Chapter 6 Electron Spin Resonance Studies Of Clay Minerals

139 Chapter 6

ELECTRON S P I N RESONANCE STUDIES OF CLAY EINERALS Thomas J . P I N N A V A I A Department o f Chemistry, M i c h i g a n S t a t e U n i v e r s i t y , East Lansing, M i c h i g a n 48824, USA. 6.1

INTRODUCTION I n r e c e n t y e a r s , e l e c t r o n s p i n resonance ( e s r ) s p e c t r o s c o p y has proven t o be

a powerful t o o l i n studies o f c l a y mineral chemistry.

The o r i e n t a t i o n s , dynamics,

and r e a c t i o n s o f a v a r i e t y o f i n t e r c a l a t e d paramagnetic s p e c i e s have been i l l u c i d a t e d b y e s r s p e c t r o s c o p y . The paramagnetic c e n t e r s on t h e basal s u r f a c e s C U ( H ~ O2+ ) ~, C U ( H * O ) ~ ~ +VO(H20),2+) , o r they may be s i m p l e h y d r a t e d c a t i o n s (*, may be metal complexes such as Cu(en)2

.

2+ , Cu(phen)3 2+ . Adsorbed o r g a n i c r a d i c a l s ,

such as t h e p e r y l e n e c a t i o n r a d i c a l o r o r g a n i c m o l e c u l e s c o n t a i n i n g t h e paramagnetic n i t r o x i d e m o i e t y ( = N - O ) , a l s o l e n d themselves t o s t u d y by e s r s p e c t r o s c o p y . The 3+ e s r s p e c t r a o f c e r t a i n t r a n s i t i o n m e t a l i o n s ( p a r t i c u l a r l y Fe ) which s u b s t i t u t e f o r aluminum o r s i l i c o n i n t h e oxygen framework can p r o v i d e u s e f u l i n f o r m a t i o n on t h e n a t u r e o f thermal processes and t h e s t a t e o f o r d e r o r d i s o r d e r o f t h e c l a y s t r u c t u r e b e i n g probed. The i n t e n t o f t h e p r e s e n t paper i s t o p r o v i d e some r e c e n t examples o f t h e k i n d o f i n f o r m a t i o n t h a t can be o b t a i n e d t h r o u g h t h e a p p l i c a t i o n o f e s r t o t h e study o f c l a y minerals.

Several r e v i e w a r t i c l e s on t h e a p p l i c a t i o n s o f e s r

s p e c t r o s c o p y t o c l a y m i n e r a l s have appeared r e c e n t l y , w h i c h complement t h e p r e s e n t work ( H a l l , 1980a, 1980b; HcBride, 1980; P i n n a v a i a , 1980).

Two e a r l i e r r e v i e w s

by Che e t a l . (1974) and by P i n n a v a i a (1976a) a r e a l s o a v a i l a b l e .

Space l i m i t a t i o n s

do n o t a l l o w f o r an adequate t r e a t m e n t o f e s r t h e o r y , b u t s e v e r a l e x c e l l e n t t r e a t i s e s a r e a v a i l a b l e (Wertz and B o l t o n , 1972; Abragam and Bleaney, 1970; Ingram, 1967)

.

HYDRATED EETAL I O N S ON BASAL SURFACES

6.2 6.2.1

Copper( 11)

Clementz e t a l . (1973) addressed t h e q u e s t i o n o f m e t a l i o n o r i e n t a t i o n on t h e i n t e r l a m e l l a r s u r f a c e s o f s m e c t i t e c l a y s c o n t a i n i n g a r e s t r i c t e d number o f w a t e r layers.

The c o p p e r ( I 1 ) i o n was s e l e c t e d as an i d e a l e s r probe, i n p a r t , because

i t has a s i n g l e u n p a i r e d e l e c t r o n w i t h a s p i n S = 1/2.

A l s o , under c o n d i t i o n s

where t h e m e t a l i o n i s s o l v a t e d by one, two o r t h r e e m o l e c u l a r l a y e r s o f w a t e r , t h e i o n i s expected t o possess t e t r a g o n a l symmetry and t o g i v e r i s e t o an a n i s o tropic esr signal.

I n t h e absence o f any i n t e r a c t i o n s between t h e e l c t r o n s p i n

and any n e i g h b o r i n g n u c l e a r s p i n s , t h e s p i n - H a m i l t o n i a n f o r t h e i o n under t e t r a g o n a l

140

symmetry may be w r i t t e n as cos

s,

+ g1 s i n 6 S z ) t h e Bohr magneton (eh/2mc), g I 6

H i s t h e magnetic f i e l d , and

(1 1

I

and g l a r e

spectroscopic s p l i t t i n g f a c t o r s ,

i s the angle between t h e magnetic f i e l d d i r e c t i o n and t h e symmetry a x i s of t h e tetragonal i o n , which i s a r b i t r a r i l y defined along z . Since two g tensors appear i n the Hamiltonian, two resonance components will appear in t h e e s r spectrum, one corresponding t o spin quantization i n a d i r e c t i o n p a r a l l e l t o t h e symmetry a x i s ( g ) and another corresponding t o spin quantizaII t i o n perpendicular t o t h e symmetry a x i s .(g ) . I f we allow f o r coupling between t h e S = 1/2 electron spin and t h e I = 3/2 nuclear spin of t h e copper nucleus, then two more t e r n s must be added t o t h e spin Hamiltonian: 6

1

A / lSZIZ

+

A 1(SXIX+ SYIY)

where A1 I and A

L are

(2)

hyperfine s p l i t t i n g c o n s t a n t s , usually expressed in cm-l o r

in gauss. I n t h e presence of a n applied magnetic f i e l d , t h e r e f o r e , t h e S = 1 / 2 ground s t a t e i s s p l i t by an amount gBH i n t o two energy s t a t e s (corresponding t o quant i z e d o r i e n t a t i o n s of t h e e l e c t r o n spin components i n a d i r e c t i o n p a r a l l e l (!Is= -1/2) or a n t i p a r a l l e l ( M s = 1/2) t o t h e magnetic f i e l d . These two s t a t e s a r e s p l i t f u r t h e r by A h / 2 due t o coupling of the e l e c t r o n spin with t h e four quantized components of t h e I = 3/2 nuclear spin ( M I = +3/2, + 1 / 2 ) . The energy level diagram i s i l l u s t r a t e d i n F i g u r e 6.1. The allowed t r a n s i t i o n s correspond t o AM^ = 0 , nMI = 1 . T h u s , we see t h a t both t h e g I I and g1 resonance components a r e s p l i t i n t o q u a r t e t s due t o hyperfine coupling. 0

From equation 1 we may conclude t h a t when 6 = o , only the g , I resonance component will be observed, a n d when e = 90 , only t h e g component will be observed. I n a random powder sample, however, a l l possible values of 6 occur, and both resonance components will be seen. Figures 6.2A and 6.28 i l l u s t r a t e t h e e s r s p e c t r a of a powder sample of Cu2'-hectorite under conditions where a s i n g l e molecular l a y e r of water occupies t h e i n t e r l a y e r s (dool = 12.4 A ) . As expected, both g II and 4 1 resonance components occur with g 1I = 2.34, A = 0.0165 cm-l ' g1 = 2-08. A i s too small t o be resolved. Figures 6.2C and 6.2D show the esr s p e c t r a o f an 0

L 0

L

oriented film sample of Cu2+-hectorite with t h e magnetic f i e l d d i r e c t i o n oriented I I and L t o t h e s i l i c a t e s h e e t s . Since g i s observed f o r t h e I I o r i e n t a t i o n and 91 I i s observed f o r the o r i e n t a t i o n , we may conclude t h a t t h e symmetry a x i s of the planar C U ( H ~ O ) ~ion ~ + i s oriented a t 90' t o the plane of t h e s i l i cate sheets.

