Surface Tensions, Critical Micelle Concentrations, and Standard Free Energies of Micellization of C8−Lecithin at Different pHs and Electrolyte Concentrations

Abstract

The micellization of the amphiphilic zwitterionic dioctanoylphosphatidylcholine (diC8PC) in aqueous solution has been studied as a function of pH and electrolyte concentration. Critical micelle concentration (cmc) determined by surface tension measurements at pH 1.2, 3.2, 5.8, 7.4, and 10.0 indicated an increase with pH. To analyze the influence of electrolyte on the cmc, measurements of surface tension were performed at pH 1.2 and KCl concentrations in the range 0.1 M to 2.0 M. Here, the cmc decreased with increase of KCl concentration. Areas per molecule at the interface decrease from (86.8 to 65.1) Å2 molecule-1 with increase of pH. Standard Gibbs energies of micellization were calculated from the surface tension data and are in the range of approximately (−30 to −34) kJ mol-1. Data for the hydrocarbon volume of lipids, the effective area per headgroup, and the length of hydrocarbon chain have been used to estimate the shape factor of diC8PC micelles. The value for the shape factor indicates a gradual change from nonspherical (shape factor 0.49) to bilayers (shape factor 0.59).