T H E INFLUENCE OF IONS CARRYING T H E SAME CHARGE AS T H E DISPERSED PARTICLES I N T H E IXVERSIOS OF EMULSIONS BY S. GHOSH AND N. R. DHAR
In previous papers1 we have emphasised that the stability of a sol depends a great deal on the adsorption of ions carrying the same charge as the sol. We have also suggested that ions carrying the same charge as an emulsion influence the stability of the emulsion markedly. In this paper we shall show that ions carrying the same charge as the emulsoid particles have a considerable influence on the inversion of the type of an emulsion. The investigations of Lewisj2 Ellis,3 Powis4 and others show that the emulsoid particles are charged and that their stability depends upon the electric potential at the surface of the globules, just as colloids remain suspended due to their electric charge. Powis studied the effect of KC1, K 4 Fe(CN)6, BaCL A1C13 and ThC14 on the velocit,y of oil particles dispersed in water under an electric field. For the pure emulsion, the velocity, of the particles first increased when KC1 or K4Fe(CI\')6 was gradually added and with greater amounts of these electrolytes it fell. A similar maximum was observed by Ellis in the presence of O.OOIX NaOH. On the other hand, the velocity of the oil globules gradually fell when BaC12, AlC13, or ThC14 mere added and finally they moved in the opposite direction showing a reversal in charge. Similar results were obtained by Powis using As2S3 sol and KC1, Kruytj with As2S3 sol and KC1, and Freundlich and Zeh6 with AszSa sol and K 4 Fe(CN)3, and with Fe(OH)3 sol
[ (S H 3 ) 4 C o ( : z 2
Cl4 where first an increase in ( > I velocity was observed on the addition of the electrolytes. and
>Co
3"
4
We have already indicated that this increase in velocity is a direct consequence of the adsorption of an ion carrying the same charge as the suspended particles on the addition of these electrolytes, in spite of the fact that there exists an electric repulsion due to the sameness of the electric charge. Let us consider the fate of an electrolyte say MCl, when added to a negatively charged colloid. Kow, if the tendency for adsorption of similarly charged C1' ions is greater than the force of repulsion due to the sameness of electric charge, the charge on the colloid particles will increase as chloride ions are adsorbed, when small quantities of KC1 are added. This increase in velocity, however, reaches a limit when the adsorption of chloride ions is maximum and Ghosh and Dhar: J. Phys. Chem., 29, 435 (1925); Ghosh, Bhattacharya and Dhar; Kolloid-Z. (1925) Phil. Mag. (6) 15, 509 (1908\. a Trans. Faraday Soc. 2, 14 (1913). Z. physik. Chem. 89, 91 (1914). 6 Kolloid-Z. 22, 81 (1918). Z. physik Chem. 114, 65 (1924). .I
I S F L U E N C E O F IONS ON INVERSION O F EMULSIONS
295
the adsorption of the oppositely charged K ion increases. If, however, the adsorption of chloride ions cannot counter-balance the repulsive effect due to the sameness in electric charge we may not get any marked increase in the velocity of the colloid particles in an electric field, but the fall in the velocity of the colloid particles when KC1 is added to the sol will not be rapid. On the other hand, if the adsorption of similarly charged chloride ions is comparatively small or nil, the fall in the velocity of the colloid particles will be rapid on the continued addition of small quantities of KC1. Consequently, from the results of Powis we can infer that oil particles in water are negatively charged and appreciably adsorb ions carrying the negative charge from such electrolytes as KC1, K4Fe(CN)6,etc. It has already been indicated that emulsions closely resemble colloidal suspensions. From the existing literature we find that emulsions behave abnormally towards electrolytes on dilution that is become more stable towards electrolytes on dilution. This behavior is certainly due to the adsorption of ions carrying the same charge as the oil globules. It is expected therefore that such emulsions would also show (i) abnormality towards a mixture of electrolytes i.e. show an antagonistic effect when coagulated by a mixture of electrolytes of varying valency, and (ii) the phenomenon of acclimatization. Bhatnagar's' results on the coagulation of aniline emulsion show that it becomes stable on dilution towards electrolytes such as KCI, BaCL and A12(S04)3. His results are given in Table I.
TABLE I Electrolyte added Grms equivs. per litre
Time of Percept'ible Change in Emulsion Emulsion Mins: Secs.
KCl
Twice diluted Mins. Sees.
Three times diluted Mins. Sees.
