Highly compressed water structure observed in a perchlorate aqueous solution

The discovery by the Phoenix Lander of calcium and magnesium perchlorates in Martian soil samples has fueled much speculation that flows of perchlorate brines might be the cause of the observed channeling and weathering in the surface. Here, we study the structure of a mimetic of Martian water, magnesium perchlorate aqueous solution at its eutectic composition, using neutron diffraction in combination with hydrogen isotope labeling and empirical potential structure refinement. We find that the tetrahedral structure of water is heavily perturbed, the effect being equivalent to pressurizing pure water to pressures of order 2 GPa or more. The Mg2+ and ClO4− ions appear charge-ordered, confining the water on length scales of order 9 Å, preventing ice formation at low temperature. This may explain the low evaporation rates and high deliquescence of these salt solutions, which are essential for stability within the low relative humidity environment of the Martian atmosphere

Temperature-Dependent Segregation in Alcohol-Water Binary Mixtures Is Driven by Water Clustering

Previous neutron scattering work, combined with computer simulated structure analysis, has established that binary mixtures of methanol and water partially segregate into water-rich and alcohol-rich components. It has furthermore been noted that, between methanol mole fractions of 0.27 and 0.54, both components, water and methanol, simultaneously form percolating clusters. This partial segregation is enhanced with decreasing temperature. The mole fraction of 0.27 also corresponds to the point of maximum excess entropy for ethanol–water mixtures. Here, we study the degree of molecular segregation in aqueous ethanol solutions at a mole fraction of 0.27 and compare it with that in methanol–water solutions at the same concentration. Structural information is extracted for these solutions using neutron diffraction coupled with empirical potential structure refinement. We show that ethanol, like methanol, bi-percolates at this concentration and that, in a similar manner to methanol, alcohol segregation, as measured by the proximity of neighboring methyl sidechains, is increased upon cooling the solution. Water clustering is found to be significantly enhanced in both alcohol solutions compared to the water clustering that occurs for random, hard sphere-like, mixing with no hydrogen bonds between molecules. Alcohol clustering via the hydrophobic groups is, on the other hand, only slightly sensitive to the water hydrogen bond network. These results support the idea that it is the water clustering that drives the partial segregation of the two components, and hence the observed excess entropy of mixing

Preference for Isolated Water Molecules in a Concentrated Glycerol-Water Mixture

Neutron diffraction coupled with hydrogen/deuterium isotopic substitution has been used to investigate the structure of a concentrated glycerol water (4:1 mole fraction) solution. The neutron diffraction data were used to constrain a three-dimensional computational model that is experimentally relevant using the empirical potential structure refinement technique. From interrogation of this model, we find that glycerol–glycerol hydrogen bonding is largely unperturbed by the presence of water in the solution. We find that glycerol–water hydrogen bonding is prevalent, suggesting that water molecules effectively take the place of glycerol molecules in this concentrated solution. In contrast, we find that water–water hydrogen bonding is significantly perturbed. While the first coordination shell of water in the concentrated solution remains similar to that of pure water, water–water hydrogen bonding is greatly diminished beyond the first neighbor distance. Interestingly, the majority of water molecules exist as single monomers in the concentrated glycerol solution. The preference of isolated water molecules results in a solution that is well mixed with optimal glycerol–water hydrogen bonding. These results highlight the importance of preferential hydrogen bonding in aqueous solutions and suggest a mechanism for cryoprotection by which glycerol effectively hydrogen bonds with water, resulting in a disrupted hydrogen-bonded water network

What happens to the structure of water in cryoprotectant solutions?

Cryoprotectant molecules are widely utilised in basic molecular research through to industrial and biomedical applications. The molecular mechanisms by which cryoprotectants stabilise and protect molecules and cells, along with suppressing the formation of ice, are incompletely understood. To gain greater insight into these mechanisms, we have completed an experimental determination of the structure of aqueous glycerol. Our investigation combines neutron diffraction experiments with isotopic substitution and computational modelling to determine the atomistic level structure of the glycerol-water mixtures, across the complete concentration range at room temperature. We examine the local structure of the system focusing on water structure. By comparing our data with that from other studies of cryoprotectant solutions, we attempt to find general rules for the action of cryoprotectants on water structure. We also discuss how these molecular scale interactions may be related to the macroscopic properties of the system