The Effect of Preadsorbed Polymers on Adsorption of Sodium

18 The Effect of Preadsorbed Polymers on Adsorption of Sodium Dodecylsulfonate on Hematite 1

Structure/Performance Relationships in Surfactants Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 02/28/19. For personal use only.

J. E. GEBHARDT and D. W. FUERSTENAU Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system. Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity. Technological problems caused by fine particulates occur in many industrial situations including mineral processing circuits. The processing of fine particle suspensions has been the topic of several technical conferences in recent years (1-2) . One method exhibiting significant potential for recovering valuable minerals from fine-particle suspensions is selective flocculation. This involves the use of a polymer or long-chained organic molecule to selectively aggregate particles of one of the minerals prior to a separation stage. Depending on the process and mineral composition, either the valuable or gangue mineral may be flocculated. To obtain a suitable concentrate, the flocculated particles must be separated from the suspension. The usual method is sedimentation of the floes combined with elutriation of the dispersed particles. Flotation of the flocculated particles is a possible method to achieve that separation. The effect of polymers used as flocculants on the flotation of a few minerals has received 1

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0097-6156/84/0253-0291 $06.00/0 © 1984 American Chemical Society

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STRUCTURE/PERFORMANCE RELATIONSHIPS IN SURFACTANTS

l i m i t e d i n v e s t i g a t i o n ( 3 - 5 ) , and d e t a i l s on the mechanism o f p o l y m e r and f l o t a t i o n c o l l e c t o r c o a d s o r p t i o n and r e l a t i o n t o f l o t a t i o n a r e l a c k i n g . T h i s p a p e r r e p o r t s the r e s u l t s o f an i n v e s t i g a t i o n u n d e r t a k e n t o d e l i n e a t e the r o l e o f polymer and c o l l e c t o r i n t e r a c t i o n s w i t h f i n e p a r t i c l e s and t h e i r r e l a t i o n t o resultant surface w e t t a b i l i t y . In p a r t i c u l a r , the a d s o r p t i o n o f an a n i o n i c s u r f a c t a n t on a s u r f a c e w i t h p r e v i o u s l y a d s o r b e d noni o n i c o r a n i o n i c polymer i s examined. Experimental

Methods and

Materials

Materials. S y n t h e t i c h e m a t i t e was o b t a i n e d from J . T. B a k e r C h e m i c a l Company, P h i l l i p s b u r g , NJ. P a r t i c l e size analysis using a HIAC i n s t r u m e n t ( M o n t c l a i r , CA) i n d i c a t e d the p a r t i c l e s t o be 80 p e r c e n t (number) f i n e r t h a n 2 m i c r o n s . U s i n g n i t r o g e n as the a d s o r b a t e , the B.E.T. s p e c i f i c s u r f a c e a r e a was found t o be 9 s q u a r e m e t e r s per gram. The p o i n t o f z e r o c h a r g e , as o b t a i n e d from e l e c t r o p h o r e t i c measurements i n the p r e s e n c e o f i n d i f f e r e n t e l e c t r o l y t e s , o c c u r r e d a t pH 8.3. P o l y a c r y l i c a c i d (PAA) was o b t a i n e d f r o m S c i e n t i f i c P o l y m e r s , I n c . , O n t a r i o , NY, as a s e c o n d a r y s t a n d a r d w i t h a m a s s - a v e r a g e d m o l e c u l a r w e i g h t o f two m i l l i o n . The p o l y a c r y l a m i d e (PAM) used was S e p a r a n MGL o b t a i n e d from Dow C h e m i c a l Company, M i d l a n d , M I . I t s r e p o r t e d m o l e c u l a r w e i g h t was i n t h e r a n g e o f 500,000 t o 5,000,000. The monomer s t r u c t u r e s o f PAA and PAM are i l l u s t r a t e d i n F i g u r e 1. Sodium d o d e c y l s u l f o n a t e (SDS) was p r e p a r e d from d o d e c y l a l c o h o l b y Ben Den C h e m i c a l Company, N a p e r v i l l e , I L . The m a t e r i a l was r e c r y s t a l l i z e d b y d i s s o l u t i o n i n hot e t h a n o l s o l u t i o n , f i l t e r i n g and c o o l i n g t o c r y s t a l l i z e the SDS. The p r e c i p i t a t e was f i l t e r e d , washed w i t h c o l d e t h a n o l , and d r i e d i n a d e s s i c a t o r under vacuum. A n a l y t i c a l - g r a d e HC1 and NaOH were used f o r pH a d j u s t m e n t and NaCl f o r c o n t r o l l i n g i o n i c s t r e n g t h . A d s o r p t i o n M e t h o d s . F i v e grams o f h e m a t i t e were f i r s t c o n d i t i o n e d i n 0.001 M N a C l a t pH 4.1. A f t e r t h e SDS had been added t o the s l u r r y and the pH a d j u s t e d as r e q u i r e d , the samples were c o n d i t i o n e d on a r o t a t i n g s h a k e r f o r two h o u r s . The s o l u t i o n s were t h e n c e n t r i f u g e d , and the s u p e r n a t a n t l i q u i d a n a l y z e d f o r i t s SDS c o n t e n t . The amount o f SDS a d s o r b e d was c a l c u l a t e d as the d i f f e r e n c e between the i n i t i a l amount added and the r e s i d u a l amount measured. E x p e r i m e n t a l r e s u l t s showed t h a t two h o u r s was s u f f i c i e n t t i m e f o r e q u i l i b r i u m t o be r e a c h e d . Somasundaran ( 6 ) observed s i m i l a r e q u i l i b r i u m a d s o r p t i o n times f o r s u l f o n a t e a d s o r p t i o n on aluminum o x i d e . F o r a d s o r p t i o n on f l o c c u l a t e d p a r t i c l e s , the p o l y m e r was added i n a d r o p - b y - d r o p - w i s e manner from a b u r e t t e c o n t a i n i n g a 50 c c s o l u t i o n t o a 50 c c s o l u t i o n c o n t a i n i n g t h e s o l i d s . Flocc u l a t i o n was p e r f o r m e d i n an u n b a f f l e d v e s s e l , 58 mm i n d i a m e t e r . A g i t a t i o n was a c h i e v e d w i t h a 3 - b l a d e d p r o p e l l e r , 35 mm i n

