Adsorption and Electrokinetic Effects of Amino Acids on Rutile and

19 Adsorption and Electrokinetic Effects of Amino Acids on Rutile and Hydroxyapatite 1

2

D. W. FUERSTENAU, S. CHANDER , J. LIN , and G. D. PARFITT

3

Downloaded by FUDAN UNIV on April 15, 2017 | http://pubs.acs.org Publication Date: May 21, 1984 | doi: 10.1021/bk-1984-0253.ch019

Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720

The mechanism of interaction of amino acids at solid/ aqueous solution interfaces has been investigated through adsorption and electrokinetic measurements. Isotherms for the adsorption of glutamic acid, proline and lysine from aqueous solutions at the surface of rutile are quite different from those on hydroxyapatite. To delineate the role of the electrical double layer in adsorption behavior, electrophoretic mobilities were measured as a function of pH and amino acid concentrations. Mechanisms for interaction of these surfactants with rutile and hydroxyapatite are proposed, taking into consideration the structure of the amino acid ions, solution chemistry and the electrical aspects of adsorption. Interest in the nature of interactions between shortchain organic surfactants and large molecular weight macromolecules and ions with hydroxyapatite extends to several fields. In the area of caries prevention and control, surfactant adsorption plays an important role in the initial states of plaque formation (1-5) and in the adhesion of tooth restorative materials (6). Interaction of hydroxyapatite with polypeptides in human urine is important in human biology as hydroxyapatite has been found as a major or minor component in a majority of kidney stones (7). Hydroxyapatite is used in column chromatography as a material for separating proteins (8-9). The flotation separation of apatite from 1

Current address: Mineral Processing Section, Department of Mineral Engineering, The Pennsylvania State University, University Park, PA 16802 Current address: Research Laboratory, IBM, San Jose, CA 95113 Current address: Department of Chemical Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213 2

3

0097-6156/84/0253-0311506.00/0 © 1984 American Chemical Society

Rosen; Structure/Performance Relationships in Surfactants ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


