THE ELECTRICAL CONDUCTANCE O F SOLS AND GELS AND ITS BEkRING ON THE PROBLEM OF GEL STRUCTURE. I GELATIN ROBERT TAFT Department of Chemistry, University of Kansas, Lawrence, Kansaa AND
LLOYD E. MALM Department of Chemistry, Utah State Agricultural College, Logan, Utah Received August 26, 1968
The problem of gel structure is one of great complexity, and there exists a considerable divergence of opinion among colloid chemists as to the interpretation of the heterogeneous data in this field. All of the theories that have been proposed may be divided into the following classes: The one-phase, or solid solution, theqry is supported by Proctor (20) and by Kats and Robinson (16), who hold that a gel is a solid solution of the dispersion medium in a second constituent, both constituents being within the range of molecular attractions of each other. This theory fails to explain many of the properties of gels, especially the loss of mobility on setting. The two-phase, liquid-liquid theory, in which gels are assumed to be similar in structure to emulsions, is supported by Quincke, Hardy, Wo. Ostwald, and Garrett. This concept has difficulty in explaining the fact that no emulsion has ever been converted into a jelly, and Hatschek (13) argues that the theory is untenable because it is physically impossible from purely geometrical considerations to account for the elasticity shown by gels. The two-phase, solid-liquid theory was first suggested by Frankenheim in 1835, who considered gels to be composed of aggregates of small crystals with pores between them. In 1879 von Niigeli proposed that gels were composed of molecular complexes or micelles with crystalline properties, separated by skins of water and forming meshes or interstices in which the water is contained by molecular attraction. Bachman (2) studied the process of gelation microscopically, and found that., as the sol began to set, there were formed small elements consisting of submicrons and, in part, microns, which gradually lost their translatory motion. Finally the 499
500
ROBERT TAPT AND LLOYD E. MALM
whole field was filled with cohering particles without any motion. Kruyt (17) concludes that this is the most satisfactory representation of gelation. Bachman and Zsigmondy (3) later observed a fibrillar structure in addition to the grainy structure, which is quite sharply defined in the case of soap jellies. Laing and McBain (18) consider the gelation of soap to result from the linking up of colloidal particles to form a filamentous structure. From his work on foams Butschli was led to the opinion that many gels have a fine honeycomb structure which could be made visible with a hardening agent. Pauli and others have raised objections to this theory, on the ground that the honeycomb structure may have been caused by the treatment by the hardening agent. Copisarow (7) has produced such a structure by Musing tanning agents into a gelatin gel. Bradford (5) has proposed the theory that the reversible sol-gel transformation is an extreme case of crystallization. He is supported in his views by the researches of von Weimarn, who hrts prepared gels of the sulfates of calcium, barium, and strontium by precipitation from solutions of proper concentration. He states that there is no fundamental difference between these gels and ordinary jellies such as agar and gelatin. Weiser does not agree that all gels are crystalline, and cites evidence by Scherrer (21) and Harrison (12) to show that the spherites of gelatin and starch are not crystalline in character. This brief summary of the more outstanding theories of gel structure illustrates the great variance of opinion and the many difficulties involved in the interpretation of the complex phenomenon involved in gelation. In the study of gel structure the measurement of many properties has been undertaken, namely, Musion, viscosity (plasticity), optical properties, ultramicroscopic behavior, tensile strength, elasticity, compressibility, setting point, etc., but little attention has been paid to the electrical conductance of gels. Apart from the work by McBain on soap jellies, no extensive investigation has been made in this field.' The work herein reported was undertaken in order to obtain such data for a number of gel system and to apply the results as evidence for one or more of the above theories. While it is hoped that the study can be extended to other gelforming materials, this report is concerned only with gelatin-water or gelatin-salt-water systems. EXPERIMENTAL PROCEDURE
The Wheatstone bridge method was used in determining conductivity, using a Leeds and Northrup slide wire of Kohlrausch design. An audion tube was the source of the oscillating current. The refinements recom1 In addition to the work of McBain, mention should be made of the brief report of Hurd and Swanker (J. Am. Chem. SOC.66, 2807 (1933)), who state that the conductances of silica 601s and gels are practically the same.
