Local Water Structures Govern the Mixing Thermodynamics of Glycerol-Water Solutions

Glycerol is a major cryoprotective agent and is widely used to promote protein stabilization. Through a combined experimental and theoretical study, we show that global thermodynamic mixing properties of glycerol and water are dictated by local solvation motifs. We identify three water populations, i.e., bulk water, bound water H-bonded to hydrophilic groups of glycerol and wrap water hydrating hydrophobic moieties. Each population provides distinct spectroscopic fingerprints in the THz/FIR spectral range, which allow to quantify their respective abundance and their partial contributions to the mixing enthalpy. We uncover a 1:1 connection between the number of bound waters and the mixing enthalpy, as deduced from experiments as well as from simulations. The balance between local hydrophobic wrap and hydrophilic bound contributions at the molecular level dictates macroscopic thermodynamics of mixing. This offers opportunities to rationally design polyol water mixtures to optimize technological applications by tuning mixing enthalpy and entropy based on spectroscopic screening.