PARTICLE SIZE AND CONSTITUTIOTU’ OF COLLOIDAL FERRIC OXIDE. 11. DIALYSIS AND AGING* BY J. B. NICHOLS, E. 0. KRAEMER, AND E. D. BAILEY
In continuation of the investigation of the factors influencing the particlesize distribution of hydrous ferric oxide,l attention has been given to the effects of dialysis and of aging. Determination of Particle Size The low-speed Baltic type of ultracentrifuge was used throughout this investigation.* All of the determinations were made at a temperature of 30’ and a speed of 10,080 r.p.m., corresponding to a centrifugal force about 5,000 times that of gravity. A preliminary value of 4.5 was used for the density of the particles. This value is admittedly uncertain, and in some cases differences in the distribution curves obtained may be due to differences in particle density as well as in particle size. The question of particle density will be considered in a separate investigation. For the determination of particlesize distribution curves of the ferric oxide, the centrifugal form of Stokes’ law was used.3 The particle-size distribution curves obtained from the ultracentrifuge are designated as wezght-optical distribution curves because in a polydisperse system of particles the light absorption may change with radius;4 only when the absorption coefficient is independent of particle size does a weight-optical curve coincide with the weight-distribution curve. Preparation of Sols A stock solution of approximately two-molar ferric chloride was used for the present investigation. The ferric oxide sols were made by diluting portions of this stock to the desired concentrations and hydrolyzing at 100’ under a reflux condenser to prevent the loss of water and of hydrochloric acid formed. The hydrolytic action was stopped, after drawing off the sample, by cooling rapidly t o room temperature. *Presented a t the Buffalo Meeting of the American Chemical Society, September, 1931. Communication No. 79 from the Experimental Station of E. I. du Pont de Xemours and Company. 1 Nichols, Kraemer, and Bailey: J. Phys. Chem., 36, 326 (1932). 2 Svedberg and Heyroth: J. Am. Chem. Soc., 51, 550 (1929). 3 For the theory underlying the determination of distribution curves by means of the ultracentrifuge, see Svedberg and Rinde: J. Am. Chem. SOC.,46, 2681-85 (1924);Rinde: “The Distribution of the Sizes of Particles in Gold Sols,” Diss., Cpsala (1928);and Svedberg: “Colloid Chemistry,” 171 (1928). For a more complete discussion see Ref. I, page 328.
J. B. NICHOLS, E. 0.KRAEMER, AND E. D. BAILEY
506
Dialysis of a Fresh Sol The fresh sols contain free hydrochloric acid and probably ferric chloride or basic chlorides in solution as is evident from the equation of the hydrolytic reaction FeCls 3Hz0 Fe(OH)3 3 HCl or FeC13 xHzO Fe(OH),C13-x xHCl
+ +
+
+
The insoluble ferric hydroxide or basic chloride initially formed immediately undergoes a process of dehydration and condensation to colloidal particles of hydrous ferric oxide. We should expect the equilibrium established a t 100' to be shifted back to the left when the system is cooled down to room temperature. This shift would occur immediately with a soluble hydroxide, but when a colloidal phase is involved, as in the case under investigation, the reversal is sluggish and may never be quite complete. Ordinarily, advantage is taken of this slowness of re-solution to stabilize the sols and to reveal the properties of the colloidal system itself by eliminating the electrolytes through dialysis. Since electrolytes exert such an important influence on the stability and flocculation of colloidal systems, the elimination of the electrolytes might be expected to be accompanied by changes in the colloidal fraction. Such changes might involve further hydrolysis of ferric chloride, aggregation of primary particles, or disaggregation of secondary particles, depending on the concentration and the nature of the electrolytes. I n all cases changes should be reflected in the particle-size characteristics. Accordingly, an ultracentrifugal analysis was made of sols of ferric oxide a t various stages of dialysis. A one-day-old sol formed from 0.037 M FeCls was used for this study. The dialysis arrangement was similar to that used by Neidle.' A slow current of distilled water was allowed to flow through a collodion bag dipping into a beaker containing 800 cc. of the sol at 35OC., which was kept in constant motion past the wall of the bag by means of a Hamilton-Beach mixer. Samples were removed after 4.5, 1 1 , 23.5, and 47.5 hours. At the end of 47.5 hours the dialysate gave no test for iron with ammonium thiocyanate solution or for chloride with silver nitrate.
TABLE I Effect of Dialysis on Freshly Prepared Ferric Oxide from 0.037 M FeCla Sol
Fe-3 I Fe-3 2 BD-V Fe-36 Fe-35
Time of Dialysis (Hours)
Total Fe g.eq./l.
Total C1 g.eq./l.
