Probing the Surface of Clays in Aqueous Suspension by Fluorescence

Chapter 19

Probing the Surface of Clays in Aqueous Suspension by Fluorescence Spectroscopy of Proflavine 1

2

3

Jos Cenens , Robert A. Schoonheydt, and Frans C. De Schryver

Downloaded by PEPPERDINE UNIV on August 24, 2017 | http://pubs.acs.org Publication Date: November 29, 1990 | doi: 10.1021/bk-1990-0415.ch019

1

Chemical Research Centre, Shell Louvain-La-Neuve, Avenue Jean Monnet 1, Β-1348 Ottignies Louvain-La-Neuve, Belgium Laboratorium voor Oppervlaktechemie, K.U.Leuven, K. Mercierlaan, 92, B-3030 Leuven, Belgium Department of Chemistry, K.U.Leuven, Celestijnenlaan, 200F, B-3030 Leuven, Belgium 2

3

Aqueous suspensions of barasym SSM-100, hectorite and laponite, exhanged with Na , K , Cs and Ca were investigated by fluorescence spectroscopy of adsorbed proflavine. Monomers, dimers and protonated species are found on the surface. The relative concentration of these species depends on the type of clay, the exchangeable cation and the loading. On the basis of the evolution of the emission intensities with loading three categories of clays are described. The first category comprises Na -, Ca - and K -hectorite, barasym at pH = 9 and Ca -laponite. For these clays the fluorescence intensity decreases with loading, following Perrin's law. Effective surface areas in aqueous suspension are estimated for the first time with Na hectorite as a reference. The second category is composed of Na - and K -laponite and barasym at pH= 7. A minimum in the fluorescence intensity is found at a characteristic loading. This is due to nonhomogeneous distribution of proflavine molecules and/or a change in the state of aggregation of the clay. Cs -laponite and Cs -hectorite comprise the third category. Here preferential adsorption of proflavine occurs in the interlamellar space of the aggregates with formation of monolayers of monomers and quenching of the fluorescence with increasing loading does not occur to an appreciable extent. +

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0097-6156/90/0415-0378$06.00/0 ο 1990 American Chemical Society

Coyne et al.; Spectroscopic Characterization of Minerals and Their Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

19. CENENS ET AL.

Probing the Surface of Clays in Aqueous Suspension +

P r o f l a v i n e or 3,6-diaminoacridine (PFH ) i s a c a t i o n i c dye, the s t r u c t u r e of which i s shown i n F i g u r e 1. In d i l u t e aqueous s o l u t i o n i t s v i s i b l e a b s o r p t i o n band a t 445 nm (22400 cm"" ) and the shoulder around 425 nm (23530 cm" ) are r e s p e c t i v e l y the 0-0 and 0-1 vibronic components of a *-π transition. The corresponding fluorescence band i s almost structureless at room temperature with a maximum a t 506 nm (19760 cm" ). Upon p r o t o n a t i o n a second proton i s attached t o one of the N's i n p o s i t i o n 3 or 6 ( P F H o ) . T h i s leads t o a red s h i f t o f both the a b s o r p t i o n ana the f l u o r e s c e n c e bands t o 455 nm (22000 cm" ) and 545 nm (18350 cm" ) r e s p e c t i v e l y . When the concentration is increased, the 425 nm band i n c r e a s e s , due t o o v e r l a p with a band a t 428 nm (23360 cm" ). I t i s the dimer ( ( P F H ) ) a b s o r p t i o n , which has no f l u o r e s c e n c e (1- 3). When the dye i s ion-exchanged on c l a y s , i t i s concentrated i n a r e l a t i v e l y small volume around the c l a y p a r t i c l e s . The s i z e of the volume depends on the s w e l l i n g c h a r a c t e r i s t i c s o f the c l a y and thus on such f a c t o r s as (1) the s i z e and the charge d e n s i t y o f the c l a y p a r t i c l e s ; (2) the type of exchangeable c a t i o n and (3) the a b i l i t y of the dye t o penetrate i n the i n t e r l a m e l l a r space o f the aggregates. T h i s c o n c e n t r a t i o n e f f e c t shows up i n the a b s o r p t i o n and emission s p e c t r a of the adsorbed dyes, because i t a f f e c t s the r e l a t i v e amounts of monomers and dimers. T h i s has been shown by a study of the a b s o r p t i o n s p e c t r a of adsorbed methylene blue and p r o f l a v i n e ( 4 , 5 ) . For Na and C a - h e c t o r i t e and barasym the a b s o r p t i o n s p e c t r a can be q u a n t i t a t i v e l y i n t e r p r e t e d i n terms of the f o l l o w i n g e q u i l i b r i u m on the s u r f a c e 1

