Evaluation of a Macroscopic Model for Polymer ... - ACS Publications

2 Evaluation of a Macroscopic Model for Polymer Adsorption R. HOGG Downloaded by INDIANA UNIV BLOOMINGTON on May 9, 2015 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch002

The Pennsylvania State University, University Park, PA 16802 An approximate analysis of polymer adsorption as a set of sequential reactions leads to a simple equation for the adsorption isotherm expressed i n terms of three parameters. Comparison of the model with recently published statistical theories reveals remarkable agreement i n both the general shape of the isotherms and the predicted effects of molecular weight. The problems of applying such models to experimental data are discussed. The a d s o r p t i o n o f s o l u b l e p o l y m e r s a t s o l i d - l i q u i d i n t e r f a c e s i s a h i g h l y c o m p l e x phenomenon w i t h v a s t numbers o f p o s s i b l e c o n f i g u r a t i o n s o f the molecules a t thesurface. Previous analyses of p o l y m e r a d s o r p t i o n have r a n g e d i n s o p h i s t i c a t i o n f r o m v e r y s i m p l e a p p l i c a t i o n s o f " s t a n d a r d " models d e r i v e d f o r s m a l l m o l e c u l e s , to d e t a i l e d s t a t i s t i c a l m e c h a n i c a l t r e a t m e n t s o f t h e p r o c e s s . The u s e o f t h e s i m p l e s t m o d e l s , s u c h a s t h e L a n g m u i r i s o t h e r m , n e g l e c t s v a r i a t i o n s i n m o l e c u l a r c o n f i g u r a t i o n a n d assumes, i n e f f e c t , t h a t there i s a s i n g l e , "average" c o n f i g u r a t i o n o f a molecule a t t h e s u r f a c e . I n p a r t i c u l a r , t h e c o n f i g u r a t i o n i s cons i d e r e d t o be i n d e p e n d e n t o f t h e e x t e n t o f c o v e r a g e o f t h e surface: a s i n g l e m o l e c u l e on an o t h e r w i s e bare s u r f a c e o c c u p i e s t h e same a r e a a s a n i n d i v i d u a l m o l e c u l e o n a s u r f a c e s a t u r a t e d w i t h polymer. Modern s t a t i s t i c a l models o f polymer a d s o r p t i o n , on t h e o t h e r h a n d , e m p h a s i z e t h e c o n f i g u r a t i o n a l a s p e c t s o f t h e p r o c e s s and have p r o v i d e d c o n s i d e r a b l e i n s i g h t i n t o the n a t u r e and s t r u c t u r e o f t h e adsorbed l a y e r s . The p r i n c i p a l d i s a d v a n t a g e s o f t h i s approach a r i s e from t h e complexity o f the process i t s e l f . Mathematical treatments a r e n e c e s s a r i l y very complicated, i n v o l v i n g i t e r a t i v e s o l u t i o n s o f complex s e t s o f e q u a t i o n s . Thus, w h i l e the s t a t i s t i c a l approach i s i n v a l u a b l e i n t h e d e s c r i p t i o n o f a d s o r p t i o n p r o c e s s e s , i t s a p p l i c a b i l i t y t o the development o f q u a n t i t a t i v e models f o r processes such as f l o c c u l a t i o n and s t e r i c s t a b i l i z a t i o n i s s t i l l somewhat l i m i t e d .

0097-6156/84/0240-0023S06.00/0 © 1984 American Chemical Society

In Polymer Adsorption and Dispersion Stability; Goddard, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Downloaded by INDIANA UNIV BLOOMINGTON on May 9, 2015 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch002

24

POLYMER ADSORPTION AND DISPERSION STABILITY

I n a r e c e n t p a p e r (1), a n a t t e m p t was made t o d e v e l o p a n " i n t e r m e d i a t e " model w h i c h a c c o u n t s f o r c o n f i g u r a t i o n a l e f f e c t s i n a s i m p l i f i e d f a s h i o n b u t r e t a i n s much o f t h e s i m p l i c i t y o f t h e " s m a l l m o l e c u l e " models. The m o d e l i s b a s e d o n an a p p r o x i m a t e a n a l y s i s o f t h e thermodynamics o f polymer a d s o r p t i o n i n w h i c h i t i s assumed t h a t t h e p r o c e s s o c c u r s a s a s e r i e s o f c o n s e c u t i v e , r e v e r s i b l e r e a c t i o n s b e t w e e n segments o f t h e p o l y m e r m o l e c u l e s and s i t e s on t h e s o l i d s u r f a c e . C l e a r l y , t h i s model r e p r e s e n t s a c o n s i d e r a b l e o v e r - s i m p l i f i c a t i o n o f t h e complex a d s o r p t i o n process. F o r e x a m p l e , no d i s t i n c t i o n i s made between m u l t i p l e a d s o r p t i o n o f a d j a c e n t segments and t h a t o f segments f r o m comp l e t e l y d i f f e r e n t l o c a t i o n s i n a molecule. Nevertheless, the model does appear t o p r o v i d e a r e a s o n a b l e d e s c r i p t i o n o f t h e p r o c e s s and i t s p r e d i c t i o n s a r e i n g e n e r a l a g r e e m e n t w i t h o b s e r v a tion. I n t h e p r e s e n t p a p e r , we w i l l a t t e m p t t o e v a l u a t e t h e g e n e r a l v a l i d i t y o f t h e m o d e l by c o m p a r i s o n w i t h t h e s t a t i s t i c a l models. The p r o b l e m s o f a p p l y i n g t h e m o d e l t o e x p e r i m e n t a l d a t a and o f p a r a m e t e r e s t i m a t i o n w i l l a l s o be d i s c u s s e d . The B a s i c

