The Effect of Hydrocarbon Chain Length on the Adsorption of

13 The Effect of Hydrocarbon Chain Length on the Adsorption of Sulfonates at the Solid/Water Interface T.

WAKAMATSU

and D . W .

FUERSTENAU

Downloaded by FUDAN UNIV on March 23, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch013

College of Engineering, University of California, Berkeley, Calif.

Isotherms

for the adsorption

of sodium carbon ture,

alkylsulfonates

atoms were and

measured

ionic

distinct

regions.

the isotherms

to association

1 where ions

of adsorbed

detergent

electrokinetic

potential dependence

repulsion

were

shown

to

occurs

control

at the

by ionic

characterized adsorption

Region

2

3, the

In Region because

by owing

is reversed and the isotherms on concentration

that

consists of three

ions denoting

chain length.

16

tempera-

mobilities

adsorption used

are approximately

on the hydrocarbon

pH,

It is clearly

The onset of the increased

a decreased trostatic

at constant

for such detergents

chloride

interface

8, 10, 12, 14, and

Electrophoretic

In Region with

the same line. depends

strength. isotherm

exchange

strength,

determined

for the same conditions.

the adsorption ion

at the alumina-water containing

exhibit of

elec-

surface.

' T p h e a d s o r p t i o n of i o n i c surfactants at s o l i d - w a t e r interfaces is of great A

t e c h n o l o g i c a l i m p o r t a n c e i n s u c h diverse fields as w a t e r r e n o v a t i o n ,

detergency, m i n e r a l

flotation,

a n d corrosion inhibition.

The

complex

n a t u r e of the a d s o r p t i o n of surfactants at t h e s o l i d - w a t e r i n t e r f a c e is c o n t r o l l e d b y the n a t u r e of the a d s o r b i n g species itself, the p r o p e r t i e s of the s o l i d adsorbent, a n d the c o m p o s i t i o n of the aqueous s o l u t i o n . D e p e n d e n t o n these factors, i o n i c s u r f a c t a n t - s o l i d systems c a n b e classified i n t o three b r o a d types, n a m e l y ( 1 ) those i n w h i c h the surfactant adsorbs as counterions i n the d o u b l e l a y e r t h r o u g h c o u l o m b i c i n t e r a c t i o n w i t h the c h a r g e d surface, ( 2 ) those i n w h i c h the surfactant adsorbs b y c o v a l e n t b o n d f o r m a t i o n w i t h the s o l i d surface, a n d ( 3 ) those i n w h i c h t h e sur factant adsorbs t h r o u g h h y d r o p h o b i c b o n d i n g of the h y d r o c a r b o n c h a i n 161

Weber and Matijevi; Adsorption From Aqueous Solution Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

162

ADSORPTION F R O M

AQUEOUS SOLUTION

to a n o n p o l a r s o l i d s u c h as g r a p h i t e . I n a l l s u c h a d s o r p t i o n processes, the h y d r o c a r b o n c h a i n of the surfactant p l a y s a d o m i n a n t r o l e i n the b e h a v i o r of the system.

T h e i m p o r t a n c e of the h y d r o c a r b o n c h a i n i n

aqueous i n t e r f a c i a l p h e n o m e n a has b e e n d e m o n s t r a t e d b y a v a r i e t y of e x p e r i m e n t a l methods, for e x a m p l e , b y e l e c t r o k i n e t i c studies (11), tion

behavior

behavior The

(2),

adsorption

measurements

(12),

and

flota

coagulation

(8). a d s o r p t i o n of

i o n i c surfactants b y

nonpolar

solids

such

as

g r a p h i t e w o u l d b e e x p e c t e d to d e p e n d m a r k e d l y o n the l e n g t h a n d c o n figuration

of the h y d r o c a r b o n c h a i n since it is t h r o u g h the h y d r o c a r b o n

c h a i n that the surfactant b o n d s to the s o l i d . T h e i n v e s t i g a t i o n of S k e w i s Downloaded by FUDAN UNIV on March 23, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch013

and Zettlemoyer (9)

is q u i t e t y p i c a l of the k i n d s of effects o b t a i n e d i n

s u c h a d s o r p t i o n systems.

O n e of the f e w investigations r e p o r t e d i n the

l i t e r a t u r e o n the a d s o r p t i o n of i o n i c surfactants of v a r y i n g c h a i n l e n g t h o n c h a r g e d solids i n aqueous m e d i a is that of J a y c o c k , O t t e w i l l , a n d Rastogi

(5).

U s i n g c o l l o i d a l silver i o d i d e

a n d aqueous solutions

of

p y r i d i n i u m b r o m i d e s of v a r i o u s c h a i n lengths, t h e y f o u n d that at l o w coverages, the a d s o r p t i o n density w a s a p p r o x i m a t e l y t h e same for e a c h of the surfactants b u t that the a d s o r p t i o n i n c r e a s e d c a t a s t r o p h i c a l l y at some c r i t i c a l c o n c e n t r a t i o n d e p e n d e n t o n c h a i n l e n g t h . T h e use of s i l v e r i o d i d e as the adsorbent for s u c h studies c o m p l i c a t e s the s i t u a t i o n c o n s i d e r a b l y because, as is n o w w e l l k n o w n , silver i o d i d e is p a r t i a l l y h y d r o p h o b i c (4,13).

