The structure of frozen phases in slit nanopores: A grand canonical Monte Carlo study (original) (raw)
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Journal of Chemical Theory and Computation, 2013
We devise a new computational approach to compute solid−liquid phase equilibria of confined fluids. Specifically, we extend the multibaric−multithermal ensemble method with an anisotropic pressure control to achieve the solid−liquid phase equilibrium for confined water inside slit nanopores (with slit width h ranging from 5.4 Å to 7.2 Å). A unique feature of this multibaric−multithermal ensemble is that the freezing points of confined water can be determined from the heatcapacity peaks. The new approach has been applied to compute the freezing point of two monolayer ices, namely, a high-density flat rhombic monolayer ice (HD-fRMI) and a high-density puckered rhombic monolayer ice (HD-pRMI) observed in our simulation. We find that the liquid-to-solid transition temperature (or the freezing point) of HD-pRMI is dependent on the slit width h, whereas that of HD-fRMI is nearly independent of the h.
Phase equilibria of water in cylindrical nanopores
Physical Chemistry Chemical Physics, 2001
Phase equilibria of water in cylindrical nanopores were simulated in the Gibbs ensemble. The decrease of the critical temperature in the conÐnement compared to the bulk value attains 35% in the pores with radius
Phase coexistence and dynamic properties of water in nanopores
The European Physical Journal E - Soft Matter, 2003
The dynamical properties of a confined fluid depend strongly on the (spatially varying) density. Its knowledge is therefore an important prerequisite for molecular-dynamics (MD) simulations and the analysis of experimental data. In a mixed Gibbs ensemble Monte Carlo (GEMC)/MD simulation approach we first apply the GEMC method to find possible phase states of water in hydrophilic and hydrophobic nanopores. The obtained phase diagrams evidence that a two-phase state is the most probable state of a fluid in incompletely filled pores in a wide range of temperature and level of pore filling. Pronounced variations of the average and local densities are observed. Subsequently, we apply constant-volume MD simulations to obtain water diffusion coefficients and to study their spatial variation along the pore radius. In general, water diffusivity slightly decreases in a hydrophilic pore and noticeably increases in a hydrophobic pore (up to about 40% with respect to the bulk value). In the range of gradual density variations the local diffusivity essentially follows the inverse density and the water binding energy. The diffusivity in the quasi-two-dimensional water layers near the hydrophilic wall decreases by 10 to 20% with respect to the bulk value. The average diffusivity of water in incompletely filled pore is discussed on the basis of the water diffusivities in the coexisting phases.
2D versus 3D Freezing of a Lennard-Jones Fluid in a Slit Pore: A Molecular Dynamics Study
2009
We present a computer simulation study of a (6,12)-Lennard-Jones fluid confined to a slit pore, formed by two uniform planes. These interact via a (3,9)-Lennard-Jones potential with the fluid particles. When the fluid approaches the liquid-to-solid transition we first observe layering parallel to the walls. In order to investigate the nature of the freezing transition we performed a detailed analysis of the bond-orientational order parameter in the layers. We found no signs of hexatic order which would indicate a melting scenario of the Kosterlitz-Thouless type. An analysis of the mean-square displacement shows that the particles can easily move between the layers, making the crystallization a 3d-like process. This is consistent with the fact that we observe a considerable hysteresis in the heating-freezing curves, showing that the crystallization transition proceeds as an activated process.
Phase diagram of supercooled water confined to hydrophilic nanopores
2012
We present a phase diagram for water confined to cylindrical silica nanopores in terms of pressure, temperature and pore radius. The confining cylindrical wall is hydrophilic and disordered, which has a destabilizing effect on ordered water structure. The phase diagram for this class of systems is derived from general arguments, with parameters taken from experimental observations and computer simulations and with assumptions tested by computer simulation. Phase space divides into three regions: a single liquid, a crystal-like solid, and glass. For large pores, radii exceeding 1 nm, water exhibits liquid and crystal-like behaviors, with abrupt crossovers between these regimes. For small pore radii, crystal-like behavior is unstable and water remains amorphous for all non-zero temperatures. At low enough temperatures, these states are glasses. Several experimental results for supercooled water can be understood in terms of the phase diagram we present.
Molecular Simulation of the Phase Behavior of Water Confined in Silica Nanopores
The Journal of Physical Chemistry C, 2007
Molecular dynamics simulations and grand canonical Monte Carlo simulations of water molecules in 1.04, 1.96, and 2.88 nm diameter silica pores (pores S, M, and L, respectively) were conducted at 300 K to reveal possible phase states of water in each pore and clarify the effect of pore diameter and hydration level on the structural and dynamical properties of water molecules confined in nanopores. Three types of phases appear in nanopores. In the first phase, which appears in pores S, M, and L, gas-phase water adsorption at the pore surface is below monolayer coverage. In the second phase, which appears in pores S and L, there is a condensed water monolayer at the pore surface. In the third phase, which appears in pores M and L, the pore is completely filled with water. Water molecules interact with silanol groups mainly via hydrogen bonds at low hydration when the number of water molecules is much smaller than that of silanol groups. With increasing number of water molecules, the number ratio of water molecules that adsorb on silanol groups via non-hydrogen-bonding interactions increases. Near full hydration, the translational mobility of water in the first adsorption layer is much smaller than that of bulk liquid water in all three pores, while those in the pore center are about 30, 80, and 100% of its bulk liquid value in pores S, M, and L, respectively.
