Competitive Adsorption of an Anionic and a Nonionic Surfactant on

17 Competitive Adsorption of an Anionic and a Nonionic Surfactant on Polystyrene Latex 1

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B. Kronberg, M . Lindström , and P. Stenius Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden

The simultaneous adsorption of two surfactants, sodium dodecyl sulfate (SDS) and nonylphenol deca(oxyethylene glycol) monoether (NP-EO ), on polystyrene latex has been determined. The competitive adsorption of surfactant mixtures at different compositions is conveniently compared at the same total surface pressure. The basic assumption is that the pressures are equal at the onset of micellization in the mixed surfactant solutions. The results show that the surface composition differs greatly from the solution composition, viz. NP-EO adsorbs in excess both in the mixed micelles and on the latex surface. Using regular solution theory of liquid mixtures it is shown that the dominating driving force in adsorption stems from the energetically unfavorable interaction between the water and the hydrocarbon part of the surfactant, i.e. it is akin to the driving force of micellization. Moreover, it is shown that this driving force also is the dominating factor determining the surface composition of surfactant mixtures. The competitive adsorption or surface composition can therefore be calculated from the cmc's of the single surfactants. 10

10

Mixed surfactant systems are frequently used in practice. For example, latexes are often prepared in the presence of an anionic surfactant. Later, a nonionic surfactant may be added in order to enhance the colloidal stability of the system. In recent papers (1-2), we have shown how the thermodynamics of adsorption of nonionic surfactants on latex surfaces can be described in terms of a few simple parameters that may be used to predict the relative strength of adsorption of surfactants with different hydrophilic/hydrophobic balance on surfaces of different polarity. We have shown that the main driving force of adsorption of surfactants in monolayers on latex surfaces is the (cooperative) interCurrent address: Kenobel AB, Box 11536, S-100 61 Stockholm, Sweden. 1

0097-6156/ 86/ 0311 -0225$06.00/ 0 © 1986 American Chemical Society

In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


226

P H E N O M E N A IN M I X E D S U R F A C T A N T S Y S T E M S

a c t i o n between the h y d r o p h o b i c groups of the s u r f a c t a n t m o l e c u l e s on the s u r f a c e . The d i r e c t i n t e r a c t i o n between the s u r f a c e and the h y d r o p h o b i c group of the s u r f a c t a n t does p l a y a r o l e i n d e t e r m i n i n g the r e l a t i v e s t r e n g t h s of a d s o r p t i o n on d i f f e r e n t s u r f a c e s ; however, a t l e a s t i n the case of l a t e x s u r f a c e s , t h i s d i r e c t r e p l a c e m e n t of s o l v e n t / s u r f a c e i n t e r a c t i o n s by s u r f a c t a n t / s u r f a c e i n t e r a c t i o n s c o r responds to o n l y 20% or l e s s of the t o t a l f r e e energy o f a d s o r p t i o n . The remainder of the a d s o r p t i o n f r e e energy i s due to the r e p l a c e m e n t of u n f a v o r a b l e c o n t a c t s between water and the h y d r o p h o b i c group of the s u r f a c t a n t s w i t h h y d r o p h o b i c g r o u p - h y d r o p h o b i c group c o n t a c t s and water-water c o n t a c t s . These r e s u l t s i n d i c a t e t h a t i t s h o u l d be p o s s i b l e t o make rough p r e d i c t i o n s of c o m p e t i t i v e a d s o r p t i o n of d i f f e r e n t s u r f a c t a n t s on l a t e x s u r f a c e s w i t h o u t any d e t a i l e d knowledge about the p r o p e r t i e s of the s u r f a c e . The major d i f f e r e n c e i n a d s o r p t i o n s t r e n g t h s h o u l d be due to d i f f e r e n c e s i n the h y d r o p h i l i c / h y d r o p h o b i c b a l a n c e of the s u r f a c t a n t s , i . e . to d i f f e r e n c e s i n t h e i r s o l u t i o n p r o p e r t i e s . In t h i s paper we a p p l y b a s i c s o l u t i o n thermodynamics to b o t h the a d s o r p t i o n of s i n g l e s u r f a c t a n t s and the c o m p e t i t i v e a d s o r p t i o n o f two s u r f a c t a n t s on a l a t e x s u r f a c e . The s u r f a c t a n t system chosen i n t h i s model study i s sodium d o d e c y l s u l f a t e (SDS) and n o n y l p h e n o l deca ( o x y e t h y l e n e g l y c o l ) monoether ( Ν Ρ - Ε Ο 1 0 ) · These two s u r f a c t a n t s have v e r y d i f f e r e n t c m c s , i . e . the b a l a n c e between t h e i r h y d r o p h o b i c and h y d r o p h i l i c p r o p e r t i e s are very d i f f e r e n t while both are s t i l l h i g h l y s o l u b l e i n water. f

