Role of the Na+ ion on phenol derivatives/hydroxypropyl-β-cyclodextrin complex formation on porous graphitic carbon phase

The reversed-phase liquid chromatography retention of phenol derivatives was investigated over a concentration range of sodium chloride (0-10(-2) M) and hydroxypropyl-p-cyclodextrin (HP-beta-CD) (0-35 x 10(-3) M) using a porous graphitic carbon (PGC) stationary phase and a methanol/water mixture (50:50 (v/v)) as the mobile phase. A theoretical treatment was developed to investigate the effect of the sodium chloride and hydroxypropyl-beta-cyclodextrin on the equilibrium between the solutes with the PGC surface and the aqueous medium, respectively. The thermodynamic parameter variations were calculated using van't Hoff plots. It was expected that the sodium ion acted on the solute-PGC association process by modifying the surface tension of both the bulk solvent and the PGC surface. The phenol derivative/HP-beta-cyclodextrin complexation was shown to be entropically controlled for all the solutes except for the one which contained the -NO2, group in its structure, i.e. the nitro phenol derivative. A comparison of the compensation temperature of the solute-PGC association process when sodium chloride and HP-beta-CD concentration changed in the mobile phase led to the conclusion that these two modifiers acted via a variation in the hydrophobic effect. (C) 2002 Elsevier Science B.V. All rights reserved.