THE ACTIOK OF IMMISCIBLE ORGAKIC L I Q r I D S Oh’ COLLOIDAL GOLD BY T. R . BOLAY AND J. CROWE
Introduction The most important paper concerning the behaviour of gold hydrosols when shaken with immiscible organic liquids is one published by Zsigmondy’ in 1916. I n the first place Zsigmondy examined the effect of purifying the liquids, which had apparently not been done by Reinders,* the first to study the subject a t all extensively. I t was found that pure benzene, toluene, ether, carbon disulphide and amyl alcohol had no action upon the alkaline sols obtained by the Zsigmondy methods of preparation. Further, that pure benzene, toluene, and ether produced no change in the acid sols prepared by Donau’s method (reduction of gold chloride by carbon monoxide) or in Zsigmondy sols acidified by the addition of hydrochloric acid, provided the concentration of hydrogen ion did not exceed about I O + grm. ion litre. On the other hand if the sols were shaken with an impure liquid, e.g. ether which had stood in the laboratory for a prolonged period, all the gold collected in the form of a blue film at the liquid-liquid interface, as had been observed by Reinders. Zsigmondy discovered that the presence of exceedingly small quantities of protein also caused the gold in the acid sols to be shaken out by the organic liquid. In searching to explain this and the fact that excessive acidity led to the same result, experiments were carried out on suspensions of coarse gold, prepared by coagulating sols by ultrafiltration, evaporation, or the addition of electrolyte. Whether the coagulum was washed free from electrolyte prior to suspension or not, the gold particles always accumulated at the junction of the two liquids. Finally a grey-violet sol was prepared by the electrical disintegration of gold wire (Bredig’s method) in pure water contained in a quartz vessel, and shaken with purified benzene. It was found that the colloidal gold was unaffected, whereas the coarser material again adhered to the organic layer. Zsigmondy concluded that the tendency of gold particles to go to the boundary between the aqueous and organic phases was primarily a function of the degree of dispersion of the gold. He suggested that attractive forces existed between the droplets of emulsified organic liquid and the gold particles, which increased as the latter became larger. Wiegner’s3 observation that coarse ultramicrons appear to act as coagulation centres in unstable sols could be interpreted in a similar fashion. Zsigmondy thus considered that the primary particles of a gold sol were unaffected by shaking with a pure organic liquid and that the effect of added substances was due to the coagulation of the sol to form the coarser secondary complexes. To quote from his paper “Die in Kasser gelosten Verunreinigungen (bei sauren Goldlosungen
IMMISCIBLE ORGANIC LIQUIDS AND COLLOIDAL GOLD
1449
z. B. Eimeisssubstanzen) bewirken ziinachst Koagulation des Goldes, dass
dann in Form groberer Teilchen stets an die Grenzflache geht, daselbst ein metallschimmerndes Hautchen bildend ” K i t h regard to ot,her possible factors Zsigmondy was not very definite. He points out that the presence of (a) negative electrical charges and (b) water sheaths (Wasserhullen) a t the surfaces of the gold particlek and of the benzene droplets will oppose the tendency of these bodies to unite. He also says “-die in der organischen Flussigkeit (Aether, Benzol, Amylalkohol-) enthaltenen Verunreinigungen konnen, falls sie an der Oberflache sich anreichern, wie fluchtige Fettsauren u.dg1, auch direkte Vereinigung der Fliissigkeitstriipfchen mit dem Gold und damit Koagulation veranlassen.” Freundlichl has remarked that it is difficult to account for the absence of effect in the case of the small colloidal particles, since these were not necessarily in a more highly charged condition than the larger ones, for the difference was shown by particles belonging to the same system (Zsigmondy’s experiment with the Bredig sol.)* Viewing the problem as essentially a question of preferential wetting, he suggests that the interfacial tension between gold and water decreases as the particle becomes smaller, which makes it more difficult for the organic liquid to displace the water from the gold surface. In other words, the water sheath is more firmly bound, the smaller the particle. The present authors commenced an investigation with the object of determining the “critical” size which, according to the views outlined above, a particle must possess if it is to be removed from the aqueous phase to the liquid-liquid interface. Preliminary experiments, however, indicated that in certain systems at least, the influence of particle size is of minor importance. The following is an account of a study of these systems.
