Molecular Simulations Probing the Thermophysical Properties of Homogeneously Stretched and Bubbly Water Systems

Molecular simulations in the canonical ensemble were performed to probe a variety of thermophysical properties of both homogeneously stretched and bubbly water systems at a temperature of 298 K. Two types of water models, the four-site TIP4P/2005 model and the coarse-grained single-site mW model, were investigated. Simulations for the computationally efficient mW model were carried out using cubic simulation boxes with linear dimensions of 4, 8, 16, and 32 nm, whereas 4, 6, and 8 nm boxes were considered for the TIP4P/2005 model. Various thermophysical properties, including pressure (P), potential energy (U), residual isochoric heat capacity (C_V,res), viscosity (η), and self-diffusion coefficient (D_self), were calculated for densities ranging from 800 kg/m³ to the saturated liquid density (ρ_sat). Following two simulation protocols starting either from a homogeneous configuration or from a heterogeneous configuration with a single spherical cavity, spinodal cavitation (SC) and bubble collapse (BC) points were located separately. This behavior of a fluid in a box of fixed volume is analogous to the hysteresis observed for adsorption-desorption isotherms of a subcritical fluid in a mesoporous adsorbent. As the system size is increased, both BC and SC points are shifted to larger densities. In terms of thermophysical properties, qualitatively similar trends are observed for the mW and TIP4P/2005 models. As the system approaches the SC point from ρ_sat, P decreases to a large negative value, U, C_V,res, and η increase, and D_self decreases. Upon cavitation, the system undergoes a relaxation as signaled by step changes in these properties. In the heterogeneous (bubbly) region, a decrease in the density leads to relatively slower increases in P and U and a slow decrease in η, but C_V,res does not exhibit significant changes, whereas a significant decrease in D_self is observed only for the mW model and boxes larger than 8 nm. For the system sizes investigated here, the thermophysical properties at a given density cannot be simply estimated from a linear combination of the corresponding properties taken from the saturated liquid and vapor phases. For the bubbly phases, the Young-Laplace relation is found to hold well, whereas the Stokes-Einstein relation is not obeyed.