1

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1

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η M

*

[M]

n

(1)

with η = 2 or 3 (6) . T h i s i s not p o s s i b l e f o r l a p o n i t e s and Κ - and Cs -exchanged h e c t o r i t e s . T h i s i n a b i l i t y has been a s c r i b e d t o the f a c t t h a t the s u r f a c e i s not f r e e l y a v a i l a b l e t o both monomers and aggregates, presumably because of aggregation of the elementary c l a y p l a t e l e t s (6,7). These data i n d i c a t e t h a t a study of the r e l a t i v e i n t e n s i t i e s of the a b s o r p t i o n bands g i v e s q u a l i t a t i v e i n s i g h t i n t o the aggregation of c l a y p a r t i c l e s i n aqueous suspensions. In p r i n c i p l e , s i m i l a r i n f o r m a t i o n can be obtained from the f l u o r e s c e n c e s p e c t r a of p r o f l a v i n e , because PFH f l u o r e s c e s i n t e n s i v e l y and ( P F H ) does not f l u o r e s c e i n aqueous s o l u t i o n . In a p r e l i m i n a r y account we have d i s c u s s e d the f l u o r e s c e n c e s p e c t r a o f p r o f l a v i n e on c l a y s i n aqueous suspension (5,7). The p o s i t i o n s of both the a b s o r p t i o n and fluorescence bands of the different forms of p r o f l a v i n e on c l a y s and i n aqueous s o l u t i o n are shown i n F i g u r e 2. The e x t i n c t i o n c o e f f i c i e n t s are g i v e n i n Table +

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379


380

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES

b F i g u r e 1: P r o f l a v i n e monomer (a) and dimer (b)


19. CENENS ET AL.

Probing the Surface of Clays in Aqueous Suspension


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I. The s m a l l changes i n band p o s i t i o n s r e f l e c t the slightly different environment offered by the clay suspensions with r e s p e c t t o aqueous s o l u t i o n s . T h i s paper i s concerned with a q u a n t i t a t i v e study o f the f l u o r e s c e n c e i n t e n s i t y o f p r o f l a v i n e on several c l a y s . The i n t e n s i t y o f the f l u o r e s c e n c e changes with l o a d i n g i n a c h a r a c t e r i s t i c way, which depends on the type o f c l a y and the type o f exchangeable c a t i o n . An attempt i s presented t o c a l c u l a t e e f f e c t i v e s u r f a c e areas i n aqueous suspension.