BTRUCTURE OF GELS
501
mended by Jones and Josephs (15),and used by Edson and Rexroth (10) in a study of the conductance of electrolytes, were employed. The capacity of the conducting cell was balanced in parallel by an adjustable condenser. Two conductivity cells of the ordinary plunge type were used, one with a cell constant of about 0.4 and the other of approximately 0.1, the latter cell being used for the pure gelatin gels and the very dilute electrolyte solutions. The electrodes in the cells were rigidly braced, and the cell constant of each cell was determined after each trial to make sure that any changes in conductance were not due to changes in the positions of the electrodes. Measurements were made on the potassium chloride solution used for determinations of the cell constant over a period of four days to determine whether any error due to absorption might occur. No change was noted. Tests of the sensitivity of the method used both on the calibrating solution and on the actual conductivity measurements showed the method to be accurate to at least 0.1 per cent. All salts used were of c. P. grade and were further purified by recrystallization. After purification they were dried carefully to constant weight. Eastman ash-free gelatin was employed in all cases. Moisture determinations on this material gave an average value of 15.16per cent. Determinations of the ash content showed that the maximum value did not exceed 0.02 per cent on any samples used. The sols of gelatin were made up according to the following standard procedure, in order to minimize any complicating effects that are dependent upon the history of gelatin sols: Gelatin waa weighed out carefully and the approximate amount of conductivity water needed was added. The system was then kept at 45°C. for 1 hr. The gelatin became swollen and with occasional agitation was completely dispersed in another hour. The flask was then cooled and weighed to obtain the amount of water present. The conductivity measurements were made at 25" and at 30"C.,the maximum variation in temperature being 0.01"C. The conductance of the specimen in the sol state was first determined. It was then allowed to set slowly overnight a t room temperature (20-25°C.). The conductance of the gel was determined the following morning. I[t was found that sols which were set more rapidly by immersing them in an ice bath would melt more rapidly and at a lower temperature. When the sols were set slowly it was comparatively easy to get readings on the gel state of even some of the most dilute gels at 30°C. These data agree with the facts reported by Olson (19)in his study of the change of setting time with the temperature at which the sol was set. Olson assumes that at a higher temperature the building up of chains of molecules during the setting process is more complete. According to him, a gel setting rapidly is composed of short chains only slightly interlaced and is thus more easily redispersed.
502
ROBERT TAFT AND LLOYD E. MALM THE ELECTRICAL CONDUCTANCE O F PURE GELATIN SOLS AND GELS
The results obtained from conductance measurements on the system gelatin-water are summarized in table 1 and shown graphically in figure 1. The specific conductance of the sol is consistently greater than that of the gel in the range of concentrations used. The Merence between sol and gel increases with increasing concentration, and the increase is more rapid at 30°C. than at 25°C. This is clearly shown in figure 1 by the spreading of the curves with increasing concentration. The change of conductivity with temperature is greater for the sol state, the temperature coefficient being on the average about 25 per cent more. TABLE 1 Speeifie conductance of the eystem gelatin-water BYPICRA-
PH
PWI
w m:
*C.
