Low-Density Water Structure Observed in a Nanosegregated Cryoprotectant Solution at Low Temperatures from 285 to 238 K

Abstract

Liquid water structure is defined by its molecular association through hydrogen bonding. Two different structures have been proposed for liquid water at low temperatures, low density liquid (LDL) and high density liquid (HDL) water. Here we demonstrate a platform which can be exploited to experimentally probe the structure of liquid water in equilibrium at temperatures down to 238 K. We make use of a cryoprotectant molecule, glycerol which when mixed with water lowers the freezing temperature of the solution non-monotonically with glycerol concentration. We use a combination of neutron diffraction and computational modeling to examine the structure of water in glycerolwater liquid mixtures at low temperatures from 285 K to 238 K. We confirm that the mixtures are nano-segregated into regions of glycerol-rich and water-rich clusters. We examine the water structure and reveal that at the temperatures studied here, water forms a low density water structure which is more tetrahedral than that at room temperature. We postulate that nano-segregation allows water to form a low density structure which is protected by an extensive and encapsulating glycerol interface.

Exploring the behaviour of water in glycerol solutions by using delayed luminescence

“…The time resolved DL decays were modelled as compressed hyperbolas [ 54 – 55 ] and analytically de-convolved in order to obtain rate constant distributions. The present results not only confirm the previous ones obtained by the authors on other aqueous solutions, but they are in agreement with computer simulations performed by other authors too [ 25 ].…”

“…It was found that glycerol and water form hydrogen bonded glycerol-rich and water-rich clusters, postulating that such nanosegregation allows water to form a low density structure that is protected by an extensive and encapsulating glycerol interface. Moreover in Literature [ 25 ] it is reported that the glycerol-rich and water-rich clusters are unchanged upon cooling. According to Towey and co-workers [ 25 ] the temperature instead affects the tetrahedrality of the water network that can be quantified by measuring the included angle formed by three water oxygen atoms.…”

“…Using a combination of neutron diffraction experiments and computational modelling in order to examine the hydrogen-bond networks in glycerol-water solutions at different concentration, it is reported in Literature that glycerol and water microsegregate to form hydrogen bonded clusters of water-rich and glycerol-rich regions, cluster sizes depending on the concentration [ 23 – 25 ]. Actually such segregation, that is the formation of nano-clusters of water surrounded by the matrix of solute molecules, was observed also with other cryoprotectants [ 26 , 27 ].…”

“…It was found that at concentrations close to the value at which the mixture exhibits a minimum in the freezing point depression, the local density of water oxygen atoms in the water-rich region is significantly lower than the bulk value. It has been postulated that nanosegregation allows water to form a low-density structure that is protected by an encapsulating glycerol interface, so preventing ice formation [ 25 ].…”

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

“…While the physical properties of glycerol and its interactions with water have been studied extensively, the H-bond patterns of glycerol-water mixtures are much less reported at a molecular level. Neutron scattering 79 and molecular dynamics simulations 80 suggest that glycerol and water form clusters depending on mixing ratios which is directed by intramolecular and intermolecular hydrogen bonding between the two systems. Very recently a hypothesis connecting the freezing of aqueous solutions and chemical stability of amorphous pharmaceuticals have invoked water clusters, 81 and the aforementioned neutron scattering and molecular dynamics calculations were used as evidence in this intriguing suggestion.…”