Adsorption of Ionic Surfactants to Porous Glass: The Exclusion of

8 Adsorption of Ionic Surfactants to Porous Glass: The Exclusion of Micelles and Other Solutes from Adsorbed Layers and the Problem of Adsorption Maxima P A S U P A T I M U K E R J E E and A R O O N S R I A N A V I L

Downloaded by RUTGERS UNIV on February 5, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0008.ch008

School of Pharmacy, University of Wisconsin, Madison, Wis. 53706

Introduction Porous glass, containing a rigid, interconnecting network of pores (1), is used extensively for chromatographic separations of polymers (2), proteins (3), polysaccharides (4), and viruses (1,5). Because of the presence of pores, these glasses have high surface areas per unit weight (6,7). The adsorption of surfactants to glass (8) is of interest for a variety of reasons. The present work is concerned with the adsorption of cationic and anionic surfactants to porous glass with special attention to concentrations above the critical micellization concentration (c.m.c). The work is expected to provide background information for chromatographic studies on micellar systems. Ionic micelles are surrounded by electrical double layers, the effective extent of which depends mainly upon the surface charge density and the ionic strength. The dimensions of the electrical double layers around charged surfaces are also similarly determined. It was hoped that a study of the adsorption to porous glass containing pores that are rigid and large compared to molecular dimensions, the interactions of micelles with adsorbed layers could be critically studied. The interpretation of adsorption isotherms of ionic surfactants to solid or liquid substrates as also "binding" isotherms to macromolecules involves a variety of factors. These include adsorbate-adsorbent interactions as also adsorbate-adsorbate interactions in the adsorbed layers. The range of an interaction may vary with its type. The nonpolar moiety of the adsorbed molecule is expected to show mostly short-range interactions. The charge interactions, however, can be of short-range as also long range. For short-range interactions, the existence of narrow pores and/or surface nonuniformities of the order of molecular dimensions can produce important curvature effects even in an otherwise homogeneous solid. The characteristic distance of the long-range electrical double layer (approximately 0.3/ nm where C is the bulk concentration of a 1:1 electrolyte). Since this Debye thickness can be much greater than molecular dimensions, the presence 107 Mittal; Adsorption at Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

108

ADSORPTION

AT

INTERFACES


o f f a i r l y wide pores and/or r e l a t i v e l y g r o s s n o n u n i f o r m i t i e s a t t h e s u r f a c e o f nonporous s o l i d s can produce c u r v a t u r e e f f e c t s f o r e l e c t r i c a l d o u b l e l a y e r s , and t h u s a f f e c t a d s o r p t i o n . A s e c o n d s e t o f p r o b l e m s we w o u l d l i k e t o draw a t t e n t i o n t o i n t h i s study a r i s e s from the i n t e r a c t i o n s o f b u l k s o l u t e s p e c i e s w i t h the adsorbed l a y e r . These problems a r e p a r t i c u l a r l y import a n t when t h e amount a d s o r b e d i s c a l c u l a t e d f r o m t h e c h a n g e i n s o l u t e c o n c e n t r a t i o n o f b u l k s o l u t i o n s f a r from adsorbed l a y e r s . F o r e x a m p l e , i t i s w e l l known t h a t c o - i o n s a r e e x c l u d e d f r o m t h e e l e c t r i c a l d o u b l e l a y e r (9., 10). I t s importance i n the c a l c u l a t i o n o f t h e t r u e amount a d s o r b e d , i . e . , t h e amount a t t h e s u r f a c e where t h e a d s o r b e d m o l e c u l e s a r e w i t h i n t h e f i e l d o f s h o r t - r a n g e i n t e r a c t i o n s w i t h t h e s u r f a c e , h a s b e e n i n d i c a t e d b y Van D o l s e n and V o i d ( l l ) . I n a t y p i c a l c a s e o f t h e a d s o r p t i o n o f an i o n i c s u r f a c t a n t t o an i n i t i a l l y u n c h a r g e d s u r f a c e , t h e c o n c e n t r a t i o n o f the s u r f a c t a n t i o n i n the b u l k s o l u t i o n f a r from the surface i s i n c r e a s e d because o f i t s e x p u l s i o n from the e l e c t r i c a l double l a y e r adjacent t o the adsorbed l a y e r . T h i s r e s u l t s i n an u n d e r e s t i m a t e o f t h e t r u e amount a d s o r b e d a t t h e s u r f a c e . The c l a s s i c a l Donnan t y p e c o r r e c t i o n s f o r c o l l o i d a l s y s t e m s a r e b a s e d o n t h i s p r i n c i p l e but use i d e a l i z a t i o n s o f the system which are not acceptable f o r h i g h l y charged surfaces (12). I f t h e a d s o r p t i o n experiment i s so c o n d u c t e d t h a t a f i n e l y d i v i d e d phase i s a l l o w e d t o sediment o r i s c e n t r i f u g e d out b e f o r e c o n c e n t r a t i o n m e a s u r e m e n t s a r e p e r f o r m e d on t h e c o n t i n u o u s p h a s e , t h e p r o b l e m becomes more c o m p l e x b e c a u s e t h e i n t e r p a r t i c l e s e p a r a t i o n s b e t w e e n t h e p a r t i c l e s o f t h e s e p a r a t e d p h a s e may b e c o n t r o l l e d i n p a r t b y g e o m e t r i c a l f a c t o r s such as a s p e r i t i e s , l o n g range van der Waals forces, g r a v i t a t i o n a l forces f o r coarse p a r t i c l e s , as a l s o t h e r e p u l s i v e i n t e r a c t i o n s o f t h e e l e c t r i c a l d o u b l e layers. To c a l c u l a t e t h e t r u e amount a d s o r b e d a t t h e s u r f a c e , i t i s necessary t o take i n t o account the n e g a t i v e a d s o r p t i o n o f coi o n s f r o m an i n t e r a c t i n g d o u b l e l a y e r s y s t e m under t h e i n f l u e n c e o f the other f o r c e s mentioned. I f t h e adsorbent phase i s s e p a r a ted too r a p i d l y , nonequilibriurn s i t u a t i o n s could also a r i s e . 1

In m i c e l l e - f o r m i n g systems, f u r t h e r c o m p l i c a t i o n s a r i s e from the i n t e r a c t i o n o f i o n i c m i c e l l e s with adsorbed l a y e r s . Ionic m i c e l l e s w i t h t h e i r e l e c t r i c a l d o u b l e l a y e r s a r e u n l i k e l y t o show p o s i t i v e a d s o r p t i o n t o uncharged or s i m i l a r l y charged s u r f a c e s . On t h e o t h e r h a n d , r e p u l s i v e i n t e r a c t i o n s b e t w e e n s i m i l a r l y c h a r g e d s u r f a c e s and t h e h i g h l y c h a r g e d s u r f a c t a n t m i c e l l e s a r e e x p e c t e d f r o m d o u b l e l a y e r t h e o r y a n d may be p r o n o u n c e d i n v i e w o f the pronounced r e p u l s i v e i n t e r a c t i o n s e x h i b i t e d by the m i c e l l e s t h e m s e l v e s ( 1 3 ) . As i n t h e c a s e o f s m a l l c o - i o n s , s u c h i n t e r a c t i o n s s h o u l d r e s u l t i n an e x c l u d e d v o l u m e t y p e i n t e r a c t i o n w h i c h i s expected t o g i v e h i g h e r c o n c e n t r a t i o n s o f m i c e l l e s f a r from t h e a d s o r b e d l a y e r s t h a n c l o s e t o i t , t h u s r e s u l t i n g i n an u n d e r e s t i mate o f t h e t r u e a d s o r p t i o n o f t h e s u r f a c t a n t . When a f i n e l y d i v i d e d adsorbent phase i s s e p a r a t e d from t h e s o l u t i o n phase, t h e i n t e r a c t i o n s o f the i o n i c m i c e l l e s with the p a r t i c l e s of the

Mittal; Adsorption at Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

8.