T h u s , the a d s o r p t i o n i n this system m u s t b e some c o m b i

n a t i o n of c o u l o m b i c i n t e r a c t i o n a n d h y d r o p h o b i c b o n d i n g w i t h the s u r face. E s s e n t i a l l y the o n l y other i n v e s t i g a t i o n of the a d s o r p t i o n of i o n i c surfactants of v a r i o u s c h a i n lengths at m i n e r a l - w a t e r interfaces is t h a t of T a m a m u s h i a n d T a m a k i (12),

w h o d e t e r m i n e d the a d s o r p t i o n of a l k y l -

a m m o n i u m c h l o r i d e s o n a l u m i n a . T h e y a t e m p t e d to e x p l a i n t h e i r iso therms i n terms of a B r u n a u e r - E m m e t t - T e l l e r ( B . E . T . ) t y p e of e q u a t i o n , b u t the v a l i d i t y of t h e i r a p p r o a c h is q u e s t i o n a b l e because of t h e i r neglect of e l e c t r i c a l effects. R e c e n t l y (10),

it w a s d e m o n s t r a t e d that the a d s o r p t i o n of

alkyl-

sulfonates at the a l u m i n a - w a t e r interface is a system i n w h i c h the sur factant adsorbs as counterions i n t h e e l e c t r i c a l d o u b l e layer. T h i s w o r k s h o w e d that the i s o t h e r m for the a d s o r p t i o n of s o d i u m d o d e c y l sulfonate at the a l u m i n a - w a t e r interface is c h a r a c t e r i z e d b y three d i s t i n c t r e g i o n s : Region

1 i n w h i c h the

detergent

ions

adsorb

individually through

c o u l o m b i c a t t r a c t i o n for the surface; R e g i o n 2 i n w h i c h the a d s o r p t i o n is e n h a n c e d t h r o u g h association of the h y d r o c a r b o n chains of t h e a d s o r b e d surfactant ions; a n d R e g i o n 3 i n w h i c h the charge i n the S t e r n p l a n e exceeds the surface

charge

w i t h the r e s u l t i n g electrostatic

repulsion

a c t i n g to r e t a r d a d s o r p t i o n . I n the e x p e r i m e n t a l i n v e s t i g a t i o n discussed i n the present p a p e r , details of the r o l e of the h y d r o c a r b o n c h a i n i n the


13.

WAKAMATSU

AND FUERSTENAU

Adsorption

163

of Sulfonates

a d s o r p t i o n process at t h e a l u m i n a - w a t e r interface h a v e b e e n s t u d i e d f o r a series of a l k y l sulfonates w i t h v a r y i n g c h a i n lengths. I n this w o r k , p H , i o n i c strength, a n d t e m p e r a t u r e w e r e m a i n t a i n e d constant. I n p a r t i c u l a r , o u r p u r p o s e has b e e n to d e t e r m i n e h o w e a c h of these a d s o r p t i o n regions d e p e n d s o n t h e h y d r o c a r b o n c h a i n of t h e surfactant. Experimental M a t e r i a l s a n d M e t h o d s . F o r t h e s o l i d adsorbent, a - a l u m i n a ( L i n d e " A " ) of 9 9 . 9 5 % p u r i t y w a s u s e d . Its specific surface area, as m e a s u r e d b y k r y p t o n gas a d s o r p t i o n a n d b y stearic a c i d a d s o r p t i o n f r o m b e n z e n e was f o u n d to b e 15 m e t e r / g r a m . Its z e r o - p o i n t - o f - c h a r g e occurs at p H 9.1. D e t a i l s a b o u t t h e c h a r a c t e r i z a t i o n of this m a t e r i a l h a v e b e e n d e s c r i b e d i n a p r e v i o u s p a p e r ( 1 5 ) . T h e alkylsulfonates w e r e p r e p a r e d b y n e u t r a l i z i n g h i g h p u r i t y s u l f o n i c acids ( C 8 , C I O , C 1 2 , C 1 4 , a n d C 1 6 ) w i t h s o d i u m h y d r o x i d e a n d b y r e c r y s t a l l i z i n g t h e s o d i u m salt f r o m h o t absolute e t h y l a l c o h o l . D e t a i l e d i n f r a r e d spectroscopic analysis of these reagents s h o w e d t h e m to b e p u r e sulfonates w i t h b u t a trace a m o u n t of w a t e r as t h e o n l y i m p u r i t y . O n l y the C 1 4 reagent appears n o t to b e c o m p l e t e l y free of shorter c h a i n h o m o l o g s . A l l i n o r g a n i c c h e m i c a l s w e r e reagent grade. C o n d u c t i v i t y w a t e r p r e p a r e d i n a q u a r t z s t i l l w a s u s e d for a l l solutions.


