Is the counterion responsible for the unusual thermodynamic behaviour of the aqueous solutions of gemini bispyridinium surfactants?

Apparent and partial molar enthalpies at 298 K of the aqueous solutions of cationic gemini surfactants 1,1′-didodecyl-2,2′-trimethylenebispyridinium dichloride (12-Py(2)-3-(2)Py-12 Cl); 1,1′-didodecyl-2,2′-tetramethylenebispyridinium dichloride (12-Py(2)-4-(2)Py-12 Cl); 1,1′-didodecyl-2,2′-octamethylenebispyridinium dichloride (12-Py(2)-8-(2)Py-12 Cl) and 1,1′-didodecyl-2,2′-dodecamethylenebispyridinium dichloride (12-Py(2)-12-(2)Py-12 Cl) were measured as a function of concentration and are here reported for the first time. The curve of the compound with the four carbon atoms spacer lies above than that of the compound with the three carbon atoms spacer, not below, as expected. We have already found this behaviour for the same compounds here studied, but having methanesulfonate as counterion. It is interpreted as an evidence of a conformation change of the molecule determined by stacking interactions between the two pyridinium rings, appearing at an optimum length of the spacer. The curves of apparent and molar enthalpies vs. concentration and surface tension measurements, here reported, show that this behaviour is independent on the counterion. In fact, the group contribution additivity for the counterion is respected, independently on the spacer length. This behaviour could have interesting effect in biological applications for which this new class of gemini surfactant has been tailored.