Mixed monolayers, intermolecular cavities

Concept of Intermolecular Cavities in Mixed Monolayers. In mixed monolayers a deviation in average area per molecule occurs if one component forms expanded and the other condensed monolayers. This reduction in average area per molecule has been attributed by previous workers to an interaction between components in the mixed monolayer. However, this need not be true in all cases where condensation occurs. In several instances the condensation can be explained on the basis of steric considerations in the mixed monolayers. Although the following discussion is based on lecithin-cholesterol monolayers, it is equally applicable to other mixed monolayers. 

Surface Pressure, Potential, and Fluidity Characteristics for Various Interactions in Mixed Monolayers. It is possible to distinguish various types of interactions which occur in mixed monolayers by measuring the surface pressure, surface potential, and surface fluidity of the monolayers. Deviation from the additivity rule of molecular areas indicates either an interaction between components or the intermolecular cavity effect in mixed monolayers. 

Intermolecular Cavity Effect. Figure 5a shows the general characteristics of mixed monolayers in which the "intermolecular cavity effect ... 

Hydrocarbon-Hydrocarbon Interaction. Figure 5c shows the general characteristics of mixed monolayers in which hydrocarbon-hydrocarbon interaction occurs—e.g., trimyristin-myristic acid monolayers (16). The average area per molecule shows a deviation, whereas the surface potential per molecule follows the additivity rule. Hydrocarbon-hydrocarbon interaction also increases the cohesive force in the lipid layer and therefore reduces the fluidity of the mixed monolayer. It is evident from Figures 3a and 3c that surface fluidity is the only parameter which distinguishes an intermolecular cavity effect from hydrocarbon-hydrocarbon interaction. 

Van Deenen has reported (48) that the mixed monolayers of dide-canoyl lecithin-cholesterol follow the additivity rule of molecular areas even though this lecithin forms expanded monolayers. This can be explained similarly by an intermolecular cavity of smaller height, which cannot accommodate cholesterol (Figures lOd and 4d). 

Even though 1,2-dilinoleoyl and l-palmitoyl-2-linolenoyl lecithins form more expanded monolayers than egg lecithin, their mixed mono-layers with cholesterol follow the additivity rule (48). This can be explained as follows. At low surface pressures, these lecithins have greater intermolecular spacing and hence form intermolecular cavities of smaller height which cannot accommodate cholesterol molecules (Figure 4e). At high surface pressure, the linoleoyl and linolenoyl chains, as opposed to oleoyl chains, do not form cavities in the monolayer (Figure lOi). 

If at a surface pressure, tt, the average area per molecule of two surfactants in their individual monolayers is Ai and A2, then in the mixed monolayer (1 1 molar ratio) of these two surfactants, the average area per molecule should be (Ax + A2)/2 at the same surface pressure provided the surfactant molecules occupy the same area in the mixed mono-layer as they do in their individual monolayers (8,9). However, in many cases, the average area per molecule in a mixed monolayer is greater or smaller than that expected from the simple additivity rule (10, 11, 12). A reduction in the average area/molecule in a mixed monolayer can be attributed to the molecular attraction between the surfactants or to the intermolecular cavity effect (9,13). An expansion in the average area/... 

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