Investigation of a relationship between surface energy terms and the thermodynamics of transfer of drugs into sodium dodecyl sulfate micelles

Abstract

The partition between aqueous fluids and sodium dodecyl sulfate micelles has been studied for four structurally related solutes. The partition has been followed by considering the solubility, in both water and aqueous micellar sodium dodecyl sulfate solutions, as a function of temperature, and also by considering parallel experiments in which the pH was buffered at the pKa of the drug. A Taylor-Aris diffusion technique has also been used to provide a direct measurement of partition between a buffered aqueous phase and the micelles, again as a function of temperature. The thermodynamics of transfer were calculated for each experimental procedure by use of the van't Hoff isochore. The partitioning process was also modeled by considering the surface energy of each drug in terms of a Lifshitz-van der Waals contribution and a polar contribution which was divided into electron donor and electron acceptor contributions. These data were obtained from contact angle experiments. The surface energy data for the solids were used along with surface 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 micelle for each of the drugs. Correlations were obtained between the measured partitioning data and the free energy of adhesion obtained from surface energy data, especially for the buffered systems. It was concluded that the ionization of the drug was an important consideration if correlations were to be obtained between surface energy data and measured partition behavior. It was shown that the partitioning process was strongly influenced by a polar repulsion energy between the monopolar drugs and the monopolar surfactant head group. This work has demonstrated that the novel approach of modeling partition from solid state measurements is practicable, and furthermore that the model provides useful information to assist in the understanding of different partitioning behavior. © 1995 American Chemical Society.