Volumetric and viscometric study of caffeine in aqueous D-sorbitol solutions

Abstract

Densities and viscosities of caffeine in various aqueous molal solution [m = (0.005–0.020) mol $\bullet$ kg⁻¹] of D-sorbitol were measured at T = (298.15–318.15) K and ambient pressure P = 101 kPa. A number of volumetric, viscometric and thermodynamic properties like apparent molar volumes, standard partial molar volumes, apparent specific volumes, standard isobaric partial molar expansibilities and their temperature dependence, viscosity B-coefficients, solvation number, etc., have been determined from the measured data. Further, the standard volume of transfer and viscosity B-coefficient of transfer for caffeine from water to aqueous D-sorbitol solutions were derived in order to have insights into the variety of interactions in the ternary solutions. In terms of solute–solvent and solute–solute interactions, the effects of molality, solute structure, temperature and taste behavior were examined. These findings showed that caffeine acts as a structure maker and that the solute–solvent interactions dominate in the ternary solutions. UV–visible and ¹H NMR spectroscopic studies were performed to substantiate these findings. Further, bond length changes due to interaction between D-sorbitol and caffeine, molecular electrostatic potential maps, Mülliken charge distribution, etc., were also studied theoretically and correlated to various interactions in the studied solutions.