Hydrophobic effect: Solubility of non-polar substances in water, protein denaturation and micelle formation

The 'hydrophobic effect' of the dissolution process of non-polar substances in water has been analysed under the light of a statistical thermodynamic molecular model. The model, based on the distinction between non-reacting and reacting systems explains the changes of the thermodynamic functions with temperature in aqueous systems. In the dissolution of non-polar substances in water, it follows from the model that the enthalpy change can be expressed as a linear function of the temperature (Delta H-app=Delta H+n(w)C(p),T-w). Experimental solubility and calorimetric data of a large number of non-polar substances nicely agree with the expected function. The specific contribution of n(w) solvent molecules depends on the size of solute and is related to destructuring (n(w)> 0) of water molecules around the solute. Then the study of 'hydrophobic effect' has been extended to the protein denaturation and micelle formation. Denaturation enthalpy either obtained by van't Hoff equation or by calorimetric determinations again depends linearly upon denaturation temperature, with denaturation enthalpy, Delta H-den, increasing with T. A portion of reaction enthalpy is absorbed by a number n(w) of water molecules (n(w)> 0) relaxed in space around the denatured moieties. In micellization, an opposite process takes place with negative number of restructured water molecules (n(w)< 0) because the hydrophobic moieties of the molecules joined by hydrophobic affinity occupy a smaller cavity.