Liquid Argon in Nanopores: The Impact of Confinement on the Pressure Dependence of the Adiabatic Longitudinal Modulus

The elastic properties of nanoconfined argon show deviations from the behavior of bulk argon. In this paper, we study the pressure dependence of the adiabatic longitudinal modulus beta(Ar,ads) of the adsorbate in nanoporous Vycor glass. Adsorption induces a normal pressure Delta p(S) acting on the pore surface. As it is well known, this pressure leads to a deformation of the porous sample, but it also influences the elastic properties of the adsorbate. Ultrasonic measurements reveal a linear relation between beta(Ar,ads) and the Laplace pressure, p(L), which is a major contribution to Delta p(S). For the whole temperature range of the confined liquid argon (76-86.9 K), the proportionality constant, alpha(Ar,ads) = d beta(Ar,ads)/dp(L), is higher than that for bulk argon, i.e., the adsorbed argon exhibits a stronger dependence on pressure than bulk argon. In addition, we observe no temperature dependence of alpha(Ar,ads), which contrasts with the clear increase of alpha(Ar,buik) with increasing temperature. Further measurements and simulations can show how the interaction strength between adsorbate and pore surface contributes to the observed effects.