4.58 4.58 4.63 4.63 5.99 5.99 6.41 8.31 8.31 13.2 13.2
30 25 30 25 30 25 30 30 25 30 25
I
I
SPPBQPIC CONDUCFANLlD
Gel mho8
4.80 4.81 4.85 4.88 4.90
-
* Per cent gelatin
=
5.96 X 5.89 X 6.10 X 5.95 x 7.90 X 7.17 X 8.49 X 10.46 X 9.27 X 12.44 x 11.68 x
mhoa
lo-' 10-5
lo-' 10-6
lo-' 10-6 10-6
6.20 x 6.13 X 6.39 X 6.20 x 8.46 x 7.53 x 9.09 x 11.26 X 9.71 X 13.35 X 12.35 X
10-6 10-6
10-6 10-6 10-6 10-6 10-5 10-5 10-6 10-6 10-6
0.24 x 0.24 X 0.29 X 0.25 x 0.56 X 0.36 X 0.80 x 0.80 X 0.44 X 0.91 X 0.67 X
lo-' lo-' lo-' lo-' lo-' 10-6 lo-' lo-'
lo-'
grama of dry gelatin (corrected for moisture content) x loo. grams of water
The pH value (determined by the use of the quinhydrone electrode) of the most dilute sol was 4.80. Slight increases in pH were observedat higher gelatin concentrations. A pH of 4.8 corresponds to a hydrogen-ion concentration of approximately 2 X gram-moles per liter, a value mhos of the specific which could not account for more than 0.7 X conductance of the sols. The ash, less than 0.02 per cent in amount, might also account for a part of the conductance. The Eastman Kodak Company informs us, however, that actual analyses of the ash content of their "ashless" gelatin, which was employed in our study in all cases, consists almost entirely of silica. The form of the silica as present in our sols and gels is unknown but in any case, whether present as free silicic acid, colloidal silica, or in some other form, its actual conductance is probably small. This conclusion may best be realized when compared with the
503
STRUCTURE OF QELS
calculated conductance of a strong electrolyte. If, for example, the ash was entirely calcium oxide, present to the amount of 0.02 per cent, the calcium ion, if we assume complete dissociation from the gelatin, would have a specific conductance of 2 X mhos, that is, about one-third of the total specific conductance. It is thus evident that both in the sol state and in the gel state, gelatin itself is the major conductor of the current. Further, the decrease in conductance from sol to gel state, in the cases studied, lies only between 4 and 8 per cent. If gelatin micellae are therefore the chief conductors in both sol and gel state, it is evident that gelation does not seriously obstruct the conduction process. Without doubt the gelation process has decreased very considerably, if not destroyed, the mobility of the gelatin micellae. The small I5
v p 13 X il 8 1 , U
2 9
8 2
L7
Y
v)
5
3
4
5
6
7
8
9
LO
LI
12
13
14
em CLNT or G L U T I N FIG.1. Conductance measurements on the system gelatin-water
decrease in conductance upon gelation appears, therefore, at first thought, to be diflicultly explainable. The two factors become reconcilable if it is assumed that the liquid phase ezists as the continuous phase in both sol and gel states. Conduction in the sol state is then largely cataphoresis; upon gelation the process has changed from cataphoresis to electroendosmosis. In the sol state the gelatin micellae move in the liquid phase; in the gel state the liquid moves past the k e d gelatin micellae, each of which may possess surface conduction. Upon the basis of such an explanation, the small decrease in conduction upon gelation would be due to the decreased surface of contact of gelatin micellae with the liquid phase, and to the somewhat greater length of path that any ion current carriers must of necessity traverse when gelation takes place. It will be understood that such an explanation assumes also the necessity of a continuous phase of the gelatin micellae in
504
ROBERT TAFT AND LLOYD E. MALM
the gel state in order to account for the rigidity of the gel. The conductance of gelatin sol and gel is thus in accord with the fibrillar theory of gel structure. TABLE 2 Specific conductance during sol-gel transformation
hours
mho8
0 0.5 1 14 16
0 3.5 5 6 9 9.25
0.00007534 0.00007451 O.OOOO736a 0.0000716~ 0.00007160
Sol Setting in parts; soft meniscus Soft jelly; part liquid Gel set solid overnight Gel; no change
