A DIELECTRIC STUDY OF T H E GELATIN-WATER SYSTEM: ASOMALOUS DISPERSION I N BOUND (OR1ENTED) WATER HUGO FRICKE
AND
LILLIAN E. JACOBSON
Walter B. James Laboratory jor Biophysics, The Biological Laboratory, Cold Spring Harbor, Long Island, New York Received January 15, 1959
Solutions of many substances of high molecular weight, such as gelatin, casein, starch, gum arabic, and Congo red, show a rapid increase in electric conductance with increasing frequency (24, 25, 53, 54). Largely independent investigations show that the dielectric constants of some of these and other similar systems increase with decreasing frequency and that values for the dielectric constant many times higher than that of the solvent ma,y be obtained a t low frequencies (7, 8, 10, 16, 25, 27, 28, 29, 31, 36, 39, 40, 44, 47, 50, 51, 63). Observers of the dispersion in electric conductance have usually interpreted their results by assuming that the electrolytic ions of the solvent are bound electrostatically to the charged molecules of the dissolved substance, Le., the dispersion is supposed to be of the same origin as that found in strong electrolytes and treated theoretically by Debye and Falkenhagen (18). On the other hand, observers of the dielectric constant have tended toward picturing the molecules of the dissolved substances as dipoles oriented by the external field. It becomes clear, however, when t he complete experimental material is considered, that these two mechanisms cannot account fully for the dielectric behavior of these systems or even be the essential factors in some of the cases. This has been recognized in some of the more recent work in this field (10, 25, 27, 36, 48). I t appears in these systems that we are dealing with the same type of interphasial process that, in a more dominant way, influences the dielectric behavior of more coarsely dispersed systems (suspensions, colloids; for literature, see reference 25). The purpose of this work is to produce support for this view by a more extended study of the dielectric properties of the gelatin-water system. Although the nature of this interphasial process is still uncertain, we believe it probable that the underlying mechanism is an abnormal polarizability of the multimolecular film of oriented water molecules a t the interphase. This conception will be discussed in some detail below. The probable bearing of this interphasial dielectric mechanism on the 781
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HUGO FRICKE AND LILLIAN E. JACOBSON
dielectric behavior of systems of wide practical importance is worth remarking upon, for the same type of phenomenon is observed in living cells, soils, and various kinds of commercially used hygroscopic insulators (paper, fabrics). EXPERIMENTAL PROCEDURE
The dielectric constants and electric conductances of the gelatin-water systems were measured in alternating fields at frequencies from 0.002 to 65.6 megacycles. The procedure has been described in recent papers (25,26). The electrolytic cell containing the unknown solution is balanced against a similar cell containing a potassium chloride solution. At frequencies over 2 megacycles the condition of equivalence is established by comparing the cells in a resonance circuit, while a bridge is used for this purpose at the lower frequencies. Equivalence is obtained by varying the concentration of potassium chloride and a parallel condenser. At frequencies over 2 megacycles the chief part of the capacity difference between the cells is compensated for on a specially designed microcondenser which is built into the cell mounting. This condenser has no scale for reading its capacity directly, but every time it is reset its value is measured on a low-frequency bridge. This calibration was sometimes omitted, leaving us then only measurements of the electric conductance. At the highest frequencies the dispersion in conductance of gelatin-water results to an appreciable extent from the dielectric absorption of the solvent. At low concentrations of the protein this factor may be considered to be substantially eliminated by the experimental procedure used, since our measurements give us directly the difference in dispersion between the gelatin solution and the potassium chloride solution. We introduce as before (cf. 26) K,d(w)
= KKCI(0)
- Kp(O)
where K g C l ( 0 ) is the low-frequency conductivity of the potassium chloride ) lowsolution that balances the gelatin a t the frequency w and ~ ~ (is0 the frequency conductivity of the gelatin. At the low concentrations of potassium chloride used, the dispersion in conductance of the potassium chloride solution may, with sufficient accuracy, be taken as equal to that of the solvent; consequently the value ~ p drepresents the amount by which the dielectric absorption of the gelatin system exceeds that of the solvent. At the higher gelatin concentrations it might be considered preferable to give the true values for the conductance dispersion as obtained by correcting ~ , for d the dispersion of the water. However, at gelatin concentrations over 5 per cent the dispersion of the gelatin solutions is so high compared to the dispersion of water that this differentiation is without practical importance. Unless otherwise stated, the measurements were carried out a t 21.OoC.