Purity Fe/C1
0.1166 0.1049
0.1112 0.0464
2.3
11.0
0.1039
23.5 47.5
0.1007
0.0185 0.0047
0.1039
0.0024
0
4.5
'Neidle: J. Am. Chem. SOC., 38,
1270 (1916).
1.0
5.6 21.6 43.8
Colloidal Fe Weight-Optical g.eq./l. Mean Radius
o.ogoo 0.1014
3.8mp 4.2 -
o.oggr o.103g
4.4 4 o
-
PARTICLE SIZE O F COLLOIDAL FERRIC OXIDE
507
Changes in Composition: Table I gives the compositions of the series of samples obtained. The iron analyses were made by the Knop dichromate method' with diphenylamine as indicator. Neither the potassium permanganate titration nor the thiocyanate colorimetric method gave reliable results with these colloidal ferric oxide samples. The purity of the sols is expressed by the ratio of equivalents of iron to equivalents of chlorine per liter. The table shows first that the stirred dialysis produced a rather rapid elimination of chlorine and that the sol reached a moderate purity in two days of dialysis. During the dialysis the sol underwent some evaporation and
4
b
b
nmus m m~~wca.0~
FIG.I EFFECT O F DIALYSIS ON
WEIGHT-OPTICAL DISTRIBUTION CURVE OF ONE-DAY-OLD FERRIC OXIDEFROM 0.037 M FeCI3
THE
Time of Dialysis Primary Primary Secondary Secondary Sol (Hours) Mean Rad. Area Mean Rad. Area Fe-31 0 3 . 8 mp 86% ca.2omp 147~ 88 ea. 20 12 Fe-32 * 4.5 42 ea. 2 0 I4 4.4 86 Fe-36 23.5 83 ca. 20 I7 Fe-35 47.5 40 *Distribution curve for Fe-32 omitted to avoid confusion of lines.
osmotic dilution. Towards the end of the dialysis, when osmotic dilution was negligible, the increase in total iron concentration was due to evaporation. The total volume of the sol recovered, including the portions removed during the dialysis was 785 cc., as compared with the starting volume of 800 cc. The total loss of ionic iron by dialysis amounted to about 137'. Changes in Particle Size: The changes in particle size of the colloidal component during dialysis are represented by the distribution curves of Fig. I . The sols usually contain a small amount of coarser, probably flocculated material which sediments very rapidly under the experimental conditions. Since only its mean size can be estimated, this portion is indicated as a rectangle of the proper relative area a t the right of the primary distribution curve. A shift in mean radius from 3.8 to 4.4 millimicrons occurred during the first 23.5 hours as a result of the dialysis. The change is so small, however, that it is safe to conclude that the particle size of the hydrous ferric oxide is not greatly affected by the elimination of the acid and salts. The slight 'Knop: J. Am. Chem. SOC.,46, 263 (1924).
508
J. B. NICHOLS, E. 0. ICRAEMER, AND E. D. BAILEY
change probably results from the appearance of new ferric oxide formed by the hydrolysis of ferric chloride in the intermicellar liquid, for, owing to the greater mobility of hydrogen ion, the acidity of the solution was reduced sufficiently to permit additional formation of ferric oxide before the ferric chloride present was dialyzed out. After 24 hours' further dialysis the mean radius appeared to decrease slightly instead of increasing further. This shift, however, may not represent a real difference in particle size but merely a hindered sedimentation caused by what may be called a Donnan effect.' A re-examination of the purified sol a month after the dialysis showed a noticeable increae in turbidity, and the mean radius of the particles was found to be nearly 6 millimicrons. Therefore, the dialysis had brought down the electrolyte content to a point of reduced stability. The absence of marked changes in particle size during dialysis is certainly not a general phenomenon. For the particular sol investigated, the concentration of electrolytes presumably was sufficient to maintain stability, but insufficient to cause aggregation, for, otherwise, reduction of the electrolyte concentration would probably have led to peptization of the aggregates and a shift in the distribution curves to smaller sizes. On the other hand, the concentration of electrolytes was more than sufficient to maintain stability, for removal of them to a point represented by a purity of 44 did not lead immediately to detectable aggregation. The absence of a change in particle size upon reversing the charge with potassium citrate2 also indicated that the particles are largely primary particles.
