Preferential Solvation in Ternary Solutions Containing Methylbenzoate. A Kirkwood-Buff Fluctuation Theory Study Begon ˜ a Garcı ´a, Santiago Aparicio, Rafael Alcalde, and Jose ´ M. Leal* Departamento de Quı ´mica, UniVersidad de Burgos, 09001 Burgos, Spain ReceiVed: January 30, 2003; In Final Form: July 1, 2003 Density, dynamic viscosity, and refractive index of the hexane/cyclohexane/methylbenzoate ternary solvent and its binary constituents were measured at 298.15 K over the whole composition range. From the experimental measurements, excess and mixing properties were evaluated and correlated with composition using different models. The structure and interactions in the ternary solvent were analyzed starting from the experimental properties by application of the Kirkwood-Buff fluctuation theory of solutions along with the UNIFAC group contribution method for predicting activity coefficients. The local composition and excess (or deficit) number of molecules aggregated around a central one were determined using the Kirkwood-Buff integrals. The properties of the ternary system were used to test the predictive ability of several models. The volumetric properties of the binary and ternary systems were correlated and predicted with the cubic equations of state by Soave and Peng-Robinson combined with two simple mixing rules. Introduction Grouping of solvents into classes often is based on the nature of the intermolecular forces because the manner whereby solute and solvent molecules are associated with one another brings about a marked effect on the resulting properties. After the introduction of the concept of ionization power of solvent, 1 much work has been devoted to solvent effects on rate and equilibrium processes. 2 Because of the close connection between liquid structure and macroscopic properties, determination of volu- metric, electrical, and rheological properties is a valuable tool to learn the liquid state. 3 On the other hand, the obtaining of reliable measurements of solvent properties over a wide range of composition, pressure, and temperature often is not feasible; hence, prediction and correlation methods constitute a valuable option to overcome such difficulties. 4,5 This work contributes to the study of the structure and interactions of ester-containing ternary mixtures. These solvents are widely used in polymer science and syntheses operations; however, the scarce literature existing makes convenient a systematic research on their properties. 6 Density, dynamic viscosity, and refractive index data of the methylbenzoate/ cyclohexane/n-hexane ternary solvent and its constituent n- hexane/cyclohexane, n-hexane/methylbenzoate, and cyclohexane/ methylbenzoate binary components are reported over the full composition range. 7,8 From these measurements, the excess and mixing properties were evaluated and interpreted on the basis of interactions and geometry factors. The volumetric properties of the binary systems were correlated using the cubic equations of state by Soave 9 (SRK) and Peng-Robinson 10 (PR) combined with two simple mixing rules. On the basis of the properties of the binary components, the same properties of the ternary solvent were approached using several models. The statistical thermo- dynamic theory by Kirkwood-Buff (KB) 11 provides a proper link between macroscopic and microscopic properties and was used to interpret intermolecular interactions and preferential solvation effects. 12 This approach describes properly deviations from ideality, provides a reliable description of the solvation effect, and can be extended to different types of particles, not only spherical ones. 13 Also it enables evaluation of the local composition of the components and has become popular for binary and multicomponent mixtures. 14 The activity coefficients were evaluated using the group contribution UNIFAC III model. 15 The excess molar volumes were applied by deriving expressions for the Kirkwood-Buff integrals and the corre- sponding excess number of molecules around a central one. 16 Experimental Section Reagents. The reactants, methylbenzoate (MB), hexane (hex), and cyclohexane (cyc), of the highest purity commercially available were used without further purification. The purity was assessed by gas chromatography (GC) with a Perkin-Elmer 990 gas chromatograph, equipped with a Hewlett-Packard 3390A integrator and also by comparing the densities, viscosities, and refractive indices with literature values (Table S1, Supporting Information, and refs 17-31 contained therein). The liquids were degassed with ultrasound for several days before use and kept out of the light over Fluka Union Carbide 0.4 nm molecular sieves. To prevent the samples from preferential evaporation, the mixtures were prepared by syringing amounts, weighed to Δm ) 10 -5 g with a Mettler AT 261 Delta Range balance, into suitably stoppered bottles. The mixtures were completely miscible over the whole composition range. Instruments and Procedures. The molar excess volumes ((10 -4 cm 3 mol -1 ) were deduced from the densities of the pure liquids and mixtures. The densities, F, were measured with a computer-controlled DMA 58 Anton Paar digital density meter ((5 × 10 -6 g cm -3 ) by introducing the samples into a U-shaped oscillating tube the natural frequency of which is influenced by the mass of the sample. The proper calibration was achieved at all working temperatures ((0.01 K) with deionized doubly distilled water (Milli-Q, Millipore) and n-nonane (Fluka, 99.2% purity by GC) as reference liquids; the apparatus takes care of * Corresponding author. E-mail: jmleal@ubu.es. Tel: +34 947 258 819. Fax: +34 947 258 831. 13478 J. Phys. Chem. B 2003, 107, 13478-13486 10.1021/jp034255l CCC: $25.00 © 2003 American Chemical Society Published on Web 11/08/2003