The contribution of hydrogen bonds to bulk and surface thermodynamic properties of dimethylsulfoxide–water mixtures

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Abstract
A simple model for a hydrogen-bonded mixture treated in mean field approximation is used to study the excess bulk and interfacial thermodynamic properties of DMSO/water solutions at various compositions. According to the model, the system is treated as an ensemble of hydrogen bonding pairs involving water and DMSO molecules. In the interfacial layer of thickness equal to the length of the hydrogen bond, the number of hydrogen bonding pairs is reduced. Considering the difference in the hydrogen bonding strength for the water–water and water–DMSO hydrogen pairs, as well as orientational restrictions at the liquid/vapor interface, the number densities of water–water and water–DMSO hydrogen bonds are calculated as functions of the distance from the interface. The concentration profile is determined by minimizing the free energy function of the system. The model describes reasonably well the experimental data for excess enthalpies and free energies of mixing, as well as the excess surface free energy over the entire composition range. The resulting excess surface free energy is considered as a contribution of the hydrogen bond interaction among the two species to the total surface tension.

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