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

Abstract

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.

Alkanediyl-α,ω-Bis(dimethylalkylammonium Bromide) Surfactants

“…• M values for DTAB compare well with previously reported ones (15,16 Table 1 are negative, indicating that micelle formation is an exothermic process for the investigated surfactants. The formation of micelles by conventional surfactants is also exothermic provided the temperature is high enough or the surfactant alkyl chain long enough (12,13,18,(19)(20)(21)(22)(23)(24)(25) The increase of − H • M for s ≥ 6 is small but noteworthy.…”

“…In their recent calorimetric studies Bai et al (15,16) Table 1. They are seen to be generally less exothermic than the one obtained in the present study.…”

“…They are seen to be generally less exothermic than the one obtained in the present study. Also the plots of the enthalpy of dilution reported in these studies (15,16) Table 1. We have no explanation for the behavior of dil H reported in Refs.…”

Dimeric (Gemini) Surfactants: Effect of the Spacer Group on the Association Behavior in Aqueous Solution

“…The data should be reanalyzed using the same method as in Ref. (65) in order to assess the validity of the above conclusion.…”

Interaction between cationic and conventional nonionic surfactants in the mixed micelle and monolayer formed in aqueous medium

“…The number of species at the interface vary with the surfactant bulk concentration (n) were taken as 2. 41 The surface excess concentration (G max ), determined at surface saturation using the Gibbs isotherm (Equation 8), is a measure of the effectiveness of the surfactant adsorption at the interface, since it is the maximum value which adsorption can attain. The maximum surface adsorption concentration obtained corresponds to the maximum packing and strong tighten surfactant molecules at the interface.…”