Molecular Dynamic Simulation of Water Vapor and Determination of Diffusion Characteristics in the Pore

Abstract

Abstract. One of the varieties of pores, often found in natural or artificial building materials, are the so-called blind pores of dead-end or saccate type. Three-dimensional model of such kind of pore has been developed in this work. This model has been used for simulation of water vapor interaction with individual pore by molecular dynamics in combination with the diffusion equation method. Special investigations have been done to find dependencies between thermostats implementations and conservation of thermodynamic and statistical values of water vapor -pore system. The two types of evolution of water -pore system have been investigated: drying and wetting of the pore. Full research of diffusion coefficient, diffusion velocity and other diffusion parameters has been made. Molecular dynamics modelIn classical molecular dynamics, the behavior of an individual particle is described by the Newton equations of motion [1]. For a simulation of particle interaction we use the Lennard-Jones potential [2] with σ = 3.17Å and ε = 6.74 · 10 −3 eV. It is the most used to describe the evolution of water in liquid and saturated vapor form. Equations of motion were integrated by Velocity Verlet method [3]. Berendsen thermostat [4] is used for temperature equilibrating and control. The coefficient of the velocity recalculation λ(t) at every time step t depends on the so called 'rise time' of the thermostat τ B which belongs to the interval [0.5, 2] ps. The Berendsen algorithm is simple to implement and it is very efficient for reaching the desired temperature from far-from-equilibrium configurations. Computer simulation of microscopic modelWe made simulation for a pore of dimensions l x = 500 nm, l y = 50 nm, l z = 50 nm with integration time step ∆t = 0.016 ps and evolution time 65.3 ns. Otherwise, we have considered the following input data for the drying process:• 1000 H 2 O molecules in the pore volume 500 × 50 × 50 nm 3 form a saturated water vapor at temperature T 0 = 25 • C and pressure p 0 = 3.17 kPa;

Numerical Investigation of the Water Vapour Diffusivity inside Homogeneous Porous Media

“…The equations (2) are written under the assumption that the behavior of the water vapour in a homogeneous porous medium is similar to the behavior of the water vapour in an individual through-type pore with suitable hydraulic properties [2][3][4].…”