Mechanism of Sulfonate Adsorption at the Silver IodideâSolution

5 Mechanism o f Sulfonate A d s o r p t i o n at the

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Silver Iodide-Solution Interface K. OSSEO-ASARE, D. W. F U E R S T E N A U , and

R. H . O T T E W I L L *

Department of Materials Science and Engineering, University of California, Berkeley, Calif. 94720

Introduction The e a r l y work on silver i o d i d e d i s p e r s i o n s (1) showed that the s o l s prepared in the presence of an excess of i o d i d e ions were more s t a b l e than those prepared w i t h silver ions present in ex cess. This was found to be due to the f a c t that the p o i n t of zero charge d i d not c o i n c i d e w i t h the equivalence p o i n t , i.e. in the presence of potential determining ions only, the equivalence p o i n t was found to be at pAg 8 whereas the net charge on the s u r f a c e became zero at pAg 5.5. Historically silver i o d i d e has been clas sified as a hydrophobic sol, a definition l a r g e l y based on the f a c t that silver i o d i d e s o l s are not r e v e r s i b l e , i.e. coagulated s o l s cannot be redispersed by diluting w i t h water (see Frens and Overbeek (2)). R e c e n t l y , s t u d i e s of the a d s o r p t i o n of water vapor on silver i o d i d e powders have shown t h a t the s u r f a c e is a l s o hydrophobic in terms of the u s u a l definition of wettability. Zettlemoyer et al. (3) found that the s u r f a c e area r a t i o s water/ argon and water/nitrogen were much l e s s than u n i t y and concluded that approximately three out of five s u r f a c e s i t e s were hydro phobic; they a t t r i b u t e d the h y d r o p h i l i c - h y d r o p h o b i c balance to oxide i m p u r i t i e s . The r e s u l t s of P r a v d i c and M i r n i k (4) showed that at pI 5 (pAg 11) and pH 5 hexylamine can reverse the charge of a silver iodide particle. This behavior i s in marked c o n t r a s t to the e f f e c t of alkylamine on q u a r t z , a h y d r o p h i l i c s u b s t r a t e , where as far as a d s o r p t i o n is concerned the octylammonium i o n appears to behave in the same way as an ammonium i o n ( 5 ) . Long chain alkyl sulfates, even at low c o n c e n t r a t i o n s , a l s o a f f e c t the z e t a p o t e n t i a l of the silver i o d i d e s u r f a c e ( 6 ) . Moreover, according to B i j s t e r b o s c h and Lyklema (7) even short c h a i n a l c o h o l s , e.g. η-butyl, adsorb w i t h t h e i r hydrophobic p a r t s towards the silver i o d i d e . Recently measurements of the contact angle of silver i o d i d e surfaces in water have shown that values of ca. 23° can be obtained (8,9). These f a c t o r s all suggest t h a t there i s strong hydrophobic i n t e r a c t i o n between hydrocarbon chains and 63

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

64

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s i l v e r i o d i d e surfaces which can play an important r o l e i n s i l v e r i o d i d e - s u r f a c e a c t i v e agent s o l u t i o n i n t e r a c t i o n s . In a d d i t i o n , recent s t u d i e s u s i n g e l e c t r o n microscopy have shown that a l k y l p y r i d i n i u m compounds appear t o undergo a chem i c a l r e a c t i o n w i t h s i l v e r i o d i d e surfaces at s u r f a c t a n t concen t r a t i o n s approaching 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 (10). I t appears t h e r e f o r e that the a d s o r p t i o n of surface a c t i v e agents at a s i l v e r i o d i d e s u r f a c e may i n v o l v e a complex i n t e r p l a y of e l e c t r i c a l , hydrophobic ( d i s p e r s i o n f o r c e s ) and chemical i n t e r actions . The purpose of the present paper i s t o e l u c i d a t e a d s o r p t i o n mechanisms i n the a l k y l s u l f o n a t e - s i l v e r i o d i d e system through e l e c t r o p h o r e t i c m o b i l i t y measurements. The s u l f o n a t e s used i n t h i s study ranged from p e n t y l to t e t r a d e c y l . Further i n f o r m a t i o n on the nature of the A g i s u r f a c e and s u l f o n a t e a d s o r p t i o n was obtained through study of contact angle phenomena i n these systems. Experimental

