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.