavior is quite similar to what has already been observed in the case of silica 23 24314 aggregation in presence of polymers.23, 24 Finally, 3D aggregates with a fractal dimension of 2.0 were 25 26315 formed, probably because at the highest grafting ratios, the inter polymerization of the precursors 27 28 29316 allowed a bridging between modified silica particles. 30 31317 32 33318 3.3. Resistance to Drying Stress. As mentioned above, the hydrophobization performed in water 34 35 36319 with hydrophobic organosilanes changed the way they interact with each other and also induced a 37 38320 progressive aggregation of the silica nanoparticles. This can be further probed by studying the response 39 40 41321 of the nanostructures to a compressive stress due to evaporation of the continuous water phase. Starting 42 43322 from a dispersion with a volume fraction Φv = 0.05, water was progressively removed by evaporation at 44 45323 90 °C (see experimental part). As the form factor of the silica particles does not evolve during the 46 47 48324 concentration process, we only focused on the evolution of the structure factor as a function of the silica 49 50325 volume fraction. Figure 4 presents the resulting structure factor, S(q), for the dispersion of pure 51 52326 hydrophilic silica nanoparticles, hydrophobized silica particles with dimethyldimethoxysilane (DDMS) 53 54 55327 at 0.9 grafters per nm² where small linear aggregates were present in solution and at 3.0 grafters per nm² 56 57328 where 3D aggregates were present. 58 59 60 ACS Paragon Plus Environment 15 Langmuir Page 16 of 34
329 Hydrophilic silica. For the untreated hydrophilic silica particles dispersion (Φv = 0.05, high pH and 1 2 3 330 low ionic strength), the structure factors are typical of a repulsive system. As illustrated in Figure 4a , 4 5 331 when increasing the silica particles volume fraction through evaporation of the liquid dispersion, the 6 7 332 position of the main peak of S(q) shifted to higher q (shorter distances) as the inverse cube root of the 8 9 10333 volume fraction, indicating that the particles come closer to each other as in a homogeneous 11 12334 compression (Figure S6 in Supporting information). The increase of the main structure peak height 13 14 Φ Φ 15335 (inset of Figure 4a ) from a value Smax equal to 1.8 ( v = 0.05) to 3 ( v = 0.26) as well as the progressive 16 17336 decrease of the S(q) plateau value at small q values (0.001 Å 1 < q < 0.01 Å 1) down to a minimum value 18 19 20337 Smin = 0.007 also confirms that the spatial distribution of particles remained homogeneous upon 21 22338 concentration. The Smax values obtained during the first steps of concentration were consistent with 23 24339 those observed in colloidal systems exhibiting an ordered structure. 25, 26 25 26 27340 At high volume fraction in the liquid state ( Φv = 0.26), the main peak height reached Smax = 3. 28 29341 According to Verlet 27 and Hansen 28 when this degree of short range order is reached, the liquid state 30 31 32342 with short range order becomes unstable with respect to a state with long range order (i.e. a colloidal 33 34343 crystal). However, the hydrophilic silica dispersion failed to crystallize, presumably because the time 35 36344 required for producing crystal nuclei was longer than the evaporation time. As a result it remained in a 37 38 39345 liquid like state where the particles were still separated by distances in the order of 9 nm. The same 40 41346 phenomenon took place in the sample that was dried to Φv = 0.6, where the particles were only 1 nm 42 43 44347 apart, on average, and had lost all the mobility required for crystallization. The decrease in the height of 45 46348 the main peak of S(q), from 3 to 1.9 (Figure 4a ) and the increase in its width confirm that the dispersion 47 48349 was on its way to a colloidal glass state. All these observations on the structural changes observed 49 50 51350 during the drying of hydrophilic silica dispersions were consistent with the observation of Li et al. on 52 53351 the drying of dip coated silica films.29 54 55352 Finally, as shown in Figure 4a , the structure factor S(q) for the final dry state presents a slow q 1.8 56 57 2 58353 decay at low q values. This departure from the q scaling may reflects large scale defects in the 59 60 ACS Paragon Plus Environment 16 Page 17 of 34 Langmuir
