Macrocluster GasâLiquid and Biliquid Foams and Their Biological

2 Macrocluster Gas-Liquid and Biliquid Foams and Their Biological Significance

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0009.ch002

FELIX SEBBA Department of Chemistry, University of the Witwatersrand, Johannesburg, South Africa

Introduction In a c o n s i d e r a t i o n of s t a t e s of matter such as crystals or liquids, i t i s usual t o assume t h a t they are composed of u n i t s such as i o n s , atoms or molecules and to i n t e r p r e t the bulk property as a c o - o p e r a t i v e summation of the p r o p e r t i e s of the individual units. However, there are a number of p h y s i c a l systems which could be b e t t e r understood by c o n s i d e r i n g them as c o n s i s t i n g o f u n i t s which are themselves s t a b l e o r metastable aggregates of s m a l l e r units, the m o l e c u l e s , and i t i s suggested t h a t such systems should be named " m a c r o c l u s t e r s y s t e m s " . ( U n f o r t u n a t e l y , the term "cluster" which would be s u i t a b l e has a l r e a d y been a p p r o p r i a t e d by statistical mechanics w i t h a very d i f f e r e n t meaning.) Examples of macrocluster systems are gas -foams, biliquid foams, scums, flocs, concentrated e m u l s i o n s , living t i s s u e , cell cytoplasm and probably biomembranes. A m a c r o c l u s t e r system i s d e f i n e d as a heterogeneous a s s o c i a t i o n composed o f u n i t s , each capable o f independent e x i s t encse but organised in such a way that they c o n s t i t u t e a more advanced system which has p h y s i c a l p r o p e r t i e s a d d i t i o n a l to the p r o p e r t i e s of the i n d i v i d u a l u n i t s . Foams provide a u s e f u l example of a macrocluster system. It w i l l be shown t h a t gas foams c o n s i s t of u n i t s which are bubbles of gas encapsulated in a t h i n f i l m of surfactant s o l u t i o n . However, i n d i v i d u a l gas bubbles are not foams, and do not e x h i b i t the s e m i - s o l i d behaviour of some foams nor t h e i r f l o w p r o p e r t i e s . It i s because these bubbles are held together by t h i n aqueous f i l m s , the adhesion being a consequence of such f i l m s , t h a t the advanced system, the foam i s produced which has p r o p e r t i e s which are more than those of the i n d i v i d u a l u n i t s . As d e f i n e d i n t h i s way, many composite m a t e r i a l s would q u a l i f y as m a c r o c l u s t e r systems, as would those a l l o y s whose p r o p e r t i e s depend upon interphase d e p o s i t s even i f these be metastable s t a t e s . On the other hand, e m u l s i o n s , w i t h the e x c e p t i o n , perhaps, of very concentrated o i l - i n - w a t e r e m u l s i o n s , 18

Mittal; Colloidal Dispersions and Micellar Behavior ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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are not macrocluster systems. T h i s Is because the p h y s i c a l p r o p e r t i e s of a d i l u t e emulsion are e s s e n t i a l l y those of the continuous aqueous phase.


Gas Foams Many a u t h o r i t i e s compare gas foams to emulsions c l a i m i n g that the only d i f f e r e n c e i s that the d i s p e r s e l i q u i d phase in an emulsion i s replaced by a gas. T h i s i s i n c o r r e c t and i t w i l l be shown that the s t r u c t u r e of a foam must be very d i f f e r e n t from that of an e m u l s i o n . However, i t i s p o s s i b l e to make emulsions of a gas in w a t e r , but f a l l i n g i n t o the same t r a p , t h i s author m i s t a k i n g l y c a l l e d them "microfoams" 0 ) · B u t , apart from the common f a c t o r t h a t gas i s the dispersed phase, t h i s system has no foam p r o p e r t i e s and to avoid c o n f u s i o n , i t is here proposed t h a t , in f u t u r e , t h i s system be r e f e r r e d to as a "microgas emulsion . Another misconception i s that a foam c o n s i s t s of gas separated o n l y by a continuous aqueous s k e l e t o n , the l a m e l l a , (Figure 1 ) . This naive model cannot e x p l a i n some simple f a c t s of foam behaviour as w i l l be d i s c u s s e d . It must be recognised that no matter what process i s used to generate a gas foam, e s s e n t i a l requirements must be f u l f i l l e d . F i r s t l y , there must be a s u r f a c t a n t d i s s o l v e d in the l i q u i d , and secondly the gas has to generate a surface w i t h i n the l i q u i d and then break through a second s u r f a c e . Consider the case of a foam generated by passing a i r through a narrow o r i f i c e i n t o a d i l u t e aqueous s o l u t i o n of the foaming agent. The f i r s t step i s the formation of a g a s - f i l l e d hole in the l i q u i d . In the absence of a foaming agent, any such holes which c o l l i d e d , w h i l e s t i l l under the surface of the l i q u i d , would immediately c o a l e s c e . T h i s i s because the l a r g e r hole thus formed would expose a s m a l l e r i n t e r f a c i a l area than the sum of the two i n t e r f a c i a l areas of the two holes from which i t was formed. However, in the presence of a s u r f a c t a n t , coalescence i s l e s s probable because of the energy b a r r i e r produced by the o r i e n t a t e d molecules at the i n t e r f a c e and consequent double l a y e r . The r e p u l s i o n of the two double l a y e r s i s the p r i n c i p a l component of the s o - c a l l e d " d i s j o i n i n g pressure which r e s i s t s t h i n n i n g of the aqueous f i l m jammed between two col 1 i d i n g h o l e s . Under the i n f l u e n c e of g r a v i t y , the g a s - f i l l e d hole r i s e s to the s u r f a c e of the s o l u t i o n . Here i t meets an o b s t a c l e to i t s p r o g r e s s , because i t has to break through the s u r f a c e . If the hole i s very small so that i t does not have enough k i n e t i c energy to penetrate the s u r f a c e , i t may be r e f l e c t e d back into the i n t e r i o r of the l i q u i d . U s u a l l y , however, i t i s l a r g e enough to have s u f f i c i e n t energy to penetrate the s u r f a c e . This i s not a simple break-through because the s u r f a c e i s a l r e a d y covered w i t h a l i q u i d expanded monolayer of the foaming agent accompanied by i t s own double l a y e r . T h e r e f o r e , r e p u l s i v e f o r c e s w i l l operate preventing the gas from d i s r u p t i n g the s u r f a c e c o m p l e t e l y . If 11