Preliminary Experiments Xordenson5 has shown that the reduction of gold chloride by hydrogen peroxide is accelerated by ultraviolet light, the size of the particles in the resulting sol or suspension decreasing as the exposure is made longer. Xdvantage was taken of this circumstance t o prepare a series of sols of different degrees of dispersion. On examining the behaviour of these systems when shaken with purified amyl alcohol (see below) it was found that in every case the whole of the gold collect’ed immediately at the water-alcohol interface. This result was surprising since, judging from their appearance, the finer grained sols were quite comparable, in regard to particle size, with those studied by Zsigmondy. It was noticed however that Zsigmondy (see Introduction) had not observed the action of amyl alcohol on acid sols, with which we are here concerned. The possibility of the existence of specific differences between organic liquids was thus indicated and it was decided to investigate the action of a considerable number of organic liquids on both Xordenson and 7sigmondy sols. * See however, Freundlich and Loebmann.”
1450
T. R. BOLAM AND J. CROWE
Examination of a Variety of Organic Liquids Preparation of the Sob.-The h’ordenson sol (AuNA) was prepared as follows. Two C.C.of 0.6 percent Merck’s gold chloride (HAuC14.4H20)and 0.6 C.C.of 0.1 M anhydrous potassium carbonate (Kahlbaum) were mixed with 1 2 0 C.C. conductivity water in a large porcelain basin, 0.4 C.C. of “perhydrol” (Merck) added, and the contents of the basin immediately subjected to direct radiation from a mercury vapour lamp. The exposure lasted for 2 5 sec. and the reduction mixture was constantly stirred during the reduction. It was found that the addition of potassium carbonate, though in quantities insufficient to produce a neutral or alkaline sol, ensured a high degree of dispersion. (cf. Beaver and Mullern). To prepare the Zsigmondy sol (Au2A), 2 . 0 C.C. of the same gold chloride solution were diluted with 1 2 0 C.C. conductivity water, heated in a Jena glass beaker and 3.0 C.C.of 0.1M potassium carbonate (as above) added. When the solution was just boiling the flame was removed and 3.0 C.C. of 0.4 percent pure formaldehyde (prepared in this laboratory by Dr. Coutie) added drop by drop with vigorous stirring. Water of spec. cond. = 0.4 - 0.8 X 10- mhos, prepared in a Bourdillon still, was used in the preparation of the sols and the solutions were made up with distilled water of particularly good quality (spec. cond. = I - 2 X rob mhos.). It will be noted that the concentration of the gold was the same in the two sols. Both preparations were of a deep cherry red colour and neither showed a trace of opalescence. They were stored in well-cleaned Jena glassstoppered bottles and proved to be very stable. Dzstribution Experiments.-Three C.C. of sol and I C.C. of organic liquid were pipetted into a tube closed with a glass stopper and the mixture shaken vigorously by hand. The tube was given 400 shakes except when all the gold was removed from the aqueous phase by less shaking.* I n order to obtain reproducible results i t was found necessary to clean the tubes and stoppers very thoroughly. After washing with hot water and rectified spirits, they were immersed in aqua regia to dissolve any gold left from a previous experiment, and then placed in chromic acid overnight. Finally they were well washed with distilled water and dried in an air oven. When first used the tubes were subjected to a prolonged steaming-out. The use of tubes closed with corks was found to be unsatisfactory, a t least in the case of alcohols. Apparently the irregularities observed could be traced to material dissolved from some of the corks by the organic liquid. While reproducible results were obtained by selecting new corks which presented an unbroken surface to the contents of the tube, it was considered advisable to get glass-stoppered tubes made for the investigation. The majority of the organic liquids as obtained were either commercially pure or else had been purified in the department for research work. Finally the following were found to have absolutely no action upon either type of sol: benzaldehyde, benzene, bromo-benzene, carbon disulphide, carbon tetrachloride, chloroform, 0.m. and p.chlorotoluene, ether, ethyl benzene, parafin, salicylalde‘This rule waa adopted throughout the work.