354 structure such as lumps, voids or cracks. In spite of those defects, the material tended towards a solid 1 2 3 355 volume fraction of 0.65, very close to the close random packing of spheres ( Φv = 0.64). 4 5 356 Intermediate grafting regime. For a significant grafting of hydrophobic precursor on the silica surface 6 7 2 8 357 (0.9 grafters per nm ), the behavior of the hydrophobized silica upon concentration was significantly 9 10358 changed (Figure 4b ). At the initial volume fraction, the position of the primary peak, its width as well 11 12359 as the slow decay at low q values reflected a more aggregated state compared to untreated silica, thus 13 14 15360 concurring with cryo TEM observations (Figure 2c ). As expected, the structure peak was shifted to 16 17361 higher q values upon increasing concentration because of the decrease of the average distance between 18 19362 particles upon concentration. However, contrary to non modified silica, the structure peak height 20 21 22363 progressively decreased until its full disappearance. Hence, the initial dispersion state composed of 23 24364 isolated particles and small linear chains was not maintained during the concentration process of the 25 26 Φ 27365 system. Furthermore, a dramatic change of behavior occurred at v = 0.15. Indeed, a strong low q 28 29366 scattering replaced the broad depression that reflected the short range order of repelling hydrophilic 30 31367 particles. This low q scattering had a slow q 1.3 decay. It was followed by a “hump” at the contact 32 33 34368 distance of the silica particles. This is consistent with the occurrence of the larger structures that 35 36369 increase the spatial inhomogeneity of the sample. This drastic change of the dispersion structure can 37 38370 have as origin an accretion of the original aggregates and particles when they are close enough to bind 39 40 41371 to each other through hydrophobic interactions. The absence of structure peak close to the characteristic 42 43372 distance of one elementary particle implies an incomplete coordination shell for each particle which is 44 45 46373 compatible with an open structure for the aggregates. The observed aggregation happens much sooner 47 48374 than with non modified particles which were able to reach a density close to the one of random packing 49 50375 before being aggregated. As already stated, the slow decay at low q values means that lumps and voids 51 52 53376 were present at large scales. However, upon further concentration and drying, it can be expected that the 54 55377 structure collapses under capillary pressure and voids progressively suppressed. Indeed, when the 56 57 Φ 58378 volume fraction reached v = 0.31, the depression in the structure factor curve deepened indicating a 59 60 ACS Paragon Plus Environment 17 Langmuir Page 18 of 34
379 decrease in the concentration fluctuations at smaller scales. In the final drying step, the steeper decay ( q 1 2 2.8 3 380 ) observed at very low q values indicates the presence of large aggregates in the final structure. It is 4 5 381 noticeable that the structure factor minimum value in the depression for the dried state ( Smin = 0.042) is 6 7 382 more than 6 times higher than the one for the dried hydrophilic silica ( S = 0.0068) meaning that even 8 min 9 10383 if, in both cases, the aggregated structure in the liquid state collapses under capillary forces, the amount 11 12384 of residual voids due to the defects in the final material are higher for hydrophobized silica particles. 13 14 15385 Hydrophobic silica. Finally, contrary to the first two previous cases where a progressive decrease of 16 17386 the average interparticle distance was obtained upon concentration, the structure factor S(q) for silica 18 19387 particles hydrophobized with the highest grafting density, i.e., 3.0 grafters per nm² ( Figure 4c ), exhibits 20 21 22388 a structure peak at an average interparticle distance of 22 nm which roughly corresponds to the particle's 23 24389 size. In plain terms, this means that particles were already in contact with a significant number of 25 26390 particles in the coordination shell of a given particle as also reflected in the significant height of the 27 28 1 29391 structure peak. On the other hand, at low q values ( q < 0.01 Å ), the structure factor S(q) followed a 30 31392 power law in q 2 corresponding to an reaction limited cluster aggregation. As we can see in Figure 4c , 32 33 Φ 34393 drying occurred in two steps. During the first step, from v = 0.05 to 0.23, the structure peak position 35 36394 remained unchanged whereas its height increased (from 1.3 to 1.9) which indicate that, at shorter 37 38395 distance, the number of neighboring particles at the same average distance increased. At longer