2

T h e a d s o r p t i o n experiments w e r e c a r r i e d o u t i n a 600 m l . b e a k e r t h a t was s t o p p e r e d w i t h a T e f l o n d i s c c o n t a i n i n g holes f o r a t h e r m o m e t e r , gas i n l e t a n d outlet, a n d p H electrodes. T h e c e l l c o n t a i n i n g t h e a l u m i n a a n d t h e surfactant s o l u t i o n w a s m a i n t a i n e d at a constant t e m p e r a t u r e b y i m m e r s i n g t h e c e l l i n a t h e r m o s t a t i c a l l y c o n t r o l l e d w a t e r b a t h . A n atmos p h e r e of p u r i f i e d n i t r o g e n w a s m a i n t a i n e d o v e r t h e c e l l . T h e e x p e r i m e n t a l c o n d i t i o n s w e r e h e l d constant at p H 7.2, 2 5 ° C , a n d 2 X 1 0 " M i o n i c strength ( a d j u s t e d w i t h s o d i u m c h l o r i d e ) . T h e system w a s s t i r r e d b y means of a m a g n e t i c , T e f l o n - c o v e r e d s t i r r i n g b a r for four h o u r s . A b o u t t w o hours w a s r e q u i r e d for a d j u s t i n g p H , a n d t h e r e m a i n i n g t i m e w a s u s e d f o r a t t a i n i n g e q u i l i b r i u m a n d for s a m p l i n g . T h e m e t h o d u s e d f o r the d e t e r m i n a t i o n of sulfonate c o n c e n t r a t i o n is t h e w e l l - k n o w n m e t h y l e n e blue complex method (6, 7 ) . 3

E l e c t r o p h o r e t i c m o b i l i t i e s of t h e a l u m i n a particles w e r e d e t e r m i n e d for t h e same c o n d i t i o n s as w e r e u s e d to o b t a i n t h e a d s o r p t i o n isotherms. F o r this p u r p o s e , a s a m p l e of t h e a l u m i n a suspension w a s t r a n s f e r r e d to the electrophoresis c e l l for m e a s u r e m e n t of t h e e l e c t r o p h o r e t i c m o b i l i t i e s . A Z e t a - M e t e r w a s u s e d for this p a r t of t h e p r o g r a m . R e s u l t s . T h e i s o t h e r m for t h e a d s o r p t i o n of s o d i u m d o d e c y l sulfonate b y a l u m i n a at constant p H ( p H 7.2) a n d i o n i c s t r e n g t h ( 2 X 1 0 " M ) is g i v e n i n F i g u r e 1 to illustrate specifically t h e n a t u r e of t h e i s o t h e r m o b t a i n e d for the a d s o r p t i o n of a detergent f r o m aqueous s o l u t i o n . I n this figure, the a m o u n t of sulfonate a d s o r b e d p e r u n i t area of a l u m i n a is p l o t t e d l o g a r i t h m i c a l l y as a f u n c t i o n of 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 of a l k y l s u l f o n a t e i n s o l u t i o n . T h e a d s o r p t i o n i s o t h e r m consists of t h r e e d i s t i n c t regions: R e g i o n 1, w h i c h is c h a r a c t e r i z e d b y a l o w increase i n a d s o r p t i o n w i t h i n c r e a s i n g surfactant c o n c e n t r a t i o n ; R e g i o n 2, b y a n 3



164

ADSORPTION F R O M

AQUEOUS SOLUTION

EQUILIBRIUM CONCENTRATION OF SODIUM DODECYL SULFONATE, MOLE/LITER Figure 1. The electrophoretic behavior and isotherm for the adsorption of sodium dodecyl sulfonate from aqueous solution at pH 7.2, 25°C, and 2 X 10~ M ionic strength (NaCl). The 95% confidence limits for the three straight-line regions of the adsorption isotherm are shown 3

