Abstract
The miscibility of Langmuir monolayers of egg yolk lecithin (eggPC) with n-hexadecanoic acid (PA), 1-hexadecanol (HD), and their equimolar mixture (PA/HD) was investigated thermodynamically and morphologically. Surface pressure (π)-molecular area (A) and surface potential (ΔV)-A isotherms for the binary and ternary systems were measured, employing the Wilhelmy method and the ionizing 241Am electrode method, respectively. From the thermodynamic perspective, eggPC was partially miscible with PA, HD, and PA/HD within the monolayer state, in terms of an excess Gibbs free energy of mixing calculated from the π-A isotherms and a two-dimensional phase diagram based on a monolayer collapse pressure. This was also directly supported by phase behavior observations using fluorescence microscopy (FM). EggPC formed a typical liquid-expanded (LE) monolayer, and the others formed liquid-condensed (LC) monolayers. The FM images exhibited miscible modes at middle molar fractions of PA, HD, and PA/HD, and immiscible patterns at large molar fractions for all the systems examined. A new finding in the present study was that the eggPC/PA, eggPC/HD, and eggPC/(PA/HD) systems exhibited partial miscibility, although the systems were made of both LE (eggPC) and LC monolayers (the others). This miscibility is considered to be attributable to the molecular species comprising eggPC.