distance 39 40 41396 (shorter q values), all the S(q) curves superimposed meaning that the intra aggregate structure was not 42 43397 affected by the sample concentration and that the inter aggregate average distance was larger than what 44 45 Φ 46398 could be probed by our experimental range in q range (up to 600 nm). In a second step, from v = 0.23 47 48399 to the dry state, as the concentration increases further until the drying state, capillary forces induce a 49 50400 drastic collapsing of the porous structure where both the larger aggregates get closer and the voids are 51 52 53401 progressively suppressed as supported by the significant depression of S(q) intensity around q = 0.01 Å 54 1 55402 . It remains that the minimal value of the structure factor, Smin = 0.26, was 6 times higher than the one 56 57403 found at a grafting density of 0.9 and 38 times higher than the one found for non modified silica 58 59 60404 particles. This revealed that silica hydrophobization led to a less dense structure associated to residual ACS Paragon Plus Environment 18 Page 19 of 34 Langmuir
405 intra aggregate porosity. In our study, the compressive stress could not be measured but a comparison 1 2 3 406 based on the minimum value of the structure factor can be made with results obtained by Madeline et al. 4 5 407 which studied the restructuring of colloidal silica cakes under compression with pressures as high as 400 6 7 408 kPa.30 In that situation, voids between aggregates are progressively compressed but a very steep decay 8 9 4 4 10409 (q ) at low q values is observed at high pressures. This q power law is attributed to the formation of 11 12410 dense lumps between aggregates which build a skeleton rigid enough to prevent any further collapsing. 13 14 15411 In our case, during the ultimate drying step, the applied pressure is due to the Laplace forces and thus 16 17412 much higher than the 400 kPa applied in Madeline's work. Nevertheless, despite a more loosely 18 19413 connected skeleton, the porous structure resisted in similar ways. This similar resistance to collapse was 20 21 22414 established by the obtained decay rate of the structure factor which did not evolve with the drying step 23 24415 even when the silica dipersion was fully dried. Moreover, the minimal structure factor value Smin was in 25 26416 the same order of magnitude than the one obtained in the case of filtrated dispersions.30 However, in our 27 28 2 4 29417 case, the slower decay rate q instead of q may be attributed to a stronger mechanical resistance of our 30 31418 aggregates, leading in our case to fractal lumps instead of a dense skeleton in a matrix of smaller 32 33419 density. 34 35 36420 To summarize, from SAXS experiments, the main conclusions concerning the drying process of 37 38421 hydrophilic and hydrophobized silica particles was that (i) the drying of hydrophilic particles leads 39 40 41422 throughout the whole process to the progressive concentration of a homogeneous structure and therefore 42 43423 the formation of a dense network (ii) in contrast, the concentration of modified silica particles only 44 45424 results in the partial suppression of voids in the dried structure and the initial aggregated structures were 46 47 48425 thus maintained. 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 19 Langmuir Page 20 of 34
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60426 ACS Paragon Plus Environment 20 Page 21 of 34 Langmuir
427 Figure 4. Structure factor S(q) of silica dispersion at different concentration (volume fraction) obtained 1 2 428 by drying (a) pure hydrophilic silica particles (the inset presents the structure factor in lin lin scale in 3 4 429 order to better visualize the evolution of the peak’s height and width with the concentration and drying); 5 6 430 (b) modified silica with dimethoxydimethylsilane (DDMS) at 0.9 grafters per nm²; (c) modified silica 7 431 with dimethoxydimethylsilane (DDMS) at 3.0 grafters per nm². 8 9 432 10 11 12433 3.4. Residual porosity in the dried state. As illustrated in Figure 4 , the final structure made from 13 14434 hydrophobized silica particles diverges from the close random packing behavior of a repulsive and fully 15 16435 dispersed system observed in the case of hydrophilic particles. The residual porosity of the obtained 17 18 19436 powders after drying has been measured by mercury intrusion porosimetry analysis on modified and 20 21437 unmodified dried silica dispersions. The final porous volume as a function of the grafting density is 22 23438 represented in