Image-plane measurements of coexistence curve diameters

The diameter of the coexistence curve has been measured for several fluids to determine differences from linear form. The experimental method consists of optical interference measurements in a Mach-Zehnder interferometer supplemented by measurements of deviation in a hollow prism. The fluid sample is contained in a temperature-stabilized cell in one branch of the interferometer. A variation in cell temperature causes the density profile within the cell to change, resulting in a change of the interference pattern, which is monitored photographically. From the relation of this pattern at any temperature to the pattern at the critical temperature, information on the refractive indices of the liquid and vapor phases is obtained. This information is combined with measurements of the Lorentz-Lorenz coefficient to obtain liquid and vapor densities along the coexistence curve. The average of liquid and vapor densities is analyzed in terms of RG theory. The results of the experiment yield information on three body interactions. Studies have been completed on ethane, ethylene, hydrogen, and fluoroform.