The surface tension of ultrapure water in its pure vapour

Abstract

The surface tension of pure water has been measured for more than a century using a wide range of experimental techniques. Despite this long history, reported surface tension values are not always in agreement; instead, they are distributed over a wider range than would be expected based on the stated accuracies of individual measurements. This discrepancy suggests the presence of systematic effects, such as contamination, that may be responsible for the observed deviations. Another unresolved issue in this context is the explanation of the Jones-Ray effect; the decrease of the surface tension of water for low concentrations of added salt. While some researchers attribute this effect to surface-active impurities, others argue that it represents a fundamental property of water itself. To gain more insight into these questions, an ultra-high vacuum (UHV) system for surface tension measurements was built in the surface-science group at TU Wien. Pendant drop tensiometry was selected as the measurement technique because it doesn't require any correction factors to calculate the surface tension and is well suited for operation in a UHV system. The aim of this diploma project is to use this system to perform highly accurate surface tension measurements of ultra-pure water in its own vapour. To enhance measurement accuracy, pneumatic vibration isolators were installed and the optical system was calibrated. Surface tension measurements were conducted over a range of temperatures, and after carefully selecting the results from the most stable pendant drops, the measured surface tension was found to be approximately 0.25 mN /m higher than the value generated from historical data. This result could indicate a contamination with surfactants in many historical experiments and can provide new insights into the Jones-Ray effect.