Model

D e t a i l s o f t h e m a t h e m a t i c a l d e v e l o p m e n t o f t h e m o d e l have b e e n g i v e n e l s e w h e r e (1) a n d w i l l n o t be r e p e a t e d h e r e . I t i s u s e f u l , however, t o p r e s e n t a b r i e f summary o f t h e p r i n c i p a l a s s u m p t i o n s and a p p r o x i m a t i o n s u s e d i n t h e d e s c r i p t i o n o f t h e p r o c e s s . The b a s i s o f the model i s the f o l l o w i n g s e t o f assumptions: i) E a c h p o l y m e r m o l e c u l e c o n s i s t s o f a f i x e d number o f i d e n t i c a l segments, ii) A d s o r p t i o n o c c u r s a t a f i x e d number o f i d e n t i c a l s i t e s on t h e s o l i d s u r f a c e , iii) I n d i v i d u a l p o l y m e r segments a d s o r b r e v e r s i b l y o n t h e surface. i v ) M u l t i p l e a d s o r p t i o n o f segments f r o m t h e same m o l e c u l e occurs sequentially. No d i s t i n c t i o n i s made b e t w e e n w h i c h p a r t i c u l a r segments a d s o r b , v ) A t any s t a g e i n t h e p r o c e s s , f u r t h e r a d s o r p t i o n c a n occur on any unoccupied s i t e . T h e r e i s no b l o c k i n g o f a d j a c e n t s i t e s by a d s o r b e d p o l y m e r s e g m e n t s , n o r i s t h e r e any a p p r e c i a b l e i n t e r a c t i o n between adsorbed segments. A s s u m p t i o n s ( i ) t o ( i i i ) a r e e s s e n t i a l l y t h e same a s t h o s e u s e d i n t h e s t a t i s t i c a l t r e a t m e n t s w h i l e ( i v ) a n d ( v ) a r e somewhat more restrictive. The m o d e l i s f o r m u l a t e d by c l a s s i f y i n g t h e m o l e c u l e s i n t h e s y s t e m a c c o r d i n g t o t h e number o f a d s o r b e d segments and w r i t i n g down a s e t o f s i m p l e m a s s - a c t i o n r e l a t i o n s h i p s t o d e s c r i b e t h e equilibrium conditions. The f i r s t s t e p i n t h e p r o c e s s c o n s i s t s o f t h e a t t a c h m e n t o f one segment o f a f r e e m o l e c u l e i n s o l u t i o n t o a s i t e o n t h e s o l i d s u r f a c e a n d t h e e q u i l i b r i u m c o n d i t i o n c a n be written:


2.

HOGG

Macroscopic Model for Polymer

25

Adsorption

where K-^ i s t h e p r i m a r y a d s o r p t i o n c o n s t a n t , Γ-^ i s t h e a d s o r p t i o n d e n s i t y f o r m o l e c u l e s a t t a c h e d t h r o u g h o n e segment o n l y , η i s t h e number o f segments p e r m o l e c u l e , c i s t h e c o n c e n t r a t i o n o f f r e e molecules i n s o l u t i o n and θ i s t h e f r a c t i o n o f s u r f a c e s i t e s occu p i e d b y a d s o r b e d p o l y m e r s e g m e n t s . A n y a p p r o p r i a t e u n i t s c a n be used f o r and c w h i l e w i l l have u n i t s o f l e n g t h . T h u s , f o r Τ ι i n m o l e s / c m a n d c i n moles/cm^, w i l l be i n cm. E q u i l i b r i u m c o n d i t i o n s f o r t h e a d s o r p t i o n o f second, t h i r d , f o u r t h , e t c . , segments f r o m m o l e c u l e s a l r e a d y a t t a c h e d t o t h e s i m i l a r fashion and lead to t h e ss uu rr rf aa cc ee c ca an n be be ww rr ii tt tt ee nn down down im n ^g step: following expression f o r thei


2

if, K

(n-i+Dr^d-θ)

i

( 2 )

th where i s theadsorption constant f o r the i s t e p and i s the a d s o r p t i o n d e n s i t y f o r m o l e c u l e s adsorbed t h r o u g h i segments. F o r ±φ1, i s dimensionless. The o v e r a l l a d s o r p t i o n d e n s i t y i s g i v e n b y η (3) Γ = Σ Γ i=l ±

and

t h e f r a c t i o n a l s u r f a c e coverage by

θ - i - Σ 1Γ si=l

(4)

±

where Ν

i s t h e t o t a l number o f s i t e s p e r u n i t a r e a o f s o l i d s s u r f a c e . The u s e o f a f u r t h e r s i m p l i f y i n g a s s u m p t i o n , t h a t a l l o f the e q u i l i b r i u m constants f o r m u l t i p l e adsorption ( i > l ) a r e t h e same, l e a d s (1) t o t h e f o l l o w i n g e x p r e s s i o n s f o r t h e o v e r a l l process : θ = c*(i-e)[i+K (i-e)] " (5) n

