1 3 9

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 ich igan S t a t e U n i v e r s i t y , East Lansing, Mich igan 48824, USA.

6.1 INTRODUCTION

I n r e c e n t years , e l e c t r o n s p i n resonance ( e s r ) spec t roscopy has proven t o be

a power fu l t o o l i n s t u d i e s o f c l a y m ine ra l chemis t r y . 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 paramagnet ic spec ies have been

i l l u c i d a t e d by e s r spec t roscopy .

may be metal complexes such as Cu(en)2 , Cu(phen)3 . Adsorbed o rgan ic r a d i c a l s , such as t h e pe ry lene c a t i o n r a d i c a l o r o rgan ic mo lecu les 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 spec t roscopy . es r spec t ra o f c e r t a i n t r a n s i t i o n meta 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 rde 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 be ing probed.

The i n t e n t o f t h e p resen 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 ob ta ined th rough t h e a p p l i c a t i o n o f es r t o t h e

s tudy o f c l a y m i n e r a l s .

spec t roscopy t o c l a y m i n e r a l s have appeared recen t l y ,wh ich complement t h e p resen t

work ( H a l l , 1980a, 1980b; HcBride, 1980; P innava ia , 1980).

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

do n o t a l l o w f o r an adequate t rea tmen t o f es r t heo ry , b u t severa 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 Bo l ton , 1972; Abragam and Bleaney, 1970;

Ingram, 1967) .

The paramagnet ic cen te rs on t h e basal su r faces may be s imp le hydra ted c a t i o n s (*, C U ( H ~ O ) ~ 2+ , C U ( H * O ) ~ ~ + , VO(H20),2+) o r t hey

2+ 2+

. The

3+

Several rev iew 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

Two e a r l i e r rev iews

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

6.2 HYDRATED EETAL I O N S ON BASAL SURFACES

6.2.1 Copper( 11)

Clementz e t a l . (1973) addressed t h e ques t i on o f meta 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 su r faces o f smec 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 water

l a y e r s . 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 unpa i red e l e c t r o n w i t h a s p i n S = 1/2.

where t h e meta l i o n i s s o l v a t e d by one, two o r t h r e e mo lecu la r l a y e r s o f water ,

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 an iso -

t r o p i c e s r s i g n a l . 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 ne ighbor ing nuc lea r sp ins , t h e sp in -Hami l ton ian f o r t h e i o n under t e t r a g o n a l

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

1 4 0

symmetry may be wr i t ten as

+ g1 s in 6 S z ) (1 1 cos 6 s, t h e Bohr magneton (eh/2mc), g I I and g l a r e spectroscopic s p l i t t i n g f a c t o r s ,

H i s the magnetic f i e l d , and 6 i s the angle between the magnetic f i e l d d i rec t ion and t h e symmetry ax is of t h e te t ragonal ion , 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 wil l appear in t h e e s r spectrum, one corresponding t o spin quant izat ion i n a d i r e c t i o n para l le l t o the symmetry ax is ( g ) and another corresponding t o spin quantiza- t i o n perpendicular t o the symmetry ax is .(g ) .

I f we allow f o r coupling between the S = 1/2 electron spin and the I = 3/2

I I 1

nuclear spin of t h e copper nucleus, then two more t e r n s must be added t o the spin Hamiltonian:

where A1 I and A a r e hyperfine s p l i t t i n g cons tan ts , usual ly expressed in cm-l o r in gauss.

I n the presence of a n applied magnetic f i e l d , therefore , the S = 1 / 2 ground s t a t e i s s p l i t by an amount gBH in to two energy s t a t e s (corresponding t o quan- t ized or ien ta t ions of the e lec t ron spin components i n a d i rec t ion para l le l (!Is = -1/2) or a n t i p a r a l l e l ( M s = 1/2) t o the magnetic f i e l d . 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 lec t ron spin with the four quantized components of the I = 3/2 nuclear spin ( M I = +3/2, + 1 / 2 ) . 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 the g I I and g1 resonance components a r e s p l i t i n t o quar te t s due t o hyperfine coupling.