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It d i d s o , the gas would simply d i s p e r s e i n t o the outer atmos phere and a foam would not be formed. However, the buoyancy of the g a s - f i l l e d hole i s enough to form a " b l i s t e r on the s u r f a c e , covered by a t h i n aqueous f i l m which owes i t s s t a b i l i t y to the d i s j o i n i n g f o r c e s induced by the p r o x i m i t y o f the charged heads o f the s u r f a c t a n t m o l e c u l e s , (Figure 2). The net r e s u l t i s that the gas encapsulated by the t h i n aqueous s h e l l , f l o a t s on the s u r f a c e very s l i g h t l y immersed and i t i s p r o p e r , a t t h i s s t a g e , to r e f e r to i t as a bubble which i s a volume of gas encapsulated in a t h i n f i l m sandwiched between two expanded monolayer s u r f a c e s , (Figure 3). Should the bubble be l a r g e enough, and there be enough momentum, i t could leave the s u r f a c e as a f r e e f l o a t i n g bubble of the kind painted by John M i l l a i s , i . e . , a sphere o f gas encapsulated in a s h e l l o f s u r f a c t a n t s o l u t i o n w i t h a monolayer of s u r f a c t a n t absorbed at both inner and outer s u r f a c e s of the s h e l l . As the bubble f l o a t s on the s u r f a c e , the question a r i s e s as to whether the bubble i s complete. It would appear to be s o , because i f a f r e e bubble from a i r i s introduced to the s u r f a c e o f a s u r f a c t a n t s o l u t i o n , i t a d j u s t s i t s e l f to f l o a t on the s u r f a c e seemingly in p r e c i s e l y the same way as a bubble generated d i r e c t l y in the bulk o f the l i q u i d . Thus the bubble s i t s in a hollow in the s u b s t r a t e l i q u i d as shown in Figure 3. When two such f l o a t i n g bubbles get c l o s e enough, they move towards each other w i t h i n c r e a s i n g v e l o c i t y u n t i l they almost t o u c h , being separated s t i l l by a very t h i n aqueous f i l m . The e x i s t e n c e o f t h i s f i l m , which i s c r i t i c a l to the behaviour of foams of a l l t y p e s , i s dependent upon the d i s j o i n i n g f o r c e s which must be overcome before the f i l m can be d i s r u p t e d . The pressure d r i v i n g the bubbles together i s o f t e n r e f e r r e d to as the Laplace c a p i l l a r y pressure which e x i s t s because o f the tendency f o r any l i q u i d s u r f a c e to expose the minimum s u r f a c e a r e a . T h i s i s the same pressure as that which causes two p a r t i a l l y immersed p l a t e s to adhere, whereas two completely immersed p l a t e s show no such tendency. In the previous c a s e , the pressure P»2X where γ i s the s u r f a c e t e n s i o n and d i s the d i s t a n c e between the p l a t e s . In the case of the bubbles which are not s p h e r e s , Ρ has not been d e r i v e d , but presumably w i l l s t i l l vary i n v e r s e l y w i t h d i s t a n c e a p a r t . The f i n a l p o s i t i o n of the b u b b l e s , t h e r e f o r e , i s determined by the balance of three f o r c e s , the Laplace f o r c e s which increase as the bubbles approach, the long range Vaif der Waal s f o r c e s which tend to t h i n the i n t e r v e n i n g f i l m and the double l a y e r r e p u l s i v e f o r c e s which r e s i s t t h e s e . The r e s u l t a n t of the l a s t two produces the d i s j o i n i n g f o r c e which prevents the f i l m t h i n n i n g to disappearance. The f a c t t h a t Laplace pressure e x i s t between bubbles on water leads to an important but unexpected c o n c l u s i o n . Because they owe t h e i r o r i g i n to the tendency of l i q u i d s to minimise t h e i r s u r f a c e a r e a , there must be an i n t e r f a c e exposed, such as a meniscus. On the other hand, the l i q u i d must adhere to another phase, otherwise i t would simply r o l l up i n t o a sphere. When


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GAS

ΔΞ=/

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GAS

GAS

Figure 1. Conventional representation of foams. Ρ represents plateau borders. Hatched areas rep resent thin aqueousfilmof surfactant with mono layer at gas interface.

WATER Figure 3. Bubblefloatingon surface. è represents surfactant molecules.



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two p l a t e s are completely immersed in a l i q u i d , there is no meniscus and thus no Laplace p r e s s u r e . Immediately, the p l a t e s become p a r t l y immersed, a l i q u i d - a i r i n t e r f a c e e x i s t s and the tendency to minimise t h i s i n t e r f a c e p u l l s the p l a t e s t o g e t h e r . However, the problem of a t t r a c t i o n between two f l o a t i n g bubbles i s not as s i m p l e . The f a c t that they do a t t r a c t one another i s obvious to anyone who has watched the bubbles on h i s cup of t e a . The water meniscus cannot j u s t adhere to the g a s , so i t must adhere to the f i l m e n c a p s u l a t i n g the bubble. T h i s f i l m , c l e a r l y , must have d i f f e r e n t i n t e r m o l e c u l a r a t t r a c t i o n s o p e r a t i n g w i t h i n i t than does the bulk water otherwise i t would simply merge w i t h the bulk and the bubble would b u r s t . It f o l l o w s , t h e n , that e f f e c t i v e l y the e n c a p s u l a t i n g f i l m behaves as though i t were a separate phase from bulk water. Though t h i s might seem at v a r i a n c e w i t h the usual d e f i n i t i o n of a phase, evidence a l r e a d y e x i s t s that the molecules of water c l o s e to an i n t e r f a c e have d i f f e r e n t p r o p e r t i e s from bulk water. T h i s has been explained as being due to a more i c e - l i k e s t r u c t u r e because of the hydrogen bonding induced by the i n t e r f a c e , and f u r t h e r evidence i s provided by the f a c t that the surface of a s u r f a c t a n t s o l u t i o n has a very much higher v i s c o s i t y than the bulk phase. As water in a t h i n f i l m would have two i n t e r f a c e s , t h i s e f f e c t should be enhanced. A surface boundary implies an abrupt change of i n t e r molecular a t t r a c t i v e f o r c e s , and i t might at f i r s t seem strange that t h i s could r e s i s t the normal k i n e t i c energy tending to mix the molecules. However, once t h i s p o s s i b i l i t y is accepted many phenomena to be described l a t e r become e x p l i c a b l e , and there are two a d d i t i o n a l experimental f a c t s , h i t h e r t o unreported which s u b s t a n t i a t e the view that s u r f a c e phases have a r e a l i t y . These are the e x i s t e n c e of bridges between microgas emulsion bubbles and the e x i s t e n c e of monolayer membranes. To avoid d i s c o n t i n u i t y these w i l l be discussed as two subsections l a t e r . For two equal s o l i d spheres of radius r, a n d , w i t h a drop of water between, the adhesive f o r c e i s 2πτγ where γ i s the s u r f a c e tension of the i n t e r v e n i n g l i q u i d , but t h i s formula does not s a t i s f y the c o n d i t i o n s of the f l o a t i n g bubbles as they are f l a t t e n e d underneath and the undersurface i s completely surrounded by a l i q u i d . A l s o the spheres are not r i g i d , but deformable gas. However, as an approximation, i t can be assumed that when they get c l o s e enough t o g e t h e r , the approaching surfaces w i l l be deformed enough to become p a r a l l e l s i d e s i . e . s i m i l a r to two wet p l a t e s , p a r t l y immersed, so the adhesional pressure w i l l be γ / d where d i s the d i s t a n c e a p a r t . T h i s w i l l be r e s i s t e d by the d i s j o i n i n g f o r c e s . If the bubbles are very s m a l l , the i n t e r n a l pressure in the bubbles which i s i n v e r s e l y p r o p o r t i o n a l to t h e i r r a d i i , w i l l become g r e a t e r , so d i s t o r t i o n w i l l be l e s s than in the case of the l a r g e r bubbles. The behaviour of monolayers of such bubbles is e a s i l y seen on a blackened Langmuir t r o u g h , by blowing a i r through a small j e t under the s u r f a c e . The sheet of bubbles i s almost mono-



