Unusual behavior of the aqueous solutions of gemini bispyridinium surfactants: Apparent and partial molar enthalpies of the dimethanesulfonates

Apparent and partial molar enthalpies at 298 K of the aqueous solutions of cationic gemini surfactants 1,1'-didodecyl-2,2'-dimethylenebispyfidinium dimethanesulfonate (12-Py(2)-2-(2)Py-12 MS); 1, 1'-didodecyl-2,2'-trimethylenebispyridinium dimethanesulfonate (12-Py(2)-3-(2)Py-12 MS); 1,1'-didodecyl-2,2'-tetramethylenebispyridinium dimethanesulfonate (12-Py(2)-4-(2)Py-12 MS); 1,1'-didodecyl-2,2'-octamethylenebispyridinium dimethanesulfonate (12-Py(2)-8-(2)Py-12 MS); 1,1'-didodecyl-2,2'-dodecamethylenebispyridinium dimethanesulfonate (12-Py(2)-12-(2)Py-12 MS) were measured as a function of concentration and are here reported for the first time. They show a very peculiar behavior as a function of the spacer length, not allowing for the determination of a -CH2- group contribution when this group is added to the spacer. The curve of the compound with a four-carbon-atom-long spacer lies between those of the compound with a spacer of 2 and 3 carbon atoms, instead of that below the latter, as expected. This surprising behavior, never found before in the literature and different from that found for the more popular m-s-m-type bisquatenary ammonium gemini surfactants, could be explained by a conformation change of the molecule, caused by stacking interactions between the two pyridinium rings, mediated by the counterion and appearing at an optimum length of the spacer. The hypothesis is also supported by the data obtained from the surface tension vs log c curves, showing that A in, the minimum area taken at the air-water interface by the molecule, is significantly lower for 12-Py(2)-4-(2)Py-12 MS than that of the other compounds of the same homologous series, and that the same compound has a greater tendency to form micelles instead of adsorbing at the air/water interface. The evaluation of the micellization enthalpies, by means of a pseudophase transition model, agrees with the exposed trends. These results confirm the great crop of information that can be derived from the study of the solution thermodynamics of aggregate systems and in particular from the curves of apparent and molar enthalpies vs concentration.