DIFFUSION ACROSS OIL-WATER
INTERFACES
897
DIFFCSION ACROSS O I L W A T E R ISTERFACES' ERIC HUTCHISSOS* Depni lment of Chemistry, Stanfoid Cnzzersaty, Calajoi iiin IiLcczted .Yovernbei I S , 1947
The experiments described here form part of an investigation into the mechanism whereby molecules diffuse across an oil-n-ater interface, and into the effect of monomolecular films on such diffusion proces'es. Diffusion problems are of fundamental importance in the study of many physiological processes and have been the subject of very extenw e rebearch (9), but in many cases the systems investigated have been so complex that experimental proof of the various mechanisms proposed to explain the transfer of material, say through a cell membrane, has becn difficultt o obtain. These processes aln-ays involve the transfer of material across some kind of interface, be i t oil- [ater or solid-water, and from the earliest day>of the ytudy of monomolecular films it has been recognized that such films muit play an important r6le in physiology. Hou-ever, no predictions as to the mode of action of theie filmb are t o be found in the literature, and direct evpeiimental inveitigation beemb desirable. The structure of plasma membranes is generally considered to be a mosaic netivork of protein matter, in the interstices of which iz retained lipoid or oily material (12), and currently held theories on diffusion through iuch memhranes are based on this conception. The or,ginal "lipoid" theory of diffusion postulated by Overton (11) (and in the special case of narcosis by lleyer (10)) buggested that there \vas a marked paranelism between the permeability of organic non-electrolytes in the membrane and the distribution coefficient of the diffusing molecule betxeen the lipoid material and water. Those compounds which are preferentially soluble in the lipoid phase diffuse most rapidly In an extensive study of the rates of penetration of a number of organic liquid5 into single cells of Cham cerafophylla Collander and Barlund (5) found a good correlation between the rates of penetration and the respective distribution caoefficienti hetn-een olive oil and water. Discrepancies, particularly in the case of molecule. of low molecular weight, in which greater rates are observed than theory predicts, have led to a modification of the Overton theory, and Collander and Bsrlund propose a "lipoid-sieve" theory according to xvhich the membrane is composed of isleti of lipJid held between sieve-like areas of protein mosaic, the 5ieve-like areas varying in degree of porosity. Thus, n-hile all molecules, are free to move by means of their lipoid solubility, some may be prevented t o some extent, ov ing to their size, from permeating by the siel e mechanism. It seems extremely probable that, since the lipoid contain* .ririace-actiw m i 1 Presented 111 part at t h e T n e n t y - h r s t S a t i o n a l Colloiti Sr i n p i i u i i i \\ 1 i i r l ~~ a .hrlti untlei t h e auspices of the Division of C'ollo~dChPmistrj of t h e i i n r i i c ' i i i C'hcmic~al%cicItl at Stanfoid I-niveisit\, J u n e . 1947 2 Bristol-1Iyei-s Company Posttfoctoiafe I'ellon ant1 IIrwai c h \\wcint P I I I lirmi\t 1 x.
898
XRIC HUTCHINSOX
terials such as lecithin, the water-lipoid interface will be covered by 8 niore 01 less oriented layer, and diffusion processes will involve penetration of these layers. On such a situation the experiments of Harkins (8) and Schulman (14) would have a direct bearing. It was decided, therefore, to restrict attention, in the first instance, to the lipoid portions of the membrane, and the follov-ing experiments were planned as a crude approximation to the system cell lipoid-water. (1) Molecules were allowed to diffuse from aqueous solution across a benzenewater interface into benzene, and the rate of diffusion was studied as a function of distribution coefficient. ( 2 ) The experiments nere repeated under the same conditions, using now not simply a benzene-water interface, but a benzene-water interface a t which a film of surface-active material was present. EXPERIUEXTAL
