Solute–solute interactions in water. II. An analysis through the Kirkwood–Buff integrals for 14 organic solutes

From accurate data of activities (a), partial molar volumes (V̄), and compressibility (K) of binary aqueous mixtures, the so-called Kirkwood–Buff integrals Gij defined by Gij=∫∞0 [ gij(r) −1] 4πr2 dr=f(a,V̄,K), (i=1,2; j=1,2), have been calculated in the whole concentration range. Fourteen water(1)-organic cosolvent(2) systems [methanol, ethanol, 1-propanol, 1-butanol, 2-methyl-2-propanol, acetonitrile, acetone, dimethylsulfoxide, tetrahydrofuran, piperidine, pyridine, 1,4-dioxane, 2-aminoethanol, 2-(dimethylamino)ethanol] have been studied at 25 °C, and two (methanol and ethanol) also at different temperatures. The Gij functions show these features in relation to the molecular structures of component 2: (1) when this component presents a large nonpolar moiety, extrema are exhibited by Gij’s at certain concentrations the more marked the larger the nonpolar portion; (2) when component two is bifunctional, Gij trend is monotonic with concentration; (3) in the temperature range 0–90 °C, G22 increases and G12 decreases with increasing T in the water-rich concentration region. By interpreting the Gij quantities as a measure of the net attraction (or repulsion) among i and j molecules and as an indication of the tendency of molecules j to cluster around a molecule i, the present results also suggested that: (a) the hydrophobic interaction (attraction) more and more increases with increasing the size of the nonpolar moiety of component two, this increase being most marked at mole fraction x2≂0.2–0.3; (b) hydrophilic groups remarkably reduce the attraction among hydrocarbon-like chains according to the sequence: OH≊NH2>NH≊O≊SO>CO >CN≊Nar; (c) the hydrophobic interaction increases with increasing temperature, in contrast to the common belief that such a phenomenon is induced by the peculiar structural features of water, and that these features fade out as temperature increases.