IMMISCIBLE ORGANIC LIQUIDS AND COLLOIDAL GOLD
1451
hyde, toluene and m.xylene. I n some instances fractional distillation was necessary in order to obtain a negative result. To avoid contamination of the liquids a distillation apparatus with ground-glass joints waa constructed. It was provided with a column of Raschig rings and an Anschute thermometer was suspended above the column by means of platinum wire. Even after distillation the toluene and xylene still possessed a slight but quite definite influence upon AuNA, which however was eliminated by the following methods of purification. The toluene was boiled with aluminium chloride and the black product distilled, the distillate washed with sodium bicarbonate and water and allowed to stand overnight in contact with sodium wire. The organic liquid was now shaken with twelve changes of pure concentrated sulphuric acid, then repeatedly washed with distilled water, and after drying over sodium wire, fractionally distilled. The xylene was shaken with an equal volume of concentrated sulphuric acid and then treated with saturated sodium chloride. After several hours the crystals of sodium xylene sulphonate were filtered off, dried, and boiled with absolute alcohol, the sodium chloride removed by filtration, and the solution evaporated to dryness. The xylene was recovered by steam distillation with superheated steam, separated from the water in the distillate, dried with anhydrous potassium carbonate, and fractionally distilled. Several immiscible alcohols were now examined. -4.R. amyl alcohol, "nitrogen and pyridine free", (by R.D.H.) was shaken with four lots of caustic soda solution, well washed with distilled water and dried with anhydrous potassium carbonate. The bulk distilled over between 130.8' and 131' (B.P. iso-amyl alcohol = 131') and this was used in the experiments. While AuzA was not affected in the slightest by this organic liquid (after 400 shakes) AuNA underwent immediate change, the whole of the gold accumulating to form a blue film a t the water-alcohol interface in about 5 shakes. Exactly the same results were obtained with n-butyl, iso-butyl, hexyl and heptyl alcohols. I n these instances A.R. materials (by B.D.H.) were distilled and the fraction boiling over a range of 0.2OC. a t the correct boiling point employed.
The Action of the Alcohols Experiments were now carried out to ascertain the cause of the difference in behaviour of AuzA and AuNA when shaken with an immiscible alcohol. As indicated above the two sols were identical in appearance. They moreover were seen to closely resemble one another when examined in the slit ultramicroscope: in both cases the majority of the particles were green, there being only a small proportion of red or orange ones.* The only obvious difference between the sols was that AuzA contained a larger amount of potassium carbonate than AuNA. Hence the effect of adding this salt to a Nordenson sol was next studied. For the reason given later a sol, AuyB, prepared with 0.8 c.c., 0.2 i T K1C03 instead of 0.6 C.C.was used. A series of tubes were set up, each tube containing *See later for determination of particle sizes.
T. R. BOLAM
1452
A S D J. CROWE
+
a mixture of 3 C.C.A ~ N B I C.C.K X O 3 (Ihhlbaum), the concentration of the latter increasing from tube to tube. After standing for 2 4 hours, the mixtures were carefully examined for any colour change, and then each was shaken with I C . C . amyl alcohol. As far as the action of potassium carbonate alone was concerned, no change could be detected in the sol, provided the final concentration (C) of the added carbonate did not exceed about 30 m.equivalents4iter. As will he seen from the following summary, within limits pot’assium carbonate prevented the shaking out of the sol. The values of C are approximate. C
0.0
Film formation
complete
O.O-O.~~
decreasing
3.0-1n.j
0.j-3.0
none
increasing
12.5
&over
complete
That portion of t.he gold which did not go to the interface in any given case appeared to be quite unaffect’ed. It will be noted that at the higher concentrations of carbonate the sol became unstable again. To complete the comparison, the effect of adding potassium carbonate to a Zsigmondy sol w-as investigated. The following results were obtained with a sol, AuzB,* prepared with I . j C.C. 0.1M,K,CO, instead of 3.0 C.C. C
Film formation
0.0- 5.0
Sone
j.0-Ij . 0
increasing
I j.0 &
over.
complete.
