Christoph Hartnig

Modifications of the hydrogen bond network of liquid water in a cylindrical SiO2 pore

Abstract We present results of molecular dynamics simulations of water confined in a silica pore. A cylindrical cavity is created inside a vitreous silica cell with geometry and size similar to the pores of real Vycor glass. The simulations are performed at different hydration levels. At all hydration levels water adsorbs strongly on the Vycor surface; a double layer structure is evident at higher hydrations. At almost full hydration the modifications of the confinement-induced site-site pair distribution functions are in qualitative agreement with neutron diffraction experiment. A decrease in the number of hydrogen bonds between water molecules is observed along the pore radius, due to the tendency of the molecules close to the substrate to form hydrogen-bonds with the hydrophilic pore surface. As a consequence we observe a substrate induced distortion of the H-bond tetrahedral network of water molecules in the regions close to the surface.

Water in porous glasses. A computer simulation study

We report molecular dynamics simulations of water confined in a cylindrical silica pore. The pore geometry and size is similar to that of typical pores in porous Vycor glass. In the present study we focus on the dependence of microscopical structural and dynamical properties on the degree of hydration of the pore. We have performed five simulations of systems between 19 and 96 % hydration. In all cases, water adsorbs strongly on the pore surface, clearly demonstrating the hydrophilic nature of the Vycor surface. Two layers of water molecules are affected strongly by the interactions with the glass surface. With decreasing degree of hydration an increasing volume in the center of the pore is devoid of water molecules. At 96 % hydration the center is a continuous and homogeneous region that has, however, a lower density than bulk water at ambient conditions. A well-pronounced mobility profile exists, where molecules in the center of the pores have substantially higher self diffusion coefficients than molecules on the pore surface. The spectral densities of center of mass and hydrogen atom motion show the signature of confinement for the molecules close to the pore surface, while the spectral densities in the center of the pore are similar to those in bulk water. The molecular dynamics results are in good agreement with recent experiments. Our data indicate that the dependence of experimental data on the level of hydration of the Vycor sample is due to the different relative contribution of molecules adsorbed on the pore surface and bulk-like molecules in the interior of the pore to the experimental averages.

Non-exponential kinetic behaviour of confined water

We present the results of molecular dynamics simulations of SPC/E water confined in a realistic model of a silica pore. The single-particle dynamics have been studied at ambient temperature for different hydration levels. The confinement near the hydrophilic surface makes the dynamic behaviour of the liquid strongly dependent on the hydration level. Upon decrease of the number of water molecules in the pore we observe the onset of a slow dynamics due to the cage effect. The conventional picture of a stochastic single-particle diffusion process thus loses its validity.

Orientational correlations near interfaces. Computer simulations of water and electrolyte solutions in confined environments

Abstract We review published work and summarize the current understanding of orientational correlations in liquid water and aqueous solutions near interfaces, as obtained from computer simulation studies. We present a number of examples in which we discuss the magnitude and the range of orientational anisotropy and its dependence on the nature of the interface (non-polar, polar, or metallic) and the geometry of the interface (planar or curved).

Evidence of glassy behaviour of water molecules in confined states

pore is presented. The simulation has been performed at different hydration levels nat ambient temperature to study the single-particle dynamics. Owing to nconfinement and to the presence of a strong hydrophilic surface, the dynamic nbehaviour of the liquid appears to be strongly dependent on the hydration level. nAt higher hydration two quite distinct subsets of water molecules are detectable. nThose belonging to the first layer close to the substrate suffer a severe slowing ndown. While the behaviour of the remaining ones is more similar to bulk water. nAt lower hydrations we observe the onset of a slow dynamics due to the cage neffect. The intermediate scattering function displays a double-step relaxation nbehaviour whose long time tail is strongly non-exponential. Moreover, for low nhydrations, the intermediate scattering function clearly displays an overshooting, nwhich can be assigned to the so called ‘boson peak’. The conventional picture of nthe stochastic single-particle diffusion therefore already loses its validity at room ntemperature for confined water.