The molecular structure of thin films.—Part VI

Abstract

In this paper the results of further experiments on mono-molecular films on water are recorded, the method of experiment being that described in former papers (1). The results are summarized at the end of the paper. The most interesting facts ascertained are that the hydrocarbon chain in the alcohols has sensibly the same cross-section as that of the acids; that the highly unsaturated acids with up to five double linkages in the molecule form expanded films having practically the same properties as the slightly unsaturated oleic acid; that in the substituted acetamides R NH CO CH 3 there is a very definite transition between two condensed films, one of which is solid and the other liquid; and that the four-tentacled molecule, pentærythritol tetrapalmitate, which normally should have its four long chains directed towards the corners of a regular tetrahedron, forms a fairly stable film, with the four chains oriented parallel to each other and perpendicular to the surface. Primary Alcohols . R·CH 2 OH. Pure specimens of five alcohols were prepared by reduction of the ethyl esters of the corresponding fatty acids with sodium and alcohol. Full details of the preparations will be given in another paper. All these compounds formed both condensed and expanded films. The condensed films consisted of two portions, a lower one, less well marked in the shorter chain compounds than in the longer, which cuts the abscissa at an area per molecule of 21·6 sq. A. U., and an upper portion, identical with the line GH of former papers, which has been shown to indicate a close packing of the long chains in the film. Fig. 1 gives the curves determined on tetradecyl and eicosyl alcohols.

The role of hydrogen bonds in condensed monolayers

“…The results are shown in figure 2. The F-A curves agree closely with those of Adam and Dyer (1924), and the marked hysteresis they described was fully confirmed; the F-A curves being very sensitive to temperature, rate of compression, and even the area at which the film was initially spread. This was especially marked for the C17 compound, which showed an inter mediate behaviour between the C16 and C18 homologues, and so no F-A curve is given for this.…”

“…The rather sharp transition temperatures shown by condensed monolayers of ureas, acetamides and acetanilides (Adam 1922(Adam , 1923Adam and Dyer 1924), the large areas of unionized acid films and the shrinkage on partial ionization, can also be readily explained. Again, a comparison between substituted amides and analogous esters with very similar head groups, but where the latter cannot form cross-linkages, showrs many striking differences.…”

“…A c e t a m i d e s , i?.NHC0 CH3 Condensed films of acetamides were found by Adam and Dyer (1924) to exist in two forms with a definite transition temperature, the hightemperature form being liquid with a limiting area of ca. 24*2 A2, the lowtemperature one solid with a limiting area of ca.…”

“…E. Alexander should thus tend to facilitate the change from II to III, that is, increasing F should raise the transition temperature. Adam and Dyer (1924) found experimentally that increasing F by 8*5 dynes/cm. raised the transition temperature about 2°.…”

“…(b) U r e a s , 7?.NHCONH2 The ureas were found by Adam (1922Adam ( , 1923 and by Adam and Dyer (1924) to resemble the acetamides in showing a fairly definite transition in the condensed region, but in this case the* high-area form (limiting area ca. 25 A2) was the stable one at low temperatures, and the low-area form (limiting area ca.…”