783
DIELECTRIC STUDY OF GELATIN-WATER EXPERIMENTAL RESULTS
The gelatin systems were prepared by dissolving purified gelatin from the Eastman Kodak Company in the solvent at approximately 50'C.; the solution was placed in the conductivity cell and allowed to cool until temperature equilibrium was reached. The concentration is given in grams of gelatin per 100 cc. The ash content of the gelatin is 0.3 mg. per gram of gelatin. The low-frequency conductivity of the gelatin gels (aged) as a function of the gelatin concentration is shown in figure 1.
Hysteresis It is well known that the gelatin-water system, when brought from one temperature to another, shows pronounced hysteresis, so that days may elapse after a new temperature has been established before constant proper-
I
ao
IO
oms
Jo
G.C.U~/IOOCC.
WIG. 1. The low-frequency electrical conductivity of isoelectric gelatin gels of different concentrations.
ties are obtained. The low-frequency conductivity and the dielectric constant are both responsive to this condition, and various types of hysteresis curves based on these two characteristics have already been published (50, 51). When the gelatin-water system is brought from a higher to a lower temperature, the after effect in both electric conductance and dielectric constant is a decrease which takes place at a continuously decreasing rate, while for a transition from low to high temperature a variation in the opposite direction is obtained. As long as too high temperatures are not used, the observations are perfectly reversible. As is true for other physical characteristics, neither the electric conductance nor the dielectric constant shows any abrupt change at any point during the sol-gel transformation. These results were confirmed in our own work, and it waa found that a corresponding behavior is shown by the dielectric absorption. For
784
HUGO FRICICE AND LILLIAN E . JACOBSON
systems of different concentrations figure 2 shows the values of K: (at 65.6 megacycles) that were obtained a t d8erent times after these systems were brought from 50'C. to 21.0'C. Even after 4 days the values of K: are not constant. In fact, when plotted in a logarithmic scale, the values decrease nearly linearly with time during the limited period over which these observations were made.
Variation of frequency The influence of the frequency of the measuring alternating field on the dielectric characteristics was determined for aged isoelectric gelatin gels of 30 %
0
Fluid
40
different experiments A
20'O:2
0:4
1.b
4
2
$0
io
40
160
Hour3
FIG.2. The effect of aging on the dielectric absorption of isoelectric gelatin. Frequency 65.6 megacycles.
different concentrations up to 46.5 per cent. At the highest concentration and K: were measured between 0.002 and 65.6 megacycles, while a t Lower concentrations measurements were made above 2 megacycles only, and in most cases only K: WM determined. Under the conditions used Ae [that is, e(w) - e ( 2 w ) ] and K: vary approximately as powers of the fiequency. e
Ae K;
N
w-*
(1)
wy
(2)
The results of measurements on different samples of gelatin show some variation from each other, and while in some experiments expressions 1
DIELECTRIC STUDY OF QELATIN-WATER
785
and 2 are found to represent the results very satisfactorily, in others the agreement is less perfect. Whether this lack of reproducibility represents actual differences between the different samples of the gelatin or is due to experimental errors is still uncertain. For the range of gelatin concentrations studied the values of the exponents x and y appear to be independent of the gelatin concentration.
41s10-6 200
FIG.3. The dielectric abeorption as a function of frequency for isoelectric gelatin gels of different concentrations.
From results on other systems it is probable that the values would show dependence on concentration at concentrations over 50 per cent. The average values of 2 and y are
786
HUGO FRICKE AND LILLIAN E. JACOBSON
x
= 0.34
y = 0.66 As is to be expected on theoretical grounds (22), the sum of x and y equals unity. Representative frequency curves are shown in figures 3 and 4. A series of measurements was carried out to determine whether the relation of ~ p to d frequency changed during the aging of the gelatin system. Isoelectric gelatin systems of different concentrations were kept for several hours a t 5OOC.; then they were brought to 21.OoC.and a t different times
Megacycles
FIG.4. The dielectric absorption (6)and dielectric constant (e) as functions of frequency for 46.5 per cent isoelectric gelatin gel.
thereafter K,” was measured between 16.4 and 65.6 megacycles. The results are shown in figure 5. As far as we can tell from these observations, the relation K,” COY remains valid during the aging of the systems and there is no change in the value of the exponent. The observations on the 1 per cent gelatin include the transition sol-gel.