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d i s p e r s e . Though they adhere l o o s e l y , such f l o a t i n g bubbles do not yet c o n s t i t u t e a foam. In order to convert them into a t y p i c a l polyederschaum, i t i s necessary to crowd the bubbles, and t h i s can r e a d i l y be achieved by compressing them between b a r r i e r s , when the bubbles w i l l climb over one another forming a m u l t i - l a y e r o f bubbles which soon d i s t o r t s to a foam. More u s u a l l y , a foam i s formed because the v e s s e l in which the bubbles are being formed has a l i m i t e d a r e a . A foam w i l l not s t a r t to form u n t i l the s u r f a c e i s completely covered by a l a y e r o f bubbles. As the bubbles are u n s t a b l e , the average l i f e - t i m e depending i n t e r a l i a on the nature and c o n c e n t r a t i o n of s u r f a c t a n t s and other s o l u t e s in the w a t e r , one of the p r e r e q u i s i t e s f o r the production of a foam i s that the r a t e of generation of bubbles must be g r e a t e r than the rate of d e s t r u c t i o n of bubbles moving from the point of generation to the w a l l s o f the v e s s e l . The geometry r e s u l t i n g from the packing of a number of deformable spheres i n t o a l i m i t e d space causes the spheres to d i s t o r t to polyhedra and the areas of c l o s e s t approach of two polyhedra are p l a n a r . The regions o f l i q u i d confined by these planar i n t e r f a c e s c o n s t i t u t e the lamellae. It should be noted that t h i s model o f independent bubbles forming a macrocluster system foam s t i l l r e t a i n s the Plateau borders as regions of low pressure because of the curvature. If the b a r r i e r s which created a foam on the trough by compression are then moved a p a r t , the foam breaks down, the bubbles s l i p back onto the water s u r f a c e and the monolayer of bubbles i s r e - e s t a b l i s h e d . In other words, there i s complete r e v e r s i b i l i t y between s p h e r i c a l bubbles and the c e l l s in a foam. It i s d i f f i c u l t to v i s u a l i s e t h i s happening i f the l a m e l l a were a s i n g l e uniform f i l m . While i t i s c o n c e i v a b l e that the outer s u r f a c e of the two approaching bubbles could be destroyed under adhesional pressure and thus produce the homogeneous l a m e l l a d e p i c t e d in t e x t b o o k s , i t i s not easy to see t h a t the reverse process could occur spontaneously. However, there i s no such d i f f i c u l t y i f the e n c a p s u l a t i n g f i l m round each bubble remains unbroken, but t h i s does mean that the l a m e l l a i s complex and c o n s i s t s of two bubble f i l m s separated by a t h i r d t h i n aqueous f i l m , (Figure k). Further evidence f o r t h i s model of a foam i s provided by the f o l l o w i n g experiment. If a v i s c o s i t y i n c r e a s i n g substance such as POLYOX i s added to the s u r f a c t a n t s o l u t i o n , the foam s t a b i l i t y i s s u f f i c i e n t l y increased f o r i t to become p o s s i b l e to remove i n d i v i d u a l bubbles from the foam w i t h a s p a t u l a , f l o a t such bubbles on a c l e a n s u r f a c e o f the s o l u t i o n , and then r e p l a c e them on the foam. A bubble r e - i n s e r t e d i n t o i t s o r i g i n a l s i t e resumes i t s o r i g i n a l shape. If a small amount of microgas emulsion i s inserted beneath a foam, the small gas bubbles w i l l r i s e through the foam bed, and i f observed i t w i l l be seen that they never penetrate a c e l l s u r f a c e d i r e c t l y , but always r i s e through the l a m e l l a e in s p i t e o f the f a c t that t h i s o f t e n means



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a longer path or along paths that are o n l y i n c l i n e d s l i g h t l y from the h o r i z o n t a l . The i m p l i c a t i o n i s that l e s s work i s needed to separate the bubbles than break c e l l w a l l s . When a foam whose foaming agent i s an a n i o n i c s u r f a c t a n t i s contacted w i t h one produced u s i n g a cat i o n i c s u r f a c t a n t , the foams mutually d e s t r o y each other (2). Though t h i s i s d i f f i c u l t to e x p l a i n on the conventional model of the s i n g l e l a m e l l a , i t i s easy to see how two bubbles, because of the o p p o s i t e l y charged s h e l l s , w i l l a t t r a c t one a n o t h e r , i . e . , w i l l r e i n f o r c e the Laplace pressures instead of r e s i s t i n g them as in a normal foam. F i n a l l y , there i s a well e s t a b l i s h e d technique f o r c o n c e n t r a t i n g ions known as foam f r a c t i o n a t i o n . This is i n e x p l i c a b l e on the c l a s s i c a l model o f a foam but can be e a s i l y under stood on the macrocluster model. The technique i s simply a p a r t i t i o n chromatography, the s t a t i o n a r y phase being the s h e l l s round the b u b b l e s , and the mobile phase the t h i n f i l m o f l i q u i d between the bubbles. Bridges Between Bubbles. Figure 5 i s a microphotograph of a microgas emulsions ( U , which i s not a macrocluster systems. However, the photograph i s o f an emulsion which i s s u f f i c i e n t l y concentrated in gas bubbles that they a c t u a l l y touch and i t w i l l be observed that whenever two bubbles meet there i s seemingly a bridge between them. They are always there no matter from what point they are i l l u m i n a t e d and a r e , t h e r e f o r e , not o p t i c a l i l l u s i o n s . The s t r u c t u r e has the same appearance as the meniscus that would be seen i f two wettable g l a s s spheres had a drop of water between them. The microgas emulsion c o n s i s t s of g a s - f i l l e d h o l e s , w i t h the s u r f a c t a n t monolayer at the i n t e r f a c e between the hole and the aqueous phase. However, i f t h i s were s o , there would not be the c o n d i t i o n s f o r forming the meniscus. In some c a s e s , the bubbles are seen to be d i s t o r t e d which means that there must be an adhesive f o r c e strong enough to achieve t h i s . This adhesive f o r c e can only be the r e s u l t of Laplace p r e s s u r e s , and such pressures can o n l y e x i s t i f there i s an i n t e r f a c e between the surface phase and the bulk water phase, and the meniscus i s formed by the s u r f a c e phase. By analogy w i t h two equal spheres of r a d i u s , γ , separated by a drop o f l i q u i d , where the adhesive f o r c e i s 2irry, the adhesive f o r c e w i l l be 2·πτγ. where r i s radius of bubble and yj the i n t e r f a c i a l t e n s i o n between s u r f a c e water and bulk water. Since i t i s the s u r f a c e water which i s producing the meniscus, the a t t r a c t i o n between i t s molecules must be g r e a t e r than t h a t between bulk water m o l e c u l e s . This i s c o n s i s t e n t w i t h the suggestion that the s u r f a c e phase i s more hydrogen-bonded than bulk water. The i n t e r f a c i a l t e n s i o n between ice and water i s of the order of 25 dynes c m " ( 3 ) · The i n t e r f a c i a l t e n s i o n between s u r f a c e phase and bulk phase must, t h e r e f o r e , l i e somewhere between 0 and 25 dynes c m " ' . As the adhesive f o r c e i s p r o p o r t i o n a l to the radius o f the bubble d i s t o r t i o n of the bubbles i s o n l y observed in the l a r g e r bubbles. 1



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The interface between shell film and continuous film is effectivily a bimolecular film layer e.g.

CONTINUOUS F . L M \

Figure 5.

