Measurements of Surface Tension of Mixtures

C.D. Holcomb, S.L. Outcalt, and M.O. McLinden

Objective: To extend significantly the accuracy, the temperature range, and the pressure range of surface tension measurements for mixtures by creating a unique phase equilibrium apparatus with the implementation of a non-visual, non-mechanical method of measuring the surface tension with the simultaneous measurement of the coexisting densities.

Problem: There are three major problems that have prevented the accurate measurement of the surface tension of mixtures over wide ranges of temperature and pressure. The first involves the difficulty of the type of method selected. The more difficult and time consuming the measurement method, the greater are the sources of measurement error. Capillary rise, maximum bubble pressure, Wilhelmy plate, Du Noüy ring, pendant drop, and sessile drop methods all require visual measurements of a height, width, and/or depth to determine accurately the surface tension. These methods require a person or camera and software to make the measurement which can introduce human, optical, or round-off errors. The second problem involves mechanical manipulation of the measurement technique. Capillary rise, Wilhelmy plate, and Du Noüy ring methods all require the measurement device to be submerged in the liquid and then withdrawn. Mechanical manipulations reduce the operating pressure of the system and add experimental complexity to the measurement of the surface tension. Finally, all methods of measuring the surface tension require knowledge of the densities of the coexisting liquid and vapor phases. Currently, the densities are either estimated or calculated from equations of state. By not measuring the densities directly, especially for more complex mixtures, uncertainties in the density prediction increase the uncertainty in the surface tension.

Approach: All three of these major problems have been eliminated in our approach which uses the differential-bubble pressure method of measuring surface tension and two vibrating tube densimeters for measuring the coexisting densities. These are incorporated into a dual-recirculation-loop high-pressure phase equilibrium apparatus that operates between 223 K and 423 K. The differential bubble pressure method is a variation of the maximum bubble pressure method that eliminates the need for a visual measurement of the depth of submersion of the dip tube. Two dip tubes of different radii are submerged in the liquid to the same depth. The difference in the maximum bubble pressures of the two dip tubes is related to the surface tension, but the pressure effect determined from the depth of submersion of the tubes is canceled. Second, the method does not require any mechanical manipulations. The tubes are mounted in a fixed position and only require that the liquid level is high enough to cover the ends of the tubes. Finally, vibrating tube densimeters are mounted in the two recirculation loops of the phase equilibrium apparatus and are used to measure the densities of the coexisting phases. This eliminates the need for predictions or an equation of state to estimate the densities. The temperature, pressure, and compositions of the phases are recorded as part of the basic phase equilibrium measurement.