Four times diluted Mins. Sees.
(a) 0.1 41 0 56 1 11.5 5 I33 10 (b) 0.139 12 8 20 5 41 0 63 9 BaClz (a) 0 . 0 2 9 IO o 18 7 27 5 40 0 (b) 0.040 2 0 4 6 7 0 I3 2 A12(S04)4(a) 0 . 0 0 2 IO o I1 I 13 5 15 9 7 2 (b) 0.005 2 0 2 53 3 58 lye have shown that with all the suspensoids so far studied in this laboratory the tervalent Al"* ion always behaves normally towards dilution. Thibehavior of Al"' ion with aniline emulsion will be discussed later on. In the following pages more evidence will be advanced in favour of the view that an emulsion can adsorb an ion or a micelle carrying the same charge as the emulsiod particles and it will be shown that our generalisation already referred to is more or less followed by emulsions. Very little systematic work has been done on the coagulation of emulsions by electrolytes. Hence it is difficult to draw very definite conclusions regarding the effect of anions on the precipitation of an emulsion. J. Phys. Chem. 25, 735 (1921).
296
S. GHOEH AND N. R. DHAR
It is obvious that when an emulsion is formed of two immiscible liquids say water and oil, it is possible to get two kinds of emulsion viz, (i) oil dispersed in water and (ii) water dispersed in oil. The existence of such two types of emulsions has been recognised by Ostwald,l Robertson2 and other. Bancroft3 has shown that sodium oleate emulsifies oil in water and calcium oleate emulsifies water in oil. The systematic work of N e ~ m a nBriggs ,~ and Schmidt,j Clowes6and others on the inversion of oil-in-water emulsion to waterin-oil type has thrown considerable light on this question. Clowes has shown that an oil-in-water emulsion produced in the presence of sodium soap can be inverted into water-in-oil emulsion by the addition of a calcium salt. The transformation of an oil-in-water emulsion to a water-inoil emulsion by means of a calcium salt has been followed microscopically. It is noted that the oil globules in water are first distorted, then elongated and as the critical point for inversion of phases approaches the Brownian movement is very marked, and finally on complete conversion of the system into an emulsion of water in oil, the Brownian movement of these oil particles dispersed in water entirely ceases. From these observtions it can be concluded that a t the critical point of the inversion of the type the coagulation of oil-inwater emulsion has set in and that the emulsion is extremely unstable. It seems that up till now very little attention has been paid to the question of charge on emulsoid particles. Several workers have shown, however, that the usual charge on the suspended oil particles is negative. MTe are of the opinion that when the electrolytes like KC1, NaC1, etc., are added to such an emulsion, at first preferential adsorption of the negative ions takes place and consequently the negative charge on the oil globules increases. If the concentration of such electrolytes is increased the influence of the oppositely charged ion becomes more prominent and the charge on the oil globules is neutralised and is followed by the coalesence of the oil globules to form bigger drops and separation into two layers. This phenomenon is the same as the coagulation of a sol by the addition of an electrolyte. It is interesting to note that reversion of emulsion oil-in-water to waterin-oil hardly takes place with KCI, NaC1, etc., but phase-reversal of oil emulsions takes place readily with bivalent and trivalent cations and H ions. Consequently, we are of the opinion that in water-in-oil emulsion the dispersed water particles are positively charged. This is further supported by the observation of Bancroft that in the presence of sodium or potassium oleate oilin-water emulsion is obtained, whilst in the presence of calcium or magnesium oleate water-in-oil emulsion is formed. Moreover Bhatnagar 7 has obtained phase-reversal of water-in-oil by KOH, K3PO4, etc. Kolloid-Z. 6 , 103 (1910); 7, 64 (1910). 7, ;(1910). 3 “Applied Celloid Chemistry,” 26; (1921). Phys. Chem. 18, 34 (1914). J. Phys. Chem. 19, 478 (1915). “roc. SOC.Exp. Biol. Med. 11, I (1913); J. Phys. Chem. 20, 407 (1916). J. Chem. SOC. 129, 1760 (1921).
* Kolloid-Z.
