POTESTI.1L DIFFERESCES AT AIR-LIQUID IXTERFACES B Y J O H S \V.kRREii
TvILLIAlfS A S D V. A .
VIGFUSSON
It is well known that the electrical phenomena with which we have to deal in colloidal chemistry are quite different from those which may be treated thermodynamically to form what we know as electrochemistry. Indeed, in colloidal chemistry it is necessary to consider just those electrical phenomena which are neglected in ordinary electrochemistry. The relationship between the electrokinetic potential and the thermodynamic potential has been the subject of much investigation, yet, in spite of this fact, there remains much to be learned about the electrokinetic potential. There exists an electrokinetic potential at the various types of interface, solid-liquid, liquid-liquid, liquid-gas, etc. This potential, called the (-potential, is measured tangentially to the interface between the two phases. In other words, it is necessary to assume that a layer of the more mobile phase adheres to the surface of the less mobile phase in order to measure it. The most familiar methods for this purpose are electroendosmose and cataphoresis. More recently there have been reported in the literature determinations of potential differences which have been measured ilerticaliy to the interface. Since in the case of both the tangential and vertical measurements adsorption exerts such a pronounced influence it might have been suspected that the two potential differences were at least quite intimately related to each other. As yet, however, the meagre experimental data indicate that such is not the case. It is not the purpose of the present article to make this comparison, rather, we shall limit the discussion to the potential differences which have been measured in a vertical direction across the liquid-air interface. These potentials are found to be absolutely dependent upon the composition of the surface layer of molecules limiting the liquid phase. This layer is definitely an adsorption layer whose composition may be determined more or less exactly by noting the changes in surface tension caused by dissolved substances in accordance with the well-known equation of Gibbs. Because of these facts it appears that it should be possible to calculate from the magnitude of the observed potential difference the electric moment of any molecule which does not dissociate and which is adsorbed in the surface layer. This calculation necessarily involves the assumption that the molecules be imagined as arranged in a parallel layer perpendicular to the interface. The following relation may be shown to hold between the potential difference, E, and the electric moment of the molecule, p ,
E
=
4n~rir, E
orp = -
4a~r
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JOHN WARREN WILLIdMS AND V. A. VIGFUSSON
I n this equation K is the Avogadro number and r is the surface concentration in gram mol9 per square centimeter. The quantity 4 7 r W may be found from surface tension measurements. Using this equation Rideal’ calculates the electric moment of the butyric acid molecule to be p = 0.3 X 1 0 - l ~e.s.u. I n recent years the Debye modification of the Clausius-Mosotti relation has made it possible to determine the electric moment of a molecule either from the temperature coefficient of the dielectric constant of its vapor or from dielectric constant’ and density data for a binary liquid mixture composed of a non-polar solvent and the molecule in question as solute. The values of the electric moments of molecules determined in this way are now universally considered to be quantitatively correct, provided, of course, that complicating chemical influences are not operative in the vapors or solutions. There has been reported recently a value for the moment of butyric acid by Wolfz, p = 0.7 X 1 0 - l ~ e.s.u., obtained from dielectric constant and density data for its benzene solutions. I t is therefore evident that the value calculated from the observed potential difference across the interface is too low by a factor greater than two. Indeed there is every reason to believe that this factor should be even greater than two, since the value reported by Wolf is undoubtedly too low due to an association of butyric acid molecules when dissolved in benzene. The effect of this association is to lower the electric moment of a dissolved molecule. I t was previously mentioned in the case of benzoic acid3 and in the case of cyclohexane carboxylic acid.4 Since the group moment5 of the C - OH linkage is 1.7 and that of the C = 0 linkage is 2 . 8 a simple spatial consideration for the carboxyl group indicates that its characteristic moment will be greater than the difference between the moments characteristic of its components, that is, 1.1. In other words the moment of the butyric acid molecule may well be taken as being greater than p = 1.1 X 1 0 - l ~e.s.u., that’ is, approximately four times the value calculated from the potential difference data. To cite a single other case, the change of potential difference when chloracetic acid is substituted in the interface for acetic acid can depend only on the value of the electric moment characteristic of the C-C1 linkage. In this way the value, p = 0.5 X 1 0 - l ~e.s.u., is found. However, the moment for methyl chloride is p = 2.0 X 1 0 - l ~ e.s.u.,6 so that again the moment calculated from the potential difference at interface data is too small by a factor of something like four. In fact, it seems that any electric moment calculated by a method similar to the one outlined by Rideal will be too small by a factor of this order of magnitude. 1 Rideal: “Surface Chemistry” (1926). *Wolf: Physik. Z., 31, 277 (1930). Williams and Allgeier: J. Am. Chem. SOC.,49, 2416 (1927) Williams: J. Am. Chem. Soc., 52, 1831 (1930). 5 Williams: J. Am. Chem. Soc., 50, 2350 (1928). Sanger: Physik. Z., 27, 556 (1926).