a b r u p t increase i n t h e slope o f the i s o t h e r m ; a n d R e g i o n 3, a g a i n b y a decreased d e p e n d e n c e o f a d s o r p t i o n o n sulfonate c o n c e n t r a t i o n . T o correlate changes i n the a d s o r p t i o n process w i t h the i s o t h e r m , the electro p h o r e t i c b e h a v i o r of a l u m i n a i n the presence o f s o d i u m d o d e c y l sulfonate is also i n c l u d e d i n F i g u r e 1. I n F i g u r e 2, isotherms f o r the a d s o r p t i o n o f C 8 t o C 1 6 sulfonates a r e p r e s e n t e d to s h o w h o w t h e c h a i n l e n g t h o f the detergent affects the a d s o r p t i o n process. T h e e l e c t r o p h o r e t i c m o b i l i t i e s o f a l u m i n a f o r the same c o n d i t i o n s as u s e d f o r d e t e r m i n a t i o n o f the isotherms are p r e s e n t e d i n F i g u r e 3. T h i s figure, together w i t h F i g u r e s 1 a n d 2, shows that the m o b i l i t y is p o s i t i v e i n s i g n i n R e g i o n s 1 a n d 2 b u t is n e g a t i v e i n R e g i o n 3. R e g i o n 1 i s characterized b y the electrophoretic m o b i l i t y being nearly independent of t h e sulfonate c o n c e n t r a t i o n . T h e t r a n s i t i o n b e t w e e n R e g i o n 1 a n d R e g i o n 2 is m a r k e d b y a s h a r p c h a n g e i n the e l e c t r o p h o r e t i c m o b i l i t y - u s . c o n c e n t r a t i o n c u r v e whereas t h e t r a n s i t i o n b e t w e e n R e g i o n s 2 a n d 3 occurs a t concentrations w h e r e the m o b i l i t y is zero. C l e a r l y , the electro p h o r e t i c b e h a v i o r o f t h e a l u m i n a depends m a r k e d l y o n t h e n u m b e r o f c a r b o n atoms i n the h y d r o c a r b o n c h a i n o f the detergent. T h e a d s o r p t i o n d e n s i t y m a r k e d as a m o n l a y e r i n F i g u r e 2 is t h a t f o r a c l o s e l y p a c k e d l a y e r of v e r t i c a l l y o r i e n t e d a l k y l s u l f o n a t e ions.


13.

WAKAMATSU

AND FUERSTENAU

Adsorption

of

Sulfonates

165

Discussion of Results The Adsorption Isotherm.

I n a p r e v i o u s p a p e r (10),

it was shown

that the i s o t h e r m for the a d s o r p t i o n of a detergent at the s o l i d - w a t e r interface c a n b e c h a r a c t e r i z e d b y three d i s t i n c t regions b u t at that t i m e the d a t a w e r e not a n a l y z e d statistically.

S t a t i s t i c a l analysis of the d a t a

p r e s e n t e d i n F i g u r e 1 shows that t h e i s o t h e r m c o r r e s p o n d i n g to e a c h of the three regions c a n b e c h a r a c t e r i z e d b y the f o l l o w i n g straight l i n e s : Region 1: log r = -8.76 + 0.66 log C Region 2: log r = -3.30 + 1.92 log C Downloaded by FUDAN UNIV on March 23, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch013

Region 3: log r = -8.03 + 0.60 log C w h e r e r is the a m o u n t of sulfonate a d s o r b e d i n m o l e p e r c m . the m o l a r c o n c e n t r a t i o n of sulfonate i n the b u l k s o l u t i o n .

2

a n d C is Statistical

analysis of these d a t a shows that the slope of these three straight lines at the 95%

confidence l e v e l (14)

is 0.66 ±

0.19, 1.92

±

0.25, a n d 0.60

± 0.22 for R e g i o n s 1, 2, a n d 3, respectively. R e g i o n 1 for s o d i u m d o d e c y l sulfonate u n d e r these e x p e r i m e n t a l c o n d i t i o n s applies u p to a detergent c o n c e n t r a t i o n of 6 X 1 0 " M . T h e a d s o r p t i o n is c h a r a c t e r i z e d b y R e g i o n 2 5

between 6 X

10" M and 3 X 5

1 0 " M u n d e r these c o n d i t i o n s , a n d b y 4

R e g i o n 3 a b o v e 3 X 1 0 " M sulfonate. 4

F u r t h e r e v i d e n c e for three d i s t i n c t m o d e s of a d s o r p t i o n c a n b e seen i n the e l e c t r o p h o r e t i c b e h a v i o r of a l u m i n a i n the presence of d o d e c y l sulfonate.

Below 6 X

sodium

10~ M, the e l e c t r o p h o r e t i c m o b i l i t y is 5

n e a r l y i n d e p e n d e n t of c o n c e n t r a t i o n , b u t at this c o n c e n t r a t i o n the slope of the m o b i h t y - t ; s . - c o n c e n t r a t i o n c u r v e a b r u p t l y changes. A t 3 X

10" M 4

d o d e c y l sulfonate c o n c e n t r a t i o n , the e l e c t r o p h o r e t i c m o b i l i t y reverses its sign, i n d i c a t i n g that the charge i n the S t e r n l a y e r n o w exceeds the surface charge i n absolute m a g n i t u d e . This experimental evidence

c l e a r l y shows the c o m p l i c a t e d n a t u r e

of the a d s o r p t i o n i s o t h e r m for a detergent at the p o l a r

solid-aqueous

s o l u t i o n interface. The Effect of A l k y l Chain Length on Adsorption. F i g u r e 2 shows that the a d s o r p t i o n isotherms a l l h a v e s o m e w h a t the same g e n e r a l c h a r acteristics o n l y the concentrations at w h i c h the effects o c c u r a p p e a r to d e p e n d o n the a l k y l c h a i n l e n g t h . C o n s e q u e n t l y , the a d s o r p t i o n b e h a v i o r i n e a c h of t h e three regions of the isotherms w i l l b e d i s c u s s e d separately a n d w i l l b e i n t e r p r e t e d i n terms of the role t h a t the h y d r o c a r b o n c h a i n p l a y s i n t h e a d s o r p t i o n process. It has a l r e a d y b e e n established (10) that a l k y l s u l f o n a t e ions adsorb at the p o s i t i v e l y c h a r g e d a l u m i n a - w a t e r interface as counterions i n the e l e c t r i c a l d o u b l e layer. C h e m i s o r p t i o n is absent. T h e e l e c t r i c a l d o u b l e



