Expanded films, liquid, 58-71 vapour

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. 

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. 

This type of expansion very frequently occurs among long-chain aliphatic compounds. All the fatty acids show it, and the expansion is very similar with the a-bromo-acids, the nitriles, alcohols, amides, ureas, oximes, amines, and acetamides. In all these series the expanded film is probably liquid, with a definite surface-vapour pressure, and the area is about 48 sq. A. at the lowest compression. Some other substances form similar films but of different areas the p-alkyl phenols tend to 39 sq. A. (j), and the a-monoglycerides (j) and the a-glyceryl ethers1 of long-chain alcohols,... 

With ethyl palmitate there is a long region of fairly small slope, nearly straight, in the place of the constant vapour-pressure region of the liquid-expanded type of films. As the temperature is raised, or the length of the hydrocarbon chain shortened, the vapour-expanded films become less like the liquid-expanded and more like the gaseous films. 

The distinction between vapour- and liquid-expanded films may be difficult to make in practice. The most sensitive tests are to plot the FA-F curves as suggested by Schofield and Rideal (.Proc. Boy. Soc. A, 110, 170 (1926) cf. h, p. 370) and see whether, when produced, these pass through the origin or to measure the surface pressure as accurately as possible between areas ranging from about 100 to 1,000 sq. A. For the distinction to be made with certainty the measurements must be made down to the second place of decimals. 

The vapour-expanded films have pressure-area curves with small values of — F0. At low values of the pressure the area becomes so large that the molecules have to lie nearly flat on the surface the thickness of the hydrocarbon part of the film thus becomes so small that there is much less opportunity for the chains to oscillate about the vertical direction than with the smaller areas of the liquid-expanded films. The hydrocarbon part of the film cannot therefore be considered as a liquid layer and the theory of duplex films tends to become inapplicable owing to the molecules being nearly fiat, the expanded state merges into the gaseous without abrupt change of slope. 

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. 

Belertser et al (1988) have observed that the electrical resistivity of amorphous chromium films at liquid-helium temperatures jumps from a value (10 3 O cm) characteristic of a poor metal by a factor 103, when the hydrogen content is increased sufficiently to increase the lattice constant by 10%. The transition is not abrupt, and is thought by these authors to be of Anderson type. They claim that it is the first time such a transition has been observed in a solid, and that it is similar to that in expanded mercury vapour (Section 4). 

The film pressure at which a gaseous film changes, or condenses, into a liquid-expanded type of film is the equivalent of a saturated vapour pressure for the surface phases. Table III gives such saturated vapour pressures at 15°C for a number of simple surface active compounds. It can be seen that... 

---