Negative Gas−Solid Adsorption and Its Influence on Wettability

Abstract

For the past 60 years it has been widely accepted among surface chemists involved with contact angle and wettability that exposure of an evacuated solid surface to any vapor will reduce the free energy of that solid surface. We show that this is a misconception which arises from the use of absolute adsorption in the free energy equation. When the adsorption is formulated as a “surface-excess” it is apparent that the free energy change on adsorption can be positive or negative, depending on whether the adsorption is negative or positive. We point out that evidence of the existence of negative adsorption in gas−solid interaction has been accumulating. Specifically we show that exposure of an evacuated Teflon (PTFE) surface to water vapor may result in negative adsorption which, of course, indicates that it has undergone an increase in surface free energy.