- M a t e r i a l s and Methods

M a t e r i a l s . T r i p l y d i s t i l l e d water used f o r a l l s o l u t i o n s was prepared by f i r s t passing tap water through a Barnstead Laboratory s t i l l followed by a two-stage Haraeus quartz s t i l l . The f i n a l d i s t i l l a t e was kept under p u r i f i e d n i t r o g e n u n t i l used. S i l v e r n i t r a t e and potassium i o d i d e of A.R. grade were used to prepare the s i l v e r i o d i d e s o l . Iodine was obtained as resublimed c r y s t a l s from M a l l i n c k r o d t and s i l v e r w i r e of 99.5 to 99.8% p u r i t y was obtained from Sargent-Welch. The sodium s a l t s of C m , 12> 10> 8 s u l f o n i c a c i d s were s u p p l i e d by A l d r i c h Chemical Company w h i l e the C 5 was from Κ & Κ L a b o r a t o r i e s . c

c

a n d

c

P r e p a r a t i o n of S i l v e r Iodide S o l . A stock s o l was prepared by adding 50 ml of 1 0 " Μ ΚΙ s o l u t i o n to an equal volume of 1 0 ~ M AgNÛ3 s o l u t i o n w i t h s t i r r i n g . A f t e r aging f o r 12 - 18 hours, t h i s was d i l u t e d to a s o l c o n c e n t r a t i o n of 10~** M Agi f o r e l e c t r o phoresis measurements. The i o n i c s t r e n g t h was c o n t r o l l e d w i t h 10~ M KNO3 and a p p r o p r i a t e volumes of AgNU3 and s u r f a c t a n t s o l u t i o n s were added to g i v e t h e r e q u i r e d pAg and s u r f a c t a n t concen tration. z

2

3

E l e c t r o p h o r e s i s . The e l e c t r o p h o r e t i c measurements were con ducted w i t h the R i d d i c k Zetameter (11), a product o f Zetameter I n c . , New York. Room temperature was maintained a t 20 ± 2°C f o r most of these experiments. I n g e n e r a l , twenty m o b i l i t y readings were taken f o r each system and averaged; the p o l a r i t y of t h e a p p l i e d v o l t a g e was reversed f o r a l t e r n a t e measurements. U s u a l l y 100 V was used; however, f o r slower p a r t i c l e s i t was o f t e n neces sary to go to v o l t a g e s as high as 300 V to observe any a p p r e c i able motion. Care was taken not to keep the s o l too long before use s i n c e on standing f o r extended periods of time, the s o l


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Sulfonate Adsorption

AL.

65

p a r t i c l e s have been found to decrease i n m o b i l i t y - probably a r e s u l t of the desorption which accompanies c o a g u l a t i o n ( 6 ) . The average diameter of the s o l p a r t i c l e s , 2a, was found by e l e c t r o n microscopy (12) to be 20 nm. The i o n i c s t r e n g t h was c o n t r o l l e d u s i n g ΙΟ" M KNO3; t h i s gave a Debye-Huckel r e c i p r o c a l l e n g t h 1/K of 9.5 nm. Based on the f a c t that under these c o n d i t i o n s the product Ka has a v a l u e of 1.05, the r e s u l t s of Wiersema et a l . (13) can be used to convert the measured 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 to z e t a p o t e n t i a l s . Below a m o b i l i t y , u , of 2 urn/ sec per volt/cm, as a good f i t to the c a l c u l a t i o n s of Wiersema et a l . , the m o b i l i t y can be converted i n t o zeta p o t e n t i a l by 3

ζ = 20u

(1)

Q

e

where ζ i s the z e t a p o t e n t i a l i n m i l l i v o l t s . At the higher mo b i l i t y v a l u e s , however, the r e l a t i o n s h i p between m o b i l i t y and zeta p o t e n t i a l becomes n o n l i n e a r . Wetting

Behavior.

