SENSITIZATION AND SURFACE CHARGE OF LYOPHILIC COLLOIDS
705
(15) PEARCE, J. N . , AND JOAXSTONE, H. F . : J. Phys. Chem. 34, 1260 (1930). (16) PEARCE, J. N . , AND KNUDSON, C. M . : Proo. Iowa Acad. Soi. 34, 197 (1928). , (17) PEARCE, J. N . , AND PETER;,P. E . : Proo. Iowa Aoad. Sci. 57,223 (1930). (18) PEARCE, J. N., AND TAYLOR, A. L.: J. Phys. Chem. 88, 1091 (1934). (19) POCKELS, A.: Physik. Z. 16,39 (1914). (20) RIDEAL, E. K.: An Introduction to Surjace Chemistry. Cambridge University Press, London (1926). AND INNES,W. B . : J . Phys. Chem. 48,537 (1942). (21) ROWLEY, H. H., (22) SCHIJMACHER, E.E.: J. Am. Chem. SOC.46,2255 (1923). (23) THOMION, W.:Phil. Mag. [41 4% 448 (1871).
SENSITIZATION AND SURFACE CHARGE OF LYOPHILIC COLLOIDS ERNST A. HAUSER
AND
MARTIN R . C I N E S
Department of Chemical Enginering, Maasachusetts Institute of Technology, Cambridge, Massachusetts Received June #, 1941
Freundlich (1)' as a result of the work of Freundlich and Brossa (3) on ferric oxide sols and serum albumin, had postulated that sensitization was to be explained on the basis that the protein sensitizers behaved as electrolytes and partially neutralized the surface charges of the lyophobic sols, thus rendering them less stable. If this concept is to apply generally, there must be provision for non-ionizing sensitizers such as carbohydrates. In the latter case, if the lyophilic colloid is to behave similarly to the protein, then the surface charge on the carbohydrates must take the place of the ionization in the protein. EXPERIMENTAL
In order to determine whether or not Freundlich's concept was correct, it was necessary to investigate sensitization using materials which would be in the class of non-ionizers. Therefore, it was decided to use Merck's reagent agar and dialyzed iron Cferrum dialysatum purum) To study the decrease of the surface charge on the lyophilic colloid, agar, viscosity measurements were made with a Hoppler viscosimeter of the rollingball type. All viscosity and subsequent measurements were made a t 50°C., 10" above the temperature of gelation, in order to prevent any gelation which might otherwise occur in the course of the investigation. The agar sols were prepared by adding approximately 0.15 g. of the reagent agar to 50 cc. of boiling conductivity water. After remaining 12 hr. in a thermostat, in Pyrex flasks, they were filtered in order to remove a fibrous material which appeared in them on standing. To these filtered sols was added 50 cc. of sodium chloride solutions of varying concentration. Before determining the viscosities of the agar-
.
1 Present address : School of Chemistry, University of Minnesota, Minneapolis, Minnesota.
706
ERNST A. HAUSER AND MARTIN R. CINES
electrolyte mixtures, they were returned to the thermostat for several hours. The results of the viscosity measurements were expressed as relative times of fall,-time for the ball to pass two fixed marks on the viscosimeter,-and are shown in figure 1. The curve shown is very similar to that obtained by Kruyt (5) in his study of the electroviscous effect on agar. The neutralization of agar particles having been determined by the disappearance of the electroviscous effect, the extent of the sensitization range of agar sols was to be found. Agar sols were prepared as before, except that the concentrations were varied. The sols containing ferric oxide of required concentrations were prepared by adding 5 CC. of dialyzed iron, or diluted dialyzed iron, to 45 cc. of agar sol. No note was taken of the electrolyte contained in the dialyzed iron, for itwould have no effect,since all determinations to be considered were of relative value only. After the sols had remained in the thermostat for 2 t o 3 hr., the amount of sodium chloride required to cause coagulation was determined by adding increasing amounts of electrolyte, with stirring, to the sol mixture. Since this work was done a t elevated temperatures, it was imIO0
$J.. 0
092
E
048
s Y)
g 0
2 4 6 8 IO CfflCENlRATlffl Cf ELECTROLYTE (NaCI) UILLIEWIV 1 LITER
-
12
FIO.1. Viscosity change of agar sols on addition of electrolyte. Concentration of agar, 1.5g. perliter.
possible’to use an ultramicroscopic technique to observe coagulation; therefore a crude method had to be used. With the sol in the standard container, a 50-cc. beaker, electrolyte was added until the ruby-red color had disappeared, when the sol was viewed by transmitted daylight: a t which point the sol was considered to be coagulated. The results of a series of determinations with three different concentrations of ferric oxide are shown in figure 2. From the reproducibility of experimental data, these results would appear accurate within 10 per cent. It will be noted that only in the case of the most dilute ferric oxide sol was it possible to obtain a protection value for the agar. The reason why this one point could be determined was that higher concentrations of ferric oxide required such large quantities of agar that the turbidity and discoloration resulting from this large quantity of lyophilic colloid could not be distinguished from coagulation as determined by the present method. Furthermore, no intermediate points between coagulation on mixing the two sols and protection could be observed; hence that portion of the curve is uncertain. 9
The northern exposure of the laboratory gave
8
reasonably constant light intensity.
