Effect of excipient porosity on melting point depression of deposited materials

The effect of excipient porosity on melting point depression of confined liquids and deposited solids in the pores of solid pharmaceutical excipients was evaluated using differential scanning calorimetry (DSC). Pore size distributions were measured using nitrogen gas adsorption. It was seen that the melting occurred over a range of temperatures, which corresponded to the distribution of pore sizes. Based on such observations, a thermodynamic model was established to explain these effects on the basis of surface area energies, which can be related to the Kelvin equation.; Various grades of porous silicon dioxide were allowed to soak in purified water, USP into their pores and DSC was conducted using various cooling rates to -30°C, isothermally maintained for 7 minutes, and heated slowly to 6°C at various heating rates. A melting point depression during the heating cycle was observed at -10°C to -2°C for the ice confined in pores of various diameters. The effect of cooling rate on the melt behavior of the confined ice was observed as a function of intensity of the "early" melting phase. (Here, early refers to the melting that occurs in a range of temperatures that is lower than the nominal freezing or melting temperature. The DSC peaks that are observed are referred to as "satellite" peaks.) Satellite melt peaks became less prevalent with slower cooling and disappeared at a cooling rate of 0.01°C/min.; The effect of porosity on melting point depressions of cyclohexane was also studied. Cyclohexane was soaked into Syloid 244 and Syloid 74 followed by freezing in DSC at a cooling rate of 1°C/min. and heating at 0.25°C/min. Similarly, two endothermic melt peaks were observed, one from each grade of Syloid-cyclohexane matrix. The satellite melt peak represented the distribution of melting of frozen confined cyclohexane starting from -7°C to -0.8°C with a peak at -3.6°C from Syloid 244, while the satellite melting phase started as low as -13°C and ended at -1.9°C, with the peak at -6.0°C from the pores of Syloid 74. This investigation further suggested that porosity can also have significant effects on the melting point depression of organic solvents (or any other solvents), which does not expand upon freezing.; The effect of pore sizes on uptake of various aqueous solutions was also studied using DSC. For a small molecule like ethanol (contrary to the bigger molecule like PEG 400), it was easy for the solution to get into the pores of Syloid, as can be seen from the satellite melt peak from DSC thermogram. However, the solutes did affect the uptake of the solvent into the pores, as evidenced in diminishing satellite melt peaks on the thermograms.; In addition to studying solvent melting behavior, the melting behavior of solid compounds deposited into the pores was also investigated. Initially, sucrose was deposited into the pores of Syloid, and it was seen that the melting peak of sucrose was significantly lowered. Similarly, melting point depressions were also observed from deposited model drugs namely: danazol, prednisolone, hydrocortisone and D(-) norgestrel. The deposition of all these model drugs into the pores of Syloid contributed a very high surface to volume ratio of the deposited material. Therefore, the surface energy was increased significantly that contributed the shift of melting in the downfield of DSC spectra. A trend in such melting point depression was found to be related to the average pore diameter of Syloid and the transport of the prepared solution into the pores of Syloid. Such decrease in the melting point of poorly soluble drugs may be an indication that their solubility has increased significantly.; Key Words. Differential Scanning Calorimetry; Melting point depression; Kelvin equation; Thermodynamic model; Stress relaxation.