N-octane diffusivity enhancement via carbon dioxide in silica slit-shaped nanopores – a molecular dynamics simulation

Abstract

Equilibrium molecular dynamics simulations were conducted to study the competitive adsorption and diffusion of mixtures containing n-octane and carbon dioxide confined in slitshaped silica pores of width 1.9 nm. Atomic density profiles substantiate strong interactions between CO2 molecules and the protonated pore walls. Non-monotonic change in n-octane self-diffusion coefficients as a function of CO2 loading was observed. CO2 preferential adsorption to the pore surface is likely to attenuate the surface adsorption of n-octane, lower the activation energy for n-octane diffusivity, and consequently enhance n-octane mobility at low CO2 loading. This observation was confirmed by conducting test simulations for pure noctane confined in narrower pores. At high CO2 loading, n-octane diffusivity is hindered by molecular crowding. Thus, n-octane diffusivity displays a maximum. In contrast, within the concentration range considered here, the self-diffusion coefficient predicted for CO2 exhibits a monotonic increase with loading, which is attributed to a combination of effects including the saturation of the adsorption capacity of the silica surface. Test simulations suggest that the results are strongly dependent on the pore morphology, and in particular on the presence of edges that can preferentially adsorb CO2 molecules and therefore affect the distribution of these molecules equally on the pore surface, which is required to provide the effective enhancement of n-octane diffusivity.

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

“…However The effect of water on the dynamics of confined propane is similar to that reported at 300 K in an earlier publication 35 . However, the effect reported here is very different to that reported for CO2 on the dynamics of confined ethane 10 , propane 13 , butane 30 and octane 29 . CO2 was found to enhance the mobility of the confined hydrocarbons in all these studies.…”

“…In natural environments, however, pores are seldom occupied by a single species. While some studies have reported the effects of presence or absence of a second species on the dynamics of one guest 10,13,[27][28][29][30] , the most ubiquitous species that can be found in the natural pore environmentswater, has largely been ignored. Although the dynamics of confined water has been studied extensively 31,32 , the effect of water on the dynamics of another confined species remains largely unexplored.…”

Confinement Effects on Carbon Dioxide Methanation: A Novel Mechanism for Abiotic Methane Formation

“…Finally, we explored the effect of the pore surface morphology/roughness by inserting two step-edges along the Y-direction 31 (hence creating a structured pore). The step edges are chosen as examples of small defects on the pore surface.…”

Structure and Dynamics of Confined C-O-H Fluids Relevant to the Subsurface: Application of Magnetic Resonance, Neutron Scattering, and Molecular Dynamics Simulations

“…Independent MD simulation studies have also confirmed that CO 2 enhances the diffusion of confined hydrocarbon. Le et al (2015Le et al ( , 2016 have shown that presence of CO 2 enhances the diffusivity of n-butane and n-octane in silica slit nanopores, by replacing the hydrocarbons from the pore walls thereby making them free to move through the pore network.…”