T H E THEORY OF EhTULSIFICATIOS.
VI1
BY ITILDER D. B.I?;CROFT
It has been noticed that after a while certain colloidal solutions become covered with an apparently solid film. Since this film forms a t the foam interface, it might be considered in connection with the adsorption of gases by liquids. Experimentally, however, the phenomenon is more akin to the emulsion formation and consequently I prefer to consider it as an intermediate case. The best experimental work on the subject has been done by Metcalf in Ostwald's laboratory.' IThen a few drops of a peptone solution are allowed to spread o\-er a surface of pure water, a solid elastic film forms in a short time on the surface of the water. Siegfried's experiments? have made it probable that ordinary peptone is a mixture of different allied substances rather than a definite chemical compound. It was, therefore, desirable t o determine whether the formation of a solid film mas dependent on the complex nature of peptone or whether a definite chemical compound is able t o form such a film. I was fortunate enough to secure a sample of pepsin-fibrinogen-peptone-u which had the formula C)21H340LI. It was prepared by Professor Siegfried and was kindly presented to me by D r . IT. Neumann. Both Profeisor Siegfried and Dr Seumann are convinced that i t is a definite chemical compound. X 3 2 percent solution of this substance was prepared and two drops were placed on the surface of pure water. A test with a piece of cork showed that the surface was still mobile. -4few more drops were then added, whereupon a solid elastic film was formed. On standing a few minutes this film disappeared completely and the surface became mobile again. On adding another drop, a solid film was formed again b u t disappeared in time, though more slowly than the preceding one. This was repeated several "
I
1IetcalE: Zcit. phys. Chem.. 52, I ( 1 9 0 j ) . Siegfried: Zeit. phy-siol. Chcrn., 35, 164 ~ 1 9 0 2 138, ; 2j9 I 19o3'1.
times until a t length a permanent film was obtained This was allowed t o stand for twenty hours, a t the end of which time it had increased perceptibly in viscosity This euperiment shows conclusii-ely t h a t the formation of solid films of peptone does not depend on peptone being a mixture " T h e experiment also shows t h a t the solid matter, of u hicli the pepsin-fibrinogen-peptone film is formed, is somei t hat soluble in water The film continues t o dissolve until a t least the upper la! ers of the water phase are saturated with the substance which forms the film. The film then becomes permanent and el-en increases in strength with time just as do films of ordinary reptone At any rate this is the most obi ious explanation This assumption of a slight solubility of the material of which the film is formed accounts for the prolnhlc miriimum o b s u i ed m ith a n ordinar! peptone film. Thi5 elplanation is niclcle more probable 11j the follon ing intercsting experimcnt n ith a i-ery concentrated peptone solution \I'heii one drcp of >uch a solution is alloned to fall on a n a t e r sarface, it spread\ out somen-hat on the suriace. hut the greater part of the drop sinks to the bottom of the dish \IThcre the solution comes in contact with water there is formed an e a i l y iisible mhite mrnibrane A4tfirst the memlirane has the form of two discs connected by a x-ertical tube Thii 5oon disappear5 and no 4gn of it remains, because the peptone has dissoll cd completely in the n atcr It seem5 probable that the white membrane is formed of the same suhttance as the ordinar!, invisible surface film of peptone If 5 0 , a certain solubility of t h e film substance has been shown ' Metcalf found that a film formed at a low temperature i i mow clastic than one formed a t a higher temperature, hiit t h c two become alike when brought to the same temperature 1' he propertics of the film varj- n i t h the time * Immediately after dropping a o j ' peptome solution on a water surface, the latter was tcsted bj- means of a piece of cork slight surface iiscosity could be detected but no elasticity The water surface in the blank test was very mohile The dishes were coxered and left for an hour and a
half The surface viscosity had increased perceptibly in the dish containing peptone. Twentj--four hours later a diitinct elasticity could be detected, and this increased perceptibly in the next twenty hours. During this whole time the water surface in the blank test remained as mobile as a t the beginning. The experiment was repeated a number of times in substantially the same way and there was always a n increase of elasticity on standing. This increase often took place more or less irregularly but it was always to be detected. The film always went through all the stage5 from a \-ery slight viscositj- a t first to a final distinct elasticit!It made no difference whether distilled or t a p water were used "n'hen one drop of a 0 . I j' solution was used inbtead of a o 3 ( ( solution, no surface 1-iscosity