Liquid expanded phase

Another interesting class of phase transitions is that of internal transitions within amphiphilic monolayers or bilayers. In particular, monolayers of amphiphiles at the air/water interface (Langmuir monolayers) have been intensively studied in the past as experimentally fairly accessible model systems [16,17]. A schematic phase diagram for long chain fatty acids, alcohols, or lipids is shown in Fig. 4. On increasing the area per molecule, one observes two distinct coexistence regions between fluid phases a transition from a highly diluted, gas -like phase into a more condensed liquid expanded phase, and a second transition into an even denser... 

S-layer proteins adsorb preferentially at lipid films in the liquid-expanded phase [138] Crystalization is observed only at the liquid-condensed phase [138]... 

Figure 4. Principle of monolayer characterization via surface pressure (n)-area (A) isotherms (a) gaseous phase, (b) liquid expanded phase, (c) condensed phase (head packing), (d) condensed phase...

On the contrary, butadiene and methacryloyl monomers (1,3,4, 10,11) can also be polymerized in the liquid expanded phase. The butadiene lipids have previously been shown to form 1,4-trans-poly(butadiene)s (40j in the monolayer (Eqn. II.). 

The monolayer behavior of A-stearoyltyrosine (Fig. 16) was more complex. Under conditions (0.0liV HCl, 22 C) where the racemic material formed a condensed film having a limiting molecular area of 39 2 A, the force-area curve of L-(+)-A-stearoyltyrosine exhibited a liquid-expanded film at large areas (ca. 100-45 per molecule) followed by a transition beginning at 16.5 dynes/cm surface pressure to a condensed phase having a smaller limiting molecular area of 34 2 A . However, both these latter samples exhibited only the liquid-expanded phase on distilled water alone. 

Contaminations are also responsible for the second difference between real and ideal isotherms. At 7rc the isotherm is not perfectly horizontal but slightly tilted, in particular at elevated temperatures. Contaminations are expelled from the liquid condensed phase. Thus, when more and more of the monolayer goes into the liquid condensed phase, contaminations are enriched in the remaining liquid expanded phase. This reduces the two-dimensional... 

As shown in the Figure 2, compared to the corresponding homopolymer, the copolymers show a liquid expanded phase depending on the comonomer content. This liquid expanded phase behaviour is not caused by a phase transition in the side chain as found with many natural and synthetic lipids. In this case it can be explained as an entropy driven coiling (B)—uncoiling (A) process of the copolymeric main chain (Frey et al., 1987). 

To describe the experimental observation [40] of the solid condensed - liquid expanded phase transitions in brush-like monolayers on silica gel a simple lattice model and the theory of orientational effects in adsorbed monolayers were used [36-38]. It was assumed that interaction between the n-octadecanol molecule and the solid could be presented as... 

Figure 7. Tapping-mode AFM image of one monolayer of -CH2OH derivatized grid Gs prepared by the LB-technique. Surface pressure at transfer 1.5 mN/m, scan size 5x5 pm (liquid-expanded phase).

FIGURE 8.16. A monomolecular film of a straight chain carboxylic acid such as myristic acid on distilled water will show a sharp transition in the n-A curve as the head groups become more closely packed. That kind of transition may be viewed as something like a reversed sublimation in which the film passes from gaseous to solid condensed without passing through the liquid expanded state. With very careful experimental work, it is sometimes possible to identify an intermediate liquid expanded phase as illustrated for myristic acid on 0.1 N HCl. 

When the gaseous monolayer is compressed, a transition into the so-called liquid-expanded phase usually takes place. This phase has been the subject of many controversies. Most of this discussion, however, took place before the structure of the lamellar liquid crystalline phase was known. It should be mentioned in this connection that Phillips et al. (1969) used such correlations in their interpretation of the monolayer structure of dipalmitoylphosphatidyl-choline which will be further discussed in Section 8.10. 

The formation of the liquid-expanded phase at compression of a monolayer means that condensation from a gaseous phase to a coherent film in the liquid state takes place. Further compression can give phases with extended hydrocarbon chains tilted in relation to the surface. Such phases are called liquid-condensed (L2) phases and a consequence of this misleading term is that the formation from a liquid-expanded phase is described as a condensation. 

For the isotherm of Figure 4.20, the surface pressure is constant in a two-phase region where the gas G and a liquid expanded phase LE coexist. For pentadecanoic acid at 15°C, such behavior occurs for jt = 0.10 mN/m. As the phase diagram of Figure 4.20 shows, there is a critical temperature (near 40°C... 

Liquid-expanded phase The monolayer phase of low surface viscosity that flows readily on the water surface and possesses only weak short-range translational order. 

Plateau (in 71-A isotherms) The flat or close to flat region upon compression of a Langmuir monolayer exhibiting a liquid expanded phase which is progressively converted into a liquid condensed phase. 

The surface rheological proprieties can be controlled by using monolayers of different lipid compositions. We used saturated and unsaturated lipids and their mixtures with different amounts of cholesterol. At the experimental temperature and film pressure the saturated lipids were in the liquid condensed or solid phase whereas the unsaturated lipids in the liquid expanded phase [3]. The mechanical properties of the monolayers can be tuned with addition of different amounts of cholesterol. The results are used for the proof of bilayer or multilayer synthesis and the conditions of their occurrence. 

DOrc monolayers, due to the unsaturation, i.e. kinks of the alkyl chains, are in the liquid expanded phase, which is a fluid phase at all film pressures FI [3,13,15]. At 21 °C and T1 >25 mN m DPPC monolayers are in the solid analogous phase [3,13,16], which is highly incompressible and condensed [13,16]. Shah and Schulman [13] show that the effect of cholesterol on either saturated or unsaturated phospholipids is strikingly different. Cholesterol increases the surface elasticity, the dilational and the shear viscosity of unsaturated phospholipid monolayers [3,13,14,17]. In saturated monolayers cholesterol disturbs the order between phospholipid molecules fluidifying the solid monolayer [13,14,18] and lowering its shear viscosity [18]. Pure cholesterol monolayers are liquid [13] and have very low surface shear viscosities which are hardly detectable [18]. 

The four possible stereomers of a chiral surfactant with two asymmetric centers within the polar head group have been synthesized and their absolute configuration determined by X-ray diffraction. One of the diastereomers exhibits a chiral discrimination when spread on water interface the monolayer racemic film undergoes a phase transition from a liquid-expanded towards a liquid-condensed phase upon compression, while the pure enantiomers only have a liquid-expanded phase, as revealed by the measured pressure-area isotherms. The transition pressure-composition diagram indicates that heterochiral interactions are favored. Our results are compared to predictions of Andelman and de Gennes based upon a statistical model. 

They also suggest that for the purpose of extracting quantitative information, one should focus on the liquid-expanded phase (LE). A closer look at the DPPC monolayer EOS in the range of 80 to 90 per molecule, where the monolayer is in the pure LE phase, reveals that the electrolytes displace the EOS in an almost parallel fashion and the degree of displacement depends on anion type (in this series of experiments) (Eig. 7). 

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