Superhydrophilic Cylindrical Confinement Effect on Solid-Liquid Coexistence of Water Through Free Energy Analysis

Abstract

Compared to bulk water, confined water exhibits unique properties, making it particularly fascinating to study, especially in terms of phase transitions. Investigating confined water systems holds significant importance for various reasons, including gaining insights into biological processes. Additionally, free energy calculations during phase transitions are crucial for understanding chemical and biochemical phenomena such as macromolecular stability, protein folding, molecular solvation, and drug discovery. However, previous studies have primarily focused on hydrophobic or moderately hydrophilic confinement. Here, we explore the cylindrical confinement effect on solid-liquid coexistence of water within superhydrophilic pore with a radius (R) varying from 1 nm to 5 nm, via Gibbs free energy analysis with monatomic water (mW) model. Molecular dynamics simulations (LAMMPS) were conducted to determine the thermodynamic melting point or solid-liquid coexistence, using multiple-histogram reweighting (MHR) along with pseudo-supercritical transformation path. Each of the cylindrically-confined water systems has been simulated for the three wall-water interaction strengths (ϵ): 1.17, 1.47, and 2.05 kcal/mol. An approximate melting temperature is obtained from the hysteresis loop of the density-temperature plot. The melting temperature exhibits an oscillatory behavior with varying pore size, which aligns well with previous literature. However, for all pore sizes and interaction strengths considered, the melting temperature is consistently lower than the recently reported melting point of bulk water. This observation is further supported by structural analysis, including order parameter calculations and visual assessments.