1

s

* η

G - - ™ - { [1+K (1-θ)] -1) η κ. s s Here, Κ i s t h e m u l t i p l e a d s o r p t i o n c o n s t a n t , C p o l y m e r c o n c e n t r a t i o n d e f i n e d by

(6) i sa relative


26 and

POLYMER ADSORPIION AND DISPERSION STABILITY G i s the r e l a t i v e a d s o r p t i o n d e n s i t y : G -

f

(8) S

By means o f E q u a t i o n s 5 and 6, t h e a d s o r p t i o n p r o c e s s c a n be d e s c r i b e d i n terms of the parameters and K . S i n c e the e f f e c t i v e area of s o l i d s u r f a c e a v a i l a b l e to polymer a d s o r p t i o n i s n o t g e n e r a l l y known i n p r a c t i c e , N i s a t h i r d p a r a m e t e r w h i c h must be f i t t e d f r o m e x p e r i m e n t and E q u a t i o n s 5 and 6 d e f i n e a three-parameter model f o r the p r o c e s s . s


g

Approximate S o l u t i o n s F o r t h e a d s o r p t i o n o f p o l y m e r s , t h e number o f segments p e r m o l e c u l e η i s l a r g e which allows f u r t h e r s i m p l i f i c a t i o n of the r e l a tionships. I f the q u a n t i t y Κ (1-θ) « 1, 8

[l+K (l-e)f * s

f o r a l l v a l u e s f o r n, and

e

n

K

s

(

1

~

e

(9)

)

f o r l a r g e n, E q u a t i o n

G * c*

e

n K s ( 1

"

9 )

(10)

E l i m i n a t i o n o f C* b e t w e e n E q u a t i o n s {1+K n K

The l a s t two g e n e r a l l y be

s

( i ^ r

s

(1-Θ) —

5 and

1

+

K

6 leads

„ .} i - e ) r l

s

(

to (11)

J

t e r m s on t h e r i g h t hand s i d e o f E q u a t i o n 11 w i l l s m a l l and t h e e x p r e s s i o n c a n be a p p r o x i m a t e d by

ηΚ which leads

[

6 reduces to

s

(12)

(1-θ)

to nK G * —1+ηΚ G s

S u b s t i t u t i o n i n Equation

10

κ

gives

nK C**nK Gexp[- ^)] s

(13) '

(

l

(14)

E q u a t i o n 14 i s an a p p r o x i m a t e , s i n g l e e x p r e s s i o n f o r t h e p o l y m e r adsorption isotherm. A t v e r y l o w c o n c e n t r a t i o n s , when G i s v e r y


HOGG

2.


27

Adsorption

s m a l l , E q u a t i o n 15 c a n be f u r t h e r r e d u c e d t o


ηΚ

e""

s

(15)

I n o t h e r w o r d s , t h e model p r e d i c t s a l i m i t i n g f o r m o f a l i n e a r ( H e n r y ' s Law) t y p e o f i s o t h e r m a s t h e p o l y m e r c o n c e n t r a t i o n t e n d s to z e r o . At very h i g h surface coverage, t h e equations reduce f o r m a l l y to t h e Langmuir form. However, i t i s v e r y u n l i k e l y t h a t t h e r e q u i r e d c o n d i t i o n s ( c o n s t a n t Κ e t c . ) would remain v a l i d i n such cases. Some t y p i c a l a d s o r p t i o n i s o t h e r m s , c a l c u l a t e d f r o m E q u a t i o n 14 ( o r f r o m E q u a t i o n s 5 a n d 6) a r e shown i n F i g u r e 1 f o r n=10^ w i t h v a r i o u s v a l u e s o f K . I n g e n e r a l , i t c a n be s e e n t h a t t h e isotherms c o n s i s t of three regions: a l i n e a r r e g i o n a t very low c o n c e n t r a t i o n ( d e s c r i b e d by E q u a t i o n 1 5 ) , a r a t h e r f l a t " p l a t e a u " r e g i o n a t intermediate concentrations and another r e g i o n o f i n c r e a s i n g a d s o r p t i o n a t v e r y h i g h c o n c e n t r a t i o n s . The e x t e n t o f the " p l a t e a u " r e g i o n i n c r e a s e s d r a m a t i c a l l y a s K i n c r e a s e s . I t s h o u l d be noted t h a t , a c c o r d i n g t o E q u a t i o n 14, t h e form o f t h e a d s o r p t i o n i s o t h e r m depends o n l y o n t h e p r o d u c t n K and n o t o n t h e i n d i v i d u a l v a l u e s o f t h e s e two q u a n t i t i e s . Thus, t h e c u r v e s shown i n F i g u r e 1 w o u l d a l s o r e p r e s e n t t h e e f f e c t o f m o l e c u l a r w e i g h t ( w h i c h d e t e r m i n e s n) a t f i x e d K . The o t h e r two p a r a m e t e r s o f t h e m o d e l , K-^ a n d N a r e s i m p l y s c a l i n g f a c t o r s f o r t h e b u l k c o n c e n t r a t i o n and t h e a d s o r p t i o n d e n s i t y r e s p e c t i v e l y . I t i s c l e a r from F i g u r e 1 that t h e slope o f t h e " p l a t e a u " r e g i o n i s l a r g e l y d e t e r m i n e d by t h e v a l u e o f n K . E x p r e s s i n g E q u a t i o n 14 i n l o g a r i t h m i c form: s

g

s

s

g

g

g

* nK I n C = I n nK + I n G - . ^ s 1+nK b s Ί

and

r

(16)

d i f f e r e n t i a t i n g , we o b t a i n dlnG dlnC*

(17)

