REVERSIBLE AGGREGATIONS OF COLLOIDAL PARTICLES. I11
ISOTHERMAL REVERSIBLE SOL SYNEREBIS~ WILFR’IED HELLER School of Chemistry, Institide of Technology, LIniversity of Minnesota, Minneapolis, Minnesoln Received November 13, 1941
I t has been shown (2) that reversible aggregates (geloids) develop in thisotropic iron oxide sols of low colloid concentration and that their subsequent association to an elastic structure leads to the transformation of the sols into gels. Subjected to the action of a centrifugal force, growing geloids settle befort? they are able to associate themselves (2). Then the system separates into a lower concentrated phase, representing a thixotropic gel (built up by the settled geloids), and into a dilute sol phase above the gel. We expected to obtain a similar phenomenon, by the influence of gravity, when only a minor amount of the dispersed phase participates in the process of aggregation. Then again, geloids should be unable to form a structure throughout the system. This time it should be so because of their low concentration, whereas in the former case it was because of thcir greater rate of settling as compared to that of association. Again, a geloid structure should develop in the lower parts of the sols after geloids have been concentrated therc by settling. It had been shown in previous papers that the amount of disperse phasc participating in the aggregation process increases, in iron oxide sols, with increasing concentration of sodium chloride. Therefore, the greatest chance for finding the phenomenon in question seemed to be in those systems where the amount of sodium chloride added was smaller than that required for obtaining a thixotropic sol-gel transformation. The present paper will deal with this phenomenon, which may be called an “isothermal reversible sol syneresis” (1). Later (section I11 A) we shall justify the application of the term “syneresis” to a system which, in its initial state, is not a gel but possibly a sol. I. EXPERIMENTAL
The same B-FeOOH sol which had already been used for some of the centiifugal experiments on thixotropic systems (2, sol A) was employed. It contained 7.56 g. of iron per liter, after addition of the sodium chloride. Three systems derived from this sol will be considered, containing 30, 38, and 46 millimoles of sodium chloride per liter. One month after the addition of sodium chloride they had a setting time of > 29 hr., -4 hr., and 105 min., respectively, in test tubes 0.75 cm. in diameter. 11. RESULTS
A . Development and general topography o j reuersibly syneretic systems Following a one-month initial period after the addition of sodium chloride, to ensure ion-interchange equilibrium, the three systems were kept standing at
.
Presented before the Division of Colloid Chemistry at the 102nd Meeting of the American Chemical Society, which was held in Atlantic City, New Jersey, September, 1941. 783
784
WISFRIED HELLEH
rest in bottles about 5 cm. in dimneter. Whcreas aliquot samples, kept in narrow test tubes, were and remained thixotropic gels, the systems in the bottles never set to typical gels. After several weeks' standing a t rest, a separation became visible ; a boundary, which moved downward slowly, appeared, separating a lower concentrated phase from an upper dilute phase. (In the system with 30 millimoles of sodium chloride per liter, the boundary waa very diffuse, and even after 7 months it was still close to the air-sol surface. Fractions taken from this system developed a far better boundary.) After the systems had stood for a t least 7 months, fractions were taken with a pipet at different heights of the colloid layer, and put into test tubea 1.2 cm. in diameter. The behavior of these fractions was then studied separately. Fractions taken from above the boundary proved to be dilute non-elastic soh, which did not separate upon TABLE 1 The change ojthe tlolume Tatio R I wzth the collozd concentratzon P-FeOOH system containing 30 millimoles of sodium chloride per liter. The syneretic sol was kept a t rest for 7 months, in a bottle with a diameter of 5 cm. Then fractions were taken at different heights, h (total height of the colloid layer = hi), and kept at rest for another 6 months, in tubes 1.2 cm In diameter PELATIVE BEIOHT (hlhr) AT WHICH FRACTION WAS TAKEN AFTER 7 MONTES
0. QR7
: O l l O I D CON CENTMTION OF FPACTION ( O M U S OF IRON P E P LITER)
0.57
0.726
0.350 O.Oo0 Fraction taken from system homogenized by brief shaking
10.47 7.56
BEHAVIOR OF FRACTIONS AFTEP 6 MONTES OF STANDINO AT P E S T I N TEST TUBES
Fluidity of lower phaae upon turning the test tubes upside down
Reversibly syneretic
0.38
Reversibly syneretic
0 70
Thixotropic gel Thixotropic gel
1 .oo
1 .oo
Flowing elastically a t once Flowing very slowly after W40 sec. Not flowing Not flowing
Reversibly syneret,ic
0.83
Not flowing
renewed standing a t rest, this time in test tubes. Fractions taken deep in the lower part of the systems were found to be truly thixotropic gels; which upon standing at rest in test tubes did not separate either. Fractions, however, which were taken from close underneath the boundary behaved aa the original system, Le., they separated in the course of time. Among these separating fractions, the final degree of separation, expressed by the volume ratio concentrated lower phaae/total volume, R1, was greater in the fractions taken a t greater heights. One example is given in table 1. From the results obtained with the fractions, a picture could be drawn as to the change of various properties with the height of the colloidal layer in the main systems. The change of consistency is depicted in figure 1. Figure 2 gives the change of the setting time with height (for a tube diameter of 0.5 cm.). Figure 3 finally shows the change of the iron concentration with height.