Effect of Aging Aging is a vague term applied to colloidal material, which in this case includes spontaneous changes in particle size, changes in distribution of ions between the intermicellar liquid and the micelle, reversal of hydrolysis or resolution of the particle, and many other factors. Data on two of these factors are given below, namely, the reversal of hydrolysis, and the change in the distribution curve. Changes in Composition: Table I1 gives the analytical results obtained for a freshly prepared, undialyzed sol and for a three-months-old, undialyzed sol from the hydrolysis of 0.037 M FeCl,. The colloidal and the ionic iron were separated by precipitating the colloidal fraction with equal volumes of 0.00~5 Al K&O4 and throwing out and washing the precipitates in a laboratory centrifuge. It is evident that the colloidal iron content dropped in the three-months interval from 7 7 % to only 37% of the total iron present, with a corresponding increase in the ferric chloride content of the intermicellar liquid. The small amount of chlorine which was found in the precipitate of 'Tiselius: Z. physik. Chem., 124, 457 (1926); Nichols: Sixth Colloid Symposium, 298 (1928).
* Nichols, Kraemer,
and Bailey, Part I loc. cit. p. 332.
509
PARTICLE SIZE O F COLLOIDAL FERRIC OXIDE
ferric oxide is designated as non-displaceable because the coagulating sulfate ion is able to displace much of the chlorine from the micelle.' The rate of reversal of course depends upon the concentration of the system. I n the same period of time, re-solution would undoubtedly occur to a greater extent in more concentrated sols and to a lesser extent in more dilute sols.
EFFECT O F AGE ON
FIG.z DISTRIBUTIOX C U R V E O F FERRIC OXIDE
THE \vEIGHT-OPTIC4L FROM 0.00j M
FeCls
Weight-Optical Mean Radius 2 6mp 3.I
Age (Days)
Sd Fe-3 Fe-7
I
15
Area 89% 99
TABLE I1 Change in Colloidal Iron Content on Aging of Ferric Oxide Sols Sol (From 0.037 M Age FeCL) (Days)
IV I1
I
95
Total Fe Content g.eq./l.
0.1163 0.1161
Total C1 Content g.eq. /1.
Colloidal Fe Non-Dis laceable Content Colloidal Content g.eq. /I. g.eq.11.
&
0.1120
0.0898
o.00090
0.1123
0.0427
0.00104
Changes in Particle Size: Aging seems to affect somewhat differently the distribution curves of ferric oxide sols prepared from different concentrations of ferric chloride. The distribution curve of ferric oxide from rather dilute (0.005 $1) ferric chloride (Fig. 2 ) undergoes only a shift t o a slightly larger mean size, but the distribution curve of the ferric oxide from the more concentrated (0.037 M) ferric chloride (Fig. 3 ) becomes somewhat flatter on aging and shows a considerable increase in the amount of secondary material of about 20 millimicrons in radius. 1 Cf. Linder and Picton: J. Chem. SOC.,87, 1908 (1905); Weiser: J. Phys. Chern., 35, 10
(1931).
J. B . NICHOLS, E. 0. KRAEMER, AND E. D. BAILEY
FIG.3 EFFECTOF AGBON THE WEIGHT-OPTICAL DISTRIBUTION OF FERRIC OXIDE FROM
sol Fe-6 Fe- I 4
0.037 M FeCls
Wmary Mean Rad. 4 4 mp
(Days) Age I
72
4 6
Primary Area 90% 76
E c s O n E n iT ca. ca.
20
mp
20
Secondary Ares, 8% 21
Effect of Dialysis on an Aged Sol I n view of the difference in composition of fresh and of aged sols, it seems likely that dialysis would have a different action in the two cases; for instance, the aged sol is conceivably more stable than the fresh sol, or it might contain secondary material which would be re-peptized during dialysis. A 2.5-monthsold sol (prepared from 0.037 M FeC13), which contained much ferric chloride formed by the re-solution of some of the ferric oxide was used for this study. Samples were removed after 4, 8 and 20 hours of dialysis of a 300 cc. portion of the sol at 25OC. After eight hours the dialysate gave only a faint test for iron with thiocyanate solution, but a positive test for chloride with silver nitrate. . At the end of twenty hours no test was obtained for either constituent in the dialysate. Changes in Composition: Table I11 gives the compositions of the series of samples obtained, after correction for the osmotic dilution occurring during
Effect of Dialysis on
2.5
TABLE I11 Months Old Ferric Oxide from 0.037 M FeCls Calcd. from Colorimetric Analyses Colloidal Ionic Fe or Hydrochloric Fe C1 as FeCL Acid Cl g.eq./l. g.eq./l. g.eq./l.
Time of Total Fe Sample Dialyas g.eq./l. (Hours)
Total C1 g.eq./l.