-
Variation of gelatin concentration The dependence of ~ ; on f the gelatin concentration is represented in figure 6; the measurements were made a t a frequency of 65.6 megacycles. Up to about 5 per cent ~ p increases d very nearly as the concentration. At
787
DIELECTRIC STUDY OF GELATIN-WATER
higher concentrations the increase ia less rapid, and from about 40 per cent on ~ p ddecreases with increasing concentration.
,
(a) S h r a . (b) 24 hrs.
P
.
(a) Z hrs.
~
2 30
2.10
2.10
30 70
Y ,
10%
la Sl.0
CS.6
32.a Megacyles
16.4
64.8
1C4
32.0
CSb
F1o.5. The effect of aging on the relation of dielectric absorption to frequency for isoelectric gelatin.
f
6oor
=lo-'
FIG.6. The dielectric absorption of isoelectric gelatin as a function of its concentration. Frequency 65.6 megacycles.
Addition of hydrochloric acid, sodium hydroxide, and sodium chloride
The effect on K$ of adding 0.002 M hydrochloric acid, 0.002 M sodium hydroxide, and 0.002 M sodium chloride to 1 per cent aged gelatin gels is
~
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HUGO FRICICE AND LILLIAN E. JACOBSON
represented in figure 7. Measurements were made between 2.05 and 65.6 megacycles. No detectable change is produced in K: by adding the acid or salt, while the addition of the base causes K$ to increase. Within the range of frequencies studied the results on all these systems may be represented by K$ U Y and the exponent is the same as in the case of the isoelectric gel. The low-frequency dielectric constant shows a similar behavior, in accordance with results already reported by Denekamp and Kruyt (10). These observers extended their measurements over a wider range of pH values, and found that for gelatin gels the dielectric constant is also increased by the addition of acid, although, in accordance with our results, the effect of adding acid was found to be smaller than that of adding base. These authors also tested the effect of adding certain ions
-
I 4
2.05
41
a2
16+
32b
65.6
t4sgasJcl.a
FIG.7. The dielectric absorption of 1 per cent gelatin gel after adding hydrochloric acid, sodium hydroxide, or sodium chloride.
of high valence, but were unable t o detect any change in the dielectric constant. Denekamp and Kruyt found that gelatin sols behaved differently from the gels as regards the effect of adding acid or base, Le., the dielectric constant of the sol was decreased by adding acid, and while it was increased by adding base, the increase was much smaller than that obtained for the gels. InJEuence of JieZd strength The intensity of the measuring field was varied between 0.5 and 2 volts per centimeter. Within this range the dielectric characteristics showed no dependence on the field intensity.