F

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G

M

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E

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F

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m

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m

Microgas emulsion


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COLLOIDAL DISPERSIONS


It should be noted that there i s a d i s t i n c t d i f f e r e n c e between f l o a t i n g bubbles and microgas emulsion bubbles. This is because the former are encapsulated in a f i l m bounded on both faces by monolayer and which i s tenacious and e l a s t i c whereas the l a t t e r i s not. It i s , t h e r e f o r e , c o n c e i v a b l e that in the case of the microgas emulsion the meniscus w i l l draw the bubble s u r f a c e w i t h i t thus forming a tube between the two gas bubbles. When two bubbles of a d i f f e r e n t s i z e t o u c h , as the pressure in the bubble i s i n v e r s e l y p r o p o r t i o n a l to the r a d i u s , the l a r g e r bubble grows a t the expense of the s m a l l e r one. This growth can a c t u a l l y be observed under the microscope. However, the growth i s not by a coalescence mechanism i . e . the w a l l s between the bubbles do not c o l l a p s e . The t r a n s f e r of gas does not proceed as might be expected u n t i l the s m a l l e r bubble v a n i s h e s , but i t seems to stop a t a minimum s i z e , a f t e r which t r a n s f e r proceeds very s l o w l y , i f at a l l . The consequence i s that the l a r g e r bubbles are observed to become coated by a l a r g e number o f t i n y b u b b l e s , which because of t h e i r s i z e , must be under higher pressure and, t h e r e f o r e , r e l a t i v e l y r i g i d . T h i s phenomenon might be caused by the connecting tube being pinched o f f below a c e r t a i n s i z e of bubble. This i s an unusual example o f a two u n i t macrocluster system. Monolayer Membranes. The s u r f a c e of a s u r f a c t a n t s o l u t i o n has s u f f i c i e n t l a t e r a l cohesion to form the b a s i s o f membranes which can be several centimetres a c r o s s , and which are remark a b l y p e r s i s t e n t . To make such a membrane a s u r f a c t a n t s o l u t i o n has to be placed upon an i d e n t i c a l s o l u t i o n without d i s t u r b i n g the s u r f a c e . Microgas emulsions provide one way of a c h i e v i n g t h i s , because they are much l e s s dense than w a t e r . If a microgas emulsion generated from a s o l u t i o n such a s , f o r example, 5 x l O ' ^ M sodium dodecyl benzene sulphonate c o n t a i n i n g 1 χ 10"3M sodium sulphate is g e n t l y placed upon an i d e n t i c a l s o l u t i o n , c o n t a i n i n g a small q u a n t i t y of i n d i c a t o r d y e , such as bromocresol b l u e , in a g l a s s c y l i n d e r , i t w i l l f l o a t upon i t . The gas s l o w l y r i s e s and escapes, u n t i l the upper s o l u t i o n becomes f r e e of gas. However, i t does not spontaneously mix w i t h the lower s o l u t i o n , as can be seen by the e x i s t e n c e of a boundary between the dyed lower s o l u t i o n and c o l o u r l e s s upper s o l u t i o n . What i s remarkable i s the r e s i s t a n c e o f the boundary to d i s t u r b a n c e such as t i l t i n g . One such membrane l a s t e d three weeks and r e s i s t e d t r a n s p o r t a t i o n from one room to another. The membranes are s l i g h t l y more e a s i l y made i f the lower s o l u t i o n i s made f r a c t i o n a l l y more dense than the upper, by i n c o r p o r a t i n g s l i g h t l y more s a l t . The s a l t appears to s t a b i l i s e the membrane. The membrane i s e l a s t i c and i f a g l a s s rod i s made to d i s r u p t the membrane to the depth of about 1 mm., on removal of the rod the membrane returns to i t s o r i g i n a l p o s i t i o n . As i n d i c a t o r dyes are themselves s u r f a c e a c t i v e , membranes have been made in which one of the s o l u t i o n s was coloured w i t h iodine and s t i l l the iodine did not d i f f u s e through



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the membrane. Though these membranes are impermeable to d y e s , they appear to be s l o w l y permeable to hydroxyl i o n s . This can be demonstrated by adding some a l k a l i to the lower s o l u t i o n and a l i t t l e p h e n o l p h t a l e i n to the upper one, and observing the pink c o l o u r developing in the upper l a y e r . However, the p o s s i b i l i t y of creep along the w a l l s of the vessel past the membrane cannot be d i s c o u n t e d . With c a r e , sandwich l a y e r s can be made such as blue and red separated by a c o l o u r l e s s s o l u t i o n . The nature of the membranes has not yet been determined and i t is impossible to say whether they are monomolecular or b i m o l e c u l a r , nor is there any assurance that the a i r may not c o n t r i b u t e to the s t a b i l i t y . What i s c l e a r i s that they are very impermeable so cannot serve as a model f o r biomembranes. What they do confirm i s that membranes such as these r e s i s t mechanical d i s t u r b a n c e and k i n e t i c bombardment, r e i n f o r c i n g the suggestion t h a t s t r u c t u r e s such as those postulated f o r foams can e x i s t f o r l o n g ' p e r i o d s of time. B i 1 i q u i d Foams. In the f o l l o w i n g d i s c u s s i o n s , i t becomes necessary to make a c l e a r d i s t i n c t i o n between two very d i f f e r e n t types of surface a c t i v e agent. On the one hand, there i s the water s o l u b l e s u r f a c t a n t , u s u a l l y ionised and t y p i f i e d by the a l k a l i soaps and water s o l u b l e d e t e r g e n t s . For b r e v i t y , t h i s type w i l l be r e f e r r e d to as WATSSA. On the other hand, there are the o i l s o l u b l e s u r f a c e a c t i v e m o l e c u l e s , s p a r i n g l y or i n s o l u b l e in w a t e r , such as f a t t y a c i d s , amines, long chai n a l c o h o l s and cholesterol. These w i l l be r e f e r r e d to as OILSSA. Biliquid foams, which d i f f e r from gas foams, o n l y in that the holes are now f i l l e d w i t h a l i q u i d , owe t h e i r s t a b i l i t y to s i m i l a r f o r c e s to those o p e r a t i v e in gas foams. The technique f o r t h e i r production has been d e s c r i b e d by Sebba ( i ) . There are two d i s t i n c t types depending on whether the hole c o n t a i n s an aqueous s o l u t i o n or an oil. In order to make a b i l i q u i d foam where the d i s p e r s e phase i s water encapsulated in a t h i n f i l m of o i l , advantage i s taken o f the f a c t that a microgas emulsion w i l l f l o a t on an o i l , so a microgas emulsion i s placed on top of an o i l such as Nujol or k e r o s i n e . As the gas leaves the microgas e m u l s i o n , i t s d e n s i t y s l o w l y r i s e s and when i t i s j u s t above that of the o i l i t s i n k s through i t . In doing s o , i t penetrates the two i n t e r f a c e s required f o r forming a foam, f i r s t l y the upper o i l - w a t e r inter face and then on l e a v i n g the o i l , i t gets i t s e n c a p s u l a t i n g o i l film. T h i s produces a b i l i q u i d bubble, and the macrocluster system, held together by the Laplace pressures i n a t h i n water f i l m between the bubbles, c r e a t e s the b i l i q u i d foam. It i s p o s s i b l e w i t h a hypodermic s y r i n g e to i n j e c t coloured s o l u t i o n s and v a r i o u s s o l u t e s i n t o the water core o f the b u b b l e s , but there i s no d i f f u s i o n outwards, showing t h a t the o i l s h e l l c o n s t i t u t e s an impermeable b a r r i e r to d i f f u s i o n . Thus, though i t was a t f i r s t thought t h a t these foams might serve as a useful model f o r b i o l o g i c a l t i s s u e , c l e a r l y the e n c a p s u l a t i n g o i l f i l m does not have the p e r m e a b i l i t y p r o p e r t i e s of a bio-membrane.