P r e p a r a t i o n of t h i n A g i f i l m s . F o l l o w i n g the method of B i l l e t t and O t t e w i l l ( 8 ) , g l a s s microscope s l i d e s , cut i n t o pieces approximately 1 cm χ 2.5 cm χ 0.1 cm, were cleaned w i t h aqua r e g i a and stored under t r i p l y d i s t i l l e d water. To p l a t e the g l a s s w i t h s i l v e r , a watch g l a s s w i t h the s l i d e s i n i t , was p o s i t i o n e d 3 cm v e r t i c a l l y below a s i l v e r source (approximately 6 cm s i l v e r w i r e ) held i n a tungsten basket. The vacuum u n i t was pumped down to about 666.6 yPa through a l i q u i d n i t r o g e n t r a p . The s i l v e r m i r r o r s were t r a n s f e r r e d q u i c k l y from the vacuum evap o r a t o r and immersed i n a 0.0025 Ν s o l u t i o n of i o d i n e i n 0.01 Μ ΚΙ s o l u t i o n f o r 20 - 30 seconds. The f i l m s were then aged i n 10" * Μ ΚΙ s o l u t i o n f o r 1.5 hours and kept under d i s t i l l e d water u n t i l used. I t was found that l e a v i n g a t h i n l a y e r of s i l v e r between the g l a s s p l a t e and the Agi f i l m improved the adhesion of the f i l m to the s u b s t r a t e . This procedure was t h e r e f o r e followed i n the p r e p a r a t i o n of the f i l m s . 1

Measurement of contact angle. The c a p t i v e bubble technique (8) was used to determine the contact angles. For making the measurement, the s o l u t i o n to be s t u d i e d was f i r s t added to a c e l l made of o p t i c a l g l a s s and then the s l i d e w i t h the A g i f i l m was placed i n t o the d e s i r e d s o l u t i o n f o r about 15 minutes before t a k i n g measurements. A g l a s s c y l i n d r i c a l tube of 3 mm i n t e r n a l diameter was placed above the f i l m s u r f a c e and the a i r pressure i n the bubble was regulated by means of a screw arrangement at the upper end of the tube. A telescope supplied w i t h an o c u l a r p r o t r a c t o r was used to observe the bubble p r o f i l e . The bubble was allowed to touch the f i l m surface by g e n t l y i n c r e a s i n g the pressure, then a f t e r s i t t i n g on the f i l m f o r about 15 seconds, the pressure was increased by a s m a l l amount. This


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Figure 1. The effect of pAg on the electro phoretic mobility of silver iodide in the pres ence of sodium pentyl sulfonate at millimolar ionic strength with potassium nitrate

3

4

5

6

7

8

ρ Ag

Figure 2. The effect of pAg on the electro phoretic mobility of silver iodide in the pres ence of sodium octyl sulfonate at millimolar ionic strength with potassium nitrate


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67

s i t u a t i o n y i e l d s the receding angle θ^. The pressure was then r e l e a s e d u n t i l the contact boundary on the p l a t e j u s t moved. The r e s u l t i n g angle i s the advancing angle, θ^. S u r f a c t a n t s o l u t i o n s of a given c o n c e n t r a t i o n were prepared and the contact angles were measured as a f u n c t i o n of pAg f o r C I Q and C ^ . In a l l cases, i n c l u d i n g the experiments i n the absence of s u r f a c t a n t , the i o n i c s t r e n g t h was c o n t r o l l e d w i t h ΙΟ" M KNO3. The contact angles reported here are the average of s i x or more values ob tained by p l a c i n g the bubbles on d i f f e r e n t samples or on d i f f e r ent p o s i t i o n s on the same sample. 3

