Films Built by Depositing Successive Monomolecular Layers on a Solid Surface by KATHARINEB
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June, i935 ~EPOSITIONOF SUCCESSIVEMONOMOLECULAR LAYERS 1007 2. In addition to showing the above change, sulfuric acid or stannic chloride is not due to the xanthone and lluorenone possess new bands in ether oxygen. sulfuric acid sohtions which are believed to be 4. The color of ketone chlorides and sulfuric associated with the stabilization of the quinonoid acid or stannic chloride is of a different nature structure for these compounds. Solutions of from that of ketones and resembles that of tri- stannic chloride produce the same changes on the arylcarbinols and salts of these carbinols. It is absorption spectrum of xanthone that sulfuric postulated that in such solutions the ketone acid does. chlorides exist in a quinonoid modification. 3. The production of color with xanthone and ANN ARBOR,MICH. RECEIVEDMARCH 28, 1935 [CONTRIBUTION FROM THE RESEARCHLABORATORY, GENERALELECTRIC COMPANY ] Films Built by Depositing Successive Monomolecular Layers on a Solid Surface BY KATHARINEB. BLODGETT’ A previous paper2 has described briefly a 5.0, approximately, the Ca ions in the water com- method of depositing successive monomolecular bine with the carboxyl. group of the adsorbed layers of “stearic acid” on glass. Subsequent molecules, converting the film to calcium stear- experiment^,^ which will be discussed in this ate. The film may be neutral soap or an acid paper, have shown that the substance deposited soap, depending on the pH of the water. The on the glass was calcium stearate and not stearic term “acid soap” is used here in the sense in which acid. The layers were deposited one at a time; it is used by McBain6 and other writers to refer thus a film could be built having a thickness of 1, to compounds of the fatty acid and the neutral 2, 3 or more layers of molecules. Further de- soap. Both types of soap will be called “calcium velopment of the method has made it possible to stearate” in this paper. deposit more than 200 layers on glass and on The film is transferred from water to glass’ by various metals. raising a clean glass slide slowly out of the water This paper will describe experiments in which on which the film is spread. A constant surface the thickness of films containing many layers was pressure is maintained on the film as the glass is measured by means of the interference of mono- raised through the surface. If the water-bath chromatic light reflected by the films. These is alkaline (PH > 7.5), the stearate molecules measurements were made with films of calcium show a ready tendency to attach themselves to or barium soaps of long-chain fatty acids deposited glass, so that the surface pressure causes them to on glass having a high refractive index. Films can be shoved off the water and onto the glass. Lang- be built with fatty acids4 when deposited on a muir, Adam and others have demonstrated that metal, but fatty acids do not adhere readily to glass. the fatty acids tend to orient themselves on a A monomolecular calcium stearate film is water surface with the -COOH “head” of each formed on water by placing a small amount of molecule in contact with the water and the chain stearic acid, dissolved in benzene, on a clean sur- tilted at a more or less steep angle with the plane face of water containing calcium salts. The of the water surface. In the present experiments stearic acid spreads rapidly in a monomolecular the monomolecular layer of calcium stearate is film.6 If the PH of the water-bath is greater than similarly oriented and the layer is transferred to (1) The experiments described in this paper were commenced in glass with the (-C00)2Ca heads attached to the collaboration with Dr. Irving Langmuir and were continued while he was traveling in the Orient. The writer is indebted to Dr. Langmuir glass, and the paired chains perpendicular to the for urging her to develop further the method described in the previous plane of the glass. The exposed upper surface paper, and for contributing many important suggestions which have been included in this paper. of the film is composed of closely packed CHs (2) K. B. Blodgett, THISJOURNAL, 56,495 (1934). groups. (3) I. Langmuir, J. Franklin Insf.,l18, 153 (1834). (4) G. L. Clark, K. K. Sterrett and P. W. Leppla, THISJOURNAL, (6) J. W. McBain, M Taylor and M. E. Laing, J. Chcm. Soc., 121, 67, 330 (1935). 621 (1922), and other papers. (5) (a) I. Langmuir, ibid., 39, 1848 (1917); (b) N. K. Adam, (7) I. Langmuir, Trans. Faraday Soc., 15, 62 (1920): reprinted in Proc. Roy. Sor. (London), A99, 336 (lYZlJ,and succeeding papers. G. E. Rev., 24, 1025 (1921). 