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

The benchmark for\naccurate density measurements is gravimetric\nmethods. However, measurements taken near the saturated-vapor line\nare affected by sorption effects, which increase as the saturation\nline is approached. Thus, a correction of the experimental data is\nrequired that accounts for the adsorbate density. To address this\nproblem, molecular dynamics simulations were performed to investigate\nthe subcritical adsorption of ethane and propane on gold surfaces.\nThe investigated temperature range was 263.15–293.15 K for\nethane and 283.15–343.15 K for propane, with densities up to\nthe saturated vapor line. To demonstrate that the simulations are\nexpedient, we conducted accurate gravimetric adsorption measurements\nfor both fluids using a novel four-sinker magnetic suspension densimeter.\nMeasurements were taken at temperatures from 283 to 293 K for ethane\nand at 293 K for propane, with pressures up to saturation pressure.\nThe measured adsorption loads range between 0.037 and 0.73 μg\ncm<sup>–2</sup> reported with combined expanded uncertainties\n(<i>k</i> = 2) ranging from 0.139 to 0.207 μg cm<sup>–2</sup>. The simulations clearly indicate that the adsorbate\ndensity increases as it nears the phase boundary and is largely unaffected\nby temperature variations within the studied range. On the basis of\nthese findings, we propose a simple, empirical model to calculate\nthe density of the adsorbate for subcritical states on nonporous gold\nsurfaces, contributing a valuable tool for future experimental considerations\nof sorption phenomena in gravimetric density measurements. Part of\nfurther studies is the validation of this model for applications to\nother surface materials.