From equation 1 we may conclude t h a t when 6 = o , only the g , I resonance com- ponent wil l be observed, a n d when e = 90 , only the g component wil l be observed. I n a random powder sample, however, a l l possible values of 6 occur , and both resonance components wil l 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 condi t ions where a s i n g l e molecular layer of water occupies the i n t e r l a y e r s (dool = 12.4 A ) . and 4 1 resonance components occur with g 1 I = 2.34, A

A

oriented film sample of Cu2+-hectorite with the magnetic f i e l d d i rec t ion or iented I I and L t o the s i l i c a t e shee ts . Since g and 91 I i s observed f o r the 1 o r i e n t a t i o n , we may conclude t h a t the symmetry a x i s of the planar C U ( H ~ O ) ~ ~ + ion i s or iented a t 90' t o the plane of t h e s i l i - c a t e shee ts .

When Cu i s par t of a two-water layer system a s in f u l l y hydrated Cu -

A / l S Z I Z + A (SXIX + SYIY) (2) 1 L

These two s t a t e s

The energy

0

0

L

0

I I As expected, both g ' g1 = 2 -08 . = 0.0165 cm-l

i s too small t o be resolved. Figures 6.2C and 6.2D show the esr spec t ra o f an

i s observed f o r the I I or ien ta t ion

L

1

2+ . 2+ 0

vermicul l i te (dool = 14.2 A ) , an iso t ropic spectra a r e observed f o r or iented f i lm

1 4 1

I f 112)

Figure 6.1. Energy level diagram for C u 2+

QII -H

Figure 6 .2 . Esr spectra o f Cu2+-hectorite: A, B are for random powders; C , D are for oriented film.

1 4 2

1 = 2.16, A 0.0145 cm- ) , b u t the spec t ra a r e indepen- I I = samples ( g 1 1 = 2.38, g

dent of sample or ien ta t ion in the magnetic f i e l d . This means t h a t t h e symmetry ax is of t h e in te rca la ted C U ( H ~ O ) ~ ~ + ion i s or iented near 45' t o the s i l i c a t e shee ts . However, when the Cu(H20l6 hydration s t a t e of MgZt-hectorite (doo l = 15.0 i), the or ien ta t ion dependent spectra shown i n Fiqure 6 . 3 a r e observed (McBride e t a l . , 1975a). From the observed or ien ta t ion dependence, i t may be concluded t h a t C U ( H ~ O ) ~ ~ + i s or iented on the basal surfaces with t h e symmetry a x i s near 90 t o the plane of t h e s i l i c a t e sheets .

2 t ion i s doped i n t o the three-water layer

Figure 6.4 summarizes the d i s t i n c t o r ien ta t ion of the Cu2+ ions hydrated by one, two, and three layers of in te r lamel la r water. smectites a r e f u l l y swollen with water (doo l = 21 A ) , qui te a d i f f e r e n t p ic ture emerges from the e s r spectrum. Under these l a t t e r condi t ions, a s ing le i so t ropic l i n e i s observed, s imi la r t o the resonance found f o r C U ( H ~ O ) ~ ~ + i n d i l u t e aqueous so lu t ion . cesses: ( 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 or ( 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 the t h r e e pr incipal axes of the C U ( H ~ O ) ~ ~ + ion through coupling of the v ibra t ion modes of t h e aquo l igands , i s responsible f o r the i so t ropic e s r l i n e observed for C U ( H ~ O ) ~ ~ + in frozen aqueous so lu t ions ( H u d s o n , 1966). As we sha l l see 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 so lu t ion- l ike environment when the i n t e r l a y e r s a r e swollen with mul t ip le layers of water.

When the i n t e r l a y e r s of Cu2+- 0

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 pro- 1

2+ and V02+ ions i n smect i te , t h e ions

6.2.2 Vanadyl , V02+ VO(H20)62+-hectorite in the f u l l y wetted s t a t e exhib i t s the blue co lor charac-

t e r i s t i c of the ion i n aqueous so lu t ion . However, under c e r t a i n condi t ions of loading and hydration s t a t e , the blue color i s l o s t and a tan-brown color develops, indicat ing t h a t a surface reac t ion takes place which depends on moisture content . The surface react ions have been invest igated in p a r t by Pinnavaia e t a l . (1974) and by HcBride (1979a).

