Influence of a buffered solution on the adsorption isotherm and overloaded band profiles of an ionizable compound

The overloaded band profiles and the adsorption isotherms of propranolol were acquired at 23 degrees C, on the endcapped C18-Kromasil stationary phase, using two aqueous solutions of methanol as the mobile phase. The first solution contained 40% methanol and no buffer. The second contained an aqueous acetate buffer at C(buffer) = 0.20 M and pH = 5.9. In both cases, 33 isotherm data points were acquired by frontal analysis (FA), to achieve an accurate description of the isotherms in the concentration range between 1.54 x 10(-3) and 1.54 x 10(-1) mol/l of propranolol. The isotherms obtained were best described by a bi-Langmuir and a bi-Moreau isotherm model, depending on whether the mobile phase was buffered or not. This shows that the adsorption of propranolol takes place on two different types of sites, a behavior similar to the one already observed with phenol and caffeine on the same column. The presence of the buffer in the mobile phase drastically changes the adsorption mechanism of propranolol. Weak adsorbate-adsorbate interactions (two and three times RT on the low- and the high-energy sites, respectively) take place in the absence of buffer but vanish when the mobile phase is buffered. As expected, the adsorption constants on the abundant low-energy sites with or without buffer are comparable because the mobile phase composition was adjusted to give similar retention times in the two cases. On the other hand, the adsorption of propranolol on the high-energy sites is stronger in presence of the buffer. The difference probably comes from ion-pair formation in the adsorbed phase between the propranolol cation and the acetate anion. The change in total saturation capacity of the adsorbent (22%) compared to that for phenol is explained by the difference in methanol content of the mobile phase.