Gerald S. Brenner

Surface area and water vapor sorption of macrocrystalline cellulose

Abstract A review of the pharmaceutical literature revealed several inconsistencies and uncertainties in values obtained for the specific surface area of microcrystalline cellulose, as well as the proposed mechanisms of water vapor sorption. In the present study, using nitrogen and krypton adsorption following several methods of sample pretreatment, a value of 1.3 m 2 /g, with no evidence of extensive microporosity, was determined. The very high values of specific surface area reported for microcrystalline cellulose using water vapor were shown not to reflect a true surface area, but rather, it is felt, to reflect penetration into the amorphous portions of the cellulose structure and interaction with individual anhydroglucose units. Analysis of water vapor sorption isotherms indicated no apparent difference in the mechanism of sorption between various starches and celluloses, including microcrystalline cellulose, after accounting for the degree of cellulose crystallinity. It appears that water sorbed to such polymers most likely exists in at least 3 states: tightly bound to an anhydroglucose unit; less tightly bound; and bulk water.

High resolution spectroscopic evidence and solution calorimetry studies on the polymorphs of enalapril maleate

Abstract Enalapril maleate, a potent angiotensin converting enzyme inhibitor, exists as polymorphs, Form I and Form II. X-Ray powder diffraction measurements have shown slightly different patterns. Differential scanning calorimetric thermograms failed to show any significant differences during melting. High resolution spectroscopic techniques, including solid state carbon-13 NMR, Fourier-transform IR and Raman, detect differences between Form I and Form II. Heats of solution data obtained also indicate measurable energy differences. It was concluded that these two polymorphic forms of enalapril maleate are energetically very similar. Virtual equivalence of in vitro dissolution rate was obtained from formulations of enalapril maleate made from either Form I, or Form II, or mixtures.