I N F L U E K C E OF IONS O N INVERSION OF EMULSIOSS
297
This is due to the fact that the oil particles have a tendency to adsorb oleate ions and pass into negatively charged oil-in-water emulsion, whilst water particles have a tendency to adsorb Mg” or Ca” ions and pass into positively charged water-in-oil emulsion. There are certain points of resemblance between the phase-reversion of emulsion and charge-reversal of colloids. It is well known that the charge reversal of a sol usually takes place by a bivalent or a polyvalent ion or H ‘ or OH’ ions; similarly, reversion in phase of emulsion also occurs with bivalent and polybalent ion or H’ and OH’ ions. Let us see what happens when CaClz is added to an emulsion of oil-in-water. The Ca“ ion mill be adsorbed by the particles of the oil on charge-neutralisation and coalesence of the oil globules will take place. Just before the separation of the substances in two layers, the water particles, which have a greater affinity for Ca” ions than that of oil particles for the same ion, will take up Ca” ion and pass into positively charged water particles forming a water-inoil emulsion, provded the system is shaken well. This is possibly the mechanism of phase-reversal of an emulsion by a polyvalent ion, I t is evident that in the case of charge reversal of a sol only the charge on the suspended particles is changed, whilst in the case of phase reversal the dispersed phase is different in the two conditions. Clowes showed that in the case of emulsions of olive oil in water stabilised by KaOH, the addition of CaClz leads to the inversion of the type. Table I1 gives the results obtained by Clowes with olive oil emulsion containing free oleic acid. TABLE I1 Vol. of M / I NaOH ~ added. C.C.
0
25
R 0-W 0-W 0-’CV
I 2
3 4 0-W= Oil-in-water emulsion. IT-0 =IT’ater-in-oil emulsion.
R= Critical
Vol. of M / r o CaCP employed. 0.5 0.75
w-0 R 0-157 0-17
w-0 w-0
C.C.
I O
R
W-0 W-0 R-0
0-W
R
(inversion) point.
These results show that the greater the concentration of the added NaOH the, greater is the amount of CaClz required to bring about the inversion of the emulsion. On plotting the logarithims of KaOH added as abscissae and the logarithims of the corresponding amounts of CaClz to bring about the inversion of the emulsion as ordinates, we find that the graph is a straight line, showing that the adsorption formula of Freundlich,‘ x/m = KC”, where x = the amount adsorbed by m grams of the adsorbent, C =concentration of the solution and IC and n are constants, is strictly followed. This therefore, simply proves that the stabilisation of the oil-in-water emulsion is due to the adsorption of OH’ ions from NaOH or oleate ions from sodium oleate formed by the action of KaOH and free oleic acid. Rolloid-Z. 3,
212
(1908).
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S. GHOSH AND N. R. DHAR
'
also shown that when NaC1 is added to an emulsion up to a concentration of O.IM preferential adsorption of chloride ions takes place and thus facilitates the formation of oil emulsion and dispersion of oil globules. At higher concentrations of NaCl the adsorption of negative ions in excess to that of positive ions does not occur to the same extent, and a t 0.35 M to 0.40 RiI NaCl the emulsion separates in two layers; that is,
Linder and Picton: J. Chem. SOC.67, 67 (1895);Weiser: J. Phys. Chem. 25, 665 Ghosh, Rhatt,acharya and Dhar: loc. cit.
(1921);
I N F L U E N C E O F IONS ON INVERSION OF EMULSIONS
299
complete coagulation goes on increasing with the increase in the concentration of KC1 up to a limiting value. After this limiting value is attained if the concentration of KC1 is further increased the amount of BaC12or SrCL necessary for complete coagulation gradually diminishes. From these results a very interesting point has been drawn out in a previous paper.’ It is well known that keeping the amount of the adsorbent constant, the greater the concentration of the electrolyte which will be adsorbed the greater is the amount of adsorption. Sow, as the concentration of the electrolytes KC1, LiC1, etc. is increased the adsorption of C1 ions goes on increasing and thus the sol becomes more and more stable. We expected that up to this maximum stability the Freundlich exponential formula for adsorption should be followed, where the concentration of KC1 added may be roughly taken as the final concentration of the electrolyte and the excess of SrC12necessary for coagulation in the presence of KC1 as the amount proportional to the adsorption of chloride ions. Beyond this maximum stability the adsorption of K ions becomes more prominent and, therefore, the adsorption formula need not be followed. We have shown that on plotting a graph with Linder and Picton’s result by taking the logarithims of the excess of SrCl2 necessary for complete coagulation as ordinates and the logarithims of the amount of KC1 added as abscissae a