Influence of cholesterol on domain shape and lattice structure in arachidic acid monolayers at high pH

Monolayer mixtures of arachidic (eicosanoic) acid with cholesterol are inve stigated on a subphase of pH 12.0 at 25 degrees C using grazing incidence X -ray diffraction (GID) and Brewster angle microscopy (BAM). GLD is applied in the low concentration range from 0 to 5 mol % cholesterol and reveals si gnificant nonmonotonic changes in the dependence of the mean molecular area and the crystallinity (expressed by the position correlation length) of th e monolayer on the cholesterol concentration. The condensation and lattice order strongly depend on the surface pressure. At zero pressure, condensati on and lattice order are relatively large below 1 mol % cholesterol and dec rease with increasing cholesterol concentration. At high pressure, this beh avior is reversed. These cholesterol effects are compared with those observ ed in phospholipid bilayers, which are not yet completely understood. With BAM, the formation of domains and the phase separation of arachidic acid an d cholesterol is observed with 1-20 mol % cholesterol. The shape of the dom ains is different from that in the absence of cholesterol, which is attribu ted to the "line activity" of cholesterol rather than to structural changes . The isolation of the domains embedded in a condensed matrix allows the ob servation of the recovery of the inner domain structure with variation of t he surface pressure.