Experimental investigation of gravity-induced concentration gradients in critical mixtures

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Abstract
The building up of concentration gradients in a critical mixture of aniline and cyclohexane is observed. A non-perturbative technique that allows the observation of the gradients along the entire size of the sample is used. The experiments are performed at three temperatures, i.e. (T-Tc)/Tc=1.8*10-3, 6.0*10-4 and 2.3*10-4. The experimental results allow the calculation of the magnitude of the barodiffusion coefficients and of the osmotic compressibility, as well as their critical behaviour. In addition, the Onsager coefficient can also be evaluated. The results are discussed in the framework of existing theories. It is shown that the observed values are close to theoretical predictions both in magnitude and temperature dependence.