O.OOO08487 0.0000860 0.0000865 O.oooO871 0.00009088 O.OOOO908e
Gel (set rapidly a t low temperature) Meniscus soft Soft jelly; viscous Viscous sol; about melted Sol state Sol; no change
SPiciric
CONDUCTANCE
x lot5
FIQ.2. Conductance measurements taken as gelation and redispersion were in progress
In the case of the 5.99 per cent and 6.41per cent sols, conductivity measurements were taken as gelation and redispersion processes were in progress. The data given in table 2 (see also figure 2) show that the gelation and redispersion processes are gradual and continuous, and the ease with which the change is accomplished is evidence for the theory that the two phases
STRUCTURE OF GELS
505
present in the sol and gel state are not extremely different in their composition. Yabuki (24),who was interested primarily in the bffect of added salts on the conductivity, includes also some data on the conductance of pure gelatin gels. His values are approximately ten timts those reported here, but since he lnakes no specifications concerning the ash content this difference may be easily accounted for by a high ash content. Greenberg and Mackey (11) have reported on the conductivity of pure gelatin sols and gels within the range of concentrations of 0.9 and 4.58 per cent. The highest percentage reported is the only result comparable to those reported here; the conductance is of the same magnitude but slightly higher. I n general their results as to the difference in conductance between sol and gel are in good agreement with those of table 1. It should also be pointed out that the difference in conductivity between sol and gel state cannot be accounted for upon any difference in the specific volume of the two phases. As we have pointed out elsewhere (22), there is a slight contraction in volume when the sol solidifies. This contraction, for gelatin concentrations up to 10 per cent at least, is less than 0.01 per cent of the volume of the sol. Furthermore, such a change in volume should produce a n increase (rather than the observed decrease) in specific conductance of the gel state over the sol state, as the number of current carriers per unit of volume would be greater in the gel state than in the sol state provided no other changes occurred. ELECTRICAL CONDUCTIVITY OF ELECTROLYTES IN GELATIN SOL8 AND GEL8
The first studies in this field were made by Arrhenius (17) in 1885 when he determined the conductivity of sodium chloride, zinc sulfate, and copper acetate in a gelatin solution which set to a gel at 24°C. He allowed a 4.2 per cent sol to cool slowly and found no discontinuity in the temperatureconductivity curve a t the setting point and no appreciable difference in conductivity of the sol and gel state. In 1889 Indeking (18) measured the viscosity and conductivity of zinc sulfate solutions containing up to 50 per cent gelatin. He found no discontinuity in the temperature-conductivity curve a t the setting point. Dumanski (9) studied the conductance of salts in gelatin sols only and made no measurements on the gel state. Further consideration of his results is given below. McBain and Laing (18) investigated the conductance of soap solutions. Their results indicate that there is no difierence in the conductance of sodium oleate in the sol and the gel state. These authors assume that the colloidal soap arises from, and is in true equilibrium with, the crystalloidal constituents. Yabuki (24) compared the conductance of potassium chloride in water and in gelatin and again in the gel state. His results will be considered later, together with those of Greenberg and Mackey (16),
506
ROBERT TAXT AND LLOYD E. MALM
TABLE 3 The electrical conductance of salts in gelatin sols and gels*
-
!!
(b- 1 x l&
I
801
F w
N
4.53 4.53 4.53 4.53 4.58 4.58 4.58 4.58 4.63 4.63 4.63 4.63 4.63 4.63 8.31 8.31 1.31 8.31 8.31 8.31 8.31 8.31
0.00101 0,00501 0.0101 0.0505 0.1008 0.1006 0.2508 0.499 0.0101 0.0101 0.1008 0.1007 0.499 0.498 0.00511 0.0051C 0.01022 0.01021 0.1018 0.1017 0.5037 0.5028
cent
00.
