1724
E. M A T I J E M. ~ , B. ABRAMSON, R.H.
~ E W I L L K. ,
reflect mme inadequacy of the model or the cat&lytic effect of certain impurities (e.g., HzO) originally present in the quartz.
[email protected] sample of synthetic quarts was kindly supplied by Dr. R. A. Laudise Of the we are 6 P b f u l ta MIS. M. Houle for metallographic prepsration of the samples, Mr. K. Murphy for experimelltal assistance and Dr. B. Hillig for valuable discussions.
w.
w.
D1SCUSSION A. (Holy cmssCollege).-mnt is the peratof thermal conductivity in this system? Have yoii tried any seeding experiments? J. D. hCgENZiE.--Tbe temperature coefficient of thermal conductivity at high temperatures is not available. However, the coefficients for both crystalline and vitreous silica are negligibly small at the lower temperatures compared to that for viscous flow, for instance. No seeding experiments were attempted nor necessary
F. Smaz IWD M. KERKER
Vol. 65
since nucleation occurred readily a t free surfaces and grnin boundaries. R. A. ORIANI(U. S. Steel Corporation) .-Can you account for the much greater difficulty of superheating in other materials? J. D. MAcKENzIE.-The primary reason is probably the high fluidity of the melt of most materials. Thus the viscosity a t the melting temperature is about 10-2 poise for most materials whereas that for quartz is about 1 0 1 0 poise. If we assume that the presently suggested mechanism is applicable and ignoring heat flow, then for 7 = IO-* and (7’ - T,) = l ” , equation (4)reduces to u l0AHf/TF, where the unik of AHf are calories per mole. The approxlmate fusion rates thus calculated for a variety of substances at 9 superheating of only one degree are given in the table. Substance - - -. .
Al Pb LiNOs NaCl Naphthalene Benzene
Fusion rate. CZU/SPC.
20 20 100
70 130 100
ADSOIWTION OF THORIUM IONS ON SILVER IODIDE SOLS BY E. MATIJEVT~, M. B. ABRAMSOX,~ R. H. O ~ E W I L LI4. It is above this pH that thorium ions show strong hydrolysis effects. Thus, i t is believed that hydrolyzed thorium species adsorb more atrongly than unhydrolyeed thonum ions. Electrophoresis and coagulation experimentn indicate a strong reversal of charge of silver iodide particle6 in accordance with the adsorption effects.
rntroduction The role of adsorption of counterions on sol
further experiments and study of the results, Freundlich himself recognized that his theory did particles in the process of coagulation has been a, not hold.” There are several reasons for the discrepancy in subject of extensive investigation; yet, the results are still indecisive. The fact that adsorption takes conclusions reached by the early investigators. place was demonstrated early in this century4J Firstly, the determination of the extremely small and attempts were made to connect the adsorb- quantities of adsorbed materials was experimentally ability of the counterions with the Schulze-Hardy precarious because they were determined by difnile.s-ll Freundlich proposed a theory of equiva- ference. Secondly. the colloidal precipitates used lent adsorption of counterions on the sol particles as adsorbents were frequently very poorly defined at and above the coagulation concentration.l 2 . I 3 systems (such as metal hydroxides, sulfides, etc.) This theory was strongly criticized by Weiser14l6 which did not justify a direct comparison of data who showed that although adsorption of counterions or a generalization of results. Finally, the results does take place, the quantities of different ions with various types of coagulating ions (such as dyes, adsorbed are by no means equivalent. After colloidal electrolytes, organic ions, complex inorganic ions, simple inorganic ions, etc.) were in(1) Suppoorkd by the Office of Ordnance Research Contract No. discriminately compared and treated as if the DA-ORD-LO. coagulation mechanism in all cases was identical. (2) Participant in the NSF Summer Research Project, 1960. (3) On leave from Department of Colloid Science, University of In connection with the last statement one must Cambridee. England. distinguish among three different groups of coagu(4) K Schulze. J . prakt. Chem.. 121 16.431 (1882): 97,32 (1883). lating ions. Firstly, there are ions and molecules (5) W. l3. Hardy, Proc. Roy. Soe. (London). A66, 110 (1900). (6) D. R. Gangulyand N. R. Dhar, 1. Phyr. Chem.. 16.836 (1922). which are preferentially adsorbed on colloidal (7) N. 0.Chatterjea and N. K. Dhar, KoU&Z.. SS. 18 (1923). particles due to strong adsorption forces (e.g., (8) N. H. Dhar. E 7 4 . 