Capillary Condensation and Interface Structure of a Model Colloid-Polymer Mixture in a Porous Medium

We consider the Asakura-Oosawa model of hard sphere colloids and ideal polymers in contact with a porous matrix modeled by immobilized configurations of hard spheres. For this ternary mixture a fundamental measure density functional theory is employed, where the matrix particles are quenched and the colloids and polymers are annealed, i.e. allowed to equilibrate. We study capillary condensation of the mixture in a tiny sample of matrix as well as demixing and the fluid-fluid interface inside a bulk matrix. Density profiles normal to the interface and surface tensions are calculated and compared to the case without matrix. Two kinds of matrices are considered: (i) colloid-sized matrix particles at low packing fractions and (ii) large matrix particles at high packing fractions. These two cases show fundamentally different behavior and should both be experimentally realizable. Furthermore, we argue that capillary condensation of a colloidal suspension could be experimentally accessible. We find that in case (ii), even at high packing fractions, the main effect of the matrix is to exclude volume and, to high accuracy, the results can be mapped onto those of the same system without matrix via a simple rescaling.Comment: 12 pages, 9 figures, submitted to PR

Wetting phenomena and interactions in phase-separate colloid-polymer mixtures

In this last chapter I will review and integrate the findings of the previous chapters and give suggestions for further research on this topic.In chapter 2 measurements of depletion interactions by means of colloidal probe atomic force microscopy (CP-AFM) are described. We found that the behaviour of the range of the depletion interaction is roughly in agreement with predictions of Fleer et aI. The strength of the depletion interaction is not in agreement with of predictions of Tuinier et al.2 and Louis3 and Bolhuis.4 It is much smaller. We explain this by the fact that the segment-surface interaction for polydimethylsiloxane (PDMS) and a stearylated silica surface is non zero. Comparison of our experimental values with values obtained by Scheutjens Fleer Self Consisted Field theory for different values of Xs leads to a reduced segment surface interaction parameter of 0.41. This corresponds to a decrease to about one order of magnitude of the maximal depletion interaction (see figure 2.13). This strong weakening is in agreement with experimental findings that significantly more PDMS is needed to reach the colloidal liquid-gas binodal than predicted, even if non ideality of the polymers is taken into account.5· 6In chapter 3 we present observations of wetting phenomena in depletion interaction driven, phase-separated colloidal dispersions. These dispersions consist of stearyl coated silica spheres in cyclohexane with PDMS as non-adsorbing polymer. The dynamic contact angle was determined by moving a thin fibre upward or downward through the colloidal liquid gas interface and drawing tangents to the meniscus at the fibre in the three phase contact line. The dynamic contact angle was measured as a function of the velocity of the fibre. From these dynamic contact angles the static contact angle was determined by interpolation to zero velocity. We observed no noticeable hysteresis. The contact angle of the colloidal liquid-gas interface at a solid substrate (coated glass) was determined for a series of compositions. We were able to locate a Cahn wetting transition7 from partial to complete wetting upon approaching the critical point. By this we have tested successfully the well known Cahn prediction regarding wetting transitions for coloid-polyrner mixtures.In the same way we determined the contact angle of the colloidal liquid-gas interface at a soft solid substrate for a series of compositions. A soft solid substrate was obtained by heating a glass fibre in a melt of PDMS. In this way we manipulated the colloidal particle-substrate interaction. Upon approach to the critical point, a transition occurs from partial to complete drying. The region where the wetting transition occurs was compared with that of density functional calculations of Brader et al. and Monte Carlo simulations of Dijkstra et al.8'

Wetting in a colloidal liquid-gas system

We present first observations of wetting phenomena in depletion interaction driven, phase separated colloidal dispersions (coated silica-cyclohexane-polydimethylsiloxane). The contact angle of the colloidal liquid-gas interface at a solid substrate (coated glass) was determined for a series of compositions. Upon approach to the critical point, a transition occurs from partial to complete wetting

Wetting behavior in colloid-polymer mixtures at different substrates

We present experimental observations on wetting phenomena in depletion interaction driven, phase separated colloidal dispersions. The contact angle of the colloidal liquid-gas interface at a solid substrate was determined for a series of compositions. Upon approach to the critical point, a transition occurs from partial to complete wetting. The interaction with the substrate was manipulated by modifying the substrate with a polymer. In that case, a transition from partial to complete drying is observed upon approach to the critical poin

Adsorption of ethoxylated styrene oxide and polyacrylic acid and mixtures there of on organic pigment

The adsorption of two polymeric surfactants on an organic pigment was investigated. As surfactants the anionic polyacrylic acid sodium salt (PANa, MW=15,000) and a non-ionic block copolymer surfactant based on styrene oxide (SO) and ethylene oxide (EO) (MW=1500) were used. The adsorption behavior was analyzed by size exclusion chromatography of the supernatant after centrifugation of the pigment dispersions. It was found that PANa has no affinity to the pigment, whereas SO–EO has a strong affinity to the pigment surface. Competitive adsorption of PANa and SO–EO was not observed. Addition of SO–EO yields stable dispersion

Depletion interaction measured by colloidal probe atomic force microscopy

We investigated the depletion interaction between stearylated silica surfaces in cyclohexane in the presence of dissolved polydimethylsiloxane by means of colloidal probe atomic force microscopy. We found that the range of the depletion interaction decreases with increasing concentration. Furthermore the depletion interaction in this system is much weaker than predicted by theories assuming a hard-wall type interaction between polymer segments and surface. We conclude that the interaction between the polymer segments and the surface is not zero, which weakens the depletion interaction

Competitive adsorption of (phosphorylated) ethoxylated styrene oxide polymer and polyacrylic acid on silica coated iron oxide pigment

The colloidal stabilization in waterbased paint is poorly understood due to its complexity in composition, usually containing mixtures of particles and of surface active agents ("dispersants"). In this study we make a step forward by analyzing the competitive adsorption of a few widely used dispersants on a typical inorganic pigment (70 nm sized Fe2O3-based red pigment; surface treated with silica; negative zetapotential at pH ~ 7). The supposition is that any particle type in paint needs sufficient adsorbed dispersant in order to be stable. Thus, we investigate, for two combinations of two dispersants, how they mutually affect their adsorption at that pigment. Also the "single" adsorption of these dispersants was investigated, thus in the absence of the other. The dispersants are an anionic, polyacrylic acid sodium salt ("PANa"; MW = 15,000 Da) in combination with a MW = 1500 Da blockcopolymer of styrene oxide (SO) and ethoxylene (EO), either or not endcapped by a phosphate group (P). The adsorption behavior was analyzed by size exclusion chromatography of the processed supernatant of the pigment dispersion. PANa and SO–EO–P adsorb for electrostatic reasons while SO–EO has affinity only to an organic surface. PANa and SO–EO–P show regular single adsorption with a plateau starting at the critical micelle concentration (cmc). SO–EO shows single adsorption only beyond its cmc based on the adsorption of full micelles. When in competition, with SO-EP-P/PANa the adsorption of SO–EO–P is lowered by ~65% while that of PANa is unchanged. With SO–EO/PANa the surface active species behave like in single adsorption below the cmc, but beyond the cmc a complicated phase separation occurs that cannot be based on mixed micelles. The adsorption data of PANa are compromised by depletion of PANa from interstices between particles. The anionic dispersants adsorb to the silicium oxide coated iron oxide pigment with negative surface potential because of the presence of a pH dependent relatively small number of positive iron oxide surface sites