ABSTRACT
We report small-angle neutron scattering studies of liquid helium mixtures confined in Mobil Crystalline Material-41 (MCM-41), a porous silica glass with narrow cylindrical nanopores (d=3.4 nm). MCM-41 is an ideal model adsorbent for fundamental studies of gas sorption in porous media because its monodisperse pores are arranged in a 2D triangular lattice. The small-angle scattering consists of a series of diffraction peaks whose intensities are determined by how the imbibed liquid fills the pores. Pure (4)He adsorbed in the pores show classic, layer-by-layer film growth as a function of pore filling, leaving the long range symmetry of the system intact. In contrast, the adsorption of (3)He-(4)He mixtures produces a structure incommensurate with the pore lattice. Neither capillary condensation nor preferential adsorption of one helium isotope to the pore walls can provide the symmetry-breaking mechanism. The scattering is consistent with the formation of randomly distributed liquid-liquid microdomains â¼2.3 nm in size, providing evidence that confinement in a nanometer scale capillary can drive local phase separation in quantum liquid mixtures.
ABSTRACT
Measurements of the proton momentum distribution n(p) in water from ambient conditions to above the supercritical point are compared with theoretical calculations based on a recently developed polarizable water model. The n(p) along the H-bond direction is narrower in the dense phases, and approaches that of the isolated molecule in the more dilute phases. The theoretical model, which includes only electrostatic interactions, is unable to explain the softening of the local potential experienced by the proton in the dense phases, but it accurately predicts the n(p) for the dilute phases.