6 > 7 6 . 6 . The stability limits (E;) of both dialyzed sols and sols in statu, nascemli are in perfect agreement a t pH 1.5. At pH 6.5 there are no data for sols in statu nascendi for comparison. At pH 2.6 there is a. disagreement betweer; the two resulk The sols in statu nnscend'i exhibit a stability limit at much higher concentrations of thorium than the dialyzed sols, and at, still lower p H , no stability limit wap observed. The mobility of t,he AgI particles was measured as a function of thorium nitrate concentration over the range 5 >: :LO--' i1.f to M a t pH values of 2.6, 4.5 and 6.5. The mobility against log concentration curves are illustrated in Fig. 4 (upper). In all throe cases, as would be expected from the coagulation el'fects, reversal of charge takes place. There is considerabled iff erence between the gradients of the curves a t the reversal of charge concentration, the steepness increasing with increasing p H . Xdditioii of distilled water to sols with a positive charge resulted in :I very slow decrease in positive mobility. the decrease during the first ten minutes after dilution being negligible. Thus it would appear that, the desorption of the thorium ion from the surface is a slow process and confirms that the errors introduced into the adsorption measuremcmts by washing the solid with distilled water are negligible Discussion Our present resulte on the adsorption of ionic (42) K. F. Schulz and E. MatijeviC. Kolloid-Z., 168, 143 (19GO). (43) G. N. Gorochowski a n d J. R . Protass. Z . physik. Chem.. Ai74, 122 (1935): I I . H . K r i i y i a n d S. .4. Troelstra, Kolloid-Beihefle, 54, 262 11843).
SILVERIODIDE SOLS
5
1727
I
I
pH.6.6-68
0
w 20 >
I
0
I
4 0
2
1
0
L 1
OO
5
3
4
I
5
I
IO
I 20
Th(NOJ4 L E F T IN SOLN. (E/Lxl$). Fig. 3.-Moles of thorium ions adsorbed on AgI sols (1.25 1 0 - 2 mole/l.) in two pH ranges: 2.5-2.7 and 6 . M . 8 plotted against thorium nitrate left in solution (calculated from the difference). G ,adsorption determined directly on the solid; 0, adsorption determined from the loss from supernatant.
x
thorium species on silver halides using dialyzed sols are in qualitat,ive agreement, as far as pH effects with earlier work on sols in statu n a ~ c e n d i . ~ ~ However, in the earlier work the measurements were not sensitive enough 00 detect adsorption a t low pH's. From Fig. 3, it is apparent that a t pH 2.6 adsorption increases with increasing concentration of Th(NOJ4 in solution to a limiting value which corresponds to 6 x g. equiv. of thorium/mole of AgI. H e ~ - a kfound *~ for several trivalent ions and one divalent ion a corresponding adsorption of -3 X 10-3 g. equiv./mole of AgI. These values are remarkably close, especially if one considers that, the sols were prepared differentiy and that different sol concentrations were used. From the surface area of this uncoagulated sol, as determined from a particle size distribution obtained by clectron microscopy, these results correspond to one thorium ion adsorbed per 400 A.2 of the surface. This is about the average derisity of potential determining ions (I-) adsorbed on a negative si1vt.r iodide SO^.^^-^^ Thus, it appears that a t low pH the limiting adsorption with thorium is reached when one thorium ion is adsorbed per approximately one potential determining ion. The thorium con(44) (1958).
K. F. Echulz a n d hf. J Herak, C-ont. Chem. Acln, 30, 127
(45) E. J. 1%'. Verwey a n d H. R. Kruyt, 2. physik. Chem., A167, 137 (1933). (46) M. Mirnik a n d B. T e i a k , Trans. Pomday Soc., 60, 65 (1954:. 147) G . 1.. Markor. Rrc. Irau. chim.. 70, iG3 (1951).