Experimental Materials. The p o l y s t y r e n e l a t e x , w i t h a mean d i a m e t e r o f 0.42 urn, was s y n t h e s i z e d by e m u l s i f i e r - f r e e e m u l s i o n p o l y m e r i z a t i o n . P o t a s s i u m p e r s u l f a t e was used as i n i t i a t o r and the s u r f a c e charge t h a t s t a b i l i z e s the l a t e x p a r t i c l e s thus o r i g i n a t e s from s u l f a t e r a d i c a l s . The s y n t h e s i s was c a r r i e d out a t the Department of Polymer Technology a t Âbo Akademi, F i n l a n d . S i n c e the l a t e x i s s l i g h t l y p o l y d i s p e r s e the s p e c i f i c s u r f a c e a r e a of the l a t e x cannot be c a l c u l a t e d w i t h s u f f i c i e n t a c c u r a c y . We w i l l t h e r e f o r e p r e s e n t the a d s o r p t i o n r e s u l t s per u n i t mass of the l a t e x . Note, t h a t we a r e i n t h i s work o n l y c o n c e r n e d w i t h the s u r f a c t a n t c o m p o s i t i o n on the s u r f a c e and not the a b s o l u t e v a l u e of the amount of adsorbed s u r f a c t a n t p e r u n i t a r e a . C o n d u c t o m e t r i e t i t r a t i o n of the s u r f a c e groups gave a s u r f a c e charge d e n s i t y of s t r o n g a c i d (-SO^"") of 0.56 C/g and of weak a c i d (-C00*") of 0.23 C/g. P r e c e d i n g the conduc tome t r i e t i t r a t i o n the l a t e x was c l e a n e d by serum r e p l a c e m e n t w i t h d o u b l y d i s t i l l e d water and 5 · 10" M HC1 as d e s c r i b e d i n r e f . ( 3 ) . The n o n i o n i c s u r f a c t a n t , n o n y l p h e n o l d e c a ( o x y e t h y l e n e g l y c o l ) monoether, NP-EOio» s u p p l i e d by B e r o l Kemi AB, Stenungsund, Sweden, was of t e c h n i c a l grade and used w i t h o u t f u r t h e r p u r i f i c a t i o n . The main i m p u r i t y i s f r e e p o l y e t h y l e n e o x i d e . A n a l y s i s of the sample gave a p o l y e t h y l e n e o x i d e c o n t e n t of * 3% ( 4 ) . Note, t h a t p o l y e t h y l e n e o x i d e a d s o r b s on p o l y s t y r e n e l a t e x e s ( 5 ) , but a monolayer i s r e a c h e d a t s o l u t i o n c o n c e n t r a t i o n s t h a t are * 10 times the c o n c e n t r a t i o n r e q u i r e d to o b t a i n a monolayer c o v e r a g e w i t h Ν Ρ - E O ^ Q · free poly e t h y l e n e o x i d e , t h e r e f o r e , i s e x p e c t e d to have n e g l i g i b l e i n f l u e n c e on the a d s o r p t i o n measurements. 4


17.

K R O N B E R G ET A L .