166

ADSORPTION F R O M

AQUEOUS SOLUTION

Figure 2. Adsorption isotherms for sodium alkylsulfonates of different hydrocarbon chain lengths on alumina at pH 7.2, 25°C, and 2 X 10~ M ionic strength 3

l a y e r m o d e l w i l l t h e n b e u s e d to i n t e r p r e t the a d s o r p t i o n b e h a v i o r o b served i n these experiments. I n R e g i o n 1, a l k y l s u l f o n a t e ions are c o n s i d e r e d to adsorb as i n d i v i d u a l counterions i n c o m p e t i t i o n w i t h t h e c h l o r i d e ions u s e d to c o n t r o l the i o n i c strength. I f the a d s o r p t i o n is of non-associated sulfonate ions b y i d e a l exchange i n t h e diffuse layer, a single l i n e i n d e p e n d e n t of c h a i n l e n g t h s h o u l d c h a r a c t e r i z e t h e a d s o r p t i o n i n R e g i o n 1. F o r t h e C 1 6 s u l fonate, at t h e lowest c o n c e n t r a t i o n w h i c h c o u l d b e s t u d i e d , t h e a d s o r p t i o n a l r e a d y has exceeded that of R e g i o n 1. O n the other h a n d , F i g u r e s 2 a n d 3 s h o w that t h e a d s o r p t i o n of C 8 sulfonate is r e s t r i c t e d e n t i r e l y t o R e g i o n 1. F o r C 8 , C I O , C 1 2 , a n d C 1 4 this a d s o r p t i o n is c h a r a c t e r i z e d a p p r o x i -


13.

WAKAMATSU

A N D FUERSTENAU

Adsorption

of

167

Sulfonates

m a t e l y b y a single l i n e , w h o s e slope is about 0.7 i n s t e a d of t h e expected slope of 1.0. S u c h deviations f r o m i d e a l exchange

are not yet clearly

u n d e r s t o o d b u t are c o n s i d e r e d to reflect differences i n t h e a b i l i t y o f sur factant ions c o m p a r e d w i t h c h l o r i d e ions to penetrate the diffuse l a y e r region. T h i s is a subject of c o n t i n u e d research. R e g i o n 2 is c h a r a c t e r i z e d b y a m a r k e d change i n t h e slope of t h e a d s o r p t i o n isotherms.

T h i s results f r o m t h e onset of association of t h e

h y d r o c a r b o n chains o f the surfactant ions a d s o r b e d i n t h e S t e r n p l a n e . T h e m e a n separation distance of a d s o r b e d ions u n d e r these c o n d i t i o n s is about 70 A . , w h i c h a p p r o x i m a t e s the m e a n separation distance i n b u l k


at the c.m.c. I n s u c h a d s o r p t i o n p h e n o m e n a , there is a r e l a t i o n s h i p b e t w e e n this asociation a n d the f o r m a t i o n of m i c e l l e s i n b u l k solution. F o r example, e l e c t r o k i n e t i c studies ( 1 ) o n q u a r t z at n e u t r a l p H s h o w e d that a l k y l a m m o n i u m ions associate i n t h e S t e r n p l a n e w h e n t h e i r b u l k c o n c e n t r a t i o n is a p p r o x i m a t e l y one h u n d r e d t h of the c.m.c. T h i s association w h i c h has b e e n c a l l e d h e m i m i c e l l e f o r m a t i o n ( 3 ) , gives rise to a specific a d s o r p t i o n p o t e n t i a l w h i c h causes t h e a d s o r p t i o n to increase m a r k e d l y a n d b r i n g s about a reversal i n the s i g n of the p o t e n t i a l at the S t e r n p l a n e . T h e h e m i m i c e l l e c o n c e n t r a t i o n , that is the b u l k c o n c e n t r a t i o n

necessary

1 A L U M I NA IONIC STRENGTH 2xlO" N pH 7.2 3

UJ Q_ O

8

-2

>-

t

0

_J

GO O

2 o \-

LLl DC O X Q_ O

cr \o UJ

+2 +4

io"

6

I

icr

_L

5

1

io~

4

CONCENTRATION OF SODIUM A L K Y L

io SULFONATE,

-3

MOLE/LITER

Figure 3. The electrophoretic mobility of alumina at pH 7.2 and 2 X 10~ M ionic strength as a function of the concentration of sulfonates with various hydrocarbon chain lengths 3


