The structure of thin films. Part IX.—Dibasic substances

Abstract

We have been anxious for some time to examine mono-molecular films of long chain molecules with a polar group at each end. In 1924, Mr. Dyer prepared specimens of the acid COOH(CH 2 ) 16 COOH, and of its mono- and di-ethyl ester, but tire publication of the results obtained on these has been deterred until a more extended series of compounds were available. Now Dr. D. A. Fairweather, who has prepared a long series of compounds of this series, including many new compounds, has most kindly placed at our disposal specimens of the acids 16, 20, 24 and 32; of the di-ethyl esters 10, 11, 16, 20 and 32; and of the mono-ethyl ester COOH(CH 2 ) 16 COOC 2 H 5 . Films of these compounds have been examined by the methods of the preceding paper. The esters gave films of fair stability and easy to work with; figs. 1 and 2 show the F—A and FA—F curves. The shorter chain substances are gaseous films at room temperature; below 2 dynes per cm. they approach much more closely to the perfectly gaseous condition than any insoluble films yet studied. The minimum for FA is about 330, the theoretical for a perfect gas being 1·372 T. or about 400. Variation of temperature from 1° to 15° made no difference appreciable within the accuracy of experiment; the fact that the curve for 10 appears below that for 11 is probably due to the former being so volatile that the molecules leave the film too rapidly for exact measurements, Above 2 dynes per cm. the correction to the gaseous state due to the size of the molecules causes the curves to take a form similar to those of the adsorbed films of the slightly soluble acids (reference 4 of preceding paper).