Competitive Adsorption on Polystyrene Latex

227


The a n i o n i c s u r f a c t a n t , sodium d o d e c y l s u l f a t e , SDS, was obtained from Merck, Darmstadt, F e d e r a l R e p u b l i c of Germany. I t has a s t a t e d p u r i t y of 99.99% and was used w i t h o u t f u r t h e r p u r i f i c a t i o n . S u r f a c e t e n s i o n measurements gave no minimum i n the s u r f a c e t e n s i o n a t the c r i t i c a l m i c e l l e c o n c e n t r a t i o n , i n d i c a t i n g t h a t the sample d i d not contain highly surface active impurities. The water was doubly d i s t i l l e d and had a c o n d u c t i v i t y l e s s than 1 yS/cm. Methods. The a d s o r p t i o n was d e t e r m i n e d by a d d i n g a s u r f a c t a n t m i x t u r e of known c o m p o s i t i o n t o the e m u l s i f i e r - f r e e l a t e x . The s o l i d / s o l u t i o n r a t i o was h e l d c o n s t a n t a t 0.17 w/w. In t h i s way a s e r i e s of a d s o r p t i o n measurements was performed w i t h i n c r e a s i n g t o t a l s u r f a c t a n t c o n c e n t r a t i o n . Note t h a t , w h i l e the r a t i o of the two s u r f a c t a n t s i n such a s e r i e s i s c o n s t a n t i n the whole system, i t i s n o t n e c e s s a r i l y c o n s t a n t on the s u r f a c e or i n the s o l u t i o n because of the p r e f e r e n t i a l a d s o r p t i o n of one of the s u r f a c t a n t s . A f t e r e q u i l i b r i u m , a t 25 i 0.1°C, f o r ~ 24 h the serum was c o l l e c t e d f o r s u r f a c t a n t a n a l y s i s by f i l t e r i n g the l a t e x through a 0.22 ym M i l l i p o r e f i l t e r . A n a l y s i s of NP-EO^o was made by U V - s p e c t r o s c o p y a t 275 nm where the p h e n y l r i n g g i v e s a s t r o n g a b s o r p t i o n . The a c c u r a c y i n the d e t e r m i n a t i o n of the a d s o r p t i o n of NP-EO-JQ i s about + 0.5 mg/g. The t o t a l s u r f a c t a n t c o n c e n t r a t i o n was d e t e r m i n e d by measuring the r e f r a c t i v e i n d e x increment on a Jena d i f f e r e n t i a l r e f r a c t o m e t e r . These measure ments g i v e the t o t a l s u r f a c t a n t a d s o r p t i o n w i t h an a c c u r a c y of about t 3 mg/g. The SDS c o n c e n t r a t i o n was o b t a i n e d from the d i f f e r e n c e ( t o t a l s u r f a c t a n t - amount of Ν Ρ - Ε Ο ^ ο ) » * hence i s d e f i n i t e l y not known t o an a c c u r a c y b e t t e r than 1 3 mg/g. The c r i t i c a l m i c e l l e c o n c e n t r a t i o n s (cmc) of the mixed s u r f a c t a n t systems were d e t e r m i n e d by measuring the s u r f a c e t e n s i o n as a f u n c t i o n of t o t a l s u r f a c t a n t c o n c e n t r a t i o n on a du Nouy r i n g b a l a n c e a t 25°C. a n c

Thermodynamic Background A d s o r p t i o n on the L a t e x S u r f a c e . The f o l l o w i n g thermodynamic t r e a t ment i s based on arguments s i m i l a r to those g i v e n by Rubingh i n h i s t r e a t m e n t of m i c e l l i z a t i o n of mixed s u r f a c t a n t s ( 6 ) . The adsorbed l a y e r i s c o n s i d e r e d to be a m i x t u r e of the two s u r f a c t a n t s i n the same way as Rubingh has t r e a t e d mixed m i c e l l e s , but an a d d i t i o n a l term i s i n c l u d e d i n the f r e e energy of the s u r f a c e phases t o a c c o u n t f o r the i n t e r f a c i a l f r e e energy. As i n R u b i n g h s treatment we do not e x p l i c i t l y take i n t o a c c o u n t the f a c t t h a t one of the s u r f a c t a n t s i s i o n i c , i . e . the e f f e c t of c o u n t e r i o n s i s i m p l i c i t l y i n c l u d e d i n the a c t i v i t y c o e f f i c i e n t o f the i o n i c s u r f a c t a n t . As w i l l be e v i d e n t from the ex p e r i m e n t a l r e s u l t s t h i s can be j u s t i f i e d i n terms o f the s t r o n g s h i e l d i n g of i o n i c i n t e r a c t i o n s i n the mixed a d s o r b e d l a y e r . T h i s has a l s o been found to be the c a s e f o r mixed m i c e l l e s ( 6 ) . Thus, the treatment i n c l u d e s some v e r y crude s i m p l i f i c a t i o n s . Our aim, however, i s to f i n d a model t h a t i s as s i m p l e as p o s s i b l e and y e t makes i t p o s s i b l e t o e s t i m a t e a d s o r p t i o n from s u r f a c t a n t m i x t u r e s w i t h i n e x p e r i m e n t a l e r r o r i n the type of systems i n v e s t i g a t e d by u s , i . e . a d s o r p t i o n a t coverages a p p r o a c h i n g monolayers on e s s e n t i a l l y h y d r o p h o b i c s u r f a c e s , the s u r f a c t a n t s a d s o r b i n g w i t h t h e i r 1


