Solubility behavior of barbituric acids in aqueous solution of sodium alkyl sulfonate as a function of concentration...

The solubility of 13 barbituric acids was determined in aqueous solutions of sodium alkyl sulfonate. The effects of concentration and temperature were investigated, and the thermodynamic functions of the solubilization process were calculated. An analysis of the location of a solubilized species within a micelle is suggested in terms of the sign and amplitude of the standard entropy of solubilization, which is strongly positive for micelle penetration and negative for adsorption. A solubilization mechanism through adsorption onto the micellar surface is suggested for most of the barbituric acids studied. The enthalpy/entropy compensation phenomenon was identical for barbituric acids in ionic and nonionic (polyoxyethylene lauryl ether )surfactant solutions with a compensation temperature of 270 degrees K, indicating common behavior of these compounds with respect to micellar solubilization. The concept of molecular surface area was used to correlate the free energy of solubilization of the solutes to their size and structure. A linear relationship was found with an excellent correlation factor for the alkane derivatives of the 5-ethyl-barbituric acids. The specific behavior of some of the barbituric acids investigated is discussed.