Insights on the mixing enthalpy of binary solvents: beyond thermodynamics and electrostatic interactions

A quantitative analysis of the mixing enthalpies for binary systems made of cyclohexane and eight other organic solvents (viz., acetone, chloroform, dichloromethane, ethyl acetate, m-xylene, o-xylene, toluene and tetrahydrofuran) at room temperature allows the estimation of some energy terms related to possible intermolecular interactions occurring in these systems. Combined with theoretical models, these energy terms enabled the derivation of the activity coefficients of cyclohexane in the infinitely diluted solution and subsequently the size of the reference solvent, i.e. cyclohexane, within a reasonable deviation from the literature values. Meanwhile, extensive quantum mechanical calculations were performed for three binary systems from above to obtain corresponding energy terms, as well as the optimised molecular geometries for potential homo- and heterodimers and trimers leaving the potential for further agglomeration among clusters. The vibrational spectra of each solvent and the binary systems were collected using a FT-IR spectrometer and compared with the quantum mechanical calculation results. The centre of the IR absorbance bands corresponding to some characteristic vibrations, such as C-Cl stretching of chloroform and dichloromethane, C = O stretching of acetone and ethyl acetate and ring stretching of THF, for the binary systems were observed to shift to higher wavenumbers relative those for the pure co-solvents. Such a vibrational frequency shift on one hand indicates the significant population of the molecular clusters and their agglomerates in pure or mixed solvents, and more complicated quantal natured intermolecular interactions in these systems on the other hand.