The concentration of carbonate required to produce the first sign of coagulation in 2 4 hours (Le. in the absence of the alcohol) was about 3 j X I O - ~ M , which, as in the case of huxB, was well above the d u e at which film formation commenced. Formation oj Red FiZnis.-Since the stabilising action of potassium carbonate might well be due to its alkalinity, the influence of sodium hydroxide (solution prepared from “pure” sticks) on h u N B and AuzB was investigated in exactly the same fashion as above. Table I contains the results for typical series. The occurrence of red films, which does not appear to have been observed by previous workers, is of particular interest since it, shows that’ coagulation to form secondary particles (blue gold) is not essential for removal to the interface. We may therefore reasonably assume that the apparent immediate formation of a blue film from a red sol is also the result of coagulation of the gold after it had reached the interface, the aggregation of the gold being extremely rapid in such a case. A change from red to blue at the irilerjace could actually be observed with .luxB and AuZB and K2C03. Immediately after shaking, the surfaces of the drops of emulsified amyl alcohol were seen to be covered with red gold, which changed to blue or bluish violet as the drops coalesced. Other considerations made it appear very unlikely that the action of amyl alcohol on a Nordenson sol was due primarily to coagulation of the *AusB and AuzB had exactly the same appearance as AuxA and AuzA The former were made to more closely resemble one another than was tlie case w t h the latter, by decreasing the difference in the amounts of K2C03used in their preparation.
IMMISCIBLE ORGANIC LIQUIDS AND COLLOIDAL GOLD
I453
TABLE I Effect of adding Sodium Hydroxide Zsigmondy sol-(B)
Sordenson sol (B) Distribution Interface Aqueous layer
C
IoBBl 4B IB t B
0.00
0.08 0.25
0.49 0.66 0.82
0 0
2.j I
0
5.02
7.53 I O 04 I2
0
SR IR
15.06 20.08 25.10
33.21 35.72 R = red B
0.00
0
2.jI
0
9R IO-R I OR I OR I OR I OR IO-R
5.02
I O .04
+R fR IR
12.55
3R
17.57 20.08 25.10 28.19 33.21 35.72 38.35 51.35
SR iR
3R
9RP
IR
9R
7R
IO-BP
+R
,,
30.70
= blue
BP = bluish-purple
I OR IOR IO-R IO-R
0
3R
IOBP
Distribution Interface Aqueous layer
6R
3R 7BP
jj
C
0
,,
11
11
,I
1,
BB1 = blue black RP reddish-purple
7.53
IORP IOBP
9R
7R 5R 0
,,
19
1, 11
,, ,,
P = purple
The number indicates approximately the proportion of gold at the interface or in the aqueous layer. I O represents the whole of the gold. The first fourteen Nordenson sols were red after twenty-four hours; the I j t h and 16th had a tinge of purple and the 17th was bluish-purple. The first eleven Zsigmondy sols were red; the 12th and 13th had a tinge of purple; and the 14th was purple.
latter by water-soluble impurities in the former, the coagulum then being brought to the interface. It was established by experiment that methyl, ethyl, n-propyl and iso-propyl alcohols, obtained by the distillation of A.R. materials, had absolutely no effect upon AuNA (or A u ~ d ) . The sharp distinction between miscible and immiscible alcohols can hardly be due merely to the absence in the one case and the presence in the other of ionising impurities. The matter was also put to the test in another way. X Xordenson sol was prepared from conductivity water which had been thoroughly shaken with a laige quantity of purified amyl alcohol. The product, though purplish red in colour, proved to be very stable, showing no change in colour or sign of sedimentation after standing for several months. Viewed in the ultramicroscope it was seen to contain about equal proportions of green and red particles. There are two objections to the assumption that the larger particles were the result of coagulation by impurities in the alcohol. In the first place a very high rate of coagulation must necessarily be assumed, and rapid coagulation usually means complete coagulation. Secondly, the dissolved alcohol
T. R . BOLAM AND J. CROWE
I454
may quite well have decreased the rate of nuclei formation during the reduction, which would lead to coarsening of the gold. Even supposing, however, that coagulation had taken place, half the gold was still in the form of primary particles and these showed no tendency to aggregate. The sol was immediately shaken out with amyl alcohol, using the routine procedure. The Influence of Electrolytes As shown in the preceding section potassium carbonate and sodium hydroxide were found to oppose or promote film formation, depending upon the concentration. A number of other electrolytes were now tested to see how far these effects were general, exactly the same procedure being employed as formerly. The results are given in Tables 11, 111, and IV.
