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

Abstract

We study by molecular dynamics simulations the thermodynamics of an anomalous fluid confined in a slit pore with one wall structured and attractive and another unstructured and repulsive. We find that the phase diagram of the homogeneous part of the confined fluid is shifted to higher temperatures, densities and pressures with respect to the bulk, but it can be rescaled on the bulk case. We calculate a moderate increase of mobility of the homogeneous confined fluid that we interpret as a consequence of the layering due to confinement and the collective modes due to long-range correlations. We show that, as in bulk, the confined fluid has structural, diffusion and density anomalies, that order in the water-like hierarchy, and a liquid-liquid critical point (LLCP). The overall anomalous region moves to higher temperatures, densities and pressure and the LLCP displaces to higher temperature compared to bulk. Motivated by experiments, we calculate also the phase diagram not just for the homogeneous part of the confined fluid but for the entire fluid in the pore and show that it is shifted towards higher pressures but preserves the thermodynamics, including the LLCP. Because our model has water-like properties, we argue that in experiments with supercooled water confined in slit pores with a width of > 3 nm if hydrophilic, and of > 1.5 nm if hydrophobic, the existence of the LLCP could be easier to test than in bulk, where it is not directly accessible