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Forces Between Silica Particles in Isopropanol Solutions of 1: 1 Forces between Silica Particles in Isopropanol Solutions of 1:1 Electrolytes Biljana Stojimirovi´c, Marco Galli, and Gregor Trefalt∗ Department of Inorganic and Analytical Chemistry, University of Geneva, Sciences II, 30 Quai Ernest-Ansermet, 1205 Geneva, Switzerland (Dated: June 8, 2020) Interactions between silica surfaces across isopropanol solutions are measured with colloidal probe technique based on atomic force microscope. In particular, the influence of 1:1 electrolytes on the interactions between silica particles is investigated. A plethora of different forces are found in these systems. Namely, van der Waals, double-layer, attractive non-DLVO, repulsive solvation, and damped oscillatory interactions are observed. The measured decay length of the double-layer repulsion is substantially larger than Debye lengths calculated from nominal salt concentrations. These deviations are caused by pronounced ion pairing in alcohol solutions. At separation below 10 nm, additional attractive and repulsive non-DLVO forces are observed. The former are possibly caused by charge heterogeneities induced by strong ion adsorption, whereas the latter originate from structuring of isopropanol molecules close to the surface. Finally, at increased concentrations the transition from monotonic to damped oscillatory interactions is uncovered. I. INTRODUCTION As described above, non-aqueous solvents are used in many practical applications. However, there is only Forces between surfaces immersed in liquids are im- scarce data in the literature on interactions between solid portant in many natural and technological processes. We surfaces across non-aqueous polar media and their mix- can find examples of such processes in biological systems, tures with water. Forces between mica sheets with SFA waste water treatment, ceramic processing, ink-jet print- across polar propylene carbonate, acetone, methanol, and ing, and particle design [1–6]. Recent advancement in the ethylene glycol were first measured by Christenson and force probing techniques such as surface force apparatus Horn [20–22]. In these measurements, two regimes were (SFA), colloidal probe technique based of atomic force observed, the long-range behavior, which was dominated microscopy (AFM), and optical tweezers enable routine by repulsive double-layer force, and the short-range be- surface force measurements with high precision and ex- havior, which included oscillatory forces. These oscil- cellent reproducibility [7–9]. lations are formed by structuring of solvent molecules A vast majority of the surface force measurements are near the solid surface [20, 23]. The forces between silica done in aqueous systems. Some examples of such mea- surfaces in ethylene glycol were measured with colloidal surements aimed to study the effects of multivalent ions probe technique [24]. At large distances, long-range re- on electrostatic interactions [10–12] or mechanisms be- pulsion was observed, and at short distances, hydration- hind oscillatory structural forces [13–15]. Although water like repulsion was measured. Attractive solvation inter- is the most important natural solvent, processes in non- actions were measured when fluorocarbon surfaces were aqueous media are equally interesting in view of techno- interacting across ethylene glycol [25]. The above cited logical as well as some natural processes. An example research has shown that double-layer and solvation forces of such a process includes ceramics processing, where or- similar to ones measured in water are also present in non- ganic polar media, such as alcohols or ketones, are used aqueous polar media. for milling and homogenization of ceramic powder mix- Forces between silica surfaces across alcohols and arXiv:2003.04058v2 [cond-mat.soft] 5 Jun 2020 tures, which permit production of high-quality complex alcohol-water mixtures were investigated with a colloidal ferroelectric or structural materials [16, 17]. Another ex- probe technique [2, 26–29]. In these systems, the range ample of a process using non-aqueous solvents is printing and magnitude of the double-layer force change by chang- of materials in 2D or 3D shapes. Material inks can be ei- ing water content in the mixture. The variation of the ther completely non-aqueous based or can contain large decay length of double-layer forces was attributed to the portions of non-aqueous phases to control surface ten- variation of the dielectric constant in the mixtures and sion, drying, or viscosity. Such inks were used to print ion association at high alcohol contents [26]. At short 3D objects from composite materials or even integrated distances a step-like repulsion was observed in pure alco- Li-ion batteries [18, 19]. hols [26, 27, 29]. These