COMPARATIVE-STUDY OF HYDROPHOBIC EFFECTS IN WATER-ALCOHOL AND WATER ETHYLENE-GLYCOL, WATER ETHANOLAMINE, WATER ETHYLENEDIAMINE, AND WATER 2-METHOXYETHANOL SYSTEMS

The magnitude of hydrophobic effects present in water/alcohol mixtures and in mixtures of water with various bifunctional cosolvents (ethylene glycol, 2-methoxyethanol, ethanolamine, and ethylenediamine) has been investigated through the Kirkwood-Buff (K-B) theory. The K-B integrals (G(ij)) reflect the overall affinity between species i and j (i.e., balance of repulsive and attractive contributions). In water/alcohol mixtures, G(ij) values exhibit complex composition dependence, with sharp extrema in G11, G12, and G22 over a narrow range of water-rich compositions; these features are further seen to be strongly dependent on the size of the hydrocarbon group of the cosolvent molecule. For mixtures of water with the bifunctional cosolvents, all systems exhibit analogous G(ij) variations, mostly a weak monotonous increase with cosolvent mole fraction. With due acknowledgment for the excluded volume (V(e)) contributions to G(ij), which vary in the different solvent mixtures examined here, it is readily apparent that in water/alcohol mixtures with 0 < x2 less than or similar to 0.3, the hydrophobic effects promote correlation among water molecules (G11) and among cosolvent molecules (G22). In water/bifunctional cosolvent mixtures, the various interactive components exhibit little correlation, and the hydrophobic effects appear greatly reduced. These observations point to important differences in the intermolecular processes occurring in aqueous mixtures of the mono- and bifunctional cosolvents studied here, in line with previous discussion on the thermodynamic properties of these mixtures.