Purity
Fe-30 o (original) Fe-26 4 Fe-28 8 Fe-29 20
0.1123
1.0
0.0426* 0.0735* 0 . 0 3 7 2
0.0271
2.8
0.0104 0.0016
7.1
0.0650 0.0698 0.0715
0.1161 0.0748 0.0735 0.0715
43.3
0.0098
0.0157
0.0037
0.0051 -
-
'Separation of colloidal and ionic iron by precipitation with potansium Rulfate solution gave 0.0427 g. eq./l. of colloidal iron and 0.0734 g. eq./l. ionic iron.
PARTICLE SIZE OF COLLOIDAL FERRIC OXIDE
5'1
the dialysis. The table shows that the sol reached the same purity as the fresh sol. It is also evident that the colloidal-iron content after twenty hours of dialysis increased nearly 70% over that of the undialyzed sol. This indicates that the removal of hydrochloric acid at the start of the dialysis permitted a re-hydrolysis of much of the ferric chloride or basic ferric chloride present. The total loss in iron during the dialysis amounted to about 39%. The colloidal-iron content (corrected for osmotic dilution) was calculated on three assumptions: First, that the chlorine and iron present, at the end of the dialysis (sol Fe-29) were all contained in the micelle; second, that the light absorption was due to the colloidal-iron content; and third, that Beers' law holds. This procedure is justified by the agreement between the colloidal iron of Fe-30 estimated colorimetrically and the value obtained by precipitation of the colloidal fraction with potassium sulfate solution. Ionic iron (001umn 7) was considered as the difference between total iron and colloidal iron. In estimating the distribut,ion of chlorine, the ionic iron was assumed to be present as ferric chloride. Accordingly, column 7 refers to both the iron and the chlorine of the ferric chloride, The remaining chlorine was present in part as chlorine bound in the colloidal micelles, which was supposed to be equal to chlorine still present at the end of dialysis (Fe-29, column 4),and in part as hydrochloric acid. The values of column 5 were therefore obtained by subtracting the bound chlorine and the chlorine as ferric chloride from the total chlorine content. The table shows that the re-hydrolysis of t,he ferric chloride, which took place as a result of the more rapid removal of hydrogen ion than of ferric ion during the dialysis, proceeded rapidly in the first four hours and practically reached completion at the end of eight hours. As long as the re-hydrolysis proceeds some of the chlorine will be present as hydrochloric acid formed during this process; accordingly, it was not possible to estimate the ionic iron in the earlier stages simply from the chlorine content found. Changes in Particle Size: The weight-optical distribution curves for the series (Fig. 4) also show that most of t,he change occurred in the first four hours. The distribution curve for the eight-hour sample (Fe-28) was practically identical with that for the four-hour dialysis; therefdre, the former was left out of the figure to avoid confusion of lines. Its mean radius was 4.5 millimicrons as compared with 4.55 millimicrons for the sample taken after four hours of dialysis. Reduction in the hydrogen-ion concentration in the early stages of the dialysis seems to produce both a re-hydrolysis of ferric chloride present and a re-peptization of some of the flocculated material, represented by the rectangles a t the right of the distribution curves. Comparison of the distribution curves for the dialyzed sols of an aged ferric oxide (Fig. 4) with those for the dialysis of a fresh ferric oxide sol (Fig. I ) , and with the curves in Fig. 3, giving the effect of aging on the distribution curve, shows that, even when the electrolytes have been largely eliminated, the flattened form of distribution curve is retained for the aged sol and the steeper type is retained for the freshly prepared sol. Aging there-
J. B. NICHOLS, E. 0. KRAEMER, A N D E. D. BAILEY
5'2
fore seems to produce some permanent change in the shape of the distribution curve for the more concentrated sol. It is not possible to decide from the distribution curves whether the material hydrolyzed during the dialysis was simply deposited on the particles already present, thus shifting the mean radius to a slightly larger size, or whether new primary particles of larger mean size were formed from the hydrolysis of the ferric chloride present in the sol. The distribution curves for both the freshly hydrolyzed and for the aged ferric oxide showed a slight
FIG.4 EFFECTOF DIALYSIS ON THE WEIGHT-OPTICAL DISTRIBUTION CURVEOF TWO-ANDONE-HALF MONTHS OLDFERRIC OXIDEFROM 0.037 M FeCls Time of Dialysis Primary Primary Secondary Secondary Sol (Hours) Mean Rad. Area Mean Rad. Area Fe-30 0 3 80mr 64 70 ca. 20 mp 28% 20 80 ca. 20 Fe-26 4 55 82 ca. 20 18 Fe-28* 4 50 Fe-29 20 4.20 82 ca. 20 18 *Distribution curve for Fe-28 omitted t o avoid confusion of lines.