short-range forces stem form the ordering of the alcohol molecules near the surface. Lately, there has been a lot of interest in surface forces ∗ E-mail: [email protected] across ionic liquids and highly concentrated aqueous salt 2 solutions. It has been shown that in very concentrated Cantilever and pure ionic liquids long-range exponential repulsion Silica Particle exists [23, 30, 31]. These repulsions have decay lengths OH much larger compared to the Debye length and further- more the decay length increases with increasing con- centration in highly concentrated systems. This behav- Isoprop yl Alcohol ior is opposite to the behavior observed in dilute elec- trolytes [31]. It was further observed that at high concen- Silica Particle trations of electrolytes the transition from monotonic to oscillatory forces is present [31] and that the wavelength of these oscillations can be abruptly changed by varying solvent composition [23]. These experiments sparked a FIG. 1. Schematic representation of a colloidal probe exper- renewed interest in theoretical description of ionic flu- iment. Force measurements between two silica particles were ids and a variety of theoretical approaches were utilized done in isopropanol solutions. to describe these new exciting experimental data [32–38]. Measurements mentioned above have been performed us- ◦ ing SFA, where one typically uses mica as a surface. Us- 1200 C for 2 hours, to burn the glue, achieve firm at- ing colloidal probe AFM would enable to use other sur- tachment and decrease surface roughness of the parti- faces during similar experiments, which would test the cles. Solutions were made in isopropanol (99.8%, Extra hypothesis that the phenomena observed in ionic liquids Dry, AcroSeal, Acros Organics) with addition of tetra- are interface independent. butylammonium bromide (TBAB, 99+%, Acros Organ- ics) or lithium chloride (LiCl, BioXtra, >99.0%, Sigma- Here, we investigate forces between silica colloids in Aldrich). isopropanol solutions of 1:1 electrolytes. Due to a lower dielectric constant of isopropanol as compared to water, Before force measurement, cantilevers and substrate the electrostatic coupling is stronger in these solutions. were cleaned in Milli-Q water and ethanol, and then The stronger electrostatic coupling induces a very rich treated in plasma for 20 minutes. When mounted into the behavior of these simple systems. Variety of different AFM, the fluid cell was filled with solution, and the par- type of forces are found. In addition to double-layer and ticle on the cantilever was centered above another one on van der Waals interactions, attractive non-DLVO and the quartz disk with a precision of around 100 nm. The short-range solvation forces are present. At increased speed of the approach of the cantilever to the substrate concentrations the transition from monotonic exponen- during the measurement was 500 nm/s for all experi- tial to damped oscillatory interactions is observed. ments, except for 50 mM solutions, where the approach speed of 100 nm/s was used. For a selected pair, the can- tilever deflection was recorded in 150 approach-retract II. MATERIALS AND METHODS cycles. For each salt concentration, measurement was done on 3-5 different pairs of particles. The approach parts of the curves were averaged and used for analy- A. Force Measurements sis. Hookes law was used to convert deflection to force. Cantilever spring constant was determined by the Sader The surface force measurements were performed on method [39]. a closed-loop atomic force microscope (MFP-3D, Asy- lum Research) using the colloidal probe technique at ◦ room temperature 23 ± 2 C, see Fig. 1. The AFM B. Analysis of the Force Curves is mounted on an inverted optical microscope (Olym- pus IX70). Spherical silica 4 µm particles (Bangs Lab- The extended DLVO theory is used to analyze the force oratories Inc., USA) were attached to tipless cantilevers curves [7, 40] (MikroMasch, Tallin, Estonia) with the help of a small amount of glue (Araldite 2000+). Some particles were F = FvdW + Fdl + Fatt, (1) spread onto quartz polished disk (Robson Scientific, Saw- bridgeworth, UK) which was used as a bottom of a liquid where the total force between two particles is a superposi- cell in which measurements were done. The quartz disk tion of van der Waals, FvdW, and double-layer, Fdl, forces was beforehand cleaned in piranha solution (3:1 mixture as in classical DLVO and we add an additional attractive of H2SO4 98% and H2O2 30%). Both cantilevers with exponential term, Fatt. The van der Waals force is cal- particles and the substrate were heated in an oven at culated with a non-retarded expression for two spherical 3 particles with radius R [7, 40] The pressure is then integrated to obtain energy per unit area for two plates HR 1 − · FvdW =
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