Experimental H e c t o r i t e (H), barasym SSM-100 (BS) from the Source Clays Repository o f the Clay M i n e r a l s S o c i e t y and l a p o n i t e Β (L) from Laporte I n d u s t r i e s were used. The c l a y s were exchanged with 1 M NaCl s o l u t i o n s t h r e e times, the < 2 mm f r a c t i o n was separated by c e n t r i f u g a t i o n and s t o r e d i n 1 M NaCl a t 277 K. The p r e p a r a t i o n o f the < 0.3 μπι f r a c t i o n , the i o n exchange with the monomer of p r o f l a v i n e , PFH , and the spectroscopy have been d e s c r i b e d i n d e t a i l (5). A l l manipulations were done i n the dark. The t o t a l dye c o n c e n t r a t i o n i n the suspensions was 2.5 * 10" M. At pH = 7 and i n the absence o f c l a y the r a t i o o f the monomer t o the protpnated monomer, [PFH ]/[PFH ]/ was l a r g e r than 3 * 10 and the s o l u t i o n contained 0.2% 1.3% dimers, ( P F H ) , the exact number depending on the d i m e r i z a t i o n constant used i n the c a l c u l a t i o n . Loadings i n the range 0 - 30% of the N a exchange c a p a c i t y (CEC) were r e a l i z e d by a d j u s t i n g the amount o f c l a y i n the suspension. A d s o r p t i o n was q u a n t i t a t i v e . The CEC v a l u e s were 0.464, 0.526 and 0.568 meq/g f o r r e s p e c t i v e l y BS, Η and L. B u f f e r e d suspensions (pH = 9) were prepared by adding 10 cm o f a s o l u t i o n , c o n t a i n i n g 1 mmol/dm NaOH/NaHC0 and 10 Mmole/dm p r o f l a v i n e t o 30 cm o f c l a y suspension. When the exchanges were done without pH c o n t r o l , we c a l l them exchanges i n n e u t r a l c o n d i t i o n s . Samples of h e c t o r i t e and l a p o n i t e s a t u r a t e d with K , C s and C a were prepared by exchanging d e s a l t e d Na-clay with the d e s i r e d C I " s a l t and d i a l y z i n g u n t i l no C I " was detected with the AgN0 t e s t . In a l l cases the absorbance o f the suspensions a t 445 nm was s m a l l e r than 0.2. T h i s small absorbance v a l u e minimizes r e a b s o r p t i o n of the emitted l i g h t . Oxygen was not removed from the suspensions, s i n c e no measurable e f f e c t on the f l u o r e s c e n c e i n t e n s i t y was found. E x c i t a t i o n s p e c t r a were recorded i n the range 350 500 nm with the emission wavelength preset at fixed p o s i t i o n s i n the range 500 - 600 nm with a SPEX F l u o r o l o g model 1691 s p e c t r o f l u o r i m e t e r . e

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19. CENENS ET AL.

Probing the Surface ofClays in Aqueous Suspension

Results Spectroscopy. The absorption and f l u o r e s c e n c e s p e c t r a o f p r o f l a v i n e on H and BS a r e p u b l i s h e d (5) and t h e p o s i t i o n s o f t h e band maxima a r e summarized i n f i g u r e 2. T y p i c a l e x c i t a t i o n s p e c t r a a r e shown i n f i g u r e 3. F o r H and L t h e maximum i s a t 456 nm with shoulders a t 430 nm and i n t h e range 400-410 nm. These a r e r e s p e c t i v e l y t h e 0-0, 0-1 and 0-2 v i b r o n i c bands o f PFH . The p o s i t i o n s are independent o f t h e emission wavelength i n t h e range 500 - 580 nm and t h e l o a d i n g . F o r n e u t r a l BS suspensions, the e x c i t a t i o n s p e c t r a have a maximum a t 460 nm and t h e r e i s a weak band a t 365 nm, which i s due PFH . The 460 nm band i s then a mixture o f t h e 0-0 v i b r o n i c components o f PFH and PFH . The l a t t e r disappears a t pH = 9. Fluorescence s p e c t r a o f p r o f l a v i n e on l a p o n i t e a r e compared with t h e spectrum o f a 2.5 * 10 M solution i n f i g u r e 4. These s p e c t r a a r e t y p i c a l f o r PFH . In g e n e r a l , the emission maximum s h i f t s t o t h e r e d and t h e i n t e n s i t y decreases with i n c r e a s i n g l o a d i n g . Both phenomena a r e due t o d i m e r i z a t i o n . The extent o f t h e r e d s h i f t depends on the type o f exchangeable c a t i o n , as shown i n f i g u r e 2. For Cs* i t i s from 490 nm a t 0.2% l o a d i n g t o 512 nm a t 8.2%; f o r K from 502 t o 512 nm and f o r Na* and C a from 508 t o 514 nm. S i m i l a r r e d s h i f t s were observed f o r h e c t o r i t e s . For BS t h e emission maximum i s a t 535 nm a t the s m a l l e s t loadings a t n e u t r a l pH. T h i s i s due t o PFH, " " (5) . A t higher loadings o r a t pH = 9 f o r a l l l o a d i n g s t h e emission maximum i s a t 502 nm, t y p i c a l f o r PFH . +