The materials used as the diffusing molecules were ethyl, n-propyl, n-butyl, and n-amyl alcohols. Ten milliliters of an aqueous solution of alcohol was placed in a graduate, thermostated at 25"C., and 20 ml. of benzene n-as carefully pipetted on top of the aqueous layer. Diffusion began immediately; the rate was folloJyed b y withdrawing 0.25-cc. >arnples of the benzene layer and determining the refractive index by means of a Zeis:: interferometer. In earlier experiments the two layers were not stirred, as it \vas felt that stirring would disturb the interface unduly. A11 the compounds iised yielded solutions, both in benzene and in water, with lower density than the pure solvents, so that for diffusion from the lower aqueous layer into the benzene, stirring by means of convection occurred. Later work in which itirring n-ah used gave results identical with those in the absence of stirring, bhon-ing that uniformity of concentration in the t1vo layers was obtained in either m>e. Stirring Ivas achieved by means of two paddle wheels about 3 cm. in diameter soldered to a thin shaft about 2 mni. in diameter, so adjusted that one paddle \va> about 1 mm. above the interface and the other about 1 mm. L e l o ~the interface. .it the speeds of stirring used, 20-30 revolutions per minute, the interface \vab only slightly disturbed and it is estimated that the ripples induced by stirring changed the interfacial area by less than 5 per cent. Experiments with colored dyebtuffs s h o w d that the speeds of stirring used \\-ere sufficient to maintain bulk uniformity of concentration. -it first, attempts were made to sprcad films at the benzene-xater interface, but none of the materials commonly used at air-water interfaces proved sufficiently stable over the several-hour periods required during these experiments. Accordingly, $table adsorbed films \rere obtained by using solutions of sodium cetyl sulfate and cetylpyridinium chloride. The concentrations of these wrface agents were low, being less than 200 mg./liter, and their effect on diffusion in the aqueous layer may be neglected. By varying the concentration of surface agent, the surface pressinc of the film was readily varied. The u-ater and benzene were mutually saturated n t the beginning of each experiment to minimize side effects due to the diffusion of water. might he expected, the rcyrodiicihilit\- of thiq type of experiment was not
899
0.8
0.6
C8
C e'
0.4
0.2
1 100
1
1
200 300 T I M E MIN.
1 40 0
FIG.2 . Coinptti~atiw I atos of diffusioii for homologous alcohols from water t o benzene
900
ERIC HUTCHISSOX
very high. In the absence of films, variations oi i. 5-7 per cent n-ere observed, but in the presence of films reproducibility was rather better. -1ccordingly it is emphasized that the results of' these experiments are semiquantitative and that deductions therefrom arc tentative. RESULTS T.
I n figure 1 curve5 are given for the concentration, GIB, of n-propyl alcohol,i11 the benzene layer as a function of time. The diffusion doe\ not appear to tit the Fick law relating rate to concentration gradient (assuming any arbitrary thickness T for the interface) anti no value- of dift'u+ion coefficient5 hac(. +o f a r been T..\BLE 1 Distribution coeficients o j normal ulcohols betweerL benzene and water
I
ALCOMOI.
Ethyl. . . ,
I .. . ...I ,
n-Propyl .
?I - B u t y l .
n-hmyl
..... .
..
~
DISTRIBDTION
IWTI
K \TI:/
.-
inolc.~,'liicr
niiilc s l i t c i
6.li 4.72 2.76 1.51
0.643 0.336 0.144 0.060
1.93 1.61 1.00 0,528
1.55 0.789 0.256 0 124
0.364 0.308 0.225 0.175
0.4b2
0.051 1 0.0306
0 . 132
0.329 0,204 0.103
0 .O6lh
1.4 X
0.757 & 0 . 1 0.940 f 0 1 0.1 1.10 1.72 i0 1
1.9
x
10-4
0.3bb 5k 0 . 2 0.49 It 0 2
9.5
x
10-2
*
-
* Gram-formulae per liter
t
\r,
= Gram-formulae per liter =
---____
MW. MB.
obtained. At lon- concentration of alcohol the initial slope of the C'n-time curveis proportional to the initial concentration but t he proportionality disappearh at higher concentrations. The initial concentrations varied for thc different wlcohols due to differences in solubility, distribution coeficlient , a n d thc nccd t o have sufficient alcohol present to make measurement of the refrartive index reasonably accurate, +o that direct comparison of the rate\ is not ea-y In figure 2, values of :t "-atmation" function, ( ' B / ( ' , r , , ale plotted ior the is thc cwnwnt r a t i o n of various alcohol> t o make a comparison pohsihle. alcohol in benzene at time f , and C,, is the final ryuilthrium (~)ncentration in thtl benzene when diqtribution has heen attained. Sincar (',,, I* piq"rtiona1 t o the (IR
DIFFUSION ACROSS OIL-WATER
I 100
901
INTERFACES
I 200 M I N .
I
I 300
Frc,. 3. I.;ff~ct of film pressure on the diffusion of n-propyl alcohol
4 - -
0.8
0.6
Ct c q
0.4
1 N-BUTYL
ALCOHOL
2 N-BUTYL AMINE 3 ETHER
4 METHYL ETHYL KETONE
0.i
I
Io0
I
I
200
30-0
TIME MIN.