228


h y d r o c a r b o n m o i e t y d i r e c t e d towards the s u r f a c e . The a d s o r p t i o n on h y d r o p h i l i c s u r f a c e s , i n v o l v i n g i n t e r a c t i o n s between t h e p o l a r end group and the s u r f a c e , has been t r e a t e d e x t e n s i v e l y by e.g. Scamehorn et a l ( 8 ) . The c h e m i c a l p o t e n t i a l , μ^, o f component i i n the b u l k s o l u t i o n i s g i v e n by

(1)


μ. = μ? + kT l n a. 1 1 1

w i t h i = 1, 2 o r w f o r the two s u r f a c t a n t s and w a t e r , r e s p e c t i v e l y . μ9 i s t h e c h e m i c a l p o t e n t i a l i n the s t a n d a r d s t a t e and a ^ i s t h e activity. The c h e m i c a l p o t e n t i a l o f component i i n the s u r f a c e phase i n e q u i l i b r i u m w i t h t h e b u l k s o l u t i o n i s g i v e n by μ. = μ ? ι *ι

δ

+ kT l n a ! - Α.γ ι ι'

(2)

where A^ i s the a r e a p e r m o l e c u l e i n the s u r f a c e phase and γ i s the surface, or i n t e r f a c i a l , tension. Adsorption of a Single Surfactant. We denote a q u a n t i t y v a l i d f o r s u r f a c t a n t i a t i t s c r i t i c a l m i c e l l e c o n c e n t r a t i o n (cmc) i n a s o l u t i o n o f o n l y s u r f a c t a n t i i n water by the s u p e r s c r i p t c ( i ) . Thus, the c h e m i c a l p o t e n t i a l o f the s u r f a c t a n t a t the cmc i n the e q u i l i b r i u m s o l u t i o n o r i n the s u r f a c e phase a t the o n s e t o f m i c e l l i z a t i o n i n t h e s o l u t i o n i s g i v e n by μ? 1

( ί )

y*

( i )

= μ? 1 = μ?

c

kT I n a

+

(

i

)

(3)

1 8

kT In a * >

+

c ( i )

- A.

c ( i ) Y

(4)

c(i) where γ denotes t h e s u r f a c e , o r i n t e r f a c i a l , t e n s i o n a t t h e cmc of s u r f a c t a n t i i n the s o l u t i o n . C h o o s i n g the pure component s t a n d a r d s t a t e , we o b t a i n the f o l l o w i n g e x p r e s s i o n f o r t h e i n t e r f a c i a l t e n s i o n by c o m b i n a t i o n o f Equations 1-4. Α^γ

A.

kï

and

c ( i )

a

Y

+

Ττ—

l n

W

=

ΈΓ

W

ΤΖτ-

a ^ +

1 l n

l n

c

(

i

)

-ΎΪΪΓ a.

( 5 )

ι

o f water on t h e l a t e x s u r f a c e , E q u a t i o n s 1

Α γ°

W

S

a '

Ϊ 7ι "

F o r the a d s o r p t i o n 2 give Α γ

S

s

/C\

W

Γ"

( 6 )

W where A

0

OS K

and

Ο

Α γ = μ - μ w'w w w γ° i s the i n t e r f a c i a l

/ - , Ν

(7)

K

t e n s i o n between t h e pure water and the


17.

Competitive Adsorption on Polystyrene Latex

K R O N B E R G ET A L .

229

s o l i d . F i n a l l y , assuming = A - A and e l i m i n a t i n g γ between Equa t i o n s 5 and 6 we f i n d t h e f o l l o w i n g e x p r e s s i o n f o r t h e s u r f a c e compo sition: w

Sj

s,c(i)

ln

4W

+

a

c

e < - ^

(

i

)

>

a. ι

w

where we have p u t a = 1 f o r t h e s e d i l u t e s u r f a c t a n t w


r

solutions.

A d s o r p t i o n o f Two S u r f a c t a n t s . We now denote a q u a n t i t y v a l i d a t t h e o n s e t o f m i c e l l i z a t i o n i n t h e e q u i l i b r i u m mixed s u r f a c t a n t s o l u t i o n by t h e s u p e r s c r i p t c . Thus, the c h e m i c a l p o t e n t i a l o f s u r f a c t a n t i i n the mixed s o l u t i o n o r i n t h e mixed s u r f a c e phase a t t h e o n s e t o f m i c e l l i z a t i o n i s g i v e n by

μ?

= μ? + kT I n a?