TABLE I1 Xordenson sol (B) Electrolyte
KCl BaC12 LaC13 HCl KH~POI NaAc Na2HP04/2 Na3Cit/3
Distribution
X
2 . 4 -26.8 0.05- 0 . 4 0 0.0005-
0.0049
1 . 4 -14.3
1 . 7 -16.7 4 . 2 -41.9 5.0 - 5 0 . 0 7 . 5 -75.0
At all concentrations gold completely removed to interface as blue, blue-black or blue film
9
7.3- 9.8
0.1-
0.15
0.0034- 0.0039
5.7- 7 . 2 8.3-10. o 12.6-16.7 2 5.0-30.0 30.0-3 7 . 5
The figures under x are the limits of concentration between which the action of the electrolyte was studied, and those under y, the concentration limits between which there appeared the first sign of change (blue tinge) in the sol, produced by the electrolyte alone in 24 hours. The concentrations are expressed in millimols per litre.
TABLE I11 Zsigmondy Sol (B) Electrolyte
KC1
Cooc. 2.4
4.9 7.3
BaC12
0.I 0.2
0.3 0.4 LaC1,
0.001 0.0018
Film
Y
IB 8B I OB
12.2-14.6
IB 3B 8B IOB
0.3
9B I OB
IMMISCIBLE ORGANIC LIQUIDS A N D COLLOIDAL GOLD
I455
TABLE Is’ Zsigmondy Sol (B) Electrolyte
HC1 KHZPO, S a Acetate S a 8 Citrate,!3 Na2HPO4;’2
Conc. *
Film
2.75
I oBBl
1.70
IOB 9B I OB I OB
4.2
7.5 5.0
Y
5 . 5 1 - 8.25 8 . 3 -10.0 20.8 -25.0
22.5 -30.0 20.0 - 2 5 . 0
*Lowest concentrations employed.
These tables show that electrolytes in general promote film formation and that the concentration of electrolyte a t which separation of the gold commences is always less, often very much less, than that (y) necessary to bring about a detectable change in the colour of the sol in 2 4 hours. The influence of certain electrolytes was now more closely investigated employing an acid sol prepared by a modification of Zsigmondy’s method. It was found possible to obtain perfectly clear, stable, yellowish-red sols by using tri-sodium citrate in place of potassium carbonate.** The following
TABLE V Sol Aurit Conc. of electrolyte * 0.00
0.25
0.50 0.75 I .oo 1.25 I ,
50
1.75 2.00 2 . 2 5
2 . 5 0
5.0
7.5 10.0 12.5
15.0
17.5 20.0
Nas Citrate Interface Aoueous Lyer
5B 3B 2B 3B j-B 5B J)
0
2R 3R 2R
+R 0
,,
,,
1)
J)
,I
I1
,I
J)
,J
Distribution of gold Naz phosphate Interface Aaueous iayer
5B +B +B +B IB IB 2B 4B 5-B
5B jB
NaOH Interface Aqueous layer
-
0
5-R 5-R 5-R 4R 4R 3R IR +R 0 0
0
,,
JJ 11
JJ
I,
Jt tJ
,, tJ
f R 3R 5B
,, ,! ,! JJ
YaCl, HC1 and KHzPOl from 0.25 to 2 . 5 0 : all gold went to interface at every concentration. *Concentration of added electrolyte (in millimols per litre) after mixing with the sol. “Anderson has prepared sols by using sodium citrate alone. Formaldehyde waa employed in the present instance in order to approximate as closely to the Zsigmondy method as poasible.
