Thermodynamics of tetramethylurea solutions in ethylene glycol: The evidence of pairwise solvophobic interaction

This paper focuses on the solute – solute interactions in dilute solutions of a typically hydrophobic solute tetramethylurea (TMU) in ethylene glycol (EG) which forms the three-dimensional hydrogen-bond network at the supramolecular level. The results of our experimental efforts based on dilution calorimetry and phase partition techniques are able to make a conclusion that thermodynamics of the TMU – TMU pairwise interaction in EG is to a great extent similar to that occurs in liquid water. All effects, however, are weaker expressed in the non-aqueous medium. In particular, the integrand of the potential of mean force, i.e. the second virial coefficient, between two TMU molecules becomes negative only at elevated temperatures indicating the appearance of a rather weak solvophobic interaction in EG. Since C=O-groups of TMU are well-incorporated into the H-bond network of EG, it is reasonable to assume that the attraction observed should be mainly induced by apolar methyl groups. The correlation between the enthalpy- and volume-related parameters of pairwise interaction provides the additional confirmation of the existence of the solvent-induced solvophobic interaction between TMU molecules in EG.