Effect of ethylene glycol on the molecular organization of H2O in comparison with methanol and glycerol: A calorimetric study

Excess partial molar enthalpies of ethylene glycol, H-EG(E), in binary ethylene glycol-H2O, and those of 1-propanol, H-IP(E), in ternary 1-propanol-ethylene glycol (or methanol)-H2O were determined at 25degreesC. From these data, the solute-solute interaction functions, H-EG-EG(E)=N(partial derivativeH(EG)(E)/partial derivativen(EG)) and H-1P-1P(E)=N(partial derivativeH(1P)(E)/partial derivativen(1P)), were calculated by graphical differentiation without resorting to curve fitting. Using these, together with the partial molar volume data, the effect of ethylene glycol on the molecular organization of H2O was investigated in comparison with methanol and glycerol. We found that there are three concentration regions, in each of which the mixing scheme is qualitatively different from the other regions. Mixing scheme III operative in the solute-rich region is such that the solute molecules are in a similar situation as in the pure state, most likely in clusters of its own kind. Mixing scheme II, in the intermediate region, consists of two kinds of clusters each rich in solute and in H2O, respectively. Thus, the bond percolation nature of the hydrogen bond network of liquid H2O is lost. Mixing scheme I is a progressive modification of liquid H2O by the solute, but the basic characteristics of liquid H2O are still retained. In particular, the bond percolation of the hydrogen bond network is still intact. Similar to glycerol, ethylene glycol participates in the hydrogen bond network of H2O via-OH groups, and reduces the global average of the hydrogen bond probability and the fluctuations inherent in liquid H2O. In contrast to glycerol, there is also a sign of a weak hydrophobic effect caused by ethylene glycol. However, how these hydrophobic and hydrophilic effects of ethylene glycol work together in modifying the molecular organization of H2O in mixing scheme I is yet to be elucidated.