2

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Quantum Y i e l d s . The quantum y i e l d i s t h e r a t i o o f t h e f l u o r e s c e n c e i n t e n s i t y t o t h e amount o f l i g h t absorbed. The r a t i o s o f t h e quantum y i e l d o f a 2.5 ΙΟ" M s o l u t i o n , c o n t a i n i n g only PFH"", and t h e quantum y i e l d s o f p r o f l a v i n e on t h e c l a y s , Φ / Φ with Φ = 0.34 ( 1 ) , a r e shown i n f i g u r e s 5-8 as a f u n c t i o n o f t h e l o a d i n g , expressed i n % o f t h e CEC. An i n c r e a s e o f Φ4 0.05

SURFACE CONCENTRATION (MOL.DM-3) F i g u r e 9 : P e r r i n p l o t s f o r Na -H ( • ) and Na -BS ( Ο ) . The s u r f a c e areas f o r t h e c a l c u l a t i o n o f t h e s u r f a c e c o n c e n t r a t i o n a r e r e s p e c t i v e l y 775 and 110 m /g. The b l a c k dots a r e t h e p o i n t s o f Na -H a t small l o a d i n g s r e c a l c u l a t e d with a s u r f a c e area o f 125 m /g. +

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SURFACE CONCENTRATION (MOL.DM~3) z +

Figure 10: P e r r i n plot of Ca -L. The s u r f a c e c o n c e n t r a t i o n i s c a l c u l a t e d with a s u r f a c e area o f 819 m /g2