Results E l e c t r o p h o r e s i s . Figures 1 to 5 present 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 Agi as a f u n c t i o n of pAg f o r v a r i o u s a l k y l s u l f o n a t e concentrations and d i f f e r e n t chain l e n g t h s . In the absence of s u r f a c t a n t , the p o i n t of zero charge of A g i was found to occur at pAg 5.6, i n agreement w i t h previous values reported i n the l i t e r a t u r e ( 1) . I n a l l cases, w i t h i n c r e a s i n g concentrations of s u r f a c t a n t at any pAg values below the pzc, the p o s i t i v e m o b i l i t y of the s o l p a r t i c l e s decreased, passed through z e r o , and then became i n c r e a s i n g l y negative. The most i n t e r e s t i n g f e a t u r e s of these r e s u l t s are (1) The short c h a i n s u l f o n a t e s are able to reverse the z e t a p o t e n t i a l on A g i although much higher concentrations are r e q u i r e d than f o r longer chain s u l f o n a t e s . (2) A l l the curves appear to c o i n c i d e i n the neighborhood of pAg 7 independent of chain l e n g t h or s u r f a c t a n t c o n c e n t r a t i o n . (3) The zeta p o t e n t i a l goes through a maximum at higher concentrations of s u r f a c t a n t , an e f f e c t which increases w i t h chain l e n g t h . Wetting Behavior. The magnitude of the contact angles ob t a i n e d both i n the absence of s u r f a c t a n t and i n the presence of v a r i o u s concentrations of and C\q have been p l o t t e d as a f u n c t i o n of pAg i n Figures 6 to 8. In the absence of s u r f a c t a n t , both the receding and advanc ing contact angles go through a maximum at about pAg 5.4. The curves shown i n F i g u r e 6 are drawn through the weighted average of the experimentally determined contact angle values f o r each pAg. The spread i n the θ values f o r a given pAg i s l a r g e l y the r e s u l t of the inherent d i f f i c u l t y of o b t a i n i n g p e r f e c t l y r e p r o d u c i b l e s u r f a c e s . These r e s u l t s are i n good agreement w i t h those reported by B i l l e t t and O t t e w i l l (14) who a l s o observed a maximum at about pAg 5.4, which i s c l o s e to the pzc. Figures 7 and 8 show that when the b u l k c o n c e n t r a t i o n of the s u r f a c t a n t i s low, the w e t t i n g behavior i s very s i m i l a r to that of the s u r f a c t a n t - f r e e system except at higher p o s i t i v e s u r f a c e charge (low pAg) where a s i g n i f i c a n t r i s e i n contact angle i s observed. In other words, i n F i g u r e 8, the receding contact


ADSORPTION

3

4

5

6

7

A T INTERFACES

8

Ρ Ag

Figure 3. The effect of pAg on the electrophoretic mobility of silver iodide in the pres ence of sodium decyl sulfonate at millimolar ionic strength with potassium nitrate

3

4

5

6

7

PAg

Figure 4. The effect of pAg on the electrophoretic mobility of silver iodide in the pres ence of sodium dodecyl sulfonate at millimolar ionic strength with potassium nitrate


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Figure 5. The effect of pAg on the electrophoretic mobility of silver iodide in the presence of sodium tetradecyl sulfonate at millimolar ionic strength with potassium nitrate

Figure 6. The contact angle on silver iodide in the absence of surfactant as a function of pAg at millimolar ionic strength with potassium nitrate


69

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120

AT

Γ

—ι C S0 No 0

3

( Ι 0 " Μ KNO3) 3

100 h

0 Δ

Ι«Ι0" Μ 4

ο 801

60|

ο

ο < 40|

2 Ο

5

pAg

6

Figure 7. The contact angle on silver iodide in the presence of sodium decyl sulfonate as a func tion of pAg at millimolar ionic strength with potassium nitrate

120

100 Ul UJ

ο ω 80(-

ζ

where a£g and a°^- are the a c t i v i t i e s of Ag+ and I r e s p e c t i v e l y at the pzc. Table I summarizes the r e s u l t s obtained f o r AG p u s i n g Equation (10) f o r pAg 3, 4, and 5, r e s p e c t i v e l y . There i s a strong dependence on c h a i n l e n g t h , the values o f AG p i n c r e a s i n g from 4.7 RT f o r C to 10.9 RT f o r C ^ . The e f f e c t o f pAg does not seem to be s i g n i f i c a n t f o r the s h o r t c h a i n s u l f o n a t e s , e.g. +

S

S

5


74

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Table I.