1

metry. I t s e s r proper t ies resemble those of C U ( H ~ O ) ~ . Under normal condi t ions, t h e ion exhib i t s g in to e ight hyperfine l i n e s due t o coupling of the S = 1/2 e lec t ron spin w i t h t h e I = 7 / 2 nuclear sp in . However, when dissolved in water , t h e ion exhib i t s only a time-averaged i so t ropic l i n e due t o rapid tumbling which averages t h e g

components.

The hydrated vanadyl ion has a d e l e c t r o n i c configurat ion and te t ragonal sym-

and gL resonances with both resonance components being s p l i t

2+

I I and 91 Fully wetted V02+-hectorite a l s o gives a n i s o t r o p i c spectrum s imi la r

Figure 6.3. Esr spectra o f Cu2+ doped in to Plg*+-hectorite f i lm with H para l le l ( A ) and perpendicular ( B ) t o s i l i c a t e . sheets (from HcBride e t a1 . , 1975a).

1 4 4

Figure 6.4. Orientat ions 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 layers of water. oxygen atoms of the s i l i c a t e sheet and 1 igat ing water molecules (from Pinnavaia, 1976b).

Open c i r c l e s a r e

1 4 5

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 igu re 6.5) However, when t h e i o n i s doped i n t o Mg2+-hec tor i te 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 wh ich

i s i n d i c a t i v e o f a h i g h l y o rdered, immobi le form. FIcBride has suggested t h a t

t h i s o rdered tan-brown fo rm o f vanadyl i s VO(OH)2(H30)3.

o r i e n t a t i o n independent o f t h e magnet ic 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 near 45 t o t h e s i l i c a t e sheets.

McBride (1979) has found t h a t t h e s o l u t i o n - 2+ i s doped i n t o Mg2+-hec tor i te a t t h e 50% l e v e l

S ince t h e spectrum i s

0

A i r - d r i e d V02+-hec tor i te g i v e s t h e o r ien ta t i on -dependen t e s r spec t ra shown i n

F igu re 6.6. S ince A l I (704 G) i s observed f o r t h e pe rpend icu la r o r i e n t a t i o n and A (81 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 , wh ich i s c o l i n e a r w i t h t h e V=O bond, l i e s pe rpend icu la 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 meta l i o n s on t h e basal

sur faces . Esr 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 mixed NR4+-Cu2+ s m e c t i t e systems (McBride, 1976a; NcBr ide and

Mor t land, 1975).

and on k a o l i n i t e su r faces have a l s o been examined (NcBr ide and t l o r t l and , 1974;

McBride, 1976b; Clementz e t a l . , 1974).

1

The e s r spec t ra o f Cu2+ on reduced charge m o n t m o r i l l o n i t e

6.3 MOBILITY OF INTERLAYER METAL IONS

6.3.1 Manganese( I I)

because e i t h e r dynamic Jahn-Te l l e r e f f e c t s o r r a p i d tumb l ing can l e a d t o averag ing

O f 91 I and g components. However, f o r !In2+ and V02+, t h e dynamic Jahn-Te l le r e f f e c t does n o t a p p l y and 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

es r t h e o r y hav ing been w e l l developed (Bur lamacch i , 1971 ; Burlamacchi e t a l . ,

1970, 1973; G a r r e t t and Morgan, 1966; Campbell and Hanna, 1976).

The hyd ra ted $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 . I n

most environments, a l l t h r e e g - tenso r components a r e equal and i s o t r o p i c spec t ra

a r e observed. Because o f i t s i s o t r o p i c na ture , !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 meta l i o n o r d e r i n g on c l a y sur faces , b u t t h e e s r l i n e w i d t h s may be

r e a d i l y r e l a t e d t o i o n m o b i l i t y .

The spec t ra o f Mn2+ i n f u l l y hyd ra ted 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 so lu -

t i o n (McBride e t a l . , 1975b).

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 nuc lea r sp in . hyper f ine 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 rom

f i v e AM, = 1 t r a n s i t i o n s .

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

As no ted 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

L

5

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

Each

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

AH = AHI + AHD ( 3 )

1 4 6

I

200 GAUSS c------1

H -----t

VO*+/ M$'- Hect.

F i g u r e 6.5. Esr spec t ra o f V02+ i n aqueous so lu t ion ( 2 x VO2+/Mg2+-Kectorite f i l m ( f r o m McBride, 1979a).

M, pH = 1.5) and i n a 50:50

1 4 7

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 ne ighbor ing Mn2+ i o n s ( H i n c k l e y and Morgan, 1966).