XnK 1+G

VL+nK GJ ^ s I t c a n be shown t h a t t h e " p l a t e a u " r e g i o n o c c u r s when t h e q u a n t i t y nK G has a value c l o s e t o u n i t y . Then, a p p r o x i m a t i n g G b y l / n K i n E q u a t i o n 17 we o b t a i n : s

g

U£G dlnC*

_ 1 _ nK 1 +

(

1

8

)

s

Thus, f o r e x p e r i m e n t a l d a t a o b t a i n e d i n t h e " p l a t e a u " r e g i o n , t h e s l o p e o f a l o g - l o g p l o t o f Γ v s C c a n be u s e d t o o b t a i n a n i n i t i a l (rough) e s t i m a t e o f Κ .



28


RELATIVE

CONCENTRATION

C*

F i g u r e 1. T y p i c a l p o l y m e r a d s o r p t i o n i s o t h e r m s f o r n=10 w i t h v a r i o u s v a l u e s o f Κ . Note the broad p l a t e a u r e g i o n s f o r the larger Κ values.


2.

HOGG


29

Adsorption


Comparison w i t h S t a t i s t i c a l Models Numerous s t a t i s t i c a l t r e a t m e n t s o f t h e a d s o r p t i o n o f p o l y m e r s a t s o l i d - l i q u i d i n t e r f a c e s have b e e n d e s c r i b e d i n t h e l i t e r a t u r e . The e a r l i e r m o d e l s (2-5) d e a l t p r i m a r i l y w i t h t h e c o n f o r m a t i o n o f a s i n g l e molecule a t an i n t e r f a c e and a p p l y a t v e r y l o w adsorp t i o n d e n s i t i e s . More r e c e n t t r e a t m e n t s (6-10) t a k e i n t o a c c o u n t polymer-polymer and p o l y m e r - s o l v e n t i n t e r a c t i o n s and have l e d t o t h e emergence o f a f a i r l y c o n s i s t e n t p i c t u r e o f t h e a d s o r p t i o n process. F o r d e t a i l s o f t h e s t a t i s t i c a l t h e o r i e s o f polymer a d s o r p t i o n , t h e r e a d e r i s r e f e r r e d t o p u b l i c a t i o n s b y L i p a t o v (11), T a d r o s (12) a n d F l e e r a n d S c h e u t j e n s ( 1 3 ) . I t i s o f c o n s i d e r a b l e i n t e r e s t t o compare t h e p r e s e n t m o d e l with thepredictions of thes t a t i s t i c a l theories. In particular, t h e r e s u l t s w i l l be compared w i t h t h e t h e o r i e s o f S c h e u t j e n s a n d F l e e r ( 9 , 1 0 , 1 3 ) . These a u t h o r s h a v e p o i n t e d o u t t h a t t h e i r r e s u l t s a r e i n g e n e r a l agreement w i t h o t h e r p u b l i s h e d models. C o m p a r i s o n o f t h e a d s o r p t i o n i s o t h e r m s shown i n F i g u r e 1 w i t h those g i v e n by S c h e u t j e n s and F l e e r (see, f o r example, F i g u r e 5 o f R e f e r e n c e 13) shows e x c e l l e n t q u a l i t a t i v e a g r e e m e n t . I n e a c h case, t h e isotherms c o n s i s t o f three r e g i o n s a s d e s c r i b e d above. I t i s c l e a r t h a t good q u a n t i t a t i v e a g r e e m e n t s h o u l d b e p o s s i b l e by s u i t a b l e c h o i c e o f t h e p a r a m e t e r s Κ·^ a n d K . Some d i r e c t c o m p a r i s o n s o f t h e p r e s e n t m o d e l w i t h t h e p u b l i s h e d r e s u l t s o f F l e e r a n d S c h e u t j e n s (13) a r e g i v e n i n F i g u r e s 2 a n d 3. I n t h e c o m p a r i s o n o f t h e a d s o r p t i o n i s o t h e r m s , shown i n F i g u r e 2, t h e amount a d s o r b e d i s r e p r e s e n t e d b y t h e number o f e q u i v a l e n t m o n o l a y e r s g ( t o t a l number o f segments i n m o l e c u l e s bound t o t h e s u r f a c e ) d e f i n e d b y s

g = nG

(19)

The b u l k p o l y m e r c o n c e n t r a t i o n i s e x p r e s s e d a s a volume f r a c t i o n φ w h i c h i s p r o p o r t i o n a l t o t h e t o t a l segment c o n c e n t r a t i o n nC. Thus, i n t h i s c o m p a r i s o n , t h e r e l a t i v e c o n c e n t r a t i o n C i s d e f i n e d by C* =

