A study of the electrokinetic and stability properties of nitrofurantoin suspensions. II: Flocculation and redispersion properties as compared with theoretical interaction energy curves.

The stability and redispersion properties of nitrofurantoin dispersions are experimentally studied using simple but reproducible techniques. Solutions of different electrolyte concentrations and pH values are employed as dispersing media. The pH appears to be a determinant factor in the properties studied. Thus, with all the electrolytes considered (NaCl, CaCl2, and AlCl3, almost optimum redispersion was achieved when the pH was maintained close to neutrality. However, the important effect of AlCl3 as compared with the other electrolytes is also clearly demonstrated. Especially interesting are the results obtained when 0.1% Carbopol is added to the suspensions. The important effect of this polymer on the surface charge of nitrofurantoin, mainly at pH 7, manifests in excellent redispersion of the suspensions with either of the electrolytes employed. Using simple expressions that approximate the equations proposed in the DLVO theory of interaction between colloidal particles, the variation of the total interaction energy between nitrofurantoin particles is calculated as a function of interparticle distance. The results are compared with the experimental determinations of the sedimentation and redispersion properties of nitrofurantoin. Although some discrepancies between theory and experiments are found, the results indicate a reasonable agreement between the general features of both.