MUKERjEE

AND

ANAviL

Adsorption of Ionic Surfactants

109


adsorbent phase m u t u a l l y i n t e r a c t i n g w i t h each o t h e r i s expected t o produce a m u l t i p l i c i t y o f e f f e c t s i n v o l v i n g changes i n t h e i n t e r p a r t i c l e separations a t equilibrium, the negative adsorption o f b o t h co-ions and m i c e l l e s , as a l s o t h e e l e c t r i c a l i n t e r a c t i o n s i n t h e a d s o r b e d l a y e r s , a n d p o s s i b l e c h a n g e s i n t h e monomermicelle equilibrium. One p u r p o s e o f t h e p r e s e n t work was t o i n v e s t i g a t e t h e n a t u r e o f t h e a d s o r p t i o n i s o t h e r m s o f m i c e l l e f o r m i n g s u r f a c t a n t s above t h e c.m.c. i n some s u i t a b l y c h o s e n s y s t e m s d e s i g n e d , i n p a r t i c u l a r , t o examine t h e s i g n i f i c a n c e o f m i c e l l a r e x c l u s i o n f r o m s i m i l a r l y c h a r g e d s u r f a c e s , a n d t o what e x t e n t t h e s e e x c l u s i o n e f f e c t s may b e i n v o l v e d i n t h e i n t e r p r e t a t i o n o f t h e p r o b l e m o f a d s o r p t i o n i s o t h e r m s s h o w i n g maxima a b o v e t h e c.m.c. (ΐί,15). The a d s o r p t i o n s u b s t r a t e u s e d was p o r o u s g l a s s ( B i o - G l a s ) c o n t a i n i n g p o r e s o f a v e r a g e d i a m e t e r s o f a b o u t l6 nm a n d 35 nm. B e c a u s e o f t h e p r e s e n c e o f an e x t e n s i v e , i n t e r c o n n e c t i n g network o f p o r e s , t h e s e g l a s s e s have h i g h s u r f a c e a r e a s p e r u n i t w e i g h t o f s o l i d even f o r coarse p a r t i c l e s . E l e c t r o n micrographs (l6_) i n d i c a t e h i g h l y u n even, p i t t e d s u r f a c e s , so t h a t a s i g n i f i c a n t f r a c t i o n o f t h e s u r f a c e may b e e x t e r n a l . B e c a u s e o f t h e r i g i d i t y o f g l a s s , a n d l a r g e d i s t a n c e s between e x t e r n a l s u r f a c e s , a l l i n t e r a c t i o n s between a d s o r p t i o n s u r f a c e s c o u l d b e assumed t o o c c u r a t c o n s t a n t g e o m e t r y , independent o f the composition o f the s o l u t i o n . Experimental Materials. B i o - G l a s samples were o b t a i n e d from B i o - R a d X a b o r atories. Two d i f f e r e n t s a m p l e s o f B i o - G l a s 200, numbered I a n d I I ( T a b l e I ) w i t h a n o m i n a l e x c l u s i o n l i m i t o f 20 nm a n d B i o - G l a s 500, w i t h a n o m i n a l e x c l u s i o n l i m i t o f 50 nm w e r e u s e d . The g l a s s powders were o f 50-100 mesh s i z e , c o r r e s p o n d i n g t o c o a r s e p a r t i c l e s o f r o u g h l y 150-300 pm i n d i a m e t e r . The p a r t i c l e s were i r r e g u l a r i n shape, and s e t t l e d r a p i d l y from suspensions. The m a n u f a c t u r e r ^ f i g u r e s s h o w i n g t h e p e n e t r a t i o n o f m e r c u r y i n t o a powder a s a f u n c t i o n o f p r e s s u r e i n d i c a t e d i n a l l c a s e s o n e d i s t r i b u t i o n o f p o r e s i z e s f r o m r o u g h l y 20-70 ym c o r r e s p o n d i n g t o the e x t e r n a l s u r f a c e . A second pore s i z e d i s t r i b u t i o n c e n t e r e d a r o u n d r o u g h l y l6 nm f o r B i o - G l a s 200 a n d 35 nm f o r B i o - G l a s 500 c o r r e s p o n d i n g t o t h e i n t e r n a l s u r f a c e . T a b l e I i n d i c a t e s some pore s i z e parameters f o r t h e i n t e r n a l pores from the mercury pene t r a t i o n data. A d s o r p t i o n measurements w i t h c a t i o n i c s u r f a c t a n t s i n d i c a t e d t h a t d i f f e r e n t l o t numbers h a d d i f f e r e n t s u r f a c e a r e a s ( T a b l e I ) . D i f f e r e n t b o t t l e s o f t h e same l o t number u s u a l l y g a v e concordant r e s u l t s i n t h e a d s o r p t i o n experiments excepting i n t h e case o f sodium t e t r a d e c y l s u l f a t e f o r which d i f f e r e n t b o t t l e s o f B i o - G l a s 200 ( i ) w i t h t h e same l o t number s o m e t i m e s g a v e c o n s i s t e n t d i f f e r e n c e s i n t h e a d s o r p t i o n , t h e maximum d i f f e r e n c e b e i n g a b o u t 9 x 10~6 e q u i v . / g , o r a b o u t 15$ o f t h e maximum a d s o r p t i o n . A l l t h e experimental data r e p o r t e d i n a p a r t i c u l a r f i g u r e are based on B i o - G l a s f r o m t h e same b o t t l e , s o t h a t i n t e r n a l c o n s i s t e n c y o f



(I)

(ID

Bio-Glas 500

Bio-Glas 500

2

2.25

4.2 χ 1 0 " 8.2 χ 1 0 "

0.40 0.50 0.51

15 34 36

13-25

20-50

25-55

4

4

15 30 5

81

140

2

Surface area (m /g)

5

χ ΙΟ""

3.9 χ 1 0 "

0.67

18.5

14-25

Adsorption^ at the c.m.c. (mole/g)

From mercury p e n e t r a t i o n data. E l e c t r o n miscroscopy (16) i n d i c a t e s wider d i s t r i b u t i o n s . From mercury p e n e t r a t i o n data, estimated a t h a l f p e n e t r a t i o n i n the i n t e r n a l pores. From mercury p e n e t r a t i o n data. TDPB f o r Bio-Glas 200 ( I ) , and HDPB f o r the o t h e r s . Assuming 0.60 nm /ion.

(ID

Bio-Glas 200

a. b. c. d. e.

(I)

0

Internal pore volume (ml/g)

5

Average* pore diameter (nm)

Range o f pore diameters (nm)

a

C h a r a c t e r i s t i c s of Porous Glass Samples Used

Bio-Glas 200

Table 1.


e

8.

the

MUKERJEE

A N D ANA VIL


111

data i s maintained.

Sodium d o d e c y l s u l f a t e (SDS) a n d s o d i u m t e t r a d e c y l s u l f a t e (STDS) w e r e u s e d a s r e c e i v e d f r o m Mann R e s e a r c h L a b o r a t o r i e s . H e x a d e c y l p y r i d i n i u m b r o m i d e (HDPB) o b t a i n e d f r o m E a s t m a n O r g a n i c c h e m i c a l s was r e c r y s t a l l i z e d t w i c e f r o m e t h y l e t h e r a n d t h e n s e v e r a l times from d i s t i l l e d water. Tetradecyl pyridinium bro m i d e (TDPB) was s y n t h e s i z e d f r o m 1 - b r o m o t e t r a d e c a n e ( E a s t m a n W h i t e L a b e l ) and p y r i d i n e (Eastman S p e c t r o grade) b y t h e method o f A n a c k e r a n d Ghose ( ΐ χ ) b y D r . J . R. C a r d i n a l o f t h i s l a b o r a t o r y . The c.m.c. v a l u e s o f t h e s u r f a c t a n t s u s e d a t 3 0 - 3 5 ° C i n w a t e r a r e e s t i m a t e d t o b e 7-8 χ 1 0 " ^ M f o r HDPB, 3.0 χ 1 0 " M f o r TDPB, 2.1 χ Ι Ο M f o r STDS a n d 8.3 x 1 0 ~ M f o r SDS (l8). In the p r e s e n c e o f a d d e d s a l t s t h e c.m.c. v a l u e s a r e much l o w e r (l8). 3


