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 bro mide (DTAB) on polystyrene latex particles modified with aromatic amin o groups has been examined as a function of surfactant concentration b y dynamic light scattering. Adsorption isotherms were determined by a surface tension method. At pH 6 without added salt, DTA(+) ions are in itially adsorbed at the ends of the polymer chains through electrostat ic attraction between the quarternary ammonium head-groups of DTA(+) i ons and the sulfate groups terminating the polymer chains as well as t hrough hydrophobic interaction between the hydrocarbon tails of the su rfactant and the polymer backbone. Combination of these forces leads t o an L-type isotherm. An extended layer of polymer chains at the latex surface favors further adsorption of DTA(+) ions which bridge neighbo ring polymer chains and lead to their aggregation. Further adsorption around the initially occupied sites results in formation of micelles a long the polymer chains. Premature aggregation of the polymer chains d uring adsorption leads to a smaller adsorbed amount at saturation comp ared with DTAB adsorption to polystyrene latex particles having a flat surface layer of polymer chains. The bridging role of the adsorbed su rfactant is enhanced on increasing the surfactant hydrocarbon chain le ngth. The adsorbed amount at saturation is thus reduced within the ser ies DTAB --> TTAB --> CTAB owing to premature aggregation of the polym er chains. Hydrophobic interaction between the polymer chains and the alkyl tails of DTAB, and also between the alkyl tails themselves, is e nhanced in the presence of NaBr. Reduced electrostatic repulsion betwe en the head-groups of the adsorbed DTA(+) ions results in close-packin g along the polymer chains. Semiglobular micelles grow on further adso rption ultimately leading to coalescence of the aggregates into a cont inuous monolayer in 32 mM NaBr. At pH 2,the polymer chains are anchore d at the bare latex surface by electrostatic interaction between the p ositive amino groups and the negative sulfate groups. Conformational c hanges of the polymer chains provide a suitable steric geometry for DT A(+) ion adsorption through hydrophobic interaction of the alkyl tails with the polymer chains and also of the alkyl tails themselves with e ach other. The adsorption isotherm is thus of the S-type. Semiglobular micelles are formed on the polymer chains at saturation. At pH 12, th e 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 hyd rophobic surface owing to extension of the alkyl tails of the adsorbed DTA(+) ions into the solution. Precipitation of the latex particles t hus occurs when the DTAB concentration is close to its critical micell e concentration. DTAB adsorption at pH 12 corresponds to a C-type isot herm.