312

STRUCTURE/PERFORMANCE RELATIONSHIPS IN SURFACTANTS

gangue m i n e r a l s a s a n i m p o r t a n t i n d u s t r i a l o p e r a t i o n i n w h i c h s u r f a c t a n t s are used to e f f e c t the s e p a r a t i o n (10). Dewatering o f t h e c o l l o i d a l p h o s p h a t i c s l i m e s t h a t a r e g e n e r a t e d i n l a r g e quant i t i e s i n the p r o c e s s i n g of phosphate r o c k i s a major i n d u s t r i a l p r o b l e m w h i c h has b e e n s t u d i e d b y a number o f r e s e a r c h e r s i n r e c e n t years (11). O n l y a few s y s t e m a t i c s t u d i e s have been c a r r i e d out o n the mechanism o f i n t e r a c t i o n o f o r g a n i c s u r f a c t a n t s and m a c r o m o l e c u l e s . M i s h r a e t a l . (12) s t u d i e d t h e e f f e c t o f s u l f o n a t e s ( d o d e c y l ) , c a r b o x y l i c a c i d s ( o l e i c and t r i d e c a n o i c ) , and amines ( d o d e c y l and d o d e c y l t r i m e t h y l ) on 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 hydroxyapatite. V o g e l e t a l . (13) s t u d i e d t h e r e l e a s e o f p h o s p h a t e and c a l c i u m i o n s d u r i n g the a d s o r p t i o n o f benzene p o l y c a r b o x y l i c a c i d s onto a p a t i t e . J u r i a a n s e et a l . ( 1 4 ) a l s o observed a s i m i l a r r e l e a s e o f c a l c i u m and p h o s p h a t e i o n s d u r i n g t h e a d s o r p t i o n o f p o l y p e p t i d e s o n d e n t a l enamel. A d s o r p t i o n of polyphosphonate on h y d r o x y a p a t i t e and t h e a s s o c i a t e d r e l e a s e o f p h o s p h a t e i o n s was i n v e s t i g a t e d b y R a w l s e t a l . ( 1 5 ) . They f o u n d t h a t p h o s p h a t e i o n s were r e l e a s e d i n t o s o l u t i o n i n amounts e x c e e d i n g t h e q u a n t i t y o f phosphonate adsorbed. The p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n t o s t u d y t h e mechan i s m o f a d s o r p t i o n o f s e l e c t e d amino a c i d s i n o r d e r t o u n d e r s t a n d t h e i r i n t e r f a c i a l b e h a v i o r a t t h e h y d r o x y a p a t i t e s u r f a c e . The p r e s e n c e o f two o r more f u n c t i o n a l g r o u p s i n amino a c i d s g i v e r i s e to s u r f a c e p r o p e r t i e s w h i c h a r e q u i t e d i f f e r e n t f r o m t h e i n t e r f a c i a l p r o p e r t i e s f o r the a d s o r p t i o n of simple s u r f a c t a n t s , t h a t i s , t h o s e c o n t a i n i n g o n l y one c h a r g e d g r o u p . The a d s o r p t i o n b e h a v i o r of h y d r o x y a p a t i t e i s f u r t h e r complicated because of i t s partial solubility. A c c o r d i n g l y , t h e i n t e r f a c i a l p r o p e r t i e s were a l s o d e t e r m i n e d f o r T1O2 and compared w i t h t h o s e o f h y d r o x y a p a t i t e . P r o p e r t i e s o f Amino A c i d s Amino a c i d s a r e c h a r a c t e r i z e d b y t h e p r e s e n c e o f a d j a c e n t carboxy l i c (-CO^H) and amino (-NH) f u n c t i o n a l g r o u p s . The e q u i l i b r i u m constant t o r p r o t o n a t i o n or d i s s o c i a t i o n of these groups i s a f u n c t i o n o f t h e i r p o s i t i o n i n t h e amino a c i d m o l e c u l e . Therefore, w i d e l y d i f f e r i n g a c i d - b a s e p r o p e r t i e s o f amino a c i d s o c c u r , d e p e n d i n g upon t h e number o f f u n c t i o n a l g r o u p s and t h e i r r e l a t i v e p o s i t i o n i n the molecule. The d i s s o c i a t i o n c o n s t a n t s f o r v a r i o u s amino a c i d s u s e d i n t h i s i n v e s t i g a t i o n a r e g i v e n i n T a b l e I . The f i r s t d i s s o c i a t i o n c o n s t a n t f o r t h e -CO2H g r o u p i s t h e more a c i d i c g r o u p w i t h a pK o f 1.8 t o 2.4. T h i s g r o u p i n amino a c i d s i s s u b s t a n t i a l l y more a c i d i c t h a n a c e t i c a c i d , w h i c h has a pK = 4 . 7 6 due t o t h e l a r g e i n d u c t i v e e f f e c t o f t h e a d j a c e n t -NH^ g r o u p . The pK f o r t h e p r o t o n a t i o n o f t h e amino g r o u p has a v a l u e o f 9.0 t o 10.0 w h i c h i s s l i g h t l y l o w e r t h a n t h a t f o r t h e c o n j u g a t e a c i d o f m e t h y l a m i n e w i t h a_ pK o f 10.4. T h u s , t h e amino a c i d s h a v e a p o s i t i v e l y c h a r g e d "NIL. g r o u p a c i d i c p H s (pH < 2) a p o s i t i v e -NH~ and a n e g a t i v e - C 0 ~ g r o u p a t n e u t r a l pH's (3 < pH < 9) a n d +

T

9


Adsorption, Electrokinetic Effects of Amino Acids 313

FUERSTENAU ET AL.


Table I .

Stability

Amino A c i d

Glutamic acid

C o n s t a n t s o f Amino A c i d s

Formula

HOOC(CH ) CHCOOH 2

2

v\

ρκ

2.13

4.32

9.95

2.16

9.20

10.80

2

ρκ

3

NHLysine

Proline

H N(CH ) CHCOOH 2

2

4

H C 2ι

CH ι2

H C

CHCOOH

0

0

1.95

10.64



314

a negative -C0 g r o u p i n a l k a l i n e pH's (pH > 1 0 ) . In a d d i t i o n , one must c o n s i d e r d i s s o c i a t i o n o f o t h e r f u n c t i o n a l g r o u p s p r e s e n t as p a r t o f t h e amino a c i d . For example, g l u t a m i c a c i d c o n t a i n s a n o t h e r - C 0 w h i c h h a s a pK ( d i s s o c i a t i o n c o n s t a n t ) o f 4.32, and l y s i n e has a n o t h e r amine g r o u p w i t h t h e pK o f i t s c o n j u g a t e a c i d b e i n g 10.80. P r o l i n e , b e c a u s e o f i t s n i t r o g e n b e i n g p a r t o f t h e c y c l i c r i n g , has p K s o f 1.95 and 10.64. The c h a r g e o n v a r i o u s f u n c t i o n a l g r o u p s a s a f u n c t i o n o f pH i s s c h e m a t i c a l l y i l l u s t r a t e d in T a b l e I I . I t i s e v i d e n t t h a t a n amino a c i d z n o l e c u l e may h a v e s t r o n g l y c h a r g e d p o s i t i v e and n e g a t i v e s i t e s e v e n t h o u g h t h e m o l e c u l e has a n o v e r a l l c h a r g e n e u t r a l i t y . The p r e s e n t s t u d y shows t h a t t h i s z w i t t e r i o n i c c h a r a c t e r o f amino a c i d s p l a y s a n i m p o r t a n t r o l e i n the a d s o r p t i o n process. 2

