Effect of Superhydrophilic Confinement on Solid-Liquid Coexistence of Water using Free Energy Analysis with mW Model

Abstract

Here, we investigate the effect of slit confinement on solid-liquid coexistence of water within strongly-hydrophilic slit pore size (H) varying from 8.5 Å to 70 Å, using Gibbs free energy analysis with monatomic water (mW) model [1]. Understanding the mechanism of water phase transformation is one of the greater interest among the researchers as water is the most abundant substance in all living organisms which plays an essential role in the survival of life, further calculations of thermodynamic free energies for phase transitioning systems are extremely important in studying a wide variety of chemical and biochemical phenomena including molecular solvation, macromolecular stability, protein folding, advancing drug discovery. We perform MD simulations (LAMMPS) in order to determine the solid-liquid coexistence or thermodynamic melting point by employing thermodynamic integration along with MHR (multiple-histogram reweighting) methods [2]. Gibbs free energy difference is computed using reversible and integrable pseudo-supercritical transformation path connecting the two phases. Each of the confined water systems is simulated for the three water-wall interaction strengths: εwf = 1.0, εwf = 1.25 and εwf = 1.75910. The estimated free energy difference is found to be different for the different confinement sizes as well as interaction strengths. The computed melting temperatures for H = 40 Å with εwf = 1.0, εwf = 1.25 and εwf = 1.75910 are 262.66 ± 1.3 K, 238.13 ± 1.3 K and 243.096 ± 1.3 K, respectively. While for H = 70 Å, the estimated melting temperatures with εwf = 1.0, εwf = 1.25 and εwf = 1.75910 are 252.48 ± 1.3 K, 246.24 ± 1.3 K and 248.68 ± 1.3 K, respectively [Fig. 1]. Oscillatory nature is observed in the melting temperature with the varying pore size, which is in well agreement with the previous literature. However, for all the pore sizes with all the three interaction strengths, the melting temperature is found to be lower compared to the recently reported melting point of bulk water. The study enthralls in attaining deeper insight into the mechanism of solid-liquid phase transition of water.