19. CENENS ET AL.

Probing the Surface of Clays in Aqueous Suspension

quantum yield increases with loading. (2) Due to a d s o r p t i o n o f p r o f l a v i n e the c l a y p a r t i c l e s change t h e i r aggregation s t a t e a t the c h a r a c t e r i s t i c l o a d i n g s o f Table II. PFH becomes i n t e r c a l a t e d and u n a v a i l a b l e f o r quenching. Additional evidence that one o r both o f the mechanisms are o p e r a t i v e comes from the data obtained on Cs - s a t u r a t e d c l a y s . Now, the i n d i v i d u a l c l a y p l a t e l e t s i n the aggregates are separated by a t most a monolayer o f water (10). The proflavine molecules adsorb p r e f e r e n t i a l l y i n t h a t i n t e r l a m e l l a r space t o r e p l a c e t h e water and t o form a monolayer o f p r o f l a v i n e molecules. No dimers are formed, except f o r a small amount on the e x t e r n a l s u r f a c e o f the aggregates, and the quantum y i e l d i s almost independent o f the l o a d i n g . Surface Areas i n Aqueous Suspension. Following Perrin's law, the slopes o f the s t r a i g h t l i n e s i n f i g u r e s 9 and 10 are p r o p o r t i o n a l t o the quenching volume (appendix 1 ) . On the other hand, the s p e c t r a o f f i g u r e s 2-4 show t h a t the s p e c t r o s c o p i c p r o p e r t i e s o f the adsorbed molecules are, on the average, independent o f the type o f c l a y and of the type o f s i t e s . Equal quenching volumes o r equal s l o p e s a r e then expected. D i f f e r e n c e s i n s l o p e s must then be a s c r i b e d t o the f a c t t h a t the s u r f a c e c o n c e n t r a t i o n s were c a l c u l a t e d with the wrong s u r f a c e areas (775, 819 and 133 m /g f o r r e s p e c t i v e l y H, L and BS) . They can be adapted t o g i v e equal slopes by c a l c u l a t i o n o f an e f f e c t i v e s u r f a c e area, keeping the t h i c k n e s s o f the adsorbed l a y e r constant a t 1 nm. T h i s can be done with the assumption t h a t f o r Na -H the s u r f a c e c o n c e n t r a t i o n , c a l c u l a t e d with the s u r f a c e area o f 775 m /g i s exact ( f i g u r e 9 ) . The slopes o f the s t r a i g h t l i n e s f o r the other c l a y s can be made equal t o t h a t o f Na -H by adapting the s u r f a c e concentrations o r the s u r f a c e areas (appendix 2 ) . The e f f e c t i v e s u r f a c e areas, so obtained, are shown i n Table I I I . For BS a t pH = 9 110 m /g i s obtained, a v a l u e which i s very c l o s e t o the N -BET s u r f a c e area. I t f o l l o w s t h a t the BS p a r t i c l e s i n aqueous suspension a r e , i n our c o n d i t i o n s , the p a r t i c l e s o f the powder: t h e r e i s no s w e l l i n g o r d i s a g g r e g a t i o n . C a - H and C a - L have almost the same s u r f a c e area as Na -H: they a r e f u l l y swollen so as t o have t h e i r t o t a l s u r f a c e area a v a i l a b l e t o monomers and dimers o f p r o f l a v i n e . K -H and Cs -H have s m a l l e r e f f e c t i v e s u r f a c e areas and thus a r e c l e a r l y aggregated i n aqueous suspension. For Na -H, C a - H and K -H, which have a second s t r a i g h t l i n e i n f i g u r e s 9 and 10 a t the lowest l o a d i n g s , e f f e c t i v e s u r f a c e areas f o r t h i s low l o a d i n g regime can be c a l c u l a t e d i n the same way: by r e c a l c u l a t i n g the s u r f a c e c o n c e n t r a t i o n so as t o make the s l o p e s o f the s t r a i g h t l i n e s equal t o t h a t o f Na -H a t h i g h e r l o a d i n g s . T h i s i s shown i n f i g u r e 9 f o r Na -H. The e f f e c t i v e 2

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392


z

Table I I I . C a l c u l a t e d s u r f a c e areas (m /q) o f category 1 c l a y s i n aqueous suspension clay +

Na -H 2+

Ca -H +

Cs -H +

K -H 2 +

Ca -L +

Na -BS

external surface

total

surface

125

775

165

725

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580

135

485

-

840

110

110


19. CENENS ET AL.

Probing the Surface of Clays in Aqueous Suspension 2

s u r f a c e areas are 125, 165 and 135 m /g. I t looks as i f , a t these small loadings, p r o f l a v i n e probes only the e x t e r n a l s u r f a c e of aggregates of c l a y p a r t i c l e s and does not penetrate i n t o the i n t e r l a m e l l a r space. The p i c t u r e of these c l a y s i n aqueous suspension, which emerges from these data, i s t h a t they occur as aggregates o f i n d i v i d u a l c l a y p l a t e l e t s . At the s m a l l e s t loadings ( < 1% of the CEC) proflavine adsorbs p r e f e r e n t i a l l y on the e x t e r n a l s u r f a c e of the aggregate. At h i g h e r l o a d i n g s i n t e r l a m e l l a r a d s o r p t i o n occurs both i n the form of PFH and of ( P F H ) on the t o t a l s u r f a c e i n the case of Na and C a ; on p a r t of the s u r f a c e i n the case of K -H. The data p o i n t s of Cs*-H above a l o a d i n g o f 2% o f the CEC can a l s o be f i t t e d with the P e r r i n equation. The r e s u l t i n g e f f e c t i v e s u r f a c e area (Table I I I ) i s s i m i l a r t o t h a t of K -H. As there are only f o u r data p o i n t s , the value of the e f f e c t i v e s u r f a c e area i s s u b j e c t t o some u n c e r t a i n t y . But, q u a l i t a t i v e l y , as f o r K -H, proflavine probes only p a r t of the s u r f a c e due t o aggregation. +

