Microcalorimetric Studies on the Thermodynamic Properties of Cationic Gemini Surfactants

Microcalorimetric measurements have been made as a function of temperature on the series of gemini surfactants [C M H 2 M +1 (CH 3 ) 2 N(CH 2 ) S N(CH 3 ) 2 C M H 2 M +1 ]Br 2, designated C M C S C M, where M and S indicate the numbers of carbon atoms in the side chains and spacer, respectively, for M = 12 and S = 3, 4, 6, 8, 10, and 12. The maximum at S = 4−6 in the values of the critical micelle concentrations (cmc's) as a function of spacer length was found to be associated with a strong minimum in the magnitude of the exothermic enthalpy of micellization, Δ H mic . A strong maximum in the corresponding entropies of micellization, Δ S mic, almost cancels out the effect of Δ H mic, so that Δ G mic and hence the values of the cmc show a much weaker variation with spacer. The values of Δ C P pass through a marked minimum and are large, corresponding to a large variation of Δ H mic with temperature. This variation of Δ H mic has some interesting consequences for the variation of the cmc. Thus, for C 12 C 12 C 12 the contribution of Δ H mic to Δ G mic at room temperature is about 36%, but this increases through 61% to 84% at 308 K. Over this range the changes in Δ G mic remain within about 10% of each other, almost within the experimental error.