1008 KATHARINEB. BLODGETT 1‘01. 57 A film in which all the molecules are oriented glass is being raised. Dr. Langmuir pointed out with the CHB groups at the upper surface pos- that pressure could be most conveniently ap- sesses the exceptional property that neither water plied by placing a tiny drop of castor oil or oleic nor oil will wet the surface. The property of acid, or some other hydrophilic oil, at one end of shedding water is an important factor in the pres- the trough. A hydrophilic oil has the property ent experiments. As a glass slide is slowly raised that the molecules of the oil spread on water until out of the water on which a film is spread, the they have covered all the available area with a water recedes from the glass wherever the film monomolecular layer, crowding together into this attaches itself to the solid surface, and if the slide layer until they have established an equilibrium be withdrawn from the water fairly slowly (5 to surface pressure.8 The equilibrium pressure ex- 10 cm. per minute), it will emerge completely dry. erted by an oil is a characteristic property of each This enables the experimenter to regulate the individual pure oil; for example, oleic acid exerts speed at which he lifts the slide so that the stearate approximately twice as much pressure as castor film may be deposited uniformly over the entire oil. After the surface is covered with a mono- surface. molecular layer, the surplus oil remains gathered The speed with which a film attaches itself to in many small lenses. If, however, new space glass is governed by three factors, @H, calcium becomes available on the water surface, oil spreads concentration, and temperature of the water- instantaneously from these lenses and covers the bath. The speed increases with increasing values new area also with a monomolecular layer. Thus of one or more of these variables. Thus at 20’ a a drop of oil placed at one end of a trough on solution nearly saturated with calcium carbonate, which a calcium stearate film has been spread, 5 X molar, gives films which coat glass with acts as a surface piston to hold the stearate film fair speed at fiH 7.5, and with greater speed at under constant pressure at every instant. Oil pH S.0; whereas a solution 1 X molar cal- used for this purpose is called “piston-oil” and its cium carbonate needs to have a value of @Habout use is illustrated in Fig. 2. 0.5 higher in order to give the same results. The After glass has been coated with one layer, speed in every case is greater at 35 than at 20’. additional layers can be added one at a time by dipping the glass in and out of the water-bath on which a monomolecular layer of calcium stearate is spread. The successive operations will be de- scribed in the following section. Apparatus and Technique The process of depositing the layers is carried out in a / // trough of the kind developed by Langmuir and by Adam Y for studying films on water. The trough T, shown in Fig. 1, is filled with water to the brim. It stands in a water- w bath D, the temperature of which is controlled. The Fig. 1.-Apparatus for transferring a monomolecular film edges and inner surface of the trough and the metal barrier from a water surface to a solid surface. B are coated with paraffin wax. Three detachable strips of glass K serve as a pen within which the benzene solution When the @Hof the water is less than pH 7.5, of stearic acid is placed on the water surface, to prevent approximately, the film shows less tendency to the benzene from reaching the waxed edges. detach itself from the water and attach itself to The glass slide G is raised and lowered by means of a the glass, and at PH < 7.0 it has very little ten- lever L operated by a hand windlass W. A simple me- chanical device of this sort is needed for the reason that it is dency to coat the glass in this manner. However, difficult to lift the slide slowly by forceps held in the hand excellent films can be deposited from water at pH without an occasional backward “hitching” motion. 6.0 to 7.0 by withdrawing the glass from the bath Hitching must be positively avoided since a 6lm is de- completely wet, the surface of the water on the posited each time the slide is raised and lowered. A glass being covered with calcium stearate, and pinch-clamp at L holds the rod H and a similar clamp J at the lower end of H holds the slide. The slide and the then removing the water from beneath the film clamp which hold it are both thoroughly cleaned in a hot by drying the slide slowly at a low heat.