Macromolecular Assemblies in Polymeric Systems - American

Chapter 13

Mixed Monolayers of Lecithin and Bile Acids at the Air—Aqueous Solution Interface Downloaded by UNIV MASSACHUSETTS AMHERST on October 12, 2012 | http://pubs.acs.org Publication Date: June 10, 1992 | doi: 10.1021/bk-1992-0493.ch013

Effect of Temperature and Subphase pH M. J . Gálvez-Ruiz and M . A. Cabrerizo-Vilchez Department of Applied Physics, Biocolloid and Fluid Physics Group, University of Granada, Granada 18071, Spain

A study of mixed monolayers at the air-aqueous solution interface has been carried out. These kind of systems could be considered the most simple model to get molecular information and hence, their study is the first step to undertand the properties and the behavior of macromolecular assemblies. Surface pressure-molecular area isotherms of mixed monolayers of lecithin with chenodeoxycholic, deoxycholic and cholic acid spreaded at aqueous solution/air interface have been recorded over a wide range of pH (2 to 12) and different temperatures (25-40°C). The monolayer properties of bile acids are influenced by the subphase pH: important changes occur in both the electric characteristics and the stability of the monolayers. Also, the behavior of these films is modified with the temperature. These changes are attributed to a decrease in the hydrophobic forces between the molecules forming the monolayer and a progressive dehydration of the polar groups as temperature increases. The addition of lecithin to a bile acid film produces molecular condensation. Consequently the mixed monolayers are more stable than the simple bile acid films. This molecular association is mainly due to the attractive forces of the van der Waals type between the hydrophobic regions. The different behavior observed is largely influenced by subphase pH, temperature and surface pressure values and, therefore, by the orientation of the molecules with respect to the liquid surface.

0097-6156/92/0493-0135$06.00/0 © 1992 American Chemical Society

In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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The understanding of the behavior of mixed monolayers a t the aqueous s o l u t i o n - a i r i n t e r f a c e i s a step more towards a general theory f o r the c h o l e l i t h i a s i s processes. I n v e s t i g a t i n g the p r o p e r t i e s of mixed monolayers i s of great i n t e r e s t , because i t enables to g a i n knowledge about the i n t e r a c t i o n s between the monolayer components. These i n t e r a c t i o n s p l a y an important r o l e i n some b i o l o g i c a l processes. The i n t e r a c t i o n between amphiphilic molecules a t the air-aqueous s o l u t i o n i n t e r f a c e i s c o n t r o l l e d by three d i f f e r e n t f o r c e s : e l e c t r o s t a t i c , hydrophobic and h y d r a t i o n f o r c e s (1). The e l e c t r o s t a t i c and h y d r a t i o n c o n t r i b u t i o n s depend on subphase pH, whereas the hydrophobic and h y d r a t i o n f o r c e s are dependent on temperature. The behavior of the simple monolayers formed by l e c i t h i n or b i l e a c i d s as a f u n c t i o n of pH and temperature has been s t u d i e d i n previous papers (2, M.J.Gâlveζ-Ruiz and C a b r e r i z o - V i l c h e z , Colloids and Surfaces, i n press). The behavior of the simple monolayers of b i l e a c i d s s t u d i e d as a f u n c t i o n of the subphase pH i s t o be expanded when the pH values are s h i f t e d away from the a c i d pK. value (M.J. Gâlvez-Ruiz and M. A. Cabrerizo-Vilchez, Colloids and Surfaces, i n press) and the l e c i t h i n simple monolayers become more expanded when the pH i n c r e a s e s (2) due, i n both cases, to the increase of repulsive e l e c t r o s t a t i c f o r c e s between the p o l a r groups. A l s o , when the temperature i s i n c r e a s e d a l l the simple monolayers become more expanded. In t h i s work we study the behavior of mixed monolayers formed by lecithin and bile acid (chenodeoxycholic, deoxycholic and c h o l i c a c i d s ) as a f u n c t i o n of pH and temperature. Those compounds are i n v o l v e d i n the c h o l e l i t h i a s i s processes. An attempt has been made f o r e x p l a i n i n g the mechanisms i n v o l v e d i n g a l l s t o n e formation on the b a s i s of the i n t e r a c t i o n s i n mixed monolayers. M a t e r i a l s and Methods A n a l y t i c a l grade chenodeoxycholic and deoxycholic a c i d s were from Serva and the other monolayer components (L-ap h o s p h a t i d y l c h o l i n e and c h o l i c a c i d ) were from Sigma. The spreading solvent was a mixture of nhexane/ethanol 4:1 (v/v) (Merck A.R. grade), and 0.05% amylalcohol was added to improve spreading ( J ) . A B r i t t o n Robinson b u f f e r was used i n a l l the experiments s i n c e i t i s s u i t a b l e f o r the p r e p a r a t i o n of s o l u t i o n s i n a wide pH range. T h i s b u f f e r c o n s i s t s of a s o l u t i o n c o n t a i n i n g a c e t i c , phosphoric and b o r i c a c i d s , and i t s pH can be adjusted between 2 and 12 by a d d i t i o n of adequate amounts of sodium hydroxide. A l l these compounds were s u p p l i e d by Merck with a n a l y t i c a l q u a l i t y . Surface pressure-molecular area isotherms were