T. R. BOL.43.I A N D J. CROWE
1456
quantities proved satisfactory: I C.C.0.6 percent chlorauric acid, diluted with 123.6 C.C.Bourdillon water, 0.4 C.C. 0.131 sodium citrat,e (Kahl3.0 C.C. 0.4 percent formaldehyde. On shaking 3 C . C . of this sol baum) (&it) with I C.C.of amyl alcohol the gold immediately collected at the interface, forming a blue film. Table Vsummarises the results obtained by adding the various electrolytes. In these instances the sol electrolyte mixtures did not stand for 24 hours but were treated after a few minutes. In no case was there any change in colour of the sol between mixing with the electrolyte and shaking with the alcohol. It will be seen from the table that, after sufficiently low concentrations, sodium citrate and secondary phosphate, as well as KaOH, exerted a stabilising influence upon the sol, whereas the other substances showed no tendency to prevent the gold from accumulating at the interface.
+
+
+
Determination of Particle Size The average particle sizes of the sols and AUXAwere determined by a counting method which differed in some respects from the methods generally employed. The particles were fixed with gelatine. accurate allowance bring m a d e for the optical heterogeneity of the latter ( c j Tuorila8). Pieces of Coignet’s “Gold Label” gelatine were cut from sheets chosen a t random from a batch of thirty, and a 0.; jt:; solution prepared from each piece, using optically empty conductivity water. Such solutions become very viscous a t room temperature but do not set. On examination in the slit ultramicroscope they were found to contain a quantity of bright white particles, the number of which in a definite volumer was determined when the Brownian movement had been sufficiently reduced (by the cooling of the gelatine) to make counting easy. The field was effectively changed after each count by running out a drop of the solution from the ultramicroscope cell. The last column of Table VI gives the average number of particles contained in 3.09 X 1o-O cubic millimeters.
TABLE VI Experiment I
2
S o . of counts
Av. no. of particles 8.2 8.0
4
8.3 8.j
5
8.0
3
8.2
For counting experiments with a gold sol, I O C.C. of the latter were mixed with I O C.C. of 1.5 percent gelatine. Judging from their appearance to the naked eye and in the ultramicroscope neither Auzh or h u N h was affected *Deliminated by means of a calibrated micrometer net. The dimensions of the net and the depth of the light beam were found by using a solution of fluorescein in the cell.
IMMISCIBLE ORGAKIC LIQUIDS AND COLLOIDAL GOLD
I457
by this treatment. The gold particles were counted under the previous conditions of light intensity, etc. A fresh sample of gold-gelatine mixture was used for each experiment, and each count made on a different portion of the sample. The average radius of the particle was calculated by adopting the usual assumptions. Table VI1 contains the results.
TABLE VI1 Sol l U Z A
h U N h
Sample
No. of counts
I
IS
2
22
I
35 40 40
2
3
Average number of particles Total In gelatine I n gold sol
40 41
19.6 19.0 19.5
8 2 ,I
>>
31,8
3ow
32.8
30
11.4
42
10.8
43
11.3
42
1
I,
Av. radius of gold particles
The above method was used because when the sols were diluted for counts of the usual type, the background was found to be so dark, (indicating the absence of amicrons) that it was extremely difficult to distinguish the micrometer net. On the other hand if this difficulty was avoided by employing the sols a t a sufficiently high concentration, it was impossible to make counts with any degree of accuracy unless the Brownian movement was considerably diminished. The method has also the advantage that the number of counts necessary is less than in the ordinary method.
Discussion I n agreement with the work of Zsigmondy, it was found necessary to purify carefully an organic liquid in order to ascertain with certainty if it possessed any action upon gold sols.* Employing organic liquids of the requisite quality, a pronounced specific action of the immiscible alcohols was shown to exist. Of all the liquids examined, they alone were found to affect Yordenson sols. Hence Zsigmondy’s conclusion that acid sols are stable when shaken with organic liquids requires modification. It seems probable that this view was based on insufficient experimental material, since the only liquids mentioned as having been studied by him with regard to their influence upon acid sols, are benzene, toluene and ether. The behaviour of a Kordenson sol when shaken with an immiscible alcohol is also obviously not in accord with Zsigmondy’s statement that “Man kann allgemein sagen: reines kolloidales Gold ist stabil gegen Ausschutteln, koaguliertes** dagegen instabil.” Moreover, as has already been indicated, the formation of red films provides very direct evidence that the primary *Reinders* and TanekQ neglected to take this precaution and their results are correspondingly limited in value. **i.e. aggregates (secondary particles) formed by the fusion of the original (primary) particles of the sol, which give a blue coagulum and appear red in the ultramicroscope.
