T H E
J O U R N A L
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PHYSICAL CHEMISTRY Registered in U . 5. P a t e n t O f i c e
@ Copyright, 1964, b y the A m e r i c a n Chemical Society
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VOLUME 68, NUMBER 12 DECEMBER 15, 1964
Soap Films and Some Problems in Surface and Colloid Chemistry’
by Karol J. Mysels Department of Chemistry, Uniuersity of Southern California, Lo8 Angeles, California 90007 (Received M a g 19, 1964)
Once their behavior is understood, soap films become a powerful tool for the study of surface and colloidal phenomena. This is because their geometry is well defined, their monolayer structure is relatively simple, and their behavior is easily observed. The formation and evolution of relatively thick films are controlled by ordinary hydrodynamics, without any indication of the existence of rigidified aqueous layers near the surface. Thinner films show the effect of both double-layer repulsiot and van der Waals attractions. These forces balance at a thickness of the order of 100 A. Some problems in the interpretation of recent measurements of these equilibrium thicknesses and some approaches leading to further information about ttese forces are discussed. Under certain conditions the films thin even further to about 45 A. This is a thermodynaniically stable state, which, though first observed by Newton, is still far from understood.
Soap films-the gossamer sheets which form a child’s soap bubble, the pleasurable head on beer, or the distressing overflowing foam of the production vesselhave a respectable scientific history since the days when Robert Hook$ first called the attention of the Royal Society and of Newton to the optical phenomena which they exhibit. They have assisted in the development of the theory of optics,a of capillary f o r ~ e s , and ~ . ~ of minimal area problems6; they have served as delicate tools for detecting the magnetism of gases7 and as analog computers in solving differential equations with coinplicated boundary conditions.8 Today soap films serve science again in the elucidation of a number of problems in surface and colloid chemistry such as those
of phase transitions in monolayers, of the structure of solvent in the neighborhood of a surface, of Gibbs film elasticity, of the ma,gnitude of the double-layer repulsion, of the law of van der Waals attraction a t ~
~~
(1) Based on the Kendali Award Lecture presented at the 148th National Meeting of the American Chemical Society, Philadelphia, Pa., April, 1964. It reviews our work on films of ionic surfactants, which began under a 1-year grant from the American Chemical Society Petroleum Research Fund, was supported for several years by the Air Force Office of Scientific Research, and now continues with support from the National Science Foundation. (2) R. Hooke, Communication t o the Royal Society, March 28, 1672; T. Birch, ”History of the Royal Society,” Vol. 111, A. Millard, London, 1757, p. 29.
(3) I. Newton, “Opticks,’: Book I, Part 2, expt. 4; Book 11,Part 1, obs. 17-21, London, 1704.
