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

Abstract

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.

Structure of Aqueous Solutions of Acetonitrile Investigated by Acoustic and Positron Annihilation Measurements

“…Below we present selected literature data showing how unclear it is. On the basis of comparative studies of many aqueous systems Matteoli and Lepori stated that the hydrophilic group of acetonitrile reduces its hydrophobicity almost as strong as the OH or NH 2 groups do [1]. Let us note that the series of hydrophilic character of different groups proposed by Matteoli and Lepori is in contradiction with findings of Wakisaka et al [2], while according to the former authors methanol and acetonitrile should be similar in hydrophobic character (methanol should be even weaker).…”

Micro-Heterogeneity in Complex Liquids

“…The middle panel shows the RKBI calculated from the various correlation functions shown in the left panel. It is obvious that, if the initial form should have been retained, it would lead to value of the KBI or the order of 2500, which correspond to the peak of the RBKI (as often used in various publications), and too large compared to the various experimental estimates from (Matteoli E. & Lepori L., 1984) shown in brown horizontal line and our own experimental value (Perera et al, 2006) shown in cyan line. The TS extension (blue curve) allows to bring the wrong initial tendency to the value very close to our experimental estimate.…”

“…In a landmark paper (Matteoli E. & Lepori L., 1984), Matteoli and Lepori have provided the behaviour of the experimental KBI for a variety of binary mixtures. The reproduction of this data by computer simulations and its interpretation is still a challenging open problem.…”

A re-appraisal of the concept of ideal mixtures through a computer simulation study of the methanol-ethanol mixtures

“…7, we report the KBI for both mixture models (symbols), together with the expected ideal KBI (lines). These ideal KBI are defined from the relations derived from the KB theory, 14,15,24 …”

“…In contrast, most aqueous mixtures are strongly nonideal. 15,24 However, some aqueous mixtures, such as aqueousdimethylsulfoxyde (DMSO) mixtures are surprisingly ideal from the KBI point of view. 25 Our interpretation of this ideality is through our finding that, in such mixtures, water forms linear clusters, and the interactions between such clusters and DMSO are near ideal.…”