Liquid-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. 

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. 

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... 

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]. 

Leontidis E, Aroti A (2009) Liquid expanded monolayers of lipids as model systems to understand the anionic Hofmeister series 2. Ion partitioning is mostly a matter of size. J Phys Chem B 113 1460-1467... 

Leontidis E, Aroti A, Belloni L (2009) Liquid expanded monolayers of lipids as model systems to understand the anionic Hofmeister series 1. A tale of model. J Phys Chem B 113 1447-1459 Levitsky VY, Panova AA, Mozhaev VV (1994) Correlation of high-temperamre stability of a-chymotrypsin with salting-in properties of solution. Eur J Biochem 219 231-236 Lipkind GM, Fozzard HA (2008) Voltage-gated Na channel selectivity the role of the conserved domain III lysine residue. J Gen Physiol 131 523-529 Lo Nostro P Fratoni L, Ninham BW, Baglioni P (2002) Water absorbency by wool fibers Hofmeister effect. Biomacromolecules 3 1217-1224... 

Leontidis E, Aroti A, Belloni L. (2008) DPPC liquid expanded monolayers as model systems to understand the anionic Hofineister series 1. A tale of models. JPhys Chem B 113 1447-1459. 

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]. 

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. 

FIG. 16 Fomation of a Langmuir lipid monolayer at the air/subphase interface and the subsequent crystallization of S-layer protein, (a) Amphiphilic lipid molecules are placed on the air/subphase interface between two barriers. Upon compression between the barriers, increase in surface pressure can be determined by a Wilhelmy plate system, (b) Depending on the final area, a liquid-expanded or liquid-condensed lipid monolayer is formed, (c) S-layer subunits injected in the subphase crystallized into a coherent S-layer lattice beneath the spread lipid monolayer and the adjacent air/subphase interface. 

When a component of interest is considerably surface active, its adsorbed amount is high even when its bulk concentration is low. The second terms on the right-hand side of Eqs. (4)-(6) are then small and the relative surface excesses are simply taken as the surface excesses, which, in turn, may be seen as the surface concentration. For example, dilaur-oylphosphatidylcholine forms a saturated monolayer in the liquid-expanded state at the nitrobenzene-water interface when its concentration in nitrobenzene is 10 moldm [30]. Then the experimentally obtained value, 1.76 x 10 °molcm, can be considered to be the surface concentration. 

Phospholipid monolayers in liquid expanded state are likely to modify mostly the interfacial concentrations, and this electrostatic effect can be described by Eq. (6). Taking a 1/2, in agreement with most experimental results, and introducing the approximation A 02 = 0, Eq. (6) simplifies to [59]... 

The effect of phospholipid monolayers on the rate of charge transfer has been the subject of several experimental studies, but still there is a need for additional experimental evidence. For large molecular areas, the effect on the rate of ion transfer seems to be negligible [5]. An increasing surface concentration of lipids leads to liquid expanded states where the electrostatic effects are noticeable. An enhanced rate of ion transfer across monolayers of pure phospholipids has then been observed both for the cases of tracer [11,12] and supporting electrolyte ion transfer [13,17]. Finally, the blocking effect is dominant in liquid condensed monolayers [15]. 

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.). 

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. 

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. 

From these descriptions, it is seen that the films may, under given experimental conditions, show three first-order transition states, such as (i) transition from the gaseous film to the liquid-expanded (Lex), (ii) transition from the liquid-expanded (Lex) to the liquid-condensed (Lco), and (iii) from Lex or Lco to the solid state if the temperature is below the transition temperature. The temperature above which no expanded state is observed has been found to be related to the melting point of the lipid 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... 

Condensed monolayer films of pure 6 polymerized rapidly, as did mixed 6/DSPE films of up to 75% DSPE, provided the monolayers were in the condensed state [33], In the liquid-expanded state, polymerization did not occur. In the condensed state, lateral diffusion of individual lipids within the monolayer is severely restricted compared to the liquid-like state. This precludes initiation of polymerization by diffusive encounter between excited-state and ground-state diacetylene lipids. In order for polymerization to occur in the condensed state, the film must be separated into domains consisting of either pure 6 or pure DSPE. A demonstration that the rates of photopolymerization for pure 6 and mixed 6/DSPE monolayers are equal would be a more stringent test for separate domains of the lipids, but no kinetic data have been reported for this system. 

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). 

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... 

Dipalmitoyl Lecithin—Cholesterol Monolayers. The average area per molecule in dipalmitoyl lecithin-cholesterol monolayers shows deviation at low surface pressures, whereas at 30 dynes per cm. it follows the additivity rule (Figures 8 and 9) (42). The surface pressure—area curve of dipalmitoyl lecithin monolayers is liquid-expanded up to 30 dynes per cm., whereas above this surface pressure it is relatively incompressible (42). Figures 10b and c represent the structures of the dipalmitoyl... 

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