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mcdiuiii rarigr, and of fa1:lors govrri~ii~g sprcilic ionic iiit i:ract,ious. IC;c:rp~ii~iicnlal1~nrlcgroiin.d. Jiist :is t IIV classical applicat ioiis of soap liliiis rquirrd t lw d r v r l o p ~ i i r ~ ~ t of appropriate urm t r e h ~ i i q i ~&rr ( ~ s ~ Srwtou first put. a snap tiutihlc undrr a hdl jar to protvet it from air currciits and cxccssivc nvaporat,ion3 sn 1hr rrcr~rt devclnpnlrnts liavc roqiiirt:d iicw t~xprriliiriital approaclirs t,o produco the drsirrd qiiantitativr data. In addition, a urw look mas 11rc(lcuiat t l i v Iiasic nircliarrisurs by which a soap liliii is formod iiiitially as a rat,har thick slicct. of t,lic order of 1 and tlmi tliiiis uiorr or less gradually to an rquilihrium structiirr whose lhicknrss may bc soiucmhat Irss than 50 ;kg The basic condit.iou for ohswviug soap filiils ovrr ally 1engt.h of t,iinc: is t,hat ovaporatioii 1)r ahsmt so that closed vcsscls aud good thrniiostats or i i i i n r i t ( . circular filum couiplet.cly surrouirdrd by a Iiirriiscus'Oarr iisrd. I'lateau'~'.~~ expcdieut of adding a hygrnscopir agriit , glycerine, t.o prevcirt bursting hy rvaporatiou has t,hr basic disadvant.age that t.ho i:oiiiposit.ioii of thr filii! t l i r rigid film eaii hr rffrottd hy a slight, change of changes all t.lie l.inir so that its hrhavior brconics n~ricli tr~~ipcratrirr" and also hy ail isotlrcr~nalchangc in t,hc urnre eomplicat~ed. surface arm availahlr pcr ~ ~ i o l t : o ~as ~ livc t : found a fcw Snap filnrs arc aliuost always ohscrvrd in reflretrd vmrs ago w i t h 111.. .I. Skewis. 7'111: apparatus used light, so t.hat the iiit.crfcrcucc (if t.he hraiils rcflcctcd hy was tlir sanw as d r v i s d for iiirasuriiig Gihbs filiri t,lieir front and rear siirfacrs prodiiccs eoloir or variaclasticity.1~ A rectangular fraiiic is rapidly raised tions of intensity by which f i l i i i thickiirss can he rstifroill tlir solution or lowcrrd into it. I3olh frames uiat,c!d and the ~irot,ionof patches of diRrrciit thickopwatc: within a s q ~ ~ a Iiottle, rr .5 X .5 cni. As t,he nrsscs obscrvrd. The direct arid rcflreted ticanis iiiay total arra of thr solutioii is qiiitc siiiall, this iiiot,iou be eit.hcr separate or coiiihiiird hy Iiirans of a sruiiof thc sccoiid fraliic producrs l a r g ~ prrcrntago charigrs t ransparcnt uiirror. in t.lrr arra availatilc pcr adsorlird iiiolcculc hrfori: surVisual ohscivatious arc rasiest wheii the fil~iisarc largr and Rat. For t,his rrasnn iiiost of our work involved lihiis several cent iuiet ers nu a side or i n diaiiirter. Vertical filuis such as shown in Fig. 1 are urnst casily produced by subiuerging a rcctaiigular fraiirc in a solut.iou and thcn withdrawing it partially. I.lorizoiital filins such as sliowii in Fig. 2 and 3 can bc siipportcd by t h e top of a funnd ivit,lr a narroivcd ~iloutlr. Very slight curvalurcs may thcn he prodiicrd by slight chaugcs of air pressurt: inside t,hc funnrl. Phase Transilinns and Film IClastici/J,. T I i t v is a striking diffrrmcc hrtwcru two types of films. Oue, thr ~iiohilo f i l i i i , t,Iiiirs in irrhiutcs, shows turhulrnt niotioiis aloug 1.lrc rdgrs, and has a horizontal lavcriug of iut,rrfrrcucc colors. The othcr, the rigid lilni, thins in hours, shows litt,lc Iiiot,ion. and has a grucrally irrrgdar arra~igo~rir~it of colors. Jlost iouic surfactant salrilioiis givc niobilo filius. Rigid filiiis arc! formed by a frn coiuhiiia1,ioiisof siirfactants eapahlr of giviug rigid umiolayrrs'? by t.iglrt pacliiiig of adsorlird inolr~ u l r s , t' Iir ~ ti~,o-diiiirrisioiialrqiiivalciit of solid hodirs. 1he trausit iou botiwrii th(, t i v w ~ ~ ~ t h"i~icltiiig" i: of
,.
