Mass spectrometric characterization of the non-ionic gemini surfactant Surfynol 465 and a microcalorimetric study of its micelle formation in water

Electrospray ionization mass spectrometry was applied to determine the molecular weight distribution and the number average molecular weight of the commercially available gemini surfactant α,α′-[2,4,7,9-tetramethyl-5-decyne-4,7-diyl]bis[ω-hydroxylpoly(oxyethylene)], Surfynol 465 (S465), with NaCl and KCl as the cationization agents. The enthalpies of S465 micelle formation in aqueous solution were measured over a range of temperature by thermometric titration with a MicroCal VP-ITC microcalorimeter. The critical micelle concentration (cmc) of S465 decreased as the temperature was increased from 298 K to 348 K. The variation of the calorimetric enthalpy of micelle formation with temperature was markedly linear in this range of temperature, which allowed extrapolation to slightly higher temperatures. The van’t Hoff enthalpies calculated from the temperature dependence of the cmc, were in excellent agreement with the directly measured calorimetric enthalpies. It could be established that S465 micellization changed from endothermic to exothermic as the cmc passed through a minimum at 357 K. The thermodynamic analysis indicated that micelle formation is favoured by entropy and opposed by enthalpy up to this temperature, after which the aggregation is favoured by both entropy and enthalpy. The enthalpy–entropy compensation resulted in a slight decrease in Gibbs free energy with increase in temperature.