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

Abstract

Relative partial molar enthalpy data were obtained at five degree intervals between 15 and 79 °C for n-C8-, C9-, and C10-dimethylphosphine oxides and between 15 and 60 °C for n-C11-dimethylphosphine oxide in H2O and D2O. 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 C12 homologue. These values were then used to calculate the temperature dependence of the cmc, which was always lower in D2O than in H2O 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 H2O to D2O 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.