1

2+ .

1

When Cu i s p a r t of a two-water l a y e r system a s in f u l l y hydrated Cu v e r m i c u l l i t e (dool = 14.2 A ) , a n i s o t r o p i c spectra a r e observed f o r oriented film 2+

0

14 1

I f 112)

Figure 6.1.

Energy level diagram for C u 2+

QII

Figure 6 . 2 .

-H

Esr spectra o f Cu2+-hectorite: A, B are for random powders; C , D are for oriented film.

142 1 0.0145 cm- ) , b u t t h e s p e c t r a a r e independent of sample o r i e n t a t i o n in t h e magnetic f i e l d . This means t h a t t h e symmetry samples ( g 1

1

=

2.38, g

=

2.16, A

II

=

~ + i s oriented near 45' t o the s i l i c a t e a x i s of t h e i n t e r c a l a t e d C U ( H ~ O ) ~ion 2t s h e e t s . However, when t h e Cu(H20l6 ion i s doped i n t o t h e three-water l a y e r hydration s t a t e of MgZt-hectorite ( d o o l = 15.0 t h e o r i e n t a t i o n dependent spectra shown i n Fiqure 6 . 3 a r e observed (McBride e t a l . , 1975a). From t h e observed

i),

o r i e n t a t i o n dependence, i t may be concluded t h a t C U ( H ~ O ) ~i ~s +oriented on t h e basal surfaces with t h e symmetry a x i s near 90 sheets.

t o the plane of t h e s i l i c a t e

Figure 6.4 summarizes the d i s t i n c t o r i e n t a t i o n of t h e Cu2+ ions hydrated by one, two, and t h r e e l a y e r s of i n t e r l a m e l l a r water. When the i n t e r l a y e r s of Cu2+0

smectites a r e f u l l y swollen with water ( d o o l = 21 A ) , q u i t e a d i f f e r e n t p i c t u r e emerges from the e s r spectrum. Under these l a t t e r conditions, a s i n g l e i s o t r o p i c l i n e i s observed, s i m i l a r t o t h e resonance found f o r C U ( H ~ O ) ~ i ~n + d i l u t e aqueous s o l u t i o n . The averaging of g l I and g may a r i s e from two very d i f f e r e n t processes: ( 1 ) rapid tumbling of t h e ion may be occurring i n a highly mobile l i q u i d - l i k e i n t e r l a y e r environment o r ( 2 ) a dynamic Jahn-Teller e f f e c t may be occurring in a r i g i d , i c e - l i k e arrangement of water molecules in t h e i n t e r l a y e r regions. The Jahn-Teller dynamic e f f e c t , which involves rapid interchange of t h e t h r e e principal axes of the C U ( H ~ O ) ~ion ~ + through coupling of the v i b r a t i o n modes of t h e aquo l i g a n d s , i s responsible f o r t h e i s o t r o p i c e s r l i n e observed f o r C U ( H ~ O ) in ~ ~frozen + aqueous s o l u t i o n s ( H u d s o n , 1966). As we s h a l l see 2+ and V02+ ions i n s m e c t i t e , t h e ions l a t e r , based on e s r s t u d i e s of hydrated Mn do i n f a c t tumble rapidly in a s o l u t i o n - l i k e environment when t h e i n t e r l a y e r s a r e swollen with m u l t i p l e l a y e r s of water.

1

6.2.2

Vanadyl

, V02+

VO(H20)62+-hectorite in t h e f u l l y wetted s t a t e e x h i b i t s t h e blue c o l o r charact e r i s t i c of t h e ion i n aqueous s o l u t i o n . However, under c e r t a i n conditions of loading and hydration s t a t e , t h e blue color i s l o s t and a tan-brown c o l o r develops, indicating t h a t a surface r e a c t i o n takes place which depends on moisture content. The surface reactions have been investigated in p a r t by Pinnavaia e t a l . (1974) and by HcBride (1979a). The hydrated vanadyl ion has a d 1 e l e c t r o n i c configuration and tetragonal sym2+ metry. I t s e s r p r o p e r t i e s resemble those of C U ( H ~ O ) ~. Under normal conditions, t h e ion e x h i b i t s g and gL resonances with both resonance components being s p l i t i n t o e i g h t hyperfine l i n e s due t o coupling of t h e S = 1 / 2 e l e c t r o n spin w i t h t h e I = 7 / 2 nuclear s p i n . However, when dissolved in water, t h e ion e x h i b i t s only a time-averaged i s o t r o p i c l i n e due t o rapid tumbling which averages t h e g I I and 91 components. Fully wetted V02+-hectorite a l s o gives a n i s o t r o p i c spectrum s i m i l a r

Figure 6.3.

Esr spectra o f Cu2+ doped i n t o Plg*+-hectorite film with H p a r a l l e l ( A ) and perpendicular ( B ) t o s i l i c a t e . sheets (from HcBride e t a1 . , 1975a).

144

Figure 6.4.

Orientations of i n t e r l a y e r aquo copper( 11) ions formed by hydration with one, two, and t h r e e l a y e r s of water. Open c i r c l e s a r e oxygen atoms of t h e s i l i c a t e sheet and 1 igating water molecules (from Pinnavaia, 1976b).

145 t o t h e aqueous s o l u t i o n spectrum. l i k e spectrum i s r e t a i n e d when VO (see F i g u r e 6.5)

2+

McBride (1979) has found t h a t t h e s o l u t i o n i s doped i n t o M g 2 + - h e c t o r i t e a t t h e 50% l e v e l

However, when t h e i o n i s doped i n t o M g 2 + - h e c t o r i t e a t t h e 5%

l e v e l , a tan-brown c o l o r develops and an a n i s o t r o p i c spectrum i s observed w h i c h i s i n d i c a t i v e o f a h i g h l y o r d e r e d , immobile form.

FIcBride has suggested t h a t

t h i s o r d e r e d tan-brown f o r m o f vanadyl i s VO(OH)2(H30)3.

S i n c e t h e spectrum i s

o r i e n t a t i o n independent o f t h e m a g n e t i c f i e l d , t h e symnetry a x i s appears t o be i n c l i n e d n e a r 45

0

t o t h e s i l i c a t e sheets.

A i r - d r i e d V 0 2 + - h e c t o r i t e g i v e s t h e o r i e n t a t i o n - d e p e n d e n t e s r s p e c t r a shown i n F i g u r e 6.6. S i n c e A l

A

I

(704 G) i s observed f o r t h e p e r p e n d i c u l a r o r i e n t a t i o n and

G ) i s observed f o r t h e p a r a l l e l o r i e n t a t i o n , t h e symmetry a x i s , w h i c h i s

1(81

colinear with the

V=O

bond, l i e s p e r p e n d i c u l a r t o t h e s i l i c a t e sheets.