straight line is obtained. This proves conclusively that chloride ions are preferentially adsorbed by As2S3 sol. The similar graph obtained for the inversion of oil-in-water emulsion by CaC12 in the presence of NaOH shows that this stabilisation of the emulsion in presence of NaOH is not materially different from that of As2S3 sol in the presence of KC1, though more markedly. Clowes has classified electrolytes into two main groups ( I ) promoting dispersion of oil-in-wa ter salts of monovalent cations Li, IYa, K, etc., alkalies, salts of divalent and trivalent anions, ( 2 ) promoting dispersion of water-inoil-salts divalent and trivalent cations Ca, Ba, Sr, Fe, Al, etc. It has been already indicated that the system water-in-oil may be looked upon as coagulated oil-in-water emulsion, and therefore can be compared to a jelly, where water is dispersed in the continuous solid phase. Hardy2believed that the concentrated gelatine jellies consisted of drops of water suspended in a gelatine-rich phase. Many workers are, however, of the opinion from the ultramicroscopic observations that jellies have a sponge structure and not a honeycomb structure peculiar to emulsion jellies, where water is the dispersed phase; but it is now believed that we cannot distinguish between the two cases by means of the ultramicroscope.3 If we regard a water-in-oil emulsion as coagulated oil-in-water emulsion, salts like NaCl, KC1, K*Fe(CIY)eetc., promote the formation of the emulsion oil-in-water due to the adsorption of similarly charged ions like Cl,’ Fe(CN)e”’ etc., whilst the salts of bivalent and trivalent cations can easily coagulate Ghosh and Dhar: J. Phys. Chem. 29, 435 (1925). Z. physik. Chem. 33, 326 (1900). Compare Bachmann: Kolloid-Z. 23, 89 (1918).
300
S. GHOSH AKD N. R. DHAR
the oil particles and help the dispersion of water particles as a positively charged water-in-oil emulsion. Clayton’ has also observed that monovalent salts like KC1, XaC1, etc., help the emulsification of oil in water. He has observed that 176KaC1 greatly facilitates the formation of margarine emulsion in water. This is ascribed by Clayton to the adsorption of chloride ions from S a c 1 and subsequent liberation of KaOH, which forms soap with the free oleic acid already present. Similar effects of monovalent cations have been observed by Ayres2 and Herschel3 on the formation of oil-in-water emulsion. TTe are of the opinion that in all these cases the charge on the oil particles and the degree of dispersions are increased due to the adsorption of small quantities of C1’ ions. Briggs4 on the other hand has observed that NaC1 possesses no emulsion-pro. ducing property with benzene. Recently Rosner and Narrat5 have shown that a 2 5 cc mixture of rape oil and mineral oil forms a stable emulsion with 2 5 cc of sea water, where water forms the continuous phase. The emulsion is fairly stable showing that the emulsification is greatly helped by the XaCl of sea-water. Increase in the amount of sea water is unfavourable towards emulsification, Harkins and Keith6have also shown that the addition of potassium iodide or sodium chloride decreases the size of the dispersed oil particles; and this shows that an oil emulsion is stabilised by the addition of monovalent electrolytes due to the adsorption of the similarly charged anions. The experiments of Parson and Wilson7 are also of much interest from this point of view. Using emulsions of pure mineral oil ‘Nujol’ in the presence of a dilute sodium oleate solution, they found that 0 . 2 2 M NaCl and 0.24 M Na2S04are necessary to precipitate an oil emulsion and separate it into two layers. It will be seen that if the concentration of these electrolytes are expressed in equivalents far greater quantities of NazS04are required to precipitate the emulsion than NaC1, which points to the fact that anions have a marked effect on the precipitation of the negatively charged emulsions. They have also studied the inversion of oil-in-water emulsion to water-in-oil emulsion by various electrolytes. Aqueous solutions of MgS04, MgC12, FeS04, AlZ(SOJ3 and FeC13 were used. With magnesium salts no absolutely sharp inversion point was found, but, rather, a time-period of instability during which inversion took place. Complete inversion took place with trivalent electrolytes. This shows that the trivalent cations are more effective precipitants and invertors of type than Mg; consequently, more magnesium salts are necessary to bring about the inversion in the type of the emulsion. The first effect observed on adding a magnesium salt was to lower the viscosity of the oil-in-water emulsion. This was followed by an increase in “Rfargarine,” 72 (1920). 2Chem. Met. Eng. 22, 1061 (1920). 3 U . S. Bur. Standards, Tech. Papers S o . 86, 17 (1917). J. Ind. Eng. Chem. 13, 1009 (1921). 6 “Petroleum,” 19, 611 (1923). Science, 59, 463 (1924). J. Ind. Eng. Chem. 13, 1119 (1921).