SOL t BOUNDARY SYNERETIC GEL
0.4 THlYOtROPlC GEL
h 0.2 l
h
~
~
~
~
relative height of level considered.
0.6
0.2
e I2 G U M S ft PER LITER 8 (-) Fro. 2 FIQ. 3 FIG.2. The change of setting time with the height. 0, 38 millimoles of sodium chloride per liter; .,46 millimoles of sodium chloride per liter. Diameter of tubes = 0.5 om. Fro. 3. The change of colloid concentration with the height. 0,s millimoles of sodium chloride; 0.098 g. of iron per liter above the boundary; 0 , 4 6 millimoles of sodium chloride; 0.016 g. of iron per liter above the boundary. 4
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WILFRIED HELLER
Upon shaking, the syneretic systems chinged into Completely homogeneous sols. Left standing thereafter, they separated again in the course of several days. (Thus, separation proceeded faster than the fiqt time, when the systems were less aged.) But it took more than a mOnth befQre the final state of syneresis was reached again, that is, before the boundary level became stationsry. (It is likely that in other syneretic systems, the final state may be reached faster, especially in those containing strictly hydrophobic particles, where the difference in the density of the geloids as compared to that of the dispersive medium is greater.) The revenibility has been tested several times, with the same result.
B . The colloid concentration at which thixotropic systems become syneretic The behavior of different fractions of the same system shows that the colloid concentration is decisive as to whether, for a given electrolyte concentration, a sol turns entirely into a thixotropic gel or separates into two layers. From figures like figure 3, in connection with figures like figure 1, we can derive the approximate colloid Concentration below which separation orcurs. Table 2
SODIUM CHLOR1DE CONCENTRATION
c tr grams ojFe
per lifn
-10 8.8
7.2
gives the data for the iron oxide sol in question, for a tube diameter of 1.2 cm. This table shows why only the system containing 46 millimoles of sodium chloride per liter behaves like a thixotropic gel in test tubes of current width (1-1.5 cm. in diameter). Only in this system is the colloid concentration (7.56g. of iron per liter in all three systems) higher than the “concentration of transition,” ctr.
C . Influence of the colloid concentration upon the rate of s p a n t a n e m reversible centrifugation syneresis and upon the rate of artijkial reversible syneresis The relative volume of the dilute phase is, in the final state, the smaller the more concentrated the system. This can be deduced from table 1. A further influence of increasing colloid concentration is that the stationary state is reached earlier. A direct test of this could not be made, because of the considerable length of time involved in awaiting the stationary state. An indirect test was made by centrifuging the various fractions taken out of the system which contained 46 millimoles of sodium chloride per liter: substitution of centrifugal acceleration for acceleration by gravity leads to an accelerated rate of syneretic separation, and it also brings almut syneresis in such systems
I
REVERSIBLE AGGREGATIONS OF COLLOIDAL PARTICLES.