The above s t u d i e s have focused on t h e o r i e n t a t i o n o f m e t a l i o n s on t h e basal surfaces.

E s r s t u d i e s have a l s o proven t o be u s e f u l i n c h a r a c t e r i z i n g mixed

Nat-Cu2+ and m i x e d NR4+-Cu2+ s m e c t i t e systems (McBride, 1976a; N c B r i d e and M o r t l a n d , 1975).

The e s r s p e c t r a o f Cu2+ on reduced charge m o n t m o r i l l o n i t e

and on k a o l i n i t e s u r f a c e s have a l s o been examined ( N c B r i d e and t l o r t l a n d , 1974; McBride, 1976b; Clementz e t a l . ,

1974).

MOBILITY OF INTERLAYER METAL IONS

6.3 6.3.1

Manganese( II)

As n o t e d e a r l i e r , Cu2+ i s n o t an i d e a l probe f o r examining i n t e r l a y e r m o b i l i t y because e i t h e r dynamic J a h n - T e l l e r e f f e c t s o r r a p i d t u m b l i n g can l e a d t o a v e r a g i n g Of

91 I and g

effect

components. L does n o t a p p l y and

However, f o r !In2+ and V02+, t h e dynamic J a h n - T e l l e r b o t h i o n s a r e s u i t a b l e probes f o r t h i s purpose, t h e

e s r t h e o r y h a v i n g been w e l l developed ( B u r l a m a c c h i , 1971 ; Burlamacchi e t a l . , 1970, 1973; G a r r e t t and Morgan, 1966; Campbell and Hanna, 1976). 5 The h y d r a t e d $ln(H20)62+ i o n has a h i g h s p i n d e l e c t r o n i c c o n f i g u r a t i o n .

In

most environments, a l l t h r e e g - t e n s o r components a r e equal and i s o t r o p i c s p e c t r a a r e observed.

Because o f i t s i s o t r o p i c n a t u r e , !h2+i s n o t w e l l s u i t e d f o r e s r

s t u d i e s o f m e t a l i o n o r d e r i n g on c l a y s u r f a c e s , b u t t h e e s r l i n e w i d t h s may be readily related t o ion mobility. The s p e c t r a o f Mn2+ i n f u l l y h y d r a t e d forms o f m o n t m o r i l l o n i t e , h e c t o r i t e , v e r m i c u l i t e and n o n t r o n i t e resemble t h e spectrum o f t h e i o n i n homogeneous s o l u t i o n (McBride e t a l . ,

1975b).

I n each case, s i x h y p e r f i n e l i n e s a r e observed due

t o c o u p l i n g o f t h e S = 5 / 2 e l e c t r o n s p i n w i t h t h e I = 5/2 n u c l e a r s p i n .

Each

h y p e r f i n e component c o n s i s t s o f t h r e e superimposed L o r e n t z i a n l i n e s a r i s i n g f r o m f i v e AM,

= 1 transitions.

I n t h e absence o f inhonogeneous l i n e broadening e f f e c t s ,

t h e l i n e w i d t h s (AH) a r e t h e sum o f t w o c o n t r i b u t i o n s AH = AHI

+

AHD

(3)

146

200 GAUSS c------1 H -----t

I

F i g u r e 6.5.

VO*+/ M$'-

Hect.

E s r s p e c t r a o f V02+ i n aqueous s o l u t i o n M, pH = 1.5) and i n a 50:50 (2 x VO2+/Mg2+-Kectorite f i l m ( f r o m McBride, 1979a).

147 where AHI

i s t h e i n t r i n s i c l i n e w i d t h due t o c o l l i s i o n a l r e l a x a t i o n processes and

nHD i s t h e l i n e w i d t h due t o d i p o l a r i n t e r a c t i o n s between n e i g h b o r i n g Mn2+ i o n s ( H i n c k l e y and Morgan, 1966). The AHD t e r m i s c o n c e n t r a t i o n dependent because t h e d i p o l a r i n t e r a c t i o n s a r e p r o p o r t i o n a l t o r-3, where r i s t h e average Hn2+ Mn2+ d i s t a n c e .

11,

In d i l u t e s o l u t i o n ( < 0.01

and d e t e r m i n e d e x c l u s i v e l y by aHI.

r > 55

i), the

e s r l i n e s a r e narrow The l i n e w i d t h s o f f u l l y h y d r a t e d Fln2+ -

h e c t o r i t e a r e a p p r e c i a b l y b r o a d e r t h a n t h o s e o f Nn2+ i n d i l u t e s o l u t i o n , as can be seen by comparing F i g u r e s 6.7a and 6.7b. Furuhata and Kuwata (1969).

A s i m i l a r o b s e r v a t i o n has been made by

D r y i n g t h e m i n e r a l decreases t h e m o b i l i t y o f t h e

i n t e r l a y e r and broadens t h e l i n e s even more ( c f . F i g u r e 6 . 7 ~and 6.7d).

2+

c l e a r t h a t t h e l i n e w i d t h s o f f u l l y s a t u r a t e d En m a i n l y b y d i p o l a r r e l a x a t i o n processes.

It i s

- h e c t o r i t e a r e determined

However, t h e d i p o l a r e f f e c t s may be

removed b y d o p i n g t h e i o n a t t h e 5% l e v e l i n t h e t l g 2 + - h e c t o r i t e . The average 2+ Mn -Mn2+ i n t h e doped m i n e r a l i s 'L 55 v e r s u s 'L 12 i n the f u l l y saturated 2+ m i n e r a l . Theory i n d i c a t e s t h a t T, t h e c o r r e l a t i o n t i m e f o r c o l l i s i o n o f Fn

i

i

i o n w i t h b u l k w a t e r m o l e c u l e s , i s d i r e c t l y p r o p o r t i o n a l t o t h e w i d t h o f t h e ?I1 = -1/2 t r a n s i t i o n

(s, the fourth-highest

i n t e r a c t i o n s a r e absent. h e c t o r i t e i s 28.6 G

E.2 2

f i e l d 1i n e ) p r o v i d e d t h a t d i p o l a r 2+ S i n c e t h e l i n e w i d t h f o r Mn2+ doped i n t o Mg G f o r Mn2+ i n d i l u t e s o l u t i o n , T i s o n l y

i n t h e i n t e r l a y e r o f f u l l y w e t t e d F1g2+-hectorite (dool

= 21

s o l u t i o n , where i t has been e s t i m a t e d t o be 3.2 x 1971).

30% l o n g e r

i n dilute

sec. ( R u b i n s t e i n e t a l . ,

We may conclude, t h e r e f o r e , t h a t t h e i n t e r l a y e r s a r e v e r y s o l u t i o n - l i k e

even when t h e i n t e r l a y e r s a r e o n l y 6.3.2

i) than

%

'i,

12

0

A thick.