28



The general behaviour of such water encapsulated b i l i q u i d foams resembles that of gas f i l l e d foams so c l o s e l y t h a t i s reasonable to t r e a t them as s i m i l a r macrocluster systems. There i s as yet no evidence to decide whether the o i l f i l m is coated w i t h an aqueous s u r f a c e l a y e r in c o n t a c t w i t h a t h i n continuous water f i l m or not. The aqueous phase c o n t a i n s WATSSA because i t s presence i s e s s e n t i a l f o r the production o f the microgas emulsion. If a small amount of OILSSA i s added to the o i l phase, the c h a r a c t e r o f the b i l i q u i d foam changes c o m p l e t e l y . The bubbles are very much s m a l l e r and more d e l i c a t e and the a d h e s i o n a l s t r e n g t h i s reduced so t h a t the bubbles e a s i l y f l o a t away as o i l encapsulated b i l i q u i d bubbles. It was t h i s o b s e r v a t i o n which led to the suggestion that something s i m i l a r , w i t h c h o l e s t e r o l as the OILSSA, was a f a c t o r in c a n c e r , Sebba {$) . It i s not c l e a r how the OILSSA reduces the a d h e s i o n , but as the Laplace pressure i s p r o p o r t i o n a l to the i n t e r f a c i a l t e n s i o n , and the OILSSA must reduce t h i s , the e x p l a n a t i o n may be the simple one of reduction of the Laplace pressures and hence the a d h e s i o n . The second type of b i l i q u i d foam i s one in which the d i s continuous phase i s o i l and the e n c a p s u l a t i n g phase i s aqueous. To produce such a foam, a microgas emulsion i s allowed to f l o a t on the o i l , j u s t as in the p r e p a r a t i o n of the f i r s t t y p e . If, however, some more o f the o i l i s poured onto the microgas e m u l s i o n , because the o i l i s heavier i t w i l l f a l l through as drops which get coated w i t h the aqueous s o l u t i o n . At the bottom of the microgas e m u l s i o n , i t passes through the second i n t e r f a c e , and completes the requirements f o r a foam. This now c o n s i s t s of o i l c e l l s in a t h i n aqueous s h e l l , the u n i t s , which have a formal resemblance to the gas f i l l e d soap b u b b l e , being separated by a t h i n f i l m of aqueous s o l u t i o n which resembles the continuous aqueous f i l m in a gas f i l l e d foam. Thus, i t i s seen that these foams are more a k i n to g a s - f i l l e d foams than i s the f i r s t type of b i 1 i q u i d foam. It i s necessary to emphasize the d i s t i n c t i o n between o i l in-water emulsions and o i l c e l l b i l i q u i d foams. In the former, there i s a s i n g l e i n t e r f a c e between the o i l drop and the c o n t i n uous aqueous medium, the e m u l s i f y i n g agent at t h i s i n t e r f a c e p r o v i d i n g an energy b a r r i e r which prevents c o a l e s c e n c e . There are none of the c o n d i t i o n s f o r a macrocluster system except in cases of very concentrated emulsions. In the b i l i q u i d foam, the o i l c e l l i s encapsulated by a t h i n aqueous f i l m producing the u n i t f o r the macrocluster system, the adhesion now being provided by the Laplace pressures a c r o s s the t h i n aqueous f i l m which c o n s t i t u t e s the continuous phase. B i l i q u i d foams of t h i s s o r t are o f t e n met w i t h in chemical and h y d r o m e t a l l u r g i c a l processes when o i l as w e l l as aqueous phases are i n v o l v e d . They have been looked upon as e m u l s i o n s , but o f t e n prove r e s i s t a n t to d e s t r u c t i o n using techniques which should destroy emulsions. It would appear that the c o r r e c t procedure would be to use techniques which are known to destroy


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foams, although t h i s i s a d m i t t e d l y an a r t and not always predictable. It w i l l be shown p r e s e n t l y that o i l encapsulated b i l i q u i d foams may provide a model f o r biomembranes and i n t r a c e l l u l a r s t r u c t u r e s may c o n s i s t o f n o n - m i s c i b l e l i q u i d s o p e r a t i n g a c c o r d i n g to the r u l e s f o r macrocluster b i l i q u i d foams. M a c r o c l u s t e r Systems and L i f e . The behaviour of lenses o f o i l spreading on water s u r f a c e s has been f u l l y d e a l t w i t h by Langmuir (6) and Harkins (7). However, t h e i r treatment concerns o n l y pure water and the behaviour o f lenses when spread on d i l u t e WATSSA s o l u t i o n s i s very d i f f e r e n t . The i n t e r a c t i o n o f two such lenses i s an example of a two-unit macrocluster system, and an understanding o f such behaviour may lead to b i o l o g i c a l i n s i g h t . A h i g h l y v i s c o u s o i l l i k e Nujol , i f f r e e of s u r f a c t a n t , does not spread on water o r d i l u t e s u r f a c t a n t s o l u t i o n . I f , however, some OILSSA i s incorporated i n t o the N u j o l , i t w i l l spread in a strange way. The nature of the spreading depends upon the c o n c e n t r a t i o n o f WATSSA in the s u b s t r a t e and as a l l the c o n d i t i o n s have not been f u l l y i n v e s t i g a t e d , a d e s c r i p t i o n of behaviour under o n l y one set o f c o n d i t i o n s i s g i v e n . The s u b s t r a t e i s water c o n t a i n i n g 0.2 g/1 o f sodium benzene sulphonate plus 0.1 g/1 of sodium sulphate decahydrate. The spreading lens i s Nujol c o n t a i n i n g \0% by volume of the TERGITOL 15-S-3. This i s a n o n - i o n i c s u r f a c t a n t , an a l k y l p o l y (ethyleneoxy) ethanol of HLB number 9 s u p p l i e d by Union Carbide C o r p o r a t i o n . The behaviour i s more dramatic i f the nujol i s coloured by an o i l s o l u b l e dye such as SUDAN 111 or WAXOLINE g r e e n , but the same behaviour can be observed in the absence of dye. The spreading behaviour i s r e a d i l y observed i f the s u b s t r a t e i s placed in a g l a s s d i s h , such as a pyrex baking d i s h , which i s i l l u m i n a t e d from below by a powerful lamp. If a drop of the Nujol s o l u t i o n i s placed on a c l e a n s u r f a c e , i t spreads r a p i d l y , but not as a c i r c u l a r lens but much more i r r e g u l a r l y . If now, a second drop i s placed on the s u r f a c e , which presumably now has a f i l m of OILSSA on i t s s u r f a c e , i t w i l l spread w i t h low contact angle more s l o w l y to a f a i r l y r e g u l a r d i s c , a couple of c e n t i metres in diameter. Spreading w i l l c o n t i n u e f o r a few moments and then suddenly the lens w i l l r e t r a c t , reducing a few m i l l i m e t e r s in diameter and w i t h an obvious increase in c o n t a c t a n g l e . From t h i s moment, spreading i s much slower and very i n t e r e s t i n g . Sometimes, a rhythmic expansion and c o n t r a c t i o n w i l l be o b s e r v e d , again w i t h i n a few m i l l i m e t r e s range o f diameter. T h i s may continue f o r a w h i l e , but i s o f t e n so s l i g h t as to be unobserved. T h i s i s not a new phenomenon. It has been observed w i t h c e t y l alcohol on water (8), and was e x p l a i n e d as due to a changing e x t e r n a l s u r f a c e pressure due to s o l u t i o n of the c e t y l a l c o h o l i n the s u b s t r a t e . As the s u r f a c e pressure dropped, the contact angle became l o w e r , and spreading of the lens could proceed r a i s i n g the s u r f a c e pressure to a point at which the contact angle