168

ADSORPTION F R O M

AQUEOUS SOLUTION

to i n d u c e association i n the interface, increases w i t h d e c r e a s i n g c h a i n l e n g t h . A t p H 7.2 o n a l u m i n a this c o n c e n t r a t i o n is 4 X 1.4 X

1 0 " M for C 1 4 , 6 X

1 0 " M for C 1 2 , 7 X

5

1 0 " M for C 1 6 , 6

1 0 " M for C I O , a n d is

5

4

a b o v e 2 X 1 0 " M for C 8 sulfonate. T h e slopes of the isotherms are 0.56 3

for C 8 , 1 . 3 3 for C I O , 1.92 for C 1 2 , 2.50 for C 1 4 , a n d 5.80 for C 1 6 sulfonate. T h u s , w i t h i n c r e a s i n g c h a i n l e n g t h , the c o m p e t i t i o n b e t w e e n R S 0 " a n d 3

C I " for sites at the surface strongly favors the a d s o r p t i o n of the detergent. I n a p r e v i o u s p u b l i c a t i o n (10) Region 2 where

i t w a s s h o w n that a d s o r p t i o n i n

association of the h y d r o c a r b o n

chains is l e a d i n g to

extensive a d s o r p t i o n a n d e v e n t u a l r e v e r s a l of the s i g n of the m o b i l i t y —


i.e.,

— i s given by (i)

w h e r e Ts is t h e a d s o r p t i o n density i n the S t e r n p l a n e i n m o l e s / c m . , r is 2

the effective r a d i u s of the a d s o r b e d i o n , C is the b u l k c o n c e n t r a t i o n i n m o l e s / c c , z is the v a l e n c e of the a d s o r b e d i o n , F is F a r a d a y ' s constant, \f/6 is the p o t e n t i a l i n the S t e r n p l a n e , is the cohesive energy p e r m o l e of C H

2

groups, n is the effective n u m b e r of associating C H

2

groups p e r

h y d r o c a r b o n c h a i n , R is the gas constant, a n d T the absolute t e m p e r a t u r e . T h u s , expressing a d s o r p t i o n o n a l o g a r i t h m i c basis, w e h a v e dlnr dlnC

_

zF RT

di// d\nC 5

_

d n RT d l n C

(2)

T h e v a r i a t i o n of n w i t h c o n c e n t r a t i o n expresses the fact that i n R e g i o n 2 c o m p l e t e r e m o v a l of e a c h C H g r o u p of the surfactant f r o m w a t e r is o n l y 2

possible at a m o n o l a y e r .

I n b u l k systems the analogous processes are the

pre-association i n t o d i m e r s , t r i m e r s , etc. just b e l o w the c r i t i c a l m i c e l l e concentration. F r o m the slopes of the a d s o r p t i o n isotherms a n d the m o b i l i t y - c o n c e n t r a t i o n curves, it is possible to evaluate w i t h E q u a t i o n 2, d n / d l n C — i.e., the t e r m w h i c h expresses the effective n u m b e r of C H m o v e d f r o m the aqueous e n v i r o n m e n t .

2

groups r e

B y this means, the values of

d n / d l n C are f o u n d to b e 9 for C 1 6 , 4 for C 1 4 , 2 for C 1 2 , 2 for C I O , a n d 0 f o r C 8 . A c o m p a r i s o n c a n b e m a d e for the C 1 6 sulfonate b y c o n s i d e r i n g the m o b i l i t y curves a n d a d s o r p t i o n isotherms. O b s e r v a t i o n of the electro p h o r e t i c m o b i l i t y c u r v e for the C 1 6 surfactant shows that h e m i m i c e l l e f o r m a t i o n begins at 6 X 1 0 " M a n d that the m o b i l i t y c u r v e a g a i n b e c o m e s 6

i n d e p e n d e n t of c o n c e n t r a t i o n at 1.5 X

1 0 " M , this latter c o n c e n t r a t i o n 5

c o i n c i d i n g w i t h m o n o l a y e r coverage i n the e x p e c t e d m a n n e r .

Thus, n

has a n effective v a l u e of zero at a b o u t 6 X 1 0 ~ M a n d a v a l u e of 15 at 6

m o n o l a y e r coverage ( a s s u m i n g that the t e r m i n a l C H

3

groups are exposed

to the s o l u t i o n ) . T h i s leads to a n e s t i m a t e d v a l u e of d n / d l n C of a p p r o x i -


13.

WAKAMATSU

A N D FUERSTENAU

Adsorption

of

169

Sulfonates

m a t e l y 16. A s p o i n t e d o u t p r e v i o u s l y ( 1 0 ) , a l i m i t a t i o n of this treatment is t h a t t h r o u g h o u t R e g i o n 2, t h e f r a c t i o n of t h e sulfonate ions t h a t adsorb i n the S t e r n l a y e r increases, r e a c h i n g u n i t y at t h e e n d of R e g i o n 2. S i n c e E q u a t i o n 2 d e p e n d s o n t h e a s s u m p t i o n that t h e a d s o r p t i o n i n t h e S t e r n layer is p r o p o r t i o n a l to t h e t o t a l a d s o r p t i o n , t h e v a l u e of d n / d l n C c a l c u l a t e d f r o m t h e a d s o r p t i o n w i l l b e too l o w . T h e e n d of R e g i o n 2 is c h a r a c t e r i z e d b y t h e fact that t h e m o b i l i t y passes t h r o u g h zero.

