A structural approach to reveal the cryoprotective action of glycerol

Glycerol-water liquid mixtures are intriguing hydrogen-bonded systems that are essential to many fields; from basic molecular research to their wide spread use in industrial and biomedical applications as cryoprotective solutions. This thesis details a study of the structure and bonding of this important system as a function of concentration and temperature. Here, aqueous glycerol has been investigated using a combination of neutron diffraction and computational modelling. When studying pure liquid glycerol no evidence for intra-molecular hydrogen bonding is found. It is shown that, contrary to previous theories, waterglycerol hydrogen bonds compensate for the loss of water-water hydrogen bonds with increasing glycerol concentration. The molecular scale clustering of the system is also investigated. It is found that there is a preference for isolated water molecules in a concentrated glycerol-water mixture and for monomeric glycerol molecules in dilute aqueous glycerol. At intermediate concentrations, the system forms a structure with percolating clusters of both glycerol and water. Interestingly, this bi-percolating structure is found over the concentration range at which many extremes of thermodynamic functions are found. On cooling, the water network adopts a more tetrahedral structure that is more ice-like. However, the molecular scale clustering persists as the system is cooled. It is, therefore, proposed that it is the mixing characteristics that allow glycerol-water systems to form a structure that prevents extended water network formation. It is likely that it is this structuring that retards ice formation as the temperature is lowered.