DIELECTRIC STUDY OF GELATIN-WATER
789
DISCUSSION
Since the aged gelatin gels can contain a t the most only small quantities of the sol, and since the amount of sol present in a gel may be assumed to decrease rapidly with aging, while the dielectric characteristics decrease comparatively little with aging, the indication is that orientation of the gelatin dipole molecules in the external field plays no appreciable part in the dielectric behavior of aged gelatin gels. On the other hand, the orientation of molecular dipoles would be expected to play some rdle in the dielectric behavior of gelatin sols. From their observations on the change in the dielectric constant of gelatin sols and gels with pH, Denekamp and Kruyt (10) concluded that the difference in polarizability of the sol and the aged gel (at 1.2 megacycles) represents the influence of the molecular dipoles. Our observations indicate that this is not wholly so. The dielectric mechanism effective in the aged gels appears to be present to an increased extent in the sol, and this accounts for a t least an appreciable part of the difference in polarizability between the two states. However, more complete measurements on the frequency dependence of the dielectric characteristics of the sols are required before the influence of the molecular dipoles can be definitely established.' Since the gelatin micelles are electrically charged, they bind the ions of the solvent electrostatically. The ions will therefore contribute to the dielectric properties of the gelatin gels in a way similar to that found in the case of strong electrolytes (Debye-Falkenhagen effect). The average number of positive and negative elementary charges on a gelatin molecule a t the isoelectric point is probably about ten (33), but the charges are so distributed that the electrostatic forces are comparatively weak. Isoelectric gelatin appears to act approximately as a 2-2 valence type electrolyte (3, 35, 45, 55). At the low electrolyte concentrations with which we have worked, the ions would therefore not be expected to contribute to any appreciable extent to the polarizability of the gelatin systems. This is confirmed by the finding thgt the addition of sodium chloride or hydrochloric acid to the isoelectric gelatin gels produces no change in e or K:, although these substances were added in sufficient quantities to increase
* Dielectric constant-frequency curves have been determined for solutions of such proteins as carboxyhemoglobin, serum albumin, and serum globulin (1, 16,47). The curves show anomalous dispersion which was assumed to be of the Debye type. It seems unlikely that these proteins should show no polarizability of the type found in gelatin, although this effect is probably not as great as i t is in gelatin. More extended frequency measurements are needed to ascertain the true state of affairs in these systems. Measurements that we have carried out on egg albumin between 2.0 and 65.6 megacycles show the presence of the interphasial dielectric effect, but t o a much smaller degree than in the case of gelatin. When the albumin is denatured by heating, i t behaves dielectrically more nearly like gelatin.
790
HeGO FRICKE AND LILLIAN E. JACOBSON
the low-frequency conductance by a factor of more than 10. I n adding hydrochloric acid, besides changing the ionic strength, we produced an appreciable change in the charges on the gelatin, which should have changed the dielectric characteristics of the gelatin if electrostatically bound ions had played an important rble. The same conclusion as to the negligible importance of the electrostatic binding of the ions of the solvent results from the fact that in all cases studied we can account very closely for the observed low-frequency conductivities of the gelatin systems (restricting ourselves to the low gelatin concentrations) on the basis of the ions known to be present in the systems if these are assumed to have the same mobilities as they have in the absence of gelatin. The agreement is so close, compared to the observed highfrequency increase in conductance, as to leave no doubt that this increased conductance cannot be produced by the ions. I n verifying this statement, let us first consider the isoelectric gelatin gels to which no electrolytes were added. The low-frequency conductivity ohm-' crn.-' (figure 1). Nearly half of of 1 per cent gelatin is 12 X this conductivity is accounted for by the hydrogen ion. The remaining part may be accounted for by electrolytic impurities, the ash content of the gelatin being 0.3 mg. per gram of gelatin. I n any case it is clear that the amount of free organic electrolytes in the gelatin must be small, and that the total amount of electrolytes is wholly insufficient to account for the high-frequency conductivity values, which a t 65.6 megacycles are over 30 X lo-' ohm-' em.-' for 1 per cent gelatin and are still increasing rapidly with increasing frequency. Let us now consider the low-frequency conductivity of gelatin gels to which known amounts of sodium chloride, hydrochloric acid, or sodium hydroxide were added. Adding 0.002 M sodium chloride to 1 per cent isoelectric gelatin increases the low-frequency conductivity from 12 X lo-' ohm-' cm.