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 Hn

Mn2+ d i s tance .

and de termined e x c l u s i v e l y by aHI.

h e c t o r i t e a r e a p p r e c i a b l y b roader than those 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 igu res 6.7a and 6.7b. A s i m i l a r obse rva t i on has been made by Furuhata and Kuwata (1969). D ry ing t h e m ine ra 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). It i s

c l e a r t h a t t h e l i n e w id ths o f f u l l y s a t u r a t e d En

m a i n l y by d i p o l a r r e l a x a t i o n processes.

removed b y dop ing t h e i o n a t t h e 5% l e v e l i n t h e t l g2+-hec to r i t e . The average

Mn -Mn2+ i n t h e doped m ine ra l i s 'L 55 i versus 'L 12 i i n t h e f u l l y s a t u r a t e d 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 o n w i t h b u l k water mo lecu les , 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, t h e f o u r t h - h i g h e s t f i e l d 1 i n e ) p rov ided t h a t d i p o l a r 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. 22 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 % 30% longer i n t h e i n t e r l a y e r o f f u l l y we t ted F1g2+-hectorite (dool = 21 i) than i n d i l u t e s o l u t i o n , where i t has been es t ima ted t o be 3.2 x sec. ( R u b i n s t e i n e t a l . ,

1971).

even when t h e i n t e r l a y e r s a r e o n l y 'i, 12 A t h i c k .

The AHD te rm i s c o n c e n t r a t i o n dependent because 2+ -

In d i l u t e s o l u t i o n ( < 0.01 11, r > 55 i), t h e e s r l i n e s a r e narrow 2+ - The l i n e w id ths o f f u l l y hydra ted Fln

2+ - 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

2+

2+ Since t h e l i n e w i d t h f o r Mn2+ doped i n t o Mg -

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 0

6.3.2 Vanadyl i o n

The m o b i l i t y o f hyd ra ted V02+ doped i n t o Mg2+-hec tor i te has a l s o been inves -

t i g a t e d by e s r l i n e broaden ing methods (ElcBride, 1979).

MI = 7 /2 t r a n s i t i o n i s p r o p o r t i o n a l t o T (Chasteen and Hanna, 1972).

l i n e w i d t h o f f u l l y hyd ra ted V02+/Mg2+-hectorite i s 35 G E. 23 G f o r V02+ i n d i l u t e s o l u t i o n , T i s 1.5 t imes l a r g e r i n t h e c l a y than i n d i l u t e s o l u t i o n .

S ince t h e c o r r e l a t i o n t i m e i s 5 x 16" sec. i n d i l u t e s o l u t i o n , T f o r t h e c l a y

environment i s 7.5 x

i n f u l l y we t ted #g2+-hec to r i t e i n a s o l u t i o n - l i k e environment.

o f t h e V02+ i o n i s decreased.

es r t i m e sca le , t h e mo t ion 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 comp le te l y

average A l l and A l l 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 umb l ing . 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 water i n Mn2+ s m e c t i t e i s rep laced by

l a r g e r o rgan ic mo lecu les such as p y r i d i n e (Pafamov e t a l . , 1971; Tarasev ich and

Ovcharenko, 1973).

I n t h i s system, t h e

S ince t h e

sec. Thus, t h e V02+ ion , l i k e !In2+, tumbles r a p i d l y

When t h e water i n V02+-t lg2+-hector i te i s rep laced 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 comp le te l y o r i e n t e d on t h e

A s i m i l a r

1 4 8

VQ2*- Hectorite n Ill (air-dry)

F igu re 6.6. Es r spec t ra o f an a i r - d r y VO*+-hec tor i te 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 hyd ra ted (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 cB r ide e t a l . , 1975b).

1 4 9

6.4 INTERLAMELLAR METAL COFIPLEXES

Esr can be an e x c e p t i o n a l l y power fu l t o o l f o r obse rv ing t h e f o r m a t i o n o f metal

The e s r parameters o f t h e com- complexes on t h e i n t e r l a m e l l a r su r faces o f c l a y s .

p lex should be d i f f e r e n t f rom those o f t h e s imp le 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 tumb l ing o f t h e complex does n o t average these parameters. S tud ies o f t h e

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.

