Density of the Subcritical Adsorbate on Gold Surfaces: A Generic Empirical Model

The benchmark for accurate density measurements is gravimetric methods. However, measurements taken near the saturated-vapor line are affected by sorption effects, which increase as the saturation line is approached. Thus, a correction of the experimental data is required that accounts for the adsorbate density. To address this problem, molecular dynamics simulations were performed to investigate the subcritical adsorption of ethane and propane on gold surfaces. The investigated temperature range was 263.15–293.15 K for ethane and 283.15–343.15 K for propane, with densities up to the saturated vapor line. To demonstrate that the simulations are expedient, we conducted accurate gravimetric adsorption measurements for both fluids using a novel four-sinker magnetic suspension densimeter. Measurements were taken at temperatures from 283 to 293 K for ethane and at 293 K for propane, with pressures up to saturation pressure. The measured adsorption loads range between 0.037 and 0.73 μg cm–2 reported with combined expanded uncertainties (k = 2) ranging from 0.139 to 0.207 μg cm–2. The simulations clearly indicate that the adsorbate density increases as it nears the phase boundary and is largely unaffected by temperature variations within the studied range. On the basis of these findings, we propose a simple, empirical model to calculate the density of the adsorbate for subcritical states on nonporous gold surfaces, contributing a valuable tool for future experimental considerations of sorption phenomena in gravimetric density measurements. Part of further studies is the validation of this model for applications to other surface materials.