Structure of the cybotactic region. A determining factor in the spectral behaviour of some azo-dyes in the presence of indifferent electrolytes

Abstract

The previously reported metachromatic effect of the addition of indifferent electrolytes to aqueous solutions of Methyl Orange (I) has been observed in other azo-dyes bearing the same chromophoric group. The extent of the effect depends on the nature of the auxochromic groups present, which also determine the structure of the cybotactic region around the dyes (i.e. the degree of hydration).

Such induced metachromatic behaviour is not observable in organic solvents, irrespective of their hydrogen-bonding ability. Nevertheless, it appears from a correlation found between the experimental data and the solvent parameters of different aqueous + non-aqueous binary solvents that the shielding effect of the organic species is not the result of preferential solvation but may be attributed to properties inherent in the structure of the cybotactic region of the dyes.