Study of Changes in Enterosgel Structure in Case of Violation of Drug Storage Conditions

The structure of enterosgel before and after freezing (samples I and II, respectively) was comparatively analyzed by gravimetric analysis and NMR methods [PMR, ¹³C NMR, diffusion-ordered NMR spectroscopy (DOSY)] to understand the reason for the negative effect of freezing on the adsorption capacity of enterosgel. The methylsiloxane moieties in samples I and II were shown to be magnetically equivalent by PMR and ¹³C NMR methods. The molecular masses of samples I and II were estimated by the DOSY method and corresponded to the cyclic dimer of methylsilicic acid (MM 152 Da) for sample I and the linear dimer of methylsilicic acid (MM 170 Da) for sample II. It was hypothesized that the conformationally rigid cyclic dimer structure was destroyed upon freezing of enterosgel due to rupture of a siloxane bond. The adsorption capacity of enterosgel deteriorated because of the structure disruption and hydrophobicity change. Thermogravimetric analysis revealed that water did not participate in hydrogen bonding with enterosgel.