or elasticit>- could he detected either a t first or after forty-eight hour5 \\-hen two drops of this solution were taken, all elastic film waq obtained a t an>- rate in some cases. \\'ith a 0.6' solution the film was distinctlv viscous from the start but not elastic At the end of six hours the surface was distinctly elastic and the elasticit>-had increased appreciably by the end of twent? four hours. Solutions containing I .z' and 2 . j( peptone differed from the preceding one only in t h a t the filnis were elastic when first formed The elasticity of these filnis increased with time ' ' X series of special experiments led Metcalf t o the helief t h a t diffusion is practicallq- negligible' under ordinan- conditions. On the other hand the following experiment seems to me to show t h a t some diffusion must take place. '.*A solid, elastic film n-as formed by bringing sex7eral drops of a 2 5' peptone solution upon a water surface and allowing it to stand for 47 hours Powdered cork was strewn over the iurfnce and then about a third of the film removed 11~-means of filter paper. Small particles of cork which were left by the filter paper were drawn ti>- capillary action to the edge of' the film and thus made this edge a clearly defined line The undi>-
>
-
l
Zeit p h v s C h e m ,
52, ' j
1905)
turbed portion of the film was just as strong and elastic as before, while the free water surface was as mobile as ordinary water. Sfter four and one-half hours, the old film was still solid, the new surface mobile, and the line separating them clearly marked. The position of this boundary line had not changed perceptibly. Sineteen hours later the old film was still there and the boundary line was as sharp as ever; but the new water surface was now covered with a distinctly elastic film. The experiment was repeated twice with one drop of a j.3‘( solution and also with one drop of a 2 . 7 ( ; solution. These films were left untouched for 89 hours and then a portion removed as described. The result was the same as before. “ A film was prepared b\- touching the surface of the water with a glass rod which had been dropped into a one percent solution. The liquid below the surface was watched carefully; but there was no sign of any peptone sinking down The dish was then covered and left for almost four days, after which time a distinctl>- elastic film could be detected. A portion of this film was removed with filter paper. The edge of the old film moved forward a little over the new water surface. This first rapid mox-ement soon came t o an e n d , but the film continued to spread slowly over the water for about twenty-four hours. At the end of this time, the free water surface had become very small: b u t was still separated from the old film by a perfectly distinct boundary line. The water surface is, however, covered with a perceptibly elastic film: The old film had thus spread t o some extent over the water surface and a new film had formed over the rest of it. There was no sudden spreading of the film over the whole water surface as is the case when a film is first formed.” Since it had been suggested by Schuttl that the film might be due t o oxidation, Metcalf made some experiments in an atmosphere of hydrogen, getting the same results as in air. IT’hile this is satisfactory as a proof that the film is not an 1
Drude’s . i n n , 1 3 , 7 x 2 ( 1 9 0 4 1
oxidation product, it shows nothing as t o whether the presence of a gas is essential or not. Metcalf did not consider the possibility of peptone adsorbing air or hydrogen and being floated up by it. At the end of his paper IIetcalf discusses five possible hTpotheses t o account for the formation of a film when a solution is allowed t o stand, or when a drop of solution is allowed to fall on a water surface. The first possibility is that the film is due to oxidation. The films that form on molten lead or on zinc amalgam are due to this cause; b u t the experiments with hydrogen exclude i t in the case of the peptone films. The second hj-pothesis-due t o llarangoni-is that no solid film is formed a t all and that it is merely a question of a lowering of the surface tension with increasing surface viscosity. Since ether lowers the surface tension of water without increasing its surface viscosity, there is no necessary connection between these two phenomena. If there were no solid film, the surface tension would keep the surface uniform and we could not long have the phenomenon of part of the surface being mobile and the other part not. The third possibility is that peptone is a mixture of two substances, one of which is soluble in water and the other not, the film being formed of the insoluble substance. This seems to be negatil-ed by the experiment with the peptone which was a definite chemical compound. Of course, hIetcalf did not consider the possibility that the peptone is not soluble to any appreciable extent in water. He considered that his 5.3(; solution, for instance, contained the peptone in true solution, whereas now we know that he was dealing with a two phase colloidal solution. The hypothesis goes by the board, however, if we can show t h a t the film formation can be accounted for without postulating a mixture of substances. As fourth hypothesis Metcalf suggests that the surface film may be a saturated solution. Since a dissolved substance concentrates in the surface layer in case it lowers the surface tension of the liquid, it is theoretically possible that the surface layer might be saturated when the mass of the solution