2


f

E x p e r i m e n t a l M e t h o d s and M a t e r i a l s S y n t h e t i c h y d r o x y a p a t i t e p r e p a r e d b y m i x i n g s t o i c h i o m e t r i c amounts o f aqueous s o l u t i o n s o f c a l c i u m n i t r a t e and ammonium p h o s p h a t e was used i n t h i s s t u d y . The pH o f t h e b o i l i n g s u s p e n s i o n was m a i n t a i n e d a t a b o u t 10 b y f l o w i n g a m i x t u r e o f NH^ and N t h r o u g h o u t the p r e c i p i t a t i o n process. The p r e c i p i t a t e was r e p e a t e d l y washed u n t i l t h e c o n d u c t i v i t y o f t h e s u p e r n a t a n t l i q u i d was o b s e r v e d t o be c o n s t a n t . The washed s a m p l e was f r e e z e - d r i e d and a n a l y z e d . A n e l e m e n t a l a n a l y s i s o f t h e b a t c h p r e p a r a t i o n showed t h e Ca/P m o l a r t o b e r a t i o 1.64, w i t h t h e p r e d o m i n a n t i m p u r i t y b e i n g S i (0.12% S i 0 ) . The s a m p l e had a BET s u r f a c e a r e a o f 19.6 m / g and a d e n s i t y o f 2.96 g/cm . The x - r a y d i f f r a c t i o n p a t t e r n was s h a r p and c h a r a c t e r i s t i c o f s y n t h e t i c h y d r o x y a p a t i t e . "Analytical g r a d e " r e a g e n t s and C 0 - f r e e d o u b l e - d i s t i l l e d w a t e r were used throughout the i n v e s t i g a t i o n . The h y d r o x y a p a t i t e s u s p e n s i o n s f o r t h e a d s o r p t i o n s t u d i e s w e r e p r e p a r e d b y a d d i n g 0.25 g h y d r o x y a p a t i t e t o 25 m l o f 1 0 ~ M KNO^ s o l u t i o n ( u n l e s s o t h e r w i s e i n d i c a t e d ) and t h e r e q u i s i t e amount o f t h e amino a c i d . After equili b r a t i o n f o r one h o u r , t h e s o l i d s w e r e s e p a r a t e d i n a c e n t r i f u g e and t h e l i q u i d was a n a l y z e d f o r i t s e q u i l i b r i u m amino a c i d c o n c e n t r a t i o n by t h e m i n h y d r i n m e t h o d . The s u s p e n s i o n was m a i n t a i n e d a t 37 ± 0.5°C t h r o u g h o u t t h e a d s o r p t i o n e q u i l i b r a t i o n p e r i o d . 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 Zeta-Meter e l e c t r o phoresis apparatus. Lower c o n c e n t r a t i o n s o f KNO^ ( g e n e r a l l y 2 χ 10 M) w e r e u s e d i n t h e s e t e s t s t o a v o i d e x c e s s i v e com p r e s s i o n o f t h e e l e c t r i c a l d o u b l e l a y e r and t o m a i n t a i n t h e r m a l s t a b i l i t y of the suspension i n an e l e c t r i c a l f i e l d . 2

2

2

2

2

R e s u l t s and

Discussions

A d s o r p t i o n and E l e c t r o K i n e t i c B e h a v i o r o f R u t i l e . Isotherms f o r t h e a d s o r p t i o n o f l y s i n e , p r o l i n e and g l u t a m i c a c i d o n r u t i l e ( T i 0 ) a r e g i v e n i n F i g u r e 1. There i s no s i m p l e r e l a t i o n s h i p b e t w e e n t h e a d s o r p t i o n d e n s i t y and 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 . The a d s o r p t i o n does n o t obey t h e L a n g m i u r , F r e u n d l i c h o r S t e r n Grahame r e l a t i o n s h i p s . The l e v e l i n g - o f f o f t h e a d s o r p t i o n 2



2.13

α!

