The Variation of Surface Tension and Contact Angle under Applied Pressure of Dissolved Gases, and the Effects of These Changes on the Rate of Bubble Nucleation

Abstract

The surface tension and contact angle of water in the presence of dissolved carbon dioxide, and of cyclohexane in the presence of dissolved ethene have been determined as a function of the applied absolute pressures in the range 1–11 bar. At these low pressures, the effect of the applied pressure itself is unimportant, and the significant changes we observe are due to the adsorption of the dissolved gases, each of which is easily soluble in its respective solvent. We have designed and built a novel apparatus to enable us to measure both contact angle and surface tension under pressure, the former with a precision of about ±2°, and the latter to ±0.2 mN m−1. The surface tension of water changes from about 72 mN m−1to about 57 mN m−1as the pressure of CO2is increased from 1 to 11 bar absolute; similarly, the surface tension of cyclohexane changes from about 27 mN m−1to about 16 mN m−1as the pressure of ethene is increased from 1 to 9 bar absolute. Over the same ranges of pressure, the advancing contact angle at the three-phase line between gas/liquid and 316 stainless steel, changes from 77° to 44° for the water/CO2system, and from 28° to 10° for the cyclohexane/ethene system. The receding contact angle measurements were unreliable for reasons discussed in the text but in all cases were considerably lower than the advancing angles under similar external conditions. The adsorption of the gases at the respective liquid/vapor interfaces are discussed in terms of the Gibbs adsorption equation, and it is shown that for the adsorption of CO2at the water/gas interface, there is little if any change in the adsorbed amount, Γ2,1, in the pressure range used here. For ethene, the adsorbed amount is a strong function of the applied pressure.J, the rate of heteronucleation of bubbles, was measured as a function of the saturation ratio, and the results are interpreted in terms of current theories. It is shown that for CO2on stainless steel, the results can be interpreted in terms of modified surface tension and contact angle, but for ethene on the same substrate, the topography of the nucleating site has to be taken into account.