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Effect of Preadsorbed Polymers

POLYACRYLIC

CH

ACID ( P A A )

-

2

293

CH I

0

C =

0

Jn

H

POLYACRYLAMIDE

-

C H

2

-

(ΡΔΜ)

CH I C =0 I NH

2

Jn

SODIUM DODECYLSULFONATE CH (CH ) 3

Figure

1.

2

| 0

(SDS)

CH S0 NQ 2

3

Chemical s t r u c t u r e o f p o l y a c r y l i c

c r y l a m i d e monomers and sodium

acid

dodecylsulfonate.

and p o l y a -


294

d i a m e t e r , w h i c h was l o c a t e d a p p r o x i m a t e l y 10 mm f r o m t h e b o t t o m o f the v e s s e l . Each b l a d e was 13 mm a c r o s s h a v i n g a p i t c h o f a b o u t 30 d e g r e e s . A f t e r a g i t a t i n g t h e s l u r r y f o r 10 m i n u t e s a t 600 rpm, the s o l u t i o n was d e c a n t e d and the r e s i d u a l p o l y m e r determined. P o l y m e r a d s o r p t i o n was r a p i d , and t h e method o f p o l y m e r a d d i t i o n t o t h e s e w e l l - m i x e d s u s p e n s i o n s was found t o yield reproducible results. The f l o e s were t r a n s f e r r e d t o 70-cc g l a s s sample b o t t l e s t o w h i c h SDS was added. The pH was a d j u s t e d as r e q u i r e d , and the samples were c o n d i t i o n e d o n a r o t a t i n g s h a k e r ( a p p r o x i m a t e l y 60 rpm) f o r two h o u r s . Two h o u r s was o b s e r v e d t o be a d e q u a t e f o r c o n s t a n t a d s o r p t i o n v a l u e s t o be r e a c h e d i n the case o f p a r t i c l e s t h a t had been p r e v i o u s l y f l o c c u l a t e d w i t h polymer. E l e c t r o k i n e t i c M e a s u r e m e n t s . 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 were measured w i t h a f l a t - c e l l a p p a r a t u s m a n u f a c t u r e d b y Rank B r o t h e r s , Cambridge, England. In a d d i t i o n , s e v e r a l m o b i l i t y v a l u e s were c h e c k e d f o r a c c u r a c y w i t h a Z e t a M e t e r , New Y o r k . M o b i l i t i e s were d e t e r m i n e d w i t h a s m a l l volume o f the s u s p e n s i o n ( a p p r o x i m a t e l y 25 c c ) t h a t had been p r e p a r e d f o r t h e a d s o r p t i o n experiments. The pH o f the s o l u t i o n was measured p r i o r t o d e t e r m i n i n g the 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 , which i n v o l v e d measuring the v e l o c i t i e s o f f i v e t o t e n p a r t i c l e s i n each d i r e c t i o n . An a v e r a g e v a l u e o f t h e m o b i l i t i e s was r e c o r d e d . Samples c o n t a i n i n g the f l o c c u l a t e d p a r t i c l e s were d i p p e d i n t o an u l t r a s o n i c b a t h f o r a p p r o x i m a t e l y one second p r i o r t o making t h e pH and m o b i l i t y measurements. C o n t a c t A n g l e M e a s u r e m e n t s . C o n t a c t a n g l e s were measured on a mounted d i s k o f s y n t h e t i c h e m a t i t e . The d i s k was p r e p a r e d b y c o l d p r e s s i n g h e m a t i t e powder i n a s t e e l d i e and s i n t e r i n g i t a t 1,000 d e g r e e s C e l s i u s f o r 8 h o u r s i n a i r . X - r a y d i f f r a c t i o n p a t t e r n s i n d i c a t e d t h a t the sample was s t i l l α-hematite. The a n g l e formed b y a b u b b l e a t t a c h e d t o t h e d i s k was measured u s i n g a goniometer. To form a b u b b l e , a J - s h a p e d g l a s s c a p i l l a r y was p l a c e d u n d e r t h e d i s k , w h i c h had been p o s i t i o n e d s u r f a c e - s i d e down i n the s o l u t i o n . A d v a n c i n g and r e c e d i n g a n g l e s were o b t a i n e d b y m a n i p u l a t i n g a i r i n and out o f t h e c a p i l l a r y t u b e . "Equilibrium a n g l e s were d e t e r m i n e d b y d e t a c h i n g the b u b b l e from the c a p i l l a r y . The d i s k was c l e a n e d b e t w e e n each measurement b y s e v e r a l t u r n s on a p o l i s h i n g w h e e l . 1 1

