Structural effects on surface and micellar properties of alkanediyl- α, ω-bis(sodium N-acyl- β-alaninate) gemini surfactants

A study of the equilibrium surface properties (in water and in the presence of 0.01 M NaCl) of a novel series of anionic gemini surfactants, alkanediyl- α, ω-bis(sodium N-acyl- β-alaninates), is described. Parameters studied include cmc (critical micelle concentration), C 20 (required to reduce the surface tension of the solvent by 20 mN/m), γ cmc (the surface tension at the cmc), Γ max (the maximum surface excess concentration at the air/aqueous solution interface), A min (the minimum area per surfactant molecule at the air/water interface), and the cmc/ C 20 ratio (a measure of the tendency to form micelles relative to adsorb at the air/water interface). The geminis with a spacer consisting of two methylene groups show premicellar self-aggregation, both in water and in 0.01 M NaCl, when the N-acyl group contains more than 12 carbon atoms; geminis with a spacer consisting of four methylene groups show no premicellar aggregation even when the N-acyl group contains 16 carbon atoms. For the acyl chain lengths where premicellar aggregation does not occur, the values of the cmc of the geminis with a two-methylene spacer are lower than those for the corresponding analogous geminis with a four-methylene spacer. The premicellar formation for the geminis with a two-methylene spacer is due to the short-chain linkage. The geminis show little or no break in their specific conductance-surfactant molar concentration plots and an increase in the pH at the cmc. This is attributed to protonation of the carboxylate group and strong Na + release during micellization.