E. Manrev16, M.B. A ~ ~ a a a sR. o ~H. , Om%-,
172s
I
I
I
I
-6
-5
LOG. MOLAR CONC. OF Th(NOJ,,. Fig. 4.--Turbidity and mobility curves of a dialped silver iodide sol (AgI: 1 X lo-‘ mole/l., PI = 4) coagulated by thonum rutrate at the ditierent p H valuea of the MI, vis., 2.6, 4.5 arid 6.5.
centration at which this “saturation” effect occurs corresponded to the reversal of charge region in the coagulation curve (Fig. 4) as one would expect. I n the concentration range for critics1 coagulation (Fig. 4, limit A), the amount of thorium adsorbed is considerably lower. The most striking effect is that of the pH influence. A t higher p H values the adsorption of thorium on AgI appears to be higher, a sharp inereage taking place a t a p H -4 (Fig. 2). It is precisely a t this p H , however, that a sharp upturn in coagulation concentration occurs (Fig. 2, ref. 27). This change was explained in terms of the following hydrolysis reaction” Th‘+ 2HzO 1_ Th(OH)’+ &O+
+
+-
which becomes predominant at very low concentrations of th0rium.2~.2~ It appears, then, that this hydrolyzed species is more readily adsorbed. Using the equilibrium constant of 5 X IO4, given by Kraus and HolmbergB for this reaction, we find thai, the increased adsorption of thorium on both glasa and silver iodide (Fig. 3), occurs at a p H where the hydrolyzed species becomes predominant. This is a further indication that hydrolyzed species of T h ion are more strongly adsorbed than the simple hydrated ion. Similar e€fects on the influence of pH on the adsorption of metal ions have been detected bef0re.~~.a*4*One common explanation was that “radiocolloids” were formed at higher pH’s and
.
(48) J. F. King and P. R. pine, J . PA- C k . ST, 851 (1933). (49) H. Le=. SI(.bar. dbd. W i u . W k . [HA]. WI, I9 (1927).
K F.b m z m M. I h m m
Vol. 65
are more strongly adsorbed than simple ions,*‘ but this ad hoc explauation is q ~ e s t i o n a b l e . ~ ~ Such “radiocolloids” are presumed to be finely dispersed metal hydroxides. Although the solubility product for thorium hydroxid@+l has been reported to be as low as -lo-”, it seems unlikely that thorium hydroxide does form at the concentrations and pH’s employed in these experiments. We have never observed the f o m t i o n of a precipitate of thorium hydroxide under our experimental conditions. Data reported on solubilities of metal hydroxides are often quite unreliable. Furthermore, the simple application of solubility product oonstsnt principle m o t be employed when equilibria with hydrolyzed species, rather than simple ions &pe involved. Also, other kinds of complexes--such as thorium-halide complex ions-will retard the formation of thorium hydroxide. For these re8sons we believe that it is the formation of hydrolyzed ionic species at higher pH that accounts for increased adsorption on both the glass and AgI surfaces. In support of this position we might point out that at low pH aluminum ions do not reverse the charge, whereas at higher pH’s where hydrolyzed aluminum complexes are formed a very strong reversal of charge is observed.” I t appears then that the hydrolyzed ionic species adsorb more strongly on the surface of silver halides than the simple hydrated ion, even though the latter may carry a higher charge. We believe this enhanced adsorption is due to the presence of the hydroxyl group. This may not be obvious in the case of thorium where the hydrolyzed ion is presumed to be a monomer, ThOH1+. However, in the case of aluminum, where the ionized species is a polynuclear complex,a~*5z and no direct attachment of the metal atom to the solid surface can occur, contact of counterion through the hydroxyl group becomes the only possibility. (It is tacitly assumed that upon adsorption, the simple metal ion loses a t least part of its hydration shell.) The dBerence in adsorption between the hydrolyzed and the non-hydrolyzed species might be due to the effect of exchange adsorption of H+ ions on the heteropolar surface. Since simple Th4+(hydrated) are present only in media of high concentrations of hydrogen ions they may actually ‘be replaced by them. It is known that highly valent ions can be desorbed by lower valent i0ns.41~~3 Recently, Herak has demonstrated45.16by direct adsorption measurement that this exchange appears to be equivalent. Another factor that should be considered in discussing the adsorption of non-hydrolyzed thorium ions is the strong complexing tendency of thorium ions with various a n i ~ n s . ~ Although ~.~ there are no data available for iodide, thorium is known to complex with chlorideP4 Such complexes between the stabilizing ions and counterions (in our case (60) Y. Olrs, 1.C h Soc. Japaa. 61.311 (1940). (SI) I. M. Korenm~n,Zkur. Olrhehd Kliirr. S6, 1801 (1955). (62) C. B m a t , G. Bisdermann and L G . deh Cb+. &ad.. 8. 1917 (1964). (63) J. F. Bhg and U. T . Grume, 1. PI- C k . S?. 1M7 (1933). (54) E. L. Zebrwki. H. W. Alter and F. K. Hsurmmn. J . A m C b m . ?S, 6646 (1951). (55) R A. Day and It. W. Stoughton, $bid.. n, 6862 (1950).
