INDUSTRIAL AND ENGINEERING CHEMISTRY
1016
oxidation due to iacreasixi velocities is insufficient to cause the formation of ferric ions from the ferrous ions, while at high oxygen concentrations the rate of formation of ferric ions is increased so that the differential change in corrosion rate becomes more pronounced as the velocity increases. Literature Cited (I! rldeuey mid Beckei, Phil. Mac., 42, 87-96 i l W l ) . $ 2 ) B ~ a n s Trons. , Filrndny Soc., 19, 789 (1923). i:31 B v m r , "Corrosion of Metals," Amoid, 1$126. (41 Evans, J . Ckem. Soc., 1927, 11l20. (51 E v a e s . f h i d . . 1929, 1151 (81 E ~ J . F ~ ~ ~~ ~ ~~ ~~ aos. I. ~ ,M i N . .~ ~ I.. ~15-58 ~ (isas).
Vol. 23, No. 9
BXD.Crmrt.. as, 6511 3 (1Sal). "Pbyiikalisch-Chemi\rhe Tabriien;' Spinper,
(7) Poirest, Koetlieii, and Brown. IN". (0) Lnndolf-Burnstein.
1923. (O! McKay, Trans. A m . 1:iecLroiinem. Sac., 41. 201 (1922). (101 M ~ Kciirm. ~ ~ ~, ~ i sa, . 432 (1925). (11) Speller. "Cuirosinn--Caures and Prevention," M c C i a w - I i i i l . 1W8. (12) Speller. I b i d . . p. 144.
an#.,
cia)
spciier, w d . , p. 368. (14) Welker, J . Am. Chrm. So'., 29, 1251 (1sOi). (15) Walker, Trans. A m . Elciirociirm. Soi., 14. 175 (191Is). (181 Whitrnm, IN". Bno. Cirilx , 11, 348 (1!?25)..
Formation of Films at Liquid-Liquid Interfaces' AXCIWYF (3) I ~ R B publislieil a sericy of papcrs on "'llie 'l'lioory of h i u ~ S i f i C d t ~ l i l i iii " wliicli lie traces
I3
lopxncut and iieciuccs t,lre uiidcriying principles
of tlie idea iiow coimnoiily Iicld that the effectiveness of
soap, by Holmes arid Cameron (15) in t,lic case of glycerol eniulsifiod in aeetonc by means of cellulose nitrate, and by Holmes and Williams (10) in the case of water crnulsified in ether (and other organic liquids) by means of iodine. Nugent (21) found that if caustic soda is added to a fresh ernulsion of benzene with gelatin as emulsifier, the separation of benzerie begins immediately, but if the emulsion is left undisturbed for some time before adding the caustic, the separation of the benzene is retarded. The duration of this inhibition period is found to depend upon the length of time which has elapsed between emulsificat'on and addition of alkali. His interpretation is that the gelatin forms a protective film which increases in thickness with time. The older the emulsion, the thicker the layer and the longer the time required for the caustic to destroy the film and break the emulsion.