Be rkhe ise r and F lo r t land (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 ine ra l

so l va ted 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 ine ra 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 tumb l ing o f t h e i o n .

and A l I = 0.0139 cm-l . Nagai e t a l . (1974) a l s o observed by e s r spectroscopy t h e fo rma t ion 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 .

Meta l complexes can be i n t e r c a l a t e d i n smec t i t es by d i r e c t i o n exchange reac-

t i o n (Berkhe ise r and Mor t land, 1977; Velghe e t a l . , 1977; Traynor e t a l . , 1978).

Berkhe iser and Mor t l and (1977) have shown t h a t t h e spectrum ofiCu( phen)32+-hec to r i t e

depends on t h e degree o f h y d r a t i o n .

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 cons ide rab le i n t e r l a y e r m o b i l i t y . As t h e

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 ob-

served 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 su r face . Heat ing

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

( g l I = 2.240; gL = 2.058; A , I = 0.0172 cm- l ) . That i s , t h e e s r da ta 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 - Cu( phen)22t + phen occu r red on t h e i n t e r l a y e r su r faces .

formed on h e c t o r i t e by i o n exchange r e a c t i o n .

con ta ined m a i n l y Cu(en)'+, 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 spec ies , Cu(en)22+ w i t h g 1 I = 2.181, A , I = 0.0204 cm-l, gl =

When

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 c l e a r l y i n d i c a t e d w i t h g 1 1 = 2.24, 9 1 = 2.06

The f u l l y we t ted exchange fo rm g i v e s a

D 2+ 2+

Velghe e t a1 . (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 A l though t h e exchange s o l u t i o n

= 0.0019 cm-' and Cu(en)'+ w i t h g 1 I = 2.261, A , I = 0.0182 cm-l, 'JI = I= A 0.0013. T h i s o b s e r v a t i o n i n d i c a t e s t h a t l i g a n d r e d i s t r i b u t i o n

r e a c t i o n s occu r on t h e c l a y su r face wh ich 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

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

cm

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)

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

2+ ( g , , = 2.20, A l I = 0.0183 -1 1 2t

, gL = 2.048, A 0.0007 cm- ) . The spectrum o f a f i l m sample o f Cu(en)3

2+

I= , Cu(en)22+ and

1 5 0

F i g u r e 6.8. Esr spec t ra o f Cu2'-hector i te 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 Berkhe ise r and Mor t land, 1975).

F i g u r e 6.9. The Fe3+ s i g n a l s o f K', Na', L i ' and Ca2+ m o n t m o r i l l o n i t e s a t va r ious 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 HcBr ide e t a1 . , 1 9 7 5 ~ ) .

1 5 1

d i l u t e s o l u t i o n . Us ing d i f f e r e n t t h e o r e t i c a l models, Schoonheydt (1978) has con-

c luded t h a t t h e e x t e n t o f ou t -o f -p lane ?I bonding i n Cu -en complexes i s s l i g h t l y

inc reased on c l a y su r faces .

2+

6.5 FRAMEWORK PARAMAGNETIC CENTERS

Na tu ra l c l a y s may c o n t a i n a v a r i e t y o f paramagnet ic i o n s . Some o f t h e i ons

may be p resen t on t h e exchange s i t e s o r i n oc tahedra 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.

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 imes be removed by a s e l e c t i v e e x t r a c t i o n techn ique, 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 ox ides .

Other paramagnet ic cen te rs may be p resen t as a separa te

3 t Fe i s by f a r t h e most abundant e s r observab le paramagnet ic i o n i n n a t u r a l

2t c lays . Other i o n s such as Mn

magnet ic spec ies (a, Fez' and T i under most c o n d i t i o n s . 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 concen t ra t i ons o f framework Fe3+.

i n t e r a c t i o n s which broaden t h e l i n e s o f su r face exchange i o n s a r e min ima l i n t h i s

c l a y . T h i s i s one reason 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 su r face bound spec ies . It must be no ted , however, t h a t some h e c t o r i t e

samples, depending on exac 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 concen t ra t i ons

o f i r o n ox ides wh ich g i v e r i s e t o a v e r y broad l i n e (AH > 1000 G) cen tered near

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

a r e non-Kramers spec ies and a r e e s r s i l e n t 2+

There fore , d i p o l a r

g = 2.0.

