Aggregation in Binary Solutions Containing Hexafluorobenzene

Theoretical study of intermolecular interactions in crystalline arene–perhaloarene adducts in terms of the electron density

The intermolecular interactions in C₆X₆–arene crystals (X = F, Cl) and the halogen substitution effect can be quantified by the electron density.

Local Composition in the Vicinity of a Protein Molecule in an Aqueous Mixed Solvent

This paper is focused on the composition of a cosolvent in the vicinity of a protein surface (local composition) and its dependence on various factors. First, the Kirkwood-Buff theory of solution is used to obtain analytical expressions that connect the excess or deficit number of cosolvent and water molecules in the vicinity of a protein surface with experimentally measurable quantities such as the bulk concentration of the mixed solvent, the preferential binding parameter, and the molar volumes of water and cosolvent. Using these expressions, relations between the preferential binding parameter (at a molal concentration scale) and the above excesses (or deficits) are established. In addition, the obtained expressions are used to examine the effect of the nonideality of the water + cosolvent mixtures and of the molar volume of the cosolvent on the excess (or deficit) number of cosolvent molecules in the vicinity of the protein surface. It is shown that at least for the mixed solvents considered (water + urea and water + glucose) the nonideality of the mixed solvent is not an important factor in the local compositions around a protein molecule and that the main contribution is provided by the nonidealities of the protein-water and protein-cosolvent mixtures. Special attention is paid to urea as cosolvent, because urea is one of only a few compounds with a concentration at the protein surface larger than its concentration in the bulk. The composition dependence of the excess of urea around a protein molecule is calculated for the water + lysozyme + urea mixture at pH = 7.0 and 2.0. At pH = 7.0, the excess of urea becomes almost composition independent at high urea concentrations. Such independence could be explained by assuming that urea totally replaces water in some areas of the protein surface, whereas on the remaining areas of the protein surface both water and urea are present with concentration comparable to those in the bulk. The Schellman exchange model was used to relate the preferential binding parameter in water + lysozyme + urea mixtures to the urea concentration.

Structures of Alkyl Benzoate Binary Mixtures. A Kirkwood−Buff Fluctuation Theory Study Using UNIFAC

The structure of the alkyl benzoate + n-alkane, and + alkan-1-ol binary mixtures were analyzed according to the Kirkwood-Buff fluctuation theory on the basis of both the mixture properties measured over a wide temperature range and the activity coefficients calculated with the modified UNIFAC (Dortmund) model as well. Application of this model reveals that both the microheterogeneous structure and the clustering effects are strongly dependent on the chain length of the n-alkane and alkan-1-ol cosolvents. Knowledge of the local composition around each type of molecule is drawn from the Kirkwood-Buff integrals and the excess (or deficit) molecules aggregated around a central one. The rather high values of the integrals evaluated for some of these systems provide first-hand evidence for phase splitting. The conclusions drawn support previous analyses and confirm the adequacy of the methodology put forward for studying liquid mixtures at microscopic level; easily measurable experimental properties can advantageously be used with the fluctuation theory.

Selective Solvent Interactions in a Fluorous Reaction System

Mixtures of chloroform and perfluoro(methylcyclohexane) can be used as solvents for "fluorous" biphase reactions since they exist as two separate phases at low temperature but become a single phase at higher temperatures. Intermolecular nuclear Overhauser effects have been used to investigate the interactions of solvent components with the protons and fluorines of 3-heptafluorobutyrylcamphor in both phases of this biphasic system at 25 degrees C as well as the single phase at 54 degrees C. The results indicate that at 25 degrees C in the perfluorocarbon-rich phase, both solvent components interact with the solute selectively. There are no indications of unusual solute interactions of either solvent component in the chloroform-rich phase and only weak suggestions of selective interactions in the high-temperature phase. Various mechanisms for the enhancement of solute spin-solvent spin cross relaxation rates in the perfluoro(methylcyclohexane)-rich phase are considered. It is suggested that the solvation layer around the solute has a composition and possibly hydrodynamic properties different from those of the bulk solution in this phase. There are indications of appreciable regioselectivity of chloroform interactions with the hydrocarbon part of the solute in all phases.