1
shift to a smaller mean size when the sols were sufficiently free from electrolytes. This shift may well be caused by the entrance of a slight Donnan potential.[ Effect of Age on Light-Absorption Relations During the aging of the more concentrated sol the light absorption in the visible decreased to about one-half its original value on account of the conversion of some of the highly absorbing hydrous ferric oxide to ferric chloride, which by comparison has a negligible absorption in the visible. This conversion is more directly revealed by comparison of the ultraviolet absorption of the sols and of the intermicellar liquid remaining after centrifuging out the colloid particles. For a fresh sol the intermicellar liquid contributed 1270 of the total absorption a t 366 millimicrons, whereas after 2 . 5 months the intermicellar liquid accounted for 3770. The absorption of the intermicellar liquid is undoubtedly largely due to the ferric chloride. 1
Tiselius: loc. cit.
PARTICLE SIZE O F COLLOIDAL FERRIC OXIDE
513
Dialysis of a freshly prepared sol brought about a large increase in light absorption without much increase in the colloidal-iron cont,ent. Evidently the composition of the particles changed during dialysis even though the rate of sedimentation did not. The change possibly involved the removal of chlorine from the colloidal particles (which would tend to decrease their density) and a simultaneous dehydration and condensation of the particles (which would tend to increase their density). The absorption relations for an aged sol were much simpler, however. The composition of the micelle had probably approached an equilibrium with the intermicellar liquid, which was not much changed by the removal of electrolytes during dialysis. Vnder these conditions, as was shown in Table 111, it was possible to calculate the colloidal-iron content from colorimeter measurements. I t is intended to investigate the light-absorption relations further with the view of obtaining quantitative data on the changes undergone by the sols during aging.
Effect of Age of Stock Solution During the course of this investigation a single stock solution of ferric chloride was used. The stock solution, however, was not entirely stable, as 1s demonstrated by particle-size analyses of sols made a t various ages of the stock (Table IV). I t is evident that there is a gradual decrease in the weightoptical mean radius from' 4.4 millimicrons for a sol prepared from the twoweeks-old stock solution to 3.8j millimicrons for the sol prepared from the T.4BLE
Iv
Effect of Age of 1.87 M FeC13 Stock Solution on the Ferric OxideSol Produced Fe?OaSol (From 0,037 Af FeC13)
Fe-6 Fe- I 9 Fe-3 I
Age of Stock Solution 2
weeks
3 months
8 months
Weight-Optical Mean Radius of One-Day-Old Sol 4 . 4 0 mp
4.05 3.85
eight-months-old stock solution. Probably the explanation for this gradual decrease in mean radius lies in the production of more and more nuclei in the stock solution as it grows older. Thus, when the diluted ferric? chloride was hydrolyzed the particles would have more centers on which to form, and therefore the mean size would be smaller since the same concentration of ferric chloride was used in each hydrolysis. The nuclei might be semi-colloidal basic ferric chlorides resulting from a slight hydrolysis. It is a pleasure to acknowledge the assistance given by Mr. E. S. Wilkins of our Analytical Department in making the chemical analyses.
Conclusions Colloidal solutions formed by boiling dilute ferric chloride solutions are unstable, the hydrolytic reaction being reversed upon standing a t room temperatures by conversion of the colloidal hydrous ferric oxide to ferric
514
J. B. NICHOLS, E. 0. KRAEMER, AND E. D. BAILEY
chloride. For instance, in a fresh sol containing 0.116 gram equivalents iron per liter, about Soyc of the iron is in the form of colloidal ferric oxide, whereas after 2 . 5 months the colloidal fraction falls to about 40%. During aging, the particle-size distribution as determined with the ultracentrifuge shifts slightly toward larger sizes, the change becoming greater and the formation of secondary aggregates being more pronounced in the concentrated sols. Apparently two opposing processes occur during aging: Re-solution of colloidal particles, especially the smaller ones, and aggregation of the larger particles. The initial particles are probably primary particles 3 to 4 millimicrons in radius for total iron concentrations below 0.037M. Dialysis of both fresh and aged sols leads to hydrolysis of some of the ferric chloride in the intermicellar liquid and to formation of additional colloidal ferric oxide; the particle-size distribution is, however, not appreciably changed a t the end of the dialysis. However, the colloidal matter in the purified sol grows more rapidly than before. Changes in the sols during aging or dialysis are also revealed by changes in light absorption, owing to the much greater absorption by colloidal iron than by ionic iron. Concentrated ferric chloride solutions change slowly a t room temperature, presumably by hydrolysis and formation of semi-colloidal particles, for the particle size of sols formed upon diluting and boiling decreases somewhat with the age of the concentrated stock solution. Wilmingtun, Delaware, September 16, 1951.