25 25 25 25 25 30 25 25 25 30 25 30 25 30 25 30 25 30 25 30 25 30
-
13.48 13.310.17134.8 133.1 7.18 7.2: 7.5: 65.30 04.900.40130.6 129.8 12.7 59.0 58.8 126.7 126.6 0.20126.7 126.6 120.4 120.4 20.3 579.6 579.0 0.00115.0 115.8 573.2 573.0 92.0 ,118. 1118. 0.00111.8 111.8 1108. 1108.0 187.0 ,236. 1236. 0.00123.6 123.8 1226. 1228. 183.0 B36. 2636. 0.00105.4 105.4 2624. 2624. 450.0 870.0 i999. 4990. 0.00 99.98 99.984993. 4993. 124.7 124.6 0.8 124.7 124.6 128.6 128.6 12.1 23.0 138.3 137.7 0.6 138.3 137.7 131.9 131.6 ,107. 1107. 0.00110.7 110.7 1101. 1101. 181.o 214.0 ,201. 1201. 0.00120.1 120.2 1196. 1196. 876.0 1990. 4990. 0.00 98.8 98.8 4984. 4984. i266. 5266. 0.00105.3 105.3 5250. 5250. 32.5 59.07 58.680.46118.16 117.26 49.36 49.0 53.89 54.1 65.16 64.600.56130.3 124.8 35.6 114.8 114.7 0.1 114.8 114.7 105.1 105.4 116.0 117.0 38.6 127.16 127.6 0.36127.16127.6 271. ,020. 1020. 0.00102.0 102.0 1017.0 1017. 303. ,117. 1117. 0.00111.7 111.7 1106. 1107. 420, L451. 4451. 0.00 89.05 89.034448. 4 4 4 . L880. 4880. 10.00 97.78 97.783878. 4878.
1.7 2.8 4.4 20.0
40.0 40.0 98.0 191.0 2.6 4.9 40.0 46.0 190.0 3.9 4.3 4.6 32.5 40.9 170.
--
CONCENTRATION
3.2 5.99 5.99 5.99
k
K%N
-
N 0.1 0.01011 0.1009 0.2008
27 25 25 25
-
956.6 955.1 108.6 108.6 973.0 973.0 ,847. 1847.
0.6 95.6 95.5 944.0 943. 0.00108.6 108.6 101.1 101.4 0.00 97.3 97.3 967. 967. 0.00 92.20 92.291840. 1840.
417, 33.3 231. 440.
32. 5.6 39. 73.
-
*A = equivalent conductance. Y = (specific conductance of salt in gelatin) (specific conductance of gelatin). y 5 (specific conductance of aqueoua salt solution) - Y ;or decrease due to
-
-
presence of gelatin.
k-y= decrease per gram of gelatin in 100 g. of sol. C
who determined the conductivity of hydrogen chloride, sodium hydroxide, and sodium chloride in sols and gels of gelatin. With this background of comparative data, the following determinations
507
STRUCTURE OF QELS
have been made: First, the conductance of the purified salts was determined and compared with the values given by the International Critical Tables and by Kohlrausch and Holborn. The agreement was satisfactory (within 0.1 per cent) and checked the purity of the salts and the experimental method used. Table 3 gives the results obtained for potassium chloride of concentrations from 0.001 N to 0.5 N in several gelatin sols and gels. These results for 25°C. are shown graphically in figure 3. The equivalent conductance curves of potassium chloride in gelatin sols and gels follow those of potassiurn chloride in aqueous solutions in general, and have about the same slopes. The presence of gelatin causes a decrease in conductance, however,
--
3
4
5
b
LOG VOLUME FIQ.3. Equivalent conductance of potassium chloride in gelatin sols and gels
and this decrease is greater as the gelatin content increases. The conductivity was also the same in the gel as in the sol state down to salt concentrations of 0.01 N to 0.001 N . Here the equivalent conductance was slightly greater in the sol than in the gel state. However, a t this low salt concentration the conductance of the gelatin itself becomes a n appreciable factor. If the specific conductance of pure gelatin is subtracted from the total specific conductance, these differences between the sol and the gel state are practically removed. It will be recalled that the conductance of pure gelatin in the sol state was greater than in the gel state, and this difference accounts for the spreading of the sol-gel curves at low salt concentrations. The fact that the conductance of potassium chloride in gelatin sol and
508
ROBERT TAFT AND LLOYD E. MALM