S u r f a c e T e n s i o n M e a s u r e m e n t s . The s u r f a c e t e n s i o n o f s u r f a c t a n t s o l u t i o n was measured b y t h e c a p i l l a r y r i s e method w i t h a g l a s s c a p i l l a r y t u b e , 18.66 cm i n l e n g t h and 0.0531 cm i n t e r n a l d i a m eter. T h i s was o b t a i n e d b y f i l l i n g t h e c a p i l l a r y w i t h a column o f m e r c u r y and w e i g h i n g the c a p i l l a r y w i t h and w i t h o u t the mer cury. The d e n s i t y o f m e r c u r y was t a k e n t o be 13.5939 grams p e r c u b i c c e n t i m e t e r a t 20 d e g r e e s C e n t i g r a d e (_7). The h e i g h t o f c a p i l l a r y r i s e was measured w i t h a c a t h e t o m e t e r m a n u f a c t u r e d b y

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Gaertner S c i e n t i f i c Corp., Chicago, I L , i n a temperaturec o n t r o l l e d room a t 21 d e g r e e s C e n t i g r a d e . A n a l y t i c a l Methods. I t was found t h a t t h e c o n c e n t r a t i o n o f b o t h p o l y m e r s c o u l d be a n a l y z e d b y d e t e r m i n i n g t h e t o t a l c a r b o n i n s o l u t i o n with a carbon d i o x i d e coulometer, C o u l o m e t r i c s , I n c . , Wheat R i d g e , CO. The a c c u r a c y o f t h i s method i s n o t good i n t h e low polymer c o n c e n t r a t i o n r a n g e , t h a t i s l e s s t h a n 10-15 ppm. F o r h i g h e r a c c u r a c y i n t h e low p o l y m e r c o n c e n t r a t i o n r a n g e , two d i f f e r e n t methods were u s e d . I n t h e c a s e o f PAA, p o t e n t i o m e t r i c t i t r a t i o n s o f s o l u t i o n s o f PAA were p e r f o r m e d w i t h 0.01 Ν NaOH u s i n g a Brinkman m o d e l , W e s t b u r y , NY, automated t i t r a t o r . Blank t e s t s i n d i c a t e d no i n t e r f e r i n g s p e c i e s . Known amounts o f PAA were used t o p r e p a r e a c a l i b r a t i o n c u r v e i m m e d i a t e l y a f t e r t i t r a t i o n o f t h e samples c o n t a i n i n g unknown amounts o f p o l y m e r . The s t a r t i n g p o i n t o f t h e t i t r a t i o n was pH 4.0, and t h e end p o i n t was r e a c h e d n e a r pH 8. T o t a l volumes o f 75 o r 100 c c were used f o r t h e t i t r a t i o n s , and t h e i o n i c s t r e n g t h was c o n t r o l l e d a t 0.01 M NaCl. F o r t h e d e t e r m i n a t i o n o f PAM, UV a d s o r b a n c e a t 189 nm was measured f o r v a r i o u s PAM c o n c e n t r a t i o n s . The n e p h e l o m e t r i c t e c h n i q u e o f A t t i a and R u b i o ( 8 ) as m o d i f i e d b y P r a d i p (9_) was used t o check t h e c a l i b r a t i o n c u r v e o b t a i n e d b y t h e UV method. The amount o f SDS i n s o l u t i o n was d e t e r m i n e d b y t h e method o f Jones ( 1 0 ) m o d i f i e d i n a manner s i m i l a r t o t h a t o f Somasundaran (6^). M e t h y l e n e b l u e c h l o r i d e was added t o a s o l u t i o n c o n t a i n i n g SDS. The r e s u l t i n g b l u e complex was e x t r a c t e d i n t o an o r g a n i c p h a s e , and t h e a b s o r b a n c e i n t h e v i s i b l e r a n g e was m e a s u r e d . O n e - h a l f c c o f a one p e r c e n t ( b y w e i g h t ) m e t h y l e n e b l u e c h l o r i d e s o l u t i o n was added t o a 4.5 c c sample o f s o l u t i o n c o n t a i n i n g 5-20 m i c r o g r a m s o f SDS. F i v e c c o f c h l o r o f o r m was t h e n added and t h e s o l u t i o n s shaken b y hand f o r 30 s e c o n d s . For the f l o c c u l a t e d s y s t e m , t h e polymer c o n t e n t was d e t e r m i n e d f i r s t and s u f f i c i e n t amounts o f p o l y m e r were added p r i o r t o SDS d e t e r m i n a t i o n t o main t a i n t h e polymer l e v e l c o n s t a n t a t 200 ppm. A c a l i b r a t i o n c u r v e was made u s i n g a b s o r b a n c e a t 652 nm as a f u n c t i o n o f SDS concentration. R e s u l t s and D i s c u s s i o n Polymer/Surfactant Interactions. I n t e r a c t i o n between p o l y m e r s and s u r f a c t a n t s was r e c e n t l y r e v i e w e d b y Robb ( 1 1 ) and s u r f a c t a n t a s s o c i a t i o n w i t h p r o t e i n s b y S t e i n h a r d t and R e y n o l d s ( 1 2 ) . P o l y m e r / s u r f a c t a n t i n t e r a c t i o n s a r e h i g h l y dependent on t h e chem i c a l n a t u r e o f t h e polymer and t h e s u r f a c t a n t . In general, sur f a c t a n t s tend t o a s s o c i a t e w i t h uncharged polymers i n aggregates r a t h e r than i n d i v i d u a l s u r f a c t a n t m o l e c u l e s i n t e r a c t i n g w i t h t h e macro-molecule. The a b i l i t y o f s u r f a c t a n t s t o f o r m m i c e l l e s i s t h o u g h t t o be an i m p o r t a n t f a c t o r i n t h e r o l e o f s u r f a c t a n t b e h a v i o r i n i n t e r a c t i o n s with polymers. Individual surfactant