13. GALVEZ-RUIZ & CABRERIZO-VILCHEZ performed u s i n g method with a balance.

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the p r e v i o u s l y d e s c r i b e d (2) Langmuir computer-controlled "Lauda Filmwaage"


R e s u l t s and D i s c u s s i o n Mixed Monolayers C h a r a c t e r i s t i c s o f Chenodeoxycholic A c i d and L e c i t h i n . In F i g u r e 1 we observe t h a t a l l mixed monolayers formed by d i f f e r e n t proportions of l e c i t h i n and chenodeoxycholic a c i d show an intermediate behavior between those and the simple monolayers. T h i s behavior i s due t o the important condensing e f f e c t o f l e c i t h i n over chenodexycholic a c i d (more expanded than the l e c i t h i n monolayers) on the whole pH (2-12) and temperature (2540°C) range. Even, when t h e parameters increase, p r a c t i c a l l y a l l isotherms, obtained by compressing of the mixed monolayers, show the l i q u i d expanded ( L E ) - l i q u i d condensed (LC) phase t r a n s i t i o n , i n o p p o s i t i o n with the behavior of the b i l e a c i d simple monolayers a t high pH and temperature v a l u e s . Then, t h i s condensing e f f e c t i s r e s p o n s i b l e f o r t h e i n c r e a s e o f t h e mixed monolayer s t a b i l i t y a t the aqueous s o l u t i o n - a i r i n t e r f a c e . The i n t e r a c t i o n between amphiphilic molecules a t the air-aqueous s o l u t i o n i n t e r f a c e w i l l be c o n t r o l l e d by the c o n t r i b u t i o n of e l e c t r o s t a t i c , hydrophobic and h y d r a t i o n forces. For t h i s mixed system, we examine t h e i n f l u e n c e o f these f o r c e s , a n a l y z i n g the e f f e c t of the subphase pH and temperature. When the pH i s 2 o r 6, and the s u r f a c e pressure value i s 5 mNm", we observe ( i n Figure 2) p o s i t i v e d e v i a t i o n s from the law o f i d e a l mixing. Such p o s i t i v e d e v i a t i o n s i n d i c a t e t h a t the p o l a r groups of the molecules i n t e r a c t more s t r o n g l y by r e p u l s i v e e l e c t r o s t a t i c f o r c e s with each other than with t h e more hydrophobic p a r t o f t h e molecules. A l s o , the h y d r a t i o n f o r c e s can r e s u l t i n the formation of Η-bonding due t o the presence of carboxyl and hydroxyl groups i n the molecules. If the pH i s 4 and the surface pressure i s 5 mNm", the d e v i a t i o n s a r e negative. Under those experimental c o n d i t i o n s the predominant f o r c e s are hydrophobic. T h i s could be explained because of the chenodeoxycholic a c i d pK value i s around pH 4 (4) and a t t h i s pH value the molecules a r e not deprotonate. When the pH i s 8, a t the same s u r f a c e pressure v a l u e , we have found negative d e v i a t i o n s a t high molar f r a c t i o n of chenodeoxycholic a c i d and p o s i t i v e d e v i a t i o n s f o r the monolayers c o n t a i n i n g higher p r o p o r t i o n of l e c i t h i n . At b a s i c pHs (10-12) the d e v i a t i o n s from the law of i d e a l mixing a r e negative. These r e s u l t s c o u l d be explained by t h e p r o g r e s s i v e disappearance of the h y d r a t i o n f o r c e s as the pH increases s i n c e , under these pH values the molecules are i o n i z e d and the d e v i a t i o n s should 1

