Alkyltrimethylammonium bromide adsorption on polystyrene latex particles studied by dynamic light scattering and adsorption isotherms: Effects of the surface polymer layer and modified aromatic amino groups

The adsorption of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) on polystyrene latex particles modified with aromatic amino groups has been examined as a function of surfactant concentration by dynamic light scattering. Adsorption isotherms were determined by a surface tension method. At pH 6 without added salt, DTA(+) ions are initially adsorbed at the ends of the polymer chains through electrostatic attraction between the quarternary ammonium head-groups of DTA(+) ions and the sulfate groups terminating the polymer chains as well as through hydrophobic interaction between the hydrocarbon tails of the surfactant and the polymer backbone. Combination of these forces leads to an L-type isotherm. An extended layer of polymer chains at the latex surface favors further adsorption of DTA(+) ions which bridge neighboring polymer chains and lead to their aggregation. Further adsorption around the initially occupied sites results in formation of micelles along the polymer chains. Premature aggregation of the polymer chains during adsorption leads to a smaller adsorbed amount at saturation compared with DTAB adsorption to polystyrene latex particles having a flat surface layer of polymer chains. The bridging role of the adsorbed surfactant is enhanced on increasing the surfactant hydrocarbon chain length. The adsorbed amount at saturation is thus reduced within the series DTAB --> TTAB --> CTAB owing to premature aggregation of the polymer chains. Hydrophobic interaction between the polymer chains and the alkyl tails of DTAB, and also between the alkyl tails themselves, is enhanced in the presence of NaBr. Reduced electrostatic repulsion between the head-groups of the adsorbed DTA(+) ions results in close-packing along the polymer chains. Semiglobular micelles grow on further adsorption ultimately leading to coalescence of the aggregates into a continuous monolayer in 32 mM NaBr. At pH 2,the polymer chains are anchored at the bare latex surface by electrostatic interaction between the positive amino groups and the negative sulfate groups. Conformational changes of the polymer chains provide a suitable steric geometry for DTA(+) ion adsorption through hydrophobic interaction of the alkyl tails with the polymer chains and also of the alkyl tails themselves with each other. The adsorption isotherm is thus of the S-type. Semiglobular micelles are formed on the polymer chains at saturation. At pH 12, the negatively charged amino and sulfate groups directly associate with the positively charged quarternary ammonium groups of DTAB. This leads to substantial reduction of the surface charge density and a more hydrophobic surface owing to extension of the alkyl tails of the adsorbed DTA(+) ions into the solution. Precipitation of the latex particles thus occurs when the DTAB concentration is close to its critical micelle concentration. DTAB adsorption at pH 12 corresponds to a C-type isotherm.