Water in nanopores. I. Coexistence curves from Gibbs ensemble Monte Carlo simulations
The Journal of Chemical Physics, 2004
Coexistence curves of water in cylindrical and slitlike nanopores of different size and watersubstrate interaction strength were simulated in the Gibbs ensemble. The two-phase coexistence regions cover a wide range of pore filling level and temperature, including ambient temperature. Five different kinds of two-phase coexistence are observed. A single liquid-vapor coexistence is observed in hydrophobic and moderately hydrophilic pores. Surface transitions split from the main liquid-vapor coexistence region, when the water-substrate interaction becomes comparable or stronger than the water-water pair interaction. In this case prewetting, one and two layering transitions were observed. The critical temperature of the first layering transition decreases with strengthening water-substrate interaction towards the critical temperature expected for two-dimensional systems and is not sensitive to the variation of pore size and shape. Liquid-vapor phase transition in a pore with a wall which is already covered with two water layers is most typical for hydrophilic pores. The critical temperature of this transition is very sensitive to the pore size, in contrast to the liquid-vapor critical temperature in hydrophobic pores. The observed rich phase behavior of water in pores evidences that the knowledge of coexistence curves is of crucial importance for the analysis of experimental results and a prerequiste of meaningful simulations.
Effect of Pressure on the Freezing of Pure Fluids and Mixtures Confined in Nanopores †
The Journal of Physical Chemistry B, 2009
Monte Carlo simulations combined with the parallel tempering technique are used to study the freezing of Ar, CH 4 , and their mixtures in a slit graphite nanopore. For all systems, the solid/liquid coexistence line is located at higher temperature than that for the bulk phase, as expected for fluids for which the wall/fluid interaction is stronger than the fluid/fluid interaction. In the case of the mixtures, the phase diagram for the confined system is of the same type as that for the bulk (azeotropic). It is also found that the freezing temperatures for the confined fluids and mixture are much more affected by pressure than those for the bulk phase. By calculating the isothermal compressibility of the confined fluids and determining the slope of the solid/liquid coexistence line (T,P) from the Clapeyron equation, we show that such a strong effect of pressure is not related to reduced compressibility within the pores. On the other hand, the pressure dependence of the in-pore freezing temperature is correctly described in the frame of the model proposed by Miyahara et al.
The Journal of Physical Chemistry B, 2001
Liquid-vapor equilibrium, criticality, and spinodal transitions in nanopores are studied by the gauge cell Monte Carlo simulation method proposed recently (Neimark, A. V.; Vishnyakov, A. Phys. ReV. E 2000, 62, 4611). As an instructive example, we consider the capillary condensation of argon in cylindrical pores of different diameters (1.5-5.5 nm) representing typical pore channels in mesoporous molecular sieves. At the subcritical conditions, the gauge cell method allows one to construct continuous phase diagrams in the form of a van der Waals-type sigmoid isotherm. The sigmoid isotherm contains stable and metastable states on the adsorption and desorption branches connected by a backward trajectory of thermodynamically unstable states which cannot be observed experimentally yet can be stabilized in simulations. The phase equilibrium is determined by thermodynamic integration along the sigmoid trajectory using the Maxwell rule. The spinodals give the true limits of stability of vaporlike and liquidlike states. A notable difference was found between the spinodals and the limits of stability of the vaporlike and liquidlike states achieved in grand canonical Monte Carlo simulations. The critical conditions of the first-order vapor-liquid transition in pores were determined. Good agreement with experimental data on argon adsorption at 87 K on mesoporous molecular sieves was found for equilibrium transitions in pores wider than 2.2 nm and for hysteretic adsorption-desorption isotherms in pores wider than 5 nm.
Quasi-One-Dimensional Phase Transitions in Nanopores: Pore-Pore Correlation Effects
Physical Review Letters, 1997
For adsorbates confined within a single, sufficiently narrow cylindrical pore, no phase transitions occur because the system is too close to the one-dimensional limit. We study the influence of intermolecular correlations between adsorbed molecules in neighboring cylindrical pores, using molecular simulation. For a simple model of methane in the molecular sieve ALPO 4-5, we find that a phase transition between two fluid states ("gas" and "liquid") occurs below a critical temperature that is depressed relative to the bulk value. [S0031-9007(97)04210-5]