- 3

3

Adsorption Experiments. A l l s o l u t i o n s were made i n d o u b l e d i s t i l l e d water by weight. B i o - G l a s s a m p l e s were c o n d i t i o n e d b y m a i n t a i n i n g i n h o t d i s t i l l e d water f o r about t h r e e h o u r s . Adsorp t i o n e x p e r i m e n t s were p e r f o r m e d i n c o n s t a n t t e m p e r a t u r e b a t h s b y a d d i n g 10-15 m l o f s u r f a c t a n t s o l u t i o n s o f known c o n c e n t r a t i o n b y weight t o small Erlenmeyer f l a s k s o r Teflon-capped v i a l s c o n t a i n ing u s u a l l y 1 g o f B i o - G l a s and o c c a s i o n a l l y 2 g o f B i o - G l a s a t high surfactant concentrations. T h e B i o - G l a s t o l i q u i d r a t i o was u s u a l l y m a i n t a i n e d c o n s t a n t i n a p a r t i c u l a r s e r i e s o f measure ments. G e n t l e s h a k i n g was e m p l o y e d t o p r e v e n t f o a m i n g . The a n i o n i c s u r f a c t a n t s were a n a l y z e d a f t e r s u i t a b l e d i l u t i o n s b y w e i g h t b y u s i n g t h e m e t h o d o f e x t r a c t i o n i n t o c h l o r o f o r m a s methy l e n e b l u e s a l t s ( 1 9 ) f o l l o w e d b y a b s o r b a n c e measurements w i t h a Cary s p e c t r o p h o t o m e t e r , Model l6. The c a t i o n i c s u r f a c t a n t s were a n a l y z e d b y a b s o r b a n c e measurements o f t h e p y r i d i n i u m c h r o m o p h o r i c s y s t e m a t 259 nm, a f t e r s u i t a b l e d i l u t i o n b y w e i g h t . The a d s o r p t i o n e x p e r i m e n t s were r u n o v e r p e r i o d s o f one t o s e v e r a l days t o f o l l o w t h e t i m e dependence. U s u a l l y t h e changes i n a d s o r p t i o n o b s e r v e d a f t e r 1-2 days b e l o w t h e c.m.c. a n d 2-4 d a y s a b o v e t h e c.m.c. were w i t h i n t h e e s t i m a t e d e x p e r i m e n t a l errors. The amount o f s u r f a c t a n t a d s o r b e d was c a l c u l a t e d f r o m t h e amount l o s t f r o m s o l u t i o n i n t h e u s u a l manner. A l l a d s o r p t i o n data reported are thus apparent a d s o r p t i o n s not c o r r e c t e d f o r coion or micellar exclusions. The a v e r a g e r e p r o d u c i b i l i t y a n d s e l f - c o n s i s t e n c y o f t h e d a t a i n a p a r t i c u l a r s e r i e s o f m e a s u r e m e n t s was o f t h e o r d e r e x p e c t e d f r o m u n c e r t a i n t i e s o f a b o u t 1-2$ o f t h e a n a l y t i c a l c o n c e n t r a t i o n determination of the surfactant. Results F i g u r e 1 shows t h e a d s o r p t i o n i s o t h e r m s o f HDPB t o two s a m p l e s o f B i o - G l a s 500 b e l o w t h e c.m.c. T h e i s o t h e r m s show t h e t w o - s t e p c h a r a c t e r i s t i c o f t e n o b s e r v e d f o r s u r f a c t a n t a d s o r p t i o n (8_,20). The i s o t h e r m s f o r s a m p l e s I a n d I I show r o u g h l y t h e same s h a p e a n d



112

ADSORPTION

EQUIL.

CONC.

OF

HDPB

(mole/i

χ

A T INTERFACES

I0 ) 3

Figure 1. Adsorption of HDPB to Bio-Ghs 500, samples I and II, at low concentrations and 35°C

6U

Figure 2. Adsorption of STDS to Bio-Glas 200 (I) at low concentrations and 30°C. Δ = 1 day, Q) = 2 days, and V = 3 days shaking.



8.

MUKERJEE

AND

ANAviL


113

i n d i c a t e t h a t t h e two s a m p l e s h a v e d i f f e r e n t s u r f a c e a r e a s . The i s o t h e r m f o r sample I I shows a s t e e p i n c r e a s e i m m e d i a t e l y "below t h e c.m.c. r e g i o n . T h e d a t a a t h i g h e r c o n c e n t r a t i o n s shows g e n t l e i n c r e a s e a b o v e t h e c.m.c. F i g u r e 2 shows t h e a d s o r p t i o n i s o t h e r m o f STDS b e l o w t h e c.m.c. T h e i s o t h e r m e x h i b i t s a p l a t e a u , o r a s l i g h t maximum, w e l l b e l o w t h e c.m.c. ( 2 . 1 χ Î C T ^ M i n w a t e r ) . F i g u r e 3 compares t h e a d s o r p t i o n i s o t h e r m s o f HDPB a n d TDPB a b o v e t h e c.m.c. f o r B i o - G l a s 500 ( i l ) . The d i f f e r e n c e s a r e s m a l l i n t h e absence o f added s a l t : for both surfactants the adsorption i n c r e a s e s s l o w l y a b o v e t h e c.m.c. I n presence o f h i g h s a l t conc e n t r a t i o n , t h e a p p a r e n t a d s o r p t i o n f o r HDPB d e c r e a s e s w i t h c o n c e n t r a t i o n above t h e c.m.c. F i g u r e h shows t h e a d s o r p t i o n i s o t h e r m s f o r TDPB f o r B i o - G l a s 200 ( i ) , a b o v e t h e c.m.c. f r o m d i s t i l l e d w a t e r a n d i n p r e s e n c e o f 0.05M N a B r . I t a l s o shows t h e i s o t h e r m f o r HDPB f o r B i o - G l a s 200 (II). The a d s o r p t i o n i n a l l cases appears t o be constant w i t h i n e x p e r i m e n t a l e r r o r a b o v e t h e c.m.c. 0.05M NaBr i n c r e a s e s t h e a d s o r p t i o n a b o v e t h e c.m.c. b y a b o u t 25$ f o r TDPB a s compared t o the a d s o r p t i o n i n t h e absence o f s a l t . F i g u r e 5 shows t h e a d s o r p t i o n i s o t h e r m o f STDS f o r B i o - G l a s 200 ( i ) a t h i g h c o n c e n t r a t i o n s . The a d s o r p t i o n a p p e a r s t o dec r e a s e r a p i d l y a b o v e t h e c.m.c. T h e e f f e c t o f a 5°C c h a n g e i n temperature appears t o be w i t h i n experimental e r r o r . F i g u r e 6 compares t h e a d s o r p t i o n o f STDS f r o m w a t e r a n d f r o m 0.01M a n d 0.03M N a C l f o r B i o - G l a s 200 ( i ) , o b t a i n e d f r o m t h e same bottle. The i s o t h e r m i n t h e absence o f s a l t i s s i m i l a r i n shape t o t h e 30° i s o t h e r m o f F i g u r e 5 and appears t o be s h i f t e d upward b y a c o n s t a n t amount o f r o u g h l y 9 x 1 0 ~ " e q u i v . / g . In presence o f s a l t , t h e apparent a d s o r p t i o n remains n e a r l y constant u n t i l t h e e q u i l i b r i u m STDS c o n c e n t r a t i o n r e a c h e s a v a l u e o f a b o u t 10~^M, above w h i c h i t d e c r e a s e s . F i g u r e 6 a l s o shows some a d s o r p t i o n d a t a f o r SDS o n B i o - G l a s 200 ( i ) a b o v e t h e c.m.c. r e g i o n . The apparent a d s o r p t i o n appears t o be n e g a t i v e a t h i g h c o n c e n t r a t i o n s . Discussion pH M e a s u r e m e n t s . I t i s w e l l known t h a t g l a s s i s n e g a t i v e l y c h a r g e d a t n e a r n e u t r a l v a l u e s o f pH, a n d t h a t H a n d 0H~ i o n s a c t as p o t e n t i a l d e t e r m i n i n g i o n s ( 2 l ) . T h e e l e c t r o s t a t i c f a c t o r s i n v o l v e d i n the adsorption o f i o n i c s u r f a c t a n t s are thus expected t o be d e p e n d e n t u p o n t h e s o l u t i o n pH. T h e s u r f a c t a n t s u s e d i n t h i s work w e r e s a l t s o f s t r o n g a c i d s w h i c h t h e m s e l v e s do n o t a f f e c t t h e pH o f t h e s o l u t i o n , a s was c h e c k e d b y m e a s u r e m e n t s o n o u r s a m p l e s . In general, the adsorption o f i o n i c surfactants t o o p p o s i t e l y c h a r g e d s u r f a c e s i s e x p e c t e d t o i n v o l v e some i o n e x c h a n g e a s p a r t o f t h e t o t a l u p t a k e o f s u r f a c t a n t s (δ.,15.,22). For cationic sur f a c t a n t s , t h i s i o n exchange w i t h t h e g l a s s s u r f a c e i s e x p e c t e d t o r e l e a s e some H i o n s i n t o t h e s o l u t i o n (8_,22). T h e i n t e r p r e t a t i o n +



ADSORPTION

AT

INTERFACES

Figure 3. Adsorption of HDPB and TDPB to Bio-Glas 500 (II) at high concentrations and 35°C. A = TDPB, φ = TDPB in 0.05M NaBr, Ο = HDPB, Δ = HDPB in 0.03M NaBr, V = HDPB on 0.05M NaBr, • = HDPB in 0.13M NaBr, and • = HDPB in 0.2M NaNQ . Dashed line indicates an exclusion volume of 0.5 ml/g. 3

/ I

I EQUIL.

CONC.

2 (mole/f

3 χ ΙΟ ) 2

Figure 4. Adsorption of TDPB to Bio-Glas 200 (I) and HDPB to Bio-Glas 200 (II) at high concentrations and 35°C. V = TDPB 4 days, Δ = TDPB 7 days, • = TDPB in 0.05U NaBr, and Q = HDPB.