111
787
as do not separate under the influence of gravity, if kept in sufficiently narrow tubes. Figure 4 shows that the syneresis by centrifugation proceeds noticeably faster in the more concentrated fractions.* It is therefore most probable that the spontaneous syneresis also proceeds faster in more Concentrated systems. Since the system containing 46 millimoles of sodium chloride per liter was a truly thixotropic gel, when kept in smaller containers, the results reported in figure 4 permit conclusions as to the influence of the colloid concentration upon 0.:
OA
0.3
4 0.2
0.I
FIG.4 . Reversible syneresis :tccelerated by centrifuging. R I = volume of concentrated phase/total volume. 1 = tinie of centrifuging. Centrifugal acceleration = 1300 g f 150 g. The numbers inserted indicate from what level of the xyneretic systeni (hlht) the respective sample was taken.
* Centrifugation \\-asstarted immediately after shaking the four samples which had been removed from the syneretic system. (Three samples had been taken a t different heights, and one had been taken from the syncret,ic system after i t had been homogenized by brief shaking.) After centrifugation for 23 hr.-the acceleration was 1300 g-the final state of syneresis waa nearly reached; an additional centrifughtion for 2 hr. did not bring about any appreciable further change of R I . In systems with leks electrolyte, however, the development of syneresis under the influence of centrifugal acceleration was 80 slow that the h a 1 state could not be reached. For a sodium chloride concentration of 30 millimoles per liter, the RI-curve had not reached its maximum even after as much as 31 hr. of centrifuging; RI, still incressingat that time, had t,hen reached the value of 0.13 in the sample homogenized by shaking, the vnlue of 0.17 in the sample taken from the bottom of the syneretic system (10.5 g. of iron per liter), and the values 0.07 and 0.03 in samples taken further up in the concentrated phase of the syneretic system.
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WILFRIED HELLER
the rate of geloid development in thixotropic systems: it is faster in more conrentrated systems. If we take the time after whkh the curve reaches its maximum as a measure for the relative rate of geloid development (2), we obtain for the three fractions t,he values 210 It 50, 130 i= 20, and 130 f 25 min.; the respective colloid concentrations are 7.23, 10.07, and 11.16 g. of iron per liter. The setting times, measured in tubes 0.75 cm. in diameter, wcre G f 0.3, 2.1, arid 1.65 hr., respectively. (These setting times apply, of course, to systems aged for 15 months after the addition of sodium chloride.) Consequently, the rate of geloid development varies less with the colloid Concentration than does the setting time. Since the setting time is determined by both the rate of geloid development and the rate of their association (2), the disagreement seems to indicate that the rate of geloid association is likewise greater in more eoncentrated systems.
D. The injluence of the electrolyte concentration upon the rate of reversible syneresis The re-separation of homogenized syneretic systems occurred' earlier and progressed faster in those systems which contained more sodium chloride. Because of the slowness of the spontaneous syneresis in our systems, quantitative measurements have been made only on syneresis accelerated in the centrifugal field. In the syatem with 46 millimoles of sodium chloride per liter, the maximum of the fl~(t) curve had been reached, for all centrifuged fractions (7.2311.16 g. of iron per liter) a t a maximum of 210 i 50 min. (figure 4),whereas in the system containing 30 millimoles of sodium chloride per liter, none of the fractions centrifuged (0.57-10.47 g. of iron per liter) had reached equilibrium after as long as 31 hr. of continuous centrifuging.
E . Structure formation i n the gel phase of reversibly synerelic systems It has been shown (2) that the gel phase which develops in the lower part of centrifuged thixotropic sols possesses a structure, and microphotometric evidence of t>hestructure has been given. The gel phase developing in the lower part of reveiwibly syncretic systems also showed a structure. The experimental evidence of the structure was, again, the exivtencc of very pronounced mosaic-like discontinuities in the light-diffracting power of the gel phase. Upon shaking, the discontinuities disappeared completely, no matter whether the entire system or the ivolated gel phase was shaken. The structure proved to be reversible, but it did not reappear simultaneously with re-gelification. In the three syneretic systems investigated, the minimum time for macroscopic reappearance of the reversible optical discontinuities was G weeks, counting from the time that gelifieation of the lower phase waa observed. This delay in the direct visibility of structure, as compared to the time required for setting, has to be explained by a slow densification of the gel structure following setting; in the freshly formed gel, the differences in the light diffraction of the gel structure on the one hand and of the included residual sol on the other, are apparently not sufficiently great aa yet to allow a direct macroscopic detection of the structure.