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Area (À /molecule) Figure 1. Surface pressure-area isotherms f o r monolayers c o n t a i n i n g l e c i t h i n and chenodeoxycholic a c i d i n d i f f e r e n t p r o p o r t i o n s . L e c i t h i n mole f r a c t i o n is indicated. Continued on next page In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.



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F i g u r e 2. Molecular area as a f u n c t i o n of the lecithin mole fraction in mixed lecithinchenodeoxycholic a c i d monolayers, a t rc=5 mNm". Varying pH at fixed temperature or varying temperature a t f i x e d pH, as i n d i c a t e d . 1

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be p o s i t i v e . Nevertheless, we have t o take i n account the p o s s i b i l i t y of s o l u b i l i z a t i o n of the chenodeoxycholic a c i d molecules i n the subphase as b a s i c pH values (8-12). In order t o study t h i s , we have obtained the l i m i t i n g molecular areas, Ao, by e x t r a p o l a t i o n of the steep l i n e a r p o r t i o n s of the π v s . A curves t o π = 0. In Table I these values f o r three mixed monolayers, a t 25°C and a t d i f f e r e n t pH values are shown. Downloaded by UNIV MASSACHUSETTS AMHERST on October 12, 2012 | http://pubs.acs.org Publication Date: June 10, 1992 | doi: 10.1021/bk-1992-0493.ch013

2

Table I. Limiting Molecular Area Values, A„ (À /molecule), for Mixed Monolayers of Lecthin and Bile Acids at 25°C Mixed Monolayer/pH

2JQ 4.0

6.0

8Ό

10.0

12.0

25% Lecithin75% Bile acid

Chenodeoxy. Deoxychol. Cholic

14.5 19.9 18.9 18.4 15.1 16.0 17.1 17.7 15.2 15.0 15.7 15.1

17.1 17.0 16.9

16.5 17.0 19.6



273 28.1 31.8 30.9 28.2 29.7 30.7 30.9 28.0 29.0 29.1 28.7

31.6 32.2 303

303 31.0 29.6



39.4 40.0 44.0 47.7 40.2 43.2 43.8 45.0 40.6 41.5 42.1 41.7

47.0 45.5 42.4

46.9 44.6 43.4

We observe t h a t the isotherms are s h i f t e d toward lower Ao values with i n c r e a s i n g pH above the 8 v a l u e . T h i s e f f e c t i s more pronounced as the p r o p o r t i o n of b i l e a c i d i n c r e a s e s . I t should be noted t h a t behavior c h a r a c t e r i z e d by c o n t r a c t i o n of the areas as a f u n c t i o n of pH has been v e r i f i e d by Tomoaia-Cotisel e t a l . (5) f o r other compounds ( f a t t y a c i d s ) and t h i s i s a t t r i b u t e d t o the d i s s o l u t i o n of the i o n i z e d a c i d s i n the subphase. Studying the e f f e c t of temperature, we observe ( i n F i g u r e 2 ) , a t s u r f a c e pressure value 5 mNm", and 25, 30 and 35 °C., p o s i t i v e d e v i a t i o n s from the law of i d e a l mixing. These d e v i a t i o n s disappear a t 40°C. A l s o , i n Table I I , the l i m i t i n g molecular area v a l u e s , Ao, a t pH = 6.00 and as a f u n c t i o n of temperature are shown. A temperature r i s e favours the f o l l o w i n g e f f e c t s : ( i ) an i n c r e a s e i n the m o b i l i t y of the hydrophobic p a r t s and, t h e r e f o r e , a decrease i n the a t t r a c t i v e energies with the i n c r e a s e d area t h a t accompanies the temperature i n c r e a s e (6"), ( i i ) a p a r t i a l o r t o t a l dehydration of the p o l a r groups with a decrease i n the number of water molecules bound t o the monolayers and the r e s u l t i n g condensation of the s u r f a c e phase (6), and ( i i i ) the d i s s o l u t i o n of the i o n i z e d molecules i n the subphase. Then, f o r these systems, when the temperature i n c r e a s e s t o 35°C the most important e f f e c t i s t h a t the a t t r a c t i v e hydrophobic f o r c e s decrease which g i v e s r i s e t o 1