MUKERJEE

AND

ANAviL


2 3 I EQUIL. CONC. OF STDS (mole/i χ ΙΟ) 2

Figure 5. Adsorption of STDS to Bio-Glas 200 (I) at high concentrations. Ç) = 4 days, 30°C; Δ = β days, 30°C; • = 7 days,35°C.

0 1 2 3 4 5 EQUIL. CONC. OF SURFACTANT (mole/i χ ΙΟ) 2

Figure 6. Adsorption of STDS and SDS to BioGlas 200 (I) at high concentrations and 35°C. Ο = STDS, A = STDS in 0.01M NaCl, V = STDS in 0.03M NaCl, and • = SDS.


116

ADSORPTION


o f a d s o r p t i o n i s o t h e r m s b e l o w t h e c.m.c. must t a k e t h i s i n t o account.

AT

INTERFACES

factor

Some pH m e a s u r e m e n t s w e r e c a r r i e d o u t t o examine t h e n a t u r e o f t h e c h a n g e s i n s o l u t i o n pH b r o u g h t a b o u t b y t h e i n t e r a c t i o n s o f the s u r f a c t a n t s w i t h Bio-Glas under t h e c o n d i t i o n s o f t h e adsorp t i o n experiments a f t e r e q u i l i b r i u m i s a t t a i n e d . F i g u r e 7 shows some t y p i c a l pH v a l u e o f t h e s u p e r n a t a n t l i q u i d . The e q u i l i b r i u m l i q u i d was u s e d t o a v o i d t h e p r o b l e m o f t h e s u s p e n s i o n e f f e c t o n pH (9,12,21). B i o - G l a s i t s e l f , i n t h e a b s e n c e o f a n y s u r f a c t a n t , makes t h e medium s l i g h t l y b a s i c , i n d i c a t i n g t h e r e l e a s e o f s m a l l amounts o f b a s i c e l e c t r o l y t e s . Somewhat more p r o n o u n c e d e f f e c t i s o b t a i n e d w i t h B i o - G l a s 200 t h a n B i o - B i a s 500. T h i s i s c o n s i s t e n t with t h e greater surface area o f t h e former. When TDPB, a c a t i o n i c s u r f a c t a n t , i s a d d e d t o B i o - G l a s 500, some p r o t o n s a r e r e l e a s e d , a s e x p e c t e d , a n d t h e s o l u t i o n pH becomes a c i d i c . With i n c r e a s i n g c o n c e n t r a t i o n o f s u r f a c t a n t i n t h e s u p e r n a t a n t , however, a b o v e t h e c . m . c , t h e pH c h a n g e s v e r y l i t t l e , i n c r e a s i n g b y a b o u t 0.1 u n i t as t h e e q u i l i b r i u m c o n c e n t r a t i o n c h a n g e s f r o m t h e c.m.c. t o more t h a n 15 x c.m.c. I n t h e case o f a n i o n i c s u r f a c t a n t s which are s a l t s o f s t r o n g a c i d s , e x t e n s i v e i o n exchanges a r e n o t ex pected. F i g u r e 7 shows t h a t i n t h e S T D S - B i o - G l a s 200 s y s t e m t h e pH r i s e s somewhat a t l o w c o n c e n t r a t i o n s . The change i s q u i t e s m a l l , h o w e v e r , a n d a b o v e t h e c . m . c , i n p a r t i c u l a r , t h e pH c h a n g e s v e r y l i t t l e a g a i n , d e c r e a s i n g b y a b o u t 0.1 u n i t a s t h e c o n c e n t r a t i o n i n c r e a s e s t o Τ x c . m . c , t o a b o u t 1.5 x 1 0 ~ M. A f u r t h e r d e c r e a s e o f 0.2 u n i t s was o b s e r v e d when t h e e q u i l i b r i u m c o n c e n t r a t i o n was k χ 10~^M, i . e . a b o u t 18 χ c . m . c The pH v a r i a t i o n r e s u l t s a r e c o n s i s t e n t w i t h t h e t h e o r e t i c a l argument t h a t t h e a c t i v i t y o f t h e p r i m a r y a d s o r b i n g s p e c i e s , t h e monomer, c h a n g e s l i t t l e a b o v e t h e c.m.c. o f l o n g - c h a i n surfactants and, t h e r e f o r e , t h e i o n e x c h a n g e a n d t h e r e s u l t a n t c h a n g e i n t h e pH o f t h e e q u i l i b r i u m a r e a f f e c t e d l i t t l e a b o v e t h e c . m . c The r e p r o d u c i b i l i t y and s e l f - c o n s i s t e n c y o f t h e a d s o r p t i o n measure ments b e l o w t h e c.m.c. ( F i g u r e s 1 a n d 2) i n d i c a t e t h a t a n y c h a n g e i n s o l u t i o n pH was s u f f i c i e n t l y r e p r o d u c i b l e t o n o t a l t e r t h e e n e r g e t i c s o f t h e a d s o r p t i o n p r o c e s s i n a random f a s h i o n . The c . m . c v a l u e s e s t i m a t e d f r o m t h e a d s o r p t i o n i s o t h e r m s ( F i g u r e s 1 a n d 6) do n o t d i f f e r much f r o m e x p e c t e d b u l k v a l u e s , showing t h a t t h e i o n i c s t r e n g t h o f t h e s o l u t i o n s i s n o t a f f e c t e d much a t t h e c . m . c b y t h e p r e s e n c e o f B i o - G l a s . F o r comparisons w i t h o t h e r s y s t e m s , i t s h o u l d b e n o t e d t h a t when s u r f a c t a n t s a r e weak e l e c t r o l y t e s , pH e f f e c t s c a n b e c o m p l e x b e c a u s e o f c o m p o s i t i o n c h a n g e s i n t h e e q u i l i b r i u m s o l u t i o n (23) a n d b e c a u s e t h e pH may b e v e r y d i f f e r e n t a t a c h a r g e d s u r f a c e when c o m p a r e d t o t h e s o l u t i o n v a l u e (2k). When i o n e x c h a n g e i s l i k e l y t o o c c u r , ini t i a l pH v a l u e s may b e q u i t e m i s l e a d i n g , a s i n d i c a t e d b y t h e pH v a r i a t i o n s b r o u g h t a b o u t b y TDPB ( F i g u r e 7). T h e pH may, i n d e e d , be d i f f i c u l t t o c o n t r o l w i t h o u t b u f f e r s o f r e a s o n a b l e c a p a c i t y . The measurement o f t h e pH s h o u l d b e c a r r i e d o u t o n t h e e q u i l i b r i u m f l u i d t o a v o i d t h e s u s p e n s i o n e f f e c t w h i c h c a n b e s e r i o u s (9,12,


8.

MUKERJEE AND

ANAviL


117


21). For the i n t e r p r e t a t i o n o f t h e a d s o r p t i o n data above the c.m.c, our reference p o i n t i s always an i n t e r n a l one, namely t h e a d s o r p t i o n a t the c.m.c. o f the same system. The above r e s u l t s and t h e supporting arguments suggest t h a t pH v a r i a t i o n s i n s o l u t i o n s above the c.m.c. w i t h respect t o t h e s o l u t i o n a t t h e c.m.c, and the e f f e c t s o f these changes on the a d s o r p t i o n , can be ignored t o a good approximation. Tadros (25,), i n a recent paper, has r e p o r t e d t h a t the adsorp t i o n s o f a c a t i o n i c and an i o n i c s u r f a c t a n t on s i l i c a a t h i g h c o n c e n t r a t i o n s , but w e l l below the c.m.c, change by l e s s than a f a c t o r o f f i v e as t h e pH changes from 3.6 t o 9.1 f o r t h e c a t i o n i c system, and 3.6 t o 10.1 f o r t h e a n i o n i c system. This e f f e c t appears t o be r a t h e r s m a l l but supports our neglect o f pH-variat i o n e f f e c t s above t h e c.m.c Surface Areas. Tamamushi and Tamaki (8^,26) found t h a t t h e l i m i t i n g a d s o r p t i o n o f dodecyl p y r i d i n i u m bromide on alumina was much l e s s than t h a t o f dodecyl and higher alkylammonium c h l o r i d e s , t h e l i m i t i n g area being 0.90 nm^/molecule. I n our case, t h e e f f e c t o f e l e c t r o l y t e s (Figure 3) i n d i c a t e s t h a t the a d s o r p t i o n a t the c.m.c. on Bio-Glas i n t h e absence o f e l e c t r o l y t e s i s much l e s s than t h e maximum p o s s i b l e v a l u e . F o r an approximate estimate o f the e f f e c t i v e surface area o f B i o - G l a s , we have used t h e value o f 0.60 nm /molecule and t h e a d s o r p t i o n o f the c a t i o n i c s u r f a c t a n t s at the c.m.c i n t h e absence o f added s a l t , which occurs from an e q u i l i b r i u m pH o f about 6. Table I r e p o r t s these estimated s u r face areas. 2

