Thermodynamic Studies of Aqueous m–s–m Gemini Surfactant Systems

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The specific conductance, surface tension, and apparent molar volume properties of aqueous solutions of two series of m–s–m gemini surfactants—one having a constant spacer s(=3) with m=8, 10, 12, and 16 and the other having a constant alkyl chain length m(=12) with variable spacer length 2≤s≤16—are reported. A surfactant with m=12 and having a p-xylyl (φ) spacer was also studied to assess the effect of rigidity in the spacer group on gemini interfacial properties. The correlations between these properties for a given series are discussed. Experimental apparent molar volume data have been modeled assuming both a mass action (8–3–8 only) and a pseudo-phase model. Predicted values for the apparent molar volume of the surfactant at the (cmc) (Vφ,CMC) obtained from the pseudo-phase model have been compared with infinite dilution volumes (V0) calculated from two additivity methods, one based on the contribution of the corresponding mono-quaternary ammonium surfactant and the other based upon the contribution of the corresponding bolaform cation. Poor agreement was obtained with the first method, while good agreement was obtained with the second. The observed variation in the volume change due to micelle formation, ΔVφ,M, is consistent with variations in the head group area and critical micelle concentrations and can be rationalized in terms of the location of the spacer—either at the micelle/water interface, or in the micelle interior. Results obtained for the 12–φ–12 surfactant indicate that rigidity of the spacer has no measurable effect on the micellization process for such a short spacerlength.