Ultrasonic Study of Water Adsorbed in Nanoporous Glasses

TL;DR

This study investigates how nanoconfinement affects water's compressibility by measuring ultrasonic wave speeds in nanoporous glasses, revealing that confined water is significantly stiffer than bulk water, with implications for various scientific fields.

Contribution

The paper introduces a method combining ultrasonic measurements and Gassmann theory to quantify the increased stiffness of water confined in nanopores, highlighting a linear relation with Laplace pressure.

Findings

Confined water has a higher bulk modulus than bulk water at the same temperature.

The bulk modulus of confined water increases linearly with Laplace pressure.

Confined fluids are generally stiffer than their bulk counterparts, regardless of intermolecular forces.

Abstract

Thermodynamic properties of fluids confined in nanopores differ from those observed in the bulk. To investigate the effect of nanoconfinement on water compressibility, we performed water sorption experiments on two nanoporous glass samples while concomitantly measuring the speed of longitudinal and shear ultrasonic waves in these samples. These measurements yield the longitudinal and shear moduli of the water laden nanoporous glass as a function of relative humidity that we utilized in the Gassmann theory to infer the bulk modulus of the confined water. This analysis shows that the bulk modulus (inverse of compressibility) of confined water is noticeably higher than that of the bulk water at the same temperature. Moreover, the modulus exhibits a linear dependence on the Laplace pressure. The results for water, which is a polar fluid, agree with previous experimental and numerical data reported for non-polar fluids. This similarity suggests that irrespective of intermolecular forces, confined fluids are stiffer than bulk fluids. Accounting for fluid stiffening in nanopores may be important for accurate interpretation of wave propagation measurements in fluid-filled nanoporous media, including in petrophysics, catalysis, and other applications, such as in porous materials characterization.