A d s o r p t i o n a t and Below t h e C.m.c. C a t i o n i c S u r f a c t a n t s . The a d s o r p t i o n isotherms f o r c a t i o n i c s u r f a c t a n t s below t h e c.m.c (Figure l ) are s i m i l a r i n shape t o those recorded f o r a v a r i e t y o f s u b s t r a t e s (8_»20 26_). The net uptake o f t h e s u r f a c t a n t i n v o l v e s i o n exchange t o some e x t e n t , as r e v e a l e d by t h e lowering o f t h e pH (Figure 7 ) · The shape o f the isotherms i n d i c a t e s c o n t r i b u t i o n s from m u l t i l a y e r formation (8,26,27) and/or a t t r a c t i v e l a t e r a l i n t e r a c t i o n s i n t h e adsorbed l a y e r between the hydrocarbon groups (28). F i g u r e 3 shows t h a t HDPB and TDPB appear t o be q u i t e s i m i l a r i n t h e i r a b i l i t y t o adsorb a t t h e i r r e s p e c t i v e c.m.c v a l u e s . I t has been noted before t h a t t h e a d s o r p t i o n isotherms o f homologous long-chain i o n i c s u r f a c t a n t s are o f t e n very s i m i l a r i f a reduced c o n c e n t r a t i o n s c a l e i s used, i . e . i f t h e concentrations a r e ex pressed as f r a c t i o n s o f the c.m.c (8,26,29). This corresponding s t a t e type o f c o r r e l a t i o n i s o f great i n t e r e s t but i t poses a problem w i t h respect t o surface coverage s i n c e d i f f e r e n t homo logues are expected t o occupy d i f f e r e n t surface areas, a t l e a s t f o r t h e f r a c t i o n of t h e adsorbed molecules which l i e p a r a l l e l t o the s u r f a c e . Shorter c h a i n homologues should thus adsorb more on 5


118

ADSORPTION

AT

INTERFACES

the b a s i s of smaller molecular surface areas. On t h e other hand, l a t e r a l i n t e r a c t i o n s may increase n o n l i n e a r l y -with chain l e n g t h . For i o n i c s u r f a c t a n t s , however, two c o r r e c t i o n s may be o f some importance. From previous e l e c t r o k i n e t i c s t u d i e s (21,28,29,30), i t i s extremely l i k e l y t h a t there i s a charge r e v e r s a l w e l l below the c.m.c. For t h e d i f f u s e double l a y e r , t h e r e f o r e , t h e s u r f a c t a n t i s u s u a l l y a co-ion when the e q u i l i b r i u m c o n c e n t r a t i o n i s near t h e c.m.c. and t h e negative a d s o r p t i o n o f t h e s u r f a c t a n t coi o n s , which i s a f u n c t i o n o f the i o n i c s t r e n g t h o f the system, me be somewhat greater f o r s h o r t e r - c h a i n homologues w i t h higher c.m.c. s. The second f a c t o r i n v o l v e s t h e i n t e r a c t i o n (overlap) between d i f f u s e double l a y e r s surrounding adsorbed s u r f a c e s . Near the c.m.c, t h i s would r e s u l t i n an i n c r e a s e i n t h e e l e c t r i c a l work opposing the adsorption of i o n i c species t o s i m i l a r l y charged s u i faces and the a d s o r p t i o n , t h e r e f o r e , w i l l be reduced. There may be a s m a l l r e d u c t i o n o f t h e e x c l u s i o n o f co-ions a l s o ( 31). I n our case, f o r Bio-Glas 500 ( I I ) , the r a t i o o f t h e average pore r a d i u s t o t h e double l a y e r t h i c k n e s s , 6, a t t h e c.m.c i s about 1.8 f o r HDPB and 3.6 f o r TDPB. These r a t i o s are such t h a t con s i d e r a b l e i n t e r a c t i o n s between t h e double l a y e r s i n s i d e t h e pores are expected a t t h e c.m.c, p a r t i c u l a r l y f o r HDPB. I t i s d i f f i c u l t , however, t o make any q u a n t i t a t i v e c a l c u l a t i o n s o f t h e effect. I n presence o f n e u t r a l e l e c t r o l y t e s a t high c o n c e n t r a t i o n , the adsorption of t h e c a t i o n i c s u r f a c t a n t s a t the c.m.c increases a p p r e c i a b l y , i n accord w i t h t h e expected r e d u c t i o n o f the 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 a c t i o n s i n v o l v e d and t h e removal o f double l a y e r overlaps. When the i o n i c s t r e n g t h i s 0.05, f o r example, 6 i s only about l.k nm. Figures 3 and h show a d s o r p t i o n data a t high i o n i c strengths f o r TDPB on Bio-Glas 500 ( i l ) and Bio-Glas 200 ( I I ) and HDPB on Bio-Glas 500 ( i l ) . At these h i g h i o n i c strengths t h e c.m.c values are very low, l e s s than 1 χ 1 0 ~ % f o r HDPB, and the adsorptions a t t h e c.m.c. i n Figures 3 and k can be obtained by e x t r a p o l a t i o n o f t h e p o s t - c m . c l i n e s t o e f f e c t i v e l y zero c o n c e n t r a t i o n . Figure 3 shows t h a t t h e a d s o r p t i o n o f HDPB i n presence o f added NaBr increases w i t h t h e c o n c e n t r a t i o n o f NaBr. The highest a d s o r p t i o n was observed f o r 0.2M NaNOg s o l u t i o n .


f

A n i o n i c S u r f a c t a n t s . U n l i k e the case o f c a t i o n i c s u r f a c t a n t s (Figure l ) , t h e adsorption o f STDS t o Bio-Glas 200 ( i ) below t h e c.m.c appears t o a t t a i n a p l a t e a u value (or a very shallow maxi mum) w e l l below t h e c.m.c (Figure 2 ) . The maximum a d s o r p t i o n i s only about lk% o f t h a t r e g i s t e r e d by t h e c a t i o n i c TDPB, c o n t a i n i n g t h e same chain l e n g t h as STDS. This i s probably due p r i m a r i l y t o the unfavorable e l e c t r i c a l i n t e r a c t i o n s o f a n i o n i c s u r f a c t a n t s on n e g a t i v e l y charted s u r f a c e s . For an estimate o f t h e c o r r e c t i o n f o r c o - i o n e x c l u s i o n f o r STDS, i t i s t o be noted t h a t as t h e s o l u t i o n pH remains high i n presence o f STDS (Figure 7 ) , t h e g l a s s surface should have a high


8.

MUKERjEE

AND

ANAviL


119

negative surface p o t e n t i a l i r r e s p e c t i v e o f the amount o f STDS ab sorbed, and t h e s u r f a c t a n t anions a r e l i k e l y t o be t h e predominant co-ions o f t h e e l e c t r i c a l double l a y e r . Co-ion e x c l u s i o n can be represented by an equivalent average d i s t a n c e , d, from the surface up t o which a l l co-ions a r e assumed t o be excluded and beyond which there i s no e x c l u s i o n (31). For p l a n a r n o n i n t e r a c t i n g double l a y e r s , when t h e surface p o t e n t i a l i s h i g h and a l l mobile ions are monovalent, t h i s e x c l u s i o n d i s t a n c e i s represented very w e l l by t h e simple formula 26 (31). The amount o f e x c l u s i o n per cm o f surface i s given by 26C χ 10"3 where C i s t h e bulk concen t r a t i o n o f t h e c o - i o n i n e q u i v a l e n t s p e r l i t e r . The amount i s thus approximately 6 χ 10"^ν^δ i n water. I f t h e surface area o f lUO m /g f o r Bio-Glas 200 ( i ) (Table I ) i s used, t h e e x c l u s i o n i s c a l c u l a t e d t o be 8 χ 10"?, 2 Λ χ 10" and 3.7 x 10"° equiv/g f o r e q u i l i b r i u m concentrations o f 1 0 , 10"^ and 2 χ 10"^ equiv/1. These c a l c u l a t i o n s are o f o n l y approximate v a l i d i t y because o f double l a y e r overlap e f f e c t s but they i n d i c a t e t h a t t h e c o r r e c t i o n s may be s i g n i f i c a n t . I n c o n t r a s t t o t h e comparable a d s o r p t i o n o f HDPB and TDPB a t the c.m.c. o f each, SDS, i n t h e neighborhood o f i t s c.m.c, seems t o adsorb much l e s s than STDS. This may be due i n p a r t t o reduced chain-chain i n t e r a c t i o n s i n the adsorbed l a y e r s f o r the a n i o n i c systems. The a d s o r p t i o n o f STDS i n presence o f 0.01M and 0.03M NaCl was s t u d i e d mainly above t h e c.m.c (Figure 6). The e x t r a p o l a t e d values a t the c.m.c i n presence o f s a l t appears t o be somewhat l e s s than the a d s o r p t i o n a t c.m.c i n the absence o f added s a l t . Here a l s o , t h e behavior o f t h e a n i o n i c systems i s a t variance w i t h t h a t observed f o r c a t i o n i c systems (Figures 3 and h). I t i s l i k e l y t h a t the e f f e c t i v e negative surface p o t e n t i a l opposing anion a d s o r p t i o n i s c o n t r o l l e d p r i m a r i l y by t h e s o l u t i o n pH, and does not change much w i t h c o n c e n t r a t i o n o f NaCl.


