Monolayers 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... 

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... 

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).

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. 

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). 

The three general states of monolayers are illustrated in the pressure-area isotherm in Fig. IV-16. A low-pressure gas phase, G, condenses to a liquid phase termed the /i uid-expanded (LE or L ) phase by Adam [183] and Harkins [9]. One or more of several more dense, liquid-condensed phase (LC) exist at higher pressures and lower temperatures. A solid phase (S) exists at high pressures and densities. We briefly describe these phases and their characteristic features and transitions several useful articles provide a more detailed description [184-187]. 

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. 

Because of the charged nature of many Langmuir films, fairly marked effects of changing the pH of the substrate phase are often observed. An obvious case is that of the fatty-acid monolayers these will be ionized on alkaline substrates, and as a result of the repulsion between the charged polar groups, the film reverts to a gaseous or liquid expanded state at a much lower temperature than does the acid form [121]. Also, the surface potential drops since, as illustrated in Fig. XV-13, the presence of nearby counterions introduces a dipole opposite in orientation to that previously present. A similar situation is found with long-chain amines on acid substrates [122]. 

The most common two-dimensional phases in monolayers are the gaseous, liquid-expanded, liquid-condensed, and solid phases. A schematic II-A isotherm is shown in Figure 3 for a fatty acid for the phase sequence gas (G) — G -l- liquid-expanded (LE) — LE — ... 

FIG. 3 An isotherm is depicted for a Langmuir monolayer of an amphiphUe showing the ft-A variation for the phase sequence gas (G) —> G + liquid-expanded (LE) —> LE —> LE + tilted condensed phase (L2) —> L2 —> vertical condensed phase (LS) —> S (solid). Schematic depictions of the molecular organization in the phases are shown above the isotherm. 

Initially, the compression does not result in surface pressure variations. Molecnles at the air/water interface are rather far from each other and do not interact. This state is referred to as a two-dimensional gas. Farther compression results in an increase in snrface pressure. Molecules begin to interact. This state of the monolayer is referred as two-dimensional liquid. For some compounds it is also possible to distingnish liqnid-expanded and liquid-condensed phases. Continnation of the compression resnlts in the appearance of a two-dimensional solid-state phase, characterized by a sharp increase in snrface pressure, even with small decreases in area per molecule. Dense packing of molecnles in the mono-layer is reached. Further compression results in the collapse of the monolayer. Two-dimensional structure does not exist anymore, and the mnltilayers form themselves in a non-con trollable way. 

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. 

Zeelen found the extent of chiral discrimination to be dependent on the type of monomolecular phase that was formed. Thus, racemic and optically active samples displayed identical force-area curves (Fig. 14) when both formed liquid-expanded films, but owed considerably different curves (Fig. 15) under conditions where both samples formed a more highly condensed monolayer. 

In Figure 2 the ir-A and AV-A plots for SODS on O.OIM NaCl sub-solutions having different pH values are shown. In all cases, phase transitions from liquid-expanded to liquid-condensed state are evident ( ). Acidification of the subsolution Increases the transition pressure but the transition is less pronounced at the lowest pH studied. This is also accompanied by an expansion of the condensed part of the curve. Small negative surface potentials are observed over most areas. The highest potential is obtained for film spread on the pH 2.2 subsolution. For small areas, the surface potential attains a positive value. This may be related to reorientation of the dipole moments of the molecules which occur once a threshold surface concentration is exceeded (O. Mlnglns and Pethlca (7) studied the monolayer properties of SODS on various sodium chloride solutions (0.1, 0.01 and O.OOIM) at 9.5 C, and they showed that the monolayer is only stable on the more concentrated salt solutions (0.1 and O.OIM). In this work, no noticeable... 

It was established in 1945 that monolayers of saturated fatty acids have quite complicated phase diagrams (13). However, the observation of the different phases has become possible only much more recendy owing to improvements in experimental optical techniques such as fluorescence, polarized fluorescence, and Brewster angle microscopies, and x-ray methods using synchrotron radiation, etc. Thus, it has become well accepted that lipid monolayer structures are not merely solid, liquid expanded, liquid condensed, etc, but that a faidy large number of phases and mesophases exist, as a variety of phase transitions between them (14,15). 

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