Osmometric Study of Aggregation in Dye Solutions*

Most dyes constitute a class of electrolytes display ing peculiar properties in their relationship with water. There is a great deal of evidence that dye ions aggregate in aqueous solutions.15 Several methods have been proposed for the study of this phenomenon. Although very promising, vapour pressure osmometry has been almost completely neglected. Milicevic and Eigenmann6 reported the values of activity coefficients of two simple acid dyes obtained by this method. Supposing that there is no specific counterion binding and using the Debye-Huckel limiting law, they have shown that Orange II, in contrast to its isomer Orange VIII, is not appreciably dimerized. In our work, the method was applied for the determination of the extent of aggregation in solutions of two direct and two acid dyes, where higher aggregates were also expected. Using the spherical cell model of collo·id solutions,110 we were able to estimate the mean aggregation numbers of the four dyes studied.