ζ-Potential and Surface Adsorbability of Styrene/Acrylamide Copolymer Latices in Aqueous Solutions of N-Alkylpyridinium Bromides

To study the surface adsorption of cationic surfactants on hydrophilic polymer latices, ζ-potentials of styrene/acrylamide [P (St/AAm) ] copolymer latices were measured in aqueous solutions of alkylpyridinium bromide (abbr. APB, CnH2n+1NC5H5Br;n=12: DPB, n=14: TPB, n=16: HPB). The thickness Δ of the AAm layer on the P(St/AAm) latex surface determined by viscosity and the distance t from the Stern layer to the slipping plane in the electirical double layer on the latex surface increased with AAm the fraction. It thus appears that the hydration layer is present on the surface of P(St/AAm) latices. The sign of the ζ-potential of the latex changed from negative to positive with increasing concentration of surfactant and the amount of surfactant adsorbed on latices decreased with increasing the thickness of PAAm layer. The surfactant adsorption increased with the hydrocarbon chain length of APB in the order, HBP>TPB >DPB. These results can be explained on the basis of electrostatic attractive force and hydrophobic interaction. The negative value of free energy of adsorption ΔG for surfactant on a latex decreased with increasing thickness of the PAAm layer and increased with chain length of the surfactant. The large positive value of entropy of adsorption ΔS indicates the importance of entropic interactions and the formation of hydrophobic bonds as driving forces for adsorption of the surfactant on a letex surface. It is concluded that the adsorbability of a surfactant onto P(St/AAm) latices is less than that on the PS latex, since hydrophilic poly-AAm layer is present on the surface of P(St/AAm) latices.