C a l c u l a t i o n of A G

g p

AT

INTERFACES

w i t h Equation (10)

f o r t h e A d s o r p t i o n of A l k y l Sulfonates on S i l v e r Iodide (C m

=

2

15yF/cm ,

r

=

0.29 nm)

3 pAg

A l k y l Sulfonate c 0

C

1 Q

C

4

5 8

10

C

12 14

C C 5

5

Q

C C C

8.0 RT

3.2xl0~

9.4 RT

7.0xl0" 2.2xl0~

6

10.9 RT 4.7 RT

3

1.5xl0"

3

5.0 RT

7.5xl0"

5

8.0 RT

2.0xl0"

5

9.4 RT

5.6xl0"

6

10.6 RT

8.0xl0"

4

4.6 RT

1.5xl0"

4

6.2 RT

D

l.lxlO"

1 2

8.1 RT 5

3.4xl0""

1 4

gp

5

2.3x10

1 Q

-AG _ 4.7 RT RT 5.0

1.3x10

17

C

C

cm

Q

32.5x10 .5xl0" -4

1 2

C

C

c x ! 0 mol 3 3

C

C

-3

8.9 RT

6

10.0 RT

C5 has a constant v a l u e of 4.7 RT f o r the three pAg values con sidered. C maintains the same v a l u e of A G = 8.0 RT f o r pAg 3 to 5. The values f o r the longer c h a i n s u l f o n a t e s , however, ap pear to show some pAg dependency. Thus a t pAg 3 and 4, A G f o r C i s 9.4 RT, but t h i s decreases to 8.9 RT a t pAg 5. This i s i n reasonable agreement w i t h the r e s u l t s of O t t e w i l l and Watanabe (6) who found A G = 8.8 RT a t pAg 3 f o r C12 s u l f o n a t e . S i m i l a r l y f o r C , AG p decreases from pAg 3 to 5 i n the order 10.9 RT, 10.6 RT, and 10.0 RT. 1 0

g p

s p

1 2

s p

li+

s

E v a l u a t i o n of A G at the p o i n t of coincidence of m o b i l i t y pAg curves. The second c o n d i t i o n f o r e v a l u a t i n g the s p e c i f i c ad s o r p t i o n f r e e energy, i . e . u t i l i z i n g the pAg a t which a l l the m o b i l i t y - pAg curves c o i n c i d e , provides a d i f f e r e n t means of a n a l y z i n g a d s o r p t i o n phenomena. At t h i s p o i n t , i t seems t h a t the A g i s u r f a c e charge i s so h i g h l y negative that the consequent a d g


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Table I I . C a l c u l a t i o n of AG Surface A c t i v e c Agent C

C

C

5 8 10

C

12

C

14

χ 10

3

0

75

f o r Hydrophobic I n t e r a c t i o n (pAg 5)

-AG from -AG Equation (10) S

- 3.1

Chainlength

R T

sp

8.0xl0~

4

4.6 RT

1.5 RT

2.5

1.5xl0~

4

6.2 RT

3.1 RT

4

2.3xl0~

5

1.1x10

c

D

3.4x10"°

8.1 RT

5.0 RT

5

8.9 RT

5.8 RT

6

10.0 RT

6.9 RT

7

l a r g e AG ^ almost completely opposes s p e c i f i c a d s o r p t i o n . T h i s means that a t t h i s p o i n t , the s p e c i f i c a d s o r p t i o n p o t e n t i a l should j u s t be counterbalanced by the e l e c t r o s t a t i c c o n t r i b u t i o n to the a d s o r p t i o n f r e e energy. Thus, under t h i s c o n d i t i o n , e