φ

(20)

where K-[ i s a m o d i f i e d p r i m a r y adsorption constant which includes t h e segment v o l u m e a n d t h e s u r f a c e s i t e d e n s i t y Ν . I t c a n be seen from F i g u r e 2 t h a t t h e p r e s e n t model, w i t h f i x e d Κ , i s i n s u b s t a n t i a l agreement w i t h t h e s t a t i s t i c a l t h e o r y ( f o r f i x e d v a l u e s o f £he i n t e r a c t i o n p a r a m e t e r s χ a n d x ) p r o v i d e d t h e p a r a m e t e r K-^ i s a l l o w e d t o v a r y w i t h c h a i n l e n g t h . Some d i s c r e p a n c y c a n b e s e e n a t t h e h i g h e r c h a i n l e n g t h ( n = 50 a n d 1000) w i t h t h e s t a t i s t i c a l m o d e l p r e d i c t i n g somewhat f l a t t e r p l a t e a u r e g i o n s w i t h g e n e r a l l y l o w e r a d s o r b e d amounts. I t s h o u l d be n o t e d , h o w e v e r , t h a t t h e " t r a n s i t i o n c o n c e n t r a t i o n " φ , d e f i n e d g

c



30


VOLUME

FRACTION

OF

POLYMER, Φ

F i g u r e 2. Comparison of a d s o r p t i o n isotherms based on the present model ( s o l i d l i n e s ) with the s t a t i s t i c a l theory of Scheutjens and F l e e r (symbols). The f o l l o w i n g values of the parameters were used: η

1

Kj

4.0

10

20

0.68

50

0.10

2.0x10

-4

1000 -87 10

Κ

1.0 1.0 1.0 1.0 1.0 s The values taken from Scheutjens and F l e e r are f o r a 0-solvent (X-0.5) and an adsorption energy parameter X =1.0.

10

I DEGREE

OF

100 POLYMERIZATION,

IOOO η

F i g u r e 3. T h e e f f e c t o f d e g r e e o f p o l y m e r i z a t i o n on s u r f a c e coverage ( f r a c t i o n a l s i t e occupancy) a t v a r i o u s polymer concen trations. The s o l i d l i n e s r e p r e s e n t t h e p r e s e n t m o d e l and t h e s y m b o l s c o r r e s p o n d t o t h e t h e o r y o f S c h e u t j e n s and F l e e r . T h e p a r a m e t e r v a l u e s a r e t h e same a s i n F i g u r e 2. In Polymer Adsorption and Dispersion Stability; Goddard, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

2.

HOGG


31

Adsorption

by F l e e r and S c h e u t j e n s (13), a s t h e i n t e r s e c t i o n o f t h e e x t r a p o l a t e d l i n e a r and p l a t e a u r e g i o n s does a g r e e v e r y c l o s e l y f o r t h e s e c a s e s (φ° * 10" f o r n=50 a n d ^ 1 0 ~ f o r n=1000). The p r e d i c t e d e f f e c t s o f m o l e c u l a r w e i g h t o n s u r f a c e c o v e r a g e θ a r e compared i n F i g u r e 3. A g a i n t h e a g r e e m e n t i s g e n e r a l l y good w i t h some d e v i a t i o n a t h i g h c o n c e n t r a t i o n s (φ >10~2) a n d f o r t h e h i g h e r c h a i n l e n g t h s . I t i s c l e a r f r o m F i g u r e s 2 and 3 t h a t t h e two a p p r o a c h e s l e a d t o v e r y s i m i l a r f o r m s f o r 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 b u l k p o l y m e r c o n c e n t r a t i o n s l e s s t h a n a b o u t 1 0 % by volume a n d v a l u e s o f t h e p r o d u c t nK l e s s t h a n a b o u t 5 0 . I t s seems l i k e l y t h a t t h e s e c o n d i t i o n s a r e e a s i l y s a t i s f i e d i n s y s t e m s i n v o l v i n g t h e u s e o f p o l y m e r s a s f l o c c u l a n t s o r d i s p e r s a n t s where concentrations are t y p i c a l l y of theorder o f parts per m i l l i o n and a d s o r p t i o n i s p r o b a b l y r e l a t i v e l y weak ( K « 1) . The p h y s i c a l s i g n i f i c a n c e o f t h e p a r a m e t e r s K^ ( o r Κχ) a n d Κ i s obviously of considerable interest. I n the formulation of the m o d e l , t h e s e a r e s i m p l y a r b i t r a r y p a r a m e t e r s w h i c h d e f i n e , r e s p e c t i v e l y , t h e extent o f primary and m u l t i p l e a d s o r p t i o n . F o r the p a r t i c u l a r c a s e e v a l u a t e d a b o v e (χ=0.5 a n d x = l . 0 ) , K i s a p p r o x i m a t e l y c o n s t a n t w h i l e K^ a p p e a r s t o d e c r e a s e e x p o n e n t i a l l y w i t h c h a i n l e n g t h n. I t i s e x p e c t e d t h a t , f o r a g i v e n c h a i n l e n g t h , t h e p a r a m e t e r s K^ a n d K w i l l b o t h depend o n χ a n d x . F u r t h e r c o m p a r i s o n s , s i m i l a r t o t h a t g i v e n h e r e , w i l l be r e q u i r e d to e s t a b l i s h t h e p r e c i s e c o r r e s p o n d e n c e o f t h e p a r a m e t e r s u s e d i n t h e two a p p r o a c h e s . The good g e n e r a l agreement between t h e s t a t i s t i c a l a n d macroscopic models i s n o t a l t o g e t h e r s u r p r i s i n g s i n c e b o t h d e v e l o p m e n t s s t a r t f r o m t h e same s e t o f b a s i c a s s u m p t i o n s . The c l o s e c o r r e s p o n d e n c e d o e s , however, i n d i c a t e t h a t t h e a d d i t i o n a l r e s t r i c t i o n s imposed o n t h e p r e s e n t model ( i . e . c o n s t a n t K and the l a c k o f d i f f e r e n t i a t i o n between t h e a d s o r p t i o n o f a d j a c e n t , a s o p p o s e d t o more d i s t a n t , segments o f a m o l e c u l e ) h a s o n l y m i n o r e f f e c t s on t h e form o f t h e a d s o r p t i o n isotherms. The s i m p l e a n a l y t i c a l form o f the p r e s e n t model, e s p e c i a l l y a s expressed by E q u a t i o n 14, i s e s p e c i a l l y a t t r a c t i v e f o r t h e c o r r e l a t i o n and i n t e r p r e t a t i o n o f e x p e r i m e n t a l d a t a and f o r i n c o r p o r a t i o n i n t o models f o r f l o c c u l a t i o n and d i s p e r s i o n p r o c e s s e s . A t t h e same time, i t i s important t o recognize that t h e s t a t i s t i c a l approach p r o v i d e s a good d e a l o f a d d i t i o n a l i n f o r m a t i o n , o n t h e r e l a t i v e d i s t r i b u t i o n s o f t a i l s , loops and t r a i n s , i n t h e adsorbed l a y e r s , f o r e x a m p l e . Such i n f o r m a t i o n c a n n o t be o b t a i n e d d i r e c t l y f r o m t h e m a c r o s c o p i c a p p r o a c h . N e v e r t h e l e s s , one c a n e n v i s a g e t h e u s e of a h y b r i d approach t o t h e e v a l u a t i o n o f polymer a d s o r p t i o n i n w h i c h t h e s i m p l e model i s u s e d t o c h a r a c t e r i z e t h e p r o c e s s a n d e v a l u a t e p a r a m e t e r s , a f t e r w h i c h t h e s t a t i s t i c a l t h e o r y c a n be u s e d t o p r o v i d e a more d e t a i l e d d e s c r i p t i o n o f m o l e c u l a r c o n f o r m a tions etc. at the interface. 12

