Interfacial Tension of Aqueous Electrolyte Solutions: Ion‐Free Layer

X-ray Reflectivity Reveals a Nonmonotonic Ion-Density Profile Perpendicular to the Surface of ErCl3 Aqueous Solutions

“…Thus, it is argued that the relatively low electron density measured in the surface region as compared to the bulk aqueous solution is consistent with a depletion of Er 3+ ions in this region. The surface reflectivity results and prediction from the Gibbs adsorption equation for such a depletion layer are consistent with the surface tension measurements on these samples, which increase with increasing solute concentration …”

“…Surface tension values of 72.6, 73.6, 74.6, and 76.0 mN/m (±0.3 mN/m) were measured, respectively, from the 0.20, 0.50, 0.74, and 1.0 M ErCl 3 aqueous solutions at room temperature (∼23 °C). The Gibbs adsorption equation indicates that an increase of surface tension with concentration is due to a negative adsorption . Although our reflectivity analysis indicates that Er ions form a well-defined subsurface layer (or layers) whose density increases with concentration, it also reveals the formation of a surface layer depleted of Er ions.…”

“…Through studies in a wide variety of important interfacial systems, spanning atmospheric aerosols, solvent-extraction processes for metal separations into contaminant transport in the environment, , it has become clear that chemistry occurring at the aqueous interface can be markedly different from that seen in the bulk . Among early studies, surface tension measurements performed at the vapor interface of aqueous electrolyte solutions demonstrated higher values than measured for pure water, as well as an increase in the surface tension with increasing electrolyte concentration . The Gibbs adsorption equation interprets this variation of tension with concentration as a negative adsorption of ions at the solution/vapor interface.…”