emulsifying agents i l i stabilizing cniulsioris is due to the forrnatioii of a film of emulsifying agent as a separate phase at the liquid-liquid interface. Since the publication of these papers by Bancroft, Langniuir (17) has suggested that the superficial layer of molecules a t a liquid-gas or a liquid-liquid interface will have a definite orientation. Langmuir also points out that an oil containing polar groups will spread out on water until it forms a monomolecular layer and argues that this layer will he definitely oriented. Langmuir does not discuss the case of emulsifiers at a liquid-liquid interface in an emulsion. Harkins (If), who had independently reached similar conclusions, extends the fundamental idea to emulsions by postulating that the film of emulsifying agent adsorbed at the interface is also definitely oriented. For example, when oil is emulsified in water by means of soap, the carboxyl group of the soap is supposed to extend toward the water and the alkyl radical of the fatty acid of the soap toward the oil. Rarkins says, "Films of solutes positively absorbed at either liquid-vapor surfaces or liquid-liquid interfaces frequently become saturated." This concept seems to imply a monomolecular layer of emulsifying agent although Harkins does not state definitely that the interfacial film can never be greater than monomolecular. Harkins and Beeman (IO), during their studies on emulsion stability, using sodium oleate in different concentrations as emulsifier, did not obtain evidence that any of their soap fiLms became thicker than monomolecular on standing. This concept is supported by GriEn (9),by Finkle, Draper, and IIildebrand (8),andby van der Meulen and Riemann ($0). Griffin even goes so far as to assert that the interfacial layer can only he one molecule thick in the case of kerosene emulsified by soap and that any surplus soap not needed to form the monomolecular laycr will be found dissolved in the aqueous phase instead of a t the interface. Analytical evidence showing an impoverishinerrt of the Figure I~ Irish Muss-Mineral 011 Interface emulsifying agent in tlie outer or continuous phase as a result Coocet~trsiion0.8 per cent, w e 12 days. temperature approxi of the formation of an omulsion has been interpreted as showniately 80' P. (26.61' C.1 ing the formation of an interfacial film by Storch (%) in the case of butter fat emulsified by casein in milk and cream, by Linihurg (fY) has carried out measurements on tlre c a b Briggs ( 4 , 5 )in the case of benzene emulsified in water by soap, phoretic velocity of oil particles emulsified by gelatin. He by Griffin (.9) in the case of kerosene emulsified in water by found that with low concentrations of gelatin the cataphoretic velocity depended on the concentration of the gelaI Received M a y 18. 1931.
:INEh'I&VO CIIJAWISTR Y
of d i d phstic filriis over tlie
drops. Wiegner ($7) liur ohserved tlint passing milk tlirougli a liomogeniaer c:iused an increase in viscosity whiclr he. interprets as d u e t o passage of rnorc casein into the film at the enlarged interface. Lewis ( I S ) has determined the drop number of mineral oil in a solution of bile salts as a function of the concentration of emulsifier. He prepared an coiicenirstion 0.1 per cent. age emulsion and separated thedis50 days, temprinture RPprOli. maiiiy 7O'F. (21.11° C.) perse phase by centrifugal action. The drop number was determined in the clarified aqueous layer and found to have decreased. He argues that this proves that the emulsifier has been adsorbed in the interface and been removed with the disperse phase. Many authors have stated that they have actually seen the 61m. Macroscopically the film has been seen around large drops of tlie internal phase with albumin as the emulsifying agent hy Ascherson (f, 2) and by Clark and Mann (7), with saponin by Ramsden (@ with I casein ), by Robertson (#4),with acacia and with gelatin by Briggs and Schmidt (e),and with cellulose nitrate by Holmes and Cameron (16). I n none of these c a m were the films photographed. An especially interesting experiment has been carried out by Eauser (14) who made moving microphotographs of particles of rubber latex while he pierced a particle with a microscopic glass needle. The microphotographs reveal a liquid droplet emerging from the interior through the rupture and leaving behind a membrane which formed a skin around the particle. Sheppard ($5) has publislied microphotographs showing particles of lead eak gathered at the interface between nitrobenzene and sulfuric acid. Hattori (12, IS) has isolated the hull substance of milk-fat globules by treatment with solvents. Altiiough tlie methods of investigation referred to seem to establish the existence and importance of interfacial films of emulsifying agent, they are troublesome and not weil adapted for a comparison of the properties of films of various emulsifying agents at water-oil interfaces. Nor are these older methods conveniently useful for determining the speed of formation of films a t such interfaces or the factors influencing the properties or rate of formation of films. This information would be extremely valuable for the understanding of emulsification and in the development of practical methods of making emulsions. Method of Preparing and Studying Films