235


c

g

f

s

g

g

g

g



32


A p p l i c a t i o n to Experimental

Data

A major advantage o f the simple model described i n t h i s paper l i e s i n i t s p o t e n t i a l a p p l i c a b i l i t y to the d i r e c t e v a l u a t i o n of experimental data. Unfortunately, i t i s c l e a r from the form of the t y p i c a l isotherms, e s p e c i a l l y those f o r high polymers (large n) that, even with a simple model, t h i s presents c o n s i d e r a b l e difficulty. The problems can be seen c l e a r l y by c o n s i d e r a t i o n of some t y p i c a l polymer a d s o r p t i o n data. Experimental isotherms f o r the a d s o r p t i o n of commercial polymer f l o c c u l a n t s on a k a o l i n c l a y are shown i n Figure 4. These data were obtained, i n the usual way, by determination of r e s i d u a l polymer concentrations a f t e r e q u i l i b r a t i o n with the s o l i d . In general, such methods are l i m i t e d at both extremes of the c o n c e n t r a t i o n s c a l e . Serious e r r o r s a r i s e at low c o n c e n t r a t i o n due to l o s s i n p r e c i s i o n of the a n a l y t i c a l technique and a t h i g h c o n c e n t r a t i o n because the amount adsorbed i s determined by the d i f f e r e n c e between two l a r g e numbers. Both sets of data show a v e r y sharp i n c r e a s e i n a d s o r p t i o n a t low c o n c e n t r a t i o n with a p l a t e a u r e g i o n at higher concentrat i o n s . For comparison with the adsorption models, i t i s more convenient to p l o t the data on l o g a r i t h m i c s c a l e s as shown i n Figure 4(b). From t h i s p l o t , i t can be seen that the r e s u l t s f o r the c a t i o n i c polymer f a l l e s s e n t i a l l y on a s t r a i g h t l i n e i n d i c a t ing that t h i s probably coresponds to part of the p l a t e a u r e g i o n of the isotherm. The r e s u l t s f o r the nonionic polymer appear to i n d i c a t e decreased a d s o r p t i o n at low c o n c e n t r a t i o n which may correspond to the t r a n s i t i o n between the l i n e a r and p l a t e a u regions. On the other hand, the apparent drop-off i n a d s o r p t i o n may simply r e f l e c t the u n c e r t a i n t y i n the measurements at low concentration. The s e r i o u s problems which can be encountered i n f i t t i n g the model to experimental data can be seen i n Figure 5. In t h i s f i g u r e , a complete t h e o r e t i c a l isotherm, using a reasonable value f o r K i s shown on the same r e l a t i v e s c a l e s as the a c t u a l data f o r the c a t i o n i c polymer. I t i s c l e a r that there i s a l a r g e number of p o s s i b l e combinations of and N which would give about the same "goodness of f i t " . I t i s obvious from the f i g u r e that, i n order to o b t a i n r e l i a b l e parameter estimates from such systems, adsorption data are r e q u i r e d over a very broad range of c o n c e n t r a t i o n s . I f p o s s i b l e , a d s o r p t i o n should be measured d i r e c t l y (e.g. by r a d i o - t r a c i n g techniques) r a t h e r than by d i f f e r ence. In t h i s way, i t should be p o s s i b l e to extend the range of measurement and to reduce the e r r o r s at low and high concentra tion. A p p l i c a t i o n of t h i s , or the equivalent s t a t i s t i c a l models, to a c t u a l polymer a d s o r p t i o n processes i s f u r t h e r complicated by very imprecise knowledge of the s o l i d surface area which i s a c t u a l l y a v a i l a b l e f o r polymer a d s o r p t i o n . Surface roughness e t c . can c e r t a i n l y be expected to have much more complex e f f e c t s than on the a d s o r p t i o n of small molecules due to r e s t r i c t i o n s on g