where u is the surface t,ension, and s the area of the film. GibbsSb has also shown that, in a two-coniponmt, systeui this elasticity should be given by
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h’ = 4I’*(dp/dG)
. Bit_ i._ ~ ,\ . 1
(2)
where r is t,he surface density of the solute, pits chemical potent,ial, and G its total amount, present per unit film surface. Verificatiou of this relat,iou requires experiment,s on rigorously purified systenis since thr presence of enough containiuant to form a fraet,ion of a monolayer is enough to complet,ely vitiat,e t h r results. We are now devrloping tcchniqucs” for working with pure surfaces, and as these are perfected it should become possihle to test eq. 2 and perhaps use soap fihns to measure r a n d dw. Thinning Mechanisms. Rigid films t,hin primarily by the gravitational outflow of the solution from between the two immobile surface layers, and this seems to obey the simple laws of h y d r ~ d y n a m i c s . ~As ~ the constrainine.. surfaces are so near each other. the ordinary viicosity of water rrducrs the rate of outflow t ( r thv poilit whcrc i t takrs hours For 1 1 1 f~i l i i 1 to thin. I n iiiohik fihns t l w situ31iot~is t n o r t b coi~iph~x.T h r i r rapid thinning involves the relative motion of whole pat.ches of film, the two surfaces and the intralaniellar solution moving toget,her over easily visihle areas and shearing against, adjacent, patches of thc film. This motion is due t,o two different causes. The first, is g r a ~ i t , y ~ ?which . * ~ causes the thiuner (and therefore light,er) areas t,o move upwards replacing the ones that are thicker (and therefore heavier), thus leading to the horizontal stratification of colors. The other is the effect of the capillary suction a t the border where there must alwavs be a curved meniscus (the so-called
. . face equilihriuni is re-cstahlishrd. Hence the surface (or parts of it) lows its rigidity when the auxiliary frame is drawn out and bcconirs rigid again as it is lowered. Figure 1 shows selected fran1c.s froin a color uiovie‘6 rccordiug t h r process. The rxpausioii and contraction of the area of the observed film as the auxiliary frame oscillates up and down cau hr rvaluatrd from n photographic record. The corrrsponding variation of surface triision of thr systrm can be rrcordrd siniultanrously. This giveslS thr two quantities uwded to calculatc: t h r film elasticity h’asdrfinrd hy GihhsSb I< = Zdu/d In s (1)
out of the border to replace the lost area of the film. This “niarginal regenerat iou” niechanismBOis best observed in almost horizontal films where the effects of gravity are Figures 2 and 3 show the reality of marginal regeneration by stills from t,wo color movies of such films. Whereas the details of the pulling-in process are difficult to analyze thcor~tically~‘ and have not yet been isolated experinrcntally, the pulling-out has been studied furt,her as we shall see short,ly. (16) First shown a t the 1800 Colloid Symwrium at Lehigh University. (17)
P. Elworthy imd K . J. Msselr. to he published
KAROLJ. ivYSELS
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3444
As the film becomes thinner it reaches a point where it appears black because of the disappearance (or more exactly great reduction) of the reflected light as the two surfaces come close together. This low intensity of the reflected beam can still be measured photoelectrically and gives a sensitive measure of film thickness. To the naked eye, however, the film is invisible. This illusion is heightened by the sharp and abrupt transition between the uniformity of this black film and the colorful and smooth variations of the neighboring thicker film. Perrin likened it to the clean cut produced with a punch.l8 However, recent careful observations by Miss McEnteelg have shown that a transition region can sometimes be seen in horizontal films. This boundary is frequently the site of a third thinning mechanism as the area of black film grows spontaneously at the expense of the thicker film. The film disproportionates, and the excess liquid, originally present in the area whose thickness decreases, is forced into a thicker welt which gradually grows in thickness and in area and then flows to a lower level by gravity (Fig. 3). van der Waals Forces. The forces causing the growth of the black film must be van der Waals attractions between molecules, as has been emphasized by Overbeek.20 Here these forces are exerted over distances of the order of the film thickness, ie., some hundreds of angstroms. This is a range in which these forces cannot be studied directly by most other techniques, yet it is also the range of significance in the interaction of colloidal particles. Furthermore this intermediate range is of more general interest because it includes the transition region in which the distance dependence of van der Waals forces-or, more specifically, London dispersion forces-changes from the inverse seventh to the inverse eighth power. Hence, quantitative measurements in this domain would provide a severe test of the theories which have predicted correctly the short range as well as the retarded, long range behavior but do also specify21the still untested details of the transition region. In this transition region, the distances are too long and the dispersion forces are too weak to be studied by their effect on the individual molecules. At the same time, the distances are too short and the forces are too strong to allow the application of the elegant and sensitive methods using macroscopic plates and lenses.22 Yet, the simplest apparatus permits the direct observation of their effects in soap films. At first sight it may be surprising that the same forces which cause the attraction of two glass plates across a thin film of vacuum should also cause the T h e Journal of Physical Chemistry
thinning of a material film having air (which is practically vacuum) on both sides. Yet quantitative calculations showBo,zB that the pressures involved are exactly the same in both cases, and the following simple picture explains qualitatively why this is so. Let us consider a transition region between thin and thick film in the range where van der Waals forces are significant as indicated in Fig. 4. A water molecule, such as A, situated in this region will be subject to attractions by all its neighbors, but within sphere I these effects all cancel each other since, for every molecule such as B, there is one B' located diametrically opposite. Beyond this sphere, however, there are molecules such as C and D which do not have any opposites, precisely because the film is wedge-shaped. Their attractions therefore do not cancel and give a resultant force directed towards the thick film and tending to move our molecule away from the black film. Thus, the thin film becomes thinner, and the thick one thicker under the influence of van der Waals forces. The rate of this growth of black film can be measured quite readily as has been done for several systems by Overbeek and McEntee'$; it depends on such factors as the thickness of the thick film and the ionic strength, but the kinetics of the process still need clarification. When this is achieved we may have a very simple way of estimating van der Waals forces in this range.