P O T E S T I A L D I F F E R E N C E S AT .4IR-LIQCID I S T E R F A C E S
347
Frumkin and Williams’ have recently sought to account for the reason, or reasons, why the electric moment of a molecule cannot at present be calculated from potential difference at’ interface data. Four possibilities were suggested, as follows: I. Incomplete orientation of the molecules a t the interface. 2. A disturbing influence of the neighboring water molecules or ions. 3. Differential nature of the measurements of potential difference. 4. The polarization of the oriented molecules by the neighboring molecules, assuming the orientation to be always complete. I t was shown that there are certain objections to most of these possibilities, If, however, the oriented molecules are polarized by the presence of the neighboring molecules a dielectric constant of the order of magnitude three or four instead of unity might be accounted for. Since the equation relating electric moment and potential difference involves the assumption that the dielectric constant, of the molecules at t’he interface is actually unity it is evident that a higher dielectric constant of magnitude three or four would account for the larger part of the discrepancy. I t will, however, be very difficult to make any statements concerning the dielectric constant of the surface layer, so in order to accept this as an explanation experimental data must be made available. Careful examination of factors I and z indicates that if they are operative the potential difference at the interface should be a function of temperature. Furthermore, it follows from our modern ideas concerning the factors which contribute t o the dielectric constant of a system that if the oriented molecules are polarized by the presence of neighboring solvent molecules this polarization, and therefore the potential difference at the interface, calculated for equal amounts adsorbed, should be practically independent’ of temperature. Therefore, if data giving the temperature coefficient of this potential difference were made available it should lead to an elimination of either possibilities I and 2 or of possibility 4. We sought to obtain data of this character. I n view of papers published by KenrickY,Guyotg, Frumkin’”, Buhlll and Garrison’? it appeared as if this might be achieved, since in these articles there are described three distinct methods to measure the interfacial potential difference. However, the general agreement between the results of these methods and the agreement’ between the results of different observers leaves much to be desired. I n our experiments we used both the method of Kenrick and the method of Guyot and Frumkin, and many modifications of each in the attempt to reproduce the results which have been reported. The experimental arrangement of Kenrick makes use of a single vertical glass tube down the inner surface of which the reference liquid is allowed to Frumkin and IVilliams Proc. S a t . Acad. Sei., 15, 400 (1929). 8Kenrick: Z. physik. Chem., 19, 625 (1896). Guyot: rlnn. P h p . , (IO) 2, j06 (1924). Frumkin: 2. physik. Chem., 111, 190 (1924); 116, 4 8 j (192j); 123, 321 (1926). 11 Buhl: Ann. Physik, (4) 84, Z I I (1927); (4) 87,877 (1928). I* Garrison: J. Phys. Chem., 29, 1j17 (192j).
JOHN WARREN WILLIAMS AND V. A. VIGFUSSOK
348
flow. The second liquid is passed from a fine glass tip in the form of a narrow jet down the axis of the tube. The two solutions are connected through calomel electrodes to a quadrant electrometer which is calibrated to read the electromotjve force of the cell. This method is claimed by Kenrick and also by Frumkin to give very satisfactory results in the case of inorganic electrolytes and readily soluble derivatives of shorter chain hydrocarbons. There are however, several serious sources of error, namely I. Streaming potential developed by the second liquid in case of dilute solutions. 2. Balloelectric effect associated with the formation of fine drops and spray by the liquid jet. 3. Liquid-liquid junction potentials. 4. Electrostatic effects of unknown origin and magnitude. We have demonstrated experimentally that these liquid junction potentials may easily be as large as the potential difference across the interface which has been reported in various cases. A second method, which is subject to less error than the method just described, was developed by Guyot and by Frumkin for determining the change in the potential difference when capillary active substances are present. In this arrangement a stationary liquid surface is used. A platinum electrode which has been coated with a radioactive material is placed directly above
TABLE I Solution
H20 KC 1 KCl KBr KI KSCS NaCl Sac1
Conc. S
Kenrick Dropping Electrode
Frumkin Dropping Electrode
Garrison Condenser Method
$40
Buhl Dropping Electrode
-5
5*0.5
Authors Ionization Method (Modified) -9
-2
- 56
.o
-10
-82
.o I .o
-31
I
.o
0.1
I I
-Si
-4
2 .O
I
-
.o
"0s
I .o
HC1 HC1 HC1 HC 1 H2S04 KOH NHnOH
2
.o
I
.o
- 53
- 48 -55
- 29
- 23
- jo
0.5 0.1
.o I .o I .o I
- 8 -13.j 0
+78
+I58
P O T E S T I A L D I F F E R E X C E S AT AIR-LIQUID I S T E R F A C E S
319