296


m o l e c u l e s can i n t e r a c t d i r e c t l y w i t h p o l y e l e c t r o l y t e s o r p o l y m e r s c a p a b l e o f some d e g r e e o f i o n i z a t i o n , s u c h as p r o t e i n s , w h i l e a s s o c i a t i o n between p o l y m e r s and s u r f a c t a n t a g g r e g a t e s f o l l o w s a t higher surfactant concentrations. E l e c t r o s t a t i c f o r c e s are p r i m a r i l y r e s p o n s i b l e f o r the d i f f e r e n t i n t e r a c t i o n b e h a v i o r o f s u r f a c t a n t s w i t h the c h a r g e d and u n c h a r g e d p o l y m e r s . The measurement o f t h e s u r f a c e t e n s i o n o f SDS s o l u t i o n s a t c o n s t a n t p o l y m e r a d d i t i o n s was p e r f o r m e d t o i n v e s t i g a t e any p o s s i b l e i n t e r a c t i o n s between SDS and the p o l y m e r s used i n t h e s e experiments. The r e s u l t s , shown i n F i g u r e 2, i n d i c a t e no i n t e r a c t i o n between SDS and e i t h e r PAA o r PAM. I n t e r a c t i o n s between s i m i l a r l y c h a r g e d s u r f a c t a n t and p o l y e l e c t r o l y t e are not common as e l e c t r i c a l e f f e c t s f r e q u e n t l y d o m i n a t e t o p r e v e n t any h y d r o phobic o r hydrogen bonding i n t e r a c t i o n . The h y d r o p h i l i c n a t u r e o f the amide d i p o l e o f p o l y a c r y l a m i d e s has been s u g g e s t e d (11) as a p o s s i b l e f a c t o r i n p r e v e n t i n g i n t e r a c t i o n w i t h sodium dodecylsulfate. A d s o r p t i o n o f SDS on H e m a t i t e . A d s o r p t i o n o f SDS on h e m a t i t e a t pH 4.1 i s i l l u s t r a t e d i n F i g u r e 3 as the z e r o p o l y m e r ( t o p l i n e f i l l e d c i r c l e s ) a d d i t i o n . The r e s u l t s i n d i c a t e t h r e e d i s t i n c t r e g i o n s o f a d s o r p t i o n and i n t h i s r e s p e c t a g r e e w e l l w i t h p r e v i o u s i n v e s t i g a t i o n s ( 1 3 , 14) o f s y s t e m s i n v o l v i n g a d s o r p t i o n o f an a n i o n i c s u r f a c t a n t f r o m an aqueous s o l u t i o n o f c o n s t a n t i o n i c s t r e n g t h onto a p o s i t i v e l y charged oxide s u r f a c e . F u e r s t e n a u and Raghavan (15) have summarized the p r o p o s e d mechanism' o f a d s o r p t i o n b y d e s c r i b i n g the t h r e e r e g i o n s o f d i f f e r i n g s l o p e . I n b r i e f , a t low SDS e q u i l i b r i u m c o n c e n t r a t i o n t h e a d s o r p t i o n p r o c ess i s d e s c r i b e d as b e i n g an e l e c t r o s t a t i c exchange o f s u r f a c t a n t i o n s f o r c o u n t e r i o n s i n t h e d o u b l e l a y e r . The i n c r e a s e d s l o p e o f the n e x t r e g i o n i s a t t r i b u t e d t o the o n s e t o f h e m i - m i c e l l e formation. A t h i g h SDS e q u i l i b r i u m c o n c e n t r a t i o n , t h e z e t a p o t e n t i a l i s o f s i m i l a r s i g n as the s u r f a c t a n t and e l e c t r o s t a t i c i n t e r a c t i o n s oppose s p e c i f i c i n t e r a c t i o n s r e s u l t i n g i n a r e d u c e d s l o p e o f the a d s o r p t i o n i s o t h e r m . A d s o r p t i o n o n H e m a t i t e W i t h P r e - A d s o r b e d PAA. The a d s o r p t i o n o f SDS on h e m a t i t e p a r t i c l e s w h i c h were p r e v i o u s l y f l o c c u l a t e d b y the a d d i t i o n o f PAA i s i l l u s t r a t e d i n F i g u r e 3. The a d s o r p t i o n o f SDS d e c r e a s e s a s i n c r e a s i n g amounts o f PAA o c c u p y s u r f a c e sites. The a d s o r p t i o n o f PAA on h e m a t i t e , w h i c h had been i n v e s t i g a t e d p r e v i o u s l y ( 1 6 ) , i s g i v e n i n F i g u r e 4 as a f u n c t i o n o f e q u i l i b r i u m PAA c o n c e n t r a t i o n f o r v a r i o u s pH v a l u e s . These r e s u l t s show t h a t the a d s o r p t i o n o f PAA i s c h a r a c t e r i z e d b y h i g h a d s o r p t i o n d e n s i t i e s a t low e q u i l i b r i u m c o n c e n t r a t i o n s . The p l a t e a u o r s u r f a c e s a t u r a t i o n v a l u e f o r pH 4.1 w o u l d be a p p r o x i m a t e l y 0.41-0.42 rag PAA per s q u a r e m e t e r . T h i s c o r r e s p o n d s a p p r o x i m a t e l y t o t h e h i g h e s t PAA a d d i t i o n shown i n F i g u r e 3, 0.44 mg PAA per s q u a r e m e t e r , where i t was d e t e r m i n e d t h a t 0.41 mg PAA

