Thermodynamic properties of aqueous solutions of organic nonelectrolytes: use of a simple molecular model at the MacMillan–Mayer level

A simple molecular model is used to describe the excess thermodynamic properties of nonelectrolyte solutions at the MacMillan–Mayer level. In this framework, the solute may be regarded formally as a gas for which the potential of interaction between particles has been averaged over the states of the solvent. Here, interactions between solute particles have been modeled by excluded volume effects and attractive van der Waals forces. This procedure involves two parameters per neutral compound. The model has been applied to some organic nonelectrolytes in water: urea, dimethylurea, some sugars, and sugar alcohols. The adjusted parameter values correlate with the size of the molecule and the number of polar groups. Analysis of data for urea at various temperatures suggests that attractive forces may be modelled by dipole–dipole interactions. The properties of mixtures could generally be represented satisfactorily without additional parameter. The prediction of enthalpies of dilution is discussed in the case of urea solutions.