Determination of the aggregation number of anionic dyes by studies of deviation from beer's law

Abstract

The aggregation of four anionic azo dyes has been studied spectrophotometrically at high aqueous concentrations, using a cell of very small path length. The dyes all show apparent deviations from Beer's law and, using narrow particle size fractions of an azo pigment, apparent deviations from the law occur only when the particle size distribution of the absorbing species changes with concentration. On the basis of the stacking of the individual dye molecules behind one another to form aggregates, a theory has been developed relating the apparent deviation from Beer's law to the degree of aggregation, and average molecular weights have been evaluated for the four dyes.