was not. Metcalf has overlooked the possibility t h a t the solubility in the surface layer might be greater than in the mass of the liquid, in which case the hypothesis would be untenable. He rejects the hypothesis on a different ground “ T h e separation of a solid phase a t the surface of an extraordinarily dilute solution is difficult t o account for on the basis of this hypothesis alone. The only possible explanation would be t h a t the surface concentration is greater than the saturation concentration, for only under these conditions could the solid precipitate. Since the surface concentration is kept in reversible equilibrium with the concentration in the body of the liquid by the surface tension, saturation in the surface layer seems very improbable when one is dealing with such extraordinarily dilute solutiom as were found t o give distinct films. In the case of pepsin fibrin-peptone-a a perceptibly elastic film was obtained with solutions which contained less than one ten-thousandth the amount necessary for saturation. In this case the ratio between the concentration in the surface and in the body of the liquid must be a t least 10,000 : I according t o the hypothesis If we consider the molecular weight3 of the compound as 514,the molar concentration of the solution is about o 0001 mol per liter. If the law for the osmotic pressure of dilute solutions holds in this case, the osmotic pressure in the body of the solution is about 0 0 0 2 a t mosphere. The osmotic pressure of the saturated solution must be a t least ten thousand times as much or about 2 0 atmospheres. The’ difference between these two pressures, practically 2 0 atmospheres or ~o,ooo,ooodynes, represents the osmotic pressure tending to make the solute diffuse out of the surface layer. The total surface energy of water is only about 81 ergs per square centimeter. The intensity factor of this energy is 81 dynes per square centimeter It is difficult t o believe t h a t this can be in equilibrium with an osmotic AXetcalf Zeit phys C h e r n , 5 2 , 4 1 ( 1 9 0 j ) 2 A saturated solution contains a t least 59‘0 substaiice ment a solid film mas obtained with a o o 0 j g f 0 solution ? Zrit phvyiol Chcm 38, 2 9 7 i r q o i l 1
In one experi-
Tlzc Theor31 of Eniuls-pothesis seems to account for the observed phenomena in connection with the formation of solid films on the surface of concentrated and ol’ dilute solutions. It avoids the difficulties which have been pointed out as besetting the
other hypotheses It is easy t o account for the formation of solid films on the surface of very dilute solutions. The only assumption that we have t o make is t h a t the solubility of thenew film substance is extraordinarily small I n t h a t case a very small amount will be sufficient to form a saturated solution and thus give rise t o the conditions essential t o the existence of a permanent, solid film “ T h e solid films form more or less slowl>- a t the surface of the solutions It ma>- be hours or days before elasticity can be detected, the time varying with the nature of the substance; the concentration, etc On the other hand solid films form instantaneously when a drop of solution spreads out on a surface, always provided the solution is not too dilute Our hypothesis accounts for the tremendous difference in the rates of formation of films formed by the two methods V-hen the drops of solution come in contact with the water surface there are three surface tensions to consider. n‘ater-air. solution-air, and solution-water. Since the first of these k larger than the resultant of the other two. it acts as a force pulling the drops out suddenly to a thin film. whereby the surface is increased enormously Gibbs’ and van der Slensbrugghe? have deduced independently that a sudden increase of the surface of a liquid lowers the temperature and increases the surface tension. This can be shown simply in a qualitative manner If work is done upon a system, the intensity factor of the energj- against which the work is directed must increase.’ force doing work develops in the system a resistance t o its own action. If work is done against the volume energy by compressing a gas, the intensity factor of that energy, the pressure, increases. If a drop of peptone soluticn is suddenly spread out into a thin film b y a force, the surface tension of water, work is done against the surface energy Thermodynamische Studien, 3 I 9.