6.5.1 Smect i t e s

O l i v i e r e t a l . (1975) have examined t h e e s r spec t ra o f seve ra l smec t i t es . All 3 t samples e x h i b i t e d prominent f e a t u r e s near g = 4.3 wh ich 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 oc tahedra 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 . S i m i l a r

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 . , 1976a, 1976b, 1977). The non-equ iva len t oc tahedra 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 hyd roxy l groups i n t h e Fe04(0H)2

c a v i t y .

e x p l a n a t i o n f o r non -equ iva len t oc tahedra l Fe3'.

resonance near 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 smec t i t es (McBride e t a1 ., 1975c, 1975d; Be rkhe ise r and Mor t l and , 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 hyd ra ted L i

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 . Moreover,

thermal m i g r a t i o n o f L i

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 wh ich 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 m i n e r a l .

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

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

+ and Ca2+ exchange

+ i n t o t h e vacant oc tahedra l p o s i t i o n o f t h e m i n e r a l ,

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

1 5 2

under t h e above c o n d i t i o n s . These obse rva t i ons 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 assoc ia ted w i t h cen te rs 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.

may a r i s e f rom oc tahedra l Fe3+ i o n s ad jacen t t o c h a r g e - d e f i c i e n t Hg

n o n - d e f i c i e n t A13+ s i t e s .

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 .

There fo re , t h e two non-equ iva len t Fe3+ s i t e s

s i t e s and

Th is model a l l o w s a l l Fe3+ i o n s t o occupy oc tahedra l

2+

6.5.2 K a o l i n i t e s

The e s r spec t ra o f k a o l i n i t e s v a r y marked ly , 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 wh ich a r e independent o f

i m p u r i t y phases.

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 wh ich a r i s e f rom 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 des ignated 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 cen te rs 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 un reso lved 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 ass igned t o

oc tahedra 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

such as 3 d isp lacements o r 120" r o t a t i o n s . w i t h oc tahedra 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 rde red s t a c k i n g .

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 f rom two d i s t i n c t o r i e n t a t i o n s

o f su r face OH groups ad jacen t t o t h e o rde red Fe3+ s i t e s (G iese and Da t ta , 1973) .

The resonances near g = 2.0 u s u a l l y c o n s i s t o f two asymmetr ic 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 these l i n e s has been des ignated c e n t e r A.

p a r a l l e l t o t h e k a o l i n i t e c - a x i s . Schwartz e t a l . (1979) have used t h i s observa-

t i o n t o de termine 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 tend 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 pe rpend icu la r t o t h e s t r e s s d i r e c t i o n .

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.

absent 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 subsequent ly 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 a l . , 1974, 1976, 1977). The e f f e c t s of Fe3+ and 11g2+ on 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 an 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 rapped 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 . ,

1974) .

Mg2+ ( o r Fe

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

nb Centers I I a and I I b a r e assoc ia ted

The

I I = The spec ies

The un ique a x i s l i e s

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 annea l i ng a t 400". Annea l ing The A c e n t e r i s

H a l l (1980a) has es t ima ted t h a t replacement o f one A13+ pe r thousand by 2+

) i s s u f f i c i e n t t o account f o r t h e c o n c e n t r a t i o n o f A cen te rs .

1 5 3

I g=4.0

6 '

D

E

I g= 2.0

Natural kaol ini te

Mg doped kaolinite (no signals)

Fe3+ doDed kaol inite

Mg doped kaolinite X-irradiated

Mg doped kaolinite X-irradiated and annea 1 ed

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

Figure 6.10. Esr spectra of synthetic kaolinites (from Angel e t al, 1976).

1 5 4

Two o t h e r d e f e c t cen te rs (B1 and B 2 ) have been observed i n k a o l i n i t e , b u t t h e es 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 la lden, 1975). These cen te rs e x h i b i t g va lues near 2.0, b u t t h e y a r e d i s t i n g u i s h e d

from t h e A cen te rs 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

unpa i red sp ins w i t h t h e nuc lea r s p i n o f A1 w i t h I = 5 / 2 . The c o n c e n t r a t i o n o f B

cen te rs can be g r e a t l y inc reased by X - i r r a d i a t i o n . 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 des t royed by i r r a d i a t i o n and annea l i ng (Angel and

H a l l , 1973) . The B cen te rs a r e c l e a r l y assoc ia ted w i t h A1 cen te rs . The most

l i k e l y assignments a r e 0

l a t t i c e . Many o t h e r e s r a c t i v e spec ies 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 ln2+-conta in ing phases (Meads and Halden, 1975), i r o n ox ides , 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 incen t , 1978).

