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.