AG

sp

«

(12)

-AG , - - zFiK el 6

Since a l l the curves come together by pAg 7 , independent of c h a i n length and c o n c e n t r a t i o n of s u r f a c t a n t , ψβ * - 7 8 mV when spe c i f i c a d s o r p t i o n ceases. This y i e l d s a v a l u e of 3 . 1 RT f o r the s p e c i f i c a d s o r p t i o n f r e e energy. I n a d d i t i o n , because t h i s ad s o r p t i o n phenomenon does not e x h i b i t any c h a i n l e n g t h dependency, i t cannot be a t t r i b u t e d to the c h a i n - c h a i n and c h a i n - s o l i d i n t e r a c t i o n s . I t i s t h e r e f o r e suggested that t h i s behavior i s caused by the s p e c i f i c i t y of the p o l a r head of the s u r f a c t a n t . I n par t i c u l a r , the i n t e r a c t i o n of the s u l f o n a t e head w i t h Ag+ i n the l a t t i c e gives r i s e to a chemisorption bond and t h i s l a t t e r c a l c u l a t i o n estimates i t s magnitude, i . e . , A G = - 3 . 1 RT. Beekley and Taylor ( 2 0 ) have reported a marked s p e c i f i c i n f l u e n c e of c e r t a i n anions, i n c l u d i n g C10H7O3", i n the a d s o r p t i o n of Ag+ on A g i . The recent work of O t t e w i l l and co-workers ( 1 0 ) a l s o i m p l i e s t h a t there might be chemisorption i n the a d s o r p t i o n of dodecyl p y r i d inium bromide on A g i . Therefore, t a k i n g t h e AG value calcu l a t e d w i t h Equation ( 1 2 ) as A G - - 3 . 1 RT, t h e hydrophobic c o n t r i b u t i o n to the t o t a l s p e c i f i c a d s o r p t i o n f r e e energy can be c a l c u l a t e d by s u b t r a c t i n g A G = - 3 . 1 RT from the A G values given by Equation ( 1 0 ) . These have been t a b u l a t e d i n Table I I . There i s a s t r i k i n g s i m i l a r i t y between the values of (-AG p - 3 . 1 RT) and the numbers obtained by d i v i d i n g the r e s p e c t i v e c h a i n lengths by 2 , e s p e c i a l l y f o r the longer c h a i n surfactants. The f r e e energy decrease accompanying the complete removal of a hydrocarbon c h a i n from water i s about 1 RT per mole of CH2 groups ( 1 7 ) . Thus, on t h i s b a s i s , i f a l l the chains are removed from water, the decrease i n f r e e energy w i l l be 5 RT, 8 RT, 1 0 RT, 1 2 RT, and 1 4 RT r e s p e c t i v e l y f o r C5, Ce, C I Q , C , and C . c n e m

c h e m

c h

sp

s

1 2


1 4

76

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AT

INTERFACES

I f on the other hand, the adsorbed s u r f a c t a n t c o n s i s t s of h o r i z o n t a l l y o r i e n t e d c h a i n s , then the accompanying decrease i n f r e e energy might be 1/2 RT per mole of CH groups ( i n s t e a d of 1 RT) s i n c e even though h a l f of the c h a i n surface i s next to the s o l i d s u r f a c e , the other h a l f i s s t i l l exposed to the water. On t h i s b a s i s t h e r e f o r e , the f r e e energy a s s o c i a t e d w i t h complete h o r i z o n t a l o r i e n t a t i o n of the chains w i l l be 2.5 RT, 4 RT, 5 RT, 2

and

7 RT

respectively

for C , 5

C I Q , 0χ2>

Ce,

a n c l

c

l*f

0 n

compar

ing these values w i t h the corresponding (-AG - 3.1 RT) values (Table I I ) v i z . 1.5 RT, 3.1 RT, 5.0 RT, 5.8 RT, 6.9 RT respec t i v e l y f o r C5, CQ CIQ, Ci2> and C^, i t seems reasonable to con clude t h a t the adsorbed s u l f o n a t e ions o r i e n t t h e i r chains p a r a l l e l to the s o l i d s u r f a c e i n d i c a t i n g the presence of c h a i n - s o l i d hydrophobic i n t e r a c t i o n s ( A G * = A G + 3.1 RT). The f a c t sp

