The measurement of caking tendencies in sulfa drug suspensions

The resuspension of sedimented solids is recognized as a problem in certain types of pharmaceutical suspension systems. The particles of a uniform suspension will slowly settle to the bottom of a container. If the product is an oral or injectable pharmaceutical suspension, the sedimented particles must be evenly resuspended before the preparation can be put to its intended use. When it becomes difficult to resuspend a sediment by simple shaking, the sedimented material is said to have "caked". The underlying factors in caking have been studied for only a few simple systems, and the existance of caking is usually determined empirically by noting the ease or difficulty of resuspension of the sedimented solid material. This thesis presents a method that can be used to demonstrate the existance of a caked sediment, and which measures the caking in terms of the force it takes to break the sediment. The method employs the recently available Fisher "Tensiomat", and the special "H" shaped test devices which were developed for this purpose in the course of the thesis work. The "H" shaped test devices were lifted through the caked sediments formed from a number of deflocculated sulfa drug suspensions, and the "Tensiomat" readings were taken as a measure of the relative hardnesses of the cakes formed. Under the conditions of the experiment, it was found that there was a statistically significant difference in the degree of caking in the various sulfa drug suspensions. Caking tendencies proved to be inversely proportional to the particle sizes of the sulfa powders. The settling times of the suspended sulfa particles were directly proportional to the particle sizes, as would be expected.