Vanadyl i o n

The m o b i l i t y o f h y d r a t e d V02+ doped i n t o M g 2 + - h e c t o r i t e has a l s o been i n v e s I n t h i s system, t h e

t i g a t e d by e s r l i n e b r o a d e n i n g methods (ElcBride, 1979). MI

= 7/2

transition i s proportional t o

T

(Chasteen and Hanna, 1972).

l i n e w i d t h o f f u l l y h y d r a t e d V02+/Mg2+-hectorite d i l u t e solution,

T

i s 35 G

E.

Since t h e

23 G f o r V02+ i n

i s 1.5 t i m e s l a r g e r i n t h e c l a y t h a n i n d i l u t e s o l u t i o n .

S i n c e t h e c o r r e l a t i o n t i m e i s 5 x 16" environment i s 7.5 x

sec.

sec. i n d i l u t e s o l u t i o n ,

Thus, t h e V02+ i o n , l i k e !In2+,

T

f o r the c l a y

tumbles r a p i d l y

i n f u l l y w e t t e d # g 2 + - h e c t o r i t e i n a s o l u t i o n - l i k e environment. When t h e w a t e r i n V02+-tlg2+-hectorite o f t h e V02+ i o n i s decreased.

i s r e p l a c e d by methanol, t h e m o b i l i t y

Though t h e i o n i s n o t c o m p l e t e l y o r i e n t e d on t h e

esr t i m e scale, t h e motion o f t h e i o n i s n o t s u f f i c i e n t l y f a s t t o completely average A l l and

All

i n d i c a t i n g an i n t e r m e d i a t e r a t e o f t u m b l i n g .

A similar

decrease i n m o b i l i t y i s i n d i c a t e d when t h e w a t e r i n Mn2+ s m e c t i t e i s r e p l a c e d by l a r g e r o r g a n i c m o l e c u l e s such as p y r i d i n e (Pafamov e t a l . , Ovcharenko, 1973).

1971; T a r a s e v i c h and

148

n

Ill

VQ2*- Hectorite (air-dry)

F i g u r e 6.6. E s r s p e c t r a o f an a i r - d r y V O * + - h e c t o r i t e f i l m ( f r o m McBride, 1979a).

F i g u r e 6.7.

Mn2+ i n methanol ( A ) and i n h e c t o r i t e f u l l y h y d r a t e d (B), a i r d r i e d (C),

and d r i e d a t 200" (D) ( f r o m I l c B r i d e e t a l . , 1975b).

149

6.4

INTERLAMELLAR METAL COFIPLEXES Esr can be an e x c e p t i o n a l l y p o w e r f u l t o o l f o r o b s e r v i n g t h e f o r m a t i o n o f metal

complexes on t h e i n t e r l a m e l l a r s u r f a c e s o f c l a y s .

The e s r parameters o f t h e com-

p l e x should be d i f f e r e n t from t h o s e o f t h e s i m p l e s o l v a t e d i o n , p r o v i d i n g t h a t r a p i d t u m b l i n g o f t h e complex does n o t average t h e s e parameters.

Studies o f the

o r i e n t a t i o n dependence o f f i l m samples can p r o v i d e i n f o r m a t i o n on t h e o r i e n t a t i o n o f t h e complex. B e r k h e i s e r and F l o r t l a n d (1975) have i n v e s t i g a t e d t h e r e a c t i o n s o f Cu2+m o n t m o r i l l o n i t e w i t h p y r i d i n e . F i g u r e 6.8 i l l u s t r a t e s t h e spectrum o f t h e m i n e r a l When

s o l v a t e d by d i m e t h y l s u l f o x i d e b e f o r e and a f t e r t h e a d d i t i o n o f p y r i d i n e .

t h e m i n e r a l i s s o l v a t e d o n l y by DHSO, an i s o t r o p i c spectrum, = 2.15, i s observed due t o r a p i d t u m b l i n g o f t h e i o n .

However, when p y r i d i n e i s added, t h e spectrum

o f an o r i e n t e d C u ( p ~ ) ~complex ~ + i s clearly indicated with g 1 and A l

I

= 0.0139 cm-l.

1

= 2.24,

9 1 = 2.06

Nagai e t a l . (1974) a l s o observed by e s r spectroscopy

t h e f o r m a t i o n o f Cu2+ complexes on m o n t m o r i l l o n i t e f o l l o w i n g t h e a d s o r p t i o n o f p y r i d i n e and c e r t a i n amino a c i d s . M e t a l complexes c a n be i n t e r c a l a t e d i n s m e c t i t e s by d i r e c t i o n exchange r e a c t i o n ( B e r k h e i s e r and M o r t l a n d , 1977; Velghe e t a l . ,

1977; T r a y n o r e t a l . , 1978).

B e r k h e i s e r and M o r t l a n d (1977) have shown t h a t t h e spectrum ofiCu( p h e n ) 3 2 + - h e c t o r i t e depends on t h e degree o f h y d r a t i o n .

The f u l l y w e t t e d exchange f o r m g i v e s a

As t h e

n e a r l y i s o t r o p i c spectrum, i n d i c a t i n g c o n s i d e r a b l e i n t e r l a y e r m o b i l i t y .

degree o f h y d r a t i o n o f t h e c l a y was decreased, an a n i s o t r o p i c spectrum was observed which i n d i c a t e d t h a t t h e complex became o r i e n t e d on t h e s u r f a c e .

2+

t h e Cu( p h e n ) 3 - h e c t o r i t e t o 200

(gl

I

= 2.240;

gL = 2.058; A ,

I

D

= 0.0172 c m - l ) .

o c c u r r e d on t h e i n t e r l a y e r s u r f a c e s .

.

2+

That i s , t h e e s r d a t a c l e a r l y

showed t h e 1 igand d i s s o c i a t i o n r e a c t i o n Cu(phen)32t Velghe e t a1

-

Heating

gave an e s r spectrum c h a r a c t e r i s t i c o f Cu(phen)2 Cu( phen)22t

+ phen

(1977) i n v e s t i g a t e d t h e n a t u r e o f Cu2+-ethylenediamine complexes

formed on h e c t o r i t e b y i o n exchange r e a c t i o n . c o n t a i n e d m a i n l y Cu(en)'+,

A l t h o u g h t h e exchange s o l u t i o n

t h e e s r spectrum o f t h e c l a y i n d i c a t e d t h e presence

o f two complex s p e c i e s , Cu(en)22+ w i t h g 1I = 2.181,

A,I

w i t h g 1 I = 2.261,

= 0.0204 cm-l,

A,

I

gl =

and Cu(en)'+

A

This observation indicates t h a t ligand r e d i s t r i b u t i o n

I =0.0013.

= 0.0182 cm-l,

'JI =

= 0.0019 cm-'

r e a c t i o n s o c c u r on t h e c l a y s u r f a c e w h i c h f a v o r s t h e Cu(enIz2+ r e l a t i v e t o homogeneous s o l u t i o n .

The a d d i t i o n o f excess en vapor t o Cu(en)22t-saturated 2+ ( g , , = 2.20, A l I = 0.0183 h e c t o r i t e gave an e s r spectrum i n d i c a t i v e o f Cu(en)3 -1 1 2t cm , gL = 2.048, A 0.0007 cm- ) . The spectrum o f a f i l m sample o f Cu(en)3

I=

was o r i e n t a t i o n independent,

i n d i c a t i n g t h a t t h e symmetry a x i s i s i n c l i n e d near

45O t o t h e s i l i c a t e sheets.

