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Alan K Soper

Publications and source records attributed to Alan K Soper.

12 recordsLinked to original sources
PubMed2006

Structural studies on the hydration of L-glutamic acid in solution.

A combination of neutron diffraction augmented with isotopic substitution and computer modeling using empirical potential structure refinement has been used to extract detailed structural information for L-glutamic acid dissolved in 2 M NaOH solution. This work shows that the tetrahedral hydrogen bonding network in water is severely disrupted by the addition of glutamic acid and NaOH, with the number of water-water hydrogen bonds being reduced from 1.8 bonds per water molecule in pure water to 1.4 bonds per water molecule in the present solution. In the glutamic acid molecule, each carboxylate oxygen atom forms an average of three hydrogen bonds with the surrounding water solvent with one of these hydrogens being shared between the two oxygen atoms on each carboxylate group, while each amine hydrogen forms a single hydrogen bond with the surrounding water solvent. Additionally, the average conformation of the glutamic acid molecules in these solutions is extracted.

Glutamic Acid↗
PubMed2006

Structural characteristics of a 0.23 mole fraction aqueous solution of tetrahydrofuran at 20 degrees C.

Hydrogen/deuterium isotopic substitution neutron diffraction techniques were used to measure the structural correlation functions in a 0.23 mole fraction solution of tetrahydrofuran in water at room temperature. Empirical potential structure refinement (EPSR) was used to build a three-dimensional model of the liquid structure that is consistent with the experimental data. Detailed analysis shows a preference for nonpolar interactions between the cyclic ether molecules plus polar interactions between the ether and solvent water and hydrophobic hydration of the nonpolar regions of the solute. The increase in the number of hydrogen-bond-acceptor sites relative to the number of hydrogen-bond-donor sites in this system, compared to the balanced situation that would be found in pure water, has a marked compressive effect on the structure of the solvent. Despite the small size of the solvent water molecules, the 0.23 mole fraction aqueous solution is still found to contain small voids akin to those in pure liquid tetrahydrofuran. In contrast to the positive surface charge of the voids in the pure system, the average void in this aqueous solution is found to have a net negative charge. This is due to contributions from the water oxygen atoms that are negatively polarized by their intramolecular bonding.

Journal Article↗
PubMed2006

Liquid structure of the ionic liquid 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]amide.

Neutron diffraction has been used to determine the liquid structure of 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]amide ([dmim][NTf2]). Significantly smaller charge ordering is found in this liquid compared with analogous chloride and hexafluorophosphate salts due to the diffuse charge density and size of the [NTf2]- anion. This is manifested in a much larger cation-cation and cation-anion separation and an overlap of the cation-cation and cation-anion shells. Comparison of the liquid structure with the crystal structure reported by Holbrey et al. (Dalton Trans. 2004, 2267) indicates little correlation, for example, the [NTf2]- anion adopts a trans orientation predominantly in the liquid whereas a cis orientation is found in the solid phase.

Crystallography, X-Ray↗
PubMed2006

Ion solvation and water structure in potassium halide aqueous solutions.

The structure of water and the nature of ionic hydration is explored in aqueous solutions of potassium fluoride, chloride, bromide and iodide over a range of concentrations up to 4.8 ion pairs per 100 water molecules, using the combined techniques of neutron diffraction with hydrogen isotope substitution. The diffraction data are interpreted using the method of empirical potential structure refinement, which attempts to build a three-dimensional model of the scattering system consistent with the diffraction data. The water structure is strongly perturbed in the first hydration shells of both anion and cation, but is found to be only mildly perturbed outside of this region, with the largest effects occurring with the smallest anion and highest concentrations. For the potassium ion there are strong orientational correlations in the first hydration shell, with the water molecules lying with their dipole moments pointing almost directly away from the cation on average, but with an angular spread of approximately +/-60 degrees which is mildly dependent on the anion type present. For all the anions the water molecules in the first shell are strongly oriented with one O-H vector pointing directly towards the anion on average, with an angular spread of approximately +/-10 degrees for F(-), increasing to approximately +/-22 degrees for I(-). For both anions and cations the second hydration shell is much more disordered than the first, but there is a weak pattern of orientational correlation which becomes more pronounced with the larger anions. There is some evidence that the fluoride ion structures water significantly in its first hydration shell, but not beyond. The findings throw further light on recent findings that the orientational relaxation time for water outside the first shell of dissolved ions is the same as in the bulk liquid.

Halogens↗
PubMed2006

Methane hydrate formation and decomposition: structural studies via neutron diffraction and empirical potential structure refinement.

Neutron diffraction studies with hydrogen/deuterium isotope substitution measurements are performed to investigate the water structure at the early, medium, and late periods of methane clathrate hydrate formation and decomposition. These measurements are coupled with simultaneous gas consumption measurements to track the formation of methane hydrate from a gas/water mixture, and then the complete decomposition of hydrate. Empirical potential structure refinement computer simulations are used to analyze the neutron diffraction data and extract from the data the water structure in the bulk methane hydrate solution. The results highlight the significant changes in the water structure of the remaining liquid at various stages of hydrate formation and decomposition, and give further insight into the way in which hydrates form. The results also have important implications on the memory effect, suggesting that the water structure in the presence of hydrate crystallites is significantly different at equivalent stages of forming compared to decomposing. These results are in sharp contrast to the previously reported cases when all remaining hydrate crystallites are absent from the solution. For these systems there is no detectable change in the water structure or the methane hydration shell before hydrate formation and after decomposition. Based on the new results presented in this paper, it is clear that the local water structure is affected by the presence of hydrate crystallites, which may in turn be responsible for the "history" or "memory" effect where the production of hydrate from a solution of formed and then subsequently melted hydrate is reportedly much quicker than producing hydrate from a fresh water/gas mixture.