1458
T. R . BOLAM AND J. CROWE
particles of a sol may be attracted to the liquid-liquid interface without loss of individuality. It may however be argued that the contrast between e.g. AUNAand Au,A is due to a difference in primary particle size. While the actual size determinations appear to support this contention it is not possible without a closer study of the influence of electrolytes, to arrive a t a definite conclusion as to the exact importance of the factor of particle size. Under certain circumstances a t least, variations in the composition of the dispersion medium of the sol outweigh any effect of size. For example AuNR was converted by suitable additions of alkali into a system closely resembling AUzB, which can hardly be due to an increase in the degree of dispersion. I n a recent paper DeutschlO suggests ( I ) that solid particles lower the interfacial tension between water and an organic liquid, ( 2 ) that the larger the particle the greater the capillary activity and therefore, (3) that the tendency of the particles to aggregate will be greater at a gold sol-organic liquid interface than in the sol itself, since this’process reduces the free energy of the system. Thus, according to Deutsch, when a gold sol containing sufficient sodium chloride to produce minimum turbidity is shaken with hexane, the gold collects a t the liquid-liquid interface because the coagulation is accelerated by the great increase in interfacial area created by the emulsification of the hexane, and since the coagulation is irreversible, the aggregated gold remains as a film between the two liquid phases after they separate. The absence of effect when a stable gold sol is shaken with hexane is explained in the following words : “die starker geladenen, innerlich weniger abgesattigten Kolloidteilchen starker durch das Wasser zuriickgehalten werden als die durch Zusammentritt mehr abgesattigten aggregierten Teilchen” and “hier die Abstossungskrafte noch so stark wirksam sind, dass sie selbst an der Grenze noch zum Stabilisieren ausreichen, oder anders ausgedriickt, dass der Zuwachs an Capillaraktivitat der durch Zusammentreten zweier Teilchen entstandener Aggregate noch immer zu klein ist, um eine Vergrosserung der Aggregationsgeschwindigkeit zu bewirken”. There appear to be a number of difficulties in applying these views to the results of the present investigation. As has been repeatedly shown, a sufficiently acid sol is coagulated by amyl alcohol, although very stable in the ordinary sense. It might be assumed that the tendency to coagulate, while slight, was sufficient for the purpose, but the evidence suggests that the particles first collect at the interface and then coagulate. At least it is certain that in some cases the aggregation of the particles a t the interface is, a t the most, only of a loose nature (formation of red films). However it might still be assumed that such a species of aggregation constituted coagulation, or even that the electrical condition of the primary particles was not such as to counterbalance their capillary activity. Even supposing this to be the case, however, we have still to account for the absence of effect in the case of, for example, paraffin, which necessitates the further assumption that gold particles diminish the interfacial tension of water-alcohol to a greater extent than that of water-paraffin.