+ cen te rs wh ich b r i d g e A l , Si and A1 , A1 p a i r s i n t h e

6.6 ORGANIC RADICALS AND NITROXIDE S P I N PROBES

6.6.1 Arene Radical Cat ions

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

aromat ic mo lecu les and c e r t a i n t r a n s i t i o n meta l i o n s i n t h e i n t e r l a y e r s o f smec-

t i t e s .

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 reac-

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 wh ich 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 ir 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 pe r tu rbed . F u r t h e r s t u d i e s (F lo r t land and Pinnavaia;

1971; P innava ia and Mor t land, 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 develop-

ment o f t h e r e d "Type 11" benzene spec ies was preceeded by t h e l o s s o f wa te r

f rom t h e s i l i c a t e su r face and phys i ca l a d s o r p t i o n o f benzene. The removal o f

water f rom t h e c o o r d i n a t i o n sphere o f t h e Eu

edge-bonded form o f coo rd ina ted benzene, wh ich was des ignated "Type I" benzene.

The removal o f s t i l l more i n n e r sphere water l e d t o t h e f o r m a t i o n o f some C6H6+

a long w i t h t h e r e d Type I 1 spec ies . Fu r the r s low r e a c t i o n o f t h e Type I 1 spec ies

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 robab ly parapo lypheny l

(Mor t l and and Ha l l o ran , 1976; S toesse l e t a l . , 1977).

Type I spec ies g i v e s a s t rong , broad Cu2+ resonance, i n d i c a t i n g t h a t t h e ox ida -

t i o n s t a t e o f t h e meta l remains unchanged a t t h i s s tage o f t h e r e a c t i o n . Reduc-

t i o n o f Cu2+ t o Cu and t h e fo rma t ion o f C6H6 and Type I 1 benzene i s accompanied

by t h e replacement o f t h e Cu2+ resonance by a sharp i s o t r o p i c resonance near g =

2.0 t h a t may be ass igned t o C H l e s s than % 3% o f t h e i n i t i a l Cu

obse rva t i on suggests t h a t most o f t h e sp ins a r e l o s t th rough s p i n p a i r i n g i n t h e

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 Mor t l and (1969)

2+ i o n s a f f o r d e d a ye l low-green

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 igu re 6.11. The

+ +

+ (Ruper t , 1973; P innava ia e t a l , 1974) . However,

62+ + sp ins a r e recovered as C6H6 . Th is l a t t e r

1 5 5

+cuL;

//

polymer

F i g u r e 6.11. React ions of benzene w i t h Cu2+ i n s rnec t i te ( f rom P innava ia , 1977) .

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.

0

F i g u r e 6.13. S t r u c t u r e o f Ternpamine+.

1 5 6

Type I 1 species. The n a t u r e o f t h e Type I 1 spec ies i s s t i l l u n c e r t a i n , b u t a

model based on th rough 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 innava ia , 1976b).

F igu re 6.12. D i s t o r s i o n s i n t h e p a i r e d Jahn-Te l le r C6H6

t o t h e unusual i r absorp t i ons i n t h e C=C s t r e t c h i n g r e g i o n .

sur faces has

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 ince

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 agent f o r r a d i c a l c a t i o n

fo rmat ion , 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 rep lace -

ments f o r Cu

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 ven i n +

spec ies may c o n t r i b u t e

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 romat i c mo lecu les w i t h Cu2+ on s m e c t i t e

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

2+

2+ (P innava ia e t a l . , 1974).

