SOME THERMOCHEMICAL AND STRUCTURAL-PROPERTIES OF PENICILLIN-V BENZATHINE TETRAHYDRATE

The thermochemical and structural properties of penicillin V benzathin e tetrahydrate were studied in the temperature interval from 20 to 200 -degrees-C. It has been found that, on increasing the temperature, the penicillin undergoes two phase transitions before reaching the meltin g point. The first is accompanied by complete dehydration and the corr esponding structural changes are small, whereas the penicillin's cryst al lattice before and after the second transition, as determined by X- ray diffraction, differs significantly. The overall enthalpy change is 250 J g-1. Both transitions are reversible; the penicillin can be suc cessively heated and cooled an indefinite number of times, retaining i ts chemical and structural properties, provided that the temperature d oes not exceed 100-degrees-C. From differential scanning calorimetry d ata one cannot resolve the kinetic parameters of the two phase transit ions; the overall process is second order and the activation energy is 207 kJ mol-1. Carroll-Freeman analysis of the thermogravimetric data indicates that the reaction order of the dehydration is 0.6 and the ac tivation energy is 111 kJ mol-1. All these parameters were obtained at a 1.25-degrees-C min-1 heating rate. The enthalpy of fusion of the an hydrous penicillin is 45 kJ mol-1. The seeming inconsistency of the X- ray data with the DSC data is found to be related to the very pronounc ed effect of heating rate on the kinetics, and is explained in detail.