Figure 5 for silica particles hydrophobized with different organosilane precursors. The 24 25 26439 typical evolution of the cumulative porous volume as a function of the pore radius as well as the pore 27 28440 size distribution for silica particles hydrophobized with dimethoxydimethylsilane (DDMS) are presented 29 30441 in Figure S7 in Supporting information. As shown in Figure 5 , it appears that, irrespective of both the 31 32 33442 functionality and the alkyl chain length of the organosilane grafter's, the residual porous volume 34 35443 gradually increased with the grafting density from 0.08 cm 3/g for powder made from pure hydrophilic 36 37 3 38444 silica particles until 0.85 cm /g for the highest grafting ratio in the case of DDMS. In addition, the pore 39 40445 radius is increasingly shifted towards higher values with the grafting ratio as illustrated in Figure S7 in 41 42446 Supporting information. Cumulative porous volume as well as the range of pore radius for the different 43 44 45447 studied organosilane precursors were summarized in Table S1 in Supporting information. For all 46 47448 grafters considered, the same trend of increasing porous volume and pore radius range was observed as 48 49449 a function of the grafting ratio. 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 21 Langmuir Page 22 of 34
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Figure 5. Porous volume of dried modified silica dispersions with the different studied precursors as a function 30 of the grafting density: ( ) dimethoxydimethylsilane (DDMS), (▲) trimethoxy(propyl)silane (PTMS) and (▼) 31 32 isobutyl(trimethoxy)silane (iBTMS). The dashed line is a guide for the eye. Concerning DDMS, the exact value 33 of the grafters density for the first two points is not known due to the detection limit of the TGA method. For 34 35 these points, values plotted are obtained by interpolation using the known introduced amount and assuming a 36 37 grafting efficiency of 18 %. 38450 39 40 41451 42 43452 Here, as shown in Figure 5 , the drying of the unmodified hydrophilic silica particles at pH 9 and 44 45 3 46453 low ionic strength leads to a very low cumulative porous volume (0.08 cm /g) and a pore size 47 48454 distribution lower than 5 nm consistent with a well ordered dense system as observed in the case of 49 50455 silica colloidal crystals. 31 With increasing the grafting density, one observed a progressive rise in 51 52 3 53456 residual porosity after drying until 0.4 cm /g and a maximum pore radius of about 20 nm for a value of 54 55457 grating ratio about 0.9 grafters per nm 2. This reflected upon how surface hydrophobization modified the 56 57458 packing of the individual particles. Indeed, the introduction of an additional attractive component to the 58 59 60459 interaction potential between particles allowed the formation of small linear chains in addition to ACS Paragon Plus Environment 22 Page 23 of 34 Langmuir
460 individual particles (see cryo TEM images in Figure 2c). A higher initial disorder at short range caused 1 2 3 461 eventually a more random, and so less dense, packing as supported by SAXS experiments (Figure 4 ) 4 5 462 leading to a second level of disorder in the structure. Then, the residual porosity obtained for low 6 7 463 grafting densities would be the direct result of this disorder caused by the hydrophobization. In that 8 9 10464 case, the pore size was expected to remain low since it would scale mainly with the initial “structure 11 12465 defects”, that are small linear chains. Indeed, pore size distributions ( Figure S7 in Supporting 13 14 15466 information) clearly showed very low pore radius values, comparable to the particle size at most. 16 17467 Finally, for higher grafting density which leads to an initial structure in solution made of large 18 19468 aggregates ( Figure 2d ), the cumulative porous volume increased to 0.8 0.9 cm 3/g and a maximum 20 21 22469 pore radius size of 40 nm was reached, commensurate to the voids size within the aggregates initially 23 24470 present in solution. 25 26471 In the light of cryo TEM, SAXS and porosimetry experiments, we can conclude that the dried 27 28 29472 samples exhibited a structure with three levels of porosity. The first one is expected at a very low scale 30 31473 due to the random packing of the aggregates, similar to the one found for the dispersed silica particles. 