Lateral adhesion

The most simple type of amphiphile film or a polymer film would be a gaseous state. This film would consist of molecules that are at a sufficient distance apart from each other such that lateral adhesion (van der Waals forces) are negligible. However, there is sufficient interaction between the polar group and the subphase that the filmforming molecules cannot be easily lost into the gas phase, and the amphiphiles are almost insoluble in water (subphase). 

A film in which molecules move about independently on the surface and their lateral adhesion for each other is very small. At low surface pressures (Jr i and large area I A), a gaseous film obeys the relation. 7.1 = kT. At higher pressures an equation of the form t, t A - A ) = t k T holds, where, v is a constant. 

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. 

Two-dimensional monolayers can exist in different physical states which bear some resemblance to the solid, liquid and gaseous states in three-dimensional matter. Surface films are best classified according to the lateral adhesion between the film molecules, including end-groups. Factors such as ionisation (and, hence, the pH of the... 

The principal requirements for an ideal gaseous film are that the constituent molecules must be of negligible size with no lateral adhesion between them. Such a film would obey an ideal two-dimensional gas equation, ttA kT, i.e. the it-A curve would be a rectangular hyperbola. This ideal state of affairs is, of course, unrealisable but is approximated to by a number of insoluble films, especially at high areas and low surface pressures. Monolayers of soluble material are normally gaseous. If a surfactant solution is sufficiently dilute to allow solute-solute interactions at the surface to be neglected, the lowering of surface tension will be approximately linear with concentration - i.e. 

Coherent films are sometimes solid, the molecules not being free to move about in the film, so that the film cannot flow on the surface unless sufficient force is applied to break it films are more frequently liquid and can flow in the surface, even under considerable compression, fairly freely Whether films are solid or liquid is often decided by small details of distribution of the adhesive forces between adjacent molecules it is the total amount of the lateral adhesion which determines whether the molecules form coherent, or gaseous, films. 

The lateral adhesion between the molecules also affects the spreading. It is not an invariable rule that purely hydrocarbon substances never spread—the shortest chain liquid paraffins do spread and when the chains become too long, spreading of compounds which contain a water-soluble group may be prevented. 

The main types of film. Langmuir established the existence of one type of coherent film, with insoluble substances, and of the gaseous films, with the adsorbed layers of soluble substances at an air-liquid surface. It is now known that a number of different types of insoluble film can exist with long-chain aliphatic substances these are best classified according to the lateral adhesion between the molecules. These are... 

The value of the surface pressure in the two-phase region where it is constant is called the surface-vapour pressure it measures the tendency of the molecules to escape from the liquid film into the vapour film, and is smaller the greater the lateral adhesion between the molecules. The adhesion increases steadily the longer the hydrocarbon chains in tride-cylic acid it first becomes great enough to form a separate liquid phase lauric acid, with twelve carbons, is at, or just above, its critical point. ... 

Table II shows the principal measurements of surface-vapour pressure recorded. While many coherent films show no measurable pressure at areas slightly greater than that at which the molecules are closely packed, owing to the lateral adhesion being very high, at least six different homologous series have been found to show surface-evaporation phenomena with a measurable constant pressure. The highest surface-vapour pressure yet recorded is 0 39 dyne this may seem small, but considering the extreme thinness of the film, it is really fairly large. The molecules are lying flat, and therefore the thickness of the film perpendicular to the surface is about 4 5 A. a surface pressure of 0 39 dyne per cm. is thus roughly 8 7 x 106 dyne per sq. cm., or 8 5 atmospheres. This is smaller than the critical pressure of most liquids in three dimensions, but is not many times smaller.

The corresponding compounds with a hydrocarbon group or hydrogen atom instead of the water-soluble group are cither coherent films or vapour-expanded with much more lateral adhesion between the molecules. 

Equation (6) does not take account of any lateral adhesive forces which may be present between the water-soluble groups in the lower interface of the duplex film, but does allow, approximately, for the area occupied by these groups. 

In the simple form given above the theory treats the lower face of the film as a gaseous film, neglecting the lateral adhesion between the water-soluble groups. This can scarcely be wholly true. The values for A0 given above are rather unexpectedly small for a correction to the area due to the space on the surface actually covered by the end groups they are less than the measured area even of chains when closely packed. Also, they increase rather rapidly... 

From a comparison of the types of end group in Table IV which give vapour-or liquid-expanded films, it appears that those compounds whose end groups possess a considerable amount of residual affinity, such as undissociated fatty acids, amides, nitriles, c., form liquid-expanded films those which have less, such as esters or methyl ketones, form vapour-expanded. The lateral adhesion between the end groups therefore appears to have a controlling influence in 1 J.C.S. (1926), 2491 (1931), 1533. 

Influence of acidity in the water on substances with ionizable end groups. There is a very large diminution in the lateral adhesion... 

A triple bond, in stearolic acid, remote from the end group, gives such an increase in attraction for the water, combined with little increase in the lateral adhesion between the chains, that the films are vapour-expanded. This triple bond produces a considerable increase of surface potential. 

In discussing the lateral adhesions between the molecules in the films, it must always be remembered that we are dealing with opposing sets of... 

In every case yet worked out, the adsorbed films of soluble substances are of the gaseous type there are always corrections, except in very dilute solutions, to the perfect gas equation due to the area actually occupied by the adsorbed molecules and in many cases also there are corrections due to the lateral adhesion between them. 

The curves of Fig. 29 show that, with all the acids, there is a steady rise of FA with increasing surface pressure above 4 dynes this is due to the repulsions between the molecules, caused by their occupying a considerable fraction of the area of the surface. The dip of FA below 400, the value for a perfect gaseous film, is inappreciable with the 4 and 5 carbon adds (determinations were so few that a slight dip here may have been missed) it increases from 6 to 12 carbons. This dip is due to the lateral adhesions between the molecules in the films, which in the case of the thirteen carbon acid first become large enough to cause actual separation of a liquid-surface phase. The equation... 

The lateral adhesion is greater, the smaller a Bis the approximate limiting value to which the curves approach as F becomes very large. For the six carbon acid, B is 25, and the actual areas are at 5 dynes pressure, 59 sq. A. at 20 dynes, 33 sq. A. at 40, 28-6 sq. A. It should be mentioned that the activity coefficients of the acids were not determined, though a correction to the approximate Gibbs s equation (7.7) was made for the ionization of the adds. 

D. C. Jones and Cutting (unpublished 1937) have very recently confirmed the predominance of the lateral adhesion in these adsorbed films, and also the validity of Traube s rule. When the vapour phase is nearly saturated with the hydrocarbon they find films more than one molecule thick. Adsorption on heavy water is rather less than on ordinary water. 

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