The e s r parameters o f Cu(en)

2+

, Cu(en)22+ and

Cu(en)32+ on h e c t o r i t e s u r f a c e s a r e v e r y s i m i l a r t o t h o s e f o r t h e i o n s i n

150

F i g u r e 6.8.

F i g u r e 6.9.

Esr s p e c t r a o f Cu2'-hectorite s o l v a t e d by d i m e t h y l s u l f o x i d e and p y r i d i n e ( f r o m B e r k h e i s e r and M o r t l a n d , 1975).

Na', L i ' and Ca2+ m o n t m o r i l l o n i t e s The Fe3+ s i g n a l s o f K', a t v a r i o u s r e l a t i v e h u m i d i t i e s . Arrows i n d i c a t e t h e weak Fe3+ resonance ( f r o m H c B r i d e e t a1 , 1 9 7 5 ~ ) .

.

151 d i l u t e solution.

U s i n g d i f f e r e n t t h e o r e t i c a l models, Schoonheydt (1978) has con2+ ?I bonding i n Cu -en complexes i s s l i g h t l y

cluded t h a t t h e e x t e n t o f out-of-plane i n c r e a s e d on c l a y s u r f a c e s . 6.5

FRAMEWORK PARAMAGNETIC CENTERS N a t u r a l c l a y s may c o n t a i n a v a r i e t y o f paramagnetic i o n s .

Some o f t h e i o n s

may be p r e s e n t on t h e exchange s i t e s o r i n o c t a h e d r a l o r t e t r a h e d r a l p o s i t i o n s i n t h e oxygen framework.

Other paramagnetic c e n t e r s may be p r e s e n t as a s e p a r a t e

i m p u r i t y phase, such as i r o n o x i d e s (Goodman, 1978).

I m p u r i t y phases can some-

t i m e s be removed b y a s e l e c t i v e e x t r a c t i o n t e c h n i q u e , such as t h e c i t r a t e d i t h i o n i t e method o f Mehra and Jackson (1960) f o r t h e removal o f i r o n o x i d e s . 3t Fe i s b y f a r t h e most abundant e s r o b s e r v a b l e paramagnetic i o n i n n a t u r a l 2t

clays.

O t h e r i o n s such as Mn

magnetic s p e c i e s

and V02' a r e a l s o e s r o b s e r v a b l e , b u t some para-

2+ (a, Fez' and T i

under most c o n d i t i o n s .

a r e non-Kramers s p e c i e s and a r e e s r s i l e n t

Among t h e r e a d i l y a v a i l a b l e n a t u r a l c l a y s , h e c t o r i t e con-

t a i n s one o f t h e l o w e s t c o n c e n t r a t i o n s o f framework Fe3+.

Therefore, d i p o l a r

i n t e r a c t i o n s which broaden t h e l i n e s o f s u r f a c e exchange i o n s a r e m i n i m a l i n t h i s clay.

T h i s i s one r e a s o n why h e c t o r i t e has been most f r e q u e n t l y used i n e s r

s t u d i e s o f s u r f a c e bound s p e c i e s .

I t must be n o t e d , however, t h a t some h e c t o r i t e

samples, depending on e x a c t l o c a t i o n , can c o n t a i n e s p e c i a l l y h i g h c o n c e n t r a t i o n s

o f i r o n o x i d e s w h i c h g i v e r i s e t o a v e r y broad l i n e (AH > 1000 G) c e n t e r e d near g = 2.0. 6.5.1

Smecti t e s

O l i v i e r e t a l . (1975) have examined t h e e s r s p e c t r a o f s e v e r a l s m e c t i t e s .

All

3t samples e x h i b i t e d p r o m i n e n t f e a t u r e s near g = 4.3 w h i c h were a t t r i b u t e d t o Fe i n two d i s t i n c t o c t a h e d r a l s i t e s and two d i f f e r e n t t e t r a h e d r a l s i t e s . resonances have been observed f o r micas and v e r m i c u l i t e (01 i v i e r e t a1 1976b, 1977).

Similar

. , 1976a,

The n o n - e q u i v a l e n t o c t a h e d r a l environments were a t t r i b u t e d t o t h e

two a l t e r n a t i v e ( c i s and t r a n s ) arrangements o f h y d r o x y l groups i n t h e Fe04(0H)2 cavity.

However, McBride e t a l . (1975c, 1975d) have o f f e r e d a n a l t e r n a t i v e

e x p l a n a t i o n f o r n o n - e q u i v a l e n t o c t a h e d r a l Fe3'.

The h i g h f i e l d a n i s o t r o p i c

resonance n e a r g = 4.3 was found t o be s e n s i t i v e t o t h e p o s i t i o n o f t h e exchange c a t i o n i n s m e c t i t e s (McBride e t a1 ., 1975c, 1975d; B e r k h e i s e r and M o r t l a n d , 1975). As can be seen from F i g u r e 6.9,

K+ and Nat exchange i o n s cause s i g n i f i c a n t decreases

i n t h e i n t e n s i t y o f t h e h i g h - f i e l d a n i s o t r o p i c resonance when t h e r e l a t i v e h u m i d i t y i s reduced t o 08, whereas t h e s t r o n g l y h y d r a t e d L i

+

and Ca2+ exchange

i o n s under t h e same c o n d i t i o n s do n o t i n f l u e n c e t h i s resonance l i n e . thermal m i g r a t i o n o f

+ Li

Moreover,

i n t o t h e vacant octahedral p o s i t i o n o f t h e mineral,

a l s o causes a r e d u c t i o n i n s i g n a l i n t e n s i t y w h i c h i s n o t r e s t o r e d upon r e s o l u t i o n o f t h e mineral.

The l o w f i e l d i s o t r o p i c Fe3' component remains u n a f f e c t e d

152 under t h e above c o n d i t i o n s .

These o b s e r v a t i o n s suggest t h a t t h e h i g h f i e l d

a n i s o t r o p i c Fe3+ resonance component i s a s s o c i a t e d w i t h c e n t e r s o f n e g a t i v e charge i n t h e s i l i c a t e framework.

T h e r e f o r e , t h e two n o n - e q u i v a l e n t Fe3+ s i t e s 2+ may a r i s e from o c t a h e d r a l Fe3+ i o n s a d j a c e n t t o c h a r g e - d e f i c i e n t Hg s i t e s and n o n - d e f i c i e n t A13+ s i t e s .

T h i s model a l l o w s a l l Fe3+ i o n s t o occupy o c t a h e d r a l

s i t e s w i t h c i s OH c o n f i g u r a t i o n . 6.5.2

Kaolinites

The e s r s p e c t r a o f k a o l i n i t e s v a r y m a r k e d l y , depending on l o c a l i t y .

However,

a l l n a t u r a l k a o l i n i t e s have two e s r f e a t u r e s i n common w h i c h a r e independent o f i m p u r i t y phases.

They a l l e x h i b i t a group o f l i n e s near g = 4 w h i c h a r e a t t r i b u t a -

b l e t o l a t t i c e Fe3+ and a second s e t o f l i n e s near g = 2 w h i c h a r i s e from l a t t i c e d e f e c t s (Heads and Malden, 1975; Jones e t a l . ,

1974).