gel is the same is interpreted by us as meaning that the ions move with equal freedom in both cases. The only difference is that in the sol state the gelatin particles move also, while in the gel state their movement is more restricted. Evidently in the sol state there are large interstices between the gelatin particles which permit the movement of ions, and during the setting process these particles orient themselves in such a way that a rigid system is produced, but the interstices are not thereby reduced to a great extent. These data, we believe, are again evidence for the “brush-heap” or fibrillar theory of gel structure, in which both phases are considered continuous. The temperature coefficient of the specific conductance of potassium chloride in gelatin (both sol and gel are the same) over the limited range of 25°C.to 30°C.is given in table 4. The temperature coefficients of conductance for gelatin sols are of the same order of magnitude as those for aqueous solutions. This is in agreeTABLE 4 Temperature coefiient of specific conductance CONCEXTRATION OFOEWTIN
‘ j
0.01 N KCl
1
0.1 N KC1
1
0.5NKC1
per cent
4.58 4.63 8.31 0.00
0.oooO270 0. oooO247
0.oooO278
0.000240 0.000190 0.ooO194 0.ooO242
0.00055 0,00087
ment with the work of Arrhenius (1) on salts in gelatin. Indeking (14) found that the temperature coefficient was no longer independent of concentration at high gelatin concentrations (50 per cent). There is a slight tendency, as shown in the data presented in table 4, for the temperature coefficient to decrease with increasing gelatin concentration. Since the loss of fluidity due to gelation has been shown not to affect the conductivity, the change in viscosity a t 25°C.and 30°C.could not account for this. However, if ion adsorption on gelatin micellae occurs, it is possible that these ions are not freely liberated a t higher temperatures; consequently, as the gelatin concentration increases, the temperature coefficient becomes lower. Further information which may be interpreted as bearing upon ion adsorp+,ionwas obtained by using solutions of potassium thiocyanate in gelatin, and comparing the decrease in conductivity in gelatin with the decrease in potassium chloride sols and gels. The last three values of table 3 were a preliminary attempt to investigate this point, where it was found that (k - g)/C (the decrease in specific conductance per unit concentration of gelatin) showed that a greater decrertse occurred in the thio-
509
STRUCTURE OF GELS
cyanate solutions containing 5.99 per cent gelatin than for potassium chloride solutions containing 8.31per cent gelatin. This suggested a more TABLE 5 The specific conductance of aqueous potassium chloride and potassium thiocyanate solutions at 46%'. MLT CONCE"FE4TION
'
1
BPECIFIC I CONDUCT- 1 ANCI = k I
A
DENBITY
Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.00495NKCl.................... 0.00989 N KCl ... . , . . , . . . . . . . . . . . . O . O 4 9 4 4 N K C l . . . . . . . . . . . . . . . . . . . . '917.5 183.5 0.09875 N KCl.. . . . . . . . . . . . . . . . . . . I 1742. 174.2 0.01~95 N KSCN.. . . . . . . . . . . . . . . 91.35 1 182.7 O.OG988NKSCN . . . . . . . . . . . . . . . . . 180.5 , 180.5 0.04932 N KSCN.. . . . . . . . . . . . . . . . 849.5 169.9 0.09835 N KSCN ... . . . . . . . . . . . . . .~ 1627. 162.7
.I
1
~
~
0.99028 1.oooO 0.99026 0.9982 0.9905 0.9930 0.9924 0.9993 1.0027 0.9948 0.9900 1.0012 0.9897 1.0005 0.9906 1.0034 1,0070 0.9936
0.00597 0. 00596 0.00593 0.005956 0.00599 0.00598 0.00597 0.00599 0.00603
* Relative viscosity.
t Absolute viscosity.
TABLE 6 The specific conductance of potassium chloride and potassium thiocyanate i n 4.6 per cent gelatin at 46°C."