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Conclusions The fluorescence spectroscopy of proflavine is a technique t o study the s u r f a c e of c l a y s i n aqueous suspensions. The c l a y s must be e s s e n t i a l l l y F e - f r e e . The information, which one can o b t a i n i s s i m i l a r t o t h a t obtained from absorption spectroscopy. The e v o l u t i o n o f the f l u o r e s c e n c e i n t e n s i t y with l o a d i n g i s c h a r a c t e r i s t i c of the type o f c l a y and the type of exchangeable c a t i o n . Three c a t e g o r i e s of samples can be d i s t i n g u i s h e d . The f i r s t category comprises Na - C a - and K -H, C a - L and BS (pH=9). The monotonous decrease of the quantum y i e l d with l o a d i n g f o l l o w s P e r r i n ' s law. From t h i s e f f e c t i v e s u r f a c e areas of these c l a y s i n aqueous suspensions can be c a l c u l a t e d . T h i s i s the f i r s t time t h a t i t has been p o s s i b l e . The second category of samples comprises Na -L, Κ -L and BS i n n e u t r a l suspension: the quantum y i e l d goes through a minimum with l o a d i n g . T h i s can be a t t r i b u t e d t o a non-homogeneous d i s t r i b u t i o n o f the molecules over the s u r f a c e or t o a change i n c l a y aggregation w i t h l o a d i n g . The Cs - c l a y s are s p e c i a l i n t h a t they p r e f e r e n t i a l l y adsorb PFH i n the i n t e r l a m e l l a r space. +

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Appendix l . +

For the quenching of the f l u o r e s c e n c e of PFH by (PFH )2 on the s u r f a c e of c l a y s P e r r i n ' s equation (11) i s In (* /* ) - V N 0

m

+

a

[(PFH ) ]

Here, the dimer concentration concentration of dimers, calculated

2

i s the surface from the dimer


393

394


a b s o r p t i o n band a t 430 nm (5,6) w i t h the f o l l o w i n g procedure. N i s Avogadro's number. Assume a random distribution of the adsorbed p r o f l a v i n e molecules over the s u r f a c e . The s u r f a c e areas are 775, 819 and 133 m /g f o r H, L and BS r e s p e c t i v e l y . The number f o r BS i s obtained from Van Olphen and F r i p i a t (12) . The two other numbers are c a l c u l a t e d f o r average p a r t i c l e s i z e s o f 300 nm and 30 nm f o r H and L r e s p e c t i v e l y . The former i s the s i z e f r a c t i o n o f H used f o r our experiments, the l a t t e r i s taken from Van Olphen and F r i p i a t (12). The s u r f a c e volume i s taken as the s u r f a c e area times 1 nm, the t h i c k n e s s o f the adsorbed l a y e r i n which the p r o f l a v i n e molecules r e s i d e . The same t h i c k n e s s has been taken by Nakamura and Thomas (13) and DellaGuardia and Thomas (14) for tris(2,2'b i p y r i d i n e ) r u t h e n i u m ( I I ) on l a p o n i t e and m o n t m o r i l l o n i t e . One c o u l d a l s o take the t h i c k n e s s o f the double l a y e r , i f i t were p o s s i b l e t o c a l c u l a t e . T h i s only changes the numbers i n the a b s c i s s a e i n f i g u r e s 9 and 10. V i s the quenching volume. I t i s the volume around the e x c i t e d molecule i n which the quencher quenches the emission with u n i t e f f i c i e n c y . Quencher molecules o u t s i d e t h a t volume don't quench the emission. P h y s i c a l l y , P e r r i n ' s equation i s v a l i d f o r s t a t i c quenching: e x c i t e d molecules and quenchers don't move i n and out the quenching volume w i t h i n the l i f e t i m e o f the excited state. a