* h l h Xcad. Belg., 43, 39
(1878).
[He should have said t h a t i t may increase and cannot decrease. If the liquid a n d vapor are compressed, there is n o increase of pressure so long as any vapor is present.-\V. D. B . ]
Therefore, there must be an increase in the intensity factor of this energy, the surface tension. The question as to the relative changes in surface tension, temperature, etc , will depend on the conditions of the experiment: but the:- must be of such nature as to act against the external force Devaux's brilliant experiments on the limit of e-ipansion of such solutions on a pure water surface are an experimental proof for the increase in the surface tension of the drop caused b>*the formation of a film. If the water surface i z large enough, the drop spreads out rapidly in the form of a complete circle to a definite size and then stops. -It first the resultant of the two surface tensions, solution-air end solution-water, is smaller than t h a t of R-ater-air. Thtrefcrc. the drop is drawn out until the resultant of the two ctlriacc' tensions is equal to that of water-air. when equilibrium is reached. lTT'hilethe drop is spreading, an increase in the resultant of the two surface tensions, solution-air and solutionwater, must take place These are the surface tensions a t the surfaces of the film.' lye now ha\-e theoretical and experimental reasons for the assumption t h a t when a drop is drawn out to a film. there is a distinct increase in its surface tension--the intensity factor of its surface energ>---as well as in the surface- the capacity factor of the surface energy.' During the formation oi the film there is a marked and sudden increase in the inteniity factor and capacity factor, both of which affect t h e chemical action This, of course, makes itself felt by a more rapid chemical change than 11-ould occur a t the surface undcr normal conditions. 1l.e. therefore, expect that the reaction I elocit! will be high a t first and will then decrease asymptotically The cause of the chemical action is the force tending to de( '
,' I t should also l ~ ?noticed t h a t , \\-it11 a lilm oi a certain t h i n n e ~ - .t h i capillary force.: oi the I n t e r in t h e iilin Inuit have sonie ili'ect 011 surface tinqiiiii, solu tioti-air. is compressed, the iiiteniity factor oi the \-olurne energy increases b u t t h e capacity factor decreases. In t h e present case both intensity lactor a n d c a p i c i t y factor iiicrease. T h e difference is due t o t h e fact t h a t :i i-i,e of temperature iiicreavs t h e pressure of a gas but rlecrcxcs t h e surface tii14oii
crease the surface tension. This force decreases in amount as the reaction goes on and as the surface tension becomes smaller. '. Our h\-pothesis offers a n explanation for the previously mentioned maximum and minimum of strength. Such a maximum and minimum calls for a t least two factors acting simultaneouslj-. I n this case the two factors are the chemical reaction which tends t o strengthen the film and the dissols-ing of the film substance, which tend3 to weaken it. Through the simultaneous action of these two factors it is easy to follow arid account for the alternating increase and decrease in .trength. IT-ith a drop of fairly concentrated peptone solution, v,-e get solid film a t once. Under these experimental condiLion,. i t is probahle t h a t the chemical reaction T-elocitJ- far C .
