The hydrophobic behaviour of Orange IV in water and in aqueous electrolyte solutions

Abstract

Unlike the stepwise manner of self-aggregation shown by homologues of the surface-active azo dye Methyl Orange in aqueous solution, cooperative self-aggregation is shown in electrolyte solutions of Orange IV (C₆H₅—NH—[graphic omitted]—NN—[graphic omitted] SO^–₃Na⁺). Evidence of a critical micellar concentration (c.m.c.) appears from the breaks in the plots of absorbance and surface tension values against the dye concentration.

The quantitative treatment of the effects of NaCl and SrCl₂ reveals that the equation log (c.m.c.)=–KC_salt+constant, derived for zwitterionics by Mukerjee (J. Phys. Chem., 1965, 69, 4038) on the basis of the salting-out of their hydrocarbon groups, is only obeyed at lower salt concentrations. An equation log (c.m.c.)=–K log C_salt+constant fits our experimental results over the whole range of salt concentrations considered and remains valid when 1.0 mol dm^–3 urea, a well known disturber of hydrophobic bonds, is introduced.