2

Appendix 2 The s u r f a c e volume i s , expressed i n dm , the s u r f a c e area i n dm times the t h i c k n e s s of the S t e r n l a y e r . We assume i t t o be 1 nm o r 10" dm. In every c l a y suspension t h a t we have i n v e s t i g a t e d the p r o f l a v i n e c o n c e n t r a t i o n was 2.5 10" M. The s u r f a c e c o n c e n t r a t i o n i s then 2.5 10~ M/surface volume o r 250/ s u r f a c e area (dm ). 2

8

6

6

2

Acknowledgment The authors thank the Fund f o r J o i n t B a s i c Research (Belgium) f o r f i n a n c i a l support o f t h i s work. R.A.S. acknowledges the N a t i o n a l Fund f o r S c i e n t i f i c Research (Belgium) f o r h i s p o s i t i o n as Research D i r e c t o r . J.C. acknowledges a pH.D. grant from the I n s t i t u u t voor Wetenschappelijk Onderzoek i n N i j v e r h e i d en Landbouw. The authors thank Laporte I n d u s t r i e s L t d f o r the g i f t o f Laponite. Literature cited

1. Haugen, G.R.; Melhuish, W.H. Trans. Faraday Soc. 60, 386 - 394.


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19. CENENS ET AL. Probing the Surface of Clays in Aqueous Suspension 395

2. Ζanker, V.; Miethke, Ε. Z. Naturforschung A 1957, 12, 385-395. 3. Bailey, M.L. In Heterocyclic Compounds - Acridines; Acheson, R.M., Ed.; J. Wiley & Sons: New York; 1973; Chapter 10. 4. Cenens, J.; Schoonheydt, R.A. Clays and Clay Min. 1988, 36, 214 - 224. 5. Cenens, J.; Vliers, D.P.; Schoonheydt, R.A.; De Schryver, F.C. in Proc. Int. Clay Conf. Denver 1985; Schultz, L.G., Van Olphen, H. and Mumpton, F.A., eds., The Clay Minerals Society, Bloomington, Indiana, 1987; pp 352 - 358. 6. Cenens, J.; Schoonheydt, R.A. Int. Clay Conf. Strasbourg 1989, submitted. 7. Cenens, J Ph.D. Thesis, Katholieke Universiteit Leuven, Leuven, 1987. 8. Lopez Arbeloa, F.; Ruiz Ojeda, P.; Lopez Arbeloa, I. J. Photochem. & Photobiol. A: Chem. 1988, 45, 313-323. 9. Wright, A.C.; Granquist W.T.; Kennedy, J.V. J. Catalysis 1972, 25, 65-80. 10 Newman, A.C.D. In Chemistry of Clays and Clay Minerals; Newman, A.C.D., Ed.; The Mineralogical Society: London, 1987; p. 237. 11. Turro, N.J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, 1978; 628pp. 12. Van Olphen, H.; Fripiat, J.J. Data handbook for clay minerals and other non-metallic minerals; Pergamon Press: New York, 1979; 346pp. 13. Nakamura, T.; Thomas, J.K. Lanomuir 1985, 1, 568-573. 14. DellaGuardia, R.A.; Thomas, J.K. J. Phys. Chem. 1983, 87, 990-998. 15. Schoonheydt, R.A.; Cenens, J.; De Schryver, F.C. J. Chem. Soc., Faraday Trans. I 1986, 82, 281-289. RECEIVED August 7, 1989


Probing the Surface of Clays in Aqueous Suspension by Fluorescence

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