Mixed monolayers condensing effects

The adsorbed layer at G—L or S—L surfaces in practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant in the adsorbed layer. Mixed monolayers are known that involve molecular complexes, eg, one-to-one complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes between fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L interfaces is an important effect in foaming, foam stabilization, and defoaming (see DEFOAMERS). 

Using an automated film balance the behavior of mixed monomolecular films exhibiting deviations from ideality was studied. Particular attention was paid to condensation effects obtained when cholesterol is mixed with a more expanded component. The deviations at various film pressures are discussed in terms of the partial molecular areas of the film components. Slope changes in these plots are caused by phase transitions of the expanded monolayer component and do not indicate the formation of surface complexes. In addition, the excess free energies, entropies, and enthalpies of mixing were evaluated, but these parameters could be interpreted only for systems involving pure expanded components, for which it is clear that the observed condensation effects must involve molecular interactions. 

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

A mixed monolayer consisting of stearic acid (9.9%), palmitic acid (36.8%), myristic acid (3.8%), oleic acid (33.1%), linoleic acid (12.5%), and palmitoleic acid (3.6%) produces an expanded area/pressure isotherm on which Azone has no apparent effect in terms of either expansion or compressibility (Schuckler and Lee, 1991). Squeeze-out of Azone from such films was not reported, but the surface pressures measured were not high enough for this to occur. The addition of cholesterol (to produce a 50 50 mixture) to this type of fatty acid monolayer results in a reduction of compressibility. However, the addition of ceramide has a much smaller condensing effect on the combined fatty acids (ratio 55 45), and the combination of all three components (free fatty acids/cholesterol/ceramide, 31 31 38) produces a liquid condensed film of moderate compressibility. The condensed nature of this film therefore results primarily from the presence of the membrane-stiffening cholesterol. In the presence of only small quantities of Azone (X = 0.025), the mixed film becomes liquid expanded in nature, and there is also evidence of Azone squeeze-out at approximately 32 mN m. ... 

The association of the acidic phospholipid PI with the cationic linear polysoap PVPC6 is cooperative in both mixed spread monolayers and mixed swollen multibilayers. This association has a condensation effect on the spread mixed monolayers on one hand and affects the properties of the interbilayer structural water of swollen mixed multibilayers on the other hand. Namely the increase in the degree of... 

Funasaki and coworkers [12] studied intermolecular interactions in mixed micelle formation of polyoxyethylene (15) dihydrocholesterol (DHC-E015) with polyoxyethylene (7) dodecanol (C12E07), by means of volumetric methods. The objective of their study was to investigate the condensing effect of cholesterol on phospholipid monolayers and the membrane-thickening action of cholesterol on lecithin bilayers. The volume... 

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