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a more expanded c h a r a c t e r of mixed monolayers. A t 40°C the more n o t i c e a b l e e f f e c t s w i l l be the dehydration of the p o l a r groups i n the monolayers and the s o l u b i l i z a t i o n of the molecules i n the subphase. When the compression of the monolayers i s h i g h , a t 30 mNm", we cannot observe any d e v i a t i o n s from the law of i d e a l mixing on the whole pH and temperature range. In the condensed s t a t e , a t t r a c t i v e and r e p u l s i v e e l e c t r o s t a t i c i n t e r m o l e c u l a r f o r c e s a r e compensated. /


1

Table II. Limiting Molecular Area Values, A,, (A /molecule), for Mixed Monolayers of Lecthin and Bile Acids at pH = 6.0 2

Mixed MonolayersA'CC)

25Ό

3ÔÔ

35Ό

4ÔÔ"


Chendeoxy. Deoxychol. Cholic

18.9 17.1 15.7

203 18.2 22.9

223 17.8 20.6

20.1 253 27.6


Chendeoxy. Deoxychol. Cholic

31.8 30.7 29.1

32.5 33.2 32.7

34.4 35.7 33.7

31.9 41.1 44.4



44.0 43.8 44.1

45.4 46.2 48.7

46.9 49.3 47.3

47.9 47.9 59.2

Mixed Monolayers C h a r a c t e r i s t i c s o f Deoxycholic A c i d and Lecithin. In F i g u r e 3, the isotherms obtained by compressing mixed monolayers formed of d e o x y c h o l i c a c i d and l e c i t h i n , are shown. These a r e s i m i l a r t o those found f o r the l a s t system (chenodeoxycholic a c i d - l e c i t h i n ) on the whole pH and temperature range. We a r e i n t e r e s t e d i n comparing the behavior of t h i s system with the l a s t one, because the o n l y d i f f e r e n c e i s t h a t the molecules of chenodeoxycholic and deoxycholic a c i d s have a hydroxyl group in a different position (transand c i s - , r e s p e c t i v e l y , with r e s p e c t t o the other hydroxyl group). At a c i d i c pH v a l u e s , we observe the same type of d e v i a t i o n s from the law of i d e a l mixing t h a t we found p r e v i o u s l y f o r the chenodeoxycholic a c i d - l e c i t h i n system. However, a t b a s i c pH values (8-12) a l l the d e v i a t i o n s are p o s i t i v e . In t h i s case, the presence of two hydroxyl groups i n c i s - p o s i t i o n provokes a i n c r e a s e of the r e p u l s i v e e l e c t r o s t a t i c f o r c e s and a l s o , the h y d r a t i o n f o r c e s appear t o be present r e s u l t i n g i n the formation of Η-bonding i n o p p o s i t i o n with the chenodeoxycholic a c i d molecules, where the hydroxyl groups are i n t r a n s - p o s i t i o n and the formation of Η-bonding i s l e s s favoured. The temperature e f f e c t on the d e o x y c h o l i c a c i d l e c i t h i n mixed monolayers i s the same as t h a t p r e v i o u s l y




Area ( À V m o l e c u l e )

Area ( À V m o l e c u l e ) Figure 3. Surface pressure-area isotherms f o r monolayers c o n t a i n i n g l e c i t h i n and deoxycholic a c i d i n d i f f e r e n t p r o p o r t i o n s . L e c i t h i n mole f r a c t i o n i s indicated. Continued on next page In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