E FIG.4. Comparative rates of
diffusion for a number of four-carbon-atom compounds
902
ERIC HUTCHIKSOK
initial concentration in the water, as are the initial values of CB within limits, this “saturation” function may be used for a rough comparison of the series of alcohols. In table 1values are given for the initial rate/initial concentration, although as stated above this may not be an exact comparison owing t o variation of this factor with concentration. The results show that, on this scale, the rates of diffusion are in the order n-amyl alcohol > n-butyl alcohol > n-propyl alcohol > ethyl alcohol. Qualitatively this is in accordance with the Overton-Meyer theory. The values obtained for the distribution coefficients of the various alcohols, however, show that the variation in distribution coefficients is much greater than that of the initial rates.
1
5 MOLE
%
IO
ALCOHOL
Fic. 5 . Interfacial teiibion of the system aqueous ethyl alcohol-ltenxenc
Turning noIv to the effect of films on the rate of diffusion, the dotted curves in figure 2 show the effect of a film of sodium cetyl sulfate a t 30 dynes/cm. pressure. Clearly, under the same conditions of concentration gradient the rate of diffusion of a given alcohol is reduced considerably. Materials other than sodium cetyl sulfate were used to give adsorbed fi1rn~e.g., cetylpyridinium chloride in water, cholesterol and palmitic acid in benzeneand in all cases similar retarding effects due t o the interfacial film were observed. The question now arises as t o how the presence of the film can exert such a marked influence on the diffusion. A number of suggestions may be made a priori. (1) Distribution equilibrium is disturbed hy t,he presence of surface agents in solution. ( 2 ) The molecules in the film occupy hpace in the interface
DIFFCSIOB ACROSS OIL-WSTER
IXTERFACES
903
which would otherwise be available to the diffusing molecules, i.e., the film acts merely as a sieve. ( 3 ) The diffusing molecule may, on penetrating the film, interact xvith the film molecules and spend a greater time in the interface before passing into the benzene phase, i.e., the film would act as a potential barrier. Experiment readily showed that the presence of sodium cetyl sulfate, a t least in such concentrations as were used, had no measurable effect on distribution coefficients, so that hypothesis 1 could be abandoned. Experiments were carried out with n-propyl alcohol a t a number of film pressures, as shown in figure 3. Variation of the film pressure over a sevenfold range produced a much smaller change in the rate of diffusion. ,It first sight this might appear to indicate that the film does not act as a sieve, since a more critical de-
b
I MOLE
'4
2 ALCOHOL
F I ~ . fi. Interfacial tpnsion of thc system aqueous n-propyl alcohol-benzene
pendence on pressure might be anticipated by analogy with experiments on the diffusion of water molecules through monolayers (15). Hou-ever, it should be pointed out that (a) the final film pressure is slowly attained by surface aging over the first 30-40 min., during which a considerable fraction of the transfer has occurred. and ( b ) apart from one experiment by Alexander (2) no data are available for force-area curves for sodium cetyl sulfate, so that little is known about the packing of the film molecules at various pressures. Rather more direct evidence was obtained by studying the rates of diffusion of a number of four-carbon-atom compounds, ciz., n-butyl alcohol, n-butylamine, ether, and methyl ethyl ketone, as shown in figure 4. It will he seen that the retardation is very much less in the cabe of n-butylamine than for n-hutyl alcohol,
90.2
ERIC HUTCHINSOS
while for ether and methyl ethyl ketone there is practically no retardation. Further, it mas found that although n-amyl alcohol is strongly retarded, tertamyl alcohol is scarcely retarded a t all. This variation in the effect of the film with molecules differing in configuration, but not very greatly in size, suggests strongly that penetration and interaction in the film are of greater importance than mere mechanical obstruction. Interfacial-tension experiment3 \\-ere carried out t o see n hether a study of the interfacial tensions would provide any evidence ab to the mechanism. In order t o make any possible effect> of penetration of the film more clearly evident, solutions of sodium cetyl sulfate \\-ere used of such concentration as to give a film
b
pressure of 23 dynes, (mi. in.teatl ot 30 dyiw-;cn~ ;1varietj. of metIio(1s \\ere used to nieahure the intertnciul tm.ion of thc y h t e n i i (1) nqueou5 alcohol-1)eiizent.w (2) (nqueou. alcoliol iocliurn wtyl 9ulfate)-benzene
+
both i n the early >tagc- oi the diffusion ant1 also a t equilibriurn. F o u r different metliodh ~ e i i rd , z'iz., ~ drop-volume. the ring method, pendant drop, and sessile bubble. Compariwn of the method5 u-ill form the -uhfance of a forthcoming paper, hit it may be ytatetl here that cwnsiderahle cliiagreement between the various metliod.; \v:t\ observed m d tlic value- quoted here xere obtained by means of the s e 4 e - d r o p method This method ha- been :tdequittely de-cribed in the literatmv (1, 4. 'i) and \vi11 not t i e cles.c~ribe.cl 1iwe Correction factorb
