抄録
Spatial distribution functions (SDFs), gOO(x, y, z) and gOH(x, y, z), which show the probability of atom-atom pair distribution between solute and solvent atoms, have been used to characterize the anisotropic structure of solutions in computer simulations. In this article, a new method for structural change analysis is proposed based on the difference spatial distribution function (DSDF) ΔgOO(x, y, z), and is applied to liquid water, and to infinitely dilute aqueous solutions of methoanol and ethanol. Based on the results of SDF in methanol and ehtanol solutions, the distribution of solvent water can be classified into three groups : hydrogen acceptor, hydrogen donor, and hydrophobic hydration regions. Regarding the effect fo hydrophobic groups, the DSDF (i.e. ΔgOO(x, y, z)=gOO(x, y, z)EtOH-gOO(x, y, z)MeOH) results indicated that the distribution volume of hydration water surrounding the alcohol molecule in the hydrogen acceptor region decreases with increasing steric bulk of the hydrophobic group. However, binding energy in this region is stabilized by an increase in the coordination number. Moreover, from the results of DSDF at 298 and 273 K (i.e. Δg<OO>(x, y, z, ΔT)=g<OO>(x, y, z, 298 K)-g<OO>(x, y, z, 273 K)), the distribution of hydration water spreads out over the region surrounding the hydrogen acceptor and donor. This result is reasonable since an increase in the fluctuation of the network structure results from a rise in temperature. The new DSDF method presented in this work should be widely applicable to structural change analysis of hydration structure for anisotropic solutions by computer siulation.
