Dynamic surface tension of aqueous surfactant solutions .8. Effect of spacer on dynamic properties of gemini surfactant solutions

Two series of cationic surfactants, each with two hydrophilic and two hydrophobic groups in the molecule (''gemini'' surfactants), have been synthesized. One series has a rigid, hydrophobic spacer, and the second, a flexible, more hydrophilic one. The dynamic behavior of their aqueous solutions has been studied by the maximum bubble pressure method. The n value (dynamic adsorption parameter) in both series increases with an increase in the alkyl chain length. This increase is larger in the geminis with a flexible hydrophilic spacer. The induction time, t(i), of the geminis with a flexible, hydrophilic spacer is larger than that of the geminis with a rigid, hydrophobic spacer under similar conditions, probably because the chain-chain interaction of the former can form larger linear aggregates, which has been confirmed by viscosity measurements. Adsorption at the air/solution interface of all the compounds studied is diffusion-controlled, as indicated by the fit of the dynamic data to the long time adsorption equation. The surface coverage at the end of the induction period, for all the compounds studied and at different concentrations, is in the range 54-69%. This means that before the interfacial tension can reach the fast fall region, the interface must be covered by about 2/3 of the possible surfactant molecules. The interfacial tension fall rate of geminis with a flexible, more hydrophilic spacer is larger than that of the geminis with a rigid, hydrophobic spacer. (C) 1996 Academic Press, Inc.