32 33474 A second one, medium scale porosity, corresponded to the large voids in the aggregates’ structure of 34 35 36475 approximately 50 nm in diameter. Finally, an even larger porosity, ranging up to 500 nm, remained after 37 38476 the imbrication of those very rough shaped, full of cavities objects. The porosity remaining after drying 39 40 41477 correlated well with the grafting density irrespective of both the length and the nature of the 42 43478 hydrophobic grafters. However, as soon as inter precursor condensation occurred leading to 3D 44 45479 aggregates, further grafting agent addition had no significant impact neither on the nanostructure nor on 46 47 48480 the residual porosity after drying. As a consequence, the porous volume gain slows down after a density 49 50481 of 3 grafters per nm² to reach a maximum value of about 0.9 cm 3/g. 51 52482 53 54 55483 3.5. Hydrophobic properties of the dried silica. The hydrophobization of silica particles in water not 56 57484 only induces structural changes from the dispersed to the fully dried state, but also provides a 58 59485 progressive macroscopic damp proof behavior. In order to investigate the influence of the performed 60 ACS Paragon Plus Environment 23 Langmuir Page 24 of 34
486 chemical modification on the wetting properties, contact angle measurements of a water drop were 1 2 3 487 performed on colloidal thin films prepared by dip coating a glass slide with modified and unmodified 4 5 488 silica. For each sample, film thicknesses were varied from 150 to 600 nm and the measurements were 6 7 489 performed 3 times for each film. The contact values were found very reproducible and independent of 8 9 10490 the film thickness. Note that the effect of hydrophobization was exclusively studied for low grafting 11 12491 densities where silica particles remained sufficiently well dispersed to allow the formation of 13 14 15492 homogeneous films. The resulting contact angles measurements as a function of the grafting ratio for 16 17493 different organosilane precursors are presented in Figure 6 whereas both contact angle and radius drop 18 19494 kinetics for thin films constituted by silica particles modified by dimethoxydimethylsilane (DDMS) 20 21 22495 precursors, at different grafting ratios are shown in Figure S8 in Supporting information. On the one 23 24496 hand, it clearly appears that, whatever the precursor's nature of the hydrophobic grafters, the apparent 25 26497 contact angle increased with the grafting density, from 3 4° for pure hydrophilic silica particle to a 27 28 29498 maximum value of 47° for hydrophobized silica with the highest grafting coverage. On the other hand, 30 31499 for slightly modified silica, when the drop was deposited on the film surface, its apparent contact angle 32 33500 rapidly decreased until reaching its equilibrium value after several seconds (see Figure S8 in Supporting 34 35 36501 information). For instance, the apparent contact angle went from 6.6° to 3.8° after 12 s for pure 37 38502 hydrophilic silica films and this phenomenon was also observed for silica modified with DDMS with 39 40 41503 low grafting ratios. This decrease corresponds to the fast spreading that occurs onto the hydrophilic 42 43504 porous thin film. However, this decrease at short time was totally suppressed for more hydrophobic 44 45505 silica (Figure S9 in Supporting information). 46 47 48506 49 50507 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 24 Page 25 of 34 Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Figure 6. (left) Water contact angle values at 50 s as a function of the grafting density for 24 25 dimethoxydimethylsilane DDMS (), trimethoxy(propyl)silane PTMS (▲) and isobutyl(trimethoxy)silane 26 iBTMS (▼). (right) Images of water contact angle on silica hydrophobized with dimethoxydimethylsilane 27 28 (DDMS) as the function of the grafting ratio. It was considered that after 50 s, the equilibrium state is reached 29 30 (static contact angle) as shown the contact angle kinetics in Supporting information. Concerning DDMS, except 31 for the first and last point, the exact value of the grafters density is not known due to the detection limit of the 32 33 TGA method. For these points, values plotted are obtained by interpolation using the known introduced amount 34 35 and assuming a grafting efficiency of 18 %. 36508 37 38 39509 40 41510 Another direct consequence of silica hydrophobization is a modification of its ability to adsorb water. 42 43511 To examine this effect, the water uptake at 25 °C with a relative humidity of 80 % of a previously dried 44 45 46512 silica dispersion was studied. The Figure 7a presents the water uptake kinetics of dried silica particles 47 48513 hydrophobized with dimethoxydimethylsilane (DDMS) at different grafting ratios. As we can see, at 49 50514 short time, the water uptake rapidly increases to reach an equilibrium plateau. The value of the plateau 51 52 53515 depends on the molar grafting ratio; the higher the grafting ratio is, the lower the plateau value at the 54 55516 equilibrium time is. Similar trend was observed whatever the nature of the hydrophobic grafter. The 56 57 58517 weight increase at equilibrium as a function of both the grafting density and the organosilane precursor's 59 60 ACS Paragon Plus Environment 25 Langmuir Page 26 of 34
