Aggregation of rose bengal ethyl ester induced by alkali metal cations in aqueous solution

Abstract

The spectrum of Rose Bengal C-2′ ethyl ester, C-6 sodium salt dimer in 2% ethanol has been observed in the presence of alkali metal cations. It is proposed that aggregation occurs in two steps: (a) formation of a neutral complex between the anion of the dye and the alkali metal cation and (b) reaction between the neutral complex and a second dye anion to produce the dimeric species. Both equilibrium constants, as well as the geometry of the dimer, are sensitive to the identity of the cation. The equilibrium constant for the first step ranges from 3.37 M −1 for Li + to 257 M −1 for Rb + and that for the second step ranges from 2.81 × 10 5 M −1 for Li + to 1.05 × 10 6 M −1 for Cs +. The driving force of the first reaction is purely electrostatic whereas the dimerization step is controlled mainly by the hydrophobicity of the dye anion.