Mixed monolayers, intermolecular

Vezenov D, Zhuk A, Whitesides G, Lieber C. Chemical force spectroscopy in heterogeneous systems intermolecular interactions involving epoxy polymer, mixed monolayers, and polar solvents. J Am Chem Soc 2002 124 10578-10588. 

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). 

The considerable efficiencies obtained apparently result from the suppression of the self-quenching of Chi a by separating the Chl-Chl intermolecular distance. Based on photoelectrochemical measurements, an extention of this study may elucidate the energy interactions between Chi and other photosynthetic pigments in the mixed monolayer systems. 

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/... 

Differences in the molecular area are of obvious relevance in mixed monolayers, where larger molecules have a larger partial molar area than smaller ones. Such differences lead to a situation where the smaller molecules cire increasingly preferentially adsorbed with increasing surface pressure, even in the absence of any surface interactions [16]. In adsorption layers consisting of a single surface-active compound similar effects can occur if, due to the asymmetry in different adsorption states the molecules can occupy different areas [3, 4, 15, 19, 21, 22], The fraction of molecules which are in the state characterised by a particular partial molar area depends on the surface pressure. In a thermodynamic study by Joos and Serrien [21] it was shown that if the molecule possesses, say, the two modifications 1 and 2, with different partial molar surface areas coi and o)2 (in absence of intermolecular interactions) their ratio in the surface layer obeys the equation... 

As the second term in Eq. (2.153) is non-zero, the chemical potential of the insoluble component does not depend on the adsorption of the soluble component provided that both surface pressure and adsorption of the insoluble component are fixed. In turn, as the surface concentration of the insoluble component is fixed, the requirement for constant activity of this component implies the independence of this activity coefficient of adsorption of the soluble component. Clearly, this requirement is satisfied not only for the trivial case of an ideal monolayer, but also for non-ideal monolayers, provided that the activity cross-coefficients of the components (or intermolecular interaction parameters) vanish. For example, if the equation of state Eq. (2.35) is used for a non-ideal (with respect to the enthalpy) mixed two-component monolayer, it follows from Eq. (2.153) that Eqs. (2.151) and (2.152) are applicable when ai2 = 0. Clearly, the condition of Eq. (2.153) imposes certain restrictions to the applicability of Pethica s model. The generalised Pethica equation (2.151) was thermodynamically analysed in [64, 65]. Moreover, an attempt to verify Eq. (2.151) experimentally was undertaken in [65], which also confirms its validity for mixed monolayers comprised of two non-ionic surfactants, or for mixtures of non-ionic and ionic surfactant, or two ionic surfactants. 

It has been shown by Harvey et al. (1989) that incorporation of palmitic acid into a monolayer spread from stearoylserine methyl ester (SSME) breaks up intermolecular SSME interactions. The palmitic acid acts as a two-dimensional diluent. Figures 52(A-C) give the Yl/A isotherms for mixtures of FE and SE C-15 6,6 -A with palmitic acid. Dilution of the monolayer cast from the second eluting isomer with 15 mol% palmitic acid separates the diacid molecules from one another on the water surface and perhaps allows for the expression of their stereochemically dependent conformations. The mixed film (15% palmitic acid/85% C-15 6,6 -A) expands at low II and behaves in much the same manner as the single-component monolayer (C-15 6,6 -A) behaves at 30°C. Addition of 15 mole% palmitic acid into a monolayer cast from the FE C-15 diacid has little effect on its energetics of compression, indicating a stronger intermolecular interaction afforded by its stereochemically dependent conformation at the air-water interface. 

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). 

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