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

Document Type

Article

Publication Date

12-26-2018

Abstract

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.

Identifier

85059526543 (Scopus)

Publication Title

Langmuir

External Full Text Location

https://doi.org/10.1021/acs.langmuir.8b02909

e-ISSN

15205827

ISSN

07437463

PubMed ID

30475628

First Page

15650

Last Page

15657

Issue

51

Volume

34

This document is currently not available here.

Share

COinS