Thermodynamics of Molecular Self-Assembly of Cationic Gemini and Related Double Chain Surfactants in Aqueous Solution

Microcalorimetric measurements have been made on the series of gemini surfactants [C M H 2M+1 (CH 3 ) 2 N(CH 2 ) S N(CH 3 ) 2 C M H 2M+1 ] Br 2, designated C M C S C M Br 2, where M and S indicate the numbers of carbons in the side chains and spacer respectively, for M = 12 and S = 3, 4, 6, 8, 10, 12. For comparison, parallel measurements have been made on the series of double-chain, singly charged surfactants dodecyldimethylalkylammonium bromides, designated as C 12 C N Br with N = 1, 2, 4, 6, 8, 10, 12, where N is the number of carbons in the secondary alkyl chain. The choice of this second series was to compare each C M C S C M Br 2 with its nearest equivalent monomer C 12 C S/2 Br. The values of the critical micelle concentrations (cmc) were found to be in good agreement with other measurements, showing a maximum at S = 4−6 for the geminis and a nonlinear variation with chain length for the C 12 C N Br series. For both series of surfactants, the enthalpies of micellization, Δ H mic, are all exothermic and show a marked minimum in magnitude at S = 4−6 or N = 4−6. The variation of Δ H mic and Δ S mic across both series shows that the balance between enthalpic and entropic contributions to the process of micellization changes substantially with S or N.