DIFFGSIOh- ACROSS OIL-WATER
k” IO
INTERFACES
t
\
- --I
I
905
-
I
FIG.9. Aging of the interface for t h e systeiii aqueous sodium c’ctyl sulfate-benzene
9O(i
ERIC HUTCHINSON
derived directly from the tables of Bashforth and Adanis (3) were obtained froin a paper by Porter (13). Bubbles were used of such size that the correction factors were less than 10 per cent, and were in the region where they may be calculated with considerable accuracy. The bubble-forming tube was placed vertically in an optical cuvette containing 100 ml. of the solution of alcohol, and 10 ml. of benzene was carefully pipetted into the tube. Measurements of the interfacial tension were made from the time of formation of the interface until equilibrium n-as established, usually u period of 3-4 hr. I n figures 5-8 values are given for the interfacial tension of the varioua alcohol solutions over a range of concentrations, as measured 5 min. from the beginning
-0-
0.5 MOLE ~'IL
10 o/o
ETHER
10 Ititcrfaclal trtlrion for t h e
tern uqueuus etlicii
lx~iiieiic
of the experiment. v;~hieb are also given for the same bysteins nfter distributiuli equilibrium had been established. The open circles refer to the 3-min. readings and the full circles t o the equilibnurn values; dotted curves shon- t h r effect of the presence of sodium cetyl sulfate on the system. (It xi11 be obberved that the values of interfacial tension correiponding t o the s p t e n i consisting of .;odiuni cetyl sulfate aqueouq alcohol and Lenzene are not the same iii both cases; this is due to the aging of S I I C ~an interface, as shown in figure 9.)
+
DISLGSSlOh
It is clear that tmth in the initial stageb of difiu-ion niid .Lt tquilibiiun1 COILsiderable penetration of the sodium cetyl 3ulfate filni occiir\. -ipplication of the
Gibbb adborption equilibrium t o such sybteni- i. iiot powble, oi\-nig to thci number of variables involved, but if the strength of the interaction i i measuied as the increase of preisure, then a t equal molar percentages the interaction increases in the order ethyl < propyl < butyl < amyl alcohol, as one might expcct. This penetration of the film in 170th equilibrium and non-equilibrium states is taken as evidence that the interaction hypothesis is indeed correct, and this is borne out by the interfacial-tension curves of one of the materiala for which retardation is almost negligiblc, &., ethyl ether in figure 10. Here there i i a correspondingly poor penetration of the filni during the early part of the difTusion process, and here i. also a rough correlation betiveen the Ion-er rate of diffusion of ether, as compared Tvith that of butyl alcohol, and its much lon-er surface activity. This again ivould be expected. Davson and Danielli (6)are a t some pains. in describing the Overton-Xeyer hypothesis, to minimize the importance of surface activity of the diffusing molecule, remarking that although with increasing chain length the surface activity of the molecule increases, yet so also does the distribution coefficient, and that the latter is the important factor. S o w if any mechanism may he deduced from the Overton-JIeyer theory it is simply that the driving forw is that tlue to osmotic pre'sure. In these experimenth, at the concentrations used, the initial lack of osmotic balance ~vouldbe in the order butyl > propyl > ethyl > amyl, \vhereas the measured initial rates:/concentration gradient are in fact amyl > M y 1 > propyl > ethyl, in agreement ivith the order of surface activities (at a given concentration). Indeed if, in biological processes, a molecule mubt pass through some kind of oil-Tvater interface, then it is difficult to see lion. surface activity can lie ignored in interpreting the phenomena. .Ittempts w r e made to investigate any possible temperature effect of the diffusion proce-, but -0 far the results have been too erratic to enahle any conclusion to be reached. Similarly, experiments m-ere carried out to investigate diffucion from the benzene layer into water. &4gainthe r e d t s \\-ere very erratic, and it is thought that greatly i m p r o d stirring is necessary to ensure uniformity of liulk concentration. SUMMARY
The rates of diffusion of a riuniber of alcohols from water into benzene across the benzene-water interface have been measured. Initial rates are in the order amyl > butyl > propyl > ethyl, in qualitative agreement with the OvertonRleyer theory. The retarding effect of films of surface-active materials on the diffusion has been investigated, and measurement of the interfacial tensions of the various systems supports the hypothesis that interaction in the film is responsible for the retardation. As yet no mathematical analysis of the diffusion curves has been possible. and the recults quoted are semiquantitative. The author rvishes to acknowledge the helpful guidance of Profesor James W. McBain throughout t'his investigation.
