Difference spatial distribution function analysis of aqueous solutions. III. Hydration structures of alcohol and ether solutions having straight chain and branched alkyl groups

Spatial distribution functions (SDFs), g(OO)(x,y, z) and g(OH)(x,y,z), obta ined from Monte Carlo simulations at 298 K were applied to characterize the anisotropic structure of infinitely dilute aqueous solutions of alcohols a nd ethers having straight chain and branched alkyl groups. In spite of the different size and shape of the hydrophobic groups, the spatial orientation of the hydrogen-bonded water molecules was found to be of linear type with a triple layer structure in the hydrogen acceptor (HA) region and a double layer structure in the hydrogen donor (HD) region. The volumes and the coo rdination number (CN) in the HA region were essentially identical for all a lcohol and ether solutions, but the volumes for the isopropyl alcohol OPA) and isopropyl methyl ether (IPE) solutions were greater than those for the other solutions. In the hydrophobic hydration OIH) region, these values inc reased with increasing size and shape of hydrophobic groups, except in the case of IPA and IPE solutions. These results indicated that the hydration s tructures around the isopropyl group in alcohol and ether solutions differe d from those in other solutions. From the results of the difference SDF (DS DF), Delta g(OO)(x,y,z), between SDFs g(OO)(x, y z) for the two states, it was apparent that the distribution of hydration water molecules in the HA r egion for ether solution was characterized by the increase of the distribut ion in the direction of lone pair electrons on the oxygen atom of the solut e molecule with increasing hydrophobicity.