SENSITIZATION AND SURFACE CHARGE OF LYOPHILIC COLLOIDS
707
The next step ww to determine the effect of the addition of partially neutralized agar to ferric oxide sols, aa contrasted to the previous study in which electrolyte-free agar ww used. The agar sols were prepared as before, and the neutralizing electrolyte was added to them M in the viscosity studies. This sol was used to dilute ferric oxide sols to required concentrations, and the coagula-
0
04
08
I5
I8
50
24
28
Or AGAR ADDED -GMS/1ooOCc FIG.2. Sensitization-protection range of agar on ferric oxide sols WANTITI
TABLE 1 Effect o f addition of neutralized apar on sensitization of. .ferric oxide sols* CONCEXIXAIION 01 A G U
ELECIPOLYTE ADDKD TO N E m L U E AGAX: CONCENTWTION WEEN YDLED
w ~ z F& i
SOL
grsmr per lilsr
m i l l i n o h pcr litn
1.503 1.508 1.505 0.583 0.588 0.582 0.599 0.588 0.588 0.591
0
* 0.501 per cent ferric
3 1 0 2 3
4 5 6 7
COAGULATION VALUE IN YILLIMOLES 01 NaCl PEP U I E P IO COAGULATE SOL m U X E
5.5 5.1 0 55 55 55 53 53 51 51
oxide. The coagulation value of the pure sol waa approximately
195 millimoles of sodium chloride per liter.
tion was determined by adding sodium chloride in varying amounts as before. The results of this series are shown in table 1. DISCUSSION
From a consideration of table 1, it will be noted that there is a slight decrease in the coagulation value of sodium chloride (column 3) on the addition of in-
708
ERNST A. HAUSER AND MARTIN R. CINES
creasingly neutralized agar, but this decrease is readily accounted for on consideration of the quantity of electrolyte which was added to the agar sols (column 2). Furthermore, a decrease in the coagulation value is in the direction opposite to that which might be expected were the surface charge of the agar effective in sensitizing the ferric oxide sol. It would be expected that if the surface charge of agar were effective in sensitization, neutralization of the particle would decrease the sensitizing action and the coagulation value should increase towards 195, the value for the pure sol. Additional indication that the surface charge cannot be of any great importance in explaining this phenomenon will be found on consideration of figures 1 and 2. In figure 1, it will be noted that the asymptotic value of the relative viscosity occurs at a ratio of agar to sodium chloride of 4 . 6 : l . The asymptotic value would be the value of the viscosity when the agar particles were com-
FIQ.3. Mechanism of sensitization pletely discharged. From point a, figure 2, it can be found that 1.40 g. of agar reduced the coagulation value from 195 millimoles of sodium chloride to 6.0 millimoles, or that 1 g. of agar is equivalent to 8.3 g. of electrolyte. From these results, it hardly seems possible that the charge on 1 g. of agar, which can be neutralized by 4.6 g. of electrolyte, could have an effectiveness in coagulation power of 8.3 g. of this same electrolyte. Recently, Overbeek (6) has published a new concept of sensitization. From studies on protection, notably the work of Freundlich and Abramson (2), it is known that the lyophilic colloid is adsorbed on and surrounds the lyophobic particles. Therefore, Overbeek believes that when the two types of sols are mixed, one lyophobic particle has one lyophilic particle adsorbed on its surface and, on the addition of electrolyte, coagulation occurs because of the formation of a coacervate. Of the four types of coacervates classified by de Jong (4), this phenomenon could only be placed in the grouping of “complex coacervates.”
SENSITIZATION
AND SURFACE CHARGE OF LYOPHILIC COLLOIDS
709
However, the coacervates reported by de Jong in this classification had initial concentrations of lyophilic colloid in the sols of about 1to 2 per cent. Although no lower limit has been found in “complex” coacervation, it seems doubtful that such minute concentrations m found in sensitization could behave as suggested. As a further objection to this concept, it will be noted that it fails to explain the coagulation of lyophobic sols on the addition of lyophilic colloid alone, that is, without further addition of electrolyte. A new concept of sensitization which will explain the latter phenomenon, in addition to ordinary sensitization, is the following: The inner layer of charges of the Gouy diffuse double layer is believed, by most people, to be firmly attached t o the colloid particle, fitting roughly into the crystalline structure or at least held in place by similar forces. The outer or diffuse layer is merely an increased density of ions in the surrounding dispersion medium. Therefore, when the lyophilic colloid is adsorbed on the surface of the lyophobic particles, it effectively covers a portion of the inner layer, that is, assuming that the particles are not completely covered, which is justifiable insofar as a completely covered particle is protected. Since a portion of the surface has been covered, the ions in the diffuse layer immediately above the portion covered must have been pushed aside to allow passage for the lyophilic particle. Furthermore, since there is still a net charge on the surface, these ions will not tend to wander off into the dispersion medium, but will remain concentrated around the particle. Thus, the double layer on the uncovered .surface in the immediate vicinity of the covered poition will have an increased density of counter-ions and the particle will have been partially neutralized or sensitized. A representation of this suggested mechanism is shown in figure 3. In this manner, it seems possible to explain all the phenomena of sensitization excepting the peptization observed by Freundlich and Brossa (3) in their work. In spite of this deficiency, it is felt that the above concept has value insofar as it can explain a greater number of data than any other which has been presented to date. REFERENCES (1) FREUNDLICH, H.: Chapter XI1 of R. H . Bogue’s CoEloidaZ Behavior. McGrsw-Hill Book Company, Inc., New York (1924). (2) FREUNDLICH, H . , AND ABRAMSON, H . A.: Z . physik. Chem. 193, 51 (1928). (3) FREUNDLICH, H.,AND BROSSA, A , : Z. physik. Chem. 89, 306 (1915). (4) JONG,H . G. BUNGENBERG DE: Coacewation. Hermann et Cie, Paris (1936). (5 KRUYT, H.R., AND JONG,H. G. BUNGENBERG DE: Kolloidchem. Beihefte 28, 1 (1929). (63 OVERBEEK, J. TH. G.: Symposium on Hydrophobic Colloids, held a t Utrecht in 1937.
