INVESTIGATION OF A RELATIONSHIP BETWEEN SURFACE-ENERGY TERMS AND THE THERMODYNAMICS OF TRANSFER OF DRUGS INTO SODIUM DODECYL-SULFATE MICELLES

The partition between aqueous fluids and sodium dodecyl sulfate micell es has been studied for four structurally related solutes. The partiti on has been followed by considering the solubility, in both water and aqueous micellar sodium dodecyl sulfate solutions, as a function of te mperature, and also by considering parallel experiments in which the p H was buffered at the pK(a) of the drug. A Taylor-Aris diffusion techn ique has also been used to provide a direct measurement of partition b etween a buffered aqueous phase and the micelles, again as a function of temperature. The thermodynamics of transfer were calculated for eac h experimental procedure by use of the van't Hoff isochore. The partit ioning process was also modeled by considering the surface energy of e ach drug in terms of a Lifshitz-van der Waals contribution and a polar contribution which was divided into electron donor and electron accep tor contributions. These data were obtained from contact angle experim ents. The surface energy data for the solids were used along with surf ace energy terms for the head groups and the hydrophobic tails of the micelle to give a free energy of adhesion to each region of the micell e for each of the drugs. Correlations were obtained between the measur ed partitioning data and the free energy of adhesion obtained from sur face energy data, especially for the buffered systems. It was conclude d that the ionization of the drug was an important consideration if co rrelations were to be obtained between surface energy data and measure d partition behavior. It was shown that the partitioning process was s trongly influenced by a polar repulsion energy between the monopolar d rugs and the monopolar surfactant head group. This work has demonstrat ed that the novel approach of modeling partition from solid state meas urements is practicable, and furthermore that the model provides usefu l information to assist in the understanding of different partitioning behavior.