REVERSIBLE AGGREQATIONS OF COLLOIDAL PARTICLES.
I11
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111. DISCUSSION
A . Sol separation as a particular case of syneresis Usually, the term “syneresis” is applied to the spontaneous separation of a gel into two phases, one of them the shrunken gel and thc other the supernatant liquid. The systems investigated, behaved, before separation, like sols when tested in large tubes, and like gels when tested in narrow tubes. This shows the inadequacy of the definition cited, in the case of “soft” gels. Therefore, in the extension of a new definition already given ( l ) ,we propose to consider t~ phenomenon of separation as a syneresis, if, in a sol or gel of a given volume, a more concentrated, coherent, and elastic phase of a smaller volume separates, and if the phase is limited by a sharp boundary. According to this definition, the Concentrated phase may be a gel or an elastic sol phase and it is not required that the concentrated phase be a homogeneous unit, but it may be a structure, intersected, in channel-like fashion, by the dilute phase (the latter appears to be the most frequent type of gel phases). If one wishes to distinguish between systems with a more gel-like or a more sol-like consistency before separation, one may use the terms “gel syneresis” and “sol syneresis,” respectively.
B . Application of the gelm’d hypothesis to reversible syneresis Four possible causes for an isothermal and reversible syneresis3can be anticipated on the basis of the geloid hypothesis: (a) the geloid concentration may be too small to allow the building up of a gel structure throughout the whole system; ( b ) the rate of settling of geloids may be greater than the rate of their association; (c) a gel structure formed by geloids throughout the whole system may be too weak to withstand compression under the influence of an external force vector, such as gravity; (d) a gel structure formed throughout the whole system may contract under the influence of forcns acting between the colloidal particles. The syneretic systems described in the experimental part appear to belong mainly to caee ( a ) , though some features suggest that cases (b) and (c) may be involved 8180.4 The most characteristic feature of an isothermal and reversible syneresis is its dependence on the colloid concentration. The phenomenon of separation is observed only within a limited range of colloid concentrations. First of all, there is an upper limit, a “concentration of transition”, ctr, above which a system forms a truly thixotropic gel and below which it separates syneretically (table 2).5 This transition is easy to understand if the geloid concentration decreases with the colloid concentration, for then it is obvious that a point will be reached where the geloids are no longer able to form a gel structure throughout the whole *The nature of non-isothermal types of syneresis will be considered in a later paper. Typical cases of types ( b ) and ( e ) can be obtained by centrifuging freshly shaken and set thixotropic systems, respectively (2). An example of a syneresis of type (d) will be discussed later. Recent experiments by Langmuir (4) on bentonite systems and by Lauffer (5) on tobacco moeaic virus systems c o n k qualitatively this transition.
‘
790
WILFRIED HELLER
system, and where consequently a separation into a structured and an unstructured part must occur. Centrifugal experiments have shown that the geloid concentration decreases, in fact, with the colloid concentration, for table 3 shows that the colloid concentration of the dilute phase (column 3) is independent of the total colloid concentration as long &s a concentrated phase is present. The latter result implies that there must exist also a lower limiting concentration, a “critical concentration”, CO, below which the concentrated phase disappears, i.e., below which geloids are not formed. It must be reached when the total colloid concentration, e, becomes equal to the concentration of the dilute phase. For the system of table 3, this critical concentration is apparently 0.23 g. of iron per liter. The probable existence of such a co is in agreement with Langmuir’s theory on the interaction between colloidal particles (4). In conTABLE 3 “Equilibrium concenlralaons” an the dilute and i n the concsntroted phose sfler syneresis by cenlrif w i n g A 6-FeOOH system had separated syneretically in a bottle 5 om. in diameter (sodium chloride concentration = 46millimoles per liter); three fractions were taken a t different relative heights hlhl; one fraction WBR taken from the syneretic system after its homogenization by brief shaking
~~
~
grms
vams
grams
0.22 0.24 0.23
46.3 45.3
0.00
7.23 10.67 11.16
Sample fcom homogenized system
7.56
0.21
51.2
0.68 0.28
0.168 0.230 0.268
40.7
I
0.144
trast to et