W h e n t h e m o b i l i t y is zero, t h e S t e r n - G r a h a m e

expression for t h e a d s o r p t i o n of a specifically a d s o r b i n g surfactant i o n c a n b e expressed as (r )„ = 2 r C e x p ( Downloaded by FUDAN UNIV on March 23, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch013

8

(3)

*£)

0

w h e r e N is t h e n u m b e r of c a r b o n atoms i n t h e h y d r o c a r b o n c h a i n . H e r e w e m u s t assume t h a t t h e effective f r a c t i o n of t h e C H groups

removed

2

f r o m a q u e o u s e n v i r o n m e n t w h e n fa is zero m u s t b e i n d e p e n d e n t of c h a i n length.

P u t t i n g E q u a t i o n 3 i n l o g a r i t h m i c f o r m a n d r e a r r a n g i n g terms

yields: lnC -]n&k

(4)

= - N ±

0

T h e n u m e r i c a l values of (1^)0 a n d C

0

are t a b u l a t e d i n T a b l e I.

Table I.

C16 C14 C12 CIO

1.6 1.1 7.9 6.0

X X X X

10" mole/cm. 10" 10" 10" 10

2

10

11

11

1.0 9.0 2.8 1.7

X X X X

10" m o l e / l i t e r IO" IO" IO" 5

5

4

3

F r o m these n u m b e r s , i t c a n b e seen that t h e c h a n g e of (Ts)

0

when

i n c r e a s i n g t h e n u m b e r of C H groups i n t h e a l k y l c h a i n l e n g t h f r o m C I O 2

to C 1 6 , is 6.0 X 10 (C ) 6

0

1 1

to 1.6 X 1 0 " , w h i l e t h e v a r i a t i o n s i n t h e values o f 10

is 10" to 1.7 X 10" . W e d o not k n o w the effective r a d i u s of e a c h r>

3

sulfonate i o n at z e r o m o b i l i t y , b u t i t c e r t a i n l y m u s t c o r r e s p o n d a p p r o x i m a t e l y to t h e thickness of t h e a d s o r b e d

layer.

Perhaps it m a y even

increase w i t h t h e n u m b e r of c a r b o n atoms i n t h e h y d r o c a r b o n c h a i n . H o w e v e r , as t h e v a r i a t i o n i n t h e v a l u e of (Cs)

is v e r y large c o m p a r e d

0

w i t h that of ( r « ) , i t w i l l b e a s s u m e d that I n ( r ) / 2 r is a constant i n d e 0

6

0

p e n d e n t of t h e c h a i n l e n g t h . T h u s , i f w e p l o t t h e l o g a r i t h m of

(Cs)

0

against N, a straight fine s h o u l d b e o b t a i n e d . A c c o r d i n g l y , i n F i g u r e 4, the c o n c e n t r a t i o n of s o d i u m a l k y l s u l f o n a t e i n s o l u t i o n c o r r e s p o n d i n g to zero m o b i l i t y is p l o t t e d as a f u n c t i o n of t h e n u m b e r of c a r b o n atoms i n the a l k y l c h a i n . F r o m t h e slope of this l i n e , t h e v a l u e of is c a l c u l a t e d


170

ADSORPTION F R O M

to b e —0.95 RT,

a v a l u e i n g o o d agreement w i t h p r e v i o u s studies of

a m i n e salts o n q u a r t z


AQUEOUS SOLUTION

(2,11).

Figure 4. Variation of the concentration of sulfonate necessary for zero electrophoretic mobility of alumina as a function of the number of carbon atoms in the alkyl chain I n R e g i o n 3 the solpe of the i s o t h e r m is l o w e r t h a n i n R e g i o n 2. H e r e the surfactant ions p r o b a b l y adsorb b y a s o m e w h a t different m e c h a n i s m . A s s h o w n i n F i g u r e 2, the values of fa must b e negative i n this r e g i o n , a n d c o n s e q u e n t l y the a d s o r b e d sulfonate ions s h o u l d be subjected to electrostatic r e p u l s i o n i n the a d s o r p t i o n process. T h u s , r e d u c i n g the slope of the isotherms i n this r e g i o n . I n R e g i o n 3 the slope of the C 1 6 i s o t h e r m is 2.16, that of the C 1 4 is 1.50 a n d that of the C 1 2 i s o t h e r m is 0.60. A l t h o u g h o n l y three values for this slope c o u l d be o b t a i n e d f r o m o u r studies, it is a p p a r e n t that the v a l u e becomes greater w i t h i n c r e a s i n g h y d r o c a r b o n c h a i n l e n g t h . T h i s t e n d e n c y a g a i n c a n be a t t r i b u t e d to the i n c r e a s e d a t t r a c t i o n b e t w e e n h y d r o c a r b o n chains w i t h increase i n c h a i n l e n g t h . F u r t h e r , the a d s o r b e d detergent m a y t e n d to orient differently at the surface because of the electrostatic r e p u l s i o n u p o n r e v e r s a l of i/^.