Lecithin & Bue Acids


13. GÀLVEZ-RUIZ & CABRERIZO-VILCHEZ

Figure 3. Continued


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d e s c r i b e d f o r the chenodeoxycholic a c i d - l e c i t h i n system. Then, we can conclude t h a t t h i s d i f f e r e n t p o s i t i o n of the hydroxyl groups, p r a c t i c a l l y has no i n f l u e n c e on the p o s s i b l e i n t e r a c t i o n s with the l e c i t h i n molecules except at high pH v a l u e s . The condensing e f f e c t of the l e c i t h i n over d e o x y c h o l i c a c i d can be seen i n F i g u r e 4. Mixed Monolayers C h a r a c t e r i s t i c s of C h o l i c A c i d and Lecithin. In F i g u r e 5, the isotherms obtained by compressing monolayers formed by c h o l i c a c i d and l e c i t h i n mixed i n d i f f e r e n t p r o p o r t i o n s are shown. I t can be observed t h a t mixed monolayers of these compounds show a somewhat d i f f e r e n t behavior compared with t h a t shown by the previous systems with other b i l e a c i d s . The main d i f f e r e n c e i s that at high pH and temperature values and when the monolayers have a high p r o p o r t i o n of c h o l i c a c i d , the monolayers are more condensed and the EL-CL phase t r a n s i t i o n d i s a p p e a r s . P r e v i o u s l y , we d i s c u s s e d (M.J. Gâlvez-Ruiz and M.A. C a b r e r i z o - V i l c h e z , Colloids and Surfaces, i n press) t h a t under those experimental c o n d i t i o n s the c h o l i c acid molecules are d i s s o l v e d i n the subphase. T h i s i s due t o the presence of three hydroxyl groups i n these molecules. However, the presence of a more hydroxyl group i n the molecules does not have an important i n f l u e n c e on the i n t e r a c t i o n s with the l e c i t h i n molecules, because s t u d y i n g the law of i d e a l mixing, we have found the same d e v i a t i o n s as f o r the previous systems. At lower pH values (2-8) the d e v i a t i o n s are p o s i t i v e and a t b a s i c pH values (10-12) the d e v i a t i o n s are negative (Figure 6). These d e v i a t i o n s are e x p l a i n e d i n the same terms as before. The o n l y d i f f e r e n c e i s , t h a t the p o s i t i v e d e v i a t i o n s are more pronounced, even at pH 4 these do not disappear where the molecules are not deprotonate. Consequently, the h y d r a t i o n f o r c e s can be more s i g n i f i c a n t i n t h i s case, due again t o the presence of a t h i r d hydroxyl group i n the molecules of the c h o l i c acid. In F i g u r e 6 we observe a l i g h t decrease of the p o s i t i v e d e v i a t i o n s when the temperature i n c r e a s e . U n l i k e the previous system, these d e v i a t i o n s no disappear a t 40°C due t o the presence of a t h i r d hydroxyl group i n the cholic acid molecules. Again, the possibility of molecules' s o l u b i l i z a t i o n i n the subphase must be taken i n t o account. Comparing the behavior of the three mixed systems, the most n o t i c e a b l e r e s u l t i s , that the same c h a r a c t e r f o r the i n t e r a c t i o n s between molecules of l e c i t h i n and b i l e a c i d has been found. When the pH i n c r e a s e s from 2 t o 12, the p o s i t i v e d e v i a t i o n s become negative and an i n c r e a s e of temperature between 25° and 40°C b r i n g s about the p r o g r e s s i v e disappearance of the p o s i t i v e d e v i a t i o n s . Then, the condensing e f f e c t of the l e c i t h i n over b i l e a c i d molecules i s v e r y s i m i l a r f o r the three a c i d s s t u d i e d . The



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F i g u r e 4. Molecular area as a f u n c t i o n of the l e c i t h i n mole f r a c t i o n i n mixed l e c i t h i n - d e o x y c h o l i c a c i d monolayers, a t τι=5 mNm'. Varying pH a t f i x e d temperature o r v a r y i n g temperature a t f i x e d pH, as indicated. ~ . , Continued on next page 1

American Chemical Society Library 1155 16th St.. N.W. In MacromolecularWashington. Assemblies in Polymeric Systems; Stroeve, P., et al.; D.C 20036

ACS Symposium Series; American Chemical Society: Washington, DC, 1992.



