Monolayers liquid-condensed phase

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]. 

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. 

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... 

A general thermodynamic theory and a statistical thermodynamic approach are presented, which describe the phase transitions in insoluble monolayers, particularly the inclined transition from a liquid-expanded to a liquid-condensed phase. 

Fig. 3. A representative pressure vs. area isotherm. The cartoons represent idealized gas-analogous, liquid-expanded, and liquid-condensed phases of the monolayer. Two phases often coexist at a given temperature and surface pressure.

Liquid-condensed phase (surface) A monolayer phase of high surface viscosity in which there is a substantial degree of translational ordering and also possibly orientational... 

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 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. 

Application of advanced microscopic and spectroscopic methods, such as Brewster Angle Microscopy (BAM), Infrared Reflection-Adsorption Spectroscopy (IRRAS) and Grazing Incidence X-ray Dififaction (GIXD) at the monolayer revealed that when two lipid phases coexist, only the disordered liqnid-expanded phase is affected by the ions. Table I snm-marises GIXD results for the crystal cell parameters of the ordered liquid-condensed phase. These results prove that the ordered DPPC phase is not strongly affected by salts even at high salt concentrations. 

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

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... 

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. 

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...

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. 

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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