Acid-base equilibria of tetracycline in sodium montmorillonite suspensions.

Changes in the partial molar free energy of the ionic species of tetracycline as a result of interaction with sodium-saturated montmorillonite were monitored by UV spectroscopic titrations. The stronger interaction between the clay and the protonated (H3T+) species relative to the zwitterionic (H2T0) species is responsible for the apparent displacement of pK1 in clay suspensions. The displacement of the first equilibrium favors in H3T+ species due to its stabilization by the negative clay surface. Little effect of the clay was observed on pK2 and pK3, because the ionic species of tetracycline associated with these equilibria (H2T0, HT-, and T2-) did not interact strongly with the negative clay surface. Therefore, the ionic species distribution of tetracycline in a sodium montmorillonite suspension is significantly different than that expected based on solution equilibria. The degree of equilibrium displacement depended on the accessibility of the negative charge at the clay surface. Fraction bound curves support the hypothesis that the clay surface causes a change in tetracycline equilibria. The H3T+ form of tetracycline was highly bound to the sodium montmorillonite, but little adsorption was observed for the H2T0 species.