Thermodynamics of Hydrophobic Interaction of Dissymmetric Gemini Surfactants in Aqueous Solutions

Abstract

We have used microcalorimetry to measure the critical micelle concentrations (CMC) and enthalpies of micellization (ΔH mic) of the series of dissymmetric cationic gemini surfactants, [C m H2 m +1(CH3)2N(CH2)6N(CH3)2C n H2 n +1]Br2, designated as C m C6C n Br2, 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.

Monolayer Behavior of Asymmetrical Ester-type Tartaric Gemini Amphiphiles

“…Temperature on π-A Isotherms of Asymmetrical Gemini As described in the previous section, C 13 -C 15 was found to form a clear solid phase upon compression at 12.3 . To discuss the effect of the temperature on π-A isotherms of asymmetric geminis, π-A measurements were carried out at various temperatures T sub 7.3 -42.…”

“…As an example of the view point of asymmetry in hydrophobic chains, Bai et al measured the dilution enthalpies of C m C 6 C n Br 2 with m n 24 and -CH 2 6 -spacer, and concluded that a large interaction energy is associated with the addition of a CH 2 to the long alkyl chain than that to the short alkyl chain 13 . According to Zana, however, the distance between two headgroups of C m C 6 C n Br 2 would be comparable to that between two similar monomer units.…”

“…5 b , at T sub 25.2 , with an increase in the molar fraction of 2C 15 X 2C15 , π-A curves varied continuously from that of 2C 13 to 2C 15 , and it was found that the monolayer mixture of 2C 13 and 2C 15 at X 2C15 0.3 was almost in the liquid state. This means that the melting temperature of the monolayer mixture of 2C 13 and 2C 15 at X 2C15 0.3 would be around 25 . For further discussion, the temperature dependence of monolayer mixture of 2C 13 and 2C 15 was studied based on π-A isotherms, and their T L values were determined.…”

“…Interestingly, monolayer formation behaviors of asymmetrical C 13 -C 15 and of an equimolar mixture of 2C 13 and 2C 15 were quite comparable, which would suggest that small perturbations in asymmetry of two hydrophobic groups of gemini might be simulated by changing the mixing ratios of 2C 13 and 2C 15 . Therefore, π-A isotherms of mixtures of 2C 13 and 2C 15 at various molar ratios were measured at various temperatures to study the effects of mixing ratio on their monolayer formation and melting temperatures of monolayer T L . Figures 5 a -c show the representative results obtained at T sub 12.3, 25.2, and 46.1 , respectively.…”

“…From a standpoint of structure, asymmetrical C 13 -C 15 might be viewed as an equivalent mixture of 2C 13 and 2C 15 , and we also measured the π-A isotherms of an equivalent mixture of 2C 13 and 2C 15 . Results obtained at T sub 25.2 are shown in Fig.…”