Neutron diffraction with hydrogen/deuterium isotope substitution on the solvent water hydrogens is used to investigate changes in water structure around the apolar groups on alcohols and tetraalkylammonium ions at room temperature. The HH and HX correlations (where X is a nonsubstituted atom) are calculated for the following solutes in water: ethanol (5 mol%), tertiary butanol (3 mol%), tetrapropylammonium bromide (2.4 mol%), and tetrabutylammonium bromide (1.7 mol%). The correlation functions suggest that the water structure in all four systems is very similar to bulk water. There is no evidence of structural enhancement in going from pure water to water in the alcohol–water mixtures. There is a small but probably significant increase in structure in the tetraalkylammonium bromide solutions compared to both pure water and the alcohol–water mixtures. There is very little measurable difference between the two alcohol–water mixtures or the two tetraalkylammonium bromide solutions in terms of water structure. The results are consistent with the view that apolar solutes are located in cavities in the hydrogen-bonded water network, but they suggest that the increase in order associated with apolar hydration is a small effect in terms of structure.

Neutron diffraction involving seven isotope substitution experiments is used to extract the methyl hydrogen to the water hydrogen partial structure factor in a 2.0 m aqueous solution of tetramethylammonium chloride, in order to investigate the orientation of water molecules around the TMA ion. The correlation function of the TMA centers with water molecule centers is obtained and shows that the average distance is about 4.7 Å, in agreement with the result found previously using a nitrogen isotope substitution. The technique of spherical harmonic reconstruction is used to obtain orientational correlation functions between TMA ions and water molecules. The results show that the TMA ion has a predominantly ‘‘apolar’’ character in water but that a range of water molecule orientations around the ion are possible.

We present neutron diffraction isotopic substitution (NDIS) and molecular dynamics (MD) simulation results on molten poly(ethylene oxide) doped with LiI in a molar ratio of five ether oxygens per Li + ion. The NDIS and the MD studies yield estimates of the partial radial distribution function about the Li + ion. Using a quantum chemistry based atomistic force field, excellent agreement between simulation and experiment is obtained. The agreement between NDIS and MD is improved significantly if ion/ion and ion/polymer polarization effects are introduced in the simulations, indicating the importance of polarization effects in this system. Results indicate a strong coordination between the Li + cations and the ether oxygen atoms. The simulation results suggest a value of nO(r < 3 Å) = 3.5 for the coordination number of oxygens about Li + . The variation of nO with Li-O separation to 2.5 Å is similar from simulation and experiment, although experimentally beyond 2.5 Å this function is impossible to determine.

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