Adsorption of a Nonionic Surfactant (C 12E 7) on Carboxylated Styrene-Butadiene Copolymer Latex Particles Using Dynamic Light Scattering and Adsorption Isotherm Measurements

Abstract

The adsorption of the nonionic surfactant (C 12E 7) to predominantly hydrophobic styrene-butadiene copolymer latex particles, surface-modified with carboxyl groups, has been examined as a function of C 12E 7 concentration by measuring the hydrodynamic radius by dynamic light scattering and adsorption isotherms by a surface tension method. Adsorption in neutral aqueous solution and at pH 4 occurs primarily through hydrophobic interaction between the hydrocarbon surfactant tail and hydrophobic sites on the polymer chains of the "hairy" layer at the latex-water interface. The isotherms are single-stepped. At pH 12 the adsorption isotherms are two-stepped and the thickness of the surface layer of surfactant is approximately twice the fully extended length of the C 12E 7 molecule. The primary micelles formed on the extended polymer chains may grow in aggregation number with increased surfactant concentration, as previously found for SDS molecules adsorbing at the latex interface. There may also be hydrogen bonding, resulting in bilayer formation on the surface chains. The observed increase in adsorption with temperature is interpreted mainly as a result of a change in the interactions between the polyethylene oxide moiety of the nonionic surfactant and water. The effect of salt (NaBr) on the strength of adsorption implies that hydrophobic interactions dominate the adsorption process and may be explained as a salting-out effect when NaBr is added. Bridging through the hydrocarbon moiety of the surfactant molecule adsorbed at the surface may lead to a more compact arrangement of the polymer chains, explaining the observed decrease in the hydrodynamic radius.