Hydration and Molar Ratio Effects in Choline Chloride-Phenol Deep Eutectic Solvents

Deep eutectic solvents (DESs) based on choline chloride and phenol (CCPhe) have attracted increasing attention due to their tunable physicochemical properties and structural versatility. In this work, molecular dynamics (MD) simulations were performed for CCPhe systems at molar ratios of 1:2, 1:3, and 1:4 with water contents ranging from 0 to 30%. Energetic, hydrogen-bond, dielectric, transport, and structural analyses were combined to establish a multiscale description of hydration effects. Hydration promotes a progressive redistribution of stabilization from chloride-phenol and choline-chloride interactions toward chloride-water interactions, accompanied by increased water-mediated hydrogen bonding and reduced hydrogen-bond lifetimes. The dielectric constant increases significantly with water content, reaching 15.93 for the 1:4 system at 30% hydration, while the infinite-system Kirkwood factor reveals enhanced cooperative dipolar correlations at high hydration levels. Diffusion coefficients increase by nearly an order of magnitude between dry and highly hydrated systems, indicating substantial mobility enhancement. Radial distribution functions show that hydration modifies the first solvation shell of choline through competitive coordination between chloride and water without altering the characteristic contact distance. These results demonstrate that controlled hydration acts as an effective tuning parameter in CCPhe systems, modulating energetic balance, hydrogen-bond connectivity, collective polarization, and molecular transport in a composition-dependent manner.