2

_i+

A d s o r p t i o n Isotherms above the C.m.c The complete i n t e r p r e t a t i o n o f a d s o r p t i o n o f i o n i c s u r f a c t a n t s above t h e c.m.c. must i n c l u d e (a) t h e e f f e c t o f i n c r e a s i n g monomer a c t i v i t y above t h e c.m.c (32,33,3*0 , (b) t h e c o n t r i b u t i o n o f t h e m i c e l l e s t o the i o n i c s t r e n g t h (2*0, (c) t h e e f f e c t o f t h e chang i n g composition o f the s o l u t i o n on t h e e l e c t r i c a l double l a y e r s adjacent t o t h e s o l i d surface and t h e i r mutual i n t e r a c t i o n , as a l s o (d) the i n t e r a c t i o n o f t h e m i c e l l e s w i t h t h e charged s u r f a c e s . Q u a n t i t a t i v e c a l c u l a t i o n s o f any o f these f a c t o r s appear t o be extremely d i f f i c u l t , and indeed, t h e f a c t o r s may not be i n dependent. I n c o n t r a s t t o many s t u d i e s o f a d s o r p t i o n t o f i n e l y d i v i d e d systems, t h e s e p a r a t i o n between a d s o r p t i o n surfaces i n our systems i s independent o f s o l u t i o n composition. In the case o f the a n i o n i c s u r f a c t a n t s , p a r t i c u l a r l y STDS (Figure 2 ) , t h e apparent a d s o r p t i o n , below t h e c.m.c, even a f t e r reasonable c o r r e c t i o n s f o r c o - i o n e x c l u s i o n s , appears t o change


120

ADSORPTION

A T INTERFACES


r e l a t i v e l y l i t t l e over a c o n c e n t r a t i o n range o f about a f a c t o r o f 2. The e f f e c t o f i o n i c s t r e n g t h v a r i a t i o n on t h e a d s o r p t i o n a t t h e c.m.c. i s a l s o q u i t e s m a l l ( F i g u r e 6). I t seems, t h e r e f o r e , t h a t t h e e f f e c t s o f a n y f u r t h e r i n c r e a s e i n monomer a c t i v i t y a b o v e t h e c.m.c. a n d a n y i n c r e a s e i n i o n i c s t r e n g t h a n d c o u n t e r i o n c o n c e n t r a t i o n on t h e t r u e a d s o r p t i o n can be i g n o r e d t o a reasonable approximation. The s t r i k i n g d e c r e a s e i n t h e a p p a r e n t a d s o r p t i o n o f STDS a n d SDS ( F i g u r e s 5 a n d 6) above t h e c.m.c. c a n b e q u a l i t a t i v e l y r a t i o n a l i z e d i n terms o f e x c l u s i o n o f m i c e l l e s from t h e s u r f a c e . The h i g h l y c h a r g e d m i c e l l e s c a n b e r e a s o n a b l y e x p e c t e d t o b e r e p e l l e d b y t h e n e g a t i v e l y c h a r g e d g l a s s s u r f a c e more e x t e n s i v e l y than monovalent c o - i o n s . Quantitative calculations are d i f f i c u l t b u t some o r d e r s o f m a g n i t u d e c a n b e e s t i m a t e d . As mentioned b e f o r e , t h e e q u i v a l e n t e x c l u s i o n d i s t a n c e f o r monovalent co-ions f r o m p l a n a r s u r f a c e s a t h i g h p o t e n t i a l s i s g i v e n b y 2δ, when t h e m o b i l e i o n s a r e a l l m o n o v a l e n t a n d when i n t e r a c t i o n s b e t w e e n d o u b l e l a y e r s i s n e g l i g i b l e , de Haan (3l) h a s shown t h a t when b i v a l e n t a n d t r i v a l e n t c o - i o n s a r e i n t r o d u c e d i n t r a c e r amounts i n s u c h s y s t e m s , t h e e x c l u s i o n d i s t a n c e becomes 2.6676 a n d 3.0676 respectively. F o r t h e h i g h l y charged m i c e l l e s , any reasonable e x t r a p o l a t i o n w o u l d i n d i c a t e a much h i g h e r v a l u e . A v a l u e o f 6δ i s used below f o r e x p l o r a t o r y purposes. The e x c l u s i o n v o l u m e f o r n o n i n t e r a c t i n g d o u b l e l a y e r s i s g i v e n by t h e s u r f a c e a r e a t i m e s t h e e x c l u s i o n d i s t a n c e . For Bio-Glas 200 ( i ) , w i t h t h e e s t i m a t e d s u r f a c e a r e a o f ihO m /g t h e e x c l u d e d v o l u m e f o r SDS (6*3.3 nm a t t h e c . m . c ) , f o r e x a m p l e , u s i n g 6δ as t h e e x c l u s i o n d i s t a n c e , i s c a l c u l a t e d t o b e 2.8 m l / g . F o r o u r p o r o u s g l a s s s y s t e m s , most o f t h e s u r f a c e i s a s s o c i a t e d w i t h t h e p o r e s a n d , t h e r e f o r e , t h e e x c l u s i o n e f f e c t s s h o u l d b e l o w e r when compared t o n o n p o r o u s s y s t e m s f o r w h i c h a l l a d s o r p t i o n s u r f a c e i s externally located. A n e s t i m a t e o f t h e e x c l u s i o n volume c a n b e o b t a i n e d from t h e e x p e r i m e n t a l data b y assuming t h a t t h e apparent d e c r e a s e i n t h e a d s o r p t i o n a b o v e t h e c.m.c. i s e n t i r e l y due t o m i c e l l a r e x c l u s i o n , and c a l c u l a t i n g t h i s e x c l u s i o n from t h e de crease i n a d s o r p t i o n from t h a t a t t h e c.m.c f o r a given equiva lent concentration o f micelles i n the equilibrium solution. F i g u r e 5 i n d i c a t e s t h a t f o r STDS t h e e x c l u s i o n v o l u m e i s b e t w e e n 1.5 t o 1 m l / g . F r o m t h e d i m e n s i o n s o f t h e p o r e s a n d t h e e x c l u s i o n d i s t a n c e s (6 = 6.6 nm a t t h e c . m . c ) , t h e minimum v a l u e f o r t h e e x c l u d e d v o l u m e s h o u l d b e o f t h e o r d e r o f t h e p o r e v o l u m e , 0.7 ml/g. The experimental values a l l appear t o be h i g h e r . F o r SDS, a l s o , about 1 m l / g o f e x c l u d e d volume i s i n d i c a t e d b y t h e d a t a ( F i g u r e 6). 2

I n p r e s e n c e o f N a C l , t h e e x c l u s i o n e f f e c t f o r t h e STDS s y s t e m n e a r t h e c.m.c. seems t o b e r a t h e r l o w a l t h o u g h t h e a p p a r e n t a d s o r p t i o n does a p p e a r t o d e c r e a s e f r o m a b o u t 10"" e q u i v / l o f t h e e q u i l i b r i u m c o n c e n t r a t i o n o f STDS. T h e r e a s o n s f o r t h i s c o m p l i cated behavior are notc l e a r . I t should be noted t h a t the surface o f g l a s s i s p r o b a b l y c o v e r e d b y a g e l - l i k e l a y e r (35.), t h e e f f e c t 2


8.