518 nature is reported in Figure 7b. As expected, unmodified silica particles were highly hydrophilic since 1 2 3 519 an increase of 13 % in weight, due to adsorbed water, was measured. However, the hydrophobic 4 5 520 grafting allowed a significant and progressive decrease of the water adsorption down to only 3 %, as 6 7 521 obtained for approximately 1 grafter per nm 2 irrespective of the used precursor. Then, an asymptotic 8 9 10522 behavior was observed meaning that the extra added precursors had no impact on the silica surface 11 12523 protection against water adsorption. 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Figure 7. (a) Water uptake kinetics on dried modified silica with dimethoxydimethylsilane at different grafting 38 39 densities: (●) hydrophilic silica dispersion, ( ) 0.23, ( ) 0.47, ( ) 0.90, ( ) 3.00 and ( ) 4.94 grafters per 40 nm². (b) Weight increase at equilibrium as a function of the grafting density for dimethoxydimethylsilane DDMS 41 42 (), trimethoxy(propyl)silane PTMS (▲) and isobutyl(trimethoxy)silane iBTMS (▼). The dashed line is a 43 44 guide for the eye. Concerning DDMS, the exact value of the grafters density for the first two points (0.23 and 45 0.47) is not known due to the detection limit of the TGA method. For these points, values plotted are obtained by 46 47 interpolation using the known introduced amount and assuming a grafting efficiency of 18 %. 48 49524 50 51525 52 53526 3.6. Discussion. As shown in this work, our original approach which consists in directly 54 55 56527 hydrophobizing in water silica nanoparticles using organosilanes allowed to precisely control the 57 58528 interactions of the modified silica particles at different stages of concentration and drying, from 59 60 ACS Paragon Plus Environment 26 Page 27 of 34 Langmuir
529 dispersed to dried state. Moreover, it also represents an original environmentally friendly route to 1 2 3 530 elaborate, upon simple drying, porous silica materials with damp proof properties. Indeed, the 4 5 531 performed hydrophobization, not only conferred damp proof properties to the final material, but also 6 7 532 had a critical impact on the manner in which silica particles interacted with each other in solution and 8 9 10533 under drying stress, thus controlling the porosity of the final material. 11 12534 For low molar grafting ratio, the inter polymerization of the precursors is limited by the very slow 13 14 15535 addition of the hydrophobic precursor in the silica dispersion. The probability for two precursors to 16 17536 interact with one another is much lower that the condensation of the hydrolyzed organosilane onto the 18 19537 silica surface. Consequently, the grafting density increases with the introduced ratio on the silica surface 20 21 22538 until maximum coverage as observed in Figure 1 whatever the nature of the organosilane's precursor. 23 24539 Above this grafting ratio, the precursor excess will polymerize, forming chains extending away from the 25 26540 silica surface and eventually inducing covalent bonding between silica particles. In that case, the added 27 28 29541 precursors strongly change the colloidal state of the dispersion ( Figure 2 ) and do not participate on the 30 31542 silica surface protection against water adsorption (Figure 7 ). Therefore, it appeared that a value of 1 32 33543 grafter per nm 2, corresponding to a water adsorption around 3 4 %, was the optimal coverage that can 34 35 36544 be obtained in those experimental conditions and that above this value, the amount of adsorbed water 37 38545 remained constant. Silica hydrophobization by organosilane precursors was thus never complete since 39 40 2 41546 only a fraction of the silanol groups (considering a hypothetical average amount of 5 SiOH per nm ) 42 18 43547 could be modified by alkyl chains. The upper limit of the modified surface sites number depends on 44 45548 various steric and kinetics parameters difficult to study independently