I n the course of studies to determine the most suitable emuisifying agents and the best method of preparation of
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stttblc a d concentrated ornulsiws o S niitx:rzd : & i d of fatt,y oils in water for the inaiiufacturo of imIiwtri:il products, air iiasy ineaiis of prepnririg and studying the Iilins st t.lie liqoidliquid interface was found. Thir; consirtcrl aiinply of plncing :about 30 cc. of a fresh aqiieoos solation of thc emulsifying ngent of known coueentration in a sinall beiikcr and then with n pipet 10 w. of the oil oii top OS the solufying a,gmt, t,aking uarc to avoid mixing. The lxskers were then allomed to staid qiiietiy Sur nimy days and exniniiied at intervals. The emulsifier solii sodium oleate, 0.0:3, 0.06 per cent (Stcrlini., sodiinir glyaiciiolate, 0.03, 0.06 per cent ('ompmy); tricthunolnmirie, 0.03, 0.06 per cent (iisu:iI comincrcial qiinlity); acacia, 0.5, 3.0 per cent; tragacanth, 0.5, 2.0 per cent; Irish moss, 0.4, 0.8 per ceut; gcLTtin, 0.5, 0.3 pcr cent; and saponin, 0.1, 2.0 per cent. Tho oils used were olive oil, castor oil, cod-liver oil, and refined heavy mineral oil (st,aniolind). All of them were the best refined oils on the market. During t h e experiments no antiseptics were used for preserving the emulsifier solutions, in order to avoid complications due to the presence of extraneous materials and in order to test the emulsifiers in their natural conditions. Similar experiments with solutions containing antiseptics are in progress. After standing for four weeks a film was formed in all of the cases tried except in that of mineral oil and an aqueous solution of triethanolamine. Since triethanolamine readily forms films and emulsions with the fatty oils hut not with mineral oils it may be inferred that the effective agent is a salt of triethanolamine and a fatty acid. Some beakers were kept a t room temperature, others at an elevated temperature, 38' C., and still others at ahout 8" C . With a few exceptions the films were kept under observation for four weeks only, as some solutions had begun to putrify hy this time. With the single exception noted, it was apparent that a solid film had formed at the water-oil interfaces as shown by Table I. Microscopic inspection showed that the films are not of hiOlOgiCal origin. I n all except one of the cases examined up to t h e p r e s e n t t i m e a film was formed at the interface, but the films were f o u n d to differ in a p p e a r a n c e and in physical properties, depending both on the e m u l s i f i e r used a n d on t h e oil. There was a l s o a m a r k e d difference in the time required for the fiLms to become visible as shown by Table 11. I n general, increasing the temperature and the concentrations seemed to cause the films to form more rapidly and to become more apparent. Concentration 0 0 8 per cent I n some cases the films were ~ ~ ~ ~ ~ ~pproxt: J ' . ~ ~ f ~ $ readily visible, but in others they were so transparent that they could not he easily seen. In studying these films it was found convenient to use a simple wire ring of about 3 cm. diameter without a handle, or a wire loop of about 3 em. diameter bent so that its plane w a at right angIes to a wire rod which served as a handle. I n some cases when these loops were pushed against the Hms, the latter were not readily ruptured, hut the entire film within the loop could be forced downward several millimeters into the water layer, and the film outside the loop was stretched and formed wrinkles which were easily visible. Figure 1 gives a photograph of such a case, made by pointing the camera downward. The lower aqueous laser contained 0.8 per cent of dissolved Irish moss and the upper layer mueral oil. After
Sodivm "leaf< Sodium gly ocholatr
Triethsnoiamine Geibtin Irish m a s Acacia
sapottin
TraisFsnth
'Table 11-~Spccdof lrilm Formatioo ef Yerioue I;nrulaitiera in Contact alth IMiorent Oil8 (nh\ervation time. 4 weeks) OLlva O1L CAST"* 01L Coo-Llvae OIL siow very slow Very quick Very quick slow very siow very w i c k very Siow Very quick Moderate slow Quick Quick Moderate Quick wick slow Qick Very quick v e v slow Quick Quick Moderate Quick NoxhNnrruae Very quick, within 24 hours Qxick, 1 to 2 days Moderate, 2 t o S d a y s Slow. 8 to 16 d e n Very */ow, over 15 days
standirig undisturbed at a temperature or about 38" C. for 12 days, a ring of platiiruin wire wit,hout a handle was dropped on the upper surface. It sank to the interface where it wan caught by the interfacial film. The ring sank several millimeters below the original level of the interface, but it dragged the film with it and did not break through and sink to the bottom. The film outside the ring stretched and formed obvious wrinklee hut did not break away from the sides of the beaker. The strength and elasticity of some of the fdms formed in this way, especially those made with Irish moss and minera.1 oil or olive oil, and acacia and col-liver oil, are truly astonishing. In otlier cases the films were miicli weaker.
