Silica Surfaces In Solutions Research Articles

Direct measurement of interaction force between hydrophilic silica surfaces in triblock copolymer solutions with salt by atomic force microscopy

Triblock copolymers composed of polyethylene oxide (PEO) and polypropylene oxide (PPO) are used in various fields as nonionic surfactants. In this study, we measured interaction forces between untreated hydrophilic silica surfaces in solutions with two typical triblock copolymers, Pluronic P123 (PEO20PPO70PEO20) and F127 (PEO99PPO65PEO99), in the presence of 1 mM and 500 mM NaCl using atomic force microscopy (AFM). In solutions at the copolymer concentration of 1 µM, which is below the critical micelle concentration (CMC), the measured interaction forces were monotonically repulsive in the presence of 1 mM NaCl, which suggested the brush-like conformation of copolymers on the surfaces. When the concentration of NaCl was increased to 500 mM, interaction forces became attractive, which indicated the bridging of adsorbed polymers onto surfaces, the strength of which varied depending on the affinity and adsorption density of copolymers. The interactions at the copolymer concentration of 1 mM, which were above the CMC of both copolymers, were steric repulsions between adsorbed micelles on the surfaces with 1 mM of NaCl. For 500 mM of NaCl, an attractive jump after a steric repulsion was observed only in the force curve for P123, which inferred that the displacement of micelles from the surfaces was presumably due to a decrease in the strength of adsorption caused by the dehydration of EO groups. These results indicated that the length of the EO group considerably affected the interactions.

Surface Chemistry and Rheology of Polysulfobetaine-Coated Silica

We have measured the viscosity of suspensions of colloidal silica particles (d = 300 nm) and the properties of silica surfaces in solutions of a polymer consisting of zwitterionic monomer groups, poly(sulfobetaine methacrylate), polySBMA. This polymer has potential use in modifying surface properties because the polymer is net uncharged and therefore does not generate double-layer forces. The solubility of the polymer can be controlled and varies from poor to good by the addition of sodium chloride salt. Ellipsometry was used to demonstrate that polySBMA adsorbs to silica and exhibits an increase in surface excess at lower salt concentration, which is consistent with a smaller area per molecule at low salt concentration. Neutron reflectivity measurements show that the adsorbed polymer has a thickness of about 3.7 nm and is highly hydrated. The polymer can be used to exercise considerable control over suspension rheology. When silica particles are not completely covered in polymer, the suspension produces a highly viscous gel. Atomic force microscopy was used to show this is caused by bridging of polymer between the particles. At higher surface coverage, the polymer can produce either a high or very low viscosity slurry depending on the sodium chloride concentration. At high salt concentration, the suspension is stable, and the viscosity is lower. This is probably because the entrainment of many small ions renders the polymer film highly hydrophilic, producing repulsive surface forces and lubricating the flow of particles. At low salt concentrations, the polymer is barely soluble and more densely adsorbed. This produces less stable and more viscous solutions, which we attribute to attractive interactions between the adsorbed polymer layers.

PH dependence of friction forces between silica surfaces in solutions

The pH dependence of the friction between a silica particle and a silica wafer was investigated using lateral force microscopy. Measurements were done in the range of 3.6 ≦ pH ≦ 10.6 and the effect of high loading force was also examined. It is found that the friction is independent of the pH of solutions and increases linearly with the applied load, when the pH is between 3.6 and 8.6. On the other hand, once the pH is above 9.0, the friction becomes extremely small and the dependence on the applied load becomes nonlinear. It is postulated that this transition is due to the development of a gel layer composed of polymer-like segments of silicilic acid anchored on the surface; at the lower applied load, this layer acts as a boundary lubricant between the surfaces, but, at the higher applied load, the entanglements of these segments and more direct contact between two solid surfaces leads to the increase of the friction. The effects found here are expected to play an important role in elucidating the basic mechanism of the planarization process of silica wafers.

Molecular Weight Dependence of the Depletion Interaction between Silica Surfaces in Solutions of Sodium Poly(styrene sulfonate)

Direct measurements of the depletion interaction between silica surfaces immersed in solutions of sodium poly(styrene sulfonate) have been made with the atomic force microscope, using the colloid probe technique. Significant variations in the interaction forces as a function of both the polymer molecular weight and the probe radius were observed. The depletion interaction yielded a secondary minimum in the observed force−distance profiles. This minimum was observed to increase in depth as a function of both the polymer molecular weight and the polymer concentration. The magnitude of the measured depletion interaction did not increase as much as expected when the size of the colloid probe was increased. The importance of surface asperities and local radii of curvature with respect to force measurements are discussed.

Heterogenization of racemic ethylenebis(1-indenyl)zirconium dichloride on trimethylaluminum vapor modified silica surface

Heterogeneous metallocene catalysts were prepared by adsorbing rac-Et(Ind) 2ZrCl 2 on a modified silica surface in solution. The modification of silica was conducted in gas phase with atomic layer chemical vapor deposition (ALCVD) technique, where the silica, preheated at either 350 or 600°C, was allowed to react with vaporized trimethylaluminum (TMA) at 250°C. Modified carriers and heterogeneous catalysts were characterized with FTIR, 1 H MAS (magic-angle spinning) NMR, 13 C , and 29 Si CP (cross-polarization) MAS NMR spectroscopies and elemental analyses. In the reaction of TMA with silica, a saturated surface was formed consisting of different (O) 4− n Si(CH 3) n ( n=1, 2 or 3) and AlCH 3 groups. The ratio of SiMe to AlMe groups was approximately 1.5 in the TMA/SiO 2 carriers. When the metallocene was adsorbed onto the carrier it seemed to react with the surface AlCH 3 groups and possibly ZrCH 3 groups were formed. Heterogeneous catalysts were tested in the polymerization of ethylene and propylene in the presence of methylalumoxane (MAO). And they produced similar polymer as the homogeneous rac-Et(Ind) 2ZrCl 2 catalyst, but with lower activity. A catalyst with the best activity was achieved from silica that was preheated at 600°C. Moreover, leaching of catalyst was examined whereupon a part of zirconium was observed to desorb from the carrier.

Depletion forces between silica surfaces in solutions of poly(acrylic acid)

The forces between silica surfaces in the presence of poly(acrylic acid) solutions have been measured as a function of polymer concentration using an atomic force microscope. Under similar solution conditions to the force–distance studies, the stability of a colloidal silica dispersion to depletion flocculation was studied. The measured force–distance curves were used to interpret the observed flocculation behaviour of the dispersion.