such as the size of the precursor, 46 47 48549 the roughness and porosity of the material to modify, the reaction conditions, the number and the nature 49 50550 of the surface silanol groups. The unreacted silanol groups result in a residual hydrophilicity of the 51 52551 surface as observed in the water adsorption experiments, even for high amounts of introduced 53 54 55552 precursors (water adsorption of 3 % in weight). Nevertheless, this contrasts with a fully hydrophilic 56 57553 surface where water molecules progressively adsorb on the hydroxyl groups and form multilayers 58 59554 (water adsorption of 13 % in weight). To support this discussion, near infrared experiments were 60 ACS Paragon Plus Environment 27 Langmuir Page 28 of 34
555 performed at equilibrium (20 days at 25 °C and a relative humidity of 80 %) on both hydrophilic silica 1 2 3 556 particles and silica particles hydrophobized with dimethoxydimethylsilane with a grafting ratio of 3.0. 4 5 557 The resulting spectra reported in Figure 8a show a set of bands in the region 7500 6500 cm 1 6 7 558 characteristics of free silanol groups (7316 cm 1), water molecules hydrogen bonded to silanol group 8 9 1 1 32 10559 (7121 cm ) and water molecules bonded with water molecules (6861 cm ) respectively. In the case of 11 12560 hydrophobized silica particles, the band at 7316 cm 1 points the presence of free silanol groups whereas 13 14 15561 this contribution is nearly inexistent for hydrophilic silica particles. This comparison supports the idea 16 17562 that the grafted hydrophobic organosilanes induce locally a steric hindrance which prevents the water 18 19563 molecules condensation on silanol groups close to the grafter. In addition, water adsorption desorption 20 21 22564 isotherms were carried out on both silica and the results are presented in Figure 8b. As shown, dried 23 24565 material made from hydrophilic silica particles exhibits a type IV isotherm presenting a hysteresis loop 25 26566 between a relative pressure range of 0.65 to 0.8, in agreement with a capillary condensation taking place 27 28 29567 in interstitial pores for a cubic packing (3 6 nm) (Figure S7 in Supporting Information). This is also 30 31568 supported by the high contribution of water hydrogen bonded water (6861 cm 1) in near infrared 32 33569 experiments (Figure 8a ). On the other hand, a type I isotherm is observed in the case of hydrophobized 34 35 36570 silica which indicates the fact that water molecules mainly adsorb onto silica surface as a monolayer. 37 38571 This observed behavior for hydrophobized silica is consistent with a reported study on porous silica 39 40 33 41572 glass grafted with trimethylsilyl groups. In sharp contrast, hydrophilic silica based materials feature a 42 43573 very low porosity with a high water adsorption capacity while hydrophobized materials exhibit a larger 44 45574 residual porosity and only adsorb a low water amount. 46 47 48575 Therefore, in the case of hydrophilic silica particles, water molecules form a two dimensional aqueous 49 50576 network whereas in the case of modified silica water molecules adsorbs on the remaining silanol groups 51 52577 and forms disconnected clusters around the hydrophobic grafters. Based on the water uptake for both 53 54 55578 hydrophilic and hydrophobized silica (13 and 3 % in weight respectively), the average number of water 56 57579 molecules per silanol groups respectively decreases from 6.2 to 1.8. Both results obtained for 58 59580 hydrophilic and hydrophobized silica are in good agreement with Takei's work which reports (i) a 60 ACS Paragon Plus Environment 28 Page 29 of 34 Langmuir
581 dramatic increase of the water adsorption obtained at high pressure (multilayers) and (ii) the rupture of 1 2 33 3 582 the two dimensional water network in presence of hydrophobic grafters. 4 5 583 6 7 584 8 9 10585 11 12586 13 14 15587 16 17588 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Figure 8. (a) Near infrared spectra at equilibrium and (b) water adsorption desorption isotherm of pure 40 41 hydrophilic silica particles (red line) and silica hydrophobized with dimethoxydimethylsilane with a grafting 42 2 43 density of 3.0 grafters per nm (blue line). For water adsorption desorption isotherms, the samples were heated at 44 100 °C during 15 h under vacuum in order to remove any traces of condensed water. 45 46589 47 48590 49 50 51591 52 53 54592 4. Conclusion 55 56 57593 In the present work, we have shown that a controlled hydrophobization of aqueous silica dispersion 58 59 60594 using organosilane precursors, allowed to finely tune the interparticle interactions and consequently to ACS Paragon Plus Environment 29 Langmuir Page 30 of 34
