Thermodynamic and Brewster angle microscopy studies of fatty acid/cholesterol mixtures at the air/water interface

Mixtures of cholesterol with stearic (STA), oleic (OA), and linoleic (LA) a cids spread as monolayers at the air/water interface were used as model sys tems to examine the hypocholesterolemic effect of fatty acids. Miscibility and interactions between the components of the cholesterol/fatty acid syste ms were studied basing on the analysis of surface pressure/area isotherms c ompleted with Brewster angle microscopy images. In monolayers, STA and chol esterol were found to be immiscible. In contrast, OA and LA were found to f orm miscible, but nonideal mixed monolayers with cholesterol. They exhibit negative deviations from ideality in the surface pressure/area plots. This reflects close-packing arrangements between bulky cholesterol molecule and the hydrocarbon chains of unsaturated fatty acids. The analysis of the exce ss free energies of mixing shows that the maximum negative value of Delta G (exc) appears at about X-chol = 0.5-0.7. Thus, the formation of the most st able 1:1 and 2:1 complexes between cholesterol and an unsaturated fatty aci d molecule may account for the hypocholesterolemic effect of the acids in h uman organism by complexing free cholesterol, thereby hindering its deposit ion on artery walls.