Control of Liquid–Liquid Phase Separation, Nucleation, and Growth of L-Menthol Single Crystals from Water–Ethanol Solvent Mixtures

L-Menthol is a commonly used material in food production for flavor enhancement and fragrances, as well as in pharmaceuticals. However, during the solution crystallization process of L-menthol, there is a major issue of oiling out or liquid–liquid phase separation (LLPS) in most organic solvents and their mixtures. The objective of the current work is to prevent the LLPS phenomenon and grow high-quality single crystals of L-menthol. A water–ethanol mixed solvent with nine different mixing ratios is used, ranging from 0.99W:0.01E to 0.91W:0.09E, through the conventional slow evaporation process. The findings indicate that the mixing composition of the solvent plays a significant role in the variation of solubility, refractive index, LLPS, nucleation, and crystallization. This is clearly demonstrated in the phase diagram. The occurrence of LLPS phenomenon, nucleation, growth behavior, and morphology of L-menthol single crystals through optical microscopy is identified. Additionally, the internal structure and thermal stability of the grown polymorphs using PXRD, SCXRD, and DSC analyses are confirmed.