g


2.

HOGG


Ί 12


1

33

0~ (α)

ο

Η

ο

Ο

ε

Γ~ο

Adsorption

1 0

Η ω 2 Lu

8 .Δ

Δ

Δ

S î α. α: ο ω

4

*

Ο

N0NI0NIC

Δ

CATIONIC

5 ο& 0

100 EQUILIBRIUM

200

I

10 Log

F i g u r e 4.

Data o f Bensley

300

C O N C E N T R A T I O N (mg/l)

100

1000

C

(14) f o r the a d s o r p t i o n of commercial

f l o c c u l a n t s on a k a o l i n c l a y (BET surface area 15 m / g ) . Figure 4(a) shows the data p l o t t e d on l i n e a r s c a l e s while Figure 4(b) shows the same data p l o t t e d on l o g a r i t h m i c s c a l e s .



34


F i g u r e 5. T y p i c a l t h e o r e t i c a l i s o t h e r m s h o w i n g t h e e x p e r i m e n t a l d a t a f o r t h e c a t i o n i c f l o c c u l a n t ( s e e F i g u r e 4) p l o t t e d on t h e same r e l a t i v e s c a l e s .



2.

HOGG


35

Adsorption

m o l e c u l a r c o n f i g u r a t i o n . P a r t i c l e s i z e c a n a l s o be e x p e c t e d t o p l a y a r o l e s i n c e t h e o p p o r t u n i t y f o r polymer b r i d g i n g c a n l e a d to an i n c r e a s e i n t h e area a v a i l a b l e p e r adsorbed molecule, i . e . t o r e d u c e d a d s o r p t i o n (Γ) a t t h e same s u r f a c e c o v e r a g e (Θ) ( 1 5 ) . The e x t e n t o f p o l y m e r b r i d g i n g i n a n y s y s t e m i s p r o b a b l y i n f l u e n c e d by f a c t o r s such as s o l i d s c o n c e n t r a t i o n , p a r t i c l e s i z e and t h e f r e q u e n c y o f p a r t i c l e - p a r t i c l e c o l l i s i o n s due t o B r o w n i a n motion o r mechanical a g i t a t i o n (16). Simultaneous f l o c c u l a t i o n and a d s o r p t i o n c a n l e a d t o c h a n g e s i n t h e e f f e c t i v e s u r f a c e a r e a due t o t h e i n a b i l i t y o f p o l y m e r m o l e c u l e s t o p e n e t r a t e a g r o w i n g floe. Summary a n d C o n c l u s i o n s The e q u i l i b r i u m m o d e l f o r t h e a d s o r p t i o n o f p o l y m e r s a t s o l i d l i q u i d i n t e r f a c e s r e c e n t l y p r e s e n t e d b y Hogg a n d M i r v i l l e ( 1 ) h a s been e v a l u a t e d a t some l e n g t h . I t h a s b e e n shown t h a t , f o r p o l y m e r s c o n s i s t i n g o f a r e a s o n a b l y l a r g e number o f s e g m e n t s , t h e a d s o r p t i o n i s o t h e r m s c a n be c l o s e l y a p p r o x i m a t e d by a n e x p r e s s i o n of t h e form: * C

nK - «*e

G

e X

[

(

P - ïïn^G

) ]

(

1

A

)

