Calorimetric and densimetric examinations of aqueous solutions of heptylene-1,7-bis(dimethyloctylammonium bromide) and octylene-1,8-bis(dimethyloctylammonium bromide)

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The critical micelle concentrations of gemini surfactants having different spacer chain lengths 8-7-8 and 8-8-8 in aqueous solutions have been determined by calorimetric and densimetric methods within a wide temperature range. The isothermal volume change upon micellisation of the title surfactants has been obtained from density measurements using pseudo-phase model. Predicted values for apparent molar volume of the surfactant at the critical micellisation concentration (c.m.c.) agree very well with infinite dilution volumes calculated from additivity method. It has been shown that the inflection point in the curves of the dependence of volume expansion coefficient on molality can be used to determine the value of c.m.c. similar as the second one of the two maxima which are observed on the concentration function of apparent molar heat capacity. The experimental determined values of c.m.c. have been compared with the values presented in the literature data for 8-s-8 gemini surfactants.