Three Fe3+ c e n t e r s have

been d i s t i n g u i s h e d and d e s i g n a t e d c e n t e r s I,I I a and I I b .

Center I g i v e s r i s e

t o an i s o t r o p i c l i n e a t g = 4.2, w h i l e c e n t e r s I I a and I I b g i v e a l i n e near g =

4.9 and two f r e q u e n t l y u n r e s o l v e d l i n e s a t g = 3.7 and 3.5.

Based on c o r r e l a -

t i o n s o f l i n e i n t e n s i t i e s w i t h t h e degree o f c r y s t a l l i n i t y and t h e e f f e c t s of OMSO and o t h e r i n t e r c a l a n t s on s i g n a l i n t e n s i t y , c e n t e r I has been a s s i g n e d t o

o c t a h e d r a l Fe3+ i n a s t r o n g c r y s t a l f i e l d i n l a y e r s w i t h l a y e r s t a c k i n g d i s o r d e r nb such as 3 d i s p l a c e m e n t s o r 120" r o t a t i o n s . Centers I I a and I I b a r e a s s o c i a t e d w i t h o c t a h e d r a l Fe3+ i n s i t e s o f h i g h c r y s t a l l i n i t y and o r d e r e d s t a c k i n g .

The

d i f f e r e n c e between c e n t e r s I I a and I I b may r e s u l t from two d i s t i n c t o r i e n t a t i o n s o f s u r f a c e OH groups a d j a c e n t t o t h e o r d e r e d Fe3+ s i t e s ( G i e s e and D a t t a , 1 9 7 3 ) . The resonances near g = 2.0 u s u a l l y c o n s i s t o f two asymmetric l i n e s w i t h g 2.05 and 9 1 = 2.0 (Angel and H a l l , 1973; Meads and Nalden, 1975). r e s p o n s i b l e f o r t h e s e l i n e s has been d e s i g n a t e d c e n t e r A. p a r a l l e l t o the k a o l i n i t e c-axis.

II

The s p e c i e s

=

The u n i q u e a x i s l i e s

Schwartz e t a l . (1979) have used t h i s observa-

t i o n t o determine t h e o r i e n t a t i o n d i s t r i b u t i o n o f t h e p l a t e l e t s i n a k a o l i n i t e p e l l e t prepared under a x i a l s t r e s s .

As expected, t h e p l a t e l e t s t e n d t o o r i e n t

w i t h t h e s i l i c a t e l a y e r s perpendicular t o t h e s t r e s s d i r e c t i o n . The

A c e n t e r i n k a o l i n i t e can be e l i m i n a t e d by a n n e a l i n g a t 400".

a l s o c o l l a p s e s t h e Fe3+ l i n e s t o a s i n g l e l i n e a t g = 4.2.

Annealing

The A c e n t e r i s

a b s e n t i n Fe3+-doped s y n t h e t i c k a o l i n i t e s , b u t s y n t h e t i c k a o l i n i t e s doped w i t h Mg2+ o f Fez+ and s u b s e q u e n t l y x - i r r a d i a t e d e x h i b i t t h e A c e n t e r resonance (Angel e t al.,

1974, 1976, 1977).

The e f f e c t s of Fe3+ and 11g2+ o n t h e e s r s p e c t r a o f

s y n t h e t i c k a o l i n i t e a r e i l l u s t r a t e d i n F i g u r e 6.10. The A c e n t e r has been a t t r i b u t e d t o a n 0'

c e n t e r bound t o Mg2+(or Fez+) s u b s t i t u t i n g f o r A13+, a l t h o u g h

a t r a p p e d 02- i o n has been suggested as an a l t e r n a t i v e e x p l a n a t i o n (Jones e t a l . , 1 9 7 4 ) . H a l l (1980a) has e s t i m a t e d t h a t replacement o f one A13+ p e r thousand by 2+ Mg2+ ( o r Fe ) i s s u f f i c i e n t t o a c c o u n t f o r t h e c o n c e n t r a t i o n o f A c e n t e r s .

153

I g=4.0

I g = 2.0 Natural kaol ini t e

6 '

Mg doped kaolinite (no signals) Fe3+ doDed kaol inite

D

Mg doped kaolinite X-irradiated

E

Mg doped kaolinite

X-irradiated and annea 1 ed

Fe3+ and Flg doped kaolinite X-irradiated and annea 1 ed

F i g u r e 6.10.

Esr s p e c t r a of synthetic kaolinites (from Angel e t al, 1976).

154 Two o t h e r d e f e c t c e n t e r s (B1 and B 2 ) have been observed i n k a o l i n i t e , b u t t h e e s r s i g n a l s a r e weak and d i f f i c u l t t o r e s o l v e (Angel and H a l l , 1973; Pleads and I l a l d e n , 1975).

These c e n t e r s e x h i b i t g v a l u e s near 2.0,

but they are distinguished

from t h e A c e n t e r s by t h e presence o f h y p e r f i n e s p l i t t i n g due t o c o u p l i n g o f t h e u n p a i r e d s p i n s w i t h t h e n u c l e a r s p i n o f A1 w i t h I = 5 / 2 . c e n t e r s can be g r e a t l y i n c r e a s e d by X - i r r a d i a t i o n .

The c o n c e n t r a t i o n o f B

They a r e s t a b l e t o 200" and

can be r e v e r s i b l y c r e a t e d and d e s t r o y e d by i r r a d i a t i o n and a n n e a l i n g (Angel and H a l l , 1973).

The B c e n t e r s a r e c l e a r l y a s s o c i a t e d w i t h A1 c e n t e r s .

l i k e l y assignments a r e lattice.

+ 0

The most

c e n t e r s w h i c h b r i d g e A l , Si and A1 , A1 p a i r s i n t h e

Many o t h e r e s r a c t i v e s p e c i e s have been observed i n n a t u r a l k a o l i n i t e

samples, i n c l u d i n g adsorbed o r g a n i c f r e e r a d i c a l s i n l o w c o n c e n t r a t i o n ( H a l l e t a1 . , 1974), framework V4+ i o n s ( H a l l ,(1930a), l l n 2 + - c o n t a i n i n g phases (Meads and Halden, 1975), i r o n o x i d e s , and i r o n - r i c h i m p u r i t y phases such as mica (Meads and Malden, 1975; Angel and V i n c e n t , 1978).

ORGANIC RADICALS AND NITROXIDE S P I N PROBES

6.6 6.6.1

Arene Radical C a t i o n s

Esr has been v e r y u s e f u l i n e l u c i d a t i n g e l e c t r o n t r a n s f e r r e a c t i o n s between a r o m a t i c m o l e c u l e s and c e r t a i n t r a n s i t i o n m e t a l i o n s i n t h e i n t e r l a y e r s o f smectites.

The f i r s t r e a c t i o n o f t h i s t y p e was r e p o r t e d by Donor and M o r t l a n d (1969)

f o r Cu2+ and benzene on m o n t m o r i l l o n i t e .

