Exploring macro- and microscopic features of binary L-menthol/xylenol isomers mixtures: An integrated experimental and computational investigation

The complex nature of hydrogen bonding interactions in cyclohexyl-phenolic alcohols mixtures motivates investigations of their distinctive characteristics. To this end, we explored macroscopic bulk properties and microscopic features of four binary eutectic mixtures formed by mixing each of the four 2,n-dimethyl phenol (xylenol) isomers (n = 3, 4, 5, 6) and (1R,2S,5R)-2-Isopropyl-5-methylcyclohexanol (L-menthol). The density and viscosity of the mixtures were measured over a wide temperature and composition range. It was found that 2,n-xylenol isomers are denser than L-menthol, and the density of the mixtures decreased with increasing L-menthol mole fraction. L-menthol/2,3-xylenol showed the highest density among the studied mixtures. The larger density of L-menthol/2,3-xylenol was attributed to the negative excess molar volume. On the other hand, 2,n-xylenol isomers are less viscous than L-menthol, and the viscosity of the mixture increased with increasing L-menthol mole fraction. The viscosity of L-menthol/2,3-xylenol, L-menthol/2,4-xylenol, and L-menthol/2,5-xylenol was very similar, while L-menthol/2,6-xylenol showed significantly lower viscosity. This experimental work was complemented with molecular dynamics (MD) simulations on all mixtures with an L-menthol mole fraction of 0.6. The atomistic insight into intermolecular structural correlations provided by MD simulations allows for extracting useful information on the nature of close-neighbor interactions. Analysis of MD simulations allows for highlighting the role of hydrogen bonding interactions and dispersive ones (including aromatic effects) on driving local organization and suggesting potential structural explanations for the experimental observations.