High pressure induced phase transition and superdiffusion in anomalous fluid confined in flexible nanopores

Abstract

The behavior of a confined spherical symmetric anomalous fluid under high external pressure was studied with Molecular Dynamics simulations. The fluid is modeled by a core-softened potential with two characteristic length scales, which in bulk reproduces the dynamical, thermodynamical, and structural anomalous behavior observed for water and other anomalous fluids. Our findings show that this system has a superdiffusion regime for sufficient high pressure and low temperature. As well, our results indicate that this superdiffusive regime is strongly related with the fluid structural properties and the superdiffusion to diffusion transition is a first order phase transition. We show how the simulation time and statistics are important to obtain the correct dynamical behavior of the confined fluid. Our results are discussed on the basis of the two length scales.

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

“…41,42 Whereas, when the nanopore has at least one degree of freedom, given by the mobility of the plates in z direction, the anomalous behavior of the fluid disappears and distinct phase transitions are observed. [43][44][45][46] CS fluids confined in nanotubes also present interesting findings, similar to obtained in atomistic models for water, as the increase in diffusion coefficient and flux for narrow nanotubes associated to a layer to single-file transition and a discontinuity in the enhancement flow factor. [47][48][49] The drawback of these core-softened potentials is that due to the simplicity of the two length scales, they are not capable to reproduce the effects related to the third coordination shell of the anomalous fluid what might be relevant under confinement.…”

Relation between boundary slip mechanisms and waterlike fluid behavior

“…An anomalous fluid is characterized by having a maximum in the density versus temperature at fixed pressure and a maximum and a minimum in the diffusion coefficient versus pressure at constant temperature [43][44][45][46][47][48]. Under high confinement, these fluids exhibit additional anomalous behaviors and new phases [49,50]. When an anomalous fluid is nanoconfined the thermodynamic and dynamic properties differ from the properties observed in the bulk [29][30][31]51].…”

“…This is not the case for confined systems. A waterlike fluid forms layers which depend on the film thickness [49,50,[52][53][54][55]. Due to the layering, particles show different behaviors in different layers, which allows for the anomalous flux observed in confined waterlike materials.…”

Different Water Networks Confined in Unidirectional Hydrophilic Nanopores and Transitions with Temperature

“…When pressure is applied using water itself as pressure transmitting medium (PTM), superhydration effects and hindering of pore collapse are observed, with a consequent major change in the mechanical properties of the system [15] . From a dynamical point of view, super-diffusion effects have been predicted to occur for water nanoconfined in the unidimensional pores [16] . Upon lowering the temperature at ambient pressure, full ordering of nanoconfined water was not observed down to 173 K [14] but water ordering and site occupancies of proximal water were shown to increase as the temperature decreases, likely because water molecules interact with the nanopore wall forming HB with O atoms of the framework.…”