30



was r a i s e d s u f f i c i e n t l y f o r spreading to s t o p . When that occurred the s u r f a c e pressure dropped once again because of s o l u t i o n of the c e t y l a l c o h o l s u f f i c i e n t l y f o r spreading to be recommenced. A s i m i l a r mechanism wi11 e x p l a i n the rhythmic p u l s i n g of the Nujol l e n s . A f t e r a short t i m e , the lens c o n t a i n i n g OILSSA begins to spread a g a i n , but t h i s time i t does not do so uniformly in a l l directions. Sometimes, many f u z z y l i k e processes w i l l appear g i v i n g the lens the appearance of a m u l t i c i 1 i a t e d organism. These " h a i r s " w i l l g r a d u a l l y grow u n t i l a f t e r some minutes they w i l l have extended from the c e n t r a l lens to a d i s t a n c e o f many centimetres as f i l a m e n t - l i k e processes o f t e n w i t h b r a n c h i n g , (Figure 6). The appearance very much resembles r o o t - h a i r s . A f t e r a t i m e , the f i l a m e n t begins to d i s i n t e g r a t e i n t o t i n y g l o b u l e s of o i l w i t h high contact a n g l e . There has thus been an enormous increase in i n t e r f a c i a l perimeter c o n t a c t . As i t is almost impossible to avoid convection c u r r e n t s or s i m i l a r d i s t u r b a n c e , the p a t t e r n soon becomes very i r r e g u l a r and an i n f i n i t e v a r i e t y of patterns i s o b t a i n e d , the c e n t r a l lens d e p a r t i n g markedly from i t s o r i g i n a l c i r c u l a r p a t t e r n . Neverthe l e s s , growth continues and o f t e n the p u l s a t i o n is n o t i c e a b l e , so the odd " c r e a t u r e " f l o a t i n g on the water appears to be a l i v e . It i s p o s s i b l e to increase the rate at which the outgrowths extend by sucking o f f from the s u r f a c e a c o n s i d e r a b l e d i s t a n c e away, a t r i c k o f t e n employed f o r c l e a n i n g contamination from s u r f a c e s . The e f f e c t of t h i s i s to reduce the surface pressure of the monolayer, thus i n c r e a s i n g the rate of spreading of the lens. Sometimes, the lens does not grow " w h i s k e r s " , but develops a puckered perimeter. As t h i s occurs when the s u r f a c e i s o l d e r and c o n t a i n s a very large number of t i n y g l o b u l e s , presumably the d i m i n u t i o n of surface pressure i s s l o w e r , being replaced from a l l the small d r o p s , so spreading from the lens i s reduced, and there i s l e s s tendency f o r outgrowths to appear. However, a complete d e s c r i p t i o n and e x p l a n a t i o n of these s t r u c t u r e s must await a separate communication as these are not themselves macrocluster s t r u c t u r e s , although they can be the u n i t s which b u i l d up to macrocluster s t r u c t u r e s . The resemblance of some o f the s t r u c t u r e s found in s p r e a d ing lenses to c y t o l o g i c a l patterns may be more than a c o i n c i d e n c e and of these the most s t r i k i n g are resemblances to the s t e l l a t e c e l l s in nerve t i s s u e , the chromosomes and the endoplasmic r e t i c u l u m systems. Each p a t t e r n i s determined by a balance of a number of f a c t o r s which have not yet been q u a n t i t a t i v e l y d e t e r mined, but the f o l l o w i n g are some of them:1) Nature of WATSSA 2) Concentration of WATSSA in s u b s t r a t e 3) Nature of OILSSA k) Concentration o f OILSSA in o i l , which determines 5) Spreading pressure of lens 6) E q u i l i b r i u m surface pressure of OILSSA on s u b s t r a t e which


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i s determined by Rate of s o l u b i l i t y or s o l u b i 1 i s a t i o n of OILSSA in WATSSA solution 8) Nature and c o n c e n t r a t i o n of d i s s o l v e d s a l t s in s u b s t r a t e 9) Temperature 10) V i s c o s i t y of lens m a t e r i a l 11) Convection c u r r e n t s in s u b s t r a t e or a i r c u r r e n t s above substrate 12) Free area between lens and neighbours or o t h e r o b s t a c l e s 13) S i z e and shape of lens 14) Reaction to spreading f o r c e s 15) Age of WATSSA s o l u t i o n . With respect to ( 1 4 ) , the momentum o f these spreading lenses i s remarkable, and can themselves s t i r up c u r r e n t s o f c o n s i d e r a b l e magnitude which in turn can cause s p i r a l i n g amongst the h a i r - l i k e l e n s e s . When these get c l o s e enough t o g e t h e r , they form macroc l u s t e r systems which in appearance s t r o n g l y resemble endoplasmic r e t i c u l u m s t r u c t u r e s . Perhaps, the most s t r i k i n g macrocluster system i s t h a t produced by two adjacent l e n s e s , because i f the c o n d i t i o n s are c o r r e c t , they can simulate the b i o l o g i c a l phenomenon of endoc y t o s i s to a remarkable degree. Endocytosis i s the process by which a c e l l e n g u l f s f o r e i g n matter and embraces p i n o c y t o s i s and phagocytosis. In the f o l l o w i n g d i s c u s s i o n the term endocytosis w i l l be used to d e s c r i b e the phenomenon, although i t does not here r e f e r to l i v i n g c e l l s . The s i g n i f i c a n t property which determines t h i s behaviour i s a d i f f e r e n c e of c o n c e n t r a t i o n of OILSSA in the two l e n s e s . It i s best observed by using d i f f e r e n t c o l o u r s f o r the two l e n s e s , one of which i s c a l l e d the gobbler and the other i s the prey. For example, a good gobbler i s a 10% by volume s o l u t i o n o f TERGITOL 15-3-3 in N u j o l , coloured red w i t h a WAXOLINE dye. A good prey i s a s o l u t i o n of T e r g i t o l in Nujol l e s s concentrated than the g o b b l e r , coloured green w i t h WAXOLINE green. A 2% s o l u t i o n works w e l 1 , but Nujol without any OILSSA w i l l a l s o work. A lens of gobbler about 2 cm. diameter i s l a i d on the WATSSA s o l u t i o n in w a t e r , and when i t has almost reached a metastable e q u i l i b r i u m w i t h the s u r f a c e , i . e . spreads s l o w l y , a small d r o p , about 2 mm. diameter of prey i s placed a few m i l l i metres away from the g o b b l e r . They begin to a t t r a c t one a n o t h e r , and each moves towards the o t h e r , but because of i t s s m a l l n e s s , the prey moves f a s t e r . As they get c l o s e r , the gobbler may extrude some t e n t a c l e s , but these a r e not e s s e n t i a l . The two lenses are being drawn towards one another by Laplace pressures modified by the f a c t that the p r e y , having a lower c o n c e n t r a t i o n of OILSSA, w i l l absorb OILSSA from the i n t e r v e n i n g s u r f a c e on the s u b s t r a t e , inducing a lower s u r f a c e pressure i . e . higher s u r f a c e t e n s i o n , thus i n c r e a s i n g the Laplace p r e s s u r e . When the two lenses get very c l o s e , the Laplace pressure i s high enough to deform the g o b b l e r , so the gobbler envelops the prey which very soon i s completely engulfed by the g o b b l e r , and moves inwards 7)