exceeds the rate of solution. There is therefore a rapid increase in strength. ,After the first rapid change the chemical reaction of course goes much slower and it is quite possible tl-,::t for a n-hile the rate of solution might be greater than the chemical reaction i-elocity. This ~t-ouldcause a decrease in the strength of the film. If the time necessary for the chemical reaction to reach its final eyuililiriuin is greater than t h a t ary i o saturate the solution, the film must begin to grow again after a certain time. It is eesq- t o draw two asymptotic c u n e s whose resultant shows such a inaxinium and minimum. i f the solution is s-cry dilute. the first rapid change of t face tension takes place relatiyely dowl>- and we shall a more gradual development of the film. This explanation is purely qualitative : hut it is hoped some day to get the Iiecessir>-data for a quantitative study of the phenomenon. It is m-orth while to consider another characteristic of the film in the light of our hypothesis. It has been shown experimentally t h a t when a thick film is brought in contact with a fresh water surface, it does not spread out instantaneously over it. The film stretches more or less depending on the conditions of the experiment and a new film forms on the fresh surface without masking the boundary line of the old film. T h e surface tension of the fresh surface is evidently "
ll-ilder D . Baticrojt greater than t h a t of the surface on which the film rests. Therefore, there is a distinct tendency for the film t o spread over the new surface. A solid film resists the pull for days without any blurring of the boundary line, and the pull ceases finally because of the formation of a new film on the fresh surface and not because the old film finally spreads over the surface. This is a distinct sign t h a t the film acts essentially like a solid and not like a liquid; and also t h a t it is partly soluble in water. Thermodynamic considerations show clearly t h a t a solid and sparingly soluble film must equalize the surface tensions by a sort of distillation through the water, which will carry the film substance from the old film where the surface tension is low t o the new surface where the surface tension is higher. The continuation of this process cuts down the high surface tension and increases the low one; it will continue until equilibrium is reached and the two surface tensions have become equal. ” I have given Metcalf’s views in his own words because this is a very interesting illustration of the way in which a man may start from false premises and yet work out a very plausible explanation of the phenomena. Metcalf starts with the assumption t h a t peptone forms a true solution and consequently he has t o account for the film formation by postulating t h a t peptone changes into an insoluble modification. This a t once makes the phenomenon a special one and not a general one, which is in itself a defect because a good many substances form these films U‘hile it is legitimate t o postulate in regard t o any one substance t h a t it occurs in two modifications, a soluble and insoluble one, this becomes very unsatisfactory when the assumption has t o be made in regard t o a large number of substances T h a t , however, is a debatable matter. The error in Metcalf’s work lies in the assumption-a perfectly proper one a t the time he made it-t h a t peptone is really soluble to an appreciable extent in water We know nowadays t h a t peptone forms a colloidal solution with water and consequently we do not have t o postulate an
insoluble modification. All we have t o do is t o get the peptone concentrated in the surface layer. There are several ways of looking a t the matter and it is quite possible t h a t each one of them is partially true IT-e may consider t h a t the colloid adsorbs gas and is floated t o the surface thereby. McBain has shown’ t h a t it is verv difficult t o remove all the air from soap solutions and t h a t previous vapor pressure determinations mere consequently worthless If the concentrating of the peptone in the surface layer is due t o adsorbed gas, no surface film should be formed in the absence of air. This experiment was not tried by Metcalf. I doubt very much whether a gas adsorption is the sole factor in the formation of a solid surface film Nagel? has shown t h a t a solid film forms over a fuchsine solution even when all air is removed Since the surface of a liquid is in a different state from the mass of the liquid, we may consider t h a t we are dealing with two liquid layers, the upper one being infinitely thin. IVe might then have the colloidal substance concentrating a t the dineric interface and coalescing there to a solid film. On this hypothesis, the