Solute–solvent and solvent–solvent interactions of menthol in isopropyl alcohol and its binary mixtures with methyl salicylate by volumetric, viscometric, interferometric and refractive index techniques
Introduction
The volumetric, viscometric and interferrometric behavior of solutes has been proved to be very useful in elucidating the various interactions occurring in pure and mixed solvents. Studies on the effect of concentration and temperature on the apparent molar volumes of solutes have been extensively used to obtain information on ion–ion, ion–solvent, and solvent–solvent interactions [1], [2], [3]. It has been found by a number of workers [4], [5], [6] that the addition of a solute could either make or break the structure of a liquid.
In this paper we have attempted to study the behavior of menthol in isopropyl alcohol (I.P.A.) and in its mixture with methyl salicylate (5, 10 and 15 mass%) at various temperatures because of their extensive use in pharmaceutical and cosmetic industries. Methyl salicylate has a long history of use in consumer products as a counterirritant and as an analgesic in the treatment and temporary management of aching and painful muscles and joints. Methyl salicylate is also used as an UV absorber and in perfumery as a modifier of blossom fragrances [7]. I.P.A. is widely used as a cleaning agent, a cost-effective preservative for biological specimens and is a major ingredient in “dry-gas” fuel additive. Menthol, an old remedy in Chinese medicine extracted from plants of the genus Mentha, is widely used as both a cooling agent and a counterirritant for relieving pain especially in the muscles, viscera or remote areas [8], as well as for the treatment of pruritus.
Section snippets
Chemicals
Menthol (Thomas Baker, >99%) was used as such without further purification. Isopropyl alcohol (Merck, >99.5%) and methyl salicylate (Sigma–Aldrich, >99%) were used with no further purification other than being dried with molecular sieves. Experimental values of viscosity (η), density (ρ), sound speed (u) and refractive indices (nD) of the pure solvents were compared with the literature values and are listed in Table 1.
Measurements
Densities (ρ) were measured with an Ostwald–Sprengel type pycnometer having a
Results and discussions
Apparent molar volumes (V∅) were determined from the solution densities using the following equation:where M is the molar mass of the solute, c is the molarity of the solution; ρ0 and ρ are the densities of the solvent and the solution respectively. The limiting apparent molar volumes was calculated using a least-square treatment to the plots of V∅ versus √c using the Masson equation [13].where is the partial molar volume at infinite dilution and
Conclusion
The values of apparent molar volume , viscosity B-coefficients and isoentropic compressibility indicate the presence of strong solute–solvent interactions and these interactions are strengthened at higher temperature and weakened with increasing mass percent of methyl salicylate in the binary solution.
Acknowledgement
The authors are grateful to the Departmental Special Assistance Scheme, Department of Chemistry, NBU under the University Grants Commission, New Delhi (No. 540/27/DRS/2007, SAP-1) for financial support in order to continue this research work.
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