Physico-chemical Approach to Biopharmaceutical Phenomena. IV. Adsorption of Barbituric Acid Derivatives by Carbon Black from Aqueous Solution

Abstract

Following the previous discussions on the adsorption of tryptophan based on the hydrophobic bonding mechanism, an examination in detail was carried out in the adsorption of barbituric and thiobarbituric acid derivatives by carbon black from aqueous solution in relation to the size of hydrocarbon chains, making the first step to discuss the correlation between the adsorption and the biopharmaceutical data. Equations were derived to express the relationship between the adsorption and the size of the respective molecule, considering the contributions of hydrophobic and hydrophilic moieties. The results were in good agreement with such equations, indicating that the larger the size of hydrophobic moiety, the more the molecule was adsorbed and that each of two 5-substituted hydrocarbon chains might adopt the parallel orientation to carbon black surface. Calculating the thermodynamic functions of adsorption, it was shown that the larger the size of 5-substituted hydrocarbon chains, the less absolute value of enthalpy change and the greater entropy change were caused by the hydration of molecule accompanying the destruction of iceberg through the adsorption process. A selectivity was not found for the adsorption of such geometrical isomers as pentobarbital and amobarbital by carbon black. Correlating the adsorption with the solubility, the partition coefficient, and the surface tension depressing activity, it was established that carbon black was of hydrophobic surface and that the more hydrophobic the molecule, the easier it was adsorbed.