The most s t r i k i n g f e a t u r e o f t h e r e a c -

t i o n was t h e development o f an i n t e n s e l y r e d complex w h i c h e x h i b i t e d anomalous a b s o r p t i o n bands i n t h e i r r e g i o n t h a t i n d i c a t e d t h e a r o m a t i c i t y o f t h e benzene r i n g was l o s t o r g r e a t l y p e r t u r b e d .

F u r t h e r s t u d i e s ( F l o r t l a n d and Pinnavaia;

1971; P i n n a v a i a and M o r t l a n d , 1971) o f t h e r e a c t i o n i n d i c a t e d t h a t t h e development o f t h e r e d "Type 11" benzene s p e c i e s was preceeded b y t h e l o s s o f w a t e r from t h e s i l i c a t e s u r f a c e and p h y s i c a l a d s o r p t i o n o f benzene. The removal o f 2+ w a t e r f r o m t h e c o o r d i n a t i o n sphere o f t h e Eu ions a f f o r d e d a yellow-green edge-bonded form o f c o o r d i n a t e d benzene, w h i c h was d e s i g n a t e d "Type I"benzene. The removal o f s t i l l more i n n e r sphere w a t e r l e d t o t h e f o r m a t i o n o f some C6H6+ a l o n g w i t h t h e r e d Type I 1 s p e c i e s .

F u r t h e r s l o w r e a c t i o n o f t h e Type I 1 s p e c i e s

and/or t h e r a d i c a l c a t i o n e v e n t u a l l y a f f o r d e d polymer, p r o b a b l y p a r a p o l y p h e n y l ( M o r t l a n d and H a l l o r a n , 1976; S t o e s s e l e t a l . ,

1977).

The o v e r a l l r e a c t i o n scheme f o r Cu2+-benzene i s summarized i n F i g u r e 6.11. The Type I s p e c i e s g i v e s a s t r o n g , broad Cu2+ resonance, i n d i c a t i n g t h a t t h e o x i d a t i o n s t a t e o f t h e m e t a l remains unchanged a t t h i s s t a g e o f t h e r e a c t i o n . t i o n o f Cu2+ t o Cu

+ and

t h e f o r m a t i o n o f C6H6

+

Reduc-

and Type I 1 benzene i s accompanied

by t h e replacement o f t h e Cu2+ resonance by a s h a r p i s o t r o p i c resonance near g = 2.0 t h a t may be a s s i g n e d t o C H

l e s s than

%

+

( R u p e r t , 1973; P i n n a v a i a e t a l , 1 9 7 4 ) . However, + 62+ 3% o f t h e i n i t i a l Cu s p i n s a r e r e c o v e r e d as C6H6 . T h i s l a t t e r

o b s e r v a t i o n suggests t h a t most o f t h e s p i n s a r e l o s t t h r o u g h s p i n p a i r i n g i n t h e

155

+cuL;

/ / polymer

F i g u r e 6.11.

R e a c t i o n s of benzene w i t h Cu2+ i n s r n e c t i t e (from P i n n a v a i a , 1 9 7 7 ) .

0 F i g u r e 6.12.

Proposed s t r u c t u r e

for t y p e I 1 benzene.

F i g u r e 6.13.

Structure o f Ternpamine+.

156 Type I 1 species.

The n a t u r e o f t h e Type I 1 s p e c i e s i s s t i l l u n c e r t a i n , b u t a

model based on t h r o u g h space p a i r i n g o f r a d i c a l c a t i o n s has been proposed e a r l i e r ( P i n n a v a i a , 1976b).

A schematic r e p r e s e n t a t i o n o f t h e p a i r i n g model i s g i v e n i n

F i g u r e 6.12. D i s t o r s i o n s i n t h e p a i r e d J a h n - T e l l e r C6H6

+

s p e c i e s may c o n t r i b u t e

t o t h e unusual i r a b s o r p t i o n s i n t h e C=C s t r e t c h i n g r e g i o n . The r e a c t i o n o f a v a r i e t y o f o t h e r a r o m a t i c m o l e c u l e s w i t h Cu2+ on s m e c t i t e s u r f a c e s has

been i n v e s t i g a t e d (Matsunaga, 1972; P i n n a v a i a , 1976b; Fenn e t a l . ,

1973; Van de Poel e t a l . , 1973; Cloos e t a l . , 1973; T r i c k e r e t a l . , 1976). S i n c e 2+ t h e r o l e o f t h e Cu i o n i s t o f u n c t i o n as an o x i d i z i n g a g e n t f o r r a d i c a l c a t i o n f o r m a t i o n , o t h e r o x i d i z i n g agents such a s V02+ and Fe3+ may be used as r e p l a c e 2+ ( P i n n a v a i a e t a l . , 1974).

ments f o r Cu 6.6.2

N i t r o x i d e S p i n Probes

McBride (1976c, 1976d, 1977a, 1977b, 1979b, 1980) s t u d i e d t h e m o b i l i t y and o r i e n t a t i o n o f n i t r o x i d e s p i n probes on s m e c t i t e c l a y s u r f a c e s . form o f 4-amino-Z,2,6,6-

tetramethyl piperidine-N-oxide

e s p e c i a l l y useful i n these studies. i n F i g u r e 6.13.

The p r o t o n a t e d

+

(Ternpami ne ) has been

The s t r u c t u r e o f Tempamine

When t h e s p i n probe i s d i s s o l v e d i n l o w c o n c e n t r a t i o n ( 1 0 -

4

+

i s illustrated

1) i n

solvents o f

l o w v i s c o s i t y , r a p i d t u m b l i n g averages t h e p r i n c i p a l components o f t h e g t e n s o r and t h e h y p e r f i n e c o u p l i n g t e n s o r , A. Thus, a t h r e e - l i n e e s r spectrum i s observed 1 1 + g2,) and A, = (Axx + A + A ) i s observed. I f t h e w i t h go = 3 ( g x x = gYY 3 Yy zz v i s c o s i t y i s l o w and t h e c o r r e l a t i o n t i m e f o r t u m b l i n g i s v e r y s h o r t ( T <~ 2, 10:” sec), t h e t h r e e h y p e r f i n e l i n e s a r e o f equal h e i g h t and w i d t h .

As t h e c o r r e l a - -

t i o n t i m e i n c r e a s e s i n t h e r e g i o n o f m o d e r a t e l y f a s t t u m b l i n g ( T <~ 5 x

lo-’

sec),

t h e l i n e s remain equal i n i n t e g r a l i n t e n s i t y b u t t h e w i d t h s and h e i g h t s o f t h e t h r e e l i n e s b e g i n t o d i f f e r because o f i n c o m p l e t e l y averaged a n i s o t r o p i c terms i n t h e magnetic H a m i l t o n i a n .

The c o r r e l a t i o n t i m e f o r t u m b l i n g may be c a l c u l a t e d

f r o m t h i s l i n e b r o a d e n i n g phenomenon ( S m i t h , 1972; N o r d i o , 1976; Sachs and L a t o r r e , 1974).

As t h e m o b i l i t y o f t h e n i t r o x i d e decreases f u r t h e r i n v i s c o u s f l u i d media,

t h e spectrum becomes more complex.

