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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.

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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.

The binding of horse ferricytochrome c to yeast cytochrome c peroxidase at pH 6.0 in 8.7 mM phosphate buffer (0.0100 M ionic strength) is characterized by a small, unfavorable enthalpy change (+1.91 +/- 0.16 kcal mol-1) and a large, positive entropy change (+37 +/- 1 eu). The free energy of binding depends strongly upon ionic strength, increasing from -9.01 to -4.51 kcal mol-1 between 0.0100 and 0.200 M ionic strength. The increase in free energy is due solely to the change in entropy over this ionic strength range, with the entropy change decreasing from 37 +/- 1 to 22 +/- 3 eu between 0.0100 and 0.200 M ionic strength. The observed enthalpy change remains constant over the same ionic strength range. At 0.0100 M ionic strength, complex formation is accompanied by the release of 0.54 +/- 0.11 proton, causing a variation in the observed enthalpy of reaction depending upon the buffer. After accounting for proton binding to the buffer, the corrected values for the enthalpy and entropy of binding are +2.84 +/- 0.26 kcal mol-1 and +21 +/- 3 eu, respectively. At 0.05 M ionic strength, the observed change in heat capacity, delta Cp, for the reaction between horse ferricytochrome c and cytochrome c peroxidase is essentially zero, 1.6 +/- 9.6 cal mol-1 K-1. The corrected delta Cp for binding is -28 +/- 10 cal mol-1 K-1 after accounting for proton binding to the buffer. No evidence for formation of a 2:1 horse ferricytochrome c/cytochrome c peroxidase complex was obtained in this study.