Insoluble Monomolecular Films

Surface tension lower on side where surfactant added 

FIGURE 8.11. If a surface-active material is added to the surface of water in a container where the surface is divided by a barrier, two types of adsorption may occur (a) if the material has significant solubility in water, diffusion through the water will occur and adsorption will result on both sides of the barrier to produce a Gibbs monolayer (b) if the material is essentially insoluble in water, adsorption will occur on the side of the barrier to which it is applied, but not on the opposite side, leaving an insoluble monolayer. 

Consider now the situation on the right in which an essentially insoluble, but still surface-active, material such as stearic acid is placed on one side of the barrier. Assuming that the barrier does not leak, the system may be left for any practical period of time and when the surface tension of each side is measured, it will be found that a for the side to which stearic acid has been added has been lowered, while the other side remains that of the pure water, (To. Stearic acid may be added until a monolayer is formed with no change in (for the clean side. The added stearic add molecules, being insoluble in water, cannot be dissolved and transported through the water to be adsorbed on the other side of the barrier. The monolayer ultimately formed is an insoluble monolayer. 

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These insoluble monomolecular films, or monolayers, represent an extreme case in adsorption at liquid surfaces, as all the molecules in question are concentrated in one molecular layer at the interface. In this respect they lend themselves to direct study. In contrast to monolayers which are formed by adsorption from solution, the surface concentrations of insoluble films are known directly from the amount of material spread and the area of the surface, recourse to the Gibbs equation being unnecessary. 

If a small amount of protein solution is suitably spread at the surface of an aqueous substrate, most of the protein will be surface-denatured, giving an insoluble monomolecular film before it has a chance to dissolve. The techniques already described for studying spread monolayers of insoluble material can, therefore, be used in... 

When 1 cm of a solution containing 8.5 mg per 100 cm of stearic acid (mol. wt. = 284.3) dissolved in a volatile organic solvent is placed on the surface of water in a Langmuir trough, the solvent evaporates off, leaving the stearic acid spread over the surface as an insoluble monomolecular film. If the surface area occupied by the film is 400 cm, calculate the area occupied by each molecule of stearic acid in the film. 

Ermakov SA, Kijashko SV, Konnov IR (1995) Laboratory study of the damping of gravity-capillary waves on the water surface, covered with insoluble monomolecular films. Preprint N382, Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod... 

The investigation of solution and surface film properties of two- or three-component liquid solutions is the subject of this chapter. In one extreme, the components in the liquid solution are completely miscible giving a one-phase solution, and in the other extreme, the components are almost completely immiscible, and an insoluble monomolecular film of one component forms on the surface of the other giving a two-phase solution. Between these two extremes, different kinds of films form on the solution surfaces depending on the extent of molecular interactions between the components. The theoretical approaches and experimental techniques that are applied to these solution types will be described in Chapters 5 and 6 respectively. 

The most familiar transitions between surface phases in fluid interfaces are those in the so-called insoluble monomolecular films that some higher alcohols and fatty adds form at a water-air interface. Distinct surface phases and transitions between them are frequently observed also in monolayers of adsorbed gases on solid substrates, and are the subject of an exuberant modem literature. ... 

A number of amphiphilic calixarenes have been shown to possess the ability to self-assemble in the form of insoluble monomolecular films at the air-water interface, which are called Langmuir monolayers. In order to exhibit the ability to form Langmuir monolayers, the amphiphilic derivative has to be designed so that it is not soluble in water. Indeed if the amphiphiles are partially water-soluble, they do not assemble at the air-water interface but (at least partially) in the water phase in the form of micelles [12, 30, 31]. Amphiphilic calixarenes, self-assembled as Langmuir monolayers at the air-water interface, typically float on the water surface with an orthogonal orientation with respect to the interface the aliphatic chains point into air while the polar functions are immerged into the subphase (Fig. 37.4). 

Foams are coarse dispersions of gas in a relatively small amount of liquid. In solid foams, the continuous phase is a solid which, at one point in time, has been a liquid. Pure liquids cannot form foams and nearly all liquid foams are thermodynamically unstable. Only liquids containing a surface-active eomponent can form foams. The surface-active component may be a solute dissolved in the liquid or insoluble matter at the interface, such as a solid in the form of insoluble particles, a liquid-crystal phase, or an insoluble monomolecular film. Surfactants can stabilize but also destabilize foams and cause their collapse. 

It was found centuries later that some lipid-like substances (almost insoluble in water) formed self-assembly monolayers (SAMs) on the surface of water (Gaines, 1966 Adamson and Gast, 1997 Birdi, 1989, 1999, 2002 Chattoraj and Birdi, 1984). A few decades ago, experiments showed that monomolecular films of lipids could be studied by using rather simple experimental methods (Figure 4.1). 

Many other applications have been described for these compounds and the hst is so extensive that it is impossible to cover each example here. Nevertheless, it is noteworthy that appropriately substituted 4-arylidene-5(4r/)-oxazolones, for example, 359 and 360 have been reported as organic Iruninophores" " and electrophotographic light-sensitive materials," respectively. The use of apoca-rotenoid derivatives such as 362 as monomolecular films has also been described." For new materials, a number of water-insoluble oxazolones have been used for dyeing or printing synthetic fibers" and, in this context, 4-(pyren-1-ylmethylene)-2-substituted-5(4/i/)-oxazolones 357, prepared from pyrene-1-carboxaldehyde, have... 