solid film would theoretically he just below the surface instead of itself forming the surface: but this difference would be very difficult t o detect experimentall>-. X third possibility is t h a t a substance with a low surface tension may tend t o concentrate in the surface ex-en though i t be not in true solution II-hile the Gibbs theorem applies only t o substances which are actually in solution, there is nothing to prevent there being a similar theorem in regard t o suspended particles. Stark3 has called attention to an interesting special case where something of this sort takes place “If one drops some soot into chloroform in a watchglass and covers the latter with a glass plate t o prevent evaporation of the chloroform, the particles of soot sink because of their higher specific weight and collect into little balls a t XIcBdin and Taylor Zeit phys Chem , 7 6 , 183 ( 1 9 1I ) . Drude’s A n n , 29, 103j (1909) \Vied .Ann , 65, 288 I rS98)
j26
TI-iZdci. Ll. Bailcroft
the bottom of the watch-gless. *!t least, this is what happens in the dark. If the watch-glass with the chloroform is brought into the sunlight, the bs!ls of soot begin t o move vertically upwards. 1Vhen they reach the surface, the particles scatter suddenly and spread themselves uniformly over the surface, remaining thus scattered so long as no 5hadow falls upon them. If a ihadow from a knife or a lead pencil, for instance, falls upon the chloroform wrface which is coated n i t h soot, the particles of soot in the shadow and on the edges of it jerk together forming a thicker mass in the shaded surface than in the illuminated surface. If the optical shadow is made to rLo\Te suficientlq- slon.ly, the soot shadow moves with it If one removes the object which is casting the shadow., the soot particles which 11 ere in the shadow scatter again iristantan (> ou s1y . “ T h e three processes of the rising of the soot particles, the scattering a t the surface, and the concentrating in the shadow are not hard to account for The soot is heated more by the sunlight than the transparent chloroform. The liquid immediately in contact with the Loot is therefore heated more than the rest of the chloroform ’Ihe chlcroform in contact with the soot hcconies less dense than the rest of the chloroform and rises. carrJing to the surface with it the soot which is only a little denser Il-herever there is a ball of soot the heating is correspondingly greater and there is a lowering of the surface tension A4tthe points of higher surl’ace tension the surface contracts, expanding where the surface tension is low and, thereby. ripping the hall of soot to pieces I n this way the soot is icattered ox-er the surface If a portion of the surface is shaded. the particles of soot in the shadomlose heat by conduction and radiation, while the surface exposed t o the sun is heated continually 11-e halre, therefore. a colder portion with a higher surface tension bordering on a warmer pcjrtion with a lower surface tension The cooler surface contracts, carrying with it the particles of soot and concentrating them The force of gravity and the surface tension act together both when the particles of soot are scat-
tered and when they are brought together, but the rapidity of the action seems t o show that the surface tension phenomena are the dominating ones n‘hile the difference in density between the soot and the chloroform is not great, this is not the important factor because Quincke’ has observed a somewhat similar action n ith silx-er in a silvering solution and has accounted for it in e l actly the same way, h>- poStulating increased heating of the silver particles and consequent decrease of the surface tension of the solution in contact with them Once we admit t h a t suspended particles tend t o conceiitrate in the surface, Metcalf’s results become quite obvious IT-ith very small amounts of peptone, the disintegrating or peptonizing action of the mass of the liquid is the predominating factor and the film disappears 113th higher concentrations of peptone, the concentration of the peptone in the surface is the important factor and the film gets stronger on standing, partly because more peptone is brought up from the bottom and partly probably because the coalescence to a solid film becomes more perfect with time Metcalf nould have adopted this explanation if he had not been frightened out of it bj- the bugbear of an ox-erwhelming osmotic pressure. T h a t disappears as soon as we recognize t h a t n e are dealing with a two-phase system throughout and not with a true solution Once the film is fairlj- formed, it does not disintegrate readily or rapidly and consequently a fresh film furms over half the surface through peptone brought there from the mass of the liquid rather t h a n through a spreading of the old film though this latter ma!- occur t o some extent The peptone solutions differ fundamentall>- from mercurJsurfaces or surfaces of zinc amalgam which become coated On the other with a solid film of oxide when exposed to air hand the caSes of albumin. ferric acetate, and saponin are analogous t o t h a t of peptone Aqueous solutions of fuchsine ”
