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 (ethyle ne glycol, 2-methoxyethanol, ethanolamine, and ethylenediamine) has be en investigated through the Kirkwood-Buff (K-B) theory. The K-B integr als (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 s harp extrema in G11, G12, and G22 over a narrow range of water-rich co mpositions; these features are further seen to be strongly dependent o n the size of the hydrocarbon group of the cosolvent molecule. For mix tures of water with the bifunctional cosolvents, all systems exhibit a nalogous G(ij) variations, mostly a weak monotonous increase with coso lvent mole fraction. With due acknowledgment for the excluded volume ( V(e)) contributions to G(ij), which vary in the different solvent mixt ures examined here, it is readily apparent that in water/alcohol mixtu res with 0 < x2 less than or similar to 0.3, the hydrophobic effects p romote correlation among water molecules (G11) and among cosolvent mol ecules (G22). In water/bifunctional cosolvent mixtures, the various in teractive components exhibit little correlation, and the hydrophobic e ffects appear greatly reduced. These observations point to important d ifferences in the intermolecular processes occurring in aqueous mixtur es of the mono- and bifunctional cosolvents studied here, in line with previous discussion on the thermodynamic properties of these mixtures .