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