S

where C i s t h e d i m e n s i o n l e s s p o l y m e r c o n c e n t r a t i o n , G i s t h e r e l a t i v e a d s o r p t i o n d e n s i t y , η i s t h e number o f segments p e r m o l e c u l e and K i s a m u l t i p l e a d s o r p t i o n c o n s t a n t ( w h i c h c h a r a c t e r i z e s t h e shape o f t h e i s o t h e r m ) . A d s o r p t i o n i s o t h e r m s o b t a i n e d f r o m t h e model have b e e n shown to agree v e r y c l o s e l y w i t h t h e p r e d i c t i o n s o f r e c e n t l y p u b l i s h e d s t a t i s t i c a l t h e o r i e s ( 9 , 1 3 ) . W h i l e t h e r e c a n b e no d o u b t t h a t t h e more s o p h i s t i c a t e d , s t a t i s t i c a l m o d e l s p r o v i d e more i n f o r m a t i o n on t h e n a t u r e o f t h e a d s o r p t i o n p r o c e s s and t h e s t r u c t u r e o f the adsorbed f i l m , because o f i t s s i m p l e form, t h e macroscopic model c a n o f f e r a p o w e r f u l t o o l f o r t h e a n a l y s i s , i n t e r p r e t a t i o n and u t i l i z a t i o n o f a d s o r p t i o n d a t a . The p r o b l e m s a s s o c i a t e d w i t h t h e a p p l i c a t i o n o f t h i s ( o r a n y o t h e r ) model have been d i s c u s s e d . Because o f t h e form o f t h e t y p i c a l isotherm, which e x h i b i t s a broad p l a t e a u r e g i o n , f i t t i n g o f e x p e r i m e n t a l r e s u l t s t o t h e model r e q u i r e s t h a t d a t a be o b t a i n e d over a very broad range o f c o n c e n t r a t i o n s . T h i s i s o f t e n v e r y d i f f i c u l t t o a c c o m p l i s h i n p r a c t i c e , e s p e c i a l l y when d i f f e r e n c e methods a r e u s e d t o d e t e r m i n e t h e amount o f p o l y m e r a d s o r b e d . E v a l u a t i o n o f a d s o r p t i o n i n r e a l systems i s f u r t h e r c o m p l i c a t e d by a l a c k o f k n o w l e d g e o f t h e a v a i l a b l e s o l i d s u r f a c e a r e a . The l a t t e r may be a f f e c t e d b y p a r t i c l e s i z e , shape and s u r f a c e topography and b y polymer b r i d g i n g between p a r t i c l e s . g


36


Acknowledgments


The research described i n t h i s paper was supported i n part by the N a t i o n a l Science Foundation under Grant No. CPE-8121731 and by the O f f i c e o f Surface Mining, U.S. Department of the I n t e r i o r , under Grant No. G5115424. The author a l s o wishes to thank Dr. C o l i n Bensley, Broken H i l l P r o p r i e t a r y Company L i m i t e d , Wallsend, NSW, A u s t r a l i a f o r supplying the experimental data given i n Figure 4 and f o r h e l p f u l d i s c u s s i o n s during the course of t h i s research. Legend of Symbols C C* g G i K-j^

E q u i l i b r i u m polymer c o n c e n t r a t i o n R e l a t i v e polymer concentration d e f i n e d by Equation 7 Amount adsorbed expressed as number o f e q u i v a l e n t (segment) monolayers R e l a t i v e polymer a d s o r p t i o n d e n s i t y d e f i n e d by Equation 8 An i n t e g e r — t h e number of bound segments on a given molecule Primary adsorption constant Adsorption constant f o r attachment o f the i segment of a molecule M u l t i p l e adsorption constant T o t a l number of segments per molecule Surface s i t e d e n s i t y ( s i t e s per u n i t area) Overall adsorption density C o n t r i b u t i o n to a d s o r p t i o n density by molecules with i segments bound to the surface F r a c t i o n of surface s i t e s which have adsorbed polymer segments t

K η N Γ

θ

φ

s

s

n

Volume f r a c t i o n of polymer i n bulk s o l u t i o n

Literature Cited 1. Hogg, R.; Mirville, R. J . "Adsorption of Macromolecules at Solid-Liquid Interfaces"; presented at 56th Colloid and Surface Science Symposium, Blacksburg, VA (1982). 2. Simha, R.; Frisch, H. L . ; Eirich, F. R. J . Phys. Chem. 1953, 57, 584. 3. Silberberg, A. J . Phys. Chem. 1962, 60, 1872. 4. Hoeve, C. A. J.; Di Marzio, E. A.; Peyser, P. J . Chem. Phys. 1965, 42, 2558. 5. Roe, R. J . J . Chem. Phys. 1965, 43, 1591. 6. Silberberg, A. J . Chem. Phys. 1968, 48, 2835. 7. Hoeve, C. A. J . J . Polym. Sci. 1970, 30, 361. 8. Roe, R. J . J . Chem. Phys. 1974, 60, 4192. 9. Scheutjens, J . M. H. M.; Fleer, G. J . J . Phys. Chem. 1979, 83, 1619. 10. Scheutjens, J . M. H. M.; Fleer, G. J . J . Phys. Chem. 1980, 84, 178.


2.

11. 12. 13. 14. 15.


16.

HOGG

Macroscopic

Model for Polymer

Adsorption

Lipatov, Y. S. "Adsorption of Polymers"; Keter Publishing House, Ltd.: Jerusalem, 1974. Tadros, Th. F . , Ed. "The Effect of Polymers on Dispersion Properties"; Academic Press, London, 1982. Fleer, C. J.; Scheutjens, J . M. H. M. Adv. Coll. Interf. Sci. 1982, 16, 341. Bensley, C., unpublished data. Scheutjens, J . M. H. M.; Fleer, G. J . Adv. Coll. Interf. Sci. 1982, 16, 360. Dirican, C. "The Structure and Growth of Aggregates in Flocculation"; M.S. Thesis, The Pennsylvania State University, University Park, PA, 1981.

RECEIVED October 7, 1983


Evaluation of a Macroscopic Model for Polymer ... - ACS Publications

Recommend Documents