Relation between Electrophoretic Behavior and Molecular Shapes of Aromatic Anions

Abstract

Capillary electrophoresis (CE) has been applied to the separation of various analyte ions. [1][2][3][4][5][6] Theoretically, the capillary electrophoretic mobility (µep) is calculated bywhere q, η and r are the electric charge of the ionized solute, the buffer viscosity and the solute radius, respectively. 7 Therefore, the CE separation of analyte ions is primarily dependent on the ionic charge and the ionic radius, unless there are any other interactions in the analyte ions, such as those with the capillary wall or counterions. When the ionic shape is not spherical, it is difficult to predict µep from Eq. (1). In addition, the Stokes radius of the ion is necessary in order to determine the r-value. However, the observed µep values of non-spherical ions are expected to provide some information regarding the ionic shapes.Factors affecting the selectivity of inorganic anions have been reviewed. 8 In addition, a number of methods of predicting µep have been reported. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] The molecular weights (MW) have been reported to correspond to the µep values for amides. [9][10][11][12][13][14] In addition, for some organic and metal complex ions, the MWs correspond to the µep values. 15 The MW value is thought to depend on the molecular volume; therefore, it could be used as an index of molecular size. However, the separation of a number of aminonaphthalenesulfonate ions having the same molecular weight and electric charge at pH 9.2 has been accomplished. 16 Moreover, empirically good linear relationships between the µep values and MW -2/3 have been reported. [10][11][12][13][14] The MW 1/3 value indirectly corresponds to the third power of the molecular volume, that is, the r-value, because the MW value generally depends on the molecular volume. Therefore, the MW -2/3 value indirectly corresponds to r -2. In other words, the r -2 -values show a good linear relationship with the µep values. A correlation between the µep values for the localized electric charge of non-spherical aromatic anions with the MW -2/3 and r -2 -values has yet to be reported. Accordingly, the MW -2/3 and r -2 -values were evaluated as the indexes to determine whether or not they would be a useful means of predicting the µep values of these anions.A simulation of the µep values with the pH of the buffer has been reported for phenolic acids. 17,18 In addition, a simulation of the µep values with the pH and the 2-hydroxyisobutyric acid (HIBA) concentration of the buffer has been reported for metal HIBA complex ions. 19The effects of the µep values on a number of water-soluble organic solvents (such as methanol) for aromatic carboxylates with various functional groups (such as hydroxyl groups) have been reported. [20][21][22][23] These results showed that the differing viscosities of the buffers and the differing pKas for phenolic acids and aromatic carboxylates largely influenced the µep values. Although it has been reported that the change in the Walden product (µepη) value is de...

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

“…Consequently, it was found that contribution of only the π-π interaction to the μepa values for the aromatic anions in the aqueous solution was very small. It was found that only the 9-AC that electro-migrates as its planar shape, 9 and has a relatively largely hydrophobic anthracene ring, produced the dimer ascribed to the π-π interaction, even in its small concentration region.…”

“…It has been reported that some aromatic carboxylate ions migrated as a planar form in a capillary, compared to some aromatic sulfonate ions. 9 This observation suggests that the planar aromatic carboxylate ions would produce the π-π interaction between these aromatic rings in the electro-migration in CZE. Therefore, a capillary electrophoretic determination of the KD values for some aromatic carboxylate and sulfonate anions in an aqueous solution was investigated.…”