Figure 2 gives a photograph showing a saponin (coneentration, 0.1 per cent)-castor oil interface which was 50 days old. The wire loop broke through and sank to the bottom of the beaker, leaving a wound which was visible for a long time and which is readily apparent in the photograph. Sodium oleate with mineral oil gave a film which was much weaker. KO wriiikles appeared when the film was pushed with t,he loop, and the film, although undoubtedly present, ruptured easily and left a wound which healed so quickly that a photograph, with sufficient contrast to justify an attempt to print it, could not he successfully obtained by pointing the camera from above. However, by pulling the film upward into the o i l by means of a hook, it became clearly risible and several seconds elapsed before it went back to the interface. The a m could be photographed as shown in Figure 3. The beaker in this photograph contained a 0.06 per
MrNaKnL
Ozr
slow
very siow N o film ailcr 4 very slow very slow Quick very slow
weeks
Very quick
cent sodiuni oleate solution on wliicli iiriiieral oil had stood for 21 days. The remarkable tliing ahout these soap films is that, although they originally formed slowly, the fdms were promptly reformed after being ruptured. Figure 4, A and B, shows Hms at a cod-liver oil-acacia, 0.5 and 2.0 per cent, interfacx. They were 28 days old. On the twenty-third day a wire loop was sent through the interface. The wound reliealed leaving protuberances which made the film casily visible. Figure 4, C and I), also shows the quite opaque films at cod-liver oil- triethanolamine, 0.03 per cent, and cod-liver oil-nodiiim oleate, O.OF per cent, interfams. The iiiflnence of the oils on the properties of the films was more definite than that of the emulsifiers. Cod-
liver oil. Castor oil iias a lesser tenkeucy to form films. The film were transparent and weak and did not wrinkle in any case. Mineral oil has the least tendency to form fdms of any of the oils studied. IIowever, film were seen with the higher concentration of the emulsifiers, except triethanolamine, during the four weeks they were observed. Thin film which were tough enough to give wrinkles were obtained with tragacanth and acacia. At the interfaces where film formation took place there is no doubt that the film was polyniolecular, as it was readily visible and coherent. It therefore seems probable that even in true emulsions the films formed may be thicker than monomolecular, provided the ratio of etnulsifying agent to interfacial area is great enough. The study of the films in this maimer has helped eonsiderably in the selection of the best emulsifier blends-that is, those which give both a quick and strong film formation for
Iil'DUSTRIAL A X D ENGINEERING CHE.VISTRY
September, 1931
the industrial preparation of a stable emulsion. There are many other factors that must also be taken into consideration such as the protection from destruction by fermentation, putrefaction or mold growth, suitable viscosity, low interfacial tension, p.H control, and degree of dispersion and means of attaining it. Quantitative measurements on the strength of the films are in progress. Summary A simple method of studying the formation and properties of films of emulsifiers at liquid-liquid interfaces has been developed and the properties and behavior of such films with eight different emulsifying agents and four different oils described. These observations have been helpful in the determination of suitable conditions for preparation of stable emulsions. Literature Cited (1) Ascherson, M e m 4cad S a , 7 , 837 (1838). ( 2 ) Ascherson, Muller's A r c h , 1840, 44 (3) Bancroft, J Phys. Chem , 16, 177, 345, 475, 739 (1912) 17, 501 (1913). 19, 275, 513 (1915)
1019
Briggs, J . Phys. Chem., 19, 210 (1915). Briggs and Schmidt, Ibid., 19, 479 (1915). Briggs and Schmidt, Ibid., 19, 496-7 (1915). Clark and Mann. J . Bioi. Chem.. 62, 179-SO (1922). Finkle, Draper, and Hildebrand, J . A m . Chem. SOC.,46, 2780 (1923). Griffin, Ibid., 45, 1648 (1923). Harkins and Beeman, Ibid., 51, 1692-3 (1929). Hdrkins. Davies, and Clark, Ibid., 39, 592 (1917). Hattori, J . Pharm. SOC.Japan, N o . 616, 123 (1925) Hattori and Ogimura, Ibid., 49, 1147-9 (1929). Hauser, IND.ENG.CHEM.,18, 1146 (1926). Holmes and Cameron, J . A m . Chem. Soc., 44, 66 (1922). Holmes and Williams, Ibid., 47, 1323 (1925). Langmuir, Met. Chem. Eng.,16, 468 (1915); J . A m . Chem. Sot., 39, 1848 (1917). Lewis, Phil. M a g . , [6] 16, 499 (1908). Limburg, Rec. fraw. chim.,46, 885 (1926). Meulen, van der, and Riemann, J . A m . Chem. SOC.,46, 876 (1924). Nugent, Trans. Faraday Soc., 17, 703-7 (1922). Prieger, Biochem. Z . , 217, 331-6 (1930). Ramsden, Proc. Roy. Soc. (London),.1,72, 160 (1903). Robertson, "Pbysikalische Chemie der Proteine," p. 308 (1912). Sheppard, J. Phys. Chem., 23, 634 (1919). Storch, .4nalysl, 22, 197 (1897). Wiegner, Kolloid-Z., 16, 118 (1914). Wilson and Ries, Colloid Symposium, Monograph, 1, 145 (1923).