6.6.2 N i t r o x i d e Sp in 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 smec t i t e c l a y su r faces . The p ro tona ted

form o f 4-amino-Z,2,6,6- t e t r a m e t h y l p ipe r id ine -N-ox ide (Ternpami ne ) has been

e s p e c i a l l y u s e f u l i n these s t u d i e s . The s t r u c t u r e o f Tempamine i s i l l u s t r a t e d

i n F i g u r e 6.13.

l o w v i s c o s i t y , r a p i d tumb l ing averages t h e p r i n c i p a l components o f t h e g tenso r

and t h e h y p e r f i n e c o u p l i n g tenso r , A. Thus, a t h r e e - l i n e e s r spectrum i s observed

+ +

4 When t h e s p i n probe i s d i s s o l v e d i n low c o n c e n t r a t i o n (10- 1) i n s o l v e n t s o f

1 = + g2,) and A,, = - (Axx + A + A ) i s observed. I f t h e w i t h go = 3 (gxx

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 umb l ing i s v e r y s h o r t ( T ~ < 2, 10:

1 gYY 3 Yy zz

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 id th .

t i o n t i m e increases i n t h e r e g i o n o f modera te ly f a s t t umb l ing ( T ~ < 5 x l o - 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 id ths and h e i g h t s o f t h e

t h r e e l i n e s beg in 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 magnet ic Hami l t on ian .

f rom t h i s l i n e broaden ing phenomenon (Smi th , 1972; Nord io , 1976; Sachs and L a t o r r e ,

1974).

t h e spectrum becomes more complex.

- < T~ 5 s e c - l , t h e s o - c a l l e d s low mot iona 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 over lapped

reg ion . The a n a l y s i s o f c o r r e l a t i o n t imes 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 ince 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

th rough Stokes law,

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

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

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 scous f l u i d media,

I n t h e range o f c o r r e l a t i o n t imes, l o p 9 sec

3 4nqr Tc =3kT ( 4 ) e s r s t u d i e s o f s p i n probes on c l a y su r faces m i g h t be expected t o y i e l d t h e

m ic roscop ic 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)

1 5 7

F igu re 6.14. Esr spect ra o f Ternpamine+ doped a t t h e 1% l e v e l i n t o f u l l y wet ted K+-hec to r i t e f i l m ( f rom McBride, 1980).

#

, 20 GAUSS , n .

+ F igu re 6.15. Esr spect ra o f Tempamine doped a t t h e 1% l e v e l i n t o

K+-hector i te d r i e d a t 110" ( f rom McBride, 1980).

1 5 8

has found 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 su r faces 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

hydra ted . 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 spec t ra o f Ternpamine+ on a f u l l y we t ted

f i l m sample o f K - h e c t o r i t e w i t h t h e magnet ic 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 pe rpend icu la r t o t h e s i l i c a t e sheets. For t h e pe rpend icu la r o r i e n t a t i o n ,

Al = 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, wh ich i s d e f i n e d

as be ing 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 apparent ang le o f 45".

sample o f K + - h e c t o r i t e d r i e d a t 110".

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 pe rpend icu la 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 magnet ic f i e l d d i r e c t i o n .

The spec t ra shapes a r e those expected f o r n i t r o x i d e probe i n t h e s low mot iona l

r e g i o n . C l e a r l y , t h e a l ignment 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 wa te 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 .

probe i s app rox ima te l y a t r i g h t angles t o t h e s i l i c a t e sheets .

f o r t umb l ing i n t h e f u l l y we t ted K - h e c t o r i t e , va lues o f 1-3 x lo - ' sec a r e ob ta ined (McBride, 1977b).

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 s t a t e . La rge r va lues o f T~ a r e found

f o r t h e pe rpend icu la 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 magnet ic f i e l d com-

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 . H - h e c t o r i t e s e x h i b i t T~ va lues about t w i c e as l o n g as M - h e c t o r i t e s , perhaps due

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 so ,

e thano 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 imes

which a r e approx ima te l y two o rde rs o f magnitude l o n g e r t h a n t h e hyd ra ted 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 mo t ion i n t h e fo rmer case.

+

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

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

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

I f t h e l i n e w i d t h s o f Ternpamine' 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 +

The va lues i n t h i s range a r e 20 t o 60 t imes lower

2+

+ 2+

+

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McBride, M.B., P innava ia , T .J . and Plort land, H.M., 1975b. E l e c t r o n s p i n r e l a x a -

Copper( 11) i n t e r a c t i o n s w i t h montmor i l -

Su r face p r o p e r t i e s of mixed c o p p e r ( I 1 ) -

Am.

Clays and C lay Miner. , 23: 103-107

C lay Miner . , 10 : 313-345.

Format ion o f copper ( 11) arene complexes

1 6 1

01

01

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