Propane–Water Mixtures Confined within Cylindrical Silica Nanopores: Structural and Dynamical Properties Probed by Molecular Dynamics

Abstract

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 −OH/nm2. 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 observe the segregation of propane near the pore center. Transport properties were quantified in terms of the mean square displacement in the direction parallel to the pore axis, which allows us to extract self-diffusion coefficients. The diffusivity of propane in the cylindrical pore was found to depend on pressure, as well as on the amount of water present. It was found that the propane self-diffusion coefficient decreases with increasing water loading because of the formation of water bridges across the silica pores, at sufficiently high water content, which hinder propane transport. The rotational diffusion, the lifespan of hydrogen bonds, and the residence time of water molecules at contact with the silica substrate were quantified from the simulated trajectories using the appropriate autocorrelation functions. The simulations contribute to a better understanding of the molecular phenomena relevant to the behavior of fluids in the subsurface.

Effects of water on the stochastic motions of propane confined in MCM-41-S pores

“…To fill the gaps in our understanding of the behavior of coexisting water and a volatile, we report here a quasielastic neutron scattering (QENS) study on the effect of D2O on the dynamics of propane confined in MCM-41-S at low temperatures (230 and 250 K). The results of these experiments are complemented by MD simulations, which build our previously reported simulation studies 35 . Both experiment and simulations suggest that water hinders the diffusion of propane in MCM-41-S pores.…”

“…The preparation of the simulation cell used in this work has been described elsewhere 35 . It consisted of two stagespreparing a cylindrical pore of amorphous silica, and then loading water and propane molecules in this pore.…”

“…Exceptions include Phan et al 33 and Bui et al, 34 who studied the effect of water on the transport of confined methane. Recently, Le et al reported MD simulation studies on the effect of water on the diffusion of propane in amorphous silica cylindrical pores of diameter 1.6 nm at 300 K 35 . This pore environment resembles the 1.5 nm wide pores in molecular sieve MCM-41-S.…”

Partial CO2 Reduction in Amorphous Cylindrical Silica Nanopores Studied with Reactive Molecular Dynamics Simulations

“…[81][82] The system was simulated as confined within a realistic cylindrical pore of diameter ~16 Å carved out of amorphous silica. A detailed description of the procedure implemented during the preparation of the cylindrical silica pore has been presented in a previous study, 83 in which we studied the transport properties of confined water-propane systems, using non-reactive force fields. In Figure 1A, we provide a schematic of the pore, which is found within a 57.28 Å x 57.28 Å x 57.28 Å simulation box with periodic boundary conditions along the 3 directions.…”

The Effect of Hydration on the Structure and Transport Properties of Confined Carbon Dioxide and Methane in Calcite Nanopores

“…With more than 80% of our energy needs being met by the subsurface environments (BP Global, 2015), there is a significant interest in environmentally benign approaches to recover and store fluids in complex materials characterized by chemical and morphological heterogeneity and nano-scale porosity. Various studies have shown that the properties and transport of confined fluids such as water (Bonnaud et al, 2010;Ho and Striolo, 2015;Hu et al, 2015;Chakraborty et al, 2017), gases such as CO 2 (Chialvo et al, 2012;Striolo and Cole, 2017;Simoes Santos et al, 2018), and hydrocarbons (Cole et al, 2013;Le et al, 2015a,b;Wu et al, 2015;Le T. T. B. et al, 2017;Herdes et al, 2018;Obliger et al, 2018;Simoes Santos et al, 2018) in nanoporous environments differs from bulk behaviors due to changes in the structure and affinity of confined liquids (Wang H. et al, 2016;Johnston, 2017) and gases (Yuan et al, 2015;Sun et al, 2016aSun et al, ,b, 2017Wang S. et al, 2016a,b) for the solid interfaces. With increasing interest in enhanced gas recovery coupled with subsurface CO 2 storage, a fundamental understanding of the changes in the structure of CO 2 and CH 4 and transport properties of these gases through water-bearing nanoporous environments provides a scientific basis for the observed fate and transport of these gases at the field scale (Glezakou and McGrail, 2013;Gadikota et al, 2017;Gadikota, 2018;Gadikota and Allen, 2018).…”