Formation constants cyclodextrin mobile phase

This paper describes the elution behavior of benzene, ortho-, meta-, and para-nitrophenol, naphthalene, and biphenyl in aqueous and methanolic cyclodextrin mobile phases. The inclusion complex formation constants of these selected compounds in e-CD mobile phases containing 0%, 10%, and 20% methanol are reported. The formation constants obtained using a CN column/CD mobile phase were determined and used to predict the elution behavior of the test compounds on C-18 columns. Also, the elution behavior of solutes on C-18, e-CD, and Y-CD stationary phases are compared. 

Strength of interaction and the greater the rate of decrease in k with increasing cyclodextrin concentration. Cyclodextrin mobile phase chromatography provides an effective way to determine inclusion complex formation constants, and their magnitudes are useful in evaluating solute-CD interactions. Also, the Kf values obtained chromatographically can be confirmed spectroscopically if identical experimental conditions are employed. 

Aqueous a-cyclodextrin solutions seem to be generally applicable for TLC separation of a wide variety of substituted aromatics on polyamide thin-layer stationary sheet (13-14). In most cases, the compounds moved as distinct spots and their R, values were dependent on the concentration of the cyclodextrin in tne mobile phase. In a given family of compounds, (o-, m-, and p-nitrophenols, for example) the isomer with the largest stability constant for a-cyclodextrin complex formation had the larger value. In general, the para-substituted isomers have larger R values than the meta-isomers, which in turn have larger R values than the ortho substituted ones. 

In conventional reversed phase HPLC, differences in the physicochemical interactions of the eluate with the mobile phase and the stationary phase determine their partition coefficients and, hence, their capacity factor, k. In reversed-phase systems containing cyclodextrins in the mobile phase, eluates may form complexes based not only on hydrophobicity but on size as well, making these systems more complex. If 1 1 stoichiometry is involved, the primary association equilibrium, generally recognized to be of considerable importance in micellar chromatography, can be applied (11-13). The formation constant, Kf, of the inclusion complex is defined as the ratio of the entrance and exit rate constants between the solute and the cyclodextrin. Addition of organic modifiers, such as methanol, into the cyclodextrin aqueous mobile phase should alter the kinetic and thermodynamic characteristics of the system. This would alter the Kf values by modifying the entrance and exit rate constants which determine the quality of the separation. 

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