Oxygen in Nanopores: A Study on the Elastic Behavior of Its Solid Phases

2018

Langmuir

Nitrogen adsorption is one of the main characterization techniques for nanoporous materials. The experimental adsorption isotherm provides information about the surface area and pore size distribution (PSD) for a sample. In this work we show that additional insight into PSD can be gained when the speed of sound propagation through a sample is measured during nitrogen adsorption experiment. We analyzed published experimental data on ultrasound propagation through a nanoporous Vycor glass sample during nitrogen adsorption experiment. Next, we calculated the change of the longitudinal and shear moduli of the sample as a function of relative vapor pressure. From this, we show that the shear modulus of the sample does not change upon filling the pores, evidencing that adsorbed nitrogen at 77 K has zero shear modulus, similarly to a bulk liquid. The longitudinal modulus of the sample behaves differently: it changes abruptly at the capillary condensation and keeps gradually increasing thereafter. We performed Monte Carlo molecular simulations to predict the compressibility of adsorbed nitrogen and then calculated the longitudinal modulus of the nitrogen-saturated Vycor using the Gassmann equation. Our theoretical predictions nicely matched the longitudinal modulus derived from the experimental data. Additionally, we performed molecular simulations to model nitrogen adsorbed in silica pores of sizes ranging from 2 to 8 nm. We found that the isothermal elastic modulus of adsorbed nitrogen depends linearly on the inverse pore size. This dependence, along with the proposed recipe for probing the modulus of adsorbed nitrogen, sets up the grounds for extracting additional information about the porous samples, when the nitrogen adsorption is combined with ultrasonic experiments.

Section: ■ Discussionsupporting

confidence: 92%

Molecular Simulations Shed Light on Potential Uses of Ultrasound in Nitrogen Adsorption Experiments

Maximov

2018

Langmuir

“…Figure (left) shows the structure of Vycor based on a combination of experimental characterization techniques (Levitz et al, ). Vycor has high porosity and narrow pore size distribution; these features made it widely used as a medium for studying confinement effects for various solid and fluid phases (Egorov et al, ; Filippov et al, ; Gruener et al, ; Soprunyuk et al, ; Schappert et al, , ; Uskov et al, ). Note also that combination of various experimental methods used to calculate the porosity of Vycor suggests that Vycor does not have isolated (occluded) porosity (Levitz et al, ; Page et al, ).…”

Section: Methodsmentioning

confidence: 99%

Gassmann Theory Applies to Nanoporous Media

Geophysical Research Letters

Gurevich

2018

Recent progress in extraction of unconventional hydrocarbon resources has ignited the interest in the studies of nanoporous media. Since many thermodynamic and mechanical properties of nanoscale solids and fluids differ from the analogous bulk materials, it is not obvious whether wave propagation in nanoporous media can be described using the same framework as in macroporous media. Here we test the validity of Gassmann equation using two published sets of ultrasonic measurements for a model nanoporous medium, Vycor glass, saturated with two different fluids, argon, and n‐hexane. Predictions of the Gassmann theory depend on the bulk and shear moduli of the dry samples, which are known from ultrasonic measurements and the bulk moduli of the solid and fluid constituents. The solid bulk modulus can be estimated from adsorption‐induced deformation or from elastic effective medium theory. The fluid modulus can be calculated according to the Tait‐Murnaghan equation at the solvation pressure in the pore. Substitution of these parameters into the Gassmann equation provides predictions consistent with measured data. Our findings set up a theoretical framework for investigation of fluid‐saturated nanoporous media using ultrasonic elastic wave propagation.

“…Recently Schappert and Pelster used ultrasonic measurements to study the changes of elastic properties of fluid and solid phases of argon, nitrogen, and oxygen confined in nanoporous materials at low temperatures [4,[18][19][20][21]. Since argon is one of the simplest systems for molecular simulations, these works stimulated the development of macroscopic [22], and molecular modeling approach to the calculation of elastic properties of confined fluids [23][24][25].…”

Section: Introductionmentioning

confidence: 99%

Effect of pore geometry on the compressibility of a confined simple fluid

Dobrzanski

Maximov

The Journal of Chemical Physics

2018

Fluids confined in nanopores exhibit properties different from the properties of the same fluids in bulk; among these properties is the isothermal compressibility or elastic modulus. The modulus of a fluid in nanopores can be extracted from ultrasonic experiments or calculated from molecular simulations. Using Monte Carlo simulations in the grand canonical ensemble, we calculated the modulus for liquid argon at its normal boiling point (87.3 K) adsorbed in model silica pores of two different morphologies and various sizes. For spherical pores, for all the pore sizes (diameters) exceeding 2 nm, we obtained a logarithmic dependence of fluid modulus on the vapor pressure. Calculation of the modulus at saturation showed that the modulus of the fluid in spherical pores is a linear function of the reciprocal pore size. The calculation of the modulus of the fluid in cylindrical pores appeared too scattered to make quantitative conclusions. We performed additional simulations at higher temperature (119.6 K), at which Monte Carlo insertions and removals become more efficient. The results of the simulations at higher temperature confirmed both regularities for cylindrical pores and showed quantitative difference between the fluid moduli in pores of different geometries. Both of the observed regularities for the modulus stem from the Tait-Murnaghan equation applied to the confined fluid. Our results, along with the development of the effective medium theories for nanoporous media, set the groundwork for analysis of the experimentally measured elastic properties of fluid-saturated nanoporous materials.