Equilibrium studies of lecithin-cholesterol interactions. II. Phase relations in surface films: analysis of the "condensing" effect of cholesterol.

From measurements of the equilibrium spreading pressure pie for dispersions of lecithin--dimyristoyl (DML) or dioleoyl (DOL)--and cholesterol (CHOL) in water, we have deduced the phase relations in both the aqueous dispersions and the equilibrium surface films. At 29.5 degrees C, when the mole fraction of cholesterol in the dispersion chi(CHOL) is 0 chi(CHOL) less than chi(CHOL) less than 0.33, pie is constant and equal to the value for pure lecithin (DOL or DML). The phase rule predicts than two bulk lipid phases coexist; these are pure lecithin and lecithin:cholesterol 2:1 complex. The equilibrium surface film contants only lecithin and therefore lecithin and 2:1 complex are immiscible in surface films. When 0.33 less than chi/CHOL) less than 1.0, pie is also contant with a value intermediate between that for pure lecithin and cholesterol. In this range of lipid composition two bulk lipid phases also coexist: lecithin:cholesterol 2:1 complex and pure cholesterol. However, the equilibrium surface film contains only the 2:1 complex and, therefore, 2:1 complex is also immiscible with cholesterol in surface films. When pi less than pie, as in the case of spread films, we deduce that two surface phases may coexist; the composition of the phases will depend on chi(CHOL). When 0 less than chi(CHOL) less than 0.33, both lecithin and 2:1 complex coexist, and when 0.33 less than chi(CHOL) less than 1.0, 2:1 complex and cholesterol coexist. The "condensing" effect of cholesterol in lecithin surface films is reexamined. The effect is attributed to formation of the lecithin:cholesterol 2:1 complex and nonequilibrium conditions in the two-phase surface film.