Abstract
The interaction of N-methyl-2-pyrrolidone (NMP) with aminobenzoic acids was studied, observing the effect on the solubility of aminobenzoic acids in ether in comparison with that of N-ethyl-2-pyrrolidone (NEP). p-Aminobenzoic acid (PABA) and p-aminosalycilic acid (PAS) formed slightly soluble complexes in ether with NMP, while the other aminobenzoic acids formed soluble complexes in ether. From the solubility data, the stoichiometrical ratio and the stability constant K of the formation of complex were calculated. K value was strongly affected by the orientation of the substituent group and the existence of the hydroxyl group in aminobenzoic acid, as the decreasing order for NMP-aminobenzoic acids complexes was as follows : PABA>anthranilic acid (ANA)>m-aminobenzoic acid. K value for NEP complexes decreased in comparison with that for NMP complexes. This may be due to the steric hindrance of ethyl group in NEP. On the other hand, the complex formation in solid phase was recognized in NMP-PABA, NMP-PAS, and NMP-ANA systems by powder X-ray diffractometry and IR absorption spectroscopy. The dissolution rate of these solid complexes was studied by stationary disk method, and it was found that the complexes dissolved fast compared with the respective original compounds. These results suggested that the complex formation with NMP may enhance the bioavailability of drugs.