4.5 per cent gelatin .... . 0.00502 N KCl.. . . . . . . . 0.01003 N KCl., . . . . . . . 0.05015 N KC1. .. . . . . . . 0.1OOO N KCl.. . . . . . . . 0.00502 N KSCN.. . . . . , 0.01003 N KSCN.. . . . . . 0.05006 N KSCN.. . . . . . 0.998 N KSCN.. . . . , .
17.1 1.0040 4.628 0.02763
*A = equivalent conductance. Y = (specific conductance of salt in gelatin)
-
(specific conductance of pure gelatin). A. = equivalent conductance calculated from Y. k - y = (specific conductance of salt in water) Y ;or decrease due to presence of gelatin. 7 , = absolute viscosity of electrolyte-gelatin system.
-
extended study of thiocyanate and chloride solutions at a lower concentration and a t a temperature well above the setting point of such systems.
610
ROBERT TAFT AND LLOYD E. MALM
Table 5 gives the data on aqueous solutions at 45OC. Table 6 (see also figure 4) gives data on 4.5 per cent gelatin-salt solutions and includes viscosity and density data as well as conductivity readings. The Hofmeister series is common to a variety of phenomena in colloidal chemistry, crystalloidal chemistry, and biology. The thiocyanate ion occurs at that end of the Hofmeister series where the ions have the ability to peptize negatively charged emulsoids, decrease the time of gelation, enhance swelling, etc. The chloride ion is located somewhat nearer the other end of the series. Column k - y of table 6 shows the decrease in conductivity of potassium chloride and potassium thiocyanate due to the presence of 4.5 per cent gelatin. The decrease is approximately 5 per cent
.--
35
4.0
45
5.0
LOG
55
40
45
50
5.5
VOLUML
FIQ.4. Equivalent conductance of 4.5 per cent gelatin-salt solution8
greater for the thiocyanate than for the chloride ion. The viscosity data show that the viscosity of gelatin-potassium thiocyanate sols was less than that of gelatin-potassium chloride sols by about 3 per cent over the range of salt concentrations; consequently this factor cannot account for the peculiarity in the case of potassium thiocyanate sols. A possible explanation may be obtained by assuming that the thiocyanate ion is more strongly adaorbed on the gelatin micellae than the chloride ion and consequently fewer ions are left free to act in the conduction process. The ions adsorbed, of course;are not completely bereft of conducting ability, for upon adsorp tion they would then possem the velocity of the gelatin particles. This, however, is appreciably lower than for the simpler inorganic ion. Adsorp tion of ions would, however, bcreaae the charge on the colloidal particle, and thia would tend to increase the velocity of the particle. Thus it is
STRUCTURE OF QELS
51 1
necessary to amume that adsorption has not proceeded to such an extent that the mobility of the colloidal particles has become equal to ionic mobility. CONCLUSIONS
The fact that the change in electrical conductivity with time in the solgel transformation is gradual and regular is in agreement with other data on this transformation (19, 23, 4). This leads to the conclusion that the process involved in the formation of the gel structure is not a discontinuous one, such as crystallization would require, but is a gradual coagulation of particles. When gels were caused to set rapidly a t low temperature, they melted more readily than those which set slowly, which may be due to the fact that the process of building up chains of particles had not proceeded to completion. It is probable that the two phaseb present in the gel state are not greatly different from those present in the sol state. The micellae as one phase are richer in gelatin content than the intermicellar phase. According to Bungenberg de Jong (8) the charge and hydration of the particles are not uniformly distributed over the surface of each particle but are localized in different areas. Removal of either charge or hydration would result in gel formation. It