716
HUGO FItICKE AND EDWARD PARKEX
T H E DIELECTRIC PROPERTIES OF T H E SYSTEM GELATIN-WATER. I1 HUGO FRICKE
AND
EDWARD PARKER
Walter B. James Laboratory f o r Biophysics, The Biological Laboratory, Cold Spring Harbor, Long Island, New York Receioed September 80, I959
In a recent paper, we described some of the polarizability properties exhibited by the system gelatin-water (2). These properties are essentially the same for the sol and the gel state and may be summarized as follows: Over the whole range of frequencies of 0.5 to nearly lo6kilocycles, for which measurements have been made, the dielectric constant increases a.~ the frequency decreases, reaching values many times higher than that of water, at low frequencies, in systems of moderately high gelatin contents. The change in dielectric constant is associated with energy absorption. The variation of the dielectric constant and conductance increments with frequency can be represented rather closely by a power of the frequency law. Orientation polarization of the gelatin appears to be of negligible importance for the polarizability of the gelatin-water system. The essential polarizability mechanism, which is present generally in dispersed systems (l), is believed to be concerted orientation of the polymolecular layer of loosely bound water molecules a t the internal surfaces, an effect probably of the same nature as that underlying the polarizability rise observed in many dipolar solids near their melting points. Anomalous dispersion of a similar character may be found in adsorption films of other dipolar substances. Our earlier work on gelatin dealt with systems of fairly high water contents. For our understanding of the underlying polarization process, observations on systems of lower water contents are desirable and such observations are reported in this paper. EXPERIMENTAL PROCEDURE
The gelatin was the purified product of the Eastman Kodak Company. This gelatin contains about 13 per cent water. The gelatin content was determined &s the weight obtained after heating at QOOC.for 2 days. The change in internal structure, produced by a temporary exposure of the system gelatin-water to a higher temperature, affects (increases, if the heat treatment is not too intense) the polarizability of the system. The effect of the heating is irreversible, at least in a practical sense, when the temperature used is over 70°C. (approximately). It is important to recognize this in the present work, since we are unable to prepare homogeneous
THE SYSTEM GELATIN-WATER. I1
717
gelatin gels of low water contents except by the use of higher temperatures, and therefore the dielectric properties of these systems depend on the method of preparation used. Examples of this will be given below. We have made no systematic study of the effect of heating on the dielectric properties of gelatin, but we shall in the following limit ourselves to show observations on systems prepared under certain specified conditions. In order to prepare a gel for measurement, finely cut gelatin was placed in the electrolytic cell, and the appropriate amount of water was added, whereafter the cell was sealed off and exposed to a suitable high temperature until perfect homogeneity was obtained. The gels were tested for homogeneity by determining the dry weight of different small portions of each. The electrolytic cells were similar to those described earlier (1, 3). It is advantageous to use cells of different design, according to the gelatin concentration and range of frequencies studied. A t low frequencies and at low gelatin concentration the electrode separation should be comparatively high to reduce the influence of electrode polarization. At high frequencies and a t high gelatin concentration, cells with small electrode separation are required to avoid errors resulting from stray capacity. All measurements were carried out a t 21.0"C. EXPERIMENTAL RESULTS
Figures 1A and 1B record results obtained on aged systems of different concentrations. The gelatin concentration is given as grams of gelatin per 100 grams of the system. The dielectric constant e and conductivity increment K~ are represented as functions of frequency, in a double logarithmic graph; the value K~ is the difference between the conductivity K a t the frequency w under consideration and at very low frequencies. The values B and K are defined by representing the complex impedance 2 of ti 1-cm. cube of the gelatin as
The points representing K~ lie closely on a straight line for each particular system. The points representing e lie on a straight line for the systems of lower concentration, whilt a straight-line representation is obtained for all the systems tested, if, instead of using E , we represent the increment in dielectric constant Ae =
E(W)
-
e(2w)
Thus, within the range of frequencies tested, the dielectric observations can be represented by the relations: Ae
N
w-x
K d N U Y
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HUGO FRICKE AND EDWARD PARKER
The same type of relation was found for the systems of lower gelatin concentrations studied earlier (2).
FIG.1A. The dielectric constant e ( 0 ) and conductance increment K~ ( 0 ) a8 functions of frequency for aged gelatin systems of different concentrations. [(a = (la) 47 g. gelatin per 100 grams; gelatin and water heated a t 95°C. I( ( w ) - K (O))]. for 45 min. (lb) 47 g. gelatin per 100 grams; dried' gelatin and water heated a t 95'C. for 89 hr. (2) 69 g. gelatin per 100 grams; gelatin and water heated a t 95OC. for 18 hr. (3) 75.9 g. gelatin per 100 grama; gelatin and water heated a t 95°C. for 45 hr. 1
Original gelatin heated a t 100°C.for 48 hr.