9

C H 2

s p

t h a t i n comparison w i t h the other c h a i n l e n g t h s , C5 and Ce have v a l u e s of AG^^ which are below t h e i r r e s p e c t i v e nRT/2 (where η i s the number of carbon atoms i n the chain) values shows t h a t C5, and Ce are l e s s surface a c t i v e than t h e i r longer c h a i n counter parts . At higher s u l f o n a t e concentrations and e s p e c i a l l y w i t h the Cm s u l f o n a t e (Figure 5 ) , the slope of the m o b i l i t y - pAg curves i s reversed as the pAg i s reduced. T h i s r e v e r s a l probably i n d i c a t e s the onset of hemimicelle f o r m a t i o n , which i s the c o n d i t i o n where the a d s o r p t i o n f r e e energy becomes more negative because of the a s s o c i a t i o n of the hydrocarbon chains of the adsorbed s u l f o n a t e i o n s . Contact angle behavior. In view of the f a c t that water seems to be only weakly bonded to the A g i surface (3,21,22), there w i l l be a strong tendency f o r incoming s u l f o n a t e ions to d i s l o d g e water molecules at the s u r f a c e . The r e s u l t w i l l be an i n t e r f a c e c o n s i s t i n g of CH groups w i t h a consequent i n c r e a s e i n the hydrophobic nature of the s o l i d - s o l u t i o n i n t e r f a c e . T h i s means t h a t i n the presence of adsorbed s u l f o n a t e s the contact angle at the s o l i d - l i q u i d - a i r i n t e r f a c e should i n c r e a s e w i t h the number of CH2 groups at the s o l i d s u r f a c e , i . e . w i t h both c h a i n l e n g t h and amount of adsorbed i o n s . F i g u r e s 7 and 8 c l e a r l y support the above a n a l y s i s . A comparison of the contact angles f o r C I Q and shows t h a t at pAg 3 a receding angle of about 50 degrees i s achieved i n 5 χ ΙΟ" M C I Q s u l f o n a t e but only 3 χ 10~ M i s necessary to a t t a i n the same contact angle w i t h Cii+ s u l f o n a t e . Again, f o r each s u r f a c t a n t c o n c e n t r a t i o n , the angles i n c r e a s e w i t h decreasing pAg, showing t h a t the amount of s u r f a c t a n t adsorbed increases w i t h higher p o s i t i v e surface charge. When the pAg i s low, the s o l i d surface has a h i g h p o s i t i v e charge and t h i s leads to an i n c r e a s e i n e l e c t r o s t a t i c a t t r a c t i o n of the a n i o n i c s u r f a c t a n t s to the A g i s u r f a c e . This means that more s u r f a c t a n t i o n s w i l l be drawn towards the s u r f a c e and consequently the c o n t r i b u t i o n s of c h a i n - s o l i d i n t e r a c t i o n s to 2

4

5


5.

OSSEO-ASARE E T A L .