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p e r s q u a r e m e t e r had been a d s o r b e d . No r e s i d u a l p o l y m e r was measured i n s o l u t i o n f o r the 0.11 mg PAA a d d i t i o n . In g e n e r a l , the a d s o r p t i o n o f a s u r f a c t a n t on p a r t i c l e s w i t h p r e v i o u s l y a d s o r b e d polymer can be i n f l u e n c e d b y ( i ) a r e d u c t i o n o f s u r f a c e a r e a a v a i l a b l e f o r a d s o r p t i o n as a r e s u l t o f the p r e s e n c e o f a d s o r b e d p o l y m e r , ( i i ) p o s s i b l e i n t e r a c t i o n s between p o l y m e r and s u r f a c t a n t i n the b u l k s o l u t i o n o r i n t h e i n t e r f a c i a l r e g i o n ( t h a t i s , s u r f a c t a n t w i t h l o o p s , t a i l s o r t r a i n s o f ads o r b e d p o l y m e r m o l e c u l e s ) , ( i i i ) the s t e r i c e f f e c t o f a d s o r b e d polymer, p r e v e n t i n g approach o f s u r f a c t a n t molecules f o r adsorpt i o n at the s u r f a c e , o r ( i v ) p o s s i b l e e l e c t r o s t a t i c e f f e c t s i f p o l y m e r and/or s u r f a c t a n t are c h a r g e d s p e c i e s . S i n c e t h e r e was no e v i d e n c e o f s u r f a c t a n t / p o l y m e r intera c t i o n s , the two most i m p o r t a n t f a c t o r s a f f e c t i n g SDS a d s o r p t i o n on P A A - f l o c c u l a t e d h e m a t i t e p a r t i c l e s a r e b e l i e v e d t o be e l e c t r o s t a t i c e f f e c t s and a r e d u c t i o n i n the number o f s u r f a c e a d s o r p t i o n s i t e s due t o a d s o r b e d p o l y m e r s e g m e n t s . P o l y e l e c t r o l y t e a d s o r p t i o n on o p p o s i t e l y c h a r g e d s u r f a c e s can i n v o l v e s t r o n g a t t r a c t i v e f o r c e s (as e v i d e n c e d b y the h i g h a f f i n i t y isotherm) and i s e x p e c t e d t o r e s u l t i n a somewhat f l a t s u r f a c e c o n f i g u r a t i o n ( 1 7 ) . U s i n g the s u r f a c e s a t u r a t i o n v a l u e o f 0.41 mg PAA per s q u a r e meter and assuming a 25 s q u a r e a n g s t r o m PAA monomer segment s i z e , 87 pet o f the t o t a l s u r f a c e a r e a w o u l d be o c c u p i e d by p o l y m e r segments i f the segments were f r e e t o make c o n t a c t individually. In a c t u a l i t y , p o l y m e r segments a r e not f r e e t o make i n d i v i d u a l c o n t a c t w i t h the s u r f a c e b e c a u s e o f the n a t u r e o f the m o l e c u l a r c h a i n s t r u c t u r e . However, the a d s o r b e d amounts shown i n the a d s o r p t i o n i s o t h e r m s o f SDS on P A A - f l o c c u l a t e d hemat i t e i n d i c a t e t h a t the p o l y m e r c h a i n s may have a s i g n i f i c a n t number o f segments i n c o n t a c t w i t h the s u r f a c e , t h e r e b y r e d u c i n g t h e number o f s i t e s a v a i l a b l e f o r SDS a d s o r p t i o n . The e f f e c t o f polymer c h a r g e on the z e t a p o t e n t i a l i s g i v e n i n F i g u r e 5 where 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 y o f h e m a t i t e p a r t i c l e s f l o c c u l a t e d w i t h PAA and c o n d i t i o n e d i n SDS i s i n d i c a t e d a s a f u n c t i o n o f SDS a d s o r p t i o n d e n s i t y . The 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 h e m a t i t e p a r t i c l e s w i t h o u t p o l y m e r i n the p r e s e n c e o f SDS a r e a l s o shown i n F i g u r e 5 ( f i l l e d c i r c l e s ) . I n the p r e s e n c e o f the a n i o n i c s u r f a c t a n t , the e l e c t r o p h o r e t i c m o b i l i t y d e c r e a s e s w i t h i n c r e a s i n g SDS a d s o r p t i o n d e n s i t y , r e v e r s e s i n s i g n and becomes n e g a t i v e i n v a l u e at h i g h SDS a d s o r p t i o n d e n s i t i e s . T h i s b e h a v i o r i s t y p i c a l f o r a n i o n i c s u r f a c t a n t i n t e r a c t i o n w i t h an o p p o s i t e l y c h a r g e d s u r f a c e and has been o b s e r v e d b y o t h e r r e s e a r c h e r s ( 1 8 ) . A t pH 4.1 i n the a b s e n c e o f s u r f a c t a n t (and p o l y m e r ) , h e m a t i t e p a r t i c l e s are p o s i t i v e l y c h a r g e d and have a m o b i l i t y o f 3.9 e l e c t r o p h o r e t i c m o b i l i t y u n i t s . For the a d d i t i o n o f 0.11 mg PAA per s q u a r e m e t e r h e m a t i t e , t h e m o b i l i t y d e c r e a s e d a t SDS a d s o r p t i o n d e n s i t i e s l e s s t h a n 0.3 m i c r o m o l e s per s q u a r e m e t e r but r e m a i n e d p o s i t i v e i n v a l u e . Above t h i s a d s o r p t i o n d e n s i t y , n e g a t i v e m o b i l i t i e s were r e c o r d e d . At PAA a d d i t i o n s o f 0.22 and 0.44 mg per s q u a r e m e t e r h e m a t i t e ,

18.


