VOl. 59
992
COMMUNICATION TO THE EDITOR MONOLAYER STRUCTURE AS REVEALED BY ELECTRON MICROSCOPY Sir :
ence of islands before transfer is further supported by Geiger-counter isotherms for radioactive stearic acid obtained by H. D. Cook in our laboratory. The second alternative suggested-that both islands and coherent film are multimolecularis not consistent with our data. Shadow widths for many samples removed before collapse provide direct and abundant evidence for monolayer thickness, Similarly, the ((fibers” observed after collapse are two molecules thick and rest on a monolayer substrate. Our mechanism for collapse seems to account for both.2 Pankhurst’s assumption that at 15 dynes per em. the film area must be less than the extrapolated area does not agree with our observations. For n-hexatriacontanoic acid the areas are about the same. For stearic acid the film area a t 15 dynes per cm. is slightly greater than the extrapolated value. Areas in these electron micrographs may not warrant quantitative treatment. Exact correspondence a t each compression could hardly be expected. Transfer strains are not the only difficulty. The monolayer a t low pressures may not be uniform, and the minute samples may not represent the entire surface. Furthermore, smaller islands and even individual molecules beyond the limit of detection with the electron microscope may occupy the ((uncovered” areas.
Pankhurst‘ points out that uncovered areas in our electron micrographs of films of n-hexatriacontanoic acid2 are anomalously large. He suggests that either (1) the monolayer has been disturbed during transfer from the film balance to the collodion support or (2) the islands and coherent film are multimolecular in thickness. We recognize that the ring method of transfer may disturb the monolayer,2 both when the ring is raised through the film and when it is lowered over the collodion support. Such strains could separate the islands and increase the uncovered area. Evaporation of water held between the monolayer and the collodion may also disturb the monolayer. Nevertheless, the sequence of our micrographs for samples removed at 15 dynes per em., 25 dynes per em., and after collapse a t 58 dynes per cm., must parallel a similar sequence of changes during compression of the film on water. The essential features of the micrographs obtained by the ring method have been reproduced ,by two more-refined techniques that effect transfer in one step. I n one, a sharp-edged cup containing the collodion support is raised through the film. In the other, which may yield more quantitative results, the collodion support is raised vertically RESEARCH DEPARTMENT HERMAN E. RIES,JR. through the film at constant surface pressure as STANDARD OILCOMPANY (INDIANA) WAYNEA. KIMBALL in the Langmuir-Blodgett technique. The exist- WHITING,INDIANA (1) (2)
X. G. A. Pankhurst, J . Phya. Chem., 69, 480 (1955). H.E.Ries, Jr., and W. A. Kimball, ibid., 69, 94 (1955).
cr1
RECEIVED JULY,5 1955 (3)
R.E. Ries, Jr., and H. D. Cook,
J . Colloid Sci., 9, 535 (1954).
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