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.

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.Significant amounts of different perchlorate salts have been discovered on the surface of Mars. Here, the authors show that magnesium perchlorate has a major impact on water structure in solution, providing insight into how an aqueous fluid might exist under the sub-freezing conditions present on Mars.

Multicomposition EPSR: Toward Transferable Potentials To Model Chalcogenide Glass Structures.

The structure of xAs40Se60-(1 - x)As40S60 glasses, where x = 1.000, 0.667, 0.500, 0.333, 0.250, and 0.000, is investigated using a combination of neutron and X-ray diffraction coupled with computational modeling using multicomposition empirical potential structure refinement (MC-EPSR). Traditional EPSR (T-EPSR) produces a set of empirical potentials that drive a structural model of a particular composition to agreement with diffraction experiments. The work presented here establishes the shortcomings in generating such a model for a ternary chalcogenide glass composition. In an enhancement to T-EPSR, MC-EPSR produces a set of pair potentials that generate robust structural models across a range of glass compositions. The structures obtained vary with composition in a much more systematic way than those taken from T-EPSR. For example, the average arsenic-sulfur bonding distances vary between 2.28 and 2.46 Å in T-EPSR but are 2.29 ± 0.02 Å in MC-EPSR. Similarly, the arsenic-selenium bond lengths from T-EPSR vary between 2.28 and 2.43 Å but are consistently 2.40 ± 0.02 Å in the MC-EPSR results. Analysis of these models suggests that the average separation of the chalcogen (S or Se) atoms is the structural origin of the changes in nonlinear refractive index with glass composition.

Correlating structure with non-linear optical properties in xAs40Se60·(1-x)As40S60 glasses.

A series of xAs40Se60·(100 - x)As40S60 glasses, where x = 0, 25, 33, 50, 67, 75 and 100 mol% As40Se60, has been studied using neutron and X-ray total scattering, Raman spectroscopy and (77)Se MAS-NMR. The results are presented with measurements of non-linear refractive indices, n2, and densities. There is no evidence for the formation of homopolar bonds in these glasses, but neutron correlation functions suggest that there is a non-random distribution of sulfur and selenium atoms in sulfur-rich glasses. The average number of sulfur atoms at a distance of 3-4 Å from a selenium atom, nSeS, deviates from a linear variation with x in glasses containing <50 mol% As40Se60; n2 for these glasses also varies non-linearly with x. Importantly, a direct comparison of n2 and nSeS gives a linear correlation, suggesting that n2 may be related to the distribution of chalcogen atoms in the glasses.

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.