~I
-
Lh-iidc 4 hchutt 1l)id A
\
~
7, S9 ~
~
I
1902 ~
13, 743 (19041
and of crystal violet have been studied by Rohde' who found that solid surface films were fcrmed which could easily be recognized by their metdlic luster. " T h e surface of the liquid becomes more concentrated, then T. iscous, and lastly solid and brittle, so that finally the solution is covered with a film of solid dye There is nothing t o conflict with the assumption that the molecular forces which cause the precipitation of the solid dye are the same which give rise t o the surface tensions of liquids The strong photoelectric action of aqueous solutions of fuchsine and methyl violet is due exclusivel!- to a surface which has aged Statements as to the photoelectric action of these solutions are, therefore, worthless unless the degree of ageing is taken into account " Sagel? has studied the formation of solid films with man!- different solutions. I n the case of cobalt chloride solution he showed that the film was a cobalt carbonate due t o ammonium carbonate either in the water or in the air 115th potassium permanganate solution, the color showed t h a t the film was an oxide of manganese, presumably hydrated. 113th iron salts we know that the surface film is due to hydrous ferric oxide Of the solutions studied by Sagel the following gave rise to solid surface films on standing. NiCL, N i i S O J ? , CoC12, CoS04, FeCl?, FeS04, K I F e ( C S 6 ) , JfnSO,, MnCl?, P b ( S O ? ) ? , AgSO,l, KMnO( I n all these cases the film was undoubtedly an insoluble salt which had been formed by oxidation, hydrolysis, or by interaction with some salt in the air or in the water. M'ith colloidal silver, Sagel observed a gradual formation of a silver film. This appears to furnish satisfactory evidence that colloidal particles tend t o concentrate on the surface. In this particular case no experiments were made t o determine whether a gas adsorption played any part or n o t , but Xagel did show that a solid film forms over a fuchsine solution even when all air is removed I n the case of fuchsine solutions, Nagel considered that there had been a change to another and insoluble modification; but the proof of this is ' Drude's A n n , 19,9 3 j (1906 - I h i d , 29, 1029 (1909)
distinctly far from satisfactory The formation of solid films by electrolysis is, of course. not analogous t o the formation of solid films with peptone and there are still SO many obscure points in regard t o these films' t h a t it is scarcely worth while t o discuss them now The general results of this paper are as follows I The Gibbs theorem in regard t o the concentration in the surface film of substances which lower the surface tension applies only to the substances in true solution It is probable t h a t a similar theorem holds for suspended particles and it is probable t h a t this second theorem accounts for some of the discrepancies observed when people have tried t o apply the theorem of Gibbs quantitatively 2 Since peptone does not dissolve in water t o any appreciable extent, the concentration of peptone in the surface film does not gi\-e rise to the enormous differences of osmotic pressure calculated by Netcalf. 3 TT-hile the presence of adsorbed gases may be a factor in causing the formation of a solid film, it is not the sole factor and is probably a minor factor in most cases 4 The solid film which forms over mercury or 01-er liquid zinc amalgams is an oxide n'hen solid films are formed in solutions of ferric salts, the film is undoubtedly hydrous ferric oxide n'ith cobalt chloride solutions the film i z a carbonate of cobalt formed by the action of ammonium carbonate j There is no reason for postulating the formation of a special insoluble modification with peptone The peptone particles concentrate in the surface and coalesce t o form a solid film -4 similar result can be obtained with colloidal silver It is not quite certain to what the solid film is due in the case of solutions of fuchsine 6. The formation of a solid film a t the surface of a liquid is an intermediate case between the adsorption of solids a t a dineric interface and the flotation of solids by adsorbed gases ('oi
'
iieii 1 u i I e r b i / \
Gubhin IVled Anii, 32, I 1 4 ( 1 8 8 j 1 , LIylius and Fromm Ihlti Freundlich and S o \ i l o \ r Zeit Elebtrochemie, 16,394 1 9 1 0
j 9 3 i 1891
51,