MUKERJEE

AND

ANAviL


121

of which i s d i f f i c u l t t o estimate. In the case o f the c a t i o n i c s u r f a c t a n t s , the m i c e l l a r e x c l u s i o n e f f e c t i s e x p e c t e d t o b e r e l a t i v e l y much s m a l l e r when com p a r e d t o a n i o n i c systems because o f t h e f a r g r e a t e r s u r f a c e c o v e r age a n d p o s s i b l y l o w e r s u r f a c e p o t e n t i a l s o f t h e p o s i t i v e l y c h a r g e d s u r f a c e a t t h e c.m.c. Thus, f o r example, t h e d e c r e a s e i n t h e a p p a r e n t a d s o r p t i o n o b s e r v e d f o r STDS o n B i o - G l a s 200 ( i ) over t h e c o n c e n t r a t i o n r a n g e o f t h e c.m.c. t o 3 x 10 e q u i v / l i s about 3 x 10~5 equiv/g. F o r TDPB ( F i g u r e k) t h i s would correspond t o a d e c r e a s e o f o n l y a b o u t Q% i n t h e a p p a r e n t a d s o r p t i o n . The a p p a r ent constancy o f t h e a d s o r p t i o n o f t h e c a t i o n i c s f o r B i o - G l a s 200 ( F i g u r e k) c o u l d t h u s b e due t o a s m a l l i n c r e a s e i n t h e t r u e a d s o r p t i o n a b o v e t h e c.m.c. t o c o m p e n s a t e f o r t h e m i c e l l a r e x c l u sion.


9

The i n c r e a s e i n t h e t r u e a d s o r p t i o n h a s t o b e f a i r l y a p p r e c i a b l e f o r B i o - G l a s 500 ( i l ) f o r b o t h TDPB a n d HDPB ( F i g u r e 3) i n t h e absence o f added e l e c t r o l y t e s because t h e apparent a d s o r p t i o n i t s e l f seems t o i n c r e a s e above t h e c.m.c. I n t h e absence o f de t a i l e d i n f o r m a t i o n about t h e e x t e r n a l and i n t e r n a l s u r f a c e , i t i s d i f f i c u l t t o e s t i m a t e t h e p r o p e r e x c l u d e d v o l u m e b u t f o r HDPB, b e c a u s e o f i t s l o w c . m . c , a n d h i g h δ, t h i s s h o u l d b e o f t h e o r d e r o f t h e p o r e v o l u m e , 0.5 m l / g . I n f a c t t h e HDPB a d s o r p t i o n a p p e a r s t o i n c r e a s e above t h e c.m.c w i t h an average s l o p e o f about 0.9 ml/g. F o r t h i s s y s t e m , h o w e v e r , somewhat b e l o w t h e c.m.c. r e g i o n ( F i g u r e l ) , t h e a d s o r p t i o n t o B i o - G l a s 500 ( I I ) seems t o i n c r e a s e v e r y r a p i d l y , w i t h a s l o p e o f a b o u t 15 m l / g , u n l i k e t h e c a s e o f STDS ( F i g u r e 2). A r o u g h l y t e n - f o l d r e d u c t i o n i n t h e s l o p e above t h e c.m.c. w o u l d a c c o u n t f o r t h e o b s e r v e d i s o t h e r m above t h e c.m.c. S e v e r a l f a c t o r s s u c h as t h e i n c r e a s e i n t h e a c t i v i t y o f monomers above t h e c.m.c. as t h e c o n c e n t r a t i o n i n c r e a s e s t o more t h a n 60 χ c . m . c (33,36), a s a l s o t h e p o s s i b l e c o n t r i b u t i o n o f t h e m i c e l l e s t o t h e i o n i c s t r e n g t h o f t h e s o l u t i o n (2^), could c o n t r i b u t e t o t h i s apparent i n c r e a s e , and t h e magnitude o f t h i s e f f e c t does n o t seem t o b e e x c e s s i v e . I n p r e s e n c e o f 0.05M N a B r , t h e a p p a r e n t a d s o r p t i o n o f TDPB t o B i o - G l a s 500 ( i l ) seems t o b e n e a r l y c o n s t a n t a b o v e t h e c . m . c ( F i g u r e 3). A t t h i s h i g h i o n i c s t r e n g t h , t h e e x c l u d e d volume i n t e r a c t i o n s f r o m t h e p o s i t i v e l y c h a r g e d s u r f a c e s s h o u l d b e much reduced. F o r t h e h i g h e r h o m o l o g u e , HDPB, h o w e v e r , t h e a p p a r e n t a d s o r p t i o n a p p e a r s t o d e c r e a s e w i t h c o n c e n t r a t i o n a b o v e t h e c.m.c. i n presence o f high concentrations o f e l e c t r o l y t e s . M i c e l l a r ex c l u s i o n i n these systems cannot be a s c r i b e d t o double l a y e r effects. S t e r i c e x c l u s i o n a p p e a r s t o b e more l i k e l y . Anacker a n d Ghose ( I T ) h a v e shown t h a t v e r y l a r g e a s y m m e t r i c m i c e l l e s form i n such systems. I t seems t h a t some o f t h e s e g i a n t m i c e l l e s , w h i c h a r e p r o b a b l y r o d - s h a p e d (37), a n d w h i c h show p r o n o u n c e d e x c l u d e d v o l u m e i n t e r a c t i o n s i n s o l u t i o n (38), a r e e x c l u d e d f r o m t h e pores because o f s t e r i c reasons. The e x c l u d e d volumes a r e o f t h e o r d e r o f t h e p o r e v o l u m e o f 0.5 m l / g , a s c a n b e s e e n b y c o m p a r i s o n o f t h e i s o t h e r m s w i t h t h e l i n e drawn t o show t h i s p o r e v o l u m e .


122

ADSORPTION

A T INTERFACES

A r e c e n t l y proposed t h e o r e t i c a l treatment i n d i c a t e s that these very l a r g e m i c e l l e s a r e p o l y d i s p e r s e and t h e i r average degree o f a g g r e g a t i o n i n c r e a s e s r a p i d l y w i t h c o n c e n t r a t i o n (38.). C h r o m a t o g r a p h i c s t u d i e s o f such systems employing porous g l a s s s h o u l d be o f some i n t e r e s t .


Adsorption

Maxima a n d t h e E x c l u s i o n o f M i c e l l e s

The e x i s t e n c e o f maxima i n a d s o r p t i o n i s o t h e r m s o f s u r f a c t a n t s a b o v e t h e c.m.c. a p p e a r s t o b e a n o m a l o u s f r o m a t h e r m o d y n a m i c p o i n t o f v i e w b e c a u s e b e y o n d t h e maximum t h e a d s o r p t i o n d e c r e a s e s w i t h i n c r e a s i n g s o l u t e a c t i v i t y (1^,15.). V a r i o u s a t t e m p t s h a v e b e e n made t o r a t i o n a l i z e t h e s e o b s e r v e d maxima i n t e r m s o f i m p u r i t i e s i n t h e s y s t e m jlk) o r p o s s i b l e r e d u c t i o n o f t h e s u r f a c t a n t u p t a k e b y s o l i d s u r f a c e s b y i o n e x c h a n g e a b o v e t h e c.m.c. (15.)· The maxima o b s e r v e d i n t h e p r e s e n t w o r k f o r t h e a n i o n i c s u r f a c t a n t s have been a s c r i b e d t o m i c e l l a r e x c l u s i o n from s i m i l a r l y charged surfaces causing a lower estimate o f t h e apparent adsorp tion. The pronounced e f f e c t s observed, r e s u l t i n g even i n appar e n t l y n e g a t i v e a d s o r p t i o n s a t h i g h c o n c e n t r a t i o n s ( F i g u r e 6), a r e u n d o u b t e d l y due t o t h e r e l a t i v e l y l o w s u r f a c e c o v e r a g e o f t h e a n i o n i c s u r f a c t a n t s , and t h e presence o f an i n i t i a l l y charged sur face. N e v e r t h e l e s s , m i c e l l a r e x c l u s i o n a b o v e t h e c.m.c. i s e x p e c t e d t o b e a g e n e r a l phenomenon a n d t h e i n t e r p r e t a t i o n o f a l l a d s o r p t i o n i s o t h e r m s a b o v e t h e c.m.c. must t a k e t h i s i n t o a c c o u n t . The q u e s t i o n a r i s e s a s t o w h e t h e r t h i s phenomenon c a n p r o v i d e a g e n e r a l e x p l a n a t i o n f o r a d s o r p t i o n maxima o b s e r v e d i n o t h e r systems. F o r s o l i d s u b s t r a t e s , a d s o r p t i o n maxima h a v e o f t e n b e e n o b s e r v e d i n s y s t e m s where t h e a d s o r p t i o n i m m e d i a t e l y a b o v e t h e c.m.c. i n c r e a s e d r a p i d l y w i t h c o n c e n t r a t i o n , i . e . , w i t h r o u g h l y t h e same s l o p e a s b e l o w t h e c.m.c. (l4,15,39) b e f o r e d e c r e a s i n g at higher concentrations. No e v i d e n c e f o r s u c h a r a p i d i n c r e a s e a b o v e t h e c.m.c. h a s b e e n o b s e r v e d i n o u r work. Whether such i n c r e a s e s a r e p o s s i b l e when f i n e l y d i v i d e d s o l i d s h a v e i n t e r a c t i n g d o u b l e l a y e r s w i t h v a r i a b l e s p a c i n g s a n d s e p a r a t i o n s i s n o t known. The a p p a r e n t r e d u c t i o n i n t h e a d s o r p t i o n i n s e v e r a l s u c h s y s t e m s b e y o n d t h e maximum seems t o o c c u r t o o r a p i d l y a l s o t o b e e x p l a i n e d i n terms o f a simple p i c t u r e o f m i c e l l a r e x c l u s i o n . Thus, f o r e x a m p l e , i n t h e o r i g i n a l work o f V o i d a n d P h a n s a l k a r (39), w h i c h drew a t t e n t i o n t o t h e p r o b l e m o f a d s o r p t i o n maxima, t h e a d s o r p t i o n i s o t h e r m o f SDS o n c a r b o n d e c r e a s e s a b o v e t h e maximum w i t h a s l o p e o f a b o u t 10"^ ml/cm w h e r e a s a r o u g h e s t i m a t e b a s e d on t h e 66 e x c l u s i o n d i s t a n c e e m p l o y e d e a r l i e r , u s i n g 3.3 nm f o r δ a t t h e c.m.c. o f SDS, w o u l d b e 2 χ 10 ml/cm . A g a i n , t h e pos s i b l e complications o f i n t e r a c t i n g s o l i d p a r t i c l e s a t variable d i s t a n c e s o f s e p a r a t i o n remain t o be examined. 2