INFLUEICCE O F IONS ON INVERSION O F EXULSIONS
301
viscosity, with oil appearing as the external phase. The initial drop in the viscosity of the oil emulsion can be attributed to the adsorption of ions carrying the same charge as the emulsion and consequent increase in the degree of dispersion. In a recent paper Dhar has emphasised that, keeping other variables constant, the increase in charge on suspended particles in water results in the partial dehydration so that there is a fall in viscosity which is intimately connected with the amount of bound and free water. Such initial drop in the viscosity on the addition of small quantities of electrolytes like XaOH, KCl and NaCl to sodium palmitate solution has been observed by Farrow.1 On the addition of more electrolyte the viscosity increases very rapidly. More or less similar results have been obtained by Woudstra2 with Fe(OH)3 sol, though less markedly. It has been observed by Clowes that chloride ions are appreciably adsorbed by soap films from NaCl solution and McBain and Salmon3 have also observed that both C1’ and OH’ ions are adsorbed by soap, The increase in viscosity is certainly due to the coagulation of the sol with the increasing concentration of the electrolyte and the subsequent abstraction of the free water by the coagulam. It is therefore, clear that an increase in viscosity on the addition of magnesium salts to ‘Nujol’ emulsion is due to the dispersion of water-in-oil medium and this can be compared with a jelly of the so called ‘hydrophile colloids’ in which water is dispersed in the solid medium. Bhatnagar* has made several experiments on the inversion of the two types of emulsion by electrolytes. It has been observed by him. that the greater the concentration of NOH the greater is the amount of Ba(N03)2 required to complete the inversion. On plotting a graph with his results, it is found that the Freundlich exponential adsorption formula is strictly followed as has been already shown with the results obtained by Clowes. Bhatnagar has got the following order A1 > Cr > Ni > P b > Ba > Sr, Ca, Mg beginning with the cation possessing the greatest influence on the inversion of phase. The coagulating S ~as obtained by Linder and Pictons is in power of these cations with A s ~ sol the order A1 > P b >Ba > Sr > Ca, Mg >Xi beginning with the cations possessing the highest coagulating power. It will be interesting here to consider some results of BhatnagaF on the effect of dilution towards the inversion of the type of emulsion by different electrolytes with a paraffin-in-water emulsion. Amount of sodium oleate in each sample=o.IoI millimole BaCL in millimole 20
cc emulsion
Standard emulsion. ’’ (twice diluted) ” (thrice diluted) ” (four times diluted)
a t the inversion point
Al,(SOa)3 in millimole at the inversion point
0.050
0.015
0.0798
0.025
0.112
0.040
0 . I 56
0.057
,J. Chem. Soc. 101,347 (1912). 2 Kolloid-Z. 8, 73 (1911). J. Chem. Soc. 119, 1369, 1374, 1669 (1921). J. Chem. Soc. 119, 61, 1760 (1921). Burton: “Physical Properties cf Colloid Solutions,” 158-160 (1920). e J. Chem. Soc. 119, 61, 1760 (1921).
