Thermodynamics of hydrophobic interaction of dissymmetric gemini surfactants in aqueous solutions

We have used microcalorimetry to measure the critical micelle concentrations (CMC) and enthalpies of micellization (DeltaH(mic)) of the series of dissymmetric cationic gemini surfactants, [CmH2m+1(CH3)(2)N(CH2)(6)N-(CH3)(2)CnH2n+1]Br-2, designated as CmC6CnBr2, with constant n + m = 24 and n = 6, 8, 10, 11, 12. There is a small decrease in CMC as the ratio m/n increases but this decrease is not much larger than the error. On the other hand there is a very large decrease in the enthalpy of micellization with m/n. This is consistent with a large increase in the hydrophobic contribution to micellization with m/n. To explain the relative invariance of the CMC. there must be a corresponding large increase in the entropy of micellization with m/n. The change in the hydrophobic contribution is explained in terms of the relative strengths of internal and external hydrophobic contacts down the series.