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A N D M I C E L L A E BEHAVIOR

towards the c e n t r e of the g o b b l e r , but does not merge w i t h i t . T h i s shows t h a t there i s an envelope surrounding the prey which protects i t . In f a c t , It i s o f t e n p o s s i b l e to see a t h i n r e s i d u a l strand of prey connecting i t w i t h the o u t s i d e of the g o b b l e r , (Figure 7). If the s u r f a c e pressure o u t s i d e the gobbler i s c a r e f u l l y reduced by s u c t i o n , i t i s p o s s i b l e to open up the c a v i t y and the prey w i l l re-emerge unscathed. P h i l o s o p h e r s may muse on the f a c t that when the amoeba eats i t s v i c t i m , i t i s the v i c t i m which f o r c e s i t s way i n t o the amoeba, thus h a p p i l y committing s u i c i d e . Figure 8 shows s c h e m a t i c a l l y the mechanism for endocytosis. If the c o n c e n t r a t i o n o f OILSSA i s the same in the two l e n s e s , there i s no a t t r a c t i o n , and i f they are placed c l o s e t o g e t h e r , they may adhere but do not e n g u l f . The c o n t a c t angle seems to play a part as d r o p l e t s which do not have a spreading contact angle i . e . i s not l e s s than 9 0 ° do not show the phenomenon. If there i s no s a l t in the s o l u t i o n , i t seems not to o c c u r , but a d d i t i o n of 3 x 10~^ sodium sulphate enables i t to proceed. This i s understandable on the hypothesis t h a t in order f o r the prey to d i s t o r t the gobbler the Laplace pressure must be high i . e . the two lenses must approach w i t h i n a minimum d i s t a n c e before i t can overcome the r e p u l s i v e f o r c e s caused by the double l a y e r . Salts w i l l reduce the t h i c k n e s s o f the double l a y e r a n d , t h e r e f o r e , enable the lenses to get c l o s e r . The prey does not have to be a l e n s . If a platinum wire or t h i n g l a s s rod i s inserted i n t o the s u b s t r a t e c l o s e to the g o b b l e r , provided the contact angle i s c o r r e c t , the gobbler w i l l s t a r t moving towards the s o l i d and engulf i t . Presumably even the small increase of s u r f a c e area provided by the platinum o r g l a s s i s enough t o lower the s u r f a c e pressure s u f f i c i e n t l y f o r a t t r a c t i o n to o c c u r . D i f f e r e n c e s of spreading pressures may a l s o o f f e r a mechanism f o r the b i o l o g i c a l phenomenon of c y t o p l a s m i c movement. Lenses a f t e r spreading end up as innumerable small g l o b u l e s immersed in a monolayer of OILSSA, there c l e a r l y being e q u i l i b r i u m between the spreading pressure o f the g l o b u l e s and the s u r f a c e pressure of the monolayer. If to such a system, a drop of o i l , Nujol o r k e r o s l n e , low in OILSSA Is added, e x t r a o r d i n a r y a c t i v i t y commences. It s t a r t s w i t h the nearest small g l o b u l e s moving towards the low pressure o i l , but t h i s movement soon spreads and even the most d i s t a n t g l o b u l e s begin to p a r t i c i p a t e . As movement i s r a r e l y uniform in a l l d i r e c t i o n , soon a streaming movement i s observed and t h i s may develop i n t o a s p i r a l movement. If there are s t i l l s t r a n d - l i k e lenses on the s u r f a c e , these f o l l o w the s p i r a l movement, and wind-up l i k e a c o i l e d s p r i n g so that c o n c e n t r i c l a y e r s of lens w i t h the low pressure o i l at the centre r e s u l t . These s t r o n g l y resemble the endoplasmic r e t i c u l u m observed in c e l l s . The motive energy i s o b v i o u s l y the movement of OILSSA from the higher pressure on the s u b s t r a t e s u r f a c e i n t o the low pressure o i l . What i s i n t e r e s t i n g i s that the small g l o b u l e s when they reach the c e n t r e o i l l e n s , o f t e n rebound from M



SEBBA


Figure 6.

Lens spreading on water

Water.

Figure 7. Model for endocytosis. Hatched circle represents victim lens. Note neck to periphery.

WATER Figure 8. Mechanism of endocytosis. OILSSA moves from lens I to lens II thus tending to increase X , This increases Laplace pressure which is 8 so lens I and lens II are attracted. Spreading pressure of I > spread ing pressure of II so when they touch I spreads around II. The higher the concentration of WATSSA, the less marked the effect of OILSSA on * so attraction is reduced.



34



i t , or e n c i r c l e i t but r a r e l y merge i n t o i t . In other words, although a macrocluster system i s formed, i t i s not a strong one. On the c o n t r a r y , there i s o f t e n a sudden spurt of a c t i v i t y at one p o i n t , and a l l the e n c i r c l i n g small g l o b u l e s get swept to a d i s t a l point on the l e n s , from which p o i n t they again s l o w l y form a r i n g around the l e n s . The reason f o r t h i s e x p l o s i v e p o l a r i z a t i o n i s not obvious but i t almost looks at though an enveloping bubble had burst. The general movement i s remarkably r a p i d , and i s an example of conversion of s u r f a c e energy i n t o movement. It i s , of c o u r s e , the same phenomenon as the camphor c r y s t a l d a r t i n g about the s u r f a c e of water. When there are thousands of l i t t l e g l o b u l e s , a l l moving and a l l a l s o showing r e a c t i o n , i . e . j e t p r o p u l s i o n , as they lose OILSSA, i t i s c l e a r that the movement i s complex, but i t must in i t s t u r n c o n t r i b u t e to the p a t t e r n and shape of other lenses which are developing or packing i n t o macroc l u s t e r systems. Not o n l y i s endocytosis simulated in t h i s way, but so too i s e x o c y t o s i s . This i s b e t t e r shown by o i l s not as v i s c o u s as N u j o l , although the Nujol w i l l show i t more s l o w l y . If some dyed k e r o s i n e , c o n t a i n i n g say 2% o f TERGITOL OILSSA i s spread on the aqueous s u b s t r a t e , i t forms a t y p i c a l lens w i t h a f a i r l y even c i r c u m f e r e n c e . T h i s s l o w l y deforms, i s l a n d s o f monolayer appear and dark zones on the circumference of the kerosine appear. This i n d i c a t e s t h i c k e n i n g of the l e n s , and i t i s i n t e r e s t i n g t h a t these regions of d i f f e r e n t t h i c k n e s s appear showing that pressure i s not u n i f o r m l y d i s t r i b u t e d , (Figure 9). If in the i n t e r i o r of such a l e n s , a drop o f kerosine of d i f f e r e n t c o l o u r , but c o n t a i n i n g a higher c o n c e n t r a t i o n of OILSSA i s p l a c e d , a f t e r a very short period the f o l l o w i n g sequence i s observed. The new lens r a p i d l y expands and then r e t r a c t s to form i s l a n d s o f o i l in a monolayer o f OILSSA. These i s l a n d s may or may not merge. They move towards the t h i n n e s t part of the lens where the monolayer, being at higher s u r f a c e pressure than o u t s i d e , causes a bulge and e v e n t u a l l y breaks through c a r r y i n g the added k e r o s i n e as a lens or lenses out with i t . The lens then reforms to an approximately c i r c u l a r shape w h i l e the added lenses remain o u t s i d e j u s t touching the reformed lens which they then because of t h e i r higher OILSSA content attempt to e n g u l f . What i s s i g n i f i c a n t i s t h a t the s u r f a c e generated w i t h i n the i n t e r i o r of the l e n s , in the readjustment now forms part of the e x t e r n a l s u r f a c e of the l e n s . T h i s i s p r e c i s e l y the o p p o s i t e to what happens in the e n d o c y t o s i s , and simulates what happens in many types of c e l l , (Figure 10). To summarise, i n t e r a c t i o n of two l e n s e s , and presumably of two c e l l s , i s in essence the formation o f a two u n i t macroc l u s t e r system, w i t h endocytosis o c c u r r i n g when the OILSSA c o n c e n t r a t i o n in gobbler i s g r e a t e r than t h a t in the p r e y , and r e j e c t i o n o f the c e l l ( e x o c y t o s i s ) when the OILSSA c o n c e n t r a t i o n i s g r e a t e r in r e j e c t e d c e l l than in the host c e l l . Fertilisation of an ovum by a spermatozoon i s presumably a s p e c i a l case of endocytosis. It i s easy to envisage a s i t u a t i o n in which the



35


Water surface.

Figure 9. Spreading lens. Lens break ing up. M represents monolayer. Rest of lens is thicker. Note sites of thickening at perimeter as lens retracts.

