Water in nanopores. I. Coexistence curves from Gibbs ensemble Monte Carlo simulations

Abstract

Coexistence curves of water in cylindrical and slitlike nanopores of different size and water-substrate 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.

Water adsorption in hydrophobic MOF channels

“…This phase diagram is based on the four following results: (i) water adsorption in Al(OH)(1,4-ndc) happens at pressure lower than P sat ; (ii) the adsorption transition is continuous, meaning the confined water is supercritical at 298 K; (iii) adding methyl groups increases the adsorption pressure; (iv) as confinement increases, the critical point shifts to lower temperature. 45 The diagram obtained, represented in Fig. 9, is markedly different from that inferred for hydrophobic zeolites, where adsorption happens at P > P sat .…”

Critical phenomena at perfect and non-perfect surfaces

“…Recent simulations of water in hydrophobic pores also illustrated the effect of curved surfaces on the values of local critical exponents [193]. In that study Brovchenko et al observed a sharp crossover for cylindrical surfaces, yielding an asymptotic value for the local order parameter larger than the value β 1 ≈ 0.82 measured for slitlike pores.…”

“…The critical behaviour characteristic of the ordinary transition is expected to be observed in this case not too close to T c as one is dealing with the liquid-vapour transition near a weakly attractive surface, see the discussion in Section 2.6. Of course, the geometry is slightly different to the wedge-shaped geometries discussed so far in the literature, but nevertheless the kind of systems discussed in [193] seems to be a good candidate to study the effect of (generalised) edges on the local critical behaviour experimentally in the future.…”

Vapor-liquid critical and interfacial properties of square-well fluids in slit pores

“…For example, capillary critical temperatures in confined geometries are reported experimentally for SF 6 in porous glass and several gases in MCM-41 by Thommes et al 13 and Morishige Findenegg, 14 respectively; the shift in the critical temperature is found to have linear dependence on the inverse pore width. Numerous theoretical studies and molecular simulations including density function theory [15][16][17][18] and Monte Carlo ͑MC͒ simulations [19][20][21][22][23][24][25] were performed to understand capillary condensation. Vishnyakov et al 26 were the first to perform MC simulations systematically on carbon slit pores and studied the shift of the vapor-liquid critical point under confinement.…”