Expanded monolayers

L. The liquid-expanded, L phase is a two-dimensionally isotropic arrangement of amphiphiles. This is in the smectic A class of liquidlike in-plane structure. There is a continuing debate on how best to formulate an equation of state of the liquid-expanded monolayer. Such monolayers are fluid and coherent, yet the average intermolecular distance is much greater than for bulk liquids. A typical bulk liquid is perhaps 10% less dense than its corresponding solid state. 

FIG. 1 Schematic showing the preparation of Langmuir films of latex particles at the air-water interface (a) Spreading of the latex and formation of an expanded monolayer (b) formation of the compressed monolayer. 

It has been shown by FM that the phase state of the lipid exerted a marked influence on S-layer protein crystallization [138]. When the l,2-dimyristoyl-OT-glycero-3-phospho-ethanolamine (DMPE) surface monolayer was in the phase-separated state between hquid-expanded and ordered, liquid-condensed phase, the S-layer protein of B. coagulans E38/vl was preferentially adsorbed at the boundary line between the two coexisting phases. The adsorption was dominated by hydrophobic and van der Waals interactions. The two-dimensional crystallization proceeded predominately underneath the liquid-condensed phase. Crystal growth was much slower under the liquid-expanded monolayer, and the entire interface was overgrown only after prolonged protein incubation. 

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. 

Phosphatidic Acid Monolayers. Phosphatidic acid, prepared from egg lecithin by the action of phospholipase D, forms considerably more expanded monolayers than egg lecithin, presumably because of ionic repulsion between the phosphate groups in the phosphatidic acid mono-layers (42). Phosphatidic acid monolayers showed about four times more increase in surface potential when CaCl2 is substituted for NaCl in the subsolution than did egg lecithin monolayers (43). This again supports the conclusion that the trimethylammonium group competes with Ca2+ for the anionic phosphate group in egg lecithin monolayers (Figure 1A). 

To understand the concept of intermolecular cavities we should consider the structure of an expanded monolayer. Since the hydrocarbon chains in an expanded monolayer are in the liquid state, they have greater... 

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). 

The relationship between surface tension and temperature in emulsifiers was observed two decades ago by Lutton et al.43. They explained that this relationship is due to a transition from a liquid-expanded type of monolayer existing at high temperatures (above 40°C) to a solid condensed monolayer existing at a lower temperature (below 20°C). In solid condensed monolayers the molecular packing of the emulsifier molecules is much denser than in the liquid expanded monolayers, and these differences result in lower or higher surface tension, respectively. 

It is seen that Eq. (15), which follows approximately from Eq. (14) (assuming low monolayer coverage and neglecting entropy non-ideality), can also describe the behaviour of monolayers which comprise particles of any size. Similarly to Eq. (14), this equation involves not the geometric parameters of amphiphilic molecules (or particles), but only monolayer coverage by these entities. Equation (15) provides a good description of the experimental results obtained for various systems. For example, for some insoluble proteins in the liquid-expanded monolayer range, the value n = 20-100 was obtained.35... 

The study of monolayers formed on a wafer surface has also provided imporfanf informahon. A fhin film of an amphiphilic (confaining both polar and nonpolar groups) compormd such as a fatty acid is prepared. This is done by depositing a small quantity of the compound dissolved in a volatile solvent on a clean aqueous surface befween fhe barriers of a Langmuir trough (Fig. 8-8). The difference in surface fension (n) across the barriers is measured with a suitable device for differenf areas of the monolayer, i.e., for differenf positions of the moveable barrier. The value of n is low for expanded monolayers and falls to nearly zero when fhe surface is no longer completely covered. The pressure reaches a plateau when a compact mono-layer is formed, after which if rises again (Fig. 8-8B). 

The diagram presented in Fig. 1 (curve A) above 58°C is a straight line up to 71°C. The increase of retention volume beginning at this temperature may be only the effect of the phase transition occurring in the n-octadecanol monolayer. The result of this transition is that monolayer transforms from a SC to LE state and that the surface area occupied by a single molecule in the monolayer increases to 0.27 nm. It means that as much as 23% of alcohol molecules must be ejected from the monolayer to the threedimensional phase. Therefore, AV is the result of the increase of the liquid n-octadecanol amount in the column. The analysis of the vs r confirms that a single n-octadecanol molecule occupies 0.27 nm in the expanded monolayer. 

Serpinet, using the inverse gas chromatography method, demonstrated the existence of oriented monolayers of long-chain hydrocarbons on silica gel surface [13], on the other hand Untz [31] showed that hydrocarbons also form solid condensed and liquid expanded monolayers on glycerol but not on the water surface. However, the addition of some amount of amphiphilic molecules to the hydrocarbon provokes the mixed monolayer formation on the water surface. The phase transition in such a monolayer occurs at the temperature higher than the melting point of bulk hydrocarbon. It also appeared that the monolayers characterized by 1 1 ratio of hydrocarbon to alcohol molecules were particularly stable [41]. 

If these expanded monolayers are considered as sets of particles that have two-dimensional translations, a compression from Ai to A2 gives a change in translational entropy ... 

Desorption kinetics provide additional information that contributes to our understanding of surface phenomena such as the specific effects of tris and bicarbonate buffers on monolayers. Stable condensed mono-layers were expanded on tris buffers, and ionization appeared enhanced (8). Unstable, expanded monolayers did not expand further on tris buffers, but K data (Table I) showed a consistent decrease in the apparent pKa when fatty acids were spread on tris buffers. 

Further, Mann and McGregor (12) suggested that the study of the ESR spectral characteristics of labeled molecules in monolayers could lead to an experimental determination of surface viscosity numbers for expanded monolayers, While we do not report surface viscosity numbers here, we do report the spectra obtained using two ellipsoidal molecules in monomolecular films spread on water. We also report the low surface pressure isotherms for these systems and comment on the extent to which isotherms in general reflect molecular motion in monolayers. 

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