302
S. GHOSH AND N. R. DHAR
The results in the above table indicate that greater the dilution of the emulsion the greater is the concentration of the electrolytes necessary to bring about an inversion in phase. The most striking point observed in these results is that more abnormality is observed with Al"' ion than with Ba" ion. The concentration of BaCL required to invert the phase of four times diluted emulsion is about three times the concentration of BaClz required to invert the strongest emulsion, whilst the concentration of A12(SO4) required to invert the phase of the diluted emulsion is about four times the concentration of Alz(S04) 3 required to invert the strongest emulsion. This peculiar behaviour of Al"' ion cannot be easily explained from the point of view that the stability of a sol entirely depends upon the increase in distance between the particles on dilution. This view advanced by Kruyt and Spek,l Mukherji and coworkers2 and others indicates that the effect will be more marked with K' ion, less with Ba"' ion, and still less with trivalent AI"' ion. Kruyt and Spek, Burton and Bishop3and others have observed that on diluting a collodial solution more of uni-valent salts like KC1, XaC1, etc., are necessary to coagulate a dilute sol than a concentrated one and the amount of tervalent ions from salts like A1C13, CrC13, required to coagulate a sol decreases with the decreasing concentration of these electrolytes, whilst practically constant amounts of such salts as BaCI2, SrCl2, etc., are necessary to coagulate a strong or a dilute sol. We are of the opinion that the results of Bhatnagar do not support the view that the stability of an emulsion is due to an increase in the distance between the dispersed particles. The following explanation, which is based on the assumption that the abnormal dilution effect, i.e. more of an electrolyte is necessary to coagulate a dilute sol or emulsion than a concentrated one is a direct consequence of the adsorption of an ion carrying the same charge as the sol or emulsion, can satisfactorily explain the behaviour of Ba" and Al"' ion. When an electrolyte say AB is added to a sol or an emulsion carrying a negative charge, it is well known that if it can adsorb the anion B' carrying the same charge as the sol or emulsion appreciably, it will tend to increase the stability of the sol or emulsion and the process will follow the ordinary adsorption law. Xow, if the sol is diluted twice, it is well known that the amount of adsorption of the anion B' is always greater than half the amount of adsorption by the concentrated sol or emulsion, consequently the amount of adsorption of anion B' adsorbed by an unit mass of the suspended matter is greater for the dilute sol or emulsion than the concentrated one. The dilute sol or emulsion is, therefore, stabilised to a greater extent than the concentrated one due to the greater adsorption of anion B'. Hence, more of an electrolyte is necessary to coagulate a dilute sol than a concentrated one in the presence of AB if the anion B' is appreciably adsorbed and stabilises the sol. The following experiments with silver sol prepared by Kohlschutter's method will show that more abnormality is observed in the presence of an
2
Kolloid-Z. 2 5 , 3 (1919). J. Chem. Soc. 115, 461 (1919). J. Phys. Chem. 24, 7c1 (1920).
INFLUENCE O F TONS ON INVERSION O F EMULSIONS
303
alkali. It is well known that AgOH is not completely decomposed by hydrogen and free OH ion is always present, which peptises the sol. Concentration of the sol = 0.08 grm of Ag per litre Sol A = 4 cc made up to 8 cc; volume=8 cc. Immediate change in colour is observed. Concentration of the sol.
A
KSC3 N/8 c.c. 0.8
KNO3 9/8 in presence of 0.1 C.C. NaOH K/5. 1.2
3A/4
0.9
1.4
A/2
I .o
2.10
From this table it will be seen that the amount of KSO3 required to coagulate the sol increases with the decreasing concentration of the sol, but the phenomenon is more marked in the presence of S a O H which is known to stabilise the sol in small quantities. It has already been observed that in the presence of KCl, KiYO3, etc., more BaClz or AlC13 is necessary to coagulate the sol of AszS3. This antagonistic effect of KC1 towards BaC12 has been ascribed by us to the adsorption of chloride ions carrying the same charge as the sol from KC1. It has been already shown that more of chloride ions are adsorbed by a dilute sol than a concentrated one; consequently, the antagonistic effect of KC1 towards BaClz or will be more marked with a diluted sol of ils~S3than a concentrated one. This has been already shown by Sen1 that the antagonistic effect of NaC1 towards CaClz is more marked for the dilute sol than the concentrated one. We have already cited in this paper that NaOH or soap exert an antagonistic effect on the inversion of an oil emulsion by Ba(N03)2. Briggs2has shown that soaps are markedly adsorbed by benzene-water interface and it is clear that this adsorption mill be more marked when the emulsion is diluted; consequently, more marked antagonism will be observed with a diluted emulsion than a concentrated one, so that more of an electrolyte is necessary to invert a dilute emulsion than a concentrated one. The abnormal behaviour of AI“’ ion towards the coagulation of aniline emulsion is more due to the peptising effect of OH’ ion obtained from the dissolved aniline than due to the adsorption of the ion carrying the same charge as the emulsion from Alz(S04)3 because the concentration of SO”4ion is very small. Now, when an aniline emulsion is diluted the amount of OH’ ions in a given volume of the emulsion increases because more of aniline dissolves due to an increase jn the amount of water phase. It is well known that OH’ ions stabilise a negatively charged emulsion and the phenomenon is more marked with a diluted sol than the concentrated one. This is the probable explanation of the abnormal behaviour of AI”’ towards dilution of aniline emulsion as observed by Bhatnagar. Private communication. J. Phys. Chem. 19, Z I O (1915).