2.16

CZ3°!

Proline

+

Lysine

°d>

; cz>

EZ3-

+

4.32

!

pH

a:

- IZZh 9.95

0

9.20

10

0

12

ia

10.8

! C±|-Î CZ]

+

i n Amino A c i d s

! - LZZ3

S c h e m a t i c I l l u s t r a t i o n o f C h a r g e s on F u n c t i o n a l G r o u p s

Glutamic Acid

Table I I .



The rutile.

F i g u r e 1.

p r o l i n e on

isotherms

f o r a d s o r p t i o n of glutamic

acid, lysine


and

C/5

Η

5ί

η

C 70

C/3

"Ό

S

δ

r

η m ?ο m

>

70

Ο

τι

m

-ο

m

C *3

Η

c η

Η ?ο

C/3

ON


19.

FUERSTENAU ET AL.


i s o t h e r m s a t h i g h c o n c e n t r a t i o n s shows a n a p p a r e n t d e c r e a s e i n t h e f r e e energy o f a d s o r p t i o n w i t h i n c r e a s e i n c o n c e n t r a t i o n . This d e c r e a s e i n o v e r a l l a d s o r p t i o n e n e r g y may be a t t r i b u t e d t o r e p u l s i o n between t h e adsorbed m o l e c u l e s . Figure 2 further i l l u s t r a t e s the e l e c t r o s t a t i c n a t u r e o f i n t e r a c t i o n s between t h e r u t i l e s u r f a c e and t h e amino a c i d s . A t pH's b e l o w t h e PZC o f r u t i l e (pH 6.7) t h e s o l i d i s p o s i t i v e l y c h a r g e d and i t i s n e g a t i v e l y c h a r g e d a t h i g h e r pH's. G l u t a m i c a c i d w h i c h h a s two n e g a t i v e c h a r g e s b e t w e e n pH 4.3 and 10.0 a d s o r b s m a i n l y a t t h e p o s i t i v e s u r f a c e . Adsorption t h r o u g h t h e amine g r o u p o n t h e n e g a t i v e s u r f a c e i s much w e a k e r , p e r h a p s b e c a u s e o f t h e r e p u l s i o n b e t w e e n t h e two n e g a t i v e l y c h a r g e d c a r b o x y l a t e g r o u p s and t h e n e g a t i v e s u r f a c e . L y s i n e a d s o r b s o n l y t h r o u g h 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 a l s o , and t h e r e i s no i n d i c a t i o n of a d s o r p t i o n through c a r b o x y l i c groups. P r o l i n e e x h i b i t s a v e r y complex b e h a v i o r , however. A l t h o u g h t h e u p t a k e o f p r o l i n e c a n be c o n s i d e r e d t o i n v o l v e s p e c i f i c a d s o r p t i o n , t h e r e s u l t s can be i n t e r p r e t e d by an e l e c t r o s t a t i c model i f r e o r i e n t a t i o n o f t h e s u r f a c t a n t molecules i s taken i n t o c o n s i d e r a t i o n . A s compared t o g l u t a m i c a c i d and l y s i n e , p r o l i n e m o l e c u l e s c o n t a i n o n l y two f u n c t i o n a l groups. A t a p o s i t i v e s u r f a c e t h e a d s o r p t i o n could occur t h r o u g h t h e c a r b o x y l a t e group, whereas a t a n e g a t i v e s u r f a c e t h e m o l e c u l e a d s o r b s t h r o u g h t h e amine g r o u p . Stronger r e p u l s i o n between t h e i d e n t i c a l l y c h a r g e d g r o u p s i n t h e a d s o r b i n g m o l e c u l e and the s u r f a c e i s perhaps t h e r e a s o n f o r t h e r e l a t i v e l y lower a d s o r p t i o n d e n s i t y o f p r o l i n e compared t o g l u t a m i c a c i d . The e l e c t r o p h o r e t i c b e h a v i o r o f T i C ^ i n g l u t a m i c a c i d , l y s i n e and p r o l i n e s o l u t i o n s a r e g i v e n i n F i g u r e s 3 , 4 and 5, r e s p e c t i v e l y . B e l o w t h e PZC o f T i 0 (pH < 6.7) a d s o r p t i o n o f g l u t a m i c a c i d makes 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 more n e g a t i v e a s a n t i c i p a t e d . A t pH s g r e a t e r t h a n t h e PZC (pH > 6.7) a weak a d s o r p t i o n t h r o u g h amine g r o u p s makes 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 e v e n more n e g a t i v e b e c a u s e f o r e a c h o f t h e g l u t a m i c a c i d m o l e c u l e a d s o r b e d , two c a r b o x y l a t e g r o u p s a r e a t t a c h e d t o t h e s u r f a c e . A t pH's n e a r 10 o r h i g h e r , t h e a d s o r p t i o n o f amine g r o u p s c e a s e s b e c a u s e t h e y h y d r o l y z e t o t h e n e u t r a l form. Therefore a l l e l e c t r o p h o r e t i c m o b i l i t y c u r v e s merge a t pH 1 0 . L y s i n e does n o t e x h i b i t any s i g n i f i c a n t i n f l u e n c e o n 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 a t pH v a l u e s b e l o w t h e PZC o f T i O ^ . A t h i g h e r pH's, 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 s l i g h t l y i n c r e a s e s b e c a u s e two amine g r o u p s a r e a d s o r b e d f o r e a c h a d s o r b e d m o l e c u l e . Proline makes 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 s l i g h t l y more n e g a t i v e i n t h e i n t e r m e d i a t e pH r a n g e . A p p a r e n t l y , c a r b o x y l a t e i o n s i n t h e a d s o r b i n g molecules a r e o r i e n t e d i n an outword d i r e c t i o n such t h a t they make 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 more n e g a t i v e e v e n t h o u g h t h e proline molecule i s o v e r a l l neutral. 2

