Propane–Water Mixtures Confined within CylindricalSilica Nanopores: Structural and Dynamical Properties Probed by MolecularDynamics

摘要

Despite the multiple length and time scales over which fluid-mineral interactions occur, interfacial phenomena control the exchange of matter and impact the nature of multiphase flow, as well as the reactivity of C–O–H fluids in geologic systems. In general, the properties of confined fluids, and their influence on porous geologic phenomena are much less well understood compared to those of bulk fluids. We used equilibrium molecular dynamics simulations to study fluid systems composed of propane and water, at different compositions, confined within cylindrical pores of diameter ∼16 Å carved out of amorphous silica. The simulations are conducted within a single cylindrical pore. In the simulated system all the dangling silicon and oxygen atoms were saturated with hydroxyl groups and hydrogen atoms, respectively, yielding a total surface density of 3.8 −OHm². Simulations were performed at 300 K, at different bulk propane pressures, and varying the composition of the system. The structure of the confined fluids was quantified in terms of the molecular distribution of the various molecules within the pore as well as their orientation.This allowed us to quantify the hydrogen bond network and to observethe segregation of propane near the pore center. Transport propertieswere quantified in terms of the mean square displacement in the directionparallel to the pore axis, which allows us to extract self-diffusioncoefficients. The diffusivity of propane in the cylindrical pore wasfound to depend on pressure, as well as on the amount of water present.It was found that the propane self-diffusion coefficient decreaseswith increasing water loading because of the formation of water bridgesacross the silica pores, at sufficiently high water content, whichhinder propane transport. The rotational diffusion, the lifespan ofhydrogen bonds, and the residence time of water molecules at contactwith the silica substrate were quantified from the simulated trajectoriesusing the appropriate autocorrelation functions. The simulations contributeto a better understanding of the molecular phenomena relevant to thebehavior of fluids in the subsurface.