Ion Association Reaction between Divalent Anionic Azo Dyes and Hydrophobic Quaternary Ammonium Ions in Aqueous Solution as Studied by Capillary Zone Electrophoresis

Ion association properties of four kinds of anionic divalent azo dyes were investigated in an aqueous solution containing hydrophobic quaternary ammonium ions. The dyes used in this study possessed a hydroxy and a sulfonate group, which existed as a divalent anion in an alkaline solution. In capillary electrophoresis measurements, the apparent electrophoretic mobility of the dyes decreased along with an increase in the concentration of quaternary ammonium ion (Q(+)) in a migrating solution. Ion association constants of the dyes with Q(+), K(ass), were determined by a simulation method of the mobility change using a non-linear least square method, when 1:1 ion associates formed. The increase in log K(ass) was about 0.07 log unit per one methylene group; that is, the hydrophobicity of a pairing ion played an important role in the ion association reaction in aqueous solutions, as well as ion association in solvent extraction systems. Nitro groups of the dyes slightly affected the ion association constants, whereas the position of the sulfonate group in the azo dyes showed a significant change in ion association constants.