F o r some s o l i d s y s t e m s a g e n t l e d e c r e a s e i n t h e a p p a r e n t a d s o r p t i o n o c c u r s o v e r e x t e n d e d c o n c e n t r a t i o n r a n g e s , 10 t o 100 t i m e s t h e c.m.c. (15.). The m i c e l l a r e x c l u s i o n e f f e c t appears t o



8.

MUKERJEE AND

ANAviL

Adsorption

of Ionic Surfactants

123

be adequate f o r such systems, although other f a c t o r s may be i n volved also. Of c o n s i d e r a b l e i n t e r e s t i n t h i s connection are some b i n d i n g s t u d i e s o f i o n i c s u r f a c t a n t s w i t h h y d r o p h i l i c polymers (40,Ul) and non-ionic s u r f a c t a n t s (ho). The apparent b i n d i n g isotherms, ob t a i n e d from d i a l y s i s e q u i l i b r i u m s t u d i e s , appear t o show maxima at concentrations c l o s e t o o r somewhat above t h e c.m.c. f o r t h e d i a l y s a t e , when t h e r e t e n t a t e contains the nonpermeable polymer. The i n t e r p r e t a t i o n o f even such s o l u b l e systems i s d i f f i c u l t f o r t h e case o f polymers because of u n c e r t a i n t i e s about the p o l y mer dimensions. Some data (ko) on the b i n d i n g o f hexadecyl p y r i dinium c h l o r i d e (HDPC) t o Tween 80, a non-ionic s u r f a c t a n t w i t h a b u l k y s o r b i t a n p o l y o x y e t h y l e n e head group and having a low c.m.c. of about 0.0013$ (k2), are thus o f p a r t i c u l a r i n t e r e s t because compact m i c e l l e s are i n v o l v e d . The data were obtained from d i a l y s i s s t u d i e s u s i n g nylon membranes which are impermeable t o Tween 80 but are permeable t o HDPC. F i g u r e 8 shows the b i n d i n g i s o therm, the data p o i n t s f o r which were read from a l a r g e s c a l e graph p u b l i s h e d by Deluca and Kostenbauder (ho). The b i n d i n g r e s u l t s i n t h e formation o f mixed m i c e l l e s . I t appears t o i n c r e a s e beyond the c.m.c. o f HDPC, which was estimated t o be 1.0 χ 10~% (ho), before decreasing at higher c o n c e n t r a t i o n s . The i n c r e a s e i n b i n d i n g immediately above t h e c.m.c. may be caused i n p a r t by the presence of homologous i m p u r i t i e s . The lower homologues are ex pected t o b i n d l e s s t o the mixed m i c e l l e s . Since about 70% o f the HDPC added i s bound t o Tween 80 a t the c.m.c, t h e f r a c t i o n o f the lower homologues may be c o n s i d e r a b l y higher i n the d i a l y s a t e than i n t h e o r i g i n a l s u r f a c t a n t , r e s u l t i n g i n a h i g h e r apparent c.m.c. The mean slope above the maximum i n d i c a t e s t h e h i g h excluded volume o f about l60 ml/g o f Tween 80. The mixed m i c e l l e s have a mole f r a c t i o n o f about 0.75 o f HDPC a t the maximum as estimated from the uptake o f HDPC, and u s i n g t h e estimated molecular weight of ikOO f o r Tween 80, and a r e , t h e r e f o r e , h i g h l y charged. The excluded volume has a value o f about 91 ml/g o f mixed m i c e l l e , and about 2^0 ml/g o f hydrocarbon core i n the mixed m i c e l l e . To examine i f these extremely h i g h excluded volumes are reasonable i n terms o f the e x c l u s i o n o f m i c e l l e s o f HDPC from the e l e c t r i c a l double l a y e r s of the mixed m i c e l l e s o f HDPC and Tween 80, some comparisons w i t h mutual e x c l u s i o n s o f m i c e l l e s i n i n t e r m i c e l l a r i n t e r a c t i o n s are r e v e a l i n g . The second v i r i a l c o e f f i c i e n t s o f a s e r i e s o f a l k y l s u l f a t e m i c e l l e s have been i n v e s t i gated (13). The v i r i a l c o e f f i c i e n t s have h i g h v a l u e s , t h a t f o r SDS, f o r example, being k3 times the value expected i f t h e mi c e l l e s , without t h e i r double l a y e r s , are t r e a t e d as hard spheres. The m i c e l l e s w i t h t h e i r e l e c t r i c a l double l a y e r s thus behave l i k e e q u i v a l e n t hard spheres o f much l a r g e r r a d i i than those o f t h e m i c e l l e s themselves. I f these r a d i i are c a l c u l a t e d from the appropriate molecular weights, d e n s i t i e s and the second v i r i a l c o e f f i c i e n t s , and expressed as r + ko where r i s t h e r a d i u s o f the m i c e l l e , δ i s t h e double l a y e r t h i c k n e s s , and k i s a propor-


ADSORPTION

A T INTERFACES

10,

ο

jo-o

. Ο —

£

7


Χ

.

Λ

Λ —

t

••crac.

0

I 2

1

1 3

J 5

I 4

EQUIL. CONC. OF SURFACTANT (mole/i χ \oft

Figure 7. Variation of equilibrium solution pH with equilibrium concentrations at 30° C. Q = STDS with Bio-Gfos 200 (I), Δ = TDPB with Bio-Gkis 500 (II).

0

2 FREE

4

6

8

HDPC (mole/i χ Ι Ο ) 3

Figure 8. Adsorption (binding) isotherm of hexadecyl pyridinium chloride to 0.2% Tween 80 from dialysis equilibrium at 30°C. Data from reference (40).


8.

MUKERJEE

AND

ANAviL


125

t i o n a l i t y constant, so t h a t the average equivalent distance be tween the m i c e l l e centers i s 2(r + kô), the value o f k i s 1.0 for o c t y l s u l f a t e s , 1.25 f o r d e c y l s u l f a t e s and 1.3^ f o r dodecyl s u l f a t e s at t h e i r r e s p e c t i v e c.m.c. s. Thus the double l a y e r con t r i b u t i o n t o the e f f e c t i v e radius i s of the order of δ or g r e a t e r . For an approximate c a l c u l a t i o n of the e x c l u s i o n o f HDPC m i c e l l e s from the mixed m i c e l l e s of HDPC and Tween 80 u s i n g an equivalent hard sphere treatment, we assume t h a t both m i c e l l e s have the same r a d i i of the hydrocarbon core, 2 nm, and the double l a y e r c o n t r i b u t i o n t o the e f f e c t i v e r a d i u s , d' i n nm, i s a l s o the same. The volume around each mixed m i c e l l e excluded t o HDPC mi c e l l e centers i s given by t h a t of a sphere of r a d i u s 2(2 + d")nm. Assuming the d e n s i t y of the m i c e l l a r cores t o be u n i t y , the ex cluded volume of 2k0 ml/g of hydrocarbon core i n d i c a t e s t h a t the r a t i o of 2(2 +

Adsorption of Ionic Surfactants to Porous Glass: The Exclusion of

Recommend Documents