I n t h e range o f c o r r e l a t i o n t i m e s ,

lop9

sec

5

s e c - l , t h e s o - c a l l e d s l o w m o t i o n a l r e g i o n , t h e shape o f t h e spectrum tends toward two w e l l - r e s o l v e d o u t e r h y p e r f i n e l i n e s and a c e n t r a l o v e r l a p p e d

< T~

region.

The a n a l y s i s o f c o r r e l a t i o n t i m e s i n t h i s t i m e domain i s d i f f i c u l t , b u t

an a p p r o p r i a t e t h e o r y has been developed (Freed, 1976; Hwang e t a l . ,

1975).

S i n c e t h e c o r r e l a t i o n t i m e can be r e l a t e d t o t h e v i s c o s i t y o f t h e medium t h r o u g h Stokes law, Tc

4nqr

3

(4)

=3kT

e s r s t u d i e s o f s p i n probes on c l a y s u r f a c e s m i g h t be expected t o y i e l d t h e microscopic v i s c o s i t y i n t h e c l a y i n t e r l a y e r s .

However, McBride (1976d, 1977a)

157

F i g u r e 6.14.

Esr s p e c t r a o f Ternpamine+ doped a t t h e 1% l e v e l i n t o f u l l y w e t t e d K + - h e c t o r i t e f i l m ( f r o m McBride, 1980).

, 20

F i g u r e 6.15.

#

GAUSS ,

n .

+

Esr s p e c t r a o f Tempamine doped a t t h e 1% l e v e l i n t o K + - h e c t o r i t e d r i e d a t 110" ( f r o m McBride, 1980).

158 has f o u n d t h a t s t r o n g i n t e r a c t i o n s w i t h t h e c l a y s u r f a c e s n o t o n l y reduces t h e r o t a t i o n a l m o b i l i t y , b u t a l s o p a r t i a l l y o r i e n t t h e probe i n t h e i n t e r l a y e r s ; t h a t i s , t h e probe does n o t tumble randomly i n t h e i n t e r l a y e r s even when f u l l y h y d r a t e d . F i g u r e 6.14 i l l u s t r a t e s t h e e s r s p e c t r a o f Ternpamine+ on a f u l l y w e t t e d

+

f i l m sample o f K - h e c t o r i t e w i t h t h e m a g n e t i c f i e l d d i r e c t i o n o r i e n t e d p a r a l l e l and p e r p e n d i c u l a r t o t h e s i l i c a t e sheets. Al

For t h e p e r p e n d i c u l a r o r i e n t a t i o n ,

= 20.5 gauss, and f o r t h e p a r a l l e l o r i e n t a t i o n A l l = 15.2 gauss.

Based on

t h e observed o r i e n t a t i o n dependence, t h e z - a x i s o f t h e probe, w h i c h i s d e f i n e d as b e i n g c o l i n e a r w i t h t h e p o r b i t a l on n i t r o g e n , i s o r i e n t e d w i t h r e s p e c t t o t h e s i l i c a t e sheets a t an a p p a r e n t a n g l e o f 45". F i g u r e 6.15 shows t h e e s r s p e c t r a l p r o p e r t i e s o f Tempatnine+ adsorbed on a f i l m sample o f K + - h e c t o r i t e d r i e d a t 110".

The arrows i n t h e f i g u r e s i n d i c a t e t h e

approximate p o s i t i o n s o f t h e t h r e e resonances f o r t h e p e r p e n d i c u l a r and p a r a l l e l o r i e n t a t i o n o f t h e h e c t o r i t e f i l m r e l a t i v e t o t h e magnetic f i e l d d i r e c t i o n . The s p e c t r a shapes a r e t h o s e expected f o r n i t r o x i d e probe i n t h e s l o w m o t i o n a l region.

C l e a r l y , t h e a l i g n m e n t o f t h e probe i n t h e i n t e r l a y e r s i s g r e a t l y

enhanced by removing w a t e r and c o l l a p s i n g t h e i n t e r l a y e r s .

The z - a x i s o f t h e

probe i s a p p r o x i m a t e l y a t r i g h t angles t o t h e s i l i c a t e s h e e t s . I f t h e l i n e w i d t h s o f Ternpamine'

f o r tumbling i n t h e f u l l y wetted o b t a i n e d (McBride, 1977b).

a r e used t o c a l c u l a t e t h e c o r r e l a t i o n t i m e

+ K -hectorite,

v a l u e s o f 1-3 x

lo-'

sec a r e

The v a l u e s i n t h i s range a r e 20 t o 60 t i m e s l o w e r

than t h e r a t e o f r o t a t i o n i n t h e s o l u t i o n state.

Larger values o f

T~

are found

f o r t h e p e r p e n d i c u l a r o r i e n t a t i o n o f t h e c l a y f i l m s i n t h e magnetic f i e l d com2+

pared t o t h e p a r a l l e l o r i e n t a t i o n , a r e s u l t o f a n i s o t r o p i c r o t a t i o n . hectorites exhibit

+

H -

v a l u e s a b o u t t w i c e as l o n g as M - h e c t o r i t e s , perhaps due 2+ t o t h e more l i m i t e d i n t e r l a m e l l a r volume f o r t h e M -exchange forms. A l s o , T~

+

e t h a n o l s o l v a t e d M - h e c t o r i t e s c o n t a i n i n g s p i n probe e x h i b i t c o r r e l a t i o n t i m e s which a r e a p p r o x i m a t e l y two o r d e r s o f magnitude l o n g e r t h a n t h e h y d r a t e d systems, d e s p i t e t h e r e l a t i v e l y more i s o t r o p i c m o t i o n i n t h e f o r m e r case. RE FE RE WC ES

Abragam, A. and Bleaney, B., 1970. E l e c t r o n Paramagnetic Resonance o f T r a n s i t i o n Metal I o n s . O x f o r d U n i v e r s i t y Press, London, 911 pp. Angel, B.R. and H a l l , P.L., 1973. E l e c t r o n s p i n resonance s t u d i e s o f k a o l i n s . Proc. I n t e r n . C l a y Conf., Madrid, 1972, pp. 47-60. Angel, B.R., Jones, J.P.E. and H a l l , P.L., 1974. E l e c t r o n s p i n resonance s t u d i e s o f doped s y n t h e t i c k a o l i n i t e I. C l a y H i n e r . , 10: 247. 1976. The s y n t h e s i s , morphology, and Angel, B.R., Richards, K. and Jones, J.P.E., g e n e r a l p r o p e r t i e s o f k a o l i n i t e s s p e c i f i c a l l y doped w i t h m e t a l l i c i o n s and defects generated b y i r r a d i a t i o n . Proc. I n t e r n . C l a y Conf., Mexico, 1975, pp. 297-304. Angel, B.R., C u t t l e r , A.H., R i c h a r d s , K. and V i n c e n t , W.E.J., 1977. S y n t h e t i c k a o l i n i t e s doped w i t h Fez+ and Fe3+ i o n s . Clays and C l a y Miner., 25: 381-383. Angel, B.R. and V i n c e n t , W.E.J., 1978. E l e c t r o n s p i n resonance s t u d i e s o f i r o n o x i d e s a s s o c i a t e d w i t h t h e s u r f a c e o f k a o l i n s . Clays and C l a y Miner., 26: 263-272.

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