this surface. The ionization of the air produced in this manner is sufficient to permit a determination of the potential difference between this air electrode and the body of the liquid. The activated electrode is directly connected to the isolated pair of quadrants of an electrometer, while the liquid is joined by a potassium chloride bridge to a calomel electrode connected to the other pair of quadrants. I n our experiments with the method of Guyot and Frumkin we were successful only to the extent of a partial agreement with the results obtained by these investigators. We did not obtain the same absolute magnitudes, but values which \\-ere shifted by a more or less constant factor from them, in spite of the fact that we tried to compare them to the same reference solution. In Table I we have shown a comparison of the results of the various investigators with our own for several solutions of inorganic electrolytes. Thus it appears that before we can hope to calculate electric moments of molecules from interfacial potential difference data a number of experimental difficulties will have to be solved. It appears that in addition to the possibility already mentioned, a difficulty due to the differential nature of the measurements of potential difference, there may be involved other factors in the experimental procedure which have not been properly controlled. To determine the value of an electric moment it will certainly be necessary to know the single potential difference. I t seems probable that any experiment will always give a differential effect which is related to a layer of pure solvent molecules or to some other quite arbitrary zero point. Represented schematically the cell whose potential difference is measured is as follows:
Pt
+ Material Radioactive
1
Molecule AB (Conc. C, in conducting solution)
I
1
Hg2Cl2 KC1 (sat’d)
Hg
1
The measured electromotive force of such a cell will evidently not be the 1 Molecule AB (Conc. C, in but a single potential difference Air conducting solution) combination of several potential differences; in other words it appears that the potential difference observed will be related to some arbitrary, even if practically constant, reference. Thus, our values differ from those of Frumkin, although we tried to reproduce them exactly. We believe that this difference can be explained, (at least in part), by the difference in radioactivity of the substances used to ionize the air in the two experiments. I t appears also that even had we used the same radioactive substance in exactly equivalent amount we could have reproduced the earlier results only if we could have adjusted the electrode to exactly the same height above
3 50
JOHN WARREH WILLIAMS AND V. A. VIGFUSSON
the surface in our experiments. It is evident from Fig. I that the measured electromotive force is a function of the distance of any given radioactive electrode from the stationary liquid surface. For comparable results it will always be necessary to use equivalent amounts of radioactive materials on an electrode which is always accurately adjusted to a predetermined height. These result8 will not be absolute in nature, rather they will be referred to a constant which appears a t the present writing to be indeterminable.
FIG.I Variation of potential with distance of electrode from surface of solution.
I t should not be necessary to mention that serious errors may affect the results if the apparatus and leads to the electrometer are not properly shielded and insulated. I n our experiments every known precaution was taken. As stated, our original intention was to study the temperature coefficient of the potential difference a t several interfaces to see if there could be eliminated any of the possible reasons why the electric moment of a molecule cannot a t present be calculated from interfacial potential difference data. Because of the time involved in the study of the various experimental methods described in the literature which do not give concordant results and in the construction of an apparatus which does give a reproducible result our data on this particular point are too meagre to report a t the present time except to say that there appears to be a real temperature effect. We are not at present willing to commit ourselves as to whether this temperature effect is due simply to the change in the junction potentials involved, or is really caused by either a decrease in the degree of orientation of the molecules a t the interface as the temperature is increased or a change in the disturbing influence of the neighboring water molecules or ions. We do feel it to be worth while, however, to point out that in spite of the tremendous difficulties involved in this type of measurement something has been and is being accomplished by workers in this field. It has been shown that there is a striking difference in the order of magnitude of the electric moment calculated from the potential difference data and that calculated according to the now familiar dielectric constant methods, yet there are a
P O T E S T I A L D I F F E R E N C E S AT AIR-LIQUID INTERFACES
35I
number of marked parallelisms between the results of the two types of study. Both indicate very definitely that, polar molecules of the organic type are made up of polar parts and non-polar parts. I n the simpler molecules which have but a single polar group the length of the hydrocarbon chain appears to have little or no effect. These are conclusions whose importance need not be discussed further. hgreement is also generally found when the conclusions concerning the sign of the charge on definite linkages is considered. Thus in the case of the carbon to oxygen linkage the carbon is charged positively with respect to the oxygen; in the case of the carbon to chlorine linkage the carbon is charged positively with respect to the chlorine; and in the case of the carbon to nitrogen linkage the carbon is positively charged and the nitrogen negatively charged. This conclusion is more definite from potential difference data than it is from electric moment data in the case of the linkages t o oxygen and nitrogen because of the structures which have to be assigned to the water and ammonia molecules and to what may be considered to be their derivatives. The two valence bonds of the oxygen atom are not' extended as has usually been assumed in the discussion of molecular orientations at interfaces, but make an angle of considerably less than 180" with each other. Thus if the alcohol, ROH, be considered to be oriented at the interface water-benzene the oxygen atom is the hydrophilic part of the molecule just as it has always been considered to be in the case of ether molecules, ROR'. Likewise, the three valences of the nitrogen atom are directed toward the corners of a tetrahedron with the nitrogen atom situated at the