13.

WAKAMATSU

AND FUERSTENAU

Adsorption

of

Sulfonates

171

Summary A s a p a r t of a s t u d y o n the m e c h a n i s m of a d s o r p t i o n of a l k y l s u l fonates at the a l u m i n a - w a t e r interface, the role of h y d r o c a r b o n c h a i n l e n g t h i n the a d s o r p t i o n process has b e e n i n v e s t i g a t e d b y a d s o r p t i o n a n d electrophoresis

measurements

with

sodium

alkylsulfonates c o n t a i n i n g

8, 10, 12, 14, a n d 16 c a r b o n atoms at constant p H , t e m p e r a t u r e , a n d i o n i c strength.

T h e a d s o r p t i o n isotherms h a v e c l e a r l y b e e n s h o w n to consist

of three d i s t i n c t regions, d e p e n d i n g u p o n the i n t e r m o l e c u l a r b e h a v i o r of the h y d r o c a r b o n c h a i n . I n R e g i o n 1 w h e r e the detergent ions adsorb i n the d o u b l e l a y e r i n c o m p e t i t i o n w i t h the c h l o r i d e ions u s e d to c o n t r o l Downloaded by FUDAN UNIV on March 23, 2017 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0079.ch013

i o n i c strength, the isotherms are c h a r a c t e r i z e d b y same straight l i n e .

a p p r o x i m a t e l y the

I n R e g i o n 2 the a d s o r b e d detergent ions associate,

r e s u l t i n g i n a sharp increase i n the a d s o r p t i o n density. T h e onset of this association occurs at l o w e r b u l k concentrations as the h y d r o c a r b o n c h a i n l e n g t h is increased. I n R e g i o n 3 the a d s o r p t i o n isotherms a g a i n h a v e a decreased slope; i n this r e g i o n the e l e c t r o k i n e t i c p o t e n t i a l is reversed, r e s u l t i n g i n electrostatic r e p u l s i o n b e t w e e n the a d s o r b e d ions. B y means of the S t e r n - G r a h a m e m o d e l of the d o u b l e l a y e r u n d e r c o n d i t i o n s w h e r e the e l e c t r o p h o r e t i c m o b i l i t y of the a l u m i n a is zero, the cohesive energy p e r m o l e of C H

2

free

groups has b e e n c a l c u l a t e d to b e —0.95 R T .

Acknowledgments T h e authors w i s h to a c k n o w l e d g e the N a t i o n a l Institute of H e a l t h (Grant No. WP-00692)

for s u p p o r t of this research.

Discussions w i t h

T . W . H e a l y are also a c k n o w l e d g e d .

Literature

Cited

(1) (2)

Fuerstenau, D. W . , J. Phys. Chem. 6 0 , 981 (1956). Fuerstenau, D . W . , Healy, T . W . , Somasundaran, P., Trans. AIME 229, 321 (1964). (3) G a u d i n , A . M., Fuerstenau, D. W . , Trans. AIME 2 0 2 , 958 (1955). (4) H a l l , P. G . , Tompkins, F. C., Trans. Faraday Soc. 58, 1734 (1962). (5) Jaycock, M. J., Ottewill, R. H., Rastogi, M. C., 3rd Intern. Congr. Surface Activity V o l . II, 283 (1960). (6) Jones, J . H., J. Assoc. Agr. Chemists 2 8 , 398 (1945). (7) Ibid., 28, 409 (1945). (8) Ottewill, R. R., Rastogi, M. C., Trans. Faraday Soc. 56, 880 (1960). (9) Skewis, J. D., Zettlemoyer, A. C., 3rd Intern. Congr. Surface Activity V o l . II, 401 (1960). (10) Somasundaran, P., Fuerstenau, D. W . , J. Phys. Chem. 70, 90 (1966). (11) Somasundaran, P . , Healy, T . W . , Fuerstenau, D. W . , J. Phys. Chem. 68, 3562 (1964). (12) Tamamushi, B . , T a m a k i , K . , Proc. 2nd Intern. Congr. Surface Activity 3, 449 (1958).


172 (13) (14) (15)

ADSORPTION

F R O M AQUEOUS

SOLUTION

Tcheurekdjian, N . Zettlemoyer, A . C., Chessick, J . J., J. Phys. Chem. 68, 773 (1964). Volk, W . , " A p p l i e d Statistics for Engineers," p. 236, M c G r a w - H i l l , N e w York, 1958. Yopps, J . A., Fuerstenau, D . W . , J. Colloid Sci. 19, 61 (1964). November 24,

1967.


RECEIVED


The Effect of Hydrocarbon Chain Length on the Adsorption of

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