The interaction between water and cephalexin in the crystalline and noncrystalline states.

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Abstract
Solid cephalexin (CEX) was obtained as crystals, the noncrystalline solid (NC) produced by freeze-drying and the noncrystalline solid (GNC) produced by grinding in a centrifugal mill for 4 h. Each product formed an anhydride, monohydrate and dihydrate. The interaction between water and CEX in the crystalline and noncrystalline states was studied by the IR spectral method and the thermal kinetic analytical method (Criado method) using differential scanning calorimetry. In the IR spectrum of CEX, the hydroxyl vOH band was observed at 3400/cm, the amido vNH band at 3100/cm, the amine vNH3+ band at 2600/cm, the .beta.-lactam vC .dbd. O band at 1760/cm, the amido vC .dbd. O band at 1680/cm, and the carboxylate vCO2- band at 1600/cm. The IR spectral intensity ratios were calculated from the peak heights of the bands with respect to the band at 1280/cm as an internal standard peak. The changes of the IR spectral intensity ratios with increasing water content led to the following conclusions. In the crystalline monohydrate (phase IV), the 1st water molecule interacted with the amine and carboxylate groups of different molecules in the crystal of CEX. In the crystalline dihydrate (phase II), the 2nd water molecule interacted with NH and C .dbd. O of the amido group of CEX. In NC - H2O, the water molecule is adsorbed at the carboxylate group of CEX, while in NC - 2H2O, the 2nd water molecule is adsorbed at the amine group. The dehydrations of phase IV, GNC-H2O, GNC-2H2O and NC-2H2O followed 1st-order kinetics and the activation energies (E) were 15.67, 14.83, 14.58 and 12.50 kcal/mol, respectively, while that of NC-H2O followed 3-dimensional diffusion kinetics and its E value was 15.81 kcal/mol. The E of phase II (2nd water molecule) was 11.74 kcal/mol as determined by the Kissinger method. The binding energies between the various forms of CEX and water apparently reflect the characteristics of the groups of the CEX molecule to which water is actually bound in each case.

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