appears probable that gelatin micellae are the chief conductors of the current in the system ash-free gelatin-water. The small decrease of conductance upon gelation is interpreted as meaning that the liquid phrtse is still continuous in the gel state, the process of conduction changing from cataphoresis to electroendosmosis. In order to account for the rigidity of the gel, it is assumed that the gelatin phase is also continuous. The conductivity data therefore, in our judgment, support the fibrillar theory of gel structure. The identity of conductance of the various salts contained in the sol and gel states of gelatin also indicates that the gel structure cannot be of the discontinuous phase or honeycomb type. It is evidence again for a structure of the fibrillar type, in which the spaces between the fibrils are of sufficient size to allow the ions to travel with nearly the same freedom in the gel as in the sol state. The decrease in the conductance of salts, owing to the presence of the gel-forming substance, is probably due to several factors. The most important of these, we believe2 are adsorption of ions by the gel-forming substance and the increased resistance or increase in length of path that the ions must take, owing to the presence of the relatively huge gelatin micellae. SUMMARY
1. The electrical conductance of ash-free gelatin in both the sol state and the gel state has been determined a t 25°C. and at 3OOC. Conductance
512
ROBERT TAFT AND LLOYD E. MALM
measurements were also carried out during the sol-gel transformation at constant temperature. I n the cases studied, no abrupt change in conductance is observable when gelation takes place. There is, however, a gradual decrease in conductance during gelation, the maximum decreases in conductance lying between 4 and 8 per cent. 2. The electrical conductance of potassium chloride and of potassium thiocyanate over a considerable range of concentrations, when present in gelatin sol and gel, has been determined at several constant temperatures. The conductance of the electrolyte is appreciably lower in the presence of gelatin, but is approximately the same in the sol and the gel state, save at very dilute concentrations of electrolyte. Potassium thiocyanate shows a relatively greater decrease in conductance in the presence of gelatin than does potassium chloride. 3. Our results are interpreted as evidence for (a) the fibrillar structure of gelatin gels and (b) the greater adsorption of thiocyanate ion than of chloride ion by gelatin. REFERENCES (1) ARRHENIUS:Brit. Assoc. Advancement Sci. Rept. 66, 544 (1906). Z. anorg. Chem. 73,125 (1925). (2) BACHMAN: AND ZSIOMONDY: Kolloid-Z. 11, 160 (1912). (3) BACHMAN (4) BOQUE:J. Am. Chem. SOC.44, 1313 (1922). (5) BRADFORD: Biochem. J. 12, 351 (1918);14,91 (1920);16,853 (1921). Physical Properties of Colloidal Solutions, 2nd edition, p. 143. Long(6) BURTON: mans, Green and Co., London (1923). (7) COPISAROW: Kolloid-Z. 44,319 (1928). Z. physik. Chem. 130,206 (1927). (8) DEJONG: Z. physik. Chem. 80, 563 (1907). (9) DUMANSKI: (10) EDEON AND REXROTH: Engineering Thesis, University of Kansaa, 1928. (11) GREENBERG AND MACKEY: J. Gen. Physiol. 16,161 (1930). J. SOC.Dyers Colourists 32, 40 (1916). (12) HARRISON: (13) HATSCHEK: Trans. Faraday SOC.12, 17 (1916-17). Wied. Ann. 97, 172 (1889). (14) INDEKINO: J. Am. Chem. SOC.43,1095 (1921). (15) JONES AND JOSEPH: Kolloid-Beihefte 9,1 (1918). (16) KATZAND ROBINSON: (17) KRUYT:Colloids, 1st edition, p. 217. John Wiley and Sons, Inc., New York (1927). (18) LAINQAND MCBAIN:J. Chem. Soc.117,1506 (1920). (19) OLSON:J. Phys. Chem. S8,529 (1932). (20) P R ~ ~ T O J. E Chem. : 500. 106, 317 (1914). (21) SCHERRER: Nachr. kgl. Ges. Wiss. Gijttingen, p. 26 (1918). (22) TAFT AND MALM: J. Phys. Chem. (2,1187(1938). (23) WALPOLE:Kolloid-2. is, 241 (1913). (24) YABUKI:J. Biochem. 8, 157 (1928).
