Surface Adsorption versus Bulk Aggregation of Stearyl-Coated Silica Spheres and Surfaces

Abstract

This thesis describes an experimental study on the competition between surface adsorption and bulk aggregation of particles and surfaces of the same material. The studied system consisted of stearyl silica spheres dispersed in cyclohexane and four different kinds of hydrophobic silica surfaces were used as substrates. Poly(dimethyl)siloxane was used to increase the attraction between the particles in the presence of a surface by means of depletion interaction. Surface adsorption from particle dispersion was found to occur as the interaction between the particles was increased. Ellipsometry experiments revealed that surface adsorption occurred at polymer concentrations far below the bulk phase boundary. As the polymer concentration was increased, an increase in the thickness and the adsorbed amount could be observed. Surface adsorption was found to occur even in the absence of depletant and arose as a consequence of the Van der Waals attraction between spheres and surfaces. The observed minimal polymer concentration needed to overcome the thermal energy correlates well with calculated values. Atomic force microscopy measurements were performed to experimentally visualize the structure of the adsorbed layer. From turbidity and viscosity measurements it was found that no bulk aggregation occurs in the window of the interaction strength where surface adsorption occurs.

The obtained results were compared to recent predictions from theory and simulations by Linse and Wennerström (Linse et al., Soft Matter, 2012, 8, 2486) and were found to correspond well.