Effects of confinement on anomalies and phase transitions of core-softened fluids

Abstract

We use molecular dynamics simulations to study how the confinement affects the dynamic, thermodynamic, and structural properties of a confined anomalous fluid. The fluid is modeled using an effective pair potential derived from the ST4 atomistic model for water. This system exhibits density, structural, and dynamical anomalies, and the vapor-liquid and liquid-liquid critical points similar to the quantities observed in bulk water. The confinement is modeled both by smooth and structured walls. The temperatures of extreme density and diffusion for the confined fluid show a shift to lower values while the pressures move to higher amounts for both smooth and structured confinements. In the case of smooth walls, the critical points and the limit between fluid and amorphous phases show a non-monotonic change in the temperatures and pressures when the nanopore size is increase. In the case of structured walls, the pressures and temperatures of the critical points varies monotonically with the pore size. Our results are explained on basis of the competition between the different length scales of the fluid and the wall-fluid interaction.

Effects of confinement between attractive and repulsive walls on the thermodynamics of an anomalous fluid

“…the structural anomaly region, towards larger densities, pressures and temperatures with respect to the bulk. This effect is commonly observed in confined fluids regardless the strength of attraction of the wall and its structure [21,36].…”

“…A similar confinement has been also considered in previous theoretical works, e.g., in relation with wetting phenomena [44][45][46]. Smooth repulsive walls have been used as coarse-grained models for hydrophobic surfaces [36,47].…”

“…In particular, the enhancement is at variance with simulation results for water in hard-core repulsive smooth slit pores with size < 7 layers (we use the fact that each water layer is on average 0.3 nm wide) [47], coarse-grained water in slit pores of width 3-7 layers [60], water in disordered carbon slit pores with a size distribution 1-3 layers range [61], water in cylindrical hydrophobic and hydrophilic pores of sizes < 3 layers [62]. Furthermore, the structured attractive wall should decrease the fluid mobility with respect to the smooth wall for isotropic potentials [36,63]. Also, some experiments show a dramatic increase of the mechanical relaxation times of water compressed between hydrophilic surfaces to less than 3-4 molecular layers [42], or a mild increase, within a factor of three of the bulk value, of the effective viscosity of water confined between curved mica surfaces even when it is confined to a number of layers ≤ 10 layers [64].…”

“…attractive, confinement [83]. However, a number of isotropic models with water-like properties in nano-confinement have shown that the LLCP shifts towards lower T independently of the surface structure [36] or the surface interaction [21][84].…”

“…The CSW is an isotropic pairwise core-softened potential with a repulsive shoulder and an attractive well [28][29][30], similar to other used for colloids [32,33] and water-like liquids [21,[34][35][36]. It is suitable for describing globular proteins in solution [37], methanol hydroxyl groups [38,39], liquid metals [40], water-like liquids [29] and can properly describe the hydrophobic effect of water as a solvent [41].…”