-'; the difference is 217 X lO-'ohrn-' em.-'. The to 229 x eonductivity of 0.002 M sodium chloride itself (without the gelatin) is 226 x 10-6 ohm-' cm.-'. The difference is 1 per cent, part of which must be due to the obstruction to the current that the gelatin offers simply by its bulk. The decrease in the conductance of the sodium chloride produced by its electrostatic binding by the gelatin can therefore be but a-few per cent at the most, while the high-frequency increase in conductance (of 1 per cent isoelectric gelatin plus 0.002 M sodium chloride) is about 15 per cent a t 65.6 megacycles. Similarly, the addition of 0.002 M hydrochloric acid to 1 per cent isoelectric gelatin causes an increase in its low-kequency conductivity of 148 x 10-6 ohm-' em.-'. The change in pH is from 4.9 to 4.3, corresponding to a change in the conductivity of the hydrogen ions of 12 X lo4 ohm-' cm.-'. The conductivity of 0.002 M chloride ion is 139 X lo-'
DlELECTRIC STUDY OF GELATIN-WATER
791
ohm-' cm.-', assuming negligible electrostatic forces. The sum of these two conductivities is 151 X 10V ohm-' em.-', agreeing to within 2 per cent with the observed change in low-frequency conductivity of the gelatin. Finally, the addition of 0.002 M sodium hydroxide to 1 per cent isor electric gelatin causes an increase in the low-frequency conductivity oE ohm-' cm.-'. The change in pH is from 4.9 to 5.8, correspondr 89 X ing to a decrease in the conductivity of the hydrogen ions of 3 X ohm-' cm.-l. The conductivity of 0.002 M sodium ion is 93 X 10-6 ohm-' em.-'. The difference between these two conductivities agrees to within 1 per cent with the observed increase in the conductivity of the gelatin. To reach an understanding of the dielectric behavior of gelatin, let u6 consider the dielectric properties of more coarsely dispersed systems (25). If we suspend a small quantity of any finely divided dielectric (as, for example, glass) in water, a striking change in dielectric properties results. The system shows strong anomalous dispersion accompanied by high energy absorption. Dielectric constant values as high as 100,000 may be obtained a t very low frequencies. An interesting feature of this phenomenon is the form of the frequency dependence. For most of the many different systems studied Ae/Aw and AKJAwvary as powers of the frequency over the wide range of frequencies studied (0.5 to 2000 kilocycles), and in all cases this law represents the observations with a considerable degree of approximation. Other results, about whose generality, however, we are not very well informed, are as follows: The values of Ac/Aw and AK/Aw increase with increasing temperature and with increasing electrification of the interphase, and are independent of the presence of moderate concentrations of sodium chloride or potassium chloride in the suspending medium. A study of the influence of the size of the suspended particles showed that, for constant volume of the suspended material, Ae/Aw and AK/Aw increase as the internal surface, for particle sizes down to about l p in diameter. From this result it was concluded that the origin of these singular dielectric properties is a polarization of the region a t or near the interphase, by the external field, in the direction parallel to the interphase. It has been known for a long time that the dielectric constant of many colloids is much higher than that of water (13, 14, 24, 25, 27, 28,30,32,4$): One of the best known systems is that of colloidal vanadium pentoxide, first investigated by Errera (13, 14). This author believed that the reason for the high dielectric constant w&s an orientation of the colloidal dipole particles in the external field. Later work made it clear that this cannot be the correct explanation (24,25,27,28,32). There seems no reason to doubt that the dielectric peculiarities shown by systems of colloidal dispersion are chiefly of the same origin m those observed in suspensions. The high dielectric constant of many
792
HUGO FRICKE AND LILLIAN E. JACOBSON
colloids is quite consistent with what we should expect from the observations on suspensions if the polarizability were determined essentially by the internal surface independent of particle size. On the other hand, very highly dispersed colloids generally show dielectric characteristics comparatively little different from those of water, so it is evident that the polarizability does not keep on increasing with the internal surface when the particle size becomes very mall, but that high polarizability is a property of comparatively large interphases only. That this must be so is of course clear, since systems of molecular dispersion show none, or only traces, of these interphasial dielectric characteristics. The polarizability appears to decrease with decreasing particle size below, say, diameters of 100 or 1000A. The knowledge that we now have of the dielectric behavior of gelatin gels leaves little doubt that the singular dielectric properties of these systems originate in this same type of interphasial process. The influence of frequency, temperature, electrification