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Why is poly(oxyethylene) soluble in water? Evidence from the thermodynamic profile of the conformational equilibria of 1,2-dimethoxyethane and dimethoxymethane revealed by Raman spectroscopy.

The origin of high solubility of poly(oxyethylene) in water has been an open question. Although it is thought that the high solubility of poly(oxyethylene) arises from an increase of the trans-gauche-trans (tgt) conformer in water, the relationship between the increase of the tgt conformer and the solubility is unclear. In this study, we have investigated the conformational equilibria of 1,2-dimethoxyethane, which is a model molecule for poly(oxyethylene), by using Raman spectroscopy, and determined the change in the population and the free energy of each conformer with the aid of density functional theory calculations. The free energy of transfer of the tgt conformer from the pure liquid to the water phase is -6.1 ± 0.2 kJ mol(-1). Furthermore, the fraction of the tgt conformer increases from 0.37 to 0.78. Thus, the net contribution of the tgt conformer is -4.8 ± 0.2 kJ mol(-1), which is 79% of the total free energy of transfer (-6.07 kJ mol(-1)). This demonstrates that the high solubility of 1,2-dimethoxyethane originates from the lowest free energy and the highest fraction of the tgt conformer in water. We also successfully explain the thermodynamic mechanism of the low solubility of dimethoxymethane, which is the model molecule for poly(oxymethylene).