Figure 10. Mechanism of exocytosis. Note that internal surface of A generated near Β becomes part of the external surface of A after Β has been ejected.



36



spermatozoon introduces i n t o the ovum some chemical which i n a c t i v a t e s the OILSSA in the ovum. The e f f e c t of that would be to reduce the spreading p r e s s u r e , and hence increase the contact a n g l e , w i t h the consequence that a second spermatozoon would not be a t t r a c t e d to enter the ovum. In a previous communication (5) i t was suggested that cancer occurred because the c o n c e n t r a t ion of c h o l e s t e r o l , a t y p i c a l OILSSA, was above t h a t in normal c e l l s due t o an Induced g e n e t i c d e f e c t . It Is easy to see now why such c e l l s are I n v a s i v e , i t Is because they behave as t y p i c a l gobbler c e l l s , normal c e l l s having l e s s OILSSA becoming the prey. If, however, these are not m o b i l e , but the cancer c e l l s a r e , the consequence would be i n f i l t r a t i o n by the cancer c e l l along the aqueous lamellae between the t i s s u e c e l l s . T w o - D î m e n s i o n a l B i l i q u i d Foams. If beneath the s u r f a c e of a d i l u t e WATSSA s o l u t i o n , a stream of high v i s c o s i t y o i l such as Nujol c o n t a i n i n g some OILSSA i s r e l e a s e d , i t does not have time to break i n t o d r o p l e t s but u s u a l l y reaches the s u r f a c e as a thread of o i l . On p e n e t r a t i n g the s u r f a c e , the thread spreads to f l a t l e n s e s , and these adhere to form two-dimensional b i l i q u i d foams, the c o n d i t i o n s of breaking two surfaces a p p l y i n g in t h i s case as w e l l . A s i m i l a r e f f e c t w i t h s m a l l e r u n i t s i s obtained by beating a lens Into the s u b s t r a t e . The threads produced by an expanding lens w i l l o f t e n adhere In the same way to form s i m i l a r s t r u c t u r e s . These foams d i s p l a y some of the c h a r a c t e r i s t i c s of s i n g l e l e n s e s . For example, the endocy t o s i s process can o c c u r . If the o u t s i d e s u r f a c e pressure i s reduced by sucking o f f the s u r f a c e , the i n d i v i d u a l lenses w i l l move a p a r t , and reform again on s t a n d i n g . It i s suggested t h a t biomembranes are s p e c i a l examples of two dimensional b i l i q u i d foams. It i s now b e l i e v e d t h a t the Davson-Daniel1i model f o r a biomembrane i s not s a t i s f a c t o r y . E l e c t r o n microscopy r e v e a l s the e x i s t e n c e of p o r e s . A twodimensional b i l i q u i d foam, in which the lenses are very t h i n , and may even be b i m o l e c u l a r , seems to s a t i s f y some o f the conditions. One of the problems of biomembranes i s how penetration is regulated. It i s suggested t h a t r e g u l a t i o n could be achieved by metabolic c o n t r o l o f OILSSA c o n c e n t r a t i o n , as i l l u s t r a t e d In F i g u r e 11 and 12. If the OILSSA c o n c e n t r a t i o n i n c r e a s e s , the lenses w i l l tend to expand and the OILSSA w i l l spread to the i n t e r - l e n s aqueous s u r f a c e , u n t i l the s u r f a c e pressure equals the h o r i z o n t a l component of the spreading pressure. If the OILSSA goes i n t o the aqueous phase f a s t e r than i t moves out o f the spreading l e n s , the lenses w i l l expand and the pores w i l l c l o s e . If It Is produced in the lens f a s t e r than i t d i s s o l v e s In the aqueous phase, the s u r f a c e pressure w i l l b u i l d up, and the lens w i l l r e t r a c t , opening the p o r e s . In f a c t , there i s s i m i l a r i t y between t h i s mechanism and the opening and c l o s i n g of stomata except that in the l a t t e r case i t i s threedimensional pressures in the g u a r d - c e l l s and not two-dimensional


2.

SEBBA

37



AIR

WATER Figure 11. Opening of pores. OILSSA moves more slowly into aqueous phase so concentration on surface builds up. Effect is to decrease V and β rises so lenses contract and pores open up.

AIR

OILSSA

OILSSA

WATER Figure 12. Closing of pores. OILSSA moves via surface into aqueous phase faster than it is spread onto surface. Effect is to increase # and to compensate & drops, lenses expand and pores close.


38



pressures that are i n v o l v e d . In another p u b l i c a t i o n , suggestions w i l l be made as to the nature of the OILSSA in l i v i n g systems and how i t s c o n c e n t r a t i o n i s c o n t r o l l e d .


Conclus ions In t h i s paper, the attempt i s made to show that the behaviour of many types of two phase systems can be b e t t e r explained by c o n s i d e r i n g them as macrocluster systems. In order to understand t h e s e , i t i s necessary to assume that the t h i n f i l m of water adjacent to another phase, gas or l i q u i d , behaves as though i t i t s e l f was a d i f f e r e n t phase to bulk water. The behaviour of l e n s e s , spread on water shows remarkable resemblance to c y t o l o g i c a l behaviour. There i s no proof that t h i s behaviour can n e c e s s a r i l y be e x t r a p o l a t e d i n t o three dimensions, but i t might be u s e f u l to assume i t c o u l d . The problem i s that when spread on w a t e r , the t h i r d phase i s a i r , whereas presumably there i s not gas in c y t o p l a s m , though the question could be asked whether the absence of gas has been unequivocably proven. In c y t o p l a s m , the three phases would be an o i l - p h a s e , an aqueous phase and the s u r f a c e phase, but one of the fundamentals of any phase study is that thermodynamics i s not concerned w i t h the nature of the phases but o n l y w i t h the number of phases. However, there i s one q u a l i t a t i v e f a c t that would j u s t i f y the e x t r a p o l a t i o n from two-dimensional to three-dimensional systems and that i s that when experimenting w i t h b i l i q u i d foams whenever three d i f f e r e n t f l u i d phases met, there was e x t r a o r d i n a r y motion between them resembling the simulated protoplasmic motion described in the twodimensional model. (No attempt has yet been made to e x p l a i n the problems met w i t h in industry in f l o t a t i o n , f l o c c u l a t i o n , and f i l t r a t i o n in terms of the concepts introduced h e r e , but these approaches might very w e l l prove p r o f i t a b l e ) . T h i s paper can o n l y be considered as a p r e l i m i n a r y i n v e s t i g a t i o n , but i t does suggest that the s e c r e t s of l i f e l i e in the behaviour of immiscible l i q u i d s under the i n f l u e n c e of t h e i r interfacial forces. Acknowledgements Thanks are expressed to Mrs. F. Horwitz who helped w i t h the b i l i q u i d experiments and to Mr. T. Ambrose who helped w i t h the photographs. Literature Cited 1. 2. 3.

Sebba, F . , J . C o l l o i d Interface Sci., (1971), 35, 643 Sebba, F., Nature (London), (1963), 197, 1195 Adamson, A.W., " P h y s i c a l Chemistry of S u r f a c e " 2nd ed., pp. 348, 383, I n t e r s c i e n c e , New Y o r k , N.Y. 1967


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4. Sebba, F., J. C o l l o i d I n t e r f a c e Sci., (1972), 40, 468 5. Sebba, F., J. Colloid Interface Sci., (1972), 40, 479 6. Langmuir, I., J. Chem. Phys., (1933), 1, 756 7. H a r k i n s , W.D., "Colloid Symposium Monograph", 6, p.24, Chemical Catalog C o . , New York, N . Y . , 1928 8. Sebba, F. and B r i s c o e , H . V . A . , J. Chem.Soc., (1940), 114.


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