Molecular clustering in binary liquid mixtures

The earlier investigations on the intensity and depolarisation of the transversely scattered light in liquid mixtures in the neighbourhood of the critical solution temperature are briefly reviewed. It is pointed out that it is important to make comparative studies of the state of polarisation of the transversely scattered light with the incident light in different states of polarisation in order to get a correct idea of the state of dispersion of the scattering elements in the medium. Measurements of the depolarisation ρu, ρv and ρh were made, using, respectively, incident unpolarised light and incident light polarised with vibrations vertical and horizontal, for mixtures of eight different polarised with vibrations vertical and horizontal, for mixtures of eight different pairs of liquids over a range of temperature of about 30° above the critical solution temperature. The results obtained fully confirm the author's preliminary report regarding the existence of clusters in liquid mixtures, not only at the critical solution temperature but also at temperatures considerably removed from it. The validity for such mixtures of the optical reciprocity principle has been verified experimentally. It is also shown that the observed values of ρ u, ρ v and ρh satisfy the relation ρ u=)1+1/ρh)/(1+1/ρv) The X-ray evidence regarding the state of dispersion of the molecules in liquid mixtures of the type phenol and water is in agreement with the optical evidence presented in this paper. The bearing of the experimental results on the various theories of critical opalescence including those which have been put forward subsequent to the work of Smoluchowski and Einstein is also discussed in the paper