The adsorption of tetracycline by clays commonly used in pharmacy can be predicted if the identity and character of the commercial clay sample are established. X-ray diffraction, IR spectroscopy, and chemical analysis were used to identify the clay component and any nonclay diluents present in a series of commercial pharmaceutical grade clays. The major clay components were montmorillonite, hectorite, attapulgite, saponite, and kaolinite. The clay structure, the nature of the exchangeable cation, and the presence of nonclay components are important factors affecting the tetracycline-clay interaction. In general, clay structures with a high surface charge lead to a greater interaction with the protonated form of tetracycline, while interaction with the zwitterionic form of tetracycline occurs in clay structures with minimal surface charge. The presence of multivalent, exchangeable cations on the clay surface diminishes interaction with the protonated form of tetracycline. Nonclay Components such as calcite and dolomite increase the interactions of the zwitterionic and anionic forms of tetracycline with the clay.
Keyphrases 0 Tetracycline hydrochloride-adsorption onto clay, interaction with pharmaceutical grade clay, adsorptive properties of clay 0 Clay-pharmaceutical grade, adsorption of tetracycline, isomorphous substitution Adsorption-tetracycline onto pharmaceutical grade clays
The unique properties of clays have led to their use in pharmaceuticals for both therapeutic effects and excipient action. The crystal structure of clay minerals is well understood, and the behavior of clays in pharmaceutical systems is predictable based on structural considerations. Thus, the identity and characterization of a clay are of prime importance to the pharmaceutical scientist.
The purpose of this study was to illustrate the importance of a complete characterization of clays used in pharmaceuticals so that their behavior in formulations and in a patient's drug regimen can be predicted. The influence of clay structure, the nature of the exchangeable cation, and the presence of nonclay components on the interaction of tetracycline with reference or commercial samples of montmorillonite, hectorite, saponite, attapulgite, and kaolinite were studied.
BACKGROUND'
The basic structure of clays consists of octahedra of aluminum and magnesium in combination wit,h silica tetrahedra to give layer-like or fibrous structures. Cationic-exchange properties can arise in these structures through isomorphous substitution, i.e., aluminum for silicon, magnesium for aluminum, etc. A manifestation of isomorphous substitution is the swelling properties of certain clays with high cation-exchange capacity.
The kaolin group of minerals consists of sheets of silica tetrahedra and alumina octahedra shared in a 1:l ratio. These minerals have little or no isomorphous substitution and normally are nonswelling in aqueous so- lutions.
Smectites belong to the 21 (ratio of silica tetrahedra to alumina and/or
The information in this section is summarized from R. E. Grim, "Clay Mineralogy," 2nd ed.