Calculations on Water-Gas Equilibrium Choice of Suitable Molecular-Heat Equations Heat of Reaction and Free Energy as a Function of the Temperature' W. M. D. Bryant CHEMICAL DEPARTMENT, Du PONTAMMONIA CORPORATION, U'ILMINGTOX, DEL.
The thermal and equilibrium data for the water-gas reaction are assembled and discussed. AHzgs.l for the reaction COn(g) H2(g) = CO(g) HZO(g) is calculated to be 9751 calories. This value is combined with molecular heat equations from several sources to yield expressions for A H a s a function of the temperature. These equations, in turn, are used to show the order of agreement existing between molecular-heat equations of different sources. For this reaction specific heat equations of Lewis a n d Randall, of Eucken, and of Eastman give similar results. Partington and Shilling's equations do not agree with these results. Expressions for the free-energy change, A F " , and its temperature coefficient are derived, as well a s for the
corresponding reaction isochores. The equilibrium value of Neumann and Kohler a t 1259.1" K. is used to determine the integration constants. It is shown that the molecular-heat equations of Lewis and Randall, of Eucken, and of Eastman, satisfactorily reproduce the experimental equilibrium constants in the water-gas reaction. It is also shown t h a t Partington and Shilling's molecular-heat equations fail to reproduce these values. AFo198.1 for the water-gas reaction is calculated. Numerical results ranging from 6828 to 6750 calories are obtained depending on the specific heats used. These values constitute the first step in a recalculation of the free energy of formation of carbon monoxide and dioxide.
H E recent appearance of valuable new thermal and equilibrium data for the water-gas reaction makes it desirable to construct new expressions for the corresponding heat of reaction and free-energy change. These new data are of such a character that they fix within narrower limits the older directly determined equilibrium values, and permit of a more reliable extrapolation of the free energy to lower temperatures. Aside from its technical value, the water-gas equilibrium affords access to the free energy of formation of the oxides of carbon. The early thermal data for the water-gas reaction are fully summarized by Haber (16). Apparently no direct experimental work has been done on this equilibrium above 1000° C. since that of Haber and Richardt (19) and that of Allner ( 1 ) . The lower range, which at present is of greater industrial importance, has been rather extensively studied. The obscure
Karlsruhe thesis of Engels (10) has been cited by Keuniann and Kohler (SO) with whose work Engels' values are in good agreement. Reinders (38) mentions an equally obscure paper of van Groningen (1;) and while it has not been possible thus far to obtain van Groningen's actual figures, Reinders declares them to be in harmony with the work of Hahn (20),and of Haber (16). In 1924 Eastman and Evans ( 7 ) suggested that the equilibrium measurements of the carbon nionoxidecarbon dioxide and water-hydrogen ratios over ferrous oxide and ferric oxide might be combined to yield true values for the water-gas equilibrium. The values so obtained were consistently about 40 per cent higher than previous direct determination would indicate. To clear up this matter, Neumann and Kohler (30) redetermined the values of the constant,
+
T
1
Received May 12, 1931.
+