= y? 1

S

(9)

c

+ kT I n a ? ' - Α.γ°

1

1

(10)

l '

c

. . .

where γ denotes t h e i n t e r f a c i a l t e n s i o n a t t h e cmc o f t h e e q u i l i b r i u m solution. We now assume t h a t t h e a r e a p e r m o l e c u l e i n t h e mixed l a y e r a t the cmc and i n t h e l a y e r s o f s i n g l e s u r f a c t a n t s a t t h e i r cmc i s e q u a l , i . e . A-| = &2 ^ · C o m b i n a t i o n o f E q u a t i o n s 3 and 4 w i t h E q u a t i o n s 9 and 10 then g i v e s t h e f o l l o w i n g e x p r e s s i o n f o r t h e i n t e r f a c i a l t e n sion: s,c > A c A c(i) i i /Λ4\ =

s

a

c

(

i

)

a

ΐ η _ - 1 η -

(11)

Ί

w

y

=

e

y

+

ΐ

Γ

Ϊ

Γ

a. a. ι ι 11 f o r b o t h s u r f a c t a n t s

Using Equation obtain s,c -. 1 A , c(2) c(1)v - T ^ kT ^ " Ύ a n

=

(

2

}

+

l

s

c

1

c, c(1) > < ) 1 . _1 , c(2) "sTc(2T a /a a a

a

l

and e l i m i n a t i n g γ we

a

ι n

1

/ a

+

C

/

2

l

2

A n

0

2

1 0

*

)

2

It c a n be shown t h a t s i n c e t h e aqueous s o l u t i o n i s v e r y d i l u t e t h e r a t i o between t h e s o l u t i o n a c t i v i t i e s ( a ^ / a . ^ ' ' ) o f t h e s u r f a c t a n t s can be r e p l a c e d by t h e c o r r e s p o n d i n g c o n c e n t r a t i o n r a t i o c ' t / c . ^ ' . In t h e s u r f a c e phase we i n t r o d u c e t h e a c t i v i t y c o e f f i c i e n t s or* t h e surfactants, i.e. 1

0

a*

- x * f S ; a* = x ^ f *

1

(13)

By a n a l o g y w i t h t h e t r e a t m e n t o f mixed m i c e l l e s , we now assume t h a t the f r e e energy o f m i x i n g o f t h e s u r f a c e phase c a n be c a l c u l a t e d using the standard r e g u l a r s o l u t i o n expression f o r the a c t i v i t y c o e f f i c i e n t s i n a binary mixture:



230

2

l n f» = (1 - x ^ ) X

l n i\

= (x^) X 2

(15)

1 2

where i s the i n t e r a c t i o n parameter f o r t h e i n t e r a c t i o n between the two s u r f a c t a n t s . By u s i n g t h e s e e q u a t i o n s we i n t r o d u c e the assumptions t h a t t h e r e i s no water i n the adsorbed l a y e r a t t h e cmc, i . e . a j » ^ ^ 1, t h a t the molar volumes o f the two s u r f a c t a n t s a r e e q u a l and t h a t t h e e f f e c t s o f the c o u n t e r i o n s o f the i o n i c s u r f a c t a n t a r e i n c l u d e d i n the i n t e r a c t i o n p a r a m e t e r . These assumptions a r e v e r y c r u d e , b u t we n o t e t h a t they have been used w i t h some s u c c e s s i n the d e s c r i p t i o n o f the f o r m a t i o n o f mixed m i c e l l e s ( 6 ) . 0


(14)

1 2

1

The c o m p o s i t i o n o f the s u r f a c t a n t m i x t u r e i n the e q u i l i b r i u m s o l u t i o n i s d e s c r i b e d by the q u a n t i t y

1

C

+

1

(16) C

2

i . e . , α i s the f r a c t i o n o f t h e t o t a l s u r f a c t a n t c o n c e n t r a t i o n i n the e q u i l i b r i u m s o l u t i o n t h a t i s due t o s u r f a c t a n t 1. Combination o f E q u a t i o n s 12-16 g i v e s

c(1) c(2) where c^ and c ^ a r e the c m c s o f two s i n g l e s u r f a c t a n t s , respectively. F i n a l l y , assuming γ '= γ and i d e a l m i x i n g o f the two s u r f a c t a n t s i n the s u r f a c e phase, i . e . 0» E q u a t i o n 17 r e d u c e s t o the f o l l o w i n g e x p r e s s i o n f o r t h e c o m p o s i t i o n i n the s u r f a c e : 1

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Competitive Adsorption of an Anionic and a Nonionic Surfactant on

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