Surface Deposition and Phase Behavior of Oppositely Charged Polyion–Surfactant Ion Complexes. Delivery of Silicone Oil Emulsions to Hydrophobic and Hydrophilic Surfaces

The adsorption from mixed polyelectrolyte-surfactant solutions at hydrophobized silica surfaces was investigated by in situ null-ellipsometry, and compared to similar measurements for hydrophilic silica surfaces. Three synthetic cationic copolymers of varying hydrophobicity and one cationic hydroxyethyl cellulose were compared in mixtures with the anionic surfactant sodium dodecylsulfate (SDS) in the absence or presence of a dilute silicone oil emulsion. The adsorption behavior was mapped while stepwise increasing the concentration of SDS to a polyelectrolyte solution of constant concentration. The effect on the deposition of dilution of the bulk solution in contact with the surface was also investigated by gradual replacement of the bulk solution with 1 mM aqueous NaCl. An adsorbed layer remained after complete exchange of the polyelectrolyte/surfactant solution for aqueous NaCl. In most cases, there was a codeposition of silicone oil droplets, if such droplets were present in the formulation before dilution. The overall features of the deposition were similar at hydrophobic and hydrophilic surfaces, but there were also notable differences. SDS molecules adsorbed selectively at the hydrophobized silica surface, but not at the hydrophilic silica, which influenced the coadsorption of the cationic polymers. The largest amount of deposited material after dilution was found for hydrophilic silica and for the least-hydrophobic cationic polymers. For the least-hydrophobic polyions, no significant codeposition of silicone oil was detected at hydrophobized silica after dilution if the initial SDS concentration was high

Surface Adsorption and Phase Separation of Oppositely Charged Polyion-Surfactant Ion Complexes: 3. Effects of Polyion Hydrophobicity.

The adsorption to hydrophilic silica surfaces in relation to the bulk phase behavior was investigated by in situ null ellipsometry and bulk turbidity measurements for four cationic copolymers of varying hydrophobicity in mixtures with anionic surfactant sodium dodecylsulfate (SDS). The purpose was to reveal the effect of polyion hydrophobicity on the association with surfactant at surfaces and in the bulk. All polyelectrolytes contained 20 wt % cationic units and had similar molecular weights. An increase in surfactant concentration by the stepwise addition of SDS to a dilute polyelectrolyte solution led to an increase in both the adsorbed amount and bulk turbidity, starting at a defined SDS concentration, as a result of the formation of insoluble polyion-surfactant ion complexes. At some higher SDS concentration, the formed aggregates started to redissolve gradually in the bulk and desorb from the surface because of the overcharging of the complexes. The SDS concentration at which the maxima in adsorption and turbidity occurred increased with decreasing polyion hydrophobicity; the more hydrophobic polyions bound excess SDS more readily, and the aggregates therefore redissolved at a lower SDS concentration. The adsorption from polyelectrolyte-SDS solutions, premixed at SDS concentrations above the adsorption maximum, which occurred on diluting the solution by "rinsing" the ellipsometer cuvette with 1 mM NaCl, was also investigated. On dilution, both the turbidity and the adsorbed amount increased as the excess surfactant in the polyion-surfactant ion complexes progressively decreased. More efficient deposition was achieved if the initial SDS concentration was close to the adsorption maximum. The latter situation could be achieved either by adjusting the SDS concentration or, at a fixed SDS concentration, by choosing a polyion with the appropriate hydrophobicity