382
COMMUNICATION TO THE EDITOR
bands in chloroform solutions of naphthalene, anthracene, phenanthrene and pyrene. They attributed the origin of these bands to singlet-triplet transitions resulting from magnetic perturbations arising from the paramagnetic nature of the chelates. Such transitions can occur with fairly high probabilities when, as a result of the inhomogeneous field of a heavy atoin, spin and orbital angular momentum interact with each other. The perturbing effect of a heavy atom or of a paramagnetic ion has been observed even when these are supplied by the solvent. More recently Evans3 re-examined the systems studied by Chaudhuri and Basu and found no traces of any bands even where increased concentrations of hydrocarbon or metal complex were used. We also have obtained the absorption spectrum of anthracene in chloroform solutions containing the acetylacetonates of a number of transition metals: Le., ferric, cupric, manganous, cobalt, nickel, vanadium, aluminum, chromium and zirconium. KO evidence of the bands described by Chaudhuri arid Basu as appearing in the spectral region 5000 to 7000 A. could be detected. In view of this further confirmation of Evans’ findings, it must be concluded that the bands which were observed by Chaudhuri and Basu are indeed spurious. Their origin is, however, obscure. J. T. Bakey C.P. chloroform was used as solvent. The stabilizer (- 1%ethanol) was removed by per(2) AI. Xasha, ./‘.Chem. Phys., 20, 71 (1952). (3) D. I’. Evans, Proc. Roy. SOC.(London), A365, 55 (19GO)
Vol. 65
colation through an alumina column. The presence of this stabilizer proved, however, to have no effect on the absorption spectra. The chelates were obtained from Mackenzie Chemical Works, Inc., and Tere dried by heating overnight a t 110”. The anthracene (Eastman Organic Chemicals) was of the blue-violet fluorescence quality and mas not further purified. The absorptiop spectra were measured between 5000 and 7000 A. in a Cary recording spectrophotometer using a 5 cm. silica cell. The chelates were present in ,?.concentrations I while the concentration of anthracene was lo-* JP. A number of spectra were also obtained at higher concentrations. I n all cases the optical density of the chelateanthracene solutions increased steeply a t the shorter wave lengths. It was also observed that the concentration of the chelate as measured by its absorption between 5000 and 17000 A. mas in all cases reduced by a small but definite amount on adding the anthracene. The most likely explanation for these observations is that a charge-transfer transition is involved. Whether a definite complex between the anthracene and the chelate is formed or whether the charge-transfer transition occurs between two adjacent molecules in the absence of true complex formation has not been ascertained. In view of the apparent decrease in the chelate concentration in the presence of anthracene, it is probable, however, that definite complexes are formed.
COMMUNICATION TO THE EDITOR ON T H E 13ETENTION OF’ SOLVENT I N MONOLAYERS O F FATTY ACIDS SPREAD 0 N WATER SURFACES Sir: It has been suggested that volatile solvents may
figures that, at sufficiently long tinieb after spreading of the mixed film, the observed area per stearic acid molecule becomes constant and equal to that in films initially containing only stearic acid and n-hexane. When the films were allowed to stand a t low pressures ( 1 4 dynes/cm.) until this constant be retained in monolayers spread on aqueous sur- area was reached. and a film comprebsion expcrifaces with the aid of such solvents, and it has bcen meiit then was performed in the usual way, thc shown that there are indeed differences in the T-h PA curves were identical with thobe for films conisotherms obi ained when different solvents are taining no added hydrocarbon. The time required for evaporation of the amount used. l r 2 Some recent experiments with stearic acid monolayers containing relatively nonvolatile non- of hydrocarbon in the film can be estimated from the polar materials may shed light on this problem. evaporation rate per unit area of frec hydrocarbon Using the previously described techniques,3 we have surface in air; such estimates have been made and spread mixtures of stearic acid with n-hexadecane, the observed times required for the films to reach 2,6,10,14-tetramethylpentadecane, or bicyclohexyl, constant area can be interpreted reasonably in on water freshly redistilled in quartz. Purified n- terms of them. We conclude, as did Myers and hexane was used to aid spreading, and the ratio of Harkins. that these hydrocarbons evaporate practhis volatile solvent to the stearic acid was held tically completely from the stearic acid films at low constant (2.0 ml. per mg. of stearic acid). The surface pressure. The evaporation process, howarea of the film a t constant surface pressure was ever, is complicated, and further studies of this measured as a function of time; typical results are phenomenon are planned. If non-polar hydrocarbon molecules were reshown in Fig. 1 and 2. It is apparent from the tained in monolayers as a result of chain-chain (1) H. D. Cook and H. E. Ries, Jr., J . Phgs. Chem.. 60, 1533 (1956). interactions with the non-polar part of the film(2) M. L. Robbins and V. K. LaMer, J . Colloid Sci., 15, 123 (1960). (3) G. L riaines Jr., J . Phyrr. Chem., 65, 1322 (1059).
(4)
R .J. Myers rtnd W. 11. IJarhim, J . Z’hvs (‘hem , 40, 950 (1936).
CORIMUKICATION TO THE EDITOR
383
M O L A 9 RATIO, ACID H Y C R O C A R B O + 011
a 16
I
STEARIC
E Q U I M O L A R MIXTURES I DYYEICM.
ACID
o
n - HEXADECA4E
0
2,6, IO, 14 TETRAMETHYLPENTAGECANE BICYCLOHEXYL
A
NO HV3ROCARBON
;-----------5
-
0 MINUTES AFTER SPREADING.
15
Fig. 2.-Effect of increasing amount of hydrocarbon in mixed films on molecular area change with time after spreading. 5
IO MINUTES, AFTER SPREADING.
I5
Fig. 1.-Molecular area change with time after spreading of mixed films of stearic acid and hydrocarbons.
forming molecules, we should expect n-hexadecane to be strongly retained by stearic acid. Robbins and LaMer2 have suggested, however, that solubility and diffusion in the subphase may be involved in the observed differences in a-A isotherms obtained with different spreading solvents. The extreme insolubility of the hydrocarbons which we have examined would then account for their lack of effect on the properties of the monolayer. In the a-h isotherms for fatty acid films on mater or dilute acids, two fairly linear portions are observed. The l o w r pressure linear portion of the a-X curve, wkich for stearic acid extends from about 25 to 20A.2/molecule, and therefore includes the region of concern to us, has been the subject of much speculation. Hydration of the carboxyl
group^,^ rotational freedomj6 and other effects have been suggested to account for the larger areas a t low pressure. These all depend, however, on the behavior of the polar head-groups immersed in the aqueous surface. Changes in the PA diagram should result if spreading solvent can affect the environment or behavior of the polar heads. The differences observed by Robbins and LaMer between stearic acid and octadecanol films, therefore, may be explainable solely on the basis of the difference between the behavior of carboxyl and alcohol groups. Verification of these ideas may be possible through monolayer experiments in which the nature of the subphase is altered. Such studies are in progress. GESERAI,ELECTRIC RESEARCH LABORATORY SCHENECTADY, YEWYORK GEORGE L. GAISEB,JR. RhCEIVED DECEJIBER 28, 1960
-~ (2)
I. Langinuir, J Chem. Phys., 1, 756 (1933). J. Collozd SCZ.,7, 196 (1952)
(b) M. J. Vold,