apex.'3 If, however, allowances are made for the stereochemistry of the various atoms involved it is found that the order of magnitude of the electrical effects for the alcohols, ketones, ethers, esters, and amines is relatively the same as the dipole effects, in order. The influence of the change when bromine is substituted for chlorine, or iodine is substituted for bromine is readily comparable in both cases. From the remarks made in an earlier part of this article it is evident that the effects in the case of the acids cannot be compared because of the difficulty in obtaining electric moment data for this type of compound. Before leaving this particular discussion it is well to note that the interpretation of the electric moment data is not as simple as might be wished because the molecules must be considered as non-rigid in practically every case. There is even in some cases evidence in favor of rotation about valence bonds which is quite unrestricted by other parts of the molecule in the case of both aliphatic and aromatic derivatives. The electric moment as measured must, be considered to be an average moment, and nothing more. If it were possible to calculate the electric moment of a molecule from interfacial potential See Debye: " Polare Molekeln," (1929); Sack: Ergebn. exakt. Naturwiss., 8,307 (1929); Williams: Fortschritte Chem., Physik, physik. Chem., 20,und S o . j. (1930); Chern. Rev., 6, 589 (1929).
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JOHN KARREN WILLIAMS AKD V. A . T'IGFUSSON
difference data this moment might be quite different because it could only be the moment of a molecule fixed in position because of the proximity of its neighbors. There is another comparison between data from two types of study which make it appear that, at least in a relative way, the interfacial potential difference data which now exist in the literature are not without significance. If one compares the results of a study of the electrocapillary curves with those obtained from the potential differences measured according to the methods described above there is a definite, even if qualitative, agreement between them. Experiments show14that the position of t,he maximum of the electrocapillary curve changes strongly with the composition of the solution. Gouy showed that these effects could be explained on the basis of an adsorption of ions or molecules at the interface, and it has developed that considerable information about, the orientations of molecules a t this mercury-water interface may be obtained by observing the changes produced in the rising and falling branches of the curves obtained when the interfacial tensions are plotted against the applied potentials. The observed shift in the maximum of this curve indicates the existence of an adsorbed layer of molecules which produces a potential difference between the mercury and the solution, such that if the positive end of the molecule is turned toward the mercury the maximum will be shifted in one direction, and if it is turned toward the water the maximum will be shifted in the other direction. Since the potential difference data discussed and described in this article are attributed to an adsorption of these compounds at the air-water interface and since this adsorption gives rise to similar electric effects Frumkin" has compared the results of the two types of study and has found it necessary to assume that at the mercury-water interface there exists the same orientation which is known to exist at the air-water interface. This means again that the polar group is attracted by the water and the non-polar group by the less active substance. The result of this comparison of data may be stated by saying that except in the cases where there is reason to believe that there exists a specific interaction between the mercury surface and definite constituents of the molecules in question there is good agreement between the conclusions which have to be drawn from the data in both these cases. I t seems probable, too, that when the potential differences across the air-water interface can be measured more accurately this agreement will become even better. I t might be mentioned that it would perhaps have been easier to study the effect of a change in temperature on the position of the maximum in the electrocapillary curve in order to account for the reason, or reasons, why the electric moment of a molecule cannot be calculated from interface potential difference data. I t was because of the fact that these specific interactions between the mercury surface and certain constituents of molecules had been previously recognized and would have complicated the situation that measurements at the mercury-water interface were not made. Furthermore, in Frumkin: Colloid Symposium Annual, 7, 89 (1929).
P O T E S T L I L D I F F E R E N C E S AT AIR-LIQUID INTERFACES
353
that event it would have been necessary to reason by analogy rather than by direct inference from the experimental data which were made available. Aleasurements of this type are now being made upon systems designed to avoid these specific interactions. Finally, it is clear that if we are t o increase our knowledge concerning the orientation and structure of molecules at interfaces the electrical properties of these interfaces must be studied as a function of temperature. While a method which seems to be suitable for this study in the case of the mercurywater interface is available it appears that considerable work still remains to be done before a technic will be developed which can be depended upon t o give reproducible and comparable results in the case of the air-liquid interface. Two methods, the air ionization met,hod of Guyot and Frumkin, and the electrostatic method of Garrison can be considered to be correct in principle, but as yet they have not given results in any agreement whatsoever, even at a single temperature. Laboratory of Colloid Chemistry, Vnioersity of Wisconsin,
Madison. Wisconsin.