&.
Oct., 1961
ADSORPTION OF THORIUM IONSON SILVER IODIDE SOLS
thorium) may play a significant role. Coagulation and electrophoresis experiments on AgI with another quadrivalent ion, uiz., zirconium, show quite different effects than observed with thorium under identical conditic~ns.~~ It is quite possible that specific complexing effects may play a role here and experiments which may give us a better insight into these problerns are a t present underway. In our earlier coagulation experiments with sols in statu nascendi, we did not detect a stability limit at the lower pll values even at high T h ion concentrations, and we concluded from this that nonhydrolyzed Th ions, which constitute the prevalent species at such pH's, do not reverse the charge. The adsorption, coagulation and mobility measurements described in this work show that a t sufficiently high concentrations, Th+4ions do actually reverse the charge. This apparent contradiction arises because the absence of a stability limit in the case of our earlier experinients is not sufficient proof that there has not been reversal of charge. We have now determined the mo'rtiiity of the particles of a AgI sol in statu nmccndi (Ag1;O3 1 X lo-* M , IiI 2 X hi?) at pH 2 in the presence of 1 X 10-3 M Th(KOa)4. The particles were positively charged showing reversal of charge but had a mobility of only 0.34 p/sec./v./cm. This is considerably lower than the mobility observed with a dialyzed sol under comparable conditions. The explanation of the absence of a stability limit for the recharged sols in statu nascendi is (56) E. Matiievi6, E. G. Mathai and M. Kerker, to be published.
1729
as follows. It is known that the concentration of anions necessary to produce coagulation decreases very sharply with decreasing surface charge density of the sol particles.b7 Apparently, then, the very weakly charged sols in statu nascendi were coagulated by the rather low concentration of anions (nitrate) present in solution. Thus, even though the particles had undergone reversal of charge, they were not stabilized. On the other hand, with the dialyzed sols, the surface charge density is sufficiently high so that coagulation by the existing anions does not occur. The coagulation curves for these sols do show a stability limit a t the lowest PH. We are planning experiments to explore these differences more fully. DISCUSSION R. K. ILER(E. I. du Pont Company).-In
regard to the hasic aluminum ions that are strongly adsorbed, does increaving the temperature of an aluminum salt solution have any effect, especially when a t pH 6? E. hIATrmvI&--\\'e have studied the influence of temperature upon aluminum salt solutions by means of coagiila;ion effects and reported these results in a previous paper (E. MatijeviC: and B. Teiak, J . Phys. Chem., 57, 951 (1953)). However, we did not study directly the adsorption changes of rtiuminuni ionic species a t different temperatures. E. D. GODDARD (Lever Brothers Company).-The explanation advanced for the pH effect on coabpdation which involves the formation of basic snlts brings to mind the work of Wolstenholme and Schulman, who advanced a similar explanation to account for the effect of a numher of different salta on fatty acid monolayers a t various pH levels. (57) J. Herak and R. TeZak, Arhiu. kern., 26, 87 (1953).
