A Calorimetric Determination of the Standard Enthalpy and Heat Capacity Changes that Accompany Micelle Formation for Four Long Chain Alkyldimethylphosphine Oxides in H₂O and D₂O Solution from 15 to 79 °C

Relative partial molar enthalpy data were obtained at five degree intervals between 15 and 79 °C for n -C 8 -, C 9 -, and C 10 -dimethylphosphine oxides and between 15 and 60 °C for n -C 11 -dimethylphosphine oxide in H 2 O and D 2 O. The results were used to calculate the standard molar enthalpy and heat capacity changes for micelle formation for comparison with the data previously reported for the C 12 homologue. These values were then used to calculate the temperature dependence of the cmc, which was always lower in D 2 O than in H 2 O solutions. A small temperature dependence of the heat capacity change for micelle formation was observed in both solvents. Changes in thermodynamic parameters that accompany the transfer of the surfactant from H 2 O to D 2 O solution were also determined. It is interesting that differences in heat capacity noted for micelle formation or transfer of monomeric surfactant between the two solvents exhibited a greater dependence on the molecular weight of the surfactants than the corresponding enthalpy and entropy changes.