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m o b i l i t y v a l u e s were n e g a t i v e f o r a l l SDS c o n c e n t r a t i o n s and became s l i g h t l y more n e g a t i v e w i t h i n c r e a s i n g s u r f a c t a n t a d s o r p tion. The p r e s e n c e o f PAA has a d r a m a t i c e f f e c t on the m o b i l i t y values, although surfactant adsorption a l s o contributed t o changes i n the m o b i l i t y , e s p e c i a l l y at PAA a d d i t i o n s l e s s t h a n 0.22 mg per s q u a r e m e t e r . Above SDS a d s o r p t i o n d e n s i t i e s o f 1-2 m i c r o m o l e s per s q u a r e m e t e r , the m o b i l i t i e s o b s e r v e d i n a l l t h e e x p e r i m e n t s were the same i n the absence o r p r e s e n c e o f p r e adsorbed polymer. The s u r f a c e c h a r g e o f t h e h e m a t i t e used h e r e was measured b y t i t r a t i o n (16) and found t o be a p p r o x i m a t e l y 18 m i c r o c o u l o m b s per s q u a r e m e t e r , e q u i v a l e n t t o a c h a r g e o f 5.40 χ 1 0 e s u per s q u a r e meter o f h e m a t i t e . Assuming one e q u i v a l e n t per monomer segment, t h e t o t a l c h a r g e o f PAA wou I d be 4.02 χ 1 0 esu p e r mg PAA f o r the t o t a l l y i o n i z e d p o l y m e r . A p p r o x i m a t e l y 0.11 mg PAA ( i f t o t a l l y i o n i z e d ) would be a l m o s t s u f f i c i e n t t o n e u t r a l i z e t h e c h a r g e o f one s q u a r e meter o f h e m a t i t e . The t r a n s i t i o n o f p o s i t i v e m o b i l i t y t o n e g a t i v e m o b i l i t y o c c u r s between 0.11 and 0.22 mg PAA per s q u a r e m e t e r h e m a t i t e , i n a c c o r d a n c e w i t h the amount o f p o l y m e r c h a r g e r e q u i r e d t o n e u t r a l i z e the s o l i d s u r f a c e charge. 8

9

A d s o r p t i o n on H e m a t i t e W i t h P r e - A d s o r b e d PAM. A d s o r p t i o n o f SDS on h e m a t i t e f l o c c u l a t e d w i t h PAM was i n v e s t i g a t e d w i t h 0.44 mg PAM per s q u a r e m e t e r h e m a t i t e . SDS a d s o r p t i o n d e n s i t y as a f u n c t i o n o f e q u i l i b r i u m SDS c o n c e n t r a t i o n i s shown i n F i g u r e 6 f o r the same c o n d i t i o n s as used i n the PAA/SDS s y s t e m . In t h e low SDS c o n c e n t r a t i o n r a n g e ( t h a t i s , l e s s t h a n 0.0003 M SDS), a d s o r p t i o n o f SDS was u n a f f e c t e d by the p r e s e n c e o f PAM. A t e q u i l i b r i u m c o n c e n t r a t i o n s g r e a t e r t h a n 0.0003 M SDS, adsorption o f SDS on the f l o c c u l a t e d p a r t i c l e s was r e d u c e d from t h a t on t h e unflocculated particles. A d s o r p t i o n o f PAM o n h e m a t i t e as a f u n c t i o n o f PAM s o l u t i o n c o n c e n t r a t i o n i s shown i n F i g u r e 7. A d s o r p t i o n o f PAM does not e x h i b i t the s t r o n g a f f i n i t y o b s e r v e d f o r PAA, a l t h o u g h the p l a t e a u o r s a t u r a t i o n v a l u e i s s i m i l a r and s l i g h t l y h i g h e r f o r PAM. These r e s u l t s s u g g e s t t h a t PAM a d s o r p t i o n may o c c u r i n a more l o o s e l y packed s u r f a c e c o n f i g u r a t i o n . Some p o l y m e r m o l e c u l e s can be r e g a r d e d t o m a i n t a i n t h e i r a p p r o x i m a t e s o l u t i o n c o n f o r m a t i o n upon a d s o r p t i o n (19). A d s o r p t i o n o f a n o n i o n i c polymer would l e a d t o a c o i l e d adsorbed p o l y m e r c o n f i g u r a t i o n w i t h a s m a l l number o f p o l y m e r segments i n a c t u a l c o n t a c t w i t h the s u r f a c e . The number o f s u r f a c e s i t e s a v a i l a b l e f o r s u r f a c t a n t a d s o r p t i o n would r e m a i n q u i t e l a r g e . At e q u i l i b r i u m s u r f a c t a n t c o n c e n t r a t i o n s o f l e s s t h a n 0.0003 M SDS where the h e m a t i t e s u r f a c e i s s t i l l p o s i t i v e l y c h a r g e d , a d s o r p t i o n o f s u r f a c t a n t f o l l o w s i t s n o r m a l p a t t e r n due t o t h e e l e c t r o s t a t i c f o r c e s w h i c h p r o v i d e the d r i v i n g f o r c e f o r a d s o r p tion. S u f f i c i e n t e f f e c t i v e s u r f a c e a r e a must be a v a i l a b l e f o r t h i s l e v e l o f SDS a d s o r p t i o n d e n s i t y . As s u r f a c t a n t a d s o r p t i o n

18.

GEBHARDT A N DFUERSTENAU

303


HEMATITE pH4.l, I Ο" M NaCl 3

mo PAM/m SOLID 2

10 -5

•

0

•

0.44

J_ JL 10" ioEQUILIBRIUM SDS CONCENTRATION, mole per d m

F i g u r e 6. E q u i l i b r u i u m a d s o r p t i o n d e n s i t i e s o f sodium s u l f o n a t e on h e m a t i t e a t pH 4.1 and

absence o f p r e - a d s o r b e d

10-2

3

4

and 0.001

M NaCl

polyacrylamide.

i n the

J

dodecylpresence

304


18.



