Fluid Phase Equilibria 183–184 (2001) 331–339 Simulation of confined water in equilibrium with a bulk reservoir I.V. Brovchenko a , A. Geiger a,∗ , D. Paschek b a Physikalische Chemie, Universitaet Dortmund, 44221 Dortmund, Germany b Department of Chemical Engineering, University of Amsterdam, 1018 WV Amsterdam, The Netherlands Abstract The properties of water in pores depend strongly on its average density, which is determined by the equilibrium with a bulk reservoir. In the present paper, Gibbs ensemble simulations were used for the equilibration of water in spherical pores with smooth surfaces and radii from 9 to 20 Å and bulk liquid water at T = 300 K and P = 1 bar. The water density profile along the pore radius shows in all cases two prominent water layers near the pore surface. The oxygen–oxygen radial distribution functions evidence a clear distortion of the tetrahedral water structure in the first (outermost) layer towards a quasi-planar square- or hexagonal-like arrangement. An analysis of the non-short-circuited hydrogen-bond polygons evidences the existence of quasi-planar polygons in the first layer. Increasing the pore radius improves the water–water interaction between the two outer water layers, destroys the quasi-planar polygons and speeds up water diffusivity in the pore. The radial distribution functions of the hexagons evidence some distortion of the inner water in the pore towards the structure of cubic ice. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Molecular simulation; Liquid–liquid equilibria; Water; Gibbs ensemble; Pores 1. Introduction The thermodynamic equilibrium with a bulk reservoir determines the properties of liquids confined in pores. Hence, correct equilibration of the confined liquid with the bulk reservoir in molecular simulations is a necessary condition for a correct reproduction of its properties. Using Gibbs ensemble simulations [1] is the most direct and correct way for the equilibration of coexisting phases. This method was used recently for the simulation of water in spherical pores with radii R from 6 to 15 Å in equilibrium with bulk water at T = 300 K and P = 1 bar [2]. A vapour–liquid phase transition was observed at some strength of the water–pore interaction. Liquid water exists in such pores only when forming two prominent layers near the pore wall. The diffusivity of water in pores with R = 12 Å was found to be lower than in the bulk. ∗ Corresponding author. Tel.: +49-231-755-3937; fax: +49-231-755-3937. E-mail address: geiger@pc2a.chemie.uni-dortmund.de (A. Geiger). 0378-3812/01/$20.00 © 2001 Elsevier Science B.V. All rights reserved. PII:S0378-3812(01)00445-9