Abstract
To study the surface adsorption of cationic surfactants on hydrophilic polymer latices, the ζ-potentials of styrene/acrylic acid [P(St/AA)] copolymer latices were measured in aqueous alkyl pyridinium bromide solutions (abbr. APB, CnH2n+1 NC5H5Br ; n=12 : DPB, n=14 : TPB, n=16 : HPB) and compared with those of polystyrene (PS), styrene/2-hydroxyethyl methacrylate copolymer [P(St/HEMA)] and styrene/acrylamide copolymer [P St/AAm] latices. Surface charge density (σ) was proportional to the amount of acrylic acid (AA) used in the copolymeation. σ for P(St/AA) latices increased with pH. The distance t from the Stern layer riz to slipping plane in the electrical double layer increased with the AA fraction. Hydrous poly acrylic acid (PAA)layers thus appear to be present on the surfaces of P(St/AA)latices. PAA layer thickness is less than that of poly-HEMA or poly-AAm layers. The amount of surfactant adsorbed per unit area of a latex was calculated from the ζ-potential value. The sign of the ζ-potential of a latex changed from negative to positive with increase in surfactant concentration. Surf actant adsorption increased with APB hydrocarbon chain length in the order, HPB>TPB>DPB. These results can be explained by elctrostatic attractive force and hydrophobic interaction. The amount of surfactant adsorbed on P(St/AA) latices was less than an the PS latex, equal to that on P(St/HEMA) and P St/AAm5 latices and larger than that on the P(St/AAm10) latex. The negative value of free energy of adsorption ΔG for a surfactant on a latex decreased with PS>P(St/HEMA)-P(St/AAm5)-P(St/AA5)>P(St/AAm10). The large posltlve value of enttopy of adsorption ΔS indicates the importance of entropic interactions and the formation of hydrophobic bonds as the driving force for adsorption of a surfactant on a latex surface. Steric prevention by a PAA layer may be less than that by poly-HEMA and poly-AAm layers.
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