THE SYSTEM GELATIN-WATER.
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The values of X and Y show no apparent dependence on gelatin concentration. However, a small degree of dependence would be difficult to detect because of the lack of complete reproducibility of the results ob-
040
0.40
too 7.
FIQ.1B. The dielectric constant e (0)and conductance increment rP (0)as functions of frequency for aged gelatin systems of different concentrations. [I@ = I (la) 79.3 g. gelatin per 100 grams; gelatin and water heated a t 95°C. ( 0 ) - I (O)]. for 14 hr. (lb) 79.3 g. gelatin per 100 grams; gelatin and water heated a t 95°C. for 127 hr. (2) 85 g. gelatin per 100 grams; dried gelatin and water heated a t 95°C. for 72 hr. (3) 86 g. gelatin per 100 grsma; original gelatin sheet. (4) 100 g. gelatin per 100 grams; dried by heating a t 100°C. for 48
+.
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HUGO FRICKE AND EDWARD PARKER
tained for gels prepared at different times. This is shown in figures 1A and 1B by small irregular differences in the slope of the curves for the different systems represented. This lack of reproducibility seems to result, in part at least, from differences between different samples of the original gelatin. The values of X for the systems shown in figures 1A and 1B lie around X = 0.20, while the values of Y lie around Y = 0.80. The values of X are lower and the values of Y higher than the values obtained for the gelatin used in our earlier work. The dielectric constant of dried gelatin is nearly independeht of frequency. The value a t 512 kilocycles is e = 2.68, corresponding to a refractive index of 1.63. This value agrees quite closely with the refractive index observed in the optical region. Figures 1A and 1B show a few examples of the way in which the dielectric properties of the gelatin systems depend on the heat treatments to which they have been exposed. In figure 1A the lower dielectric constant curve for 47 per cent gelatin is for a system prepared by heating the original gelatin with water a t 95°C. for 45 min. The upper curve is for a system prepared by heating dried gelatin with water a t 95°C. for 89 hr. This system remained fluid for more than 2 weeks and was measured in this state. The slope of the curves is the same for the two systems, within the limits of reproducibility, but the dielectric constant values of the more highly heatcd system are about 50 per cent higher than those of the less highly heated system. Figure 1B shows two series of experiments on 79.3 per cent gelatin. The upper curve is for a system prepared by heating at 95°C. for 41 hr.; the lower curve is for the same system after it had been exposed to this same temperature for an additional 86 hr. The effect of this prolonged heating is a decrease in the polarizability of the system. The effect of heating on the dielectric properties of gelatin is particularly well brought out by the measurements on the original gelatin sheet, which are included in figure 1B. At the time these measurements were made, the gelatin had taken up a small amount of water in addition to that which it contained originally, and the water content was 14 per cent. Although, therefore, this sheet has nearly the same composition as the 85 per cent gelatin gel, which had undergone the regular prolonged heating, the polarizability of the sheet is very much smaller than that of the heated system. A few words should be said about the dielectric properties exhibited by gelatin systems during the period of internal change following a temporary exposure to high temperature. If we plot instantaneous values of A6 and Kd against frequency in the double logarithmic scale of figures 1A and l B , a t different times during the transition period, a series of (approximately) straight lines is obtained, which move downward with
THE SYSTEM GELATIN-WATEH.
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time a t a steadily decreasing rate. No systematic change in the slope of these lines with the aging of the systems could be seen, although small irregular changes were found in some cases. Transitions involving a change from thesol to the gel state show no particular indication of this change in these dielectric graphs. Within certain limits, the more intense the heat treatment, the higher the initial values of dielectric constant and absorption. The dielectric properteq €
4
-6 64 Kilocycles
256
101+
FIG.2. The dielectric constant of 10 per cent gelatin heated a t 78°C. for 17 hr. Observations made 2 hr. ( 0 )and 7 hr. ( 0 )after return to room temperaturc. I h r i n g the first series of observations the system was in the sol stato. The sccond series of observations were taken after gelatination had set in.
ties are reversible if the temperature is kept below 70°C. (approximately). With higher temperatures the heat treatment produces a permanent increase in the dielectric constant and absorption values, as far as can be judged from observations extending over a few weeks. The correlation between the polarizability and the mechanical properties of the gelatin systems should be remarked upon. With increasing
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HUGO FRICIW AND EDWARD PARKER
heat treatment, the gelatin becomes increasingly softer (finally passing into the sol state) and the polarizability increases correspondingly. With aging, the gelatin becomes harder and the polarizability decreases. The irreversible increase in polarizability resulting from intense heat treatment is bound up with a permanent increase in the degree of softness of the gelatin, and with sufficient exposure to high temperature the gelatin loses ita power to gel and remains indefinitely in the sol state.