IMMISCIBLE ORGANIC L I Q U I D S AKD COLLOIDAL GOLD
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Moreover the coagulation, if such, resulting in the formation of a red film is in all probability reversible, as Whitney and Blake’? have shown t h a t precipitated red gold may be peptised. It may be questioned, in any case, whether Deutsch is justified in assuming that the influence of the particles in the liquid-liquid interfacial tension is the factor of primary importance in the distribution of solid particles between two liquid phases. Apart from the difficulty of proving that such an effect exists, it does not seem reasonable to ignore the solid-liquid int’erfaces, even when dealing with the small particles of a gold sol. Regarding the problem as essentially one of “preferential wetting” the observations described in the preceding sections may be interpreted as follows. The alcohols are more strongly adsorbed a t the surface of a gold particle than are the other organic liquids: this is concordant with the pronounced capillary activity of the alcohols. Hence the particles are more easily wetted by the alcohols and made to adhere to the organic layer. It is, however, to be expected that the adsorption of the alcohol molecules will be influenced by the electrical condition of the solid surface. As a rule organic substances depress the electrocapillary curve of mercury, their action being greatest in the neighbourhood of its maximum. This is ascribed to the effect of the electric field at the interface upon the adsorption of the organic molecules. That is to say, the adsorption is greatest when the field has some small value and decreases as the field becomes more positive or n e g a t i ~ e . ’ In ~ this may it is possible t o account for the influence of the nature of the dispersion medium upon the distribution of the gold particles when a gold sol is shaken with an immiscible alcohol, i.e. the effect of any electrolyte may be attributed to its influence upon the electrical charges of the particles. If this view is correct we would expect to find much the same relationships holding as in the direct coagulation of a negative sol. This proves to be the case. In the first place the order of efficiency of the chlorides in promoting accumulation of the gold a t the alcohol-water interface is La C1, >BaC12 > KCl (Table 111). Secondly, those electrolytes (KaOH, K2C03,?r‘alHP04and NaKit) which oppose the action of the alcohol, have high coagulation values compared wit’h potassium chloride (Tables I, I1 and IY). It will be observed that the gold is not necessarily either completely brought to the alcohol-water interface or else left quite unaffected. All degrees of distribution are possible. This may be a size effect but another possible explanation is that the gold particles are not all equally charged, since Anderson’, Garner and Lewis,I4 and Garner’j found that they could account satisfactorily for the behaviour of gold sols when slowly coagulated by electrolytes, if they assumed that the primary particles carried unequal charges. Since, of the salts examined only those which give alkaline solutions possessed the power of “protecting” a sol, the question arises as to whether this effect is due simply to decrease in the hydrogen ion concentration or is partly produced by adsorption of the salt anions. This problem is being investigated.
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T. R. BOLAM AND J. CROWE
Summary A considerable number of immiscible organic liquids have been examined with respect to their action on certain gold hydrosols. 2. Zsigmondy’s conclusion that unambiguous results are obtained only if very pure liquids are employed has been confirmed. 3. It has been established that the immiscible alcohols exert a specific influence, since they may cause the gold of a stable sol to collect a t the liquid-liquid interface. 4. The formation of red interfacial films under certain conditions has been observed. 5 . The influence of various electrolytes upon the action of amyl alcohol has been studied. 6. It is suggested that the action of an immiscible alcohol is due to its strong tendency to be adsorbed a t the surfaces of the gold particles and that the degree of adsorption depends upon the electrical condition of these surfaces. An electrolyte thus promotes or opposes accumulation of the gold a t the interface according as it decreases or increases the particle charge. I.
References Zsigmondy: Nachricht. kgl. Ges. Wiss. Gottingen, Math-Phys. Kl., 1916, 38. Also:-Z. anorg. Chem., 96, 265 (1916); Z. Elektrochemie, 22, IO? (1916);Zsigmondy-Thieeaen: Das kolloide Gold,” 168 (1925). Reinders: Kolloid-Z., 13, 235 (1913). 3Wiegner: Kolloid-Z., 8,227 (1911). Freundlich: “Colloid and Capillary Chemistry,” 484 (1926). 6 Nordenson: “Uber die Bedeutung des Lichtes fur die Bildung und Stabilitit kolloider Losungen.” Diss. Upsala, 123 (1914).Also:-Svedberg “The Formation of Colloids,” 70 (1921). Beaver and Muller: J. Am. Chem. Soc., 50,30j (1928). 7 Anderson: Trans. Faraday SOC., 19, 623 (1924). * Triorila: Kolloidchem. Beihefte, 22, 191 (1926). 9 Yanek: Ann. Beole mines Oural, 1, 45 (1919). Deutsch: Z. physik. Chem., 136,353 (1928). See also:-Freundlich and Loehmann.” l1 Freundlich and Loebmann: 2. physik. Chem., 139, 368 (1929); Kolloidchem. Beihefte 28, 391 (1929). I* Whitney and Blake: J. Am. Chwn. SOC., 26, 1339 (1904). l3 Butler: Proc. Roy. Soc., 122A, 394 (1929). l4 Gamer and Lewis: J. Phys. Chem., 30, 1401 (1926). 15 Gamer: J. Phys. Chem., 30, 1410(1926). I
Chemistry Department, The University, Edinburgh, December 18, 1930.
