ORIENTATIONAL ORDER OF WATER CONFINED IN ANISOTROPIC CAVITIES

Water confined to a repulsive ellipsoidal cavity with a variable aspec t ratio is studied by computer simulation. The orientational order par ameter of the O-H bond with respect to the symmetry axis of the cavity , S-OH,is examined as a function of the cavity dimensions for finite n umbers of water molecules (N = 6 to 38 at constant density). We observ e that the absolute value of S-OH is increased by deforming a spherica l cavity into either a prolate or an oblate ellipsoidal cavity. This t rend is method independent, i.e., it is obtained using Monte Carlo sim ulations with TIP4P or SPC water and using molecular dynamics simulati ons. Generally \S-OH\ decreases with increasing N and temperature for a fixed cavity aspect ratio. The calculated magnitudes of S-OH and its temperature dependence are in order-of-magnitude agreement with exper imental NMR determinations of S-OH in hydrated fibers. Hence such spat ial anisotropies should be considered in models of the residual orient ation exhibited by probe molecules in fibrous synthetic and biological materials.