Experimental Determination of the P–T Melting Curve of Argon

Experimental Method for the Determination of the Saturation Vapor Pressure above Supercooled Nanoconfined Liquids

For sorption studies, the saturation vapor pressure p ₀ above an adsorbate is of great significance. For example, it is needed for the determination of the pore size distribution, the Laplace pressure, and the chemical potential. Above the bulk triple point, T ₃ ^bulk, this pressure is identical with the saturation vapor pressure above the bulk liquid. However, below T ₃ ^bulk, the correct value of p ₀(T) is controversial. Nanoconfined fluids exhibit a shift of the freezing and melting temperatures in comparison to the bulk state. Thus, the adsorbed fluid is supercooled in a certain temperature range below T ₃ ^bulk. Here, we show that it is possible to determine the appropriate saturation vapor pressure above the nanoconfined supercooled liquid experimentally. For this purpose, we have performed sorption measurements with liquid argon in nanoporous Vycor glass in the temperature range of the supercooled liquid and at temperatures above the bulk triple point. In order to determine the unknown and temperature-dependent saturation vapor pressure of the supercooled confined adsorbate, p ₀(T), we use the Kelvin equation relating this quantity to the pore radius, r _P(p ₀), that is independent of temperature. The knowledge of the absolute values for the liquid-vapor surface tension of the supercooled adsorbate, γ_lv(T), is not required. However, we presuppose that its dependence on the unknown vapor pressure, γ_lv(p ₀), is bulk-like. Our results indicate that the saturation vapor pressure above the supercooled nanoconfined liquid corresponds to that above supercooled bulk argon (i.e., to the pressure obtained by an extension of the usual vaporization curve to T < T ₃ ^bulk). We expect that this method can be used for the determination of p ₀ above other supercooled adsorbates.

Atomistic study of the solid state inside graphene nanobubbles

A two-dimensional (2D) material placed on an atomically flat substrate can lead to the formation of surface nanobubbles trapping different types of substances. In this paper graphene nanobubbles of the radius of 7-34 nm with argon atoms inside are studied using molecular dynamics (MD). All modeled graphene nanobubbles except for the smallest ones exhibit an universal shape, i.e., a constant ratio of a bubble height to its footprint radius, which is in an agreement with experimental studies and their interpretation using the elastic theory of membranes. MD simulations reveal that argon does exist in a solid close-packed phase, although the internal pressure in the nanobubble is not sufficiently high for the ordinary crystallization that would occur in a bulk system. The smallest graphene bubbles with a radius of 7 nm exhibit an unusual "pancake" shape. Previously, nanobubbles with a similar pancake shape were experimentally observed in completely different systems at the interface between water and a hydrophobic surface.

The Lennard-Jones melting line and isomorphism

The location of the melting line (ML) of the Lennard-Jones (LJ) system and its associated physical properties are investigated using molecular dynamics computer simulation. The radial distribution function and the behavior of the repulsive and attractive parts of the potential energy indicate that the ML is not a single isomorph, but the isomorphic state evolves gradually with temperature, i.e., it is only "locally isomorphic." The state point dependence of the unitless isomorphic number, X̃, for a range of static and dynamical properties of the LJ system in the solid and fluid states, and for fluid argon, are also reported. The quantity X̃ typically varies most with state point in the vicinity of the triple point and approaches a plateau in the high density (temperature) limit along the ML.

Homogeneous nucleation and droplet growth in supersaturated argon vapor: The cryogenic nucleation pulse chamber

We built a cryogenic nucleation pulse chamber for measuring homogeneous nucleation rates of argon. First measurements show that the growth rate of argon droplets at nucleation conditions is rather high so that nucleation and growth could not yet be decoupled. Nevertheless, the experiments permit an estimate of the onset of nucleation corresponding to a nucleation rate of J=10(7(+/-2)) cm(-3) s(-1) at temperatures 52<TK<59 and supersaturations around S approximately 10. Despite their preliminary nature these experiments indicate a severe failure of the classical nucleation theory, which predicts nucleation rates on the order of 10(-28)-10(-13) cm(-3) s(-1) for the quoted conditions. Recent calculations based on density functional theory can only partially explain the discrepancy. In addition to the first nucleation experiments, we obtained and analyzed growth curves for argon droplets from constant angle Mie-light scattering. The good agreement of the experimental growth curves with model calculations according to Fuchs and Sutugin [Highly Dispersed Aerosols (Ann Arbor Science, Ann Arbor, MI, 1970)] permits a near-quantitative description of the experimental light-scattering signal. The procedure provides an estimate for the number density of the droplets along with a measure of their polydispersity.