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S. GHOSH AND N . R. DHAR
Bhatnagar has studied the effect of various soap solutions on the stability of paraffin oil emulsions and his results indicates that soaps can be arranged in order of their protective effect. It is found from his results that the stability of the emulsion increases with the increase in the concentration of the added soap and that on allowing for experimental errors straight lines are obtained when the logarithims of the concentrations of the soap solution are plotted as ordinates and the logarithims of the corresponding of amounts of the electrolytes required to bring about the inversion as obscissae. This shows that here we are dealing with the pure adsorption phenomenon and subsequent stabilisation due to the adsorption of the highly colloidal soap micelles. It will be interesting at this stage to describe the experimental results of Briggs on the adsorption of soap in the interface between benzene and water. Briggs has shown that the amount of soap adsorbed agrees fairly well with Freundlich's adsorption formula. It is, therefore, certain that the formation of an oil-in-water emulsion depends upon the adsorption of the highly charged soap micelles, just as Fe(OH)3 is peptised to a celloidal solution by an acid due to the preferential adsorption of hydrogen ions. The greater peptising effect of a soap solution than that of an alkali is due to the colloidal nature of soap. It has been shown by us1 that an adsorbent like Bas04 can adsorb colloids in greater quantities than molecules or ions from molecular solutions. In a recent paper Seifriz2has studied the reversibility of hydrocarbon oil emulsions prepared from hydrocarbons of varying densities. It has been shown that heavier petroleum oils form water-in-oil emulsion in the presence of casein. The author has observed that the relative potency of ions in their ability to reverse the stable water in oil type to oil in water type is in the order S a ( 0 H ) > Ba(OH)2> Th(X03)4>A12(S04)3,whilst S a C l and BaCl2 cannot reverse the type at all. Seifriz concludes from these results that there is nothing to suggest the effect of valency of electrolytes on emulsions. On careful consideration we find that the above order of the electrolytes is quite reasonable as will be evident from the following point of view:
It is well known that the stability of a suspensoid or an emulsion is chiefly due to the electric charge on the suspended particles. It is, evident that the greater inverting influence of either NaOH or Th(N03)4 is certainly due to the marked adsorption of OH' or of polyvalent Th"" ions by the particles of oil. The experiments of Powis in which he has shown that Th"" ions can reverse the charge of oil globules has been already cited. XaC1 and BaClz on the other hand possess no marked inverting effect because chloride ions are not so markedly adsorbed as OH' ions and the other cations are also not as effective as Th"" ion. It is quite probable that the oil-in-water emulsion obtained with an hydroxide is negatively charged and that obtained with Th(N03)4 or A12(S04)3is a positively charged emulsion. 1 2
Compare Ghosh and Dhar: Kolloid-Z. 35, 144 (1924). J. Phys. Chem. 39, 587 (1925).
I N F L C E N C E O F IONS ON INVERSION O F EMULSIONS
305
Summary I. It is shown that an oil emulsion closely resembles a colloidal suspension and the generalisations advanced for a sol is followed by an emulsion. 2. The separation of oil-in-water emulsion into two layers can be regarded as the coagulation of the dispersed oil globules. The reversal of phase of oil-in-water is probably the dispersion of water particles carrying a positive charge in the oil. 3 . From the experimental results of various workers it is concluded that salts like NaCl, KC1, Na citrate, NaOH, etc., favour the formation of an emulsion of the type oil-in-water. This shows that anions viz, Cl’, Cit”’, OH’, etc., are appreciably adsorbed by the oil globules. 4. It has been deduced that the adsorption of the anions is mainly responsible for the antagonism observed between the electrolytes of monovalent cations like NaC1, KC1, NaOH, etc., towards salts of bivalent and trivalent cations like Ba(N03)2AICl,, A12 (S04)3,etc., on the inversion of an oil emulsion. 5 . It is shown that the (a) abnormal dilution effect i.e. more of an electrolyte is necessary to coagulate a dilute emulsion than a concentrated one and (b) the antagonistic effects observed between salts of monovalent cations and bivalent cations on the inversion of the types of the emulsion are essentially connected and go hand in hand. This generalisation has already been advanced by us in a series of papers for ordinary sols. 6. It has been emphasised that the stability of an oil emulsion in the presence of soaps mainly depends on the electric charge of highly adsorbed soap micelles. Chemical Laboratory, Cnioerszty of Allahabad, AElahabad, India. J u n e 24, 1916.
