Electrophoretic mobility study of ion association between aromatic anions and quaternary ammonium ions in aqueous solution

The ion association reaction between aromatic anions and quaternary ammonium ions in aqueous solutions was investigated through mobility changes in capillary electrophoresis. The electrophoretic mobility of aromatic anions decreased with increasing amount of quaternary ammonium salt added to the migrating solutions. The change in mobility of anions due to the ion association was found and was treated with a least-squares method, giving ion association constants. The ion associability order of the isomers of aromatic anions was found to be naphthalene-2,6-dicarboxylate > naphthalene-2,3-dicarboxylate, naphthalene-2-sulfonate > naphthalene-1-sulfonate and terephthalate > isophthalate > phthalate. The order of the ion associability of naphthalene-1,5- and -2,6-disulfonate reversed depending on the alkyl chain length of the pairing cation. The order of ion associability of quaternary ammonium ions was found to be tetraamylammonium > tetrabutylammonium > tetrapropylammonium approximate to octyltrimethylammonium approximate to hexyltrimethylammonium > tetraethylammonium > tetramethylammonium, and suggested that ion association in aqueous solution was governed by the hydrophobicity of the pairing cation.