Comparison of the Calorimetric and van't Hoff Enthalpy of Micelle Formation for a Nonionic Surfactant in H₂O and D₂O Solutions from 15 to 40 °C

Relative partial molar heat content curves were obtained for a nonionic surfactant, dodecyldimethylphosphine oxide, in H 2 O and D 2 O solutions from 15 to 40 °C by titration calorimetry. The critical micelle concentration (cmc) was always lower in D 2 O than in H 2 O. The enthalpy change for micelle formation was determined at 25 °C from integration of an abbreviated form of the van't Hoff equation assuming a temperature-independent aggregation number and heat capacity change to be 1.13 ± 0.14 and 1.75 ± 0.14 kcal/mol in H 2 O and D 2 O, respectively. The corresponding calorimetric values were 1.66 ± 0.03 and 2.07 ± 0.02 kcal/mol. The change in heat capacity obtained from the van't Hoff equation was −113 ± 17 cal/mol-K in H 2 O and −140 ± 11 cal/mol-K in D 2 O. The corresponding values determined from the temperature dependence of the molar enthalpy were −161 ± 2 cal/mol-K in H 2 O and −171 ± 2 cal/mol-K in D 2 O. The temperature dependence of the cmc was fairly well described in both solvents using the partial molar enthalpy and heat capacity changes that accompany micelle formation.