Films liquid expanded

It is sometimes found convenient to designate two subclasses of liquid films—liquid-expanded (Li) and liquid condensed (L2)—based on subtle differences in respective ti-A curves. The Li curve is one which typically extrapolates to a limiting value of is (sometimes zero) at a molecular area of about 0.5 nm. In contrast to the bulk liquid analog, such films exhibit a significant degree of compressibility but show no signs of island or hemimicelle formation that is, it appears to maintain the characteristics of a uniform phase. In many cases, Li films show a transition to a gaseous film at low pressures and perhaps to an L2 film as the available area per molecule is decreased. 

On compression, a gaseous phase may condense to a liquid-expanded, L phase via a first-order transition. This transition is difficult to study experimentally because of the small film pressures involved and the need to avoid any impurities [76,193]. There is ample evidence that the transition is clearly first-order there are discontinuities in v-a plots, a latent heat of vaporization associated with the transition and two coexisting phases can be seen. Also, fluctuations in the surface potential [194] in the two phase region indicate two-phase coexistence. The general situation is reminiscent of three-dimensional vapor-liquid condensation and can be treated by the two-dimensional van der Waals equation (Eq. Ill-104) [195] or statistical mechanical models [191]. 

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]. 

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

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. 

When the area available for each molecule is many times larger than molecular dimension, the gaseous-type film [state 1] would be present. As the area available per molecule is reduced, the other states, for example, liquid-expanded [Lex], liquid-condensed [Lco], and, finally, the solid-like [S or solid-condensed] states would be present. 

FIGURE 4.6 II versus A isotherms for different types of states (a) gas film (b) liquid-expanded (Lex) and liquid-condensed (Lco) solid films collapse state. 

Liquid Expanded Films (Lexp) In general, there are two distinguishable types of liquid films. The first state is called the liquid expanded (Lexp) (Gaines, 1966 Chattoraj and Birdi, 1984 Adamson and Gast, 1997). If the Il-A isotherm is extrapolated to zero n, the value of A obtained is much larger than that obtained for close-packed films, shows that the distance between the molecules is much larger than that in the solid him (to be discussed in later text). These films exhibit very characteristic elasticity. 

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. 

Devaux also advanced the important theory that the characteristics of the solid, liquid and gaseous states of matter are retained so long as one continuous layer of molecules remains unbroken. This conception has been partially confirmed by the work shortly to be described. A film may be solid, liquid, expanded or gaseous, and one kind is readily distinguished from another. In certain properties, a solid film of unimolecular thickness resembles quantitatively a three-dimensional solid mass of the same substance, but these properties are necessarily limited to such as can be measured in any given direction. 

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. 

Films which are still coherent but occupy a much larger area than condensed films. They have no real three-dimensional equivalent, since they act as highly compressible liquids. A number of distinct types of these expanded films have been recognised21, the most important being the liquid-expanded state, but these will not be considered in detail. 

There are a number of instances in which (with the aid of sensitive measurements) well-defined transitions between gaseous and coherent states are observed as the film is compressed. The tt-A curves show a marked resemblance to Andrews p-V curves for the three-dimensional condensation of vapours to liquids. The tt-A curve for myristic acid, given as an example, has been drawn schematically to accentuate its main features (Figure 4.26). Above 8 nm2 molecule-1 the film is gaseous and a liquid-expanded film is obtained on compression to 0.5 nm2 molecule-1. Fluctuating surface film potentials verify the heterogeneous, transitional nature of the surface between 0.5 nm2 molecule-1 and 8 nm2 molecule-1. 

Liquid-expanded films obey the equation of state,... 

Figure 3.24 Illustration ofthe origin of surface elasticity as a liquid film is expanded.

As the compression of the surface continues, the vapor phase disappears and the film is in the liquid state, also referred to as liquid expanded (LE, E or Li) state. The area per molecule in this state is in the range of 25-40 A2 for fatty acid films, which is intermediate between those in gaseous and solid (i.e. condensed) states. The surface pressure increases as the area per molecule decreases. The molecules in the liquid expanded state interact strongly with each other but are not closely packed. Defects, such as gauche conformations, can be found in the alkyl tails of the molecules. 

A single substance may, under appropriate conditions of temperature and surface pressure, be obtained as a condensed film, either a liquid- or a vapour-expanded film, but not both, and a gaseous film. The class of vapour-expanded film is rather ill defined, and shades into the gaseous films but the properties of these films are often so different from those of the gaseous films, and so close to the liquid-expanded films, that a separate name seems desirable. The expanded films form a state without any close parallel in three-dimensional matter. So far as is known, they are only found with long-chain aliphatic substances. 

The third evidence is that it is often possible to change a coherent film, condensed or liquid-expanded, into a gaseous film simply by introducing into the molecule a second point of attraction to the water, some distance from the first. These two water-soluble groups would naturally make the molecules lie flat. Thus the following form good gaseous films ... 

---