The present paper is devoted to the local composition of liquid mixtures calculated in the framework of the Kirkwood-Buff theory of solutions. A new method is suggested to calculate the excess (or deficit) number of various molecules around a selected (central) molecule in binary and multicomponent liquid mixtures in terms of measurable macroscopic thermodynamic quantities, such as the derivatives of the chemical potentials with respect to concentrations, the isothermal compressibility, and the partial molar volumes. This method accounts for an inaccessible volume due to the presence of a central molecule and is applied to binary and ternary mixtures. For the ideal binary mixture it is shown that because of the difference in the volumes of the pure components there is an excess (or deficit) number of different molecules around a central molecule. The excess (or deficit) becomes zero when the components of the ideal binary mixture have the same volume. The new method is also applied to methanol + water and 2-propanol + water mixtures. In the case of the 2-propanol + water mixture, the new method, in contrast to the other ones, indicates that clusters dominated by 2-propanol disappear at high alcohol mole fractions, in agreement with experimental observations. Finally, it is shown that the application of the new procedure to the ternary mixture water/protein/cosolvent at infinite dilution of the protein led to almost the same results as the methods involving a reference state.

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Thermodynamic data were used to calculate the Kirkwood−Buff integrals for the aqueous solutions of methanol, ethanol, propanols, and butanols. The calculated values have been compared to those obtained from small-angle X-ray scattering (SAXS) experiments, and satisfactory agreement was found. Improved expressions have been suggested and used to calculate the excess number of molecules around central ones. On this basis, information about the structure of the solutions was gathered, the main conclusion being that the interactions among similar molecules prevail; this leads to clustering dominated by the same kind of molecules.

), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Library of Congress Control Number: 2009929313 PrefaceThis book consists of a number of papers regarding the thermodynamics and structure of multicomponent systems that we have published during the last decade. Even though they involve different topics and different systems, they have something in common which can be considered as the "signature" of the present book. First, these papers are concerned with "difficult" or very nonideal systems, i.e. systems with very strong interactions (e.g., hydrogen bonding) between components or systems with large differences in the partial molar volumes of the components (e.g., the aqueous solutions of proteins), or systems that are far from "normal" conditions (e.g., critical or near-critical mixtures). Second, the conventional thermodynamic methods are not sufficient for the accurate treatment of these mixtures. Last but not least, these systems are of interest for the pharmaceutical, biomedical, and related industries.In order to meet the thermodynamic challenges involved in these complex mixtures, we employed a variety of traditional methods but also new methods, such as the fluctuation theory of Kirkwood and Buff and ab initio quantum mechanical techniques.The Kirkwood-Buff (KB) theory is a rigorous formalism which is free of any of the approximations usually used in the thermodynamic treatment of multicomponent systems. This theory appears to be very fruitful when applied to the above mentioned "difficult" systems. Indeed, in some cases (see Chapters 3-5) this theory allows one to obtain results that are scarcely attainable by traditional thermodynamic methods. For example, in mixtures of three or more components, thermodynamics can not provide any rigorous relations connecting the thermodynamic properties, such as the activity coefficients of components in multicomponent mixtures to those in binary mixtures of the constituents, whereas the KB theory of solutions provides such relations. It allows one, for example, to develop a method for predicting or correlating the solubility of drugs in multicomponent solvents. Let us emphasize again that the above results are possible because the KB theory provides a rigorous method for the treatment of multicomponent (not only binary!) mixtures.We have employed ab initio quantum mechanical techniques to obtain information about the intermolecular interactions and the geometry of large molecular clusters. Such information was ver...

In the present paper a procedure to calculate the properties of proteins in aqueous mixed solvents, particularly the excesses of the constituents of the mixed solvent near the protein molecule and the preferential binding parameters, is suggested. Expressions for the Kirkwood-Buff integrals in ternary mixtures and for the preferential binding parameter were derived and used to calculate various properties of infinitely dilute proteins in aqueous mixed solvents. The derived expressions and experimental information regarding the partial molar volumes and the preferential binding parameters were used to calculate the excesses (deficits) of water and cosolvent (in comparison with the bulk concentrations of protein-free mixed solvent) in the vicinity of ribonuclease A, ribonuclease T1, and lysozyme molecules. The calculations showed that water was in excess in the vicinity of ribonuclease A for water/glycerol and water/trehalose mixtures, and the cosolvent urea was in excess in the vicinity of ribonuclease T1 and lysozyme. The derivative of the activity coefficient of the protein with respect to the mole fraction of water was also calculated. This derivative was negative for the water/glycerol and water/trehalose mixed solvents and positive for the water/urea mixture. The mixture of lysozyme in the water/urea solvent is of particular interest, because the lysozyme at pH 7.0 is in its native state up to 9.3M urea, while at pH 2.0 it is denaturated between 2.5 and 5M and higher concentrations of urea. Our results demonstrated a striking similarity in the hydration of lysozyme at both pHs. It is worthwhile to note that the excesses of urea were only weakly composition dependent on both cases.