Analytical Sciences

Capillary Electrophoretic Determination of Self-dimerization Constants of Aromatic Carboxylate and Sulfonate Ions

Takashi YOKOYAMA, Yasuhide TSUCHIYA, Gen MURAKAMI, and Michio ZENKI

Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan

Self-dimerizations of twenty three aromatic carboxylate and sulfonate ions from their electrophoretic mobilities in aqueous solution were estimated by capillary zone electrophoresis (CZE). The magnitudes of the self-dimerizations ascribed to π-π interactions of these aromatic anions were determined by CZE as dimerization constants (K_D). Although the largest K_D value of 1.2 dm³ mol^-1 for 9-anthracenecarboxylate ion (9-AC) in these aromatic anions was found, almost all of the K_D values were zero, or near to zero. It was found that the π-π interactions of the aromatic anions were relatively small at zero ionic strength, in which the contribution of an ionic association between the cation and aromatic anions could be excluded from the K_D values, since the contribution of the electric repulsion between the aromatic anions on the K_D values was large. The relatively large K_D value of 9-AC caused that it electro-migrates as its planar shape, and has an anthracene ring of a largely hydrophobic aromatic ring.

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