Effects of Cs and Na ions on the interfacial properties of dodecyl sulfatesolutions

The competitive binding of counterions to anionic dodecyl sulfate ions in a queous solutions of cesium dodecyl sulfate (CsDS) and sodium dodecyl sulfat e (SDS) mixtures, which significantly influences the critical micelle conce ntration (cmc) and surface (or interfacial) tension of surfactant solutions , was investigated. The cmc and degree of counterion binding were obtained through electrical conductivity measurements. The curve of cmc versus the m ole fraction of CsDS in the surfactant mixture was simulated by Rubingh's e quations, which enabled us to estimate the interaction parameter in micelle s (W-R) based on the regular solution approximation. The curve-fitting exhi bited a slightly negative value (W-R = -0.1), indicating that the mixing (S DS + CsDS) enhances micelle formation owing to a greater interaction betwee n surfactant molecules and counterions than in pure systems (SDS). On going from SDS, SDS:CsDS(75:25), SDS:CsDS(50:50), SDS:CsDS(25:75) to CsDS, inter facial tension at the hexadecane/surfactant-solution interface showed a neg ative deviation from the mixing rule (interaction parameter in adsorbed fil m W-A = -0.38), indicating the replacement of Na+ bound to anionic dodecyl sulfate by Cs+ ions owing to the stronger interaction between the Cs+ and t he dodecyl sulfate ions. Droplet sizes of emulsion formed with hexadecane a nd aqueous dodecyl sulfate solutions were investigated using the light scat tering spectrophotometer. The higher binding capacity of Cs+ having a small er hydrated ionic size than Na+, also resulted in a negative deviation in e mulsion droplet size in mixed systems.