305

increases with i n c r e a s i n g SDS concentration, the surface charge is reversed (see Figure 8), and a d d i t i o n a l surfactant adsorption i s reduced due to e l e c t r i c a l forces and also to a reduction of a v a i l a b l e surface area due to adsorbed PAM segments. No s i g n i f i cant change i n slope i s observed, and i t i s p o s s i b l e that the presence of adsorbed polymer prevents or reduces the e f f e c t o f hemi-micelle formation. The number of polymer segments i n contact with the surface can be estimated from Figure 6 at high SDS concentrations. It i s assumed that reduction i n sulfonate adsorption, f o r the nonionic polymer case, i s due only to the presence of adsorbed polymer segments. The amount of reduction i n SDS adsorption at high SDS concentrations i s taken (from Figure 6) to be an average of 50 percent, that i s the e f f e c t i v e area a v a i l a b l e f o r SDS adsorption is reduced by 50 percent. Using a monomer size of 25 square angstroms, a t o t a l o f 10.7 mg o f PAM would be required to occupy the area reduced which i n t h i s case i s 22.5 square meters ( t o t a l sample surface area was 45 square meters). Since the uptake o f PAM was an average of 18 mg PAM, i t i s estimated that, at the most, approximately 50 to 60 percent of the PAM segments are i n contact with the surface. Surface W e t t a b i l i t y . Contact angles on hematite at pH 4.1 were measured as a f u n c t i o n o f PAA and PAM concentration at constant surfactant concentration of 0.001 M SDS. The r e s u l t s are given i n Figures 9 and 10 f o r PAA and PAM, r e s p e c t i v e l y . The hematite disk was conditioned for 15 minutes i n a s o l u t i o n containing the polymer and then t r a n s f e r r e d to the SDS s o l u t i o n f o r contact angle measurement. The angles were measured a f t e r 15 minutes c o n d i t i o n i n g with SDS and were not observed to vary with longer c o n d i t i o n i n g times. With no polymer present, the bubble contact angle was high, i n d i c a t i v e o f a hydrophobic surface. It i s evident that adsorbed polymer caused a dramatic decrease i n the contact angle. No bubble contact, f o r the e q u i l i b r i u m angle, could be achieved above polymer s o l u t i o n concentrations of 2 ppm for e i t h e r PAA or PAM. Both polymers, being h y d r o p h i l i c i n character, prevent bubble attachment when a s u f f i c i e n t amount o f polymer has been adsorbed. These r e s u l t s p a r a l l e l f i n d i n g s by Somasundaran and coworkers (20, 21) that adsorption of a large h y d r o p h i l i c molecule masks any e f f e c t that adsorbed s u r f a c t a n t molecules may have on the surface w e t t a b i l i t y . A d d i t i o n a l contact angles were measured at 0.001 M SDS conc e n t r a t i o n . The hematite disk was conditioned i n the SDS s o l u t i o n before adding polymer to the s o l u t i o n . The contact angle was measured a f t e r adding s u f f i c i e n t q u a n t i t i e s of polymer f o r concentrations of 1, 3, and 5 ppm. For both PAA and PAM, the contact angle remained constant and i d e n t i c a l to the value obtained i n the absence o f polymer. No polymer was able to adsorb on the surfactant-coated d i s k . It can be concluded that whichever species, surfactant or polymer, adsorbed f i r s t was not


306

Ε :>

HEMATITE ρΗ4.Ι, lO-^MNoCI

+4

mg ΡΑ M/m

ο. Ε 4.

SOLID

2

Φ

+2

•

Ο

•

0.44

m ο 5 LU

-2

oc ο I Ο Ο (Τ

b

-4 OJOI

Figure 0.001

0.1 1.0 10 SULFONATE AOSORPTION DENSITY, micromoles per m*

8. E l e c t r o p h o r e t i c m o b i l i t y o f h e m a t i t e a t pH 4.1 M NaCl as a f u n c t i o n o f s u l f o n a t e

a b s e n c e and p r e s e n c e o f p r e - a d s o r b e d

adsorption

density

polyacrylamide.

and i n the

18.


F i g u r e 9. 4.1,

C o n t a c t a n g l e s on a h e m a t i t e d i s k a t 0.001

and 0.001

centration


M NaCl

i n which

307

M SDS, pH

as a f u n c t i o n o f t h e p o l y a c r y l i c a c i d con-

the h e m a t i t e d i s k had been

preconditioned.


308

100

1

Γ

HEMATITE pH 4.1, I0" M NoCI I*I0~M SDS 3

3

Ο Δ •

REC. EQUIL. ADV.

—o-L

PAM Figure 10. 4.1,

Contact

and 0.001

CONCENTRATION, mg per d m °

angles

on a hematite disk at 0.001

SDS pH

M NaCl as a f u n c t i o n o f the polyacrylamide

t r a t i o n i n which the hematite disk had

been

concen

preconditioned.

