Abstract
Tension at the surface is a most fundamental physicochemical property of a liquid surface. The concept of surface tension has widespread implications in numerous natural, engineering and biomedical processes. Research to date has been largely focused on the liquid side; little attention has been paid to the vapor—the other side of the surface, despite over 100 years of study. However, the question remains as to whether the vapor plays any role, and to what extent it affects the surface tension of the liquid. Here we show a systematic study of the effect of vapor on the surface tension and in particular, a surprising observation that the vapor, not the liquid, plays a dominant role in determining the surface tension of a range of common volatile organic solutions. This is in stark contrast to results of common surfactants where the concentration in the liquid plays the major role. We further confirmed our results with a modified adsorption isotherm and molecular dynamics simulations, where highly structured, hydrogen bonded networks, and in particular a solute depletion layer just beneath the Gibbs dividing surface, were revealed.
Highlights
Surface tension is a macroscopic, thermodynamic manifestation of molecular structure and interaction at a surface; it relates to all other physical and chemical properties
These results suggest that initially the surface tension is controlled by a combination of the liquid and vapor phase concentrations, whereas at the final steady-state the surface tension is determined primarily by the vapor phase
We have shown that the vapor phase, or adsorption from the vapor phase, represents a significant dynamic affecting the aqueous surface tension of this class of rather common volatile organic molecules, at the final steady-state or ‘‘equilibrium’’ where it seems to be the primary factor
Summary
Surface tension is a macroscopic, thermodynamic manifestation of molecular structure and interaction at a surface; it relates to all other physical and chemical properties. Surface tension has been studied extensively for over a century, in the fields of colloid and surface chemistry, most research focuses on the effect of the liquid phase surfactant concentration, perhaps because of the much lower density of surfactant in the vapor phase [7,8,9,10,11,12,13,14,15] Even though both liquid and vapor phase adsorption are examined in many physical chemistry textbooks, they are almost always considered exclusive of one another. Previous studies have reported that aqueous alcohol solutions are susceptible to errors during surface tension measurements due to solute evaporation into the vapor phase [8,9] and there have been numerous studies on these systems [10,11,12,13,14,15], the possible influence of the vapor phase on the liquid surface tension has not been considered carefully in designed experiments
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