The adsorption of amphiphilic molecules at the surface of a liquid can be so strong that a compact monomolecular film, abbreviated as monolayer, is formed. There are amphiphiles which, practically, do not dissolve in the liquid. This leads to insoluble monolayers. In this case the surface excess T is equal to the added amount of material divided by the surface area. Examples of monolayer forming amphiphiles are fatty acids (CH3(CH2) c 2COOH) and long chain alcohols (CH3(CH2)nc iOH) (see section 12.1). 

Amphiphilic molecules form monomolecular films on liquid surfaces. Some amphiphiles, such as phospholipids, with a large hydrophobic tail are practically insoluble in water and therefore form insoluble monolayers at the air-water interface. 

As can be seen from Fig. 6.3, it was found that the partially purified, microbubble-surfactant mixture does in fact form stable monomolecular films at an air/(distilled) water interface. During the first compression-expansion cycle a minor degree of hysteresis was observed, but this effect was essentially absent during recompression (Fig. 6.3) and is probably due to the presence of various contaminants in the microbubble-surfactant mixture (see Section 6.3). It was further found that these microbubble-surfactant monolayers remain quite insoluble (cf. Section 6.1.3) when highly compressed, i.e., up to measured surface pressures of 24 dyne/cm. 

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. 

Fatty acids spread spontaneously on water to form a monomolecular film. A benzene solution containing 0.10 mm3 of stearic acid is dropped into a tray full of water. The acid is insoluble in water, but spreads on the surface to form a continuous film covering an area of 400 cm2 after all of the benzene has evaporated. What is the average film thickness in (a) nanometers and (b) angstroms ... 

In the method developed by Richter, Platikanov and Kretzschmar [143] the insoluble monomolecular layer is formed by a conventional technique at the aqueous surface in a Teflon trough. Its density is defined by a barrier and its surface pressure is measured by Wilhelmy plate (Fig. 2.27). The microscopic foam film is formed when the gas semi-bubble approaches... 

Eqs. (3.139)-(3.141) suggest that the rate of diffusion is much lower than the rate of gas dissolution and gas evolution from both film surfaces and the adsorption surfactant layers do not affect gas transfer. However, it is known that monomolecular films from some insoluble surfactants (e.g. cetyl alcohol) considerably decrease the rate of evaporation of the water substrate [204]. At high surface pressures the rate of evaporation can be reduced 5 to 10 times. Lipid bilayers, water and electrolytes can exert a significant effect on gas permeability, as was found in the study of the properties of vesicles (lyposomes) and flat black hydrocarbon films in aqueous medium [479]. 

In the LB technique, a monolayer of amphiphilic molecules, prepared at the air-water interface, is transferred to a substrate, thus giving a monomolecular film. The molecules must be solvable in a volatile, water-insoluble (organic) solvent, but not, or to a very limited extent, in water. Thus, when the solution of the molecules in the organic solvent is spread over the surface of water, the solvent evaporates, leaving a monolayer of molecules at the air-water interface. This monolayer can be compressed and transferred to a substrate. When the molecules are replaced by colloidal, nanosized particles, monolayers of these particles on a substrate are obtained. Smectites are especially well suited for the LB technique. The elementary clay sheets are about 1 nm thick and a few tens to hundreds nm wide and long. In the alkali- or alkaline earth form, they are hydrophilic, but by ion exchange with suitable amphiphilic cations, they become hydrophobic. There are then two ways to prepare mono-layers of smectite clay particles by the LB technique. 

Gibbs and Insoluble Monolayers The adsorption of surfactant molecules at the surface of a liquid can be so strong that a monomolecular film (Gibbs monolayer) of unidirectionally ordered surfactants is formed (Fig. 5). Since the decrease in surface tension is directly related to the surface excess adsorption of the surfactant by the Gibbs adsorption equation (Eq. 6), the formation of the Gibbs mono-layer can be monitored by decrease of the surface tension. The maximum number of molecules filling a given area depends upon the area occupied by each molecule. 

Surface films of proteins. Although most proteins are soluble in water they are usually sufficiently adsorbed to leave the interior of a solution almost completely for the surfaoe, provided this is large enough. Also, if a protein is placed in a suitable manner at the surface of water, it will often spread out to a thin film, which is of the order one amino-acid in thickness not only is the protein arranged in a monomolecular layer on the surface, but the complex protein molecules are themselves unfolded so that every amino-acid has its own place on the surface. Hence proteins can be studied by methods appropriate for insoluble films. 

Typically, these films are deposited using the following procedure. A flat, shallow container, such as a Langmuir-Adam surface balance, is filled with water (or other suitable liquid) and the substrate to be coated is immersed. Then a solution of the amphiphilic material, in a solvent that is insoluble in water, is deposited dropwise onto the water, thereby forming an oriented monomolecular surface film upon evaporation of the solvent. This film can then be compacted... 

A Langmuir film consists of a monomolecular layer of amphiphiles spread onto a liquid subphase (usually water) via deposition from a volatile solvent. The solvent is allowed to evaporate, leaving the molecules free to orient themselves in the two-dimensional environment at the interface (with their hydrophilic headgroups in the water and their hydrophobic tails in the atmosphere above the water surface). The molecules dissolve or evaporate only to a very limited extent, due to the insolubility of the hydrocarbon chains and the strong headgroup interaction with the water, respectively. 

When an insoluble liquid a is placed at the liquid air interface of another liquid P, the first liquid can spread out as a thin film which becomes monomolecular if the area of liquid P is sufficiently large. Everyone has experienced the colors which are seen when oil is spread on water in a muddy puddle on the road. The colors result from the interference of the light rays reflected from the oil air interface with those reflected at the water oil interface. The interference occurs because the oil film is very thin. The properties of thin films are especially interesting when their thickness corresponds to one molecule. 

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