of the micelles, and added salts is similar to that found in suspensions. Even though the exact significance of these daerent results is somewhat uncertain, taken together they seem sufficient to show that we are dealing with the same phenomenon. Since all interphases examined 90 far show this characteristic high polarizability, we must clearly look for an explanation of a very general character. A likely possibility is that the polarizability is a property of the layer of comparatively highly oriented water molecules which, there are good reaaons to believe, is present a t all interphases (2, 4, 5 , 11, 12, 20, 38, 60). (This water is to be more or less closely identified with “adsorbed” or “bound” water.) In various mechanical characteristics this layer appears to behave more like a solid than a fluid, and so it does not seem unreasoqable to assume that under the influence of an electric field the layer would show polarizability properties approximating those of many dipolar crystalline solids, Le., it is less polarizable than normal water at very high frequencies, but it becomes more highly polarizable than water when the frequency is low. It may be questioned whether an increased orientation of the water molecules alone would be sufficient to produce such marked changes in po1,mizability as we observe at interphases. To this the reply should be made, first, that it is possible that the changed properties of the water near interphases may lie in more than an increased orientation, Le., there may be an actual change in molecular pattern. For the present, however, the weight of the evidence seems to be against such a conception. Furthermore, it should be recognized that the high electric moment of the water molecule gives water a tendency toward self-polarization, so that a comparatively small change in molecular arrangement may produce a marked change in its polarizability. The remarkable “ferromagnetic” dielectric
DIELECTRIC STUDY OF GELATIN-WATER
793
properties of certain crystals (Rochelle salts, magnesium platinocyanide, and yttrium platinocyanide) containing large amounts of crystal water may be remarked upon in this connection, for these properties appear t o originate in the loosely bound water and, within certain ranges of temperature and frequency, are very similar to those found a t interphases (6, 17, 62). The observations of Smyth (57) on the marked increase in the low-frequency dielectric constant of ice resulting from the presence of small quantities of such salts as potassium chloride in the ice are also interesting. The situation in the water near the interphase may in certain respects be similar to that found in dipole solids near their melting points or near temperatures at which changes from one molecular arrangement to another take place. Around these transition points marked increases in dielectric constant and dielectric absorption are often observed (15,49, 52,57). Inside regions of anomalous dispersion the dielectric constant and dielectric absorption of dipole systems generally increase with increasing temperature, reflecting, according to Debye, an influence of the viscosity on the orientation of the dipoles in the external field This law generally holds for dipolar solids also. The result that the interphasial dielectric characteristics increase with increasing temperature is therefore consistent with our conception of their origin, although the influence of the temperature, by its effect on the viscosity, presumably would be masked in part by a direct influence of the temperature on the structure of the interphasial water. The fact that the interphases lose their high polarizability when the suspended particles are very small is explained partly because the depth of the interphasial water film decreases at high dispersions and partly because the high polarizability of the interphasial water would be assumed to result from a “cooperative” behavior of the water molecules, which would require a certain extension of the interphase to be effective. If our theory of the interphasial polarizability is correct, then we should be able to obtain confirmatory evidence by experimenting with dispersed systems containing such small quantities of water that the interphasial water films cannot be developed fully. Consider the observations on gelatin gels shown in figure 6. The dielectric absorption reaches its highest value a t a gelatin concentration of 40 per cent. The reason may be that the internal surface of the gelatin gels decreases with increasing concentre tion a t concentrations over this value. However, the presence of the maximum may also be interpreted as meaning that a t the concentration a t which it occurs all the water is “bound,” in the sense of its dielectric characteristics. This gives a value for the ‘(bound” water of 1.5 g. of water per gram of gelatin. This value is three or four times higher than the generally accepted value for the hydration, but it is quite consistent with the values for bound water obtained by the various other methods that have been used to differentiate between free and bound water in gelatin (34,37,41, 46, 58, 59).