GRAMS GELATIN PER 100 GRLHS
FIQ.3. The dielectric constant c at 16 kilocycles, the dielectric constant increment 6c [6c = c (16 kilocycles) - c (1024 kilocycles)] and conductance increment K~ at 16 kilocycles [e' (16 kilocycles) = K (16 kilocycles) - x (O)] of the gelatin systems in figures 1A and lB, represented as functions of gelatin concentration. The curve marked V.P. represents the pressure of water vapor over gelatin gels, according to measurements by Katz (4). i k just stated, our measurements so far show no distinctive features in the dielectric properties of gelatin sols as compared to the gels. Observations on sols were shown in our earlier paper (2). This work dealt with reversible systems, which were not exposed to temperatures over 50°C. The work was limited, therefore, to sols of low concentration and the measurements could be carried out over a rather narrow range of high frequencies only, owing to the error from electrode polarization present at
THE SYSTEM GELATIN-WATER.
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lower frequencies. By heating to higher temperatures, $oh of higher concentration can be prepared and it is possible to extend the measurements to lower frequencies. The results of dielectric constant observations on a 10 per cent gelatin sol are recorded in figure 2. The system was prepared by heating gelatin and water a t 78OC. for 17 hr. One series of observations was carried out 2 hr. after the gelatin was returned to room temperature, a t which time the system was still in the sol state. No observable difference in dielectric properties wm found 5 hr. later, at which time gelatination had set in. In figure 3 dielectric characteristics of some of the systems of figures 1A and 1B are represented as functions of gelatin concentration. The qualities shown are e and Kd a t 16 kilocycles and the dispersion in dielectric constant as represented by BE = E (16 kilocycles) - e (1024 kilocycles). In addition to the data for the systems of figures 1A and 1B we have included in figure 3 the results of measurements on aged gels of lower concentration, prepared by dissolving gelatin in water at 5OOC. Even though the form of the curves of figure 3 depends to a great extent on the procedure used in preparing the gels, the curves exhibit certain general features which should be remarked upon. The addition of small quantities of water to gelatin-up to 10 to 20 per cent water-causes hardly any change in the dielectric properties. As more water is added the dispersion becomes rapidly more pronounced and reaches its highest value a t about 45 per cent gelatin, whereafter the.polarizabi1ity decreases with further addition of water. The small dielectric effect of the first addition of water to gelatin may be explained by the strong binding of this water by the gelatin. For comparison, we have shown in figure 3 the water vapor pressure over the gels, according to measurements by Kat5 (4). DISCUSSION
We remarked above upon the correlation exhibited between the polarizability of a gelatin system and its mechanical properties, as the system is changed internally by a temporary exposure to high temperature. A more consistent relation is obtained b y comparing the polarizability to the intermolecular forces, as represented by the volume of the system. Heat treatment results in expansion and increased polarizability. Aging results in contraction and decreased polarizability (for systems of low gelatin concentration the volume change is generally considered to result from a change in the forces between the gelatin and the water, i.e., it is supposed to represent the change in hydration). This relation gives also an explanation of the effect of acid and base on the polarizability of gelatin.
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HUGO m I C K E AND EDWAR0 PARKER
When the pH is changed from the isoelectric point, whether to higher or lower values, the gelatin expands and the polarizability increases. The existence of a correlation of this type is indicative of orientation polarization. Because of the zwitter-ion character of the gelatin molecule, its moment may be high, and therefore it might appear likely that we are dealing with orientation of the gelatin molecule. The dielectric effect of adding water to gelatin would result, then, partly because the water increases the moment of the gelatin molecule and partly because the water loosens the forces between the gelatin molecules, thus facilitating the orientation of the gelatin molecules in the field. In considering this hypothesis, we should keep in mind that the dielectric properties of the gelatin sol are essentially the same as those of the gel and, as regards the form of the dispersion curve and the high polarizability values reached a t lower frequencies, these properties are of a type shown by dipolar systems in the solid rather than in the fluid state.* Obviously, therefore, the essential underlying dielectric mechanism is the same in the sol as in the gel and, if this mechanism is orientation polarization of the gelatin, the state of affairs in the sol must be wholly different from that found in simple solutions of dipolar substances, such as amino acids, whose dispersion properties can be, in the main, described by Debye's theory. Furthermore, since the polarizability of the gel is only slightly smaller than that of the sol, we should have to accept that there is no great change in the binding forces between the gelatin molecules aa we pass from the gel to the sol state. These considerations raise some doubt that orientation of the gelatin molecules can be responsible for the dielectric properties of the gelatin systems, and this doubt is increased in view of the fact that dielectric properties similar to those found for gelatin are observed when water is added to other high-molecular substances, such as cellulose or starch, whose molecules are not zwitter ions and can not be expected to be distinguished by particularly high moments (1). This indicates that the high polarizability of the gelatin systems is not conditioned by the possibly high moment of the gelatin molecule. In our earlier paper (2) we discussed evidence indicating that the dispersion in gelatin and other high-molecular systems is a property of the water molecules a t the internal surfaces. This hypothesis is supported by the observation that dispersion of a similar character is found much
-
2 The dispersion in solids is often studied by measuring the anomalous charging From this current. Generally this is found t o vary with time according t o i t-". i t follows, theoretically, t h a t the dielectric constant, as obtained from observation in an alternating field, varies inversely a8 a power of the frequency with an exponent equal t o 1 - m.