77

the f r e e energy of a d s o r p t i o n w i l l be expected t o i n c r e a s e w i t h increased surface charge, and the longer the chain l e n g t h , the more t h i s i n c r e a s e w i l l be apparent, both i n e l e c t r o k i n e t i c be h a v i o r and i n contact angle behavior. Summary The e f f e c t of n - a l k y l s u l f o n a t e s on the e l e c t r o p h o r e t i c and w e t t i n g behavior of s i l v e r i o d i d e has been s t u d i e d . The r e s u l t s show that i n the r e g i o n of negative charge, a l l the s u r f a c t a n t s cease to be surface a c t i v e a t the same pAg, i n the neighborhood of pAg 7. This f a c t has been used to p o s t u l a t e a chemical con t r i b u t i o n of the p o l a r head to the t o t a l s p e c i f i c f r e e energy of a d s o r p t i o n . I t i s demonstrated by means of the Stern-Grahame theory of s p e c i f i c a d s o r p t i o n a t the double l a y e r that the s u l fonates adsorb a t lower coverages w i t h t h e i r chains h o r i z o n t a l l y o r i e n t e d to the A g i s u r f a c e . The f r e e energy change asso c i a t e d w i t h t h i s s p e c i f i c c h a i n - s o l i d i n t e r a c t i o n i s found to be 0.5 RT per mole of CH2 group. At higher a d s o r p t i o n d e n s i t i e s , and e s p e c i a l l y f o r longer chain l e n g t h s , a s s o c i a t i o n c o n t r i b u t e s to the a d s o r p t i o n process. Acknowledgements The support of t h i s research by the N a t i o n a l Science Founda t i o n i s g r a t e f u l l y acknowledged. Literature Cited

1. Kruyt, H. R., Ed., "Colloid Science", Vol. I, Elsevier, Amsterdam, 1952. 2. Frens, G., Overbeek, J . Th. G., J . Colloid Interface Sci., (1971), 36, 286 3. Zettlemoyer, A. C., Tcheurekdjian, Ν., Chessick, J . J., Nature (London), (1961), 192, 653 4. Pravdic, V., Mirnik, Μ., Croat. Chem. Acta, (1960), 32, p. 1. 5. Fuerstenau, D. W., J . Phys. Chem., (1956), 60, 981 6. Ottewill, R. H., Watanabe, Α., Kolloid-Z., (1960), 170, 132 7. Bijsterbosch, B. H., Lyklema, J., J. Colloid Sci., (1965), 20, 665 8. Billett, D. F . , Ottewill, R. H., in "Wetting", S. C. I. Mono graph No. 25, p. 253, Society of Chemical Industry, London, 1967. 9. Billett, D. F . , Hough, D. B., Lovell, V., Ottewill, R. H., (1973), unpublished data. 10. Billett, D. F . , Ottewill, R. H., Thompson, D. W., in "Parti cle Growth in Suspensions", S. C. I. Monograph No. 38, p. 195, Academic Press, London, 1973. 11. Zeta-Meter Manual, Zeta-Meter Inc., New York, N.Y.


78

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

ADSORPTION AT INTERFACES

Horne, R. W., Ottewill, R. H . , J. Photo. S c i . , (1958), 6, 39 Wiersema, P., Loeb, Α . , Overbeek, J . Th. G . , J. Colloid Interface S c i . , (1966), 22, 78 Billett, D. F., Ottewill, R.H., (1971), 161st A. C. S. Na tional Meeting, Los Angeles, Ca. Stern, O., Z. Elektrochem., (1924), 30, 508 Grahame, D. C . , Chem. Rev. (1947), 41, 441 Fuerstenau, D. W., Journal of the International Union of Pure and Applied Chemistry, (1970), 24, 135 Osseo-Asare, Κ., Fuerstenau, D. W., Croat. Chem. Acta, (1973), 45, 149 Mackor, E., L., Recl. Trav. Chim., (1951), 70, 763 Beekley, J . S., Taylor, H. S., J . Phys. Chem., (1925), 29, 942 Tcheurekdjian, N., Zettlemoyer, A. C . , Chessick, J . J., J. Phys. Chem., (1964), 68, 773 Hall, P. G . , Tomkins, F. C . , Trans. Farad. Soc., (1962), 58, 1734

*Visiting Professor, Department of Materials Science and Engineering, University of California, Berkeley; Permanent address: School of Chemistry, University of Bristol, Bristol BS8 1TS, England


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