18.



r e p l a c e a b l e b y t h e o t h e r , a t l e a s t under conditions.

these

309

particular

Summary The a d s o r p t i o n b e h a v o r o f s u r f a c t a n t o n t o p a r t i c l e s i n t h e a b s e n c e and p r e s e n c e o f p r e - a d s o r b e d p o l y m e r was d e t e r m i n e d . E l e c t r o k i n e t i c s t u d i e s o f t h e s y s t e m were made. C o n t a c t a n g l e measurements y i e l d e d i n f o r m a t i o n on t h e l e v e l o f h y d r o p h o b i c i t y a c h i e v e d a t v a r i o u s a d d i t i o n s o f polymer and c o l l e c t o r . The r e s u l t s a r e summarized as f o l l o w s : 1. S u r f a c e t e n s i o n measurements i n d i c a t e d no b u l k i n t e r a c t i o n b e t w e e n t h e a n i o n i c s u r f a c t a n t and t h e a n i o n i c o r n o n i o n i c polymer. 2. The a d s o r p t i o n o f an a n i o n i c s u r f a c t a n t on a p o s i t i v e l y charged oxide s u r f a c e i s s i g n i f i c a n t l y reduced by t h e presence o f a pre-adsorbed a n i o n i c polymer. 3. The p r e s e n c e o f a p r e - a d s o r b e d n o n i o n i c polymer h a s a l m o s t n e g l i g i b l e e f f e c t s on s u r f a c t a n t a d s o r p t i o n e x c e p t a t h i g h s u r f a c t a n t c o n c e n t r a t i o n s where s u r f a c t a n t a d s o r p t i o n i s reduced. 4. E l e c t r o k i n e t i c s t u d i e s r e v e a l e d that the m o b i l i t i e s o f p a r t i c l e s w i t h p r e - a d s o r b e d a n i o n i c polymer i n t h e p r e s e n c e o f s u r f a c t a n t were c o n t r o l l e d b y t h e c h a r g e a s s o c i a t e d w i t h t h e p o l y m e r , w h i l e the m o b i l i t i e s were u n a f f e c t e d b y t h e p r e s e n c e of pre-adsorbed n o n i o n i c polymer. 5. C o n t a c t a n g l e measurements i n d i c a t e d t h a t c o n d i t i o n i n g w i t h i n c r e a s i n g amount o f p o l y m e r b e f o r e c o n d i t i o n i n g w i t h s u r f a c tant r e s u l t e d i n reduced s u r f a c e h y d r o p h o b i c i t y . Pre c o n d i t i o n i n g with s u r f a c t a n t r e s u l t e d i n a hydrophobic s u r f a c e w h i c h was n o t a f f e c t e d b y s u b s e q u e n t p o l y m e r additions. Acknowledgments The a u t h o r s w i s h t o acknowledge s u p p o r t o f t h i s r e s e a r c h b y t h e N a t i o n a l S c i e n c e F o u n d a t i o n and t h e Department o f t h e I n t e r i o r f o r a G r a n t t o t h e C a l i f o r n i a MMRRI, U n i v e r s i t y o f C a l i f o r n i a .

Literature Cited 1. "Fine Particles Processing"; Somasundaran, P., Ed.; Society of Mining Eng. of AIME: New York, 1980; 1865 pp. 2. "Benefication of Minerals Fines"; Somasundaran, P.; Arbiter, Ν., Eds.; American Institute of Mining & Metallurgical Engineers: New York, 1979; 406 pp. 3. Usoni, L.; Rinelli, G.; Ghigi, G. Proc. 8th International Mineral Processing Congress 1968, 14 pp. 4. Osborne, D. G. Trans. Instn. Min. Metal. 1978, 87, C189C193.

S T R U C T U R E / P E R F O R M A N C E R E L A T I O N S H I P S IN S U R F A C T A N T S

310

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16. 17. 18. 19. 20. 21. 22.

Balajee, S. R.; Iwasaki, I., Trans. A.I.M.E., 244, 401-406; 407-411 (1969). Somasundaran, P., Ph.D. Thesis, University of California, Berkeley, 1961. "CRC Handbook of Chemistry and Physics"; 60th Edition, 1981. Attia, Υ. Α.; Rubio, J. Br. Polymer J. 1975, 7, 135-138. Pradip, M. S. Thesis, University of California, Berkeley, 1977. Jones, J. H. J. Assoc. Official Agricultural Chem. 1945, 28, 398-409. Robb, I. D. in "Anionic Surfactants: Physical Chemistry of Surfactant Action"; Luscassen-Reynders, E. H., Ed.; Marcel Dekker, Inc.: New York, 1981; Chap. 3, pp. 109-142. Steinhardt, J . ; Reynolds, J. A. "Multiple Equilibria in Proteins" Academic Press: New York, 1969; 234 pp. Wakamatsu, T.; Fuerstenau, D. W., in ADVANCES IN CHEMISTRY SERIES Vol. 79, Gould, R. F., Ed.; 1968, pp. 161-172. Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90-96. Fuerstenau, D. W.; Raghavan, S. "Flotation: A.M. Gaudin Memorial Volume"; Fuerstenau, M. C., Ed.; American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.: New York, 1976: Vol. 1, pp. 21-65. Gebhardt, J. E., M.S. Thesis, University of California, Berkeley, 1979. Eirich, F. R. J. Colloid Interface Sci. 1977, 58, 423-435. Hunter, R. J. "Zeta Potential in Colloid Science"; Academic Press: New York, 1981. Rowland, F.; Bulas, R.; Rothstein, Ε.; Eirich, F. R. in "Chemistry and Physics of Interfaces"; Ross, S., Ed.; American Chemical Society: Washington, D.C., 1965. Somasundaran, P. J. Colloid Interface Sci. 1969, 31, 557565. Somasundaran, P.; Lee, L. T. Separation Sci. & Tech. 1981, 16, 1475-1490. Gebhardt, J. E.; Fuerstenau, D. W. Colloids and Surfaces 1983, 7, 221-231.

RECEIVED

February 3, 1984

The Effect of Preadsorbed Polymers on Adsorption of Sodium

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