794
AUQO FRICKE AND LILLIAN E. JACOBSON
I n a previous paper (25) we showed curves representing the dielectric constant of suspensions of finely powdered dielectric materials (Pyrex glass, kaolin) as a function of their volume concentration. The curve for Pyrex glass shows a maximum at a concentration of 26 per cent. The curves for kaolin were not brought to sufficiently high concentrations to reach a maximum, but the part of the curves obtained indicates that a maximum will be reached a t a concentration not greatly different from that for Pyrex glass. These measurements were carried out a t a frequency of 2 kilocycles and the average particle size appeared, by microscopic examination, to be around 0.5 to 1 p in diameter, although it may possibly have been very much smaller. At the time these observations were made it was assumed that this decrease in the dielectric constant a t high concentrations resulted from a decrease in internal surface, because of aggregation. On the basis of our present oonception the decrease may be interpreted as resulting from an “overhpping” of the interphasial water films of the different particles. If this is the correot explanation, we should expect the relation ,of dielectric c0nstW.t to frequency to change when the volume concenwation is high. This has been found to be the case. In the concentrated suspensions the dielectric constant decreases less rapidly with increasing frequency than in suspensions of low concentration. Or, expressed in another way, the curve representing the dielectric constant as a function of concentration reaohes its maximum a t a volume concentration which is lower as the frequency is lower. If we accept the interpretatibn given, it would appear that the depth of the interphasial water film inust be of the order of 1000 A. This is a very high value, though abnormalities in various physical characteristics of thin water films have been observed to set in a t thicknesses of this order of magnitude (2, 5, 11, 12, 20, 38, 60). Observations of interest in this connection are also found in the literature dealing with the influence of humidity on the dielectric properties of fibrous or gel-like insulating substances such as Bakelite, paper, fabrics, or rubber. The consideration of these data must, however, be reserved for another place. In conclusion, it should be remarked that the interphasial dielectric phenomena dealt with in this paper probably play an essential r6le in the dielectric behavior of a number of important systems. (1) It has been known for a long time that the dielectric constant and dielectric absorption of living cells are very high a t low frequencies and that these high values originate in the water-permeable gel-like membranes that surround the cells (9, 21, 23). (2) Because of their importance in radio communication, the dielectric properties of soils have been studied extensively during recent years over wide ranges of frequency. Very high dielectric constant values were observed at low frequencies, and marked increases in electric
DIELECTRIC STI'DY OF GELATIN-WATER
795
conductance were obtained a t very high frequencies (19, 56). (3) I t is well known that the didectric properties of many commercially used dielectrics of a gel-like or fibrous structure, such as Bakelite, paper, and fabrics, are influenced by their water content to an extent wholly unexplainable by any mixture law if the adsorbed water had anything like its usual dielectric properties (42,43, 61). (4) Metal electrodes in inactive solutions (such as nickel in sodium chloride) act dielectrically as if a capacity and a series resistance were interposed between themetal and the solution. The capacity and the resistance both vary as powers of the frequency (22). This behavior may be accounted for by the presence of thin, poorly conducting, surface films (metal oxides) containing a considerable quantity of adsorbed water. SUMMARY
At frequencies between 0.002 and 65.6 megacycles and for field intensities between 0.5 and 5 volts per centimeter the dielectric constant and dielectric absorption of gelatin-water systems were determined under different conditions of gelatin concentration, pH, ionic strength, and structural state. The results indicate that the dielectric properties of these systems are dependent on a polarization a t the interphases similar to thqt already known to be present in more coarsely dispersed systems. It is suggested that this polarization originates in the interphasial layer of oriented water molecules, and various types of evidence in support of this hypothesis are discusse8. The bearing of this on the dielectric behavior of hygroscopic insulators (paper, fabrics), soil, cell membranes, and metal electrodes is pointed out. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
ARRHENIUS: Physik. Z . 39, 559 (1938). AND SAWERIS: Nature 140, 237 (1937). BANGHAM, MOSALLAIN, BIGWOOD: Trans. Faraday SOC.31, 335 (1935). BRESLER AND POCHIL: Acta Physicochim. (U.R.S.S.) 8, 129 (1938). J. Phys. Chem. 41,463 (1937). BULLAND WRONSKI: BUSCH:Helv. Phys. Acta 11,269 (1938). CAVALLARO: Boll. soc. ital. biol. sper. 9,925 (1934). CAVALLARO: Arch. sci. biol. 20, 572 (1934). COLE:Cold Spring Harbor Symposia Quant. Biol. I, 107 0933). DENEKAMPAND KRUYT:Kolloid-Z. 81,62 (1937). DERJAGUIN: Z. Physik 84, 657 (1933). DERJAGUIN: Acta Physicochim. (U.R.S.S.) 6, 1 (1936). ERRERA: Kolloid-Z. 31,58 (1922); 32, 157, 240,373 (1923). J . phys. radium4,225 (1923); 6,304 (1924). ERRERA: ERRERA: Trans. Faraday SOC.24, 162 (1928). ERRERA: J Chem. Phys. 29,577 (1932). AND SACK:Trans. Faraday SOC.30,687 (1934). ERRERA FALKENHAGEN: Elektrolyte. S. Hirzel, Leipzig (1932). FELDMAN: Proc. Inst. Radio Engrs. 21, 764 (1933).
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HUQO FRICKE AND LILLIAN E. JACOBSON
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