Direct Measurements of Pore Fluid Density by Vibrating Tube Densimetry

et al. 2015

Molecular Simulation

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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.

Section: Introductionsupporting

confidence: 87%

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

Ogbe

et al. 2015

Molecular Simulation

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“…We previously conducted extensive molecular dynamics (MD) simulations to study propane adsorption, structure and diffusion in slit-shaped silica pores at sub-, near-, and super-critical conditions. 28 The results were qualitatively consistent with the experimental adsorption isotherms reported by Gruszkiewicz et al, 29 and the SANS data reported by Rother et al 30 We recently conducted MD simulations for the structure and dynamics of CO 2 -octane mixtures confined within slit-shaped silica pores (unpublished results). Building on these efforts, we present here structural (i.e., density profiles, molecular orientation, and preferential adsorption sites) and dynamic properties (i.e., self-diffusion coefficients and residence times at contact with the solid surface) for mixtures containing n-butane and CO 2 confined in slit-shaped silica pores.…”

Section: Introductionsupporting

confidence: 86%

CO₂–C₄H₁₀ Mixtures Simulated in Silica Slit Pores: Relation between Structure and Dynamics

Cole

2015

J. Phys. Chem. C

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Equilibrium molecular dynamics simulations were conducted for pure n-butane and for mixtures containing n-butane and carbon dioxide confined in 2 nm wide slit-shaped pores carved out of cristobalite silica. A range of thermodynamic conditions was explored, including temperatures ranging from sub-critical to super-critical, and various densities. Preferential adsorption of carbon dioxide near the -OH groups on the surface was observed, where the adsorbed CO 2 molecules tend to interact simultaneously with more than one -OH group. Analysis of the simulation results suggests that the preferential CO 2 adsorption to the pore walls weakens the adsorption of n-butane, lowers the activation energy for n-butane diffusivity, and consequently enhances n-butane mobility. The diffusion results obtained for pure CO 2 are consistent with strong adsorption on the pore walls, as the CO 2 self-diffusion coefficient is low at low densities, increases with loading, and exhibits a maximum as the density is increased further because of hindrance effects. As the temperature increases, the maximum in self-diffusion coefficient is narrower, steeper and shifted to lower loading. The simulation results are also quantified in terms of molecular density profiles for both butane and CO 2 and in terms of residence time of the various molecules near the solid substrate. Our results could be useful for designing separation devices and also for better understanding the behavior of fluids in sub-surface environments.

“…In the present work, we use equilibrium MD simulations to study propane adsorption isotherms in slitshaped silica pores. The results are used to qualitatively interpret the experimental adsorption isotherms reported by Gruszkiewicz et al (Gruszkiewicz et al, 2012) and to support part of the insights obtained using SANS by Rotheret et al (Rother et al, 2007). We report details concerning structural (i.e., density profiles and molecular orientation) and dynamic properties (i.e., self-diffusion coefficients and residence time at contact with the pore surface) of confined propane.…”

Section: Introductionsupporting

confidence: 60%

“…Up to 100 kPa, their results are consistent with type I isotherms (Rouquerol et al, 1994;Sing et al, 1985). In 2012, Gruszkiewicz et al (Gruszkiewicz et al, 2012) used the vibrating tube densimeter to measure propane adsorption isotherms in hydrophobic silica aerogels with average pore size between 7 and 9 nm (although it is possible that the materials used had pores of size as large as 15 nm). Their results were interpreted as a function of the excess adsorption.…”

Section: Introductionmentioning

confidence: 71%

“…In Fig. 3 we reproduce, for comparison, the experimental data by Gruszkiewicz et al (Gruszkiewicz et al, 2012) for the adsorption isotherms for propane in silica aerogels, panel (a), and the corresponding excess adsorption data, panel (b). These experimental results were obtained at 343, 368, and 370 K. These data will be used for qualitative comparison against the simulation results discussed below.…”

Section: Resultsmentioning

confidence: 95%

See 1 more Smart Citation

Propane simulated in silica pores: Adsorption isotherms, molecular structure, and mobility

Chemical Engineering Science

Cole

2015

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Molecular dynamics simulations were conducted for bulk propane in contact with fully protonated slitshaped silica pores. The effective pore width was of either 0.8 or 2.7 nm. The temperature was set at 343, 368, and 373K. The TraPPE-UA and CLAYFF force fields were implemented to model propane and silica, respectively. Each individual simulation yields the density of confined fluid as a function of the bulk pressure. For a given temperature, adsorption isotherms were estimated by repeating the simulations at various bulk pressures. The results qualitatively agree with available experimental data; namely, at fixed temperature the excess sorption is found to show a maximum near the pressure at which the pores fill; at fixed pressure the excess adsorption is found to decrease as the temperature increases and as the pore width expands. At equilibrium, pronounced layering was observed for propane near the pore surface, especially in the narrower pore and at the highest densities considered. The propane molecules at contact with silica tend to lay with their CH 3-CH 3 vector parallel to the pore surface. The mean square displacement as a function of time was used to quantify the self-diffusion coefficient of confined propane as a function of temperature, pressure and pore width. These results will be useful for enhancing the interpretation of experimental data.