Orientational order of water confined in anisotropic cavities

Water confined to a repulsive ellipsoidal cavity with a variable aspect ratio is studied by computer simulation. The orientational order parameter 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 numbers of water molecules (N = 6 to 38 at constant density). We observe that the absolute value of S-OH is increased by deforming a spherical cavity into either a prolate or an oblate ellipsoidal cavity. This trend is method independent, i.e., it is obtained using Monte Carlo simulations with TIP4P or SPC water and using molecular dynamics simulations. 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 experimental NMR determinations of S-OH in hydrated fibers. Hence such spatial anisotropies should be considered in models of the residual orientation exhibited by probe molecules in fibrous synthetic and biological materials.