Figure 4. van der Waals forces in a transition region cause the intralamellar liquid to flow towards the thicker film. J. Perrin, Ann. P h y s . (Paris), [9] 10, 165 (1918). Unpublished results. J. Th. G. Overbeek, J . Phus. Chern., 64, 1178 (1960). H. B. G . Casimir and D. Polder, P h y s . Rev.,73, 360 (1948); H. B. G. Casimir, Proc. K o n i n k l . Ned. A k a d . Wetenschap.. 51, 793 (1948) ; I. E. Dzyaloshinskii, E. M. Lifshits, and L. P. Pitaevskii, Usp. Fiz. Naulc, 73, 381 (19131). (22) E.g., B. V. Derjaguin and I. I. Abrikosova, Discussions Faraday Soc., 18,24 (1954); J. A. Kitchener and A. P. Prosser, Proc. Roy. SOC. (London), A242,403 (1957); W. Black, J. G . V. de Jongh, J. Th. G. Overbeek, and M . J. Sparnay, T r a n s . Faraday SOC.,56, 1597 (1960). (23) A. Seheludko and D . Exerowa, Kolloid-Z., 168, 24 (1960). (18) (19) (20) (21)
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SOAP
FILMS AND
3445
S O M E PROBLEMS Ilr; SURFACE AND COLALOIDCHENISTRT’
The Nonrigidity of Water near the Surface. Acting alone, the van der Waals forces would cause a selfaccelerating thinning and bursting of the film. In fact, however, once the black-film stage is reached, many filnis remain unchanged for an indefinite time. The fact that the limiting thickness depends on the ionic strength has naturally led to the invocation of the double-layer repulsion theory. This is especially coiivincing when the surfactant is ionogenic, although water surfaces and those of solutions of nonionic surfactants probably always have a surface potential.24 However, there has been also a massive body of arguments that a rigidification of water near the surface, perhaps an ice-like formation, is important in preventing the approach of the two surfaces.2h A very direct refutation of this latter idea can be obtained by a careful study of the pull-out of soap films.*6 Frankel has analyzed the hydrodynamics in the narrow transition region between the Plateau border and the film proper and has showngb~27 that if one makes the assumptions that the surface of the solution is inextensible in this region and that the viscosity of the solution remains the same as in the bulk clear up to the surface, then it follows that the thickness 6 is related to the velocity of pull-out, v, the viscosity, 7, surface tension, y, density, p , and gravity, g , by
(3) Except for the numerical constant, this is the same formula which had been developed much earlier by Derjaguin2s in connection with the very different problem of a liquid layer left on a solid inextensible substrate withdrawn from a liquid having a completely extensible surface and thus incapable of forming soap fiInis. The experimental test of Frankel’s relation is somewhat difficult for thick (0.2-10 p) films because of the need to extrapolate to zero film height where the experimentally accessible velocity of the frame approaches that of the film being withdrawn. Nevertheless, good agreement has been obtained for a variety of films having very odifferent surface proper tie^.^^ For thin films (
