States of Monomolecular Films

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]). 

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]. 

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It is found that, even a monolayer of lipid (on water), when compressed can undergo various states. In the following text, the various states of monomolecular films will be described as measured from the surface pressure, n, versus area, A, isotherms, in the case of simple amphiphile molecules. On the other hand, the Il-A isotherms of biopolymers will be described separately since these have a different nature. 

Factors influencing the physical state of monomolecular films... 

The reaction of interest—response of the monomolecular complex film to changes of pH—occurred more rapidly than the measurement technique could follow—i.e., in less than ca. 10 seconds. Such rapid response implied that the dissociation equilibrium was mobile and depended principally upon the electrostatic state of the film. 

The molecules in a monomolecular film, especially at high surface concentrations, are often arranged in a simple manner, and much can be learned about the size, shape and orientation of the individual molecules by studying various properties of the monolayer. Monomolecular films can exist in different, two-dimensional physical states, depending mainly on the magnitude of the lateral adhesive forces between the film molecules, in much the same way as three-dimensional matter. 

Surface viscosity is the change in the viscosity of the surface layer brought about by the monomolecular film. Monolayers in different physical states can readily be distinguished by surface viscosity measurements. 

Insoluble surface films can be studied by electron microscopy. The films are transferred from the substrate on to a collodion support and shadow-cast by a beam of metal atoms directed at an angle a (about 15°) to the surface (Figure 4.21). If the width x of the uncoated region is measured, the thickness of the film, x tan a, can be calculated for example, a /i-C36H73COOH film has been shown to be about 5 nm thick - i.e. consistent with a vertically orientated monomolecular layer. The technique has also been used for following the state of the surface as a film is compressed. 

These monomolecular films are a most interesting state of matter. In them the molecules are often arranged in a simple manner, so that from a study of the films much can be learned as to the size, shape, and other properties of the individual molecules. It is largely because they offer a simple way of studying the properties of molecules that so much attention has been devoted to these films during recent years. 

Monomolecular films may exist in many different forms, which correspond, in the two dimensions of the surface, to the three principal states of matter in three dimensions, solid, liquid, and gaseous. The principal factor determining whether or not the films are stable is the strength of the anchorage of the molecules to the surface, an attraction perpendicular to the surface the principal factors deciding the state of the surface films... 

A detailed account of the mechanism of spreading will be given in Chapter VI the final state of a monomolecular film with the excess collected locally in small drops seems always to be found with pure substances. Complex mixtures may, however, form much thicker films of considerable durability, e.g. kerosene on water. 

It has been stated that the monomolecular theory arose through Rayleigh noticing that the thickness of the films is about the known dimensions of a molecule. The passage quoted above from his paper shows that the theory rests not on a mere... 

The exact state of the monomolecular layers on the two faces of soap films is unknown, but would almost certainly be, on bulk solutions of the same concentration, a very highly compressed gaseous film. Possibly the attraction between the molecules on opposite faces of the films exercises some restraint on the motions in the surface layers but it is usually insufficient to solidify the films. 

Leslie s explanation,8 that the spreading is due to an attraction exerted by the lower liquid on molecules of the upper liquid beyond the first layer, squeezing out those in the first layer, cannot be correct, or the final state could not be a monomolecular film in equilibrium with excess liquid in drops—the drops would be squeezed out along the surface. 

Monolayers of distearoyl lecithin at hydrocarbon/water interfaces undergo temperature and fatty acid chain length dependent phase separation. In addition to these variables, it is shown here that the area and surface pressure at which phase separation begins also depend upon the structure of the hydrocarbon solvent of the hydrocarbon oil/aqueous solution interfacial system. Although the two-dimensional heats of transition for these phase separations depend little on the structure of the hydrocarbon solvent, the work of compression required to bring the monomolecular film to the state at which phase separation begins depends markedly upon the hydrocarbon solvent. Clearly any model for the behavior of phospholipid monolayers at hydrocarbon/water interfaces must account not only for the structure of the phospholipid but also for the influence of the medium in which the phospholipid hydrocarbon chains are immersed. 

The Gibbs adsorption isotherm shows the dependence of the extent of adsorption of an adsorbent on its bulk concentration or pressure. However, we also need to know the state of the adsorbate at the surface. These are interrelated because the extent of material adsorb-tion on a surface depends on the state of the surface. The behavior of the molecules in the surface film is expressed by a surface equation of state which relates the spreading pressure, n, which is the difference between the solvent and solution surface tensions, %= % - y to the surface concentration of the adsorbent. This equation is concerned with the lateral motions and interactions of the molecules present in an adsorbed film. In general, the surface equation of state is a two-dimensional analogue of the three-dimensional equation of state of fluids, and since this is related to monomolecular films, it will be described in Sections 5.5 and 5.6. It should be remembered that on liquid surfaces, usually monolayers form, but with adsorption on solid surfaces, usually multilayers form (see Section 8.3). 

The crucial condition that characterizes failure of a monomolecular film is inability to persist in a state of structured adsorption on the bounding surfaces consequently atoms in these surfaces can approach one another and interact directly. Molecules of non-polar liquids are not adsorbed as persistently at the bounding surfaces as are molecules of polar "boundary" lubricants and are more easily disoriented and desorbed under the influence of increased temperature and shear stress. 

FIGURE 8.13. Molecules in a monomolecular film are usually considered to exist in one of three principle states. (a) The gaseous state is that in which the molecules are relatively far apart and have little mutual interaction the film is compressible. (b) The liquid expanded state is that in which the head groups are relatively closely packed, but there is significant degree of tail mobility the film is compressible to a limited extent, (c) The condensed state in which the molecules are closely packed and have very limited mobility the film is essentially incompressible. 

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