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Area

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Figure 5. Surface pressure-area isotherms for monolayers c o n t a i n i n g l e c i t h i n and c h o l i c a c i d i n d i f f e r e n t p r o p o r t i o n s . L e c i t h i n mole f r a c t i o n i s indicated. Continued on next page In Macromolecular Assemblies in Polymeric Systems; Stroeve, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.



Area

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F i g u r e 6. Molecular area as a f u n c t i o n of the l e c i t h i n mole f r a c t i o n i n mixed l e c i t h i n - c h o l i c a c i d monolayers, a t τι=5 mNm'. Varying pH a t f i x e d temperature o r v a r y i n g temperature a t f i x e d pH, as indicated. 1


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152


presence o f d i f f e r e n t number of hydroxyl groups and/or t h e i r d i f f e r e n t p o s i t i o n s i n the b i l e a c i d molecules, have no i n f l u e n c e on the i n t e r a c t i o n s with l e c i t h i n molecules. A l s o , i n Tables I and I I we observe t h a t t h e l i m i t i n g molecular area values f o r the same p r o p o r t i o n o f l e c i t h i n are v e r y s i m i l a r .


Conclusions A l l mixed monolayers formed by d i f f e r e n t p r o p o r t i o n s o f l e c i t h i n and b i l e a c i d show an i n t e r m e d i a t e behavior between those of the simple monolayers. T h i s i s due t o the important condensing e f f e c t of l e c i t h i n over b i l e a c i d s on the whole pH (2-12) and temperature (25-40°C) range. The mixed monolayers are more s t a b l e than the simple bile acid films. The i n t e r a c t i o n s between l e c i t h i n and b i l e a c i d molecules, a t the aqueous s o l u t i o n - a i r i n t e r f a c e , depend on e l e c t r o s t a t i c , hydrophobic and h y d r a t i o n f o r c e s . The h y d r a t i o n and r e p u l s i v e e l e c t r o s t a t i c f o r c e s seem to be dominant. T h i s c o n c l u s i o n i s supported by t h e d e v i a t i o n s , from the i d e a l mixing, obtained a t d i f f e r e n t pH and temperatures. S o l u b i l i z a t i o n o f t h e b i l e a c i d molecules i n t h e subphase has t o taken i n t o account, a t high pH and temperature v a l u e s , mainly f o r the c h o l i c a c i d molecules. A t t r a c t i v e and r e p u l s i v e e l e c t r o s t a t i c i n t e r m o l e c u l a r f o r c e s a r e compensated, a t high s u r f a c e - p r e s s u r e values (LC s t a t e ) , f o r a l l monolayers. The presence of d i f f e r e n t number of hydroxyl groups and t h e i r d i f f e r e n t p o s i t i o n s i n the b i l e a c i d molecules e x e r t p r a c t i c a l l y no i n f l u e n c e on the i n t e r a c t i o n s w i t h l e c i t h i n molecules, but we observe an i n c r e a s e o f these when the c h o l i c a c i d molecules are present i n the f i l m s a t low pH. Literature (1) (2) (3) (4) (5) (6)

Cited

A.D. Sorokina, N.D. Yanopolskaya and G.A. Deborin, Bioelectrochemistry and Bioenergetics, 23 (1990) 271 A Section of J. Electroanal. Chem., 298 (1990) M.J. Gálvez-Ruiz and M.A. Cabrerizo-Vílchez, Colloid and Polym. Sci., 269 (1991) 77 J. Miñones-Trillo, S. García-Fernández and P. SanzPedrero, J. Colloid Interf. Sci., 26(4) (1968) 518 A. Fini, A. Roda and P. De María, Eur. J. Med. Chem.Chim. Ther. 17(5) (1982) 465 M. Tomoaia-Cotişel, J. Zsakó, A. Mocamo, M. Lupea and E. Chifu, J. Colloid Interf. Sci. 117(2) (1987) 464 C. Gabrielli, M. Puggelli, E. Ferroni, G. Carubia and L. Pedocchi, Colloids and Surfaces, 41 (1989) 1

RECEIVED September 24, 1991


Macromolecular Assemblies in Polymeric Systems - American

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