Journal Article↗
PubMed2006

The structure of liquid tetrahydrofuran.

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the structural correlation functions of liquid tetrahydrofuran at room temperature. Empirical potential structure refinement (EPSR) has been used to build a three-dimensional model of the liquid structure that is consistent with the experimental data. Analysis to the level of the orientational correlation functions shows that the liquid displays a preference for T-like configurations between the tetrahydrofuran molecules, a local structure that results in void-like regions of approximately 1.25 angstroms radius within the bulk liquid. The surface chemistry of these voids suggests a slightly positive electrostatic character. These findings are consistent with the known propensity of the liquid to solvate free electrons.

Journal Article↗
PubMed2006

Excess entropy in alcohol-water solutions: a simple clustering explanation.

We show that the anomalous negative excess entropy of mixing characteristic of aqueous lower alcohols containing hydrophobic groups is quantitatively consistent with a model exploiting only the experimentally observed molecular-scale segregation of the components across the entire concentration range. The simple model presented here, which uses plausible interatomic distances as its only free parameters, obviates the need to invoke "iceberg" or other water restructuring concepts which, though frequently postulated in explaining the hydrophobic interaction, are unsupported by recent experiments.

Journal Article↗
PubMed2006

Orientational correlations in liquid acetone and dimethyl sulfoxide: a comparative study.

The structure of acetone and dimethyl sulfoxide in the liquid state is investigated using a combination of neutron diffraction measurements and empirical potential structure refinement (EPSR) modeling. By extracting the orientational correlations from the EPSR model, the alignment of dipoles in both fluids is identified. At short distances the dipoles or neighboring molecules are found to be in antiparallel configurations, but further out the molecules tend to be aligned predominately as head to tail in the manner of dipolar ordering. The distribution of these orientations in space around a central molecule is strongly influenced by the underlying symmetry of the central molecule. In both liquids there is evidence for weak methyl hydrogen to oxygen intermolecular contacts, though these probably do not constitute hydrogen bonds as such.

Journal Article↗
PubMed2005

Search for memory effects in methane hydrate: structure of water before hydrate formation and after hydrate decomposition.

Neutron diffraction with HD isotope substitution has been used to study the formation and decomposition of the methane clathrate hydrate. Using this atomistic technique coupled with simultaneous gas consumption measurements, we have successfully tracked the formation of the sI methane hydrate from a water/gas mixture and then the subsequent decomposition of the hydrate from initiation to completion. These studies demonstrate that the application of neutron diffraction with simultaneous gas consumption measurements provides a powerful method for studying the clathrate hydrate crystal growth and decomposition. We have also used neutron diffraction to examine the water structure before the hydrate growth and after the hydrate decomposition. From the neutron-scattering curves and the empirical potential structure refinement analysis of the data, we find that there is no significant difference between the structure of water before the hydrate formation and the structure of water after the hydrate decomposition. Nor is there any significant change to the methane hydration shell. These results are discussed in the context of widely held views on the existence of memory effects after the hydrate decomposition.

Chemistry, Physical↗
PubMed2005

Structure of ionic liquid-benzene mixtures.

Neutron diffraction has been used to investigate the structure of liquid mixtures of 1,3-dimethylimidazolium hexafluorophosphate with benzene. Two concentrations of benzene were investigated, namely, 33 mol % and 67 mol %, and show similar structures in each case. The presence of benzene significantly alters the ionic liquid structure, in particular, in the cation-cation interactions, in agreement with the single-crystal structure described recently (Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Chem. Commun. 2003, 476). In each case, the data was analyzed using an empirical potential structure refinement process.

Journal Article↗
PubMed2003

Structural studies of ammonia and metallic lithium-ammonia solutions.

The technique of hydrogen/deuterium isotopic substitution has been used to extract detailed information concerning the solvent structure in pure ammonia and metallic lithium-ammonia solutions. In pure ammonia we find evidence for approximately 2.0 hydrogen bonds around each central nitrogen atom, with an average N-H distance of 2.4 A. On addition of alkali metal, we observe directly significant disruption of this hydrogen bonding. At 8 mol % metal there remains only around 0.7 hydrogen bond per nitrogen atom. This value decreases to 0.0 for the saturated solution of 21 mol % metal, as all ammonia molecules have then become incorporated into the tetrahedral first solvation spheres of the lithium cations. In conjunction with a classical three-dimensional computer modeling technique, we are now able to identify a well-defined second cationic solvation shell. In this secondary shell the nitrogen atoms tend to reside above the faces and edges of the primary tetrahedral shell. Furthermore, the computer-generated models reveal that on addition of alkali metal the solvent molecules form voids of approximate radius 2.5-3.0 A. Our data therefore provide new insight into the structure of the polaronic cavities and tunnels, which have been theoretically predicted for lithium-ammonia solutions.

Journal Article↗