595 change the way the particles assembled under applied drying stress. The resulting nanostructure 1 2 3 596 observed in solution was directly related to the grafting ratio. Indeed, the colloidal state of the modified 4 5 597 silica changed from well dispersed particles for low grafting ratio to small linear chains and finally 6 7 598 three dimensional network for high grafting density. The induced pre aggregation as well as the 8 9 10599 interparticle interactions changed upon hydrophobization. This led to the formation of heterogeneous 11 12600 structures during drying resulting in a residual porosity of the fully dried material. Finally, we have 13 14 2 15601 demonstrated that the hydrophobization of silica particles with a grafting density of 1 grafter per nm is 16 17602 enough in order to suppress the water condensation onto the silica surface and therefore provides damp 18 19603 proof properties in the final dried state. 20 21 22604 23 24605 25 26606 27 28607 29608 30 31609 32 33610 34 35611 36612 37 38613 39 40614 41 42615 AUTHOR INFORMATION 43 44616 *E mail: [email protected] 45 46617 *E mail: jean [email protected] 47 48618 Notes 49 50619 The authors declare no competing financial interest. 51 52620 53 54621 ACKNOWLEDGMENTS 55622 Electron microscopy was performed at the "Service de Microscopie electronique de l’Institut de 56 57 58623 Biologie Intégrative IFR 83 (University Pierre and Marie Curie, Paris). The authors thank Dr. G. 59 60 ACS Paragon Plus Environment 30 Page 31 of 34 Langmuir
624 Frébourg for the cryo TEM experiments, Dr. F. Meneau for his help on SWING beamline at ESRF. 1 2 3 625 Water adsorption desorption isotherms were performed by C. Carteret (Laboratoire de Chimie Physique 4 5 626 et Microbiologie pour l’Environnement, Université de Lorraine, Nancy). We thank F. Lequeux for 6 7 627 valuable discussions. We gratefully acknowledge Saint Gobain for financial support. 8 9 10628 11 12629 REFERENCES 13 14630 15 16631 17632 (1) Iler, R. K., The Chemistry of silica . Wiley: 1979. 18 19633 (2) Johnsson, A.; Camerani, M. C.; Abbas, Z., Combined Electrospray SMPS and SR SAXS 20 21634 Investigation of Colloidal Silica Aggregation. Part I. Influence of Starting Material on Gel Morphology. 22 23635 Journal of Physical Chemistry B 2011, 115, 765 775. 24 25636 (3) Parneix, C.; Persello, J.; Schweins, R.; Cabane, B., How Do Colloidal Aggregates Yield to 26637 Compressive Stress? Langmuir 2009, 25, 4692 4707. 27 28638 (4) Lafuma, F.; Wong, K.; Cabane, B., Bridging of Colloidal Particles through Adsorbed Polymers. 29 30639 Journal of Colloid and Interface Science 1991, 143, 9 21. 31 32640 (5) Bauer, D.; Killmann, E.; Jaeger, W., Flocculation and stabilization of colloidal silica by the 33641 adsorption of poly diallyl dimethyl ammoniumchloride (PDADMAC) and of copolymers of DADMAC 34 35642 with N methyl N vinyl acetamide (NMVA). Colloid and Polymer Science 1998, 276, 698 708. 36 37643 (6) Meszaros, R.; Varga, I.; Gilanyi, T., Adsorption of poly(ethyleneimine) on silica surfaces: Effect 38 39644 of pH on the reversibility of adsorption. Langmuir 2004, 20, 5026 5029. 40 41645 (7) Tavacoli, J. W.; Dowding, P. J.; Routh, A. F., The polymer and salt induced aggregation of silica 42646 particles. Colloids and Surfaces a Physicochemical and Engineering Aspects 2007, 293, 167 174. 43 44647 (8) Gupta, J. K.; Basu, S., Simultaneous aggregation and sedimentation of silica particles in the 45 46648 presence of surfactants. Colloids and Surfaces a Physicochemical and Engineering Aspects 2005, 255, 47 48649 139 143. 49650 (9) Zhou, Z. K.; Wu, P. Q.; Ma, C. M., Hydrophobic interactions and stability of colloidal silica. 50 51651 Colloids and Surfaces 1990, 50, 177 188. 52 53652 (10) McGovern, M. E.; Kallury, K. M. R.; Thompson, M., Role of solvent on the silanization of glass 54 55653 with octadecyltrichlorosilane. Langmuir 1994, 10, 3607 3614. 56 57 58 59 60 ACS Paragon Plus Environment 31 Langmuir Page 32 of 34
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710 1 2 711 "for Table of Contents use only" 3 4 712 Hydrophobization of Silica Nanoparticles in Water: Nanostructure and Response to Drying Stress 5 6 7 713 Solenn Moro, Caroline Parneix, Bernard Cabane, Nicolas Sanson,* and Jean Baptiste d'Espinose de 8 9 714 Lacaillerie* 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28715 29 30716 31 32 33717 34 35718 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 34