f

A d s o r p t i o n and E l e c t r o k i n e t i c B e h a v i o r o f H y d r o x y a p a t i t e . The a d s o r p t i o n d e n s i t i e s o f g l u t a m i c a c i d and l y s i n e o n h y d r o x y a p a t i t e a r e shown i n F i g u r e s ^ 6 and 7. The change i n s l o p e o f t h e a d s o r p t i o n i s o t h e r m a t 1 0 ~ M g l u t a m i c a c i d i s c o n s i d e r e d t o b e due t o a


STRUCTURE/ PERFORMANCE RELATIONSHIPS IN SURFACTANTS


318

Figure 2. lysine

The e f f e c t of pH on the adsorption of glutamic a c i d ,

and p r o l i n e on r u t i l e .


FUERSTENAU ET AL.



19.



F i g u r e 4. of

The

e f f e c t of l y s i n e on

the e l e c t r o p h o r e t i c

mobility

rutile.


FUERSTENAU ET AL.



19.

F i g u r e 5. of

The e f f e c t o f p r o l i n e on t h e e l e c t r o p h o r e c t i c

mobility

rutile.



STRUCTURE/ PERFORMANCE RELATIONSHIPS IN SURFACTANTS

F i g u r e 6.

The

isotherm

f o r a d s o r p t i o n of glutamic

a c i d on

hydroxyapatite.


FUERSTENAU ET AL.



19.

Figure 7. The e f f e c t of pH on the a d s o r p t i o n of glutamic a c i d and l y s i n e on hydroxyapatite.




324

change i n t h e a d s o r p t i o n mechanism. A t l o w c o n c e n t r a t i o n s , we propose t h a t the g l u t a m i c a c i d adsorbs whereas a t h i g h concent r a t i o n the system undergoes c h e m i c a l r e a c t i o n i n v o l v i n g the c a l c i u m i o n s a t t h e s u r f a c e . T h i s h y p o t h e s i s may e x p l a i n t h e comp l e x e l e c t r o k i n e t i c b e h a v i o r o f h y d r o x y a p a t i t e shown i n F i g u r e 8. At low c o n c e n t r a t i o n of g l u t a m i c a c i d , the e l e c t r o p h o r e t i c m o b i l i t y becomes more n e g a t i v e a s w o u l d b e e x p e c t e d i f g l u t a m i c a c i d adsorbs. At higher concentrations, a chemical r e a c t i o n occurs w i t h the s u r f a c e becoming p o s i t i v e l y charged, a p p a r e n t l y because of a h i g h c o n c e n t r a t i o n of coadsorbed calcium i o n s . Further s t u d i e s a r e r e q u i r e d t o c o m p l e t e l y understand t h i s h i g h l y complex system. The a d s o r p t i o n o f b o t h g l u t a m i c a c i d and l y s i n e i s a l m o s t a n order of magnitude s m a l l e r on h y d r o x y a p a t i t e than i t i s on TiO^ (compare F i g u r e s 2 and 7 ) . G l u t a m i c a c i d a d s o r b s e v e n a t pH 7, s u g g e s t i n g t h a t the a d s o r p t i o n can o c c u r i n d i f f e r e n t o r i e n t a t i o n s of the g l u t a m i c a c i d i o n . L y s i n e adsorbs o n l y a t a n e g a t i v e s u r f a c e t h r o u g h t h e amine g r o u p s and a d s o r p t i o n c e a s e s a t pH 10 b e c a u s e t h e amine g r o u p s h y d r o l y z e . The i n f l u e n c e o f l y s i n e 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 of h y d r o x y a p a t i t e i s presented i n F i g u r e 9. The m o b i l i t y becomes p o s i t i v e b e c a u s e o f t h e p r e s e n c e o f two p o s i t i v e l y c h a r g e d amine g r o u p s f o r e a c h a d s o r b i n g i o n . Concluding

