Expanded film

Expanded films. A very interesting state of the films, intermediate in area and other properties between the gaseous and the condensed states, is very often found with the long-chain aliphatic substances. 

This is the expanded state. Some of Langmuir s original surface pressure-area curves, notably that of oleic acid, were of the expanded type in 19201 Labrouste noted its occurrence in a number of cases and described the change from condensed to expanded films at low surface pressures Adam and others (esp. c, A)2 mapped the course of the change more thoroughly, and have shown that two types of expanded film may occur 

The expanded films tend to a definite area at low compressions, about 48 sq. A., at room temperature they are coherent, and show a surface-vapour pressure of about 0-19 dyne (compare Fig. 13). At areas greater than this the surface is heterogeneous and can be shown by electrical or optical methods to consist of two types of film. As the temperature is increased there is no change of form in the expanded curves, but they move nearly parallel to themselves to somewhat larger areas. The pressure required to initiate the change from expanded to condensed films increases by approximately 1 dyne for every degree rise in temperature. 

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, 

CJI COO CIlt-CaOK-CKflJl and CnH2ltKOCH2.CHOH.CHaOH, 

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It is evident that boundary lubrication is considerably dependent on the state of the monolayer. Frewing [48] found that, on heating, the value of fi rose sharply near the melting point sometimes accompanied by a change from smooth to stick-slip sliding. Very likely these points of change correspond to the transition between an expanded film and a condensed film in analogy with... 

The original expanded film membranes were sold ia roUs as flat sheets. These membranes had relatively poor tear strength along the original direction of orientation and were not widely used as microfiltration membranes. They did, however, find use as porous inert separating barriers ia batteries and some medical devices. More recentiy, the technology has been developed to produce these membranes as hoUow fibers, which are used as membrane contactors (12,13). 

Expanded-film membranes, 15 802-803 Expanded graphite, 12 794-795 Expanders, in nitric acid production, 17 178... 

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. 

A novel procedure, called LISIM (Linear Motor Simultaneous Stretching) technique, for the manufacture of expanded, biaxiaUy oriented films of PP and propylene copolymers, is reported. Examples, which demonstrate the the range of possibilities for varying the final film properties through formulations, machine settings and processing are demonstrated and the mechanical and optical properties of expanded films made by the above technique tabulated. (Kunststoffe, 91, No.l 1,2001, p.68-73)... 

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. 

Liquid Condensed Films (Lco) As the area per molecule (or the distance between molecules) is further decreased, a transition to a so-called liquid condensed (Lco) state is observed. These states have also been called solid expanded films (Adam, 1941 Gaines, 1966 Birdi, 1989, 1999 Adamson and Gast, 1997). The n versus A isotherms of n-pentadecylic acid (amphiphile with a single alkyl chain) have been studied, as a function of temperature (Birdi, 1989). 

Labrouste was the first to notice that below a definite temperature the area of a continuous film of oil remained constant, the area commencing to increase above this temperature until it becomes practically constant at a higher value. This change in area took place over a temperature range of about 18°. This phenomenon has been investigated in detail by Adam who has assigned the name of expanded film to the superficial phase in this state. 

Substance Temperature of half expansion Area of expanded film corrected to 0° 0. in A. (on assumption of gaseous expansion)... 

Adam has likewise examined the F, A curves for a substance such as myristic acid (temperature of half expansion under 1 4 dynes per cm. 9°) over both the region of condensed and that of expanded film. A few of the isotherms obtained for myristic acid are given... 

The apparent similarity of the F, A curves of myristic acid to the F, V curves for an easily liquefiable gas such as carbon dioxide has led Adam to suggest that the molecules in the expanded films of these substances are in a similat state to those of say carbon dioxide in the vapour state. He adduces as further argument in support of this hypothesis, the much greater coefficient of compression of expanded films. Again it is evident that the molecules are not lying down flat in the expanded state, for the increased area even under low compressions is only 50 A., whilst for stearic acid of length 21 A. and of cross-section 5 2 A. a horizontal molecule would occupy at least 109 A. of area at the same time the area per molecule decreases somewhat as the length of the hydrocarbon chain increases. 

An examination of the rate of such compression for expanded films reveals the fact that they obey a law of the type dF... 

If the film formed by superficial solution is an expanded film at low pressures but condensed at high pressures, it is found that the unimolecular character of the compression time curve is obeyed... 

Monolayer properties of octadecyldimethylamine oxide alone and in combination wth sodium alkyl sulfate on aqueous substrate have been investigated. Nonionised amine oxide produces more expanded film than the ionised species minimum film expansion and highest surface potential are obtained at half ionisation. 

The expansion of the film with Increasing acidity of the substrate may be due to the competition of counterions at the interface. The swamping amount of H ions in low pH subsolution competes with Na ions at the negatively charged interface. Such competition has been shown to exist between H ions and K" " ions at the negatively charged micelle-solution interface (11). Studies on the counterion effects in sodium docosyl sulfate monolayers (, 12) have shown that the film expansion follows the sequence Li > Na > K ". It follows that H" should give rise to the most expanded film. 

Figure 5 express the phase transition from the condensed to the expanded film and from the expanded to the condensed film, respectively. It is found for the expanded film that both the F value at constant decreases with increasing and the F value at... 

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. 

Oleic acid (Figure 4.25) gives a much more expanded film than the corresponding saturated acid, stearic acid - i.e. ir is greater for any value of A. Because of the double bond there is less cohesion between the hydrocarbon chains than for stearic acid and a greater... 

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,... 

Lateral cohesion also depends on the geometry and orientation of the film molecules, so that the following factors will favour the formation of an expanded film ... 

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