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725
more strongly in more coarsely dispersed systems. The systems investigated include a number of different suspensions and colloids, and a sufficiently extensive study has been made to justify us in concluding that dispersion of the type found in gelatin is a general property of dispersed systems containing water as the dispersion medium (1). By studying the dielectric properties of suspensions as functions of particle size and volume concentration, it was possible to show that the dispersion is a surface effect and to obtain evidence that the region in which the dispersion resides lies in the water and is of multimolecular dimension. For the case of systems so highly dispersed as gelatin, the depth of the highly polarizable region would have to be much smaller than it is for suspensions. From the value of the gelatin concentration (45 per cent gelatin) a t which the polarizability of the gelatin is a maximum it would appear that the layer is of the order of five water molecules thick. The small dielectric effect produced by adding as much as 10 to 20 per cent water to gelatin indicates that the strongly bound water molecules directly a t the gelatin surface are without importance for the high polarizability of this layer, but that the effective water molecules are at some small distance from the gelatin. These molecules may differ from those in the bulk of the water only by an increased orientation with respect to the gelatin surface. The simplest explanation of the decrease in polarizability which takes place when gelatin ages or when the pH is changed toward the isoelectric point is to assume a decrease in internal surface while the highly polarizable water layer a t the remaining part of the internal surface is not greatly changed. This may not be a wholly correct picture. The aggregation which takes place in these processes may be caused not by direct forces between the gelatin molecules but by a kind of interlocking effect between the water layers of the different gelatin molecules. On this conception, the decrease in polarizability and associated contraction in volume should be connected with the increased rigidity of these interlocking water layers. We may hope that a more extensive dielectric study of gelatin will lead to a clearer understanding of the exact rBle played by these water layers in structural changes of the system. SUMMARY
In continuation of earlier work (2), dielectric constant and electric conductance measurements were carried out on the system gelatin-water in the range of high gelatin concentrations (17 to 100 per cent gelatin) between 2 and 1024 kilocycles. The dielectric constant of dried gelatin is low and nearly independent of frequency (e = 2.68 a t 512 kilocycles). The addition of water results in increased polarizability and strong anom-
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HUGO FRICKE m D EDWARD PARKER
alow dispersion which reaches its maximum value at approximately 45 per cent gelatin. At this concentration e = ca. 500 at 2 kilocycles. The addition of the first 10 to 20 per cent of strongly bound water has little effect on the dielectric properties of the gelatin. The dispersion in dielectric constant and in conductance varies with frequency according to a power of the frequency law, as found earlier for systems of lower gelatin concentration. The internal change produced in the gelatin-water system by a temporary exposure to high temperature results in increased polarizability, which is practically reversible only if the temperature used is not over 70°C. (approximately). Under the conditions studied, no distinctive features could be seen in the dielectric properties of the sols as compared to the gels. The change in polarizability resulting from heat treatment or from a change in pH runs parallel to the change in internal forces as represented by the change in volume. Contraction is associated with decreased polarizability, expansion with increased polarizability. Anomalous dispersion, similar in character to that in gelatin, is found generally in dispersed systems containing water as the dispersion medium (suspensions, colloids, high-molecular solutions) (1) and appears to be a property of the polymolecular layer of loosely bound water a t the internal surfaces. Dispersion connected with orientation polarization of the gelatin is, if present, not sufficiently pronounced to be detected in the presence of this greater effect, under the experimental conditions so far studied. REFERENCES (1) FRICKE AND CURTIS:J. Phys. Chem. 41, 729 (1937). (2) F B I C KAND ~ JACOBSON: J. Phys. Chem. 43, 781 (1939). (3) FRICKE AND PARTS:J. Phys. Chem. a,1171 (1938). (4) KATZ:Kolloidohem. Beihefte 9, 1 (1917).
