ELECTRICAL PROPERTIES
Dec., 1938
[CONTRIBUTION FROM
THE
OF
STEARATE FILMSDEPOSITED ON METAL
2855
BIOLOGICAL LABORATORIES, HARVARD UNIVERSITY ]
Further Studies on the Electrical Properties of Stearate Films Deposited on Metal BY ELIOT F. PORTER AND
I. Introduction In two previous communications1i2we have discussed a variety of phenomena assoc‘iated with the contact potentials of multilayer stearate films built on metal slides by dipping the slides through stearate monolayers spread on liquid surfaces. Langmuir3 recently has pointed out the advantages of interpreting these phenomena on the hypothesis that the potentials are due to the production of surface charges. He also has outlined a mathematical analysis of the electrostatic forces brought into play in consequence of the potentials while the films are being dipped. This analysis suggests that by introducing an arbitrary voltage between the submerged metal slides and the liquid it should be possible to modify the behavior of the films and to control the potentials which they develop, and Langmuir refers to observations pointing to this. Thus, speaking in terms of the surface charge hypothesis, he states that “By applying potentials of a few volts between the plate and the metallic tray during the deposition of stearate monolayers it has been found possible t o alter the surface charge u on X or Y films and to obtain either positive or negative values of u over wide ranges of pH.” In the present paper we describe a variety of experiments which reveal new phenomena associated with the films, and which have a bearing on the problem of the source and nature of the potentials, particularly in relation to the surface charge hypothesis . 11. Effects of Liquids on Potentials Associated with the Films
In this section we discuss studies on the effect of dipping films into various liquids, as well as electromotive force measurements involving films covered with liquids. In these electromotive force measurements, owing to the very high resistance of the circuits due to the films themselves and in some cases to the liquids as well, it was impossible to make direct measurements with a potentiometer. For this reason we employed a (1) Porter and Wyman,
THIS JOURNAL, 18, 2746 (1937).
(2) Porter and Wyman, ibid., 60, 1083 (1938). (3) Langmuir, i b i d . , 60, 1190 (1938)
JEFFRIES
WYMAN,JR.
bridge circuit of the type described by Du Bridge,* involving an F. P. 54 Pliotron. We already have pointed out2 that dipping an X film of high contact potential into water or the aqueous substrates from which flms are obtained does not greatly alter its subsequent contact potential in air. Nevertheless while the film is under the liquid its electrical behavior is like that of a film of zero potential. The same is true of Y films and X films of lower potential. Thus the e. m. f . of the arrangement X or Y iilm/liquid/X or Y film
is always very close to zero, irrespective of the contact potentials of the films. This is illustrated by the case of two X films of potentials 1.79 and 0.175 v. separated by a few drops of a solution used for forming X films. The voltage of the combination was close to zero, i. e., 0.055-0.075. If we think of the potential as due to internal charges, or to the orientation of dipoles within the film, the effect may be interpreted as due to the accumulation of a layer of compensating free charge, or of oriented dipoles, at the outer face of the film while it is in contact with the liquid. On the other hand, if we regard the potential as due to the presence of a layer of free charge on the surface, we may think of this effect as due to the surface being discharged by contact with the liquid. It is then, of course, necessary to assume that the surface of the film is recharged by the act of separating it from the liquid, since contact potentials persist after the slides a J-*.-_>. total length of time that the slides -I - _._ __ - - __ j. *--. q . were submerged with the applied * * , -2 -___ ---__.__ __ voltage. ” 0 40 80 120 170 Potential-layer curves for Y films Y layers on platinum with applied voltages of Fig. 7 +6 and - 6 v. are shown in Fig. ‘7. Two different solutions were used: solution (3)) such layers is somewhat different from that obpH 8.6, with copper; solution (4)) $G3 7.7, without served in the case of X and Y films. If the apcopper. The small initial differences between the plied voltage is between +6 and -6 v., the initwo pairs of curves from solutions (3) and (4) are tial layer always causes an increase in the contact due probably to the difference iii pH rather than potential but by a somewhat smaller amount for the copper content of the solution, for curves simi- positive than for negative voltages. We have lar to the pair without copper at pH 7.7 are ob(9) Germer and Storks, Proc. Nail. Acad. Sci., 28, 390 (1937). r+4a ----
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-I
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