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

• Published in: The journal of physical chemistry. B Vol. 105; no. 15; pp. 3105 - 3108
• Main Authors: Bai, Guangyue, Wang, Jinben, Yan, Haike, Li, Zhixin, Thomas, Robert K
• Format: Journal Article
• Language: English
• Published: American Chemical Society 19.04.2001
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp0043017

Microcalorimetric measurements have been made on the series of gemini surfactants [CMH2M+1(CH3)2N(CH2)SN(CH3)2CMH2M+1] Br2, designated CMCSCMBr2, 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 C12CNBr 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 CMCSCMBr2 with its nearest equivalent monomer C12CS/2Br. 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 C12CNBr series. For both series of surfactants, the enthalpies of micellization, ΔH mic, are all exothermic and show a marked minimum in magnitude at S = 4−6 or N = 4−6. The variation of ΔH mic and ΔS mic across both series shows that the balance between enthalpic and entropic contributions to the process of micellization changes substantially with S or N.