Discussion

The a d s o r p t i o n o f amino a c i d s o n r u t i l e and h y d r o x y a p a t i t e e x h i b i t s some c h a r a c t e r i s t i c s o f s p e c i f i c a d s o r p t i o n . The r e s u l t s c a n b e i n t e r p r e t e d i n terms o f e l e c t r o s t a t i c m o d e l s o f a d s o r p t i o n , however, i f r e o r i e n t a t i o n of adsorbed molecules i s taken i n t o consideration. The e l e c t r o k i n e t i c b e h a v i o r o f h y d r o x y a p a t i t e i n g l u t a m i c a c i d i s c o m p l i c a t e d because of a chemical r e a c t i o n , possibly involving calcium i o n s . The s t u d y shows t h a t i t i s necessary to take i n t o c o n s i d e r a t i o n the o r i e n t a t i o n of adsorbed molecules, p a r t i c u l a r l y f o r z w i t t e r i o n i c surfactants. Acknowledgments The a u t h o r s w i s h t o a c k n o w l e d g e 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 N a t i o n a l I n s t i t u t e o f H e a l t h , G r a n t NIH ROI DE 03708-06 f o r the support of t h i s r e s e a r c h .




FUERSTENAU ET AL.




326

F i g u r e 9. The

e f f e c t of l y s i n e on the 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

hydroxyapatite.


19.

FUERSTENAU ET AL.



Literature Cited 1. Hay, D. I. Arch. Oral Biol. 1967, 12, 937. 2. Francis, M. D. Calif. Tissue Res. 1969, 3, 151. 3. Anbar, M.; St. John, G. Α.; Elward, T. E. J. Dental Res. 1974, 53, 1240. 4. Quintana, R. P. in "Applied Chemistry at Protein Interface"; Baier, R. Ε., Ed.; ACS SYMPOSIUM SERIES No. 145, American Chemical Society: Washington, D. C., 1975; p. 290. 5. Bartels, T.; Schudhof, J . ; Arends, J. J. Dentistry 1979, 7, 221. 6. Farley, E. P.; Jones, R. L.; Anbar, M. J. Dental Res. 1977, 56, 943. 7. Malek, R. S.; Boyce, W. H. J. Urol. 1977, 117, 336. 8. Bernardi, G.; Kawasaki, T. Biochem. Biophys. Acta 1968, 160, 301. 9. Bernardi, G. in "Methods in Enzymology"; Hirs, Ch. W.; Timasheff, S. Ν., Eds.; Academic: New York, 1972; Vol. 27, p. 471. 10. Smith, P. R., Jr., in "Flotation, A. M. Gaudin Memorial Volume"; Fuerstenau, M. C., Ed.; AIME: New York, 1976; Vol. 2, p. 1265. 11. Nagraj, D. R.; McAllister, L.; Somasundaran, P. Int. J. Mineral Processing 1977, 4, 111. 12. Mishra, R. K.; Chander, S.; Fuerstenau, D. W. Colloids and Surfaces 1980, 1, 105. 13. Vogel, J. C.; Frank, R. M. J. Colloid Interface Sci. 1981, 83, 26. 14. Juriaanse, A. C.; Arends, J.; Tenbasch, J. J. J. Colloid Interface Sci. 1980, 76, 212. 15. Rawls, H. R.; Bartels, T.; Arends, J. J. Colloid Interface Sci. 1982, 87, 339. RECEIVED January 20, 1984


Adsorption and Electrokinetic Effects of Amino Acids on Rutile and

Recommend Documents