Spectroscopic Characterization of Minerals and Their Surfaces

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Chapter 18

Surface Activities of Clays

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Jose J. Fripiat Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, WI 53201

Clays, and especially swelling clays, are made from anisotropic microcrystals displaying large specific surface areas. However, because these microcrystals have a strong tendency to aggregate into tactoids, only a fraction of the surface area is available for surface reactions. Therefore, not only the nature of the surface active sites, as related to the structure, but their availability, as related to the particular texture are discussed as well. The relationship between these two aspects of clay surface reactivity is more heavily emphasized here than is the specific chemistry of reactions occurring on the surface of clays. The hydration water in the interlamellar space is acidic and, therefore, able to work as an acid catalyst. The nature of the acid sites is discussed with respect to the origin of the lattice charge (tetrahedral or octahedral). The mechanism of crosslinking smectites with oligomeric inorganic cations shed light on the reactivity of the basal planes as well. In addition to acid catalyzed reactions, this paper deals with charge and/or energy transfer reactions between adsorbed species and the lattice, or between co-adsorbed species. Reduction of lattice iron(III), as well as luminescence quenching of Ru(bpy) , photoaquation of Cr(bpy) and photooxidation of water, are examples chosen for illustrating this aspect of the reactivity of clays. 2+

3

3+

3

Among the minerals found i n the earth's crust, those belonging to the p h y l l o s i l i c a t e family, namely the c l a y s , are e s p e c i a l l y i n t e r e s t i n g from the point of view o f t h e i r surface a c t i v i t y . Being f i n e l y divided, t h e i r s p e c i f i c surface area i s large. Their 0097-6156/90/0415-0360$06.00A) ο 1990 American Chemical Society

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

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18.

FRIPIAT

Surface Activities of Clays

361

l a t t i c e s contain a large number of defects generating surface active s i t e s of various kinds. The microcrystals are a n i s o t r o p i c and t h e i r morphologies are quite diverse, being t h i n extended sheets, lathes or f i b e r s . This review paper w i l l deal e x c l u s i v e l y with c l a y minerals (1). Other minerals occurring i n nature, such as aluminum or i r o n oxides which are very often associated intimately with c l a y s , also d i s p l a y large surface areas. They w i l l be ignored here, i n s p i t e of the f a c t that t h e i r surface a c t i v i t y can not be neglected. In a d d i t i o n , the review w i l l cover the domain f a m i l i a r to the author. The l a t t i c e s of clay minerals have a common feature: they contain continuous planes made from oxygen atoms l i n k e d to cations ( s i l i c o n or aluminum) i n tetrahedral coordination and to cations (such as magnesium, aluminum, l i t h i u m , or iron) i n octahedral coordination. Because of isomorphic s u b s t i t u t i o n s , f o r instance Si by A l i n t e t r a h e d r a l coordination, the o v e r a l l charge o f these l a t t i c e s i s negative. At the edges of the sheets dangling bonds may generate a p o s i t i v e charge, according to the pH of the surrounding medium. Because the o v e r a l l charge i s negative, the e l e c t r o - n e u t r a l i t y i s obtained by the adsorption of c a t i o n s , such as Na , Κ , Ca , etc. which are, i n most cases, exchangeable. I t i s commonly accepted that isomorphic s u b s t i t u t i o n s i n the tetrahedral layer give r i s e to negative charges l o c a l i z e d near the s u b s t i t u t i o n s , whereas isomorphic s u b s t i t u t i o n s w i t h i n the octahedral layer y i e l d negative charges smeared out on the surface of i n d i v i d u a l sheets. These i n t u i t i v e ideas have r e c e n t l y received support from t h e o r e t i c a l c a l c u l a t i o n s by Bleam and Hoffman.(2) Note that the negative charges a r i s i n g from isomorphic s u b s t i t u t i o n s are permanent charges, independent of the pH. In summary, there are two kinds of charges, namely those which are pH independent and those which are pH dependent. The l a t t e r may be e i t h e r p o s i t i v e or negative, depending on the value of the i s o e l e c t r i c point (3). The former are negative and e i t h e r l o c a l i z e d or d e l o c a l i z e d . One more source of complexity comes from the f a c t that the l a t t i c e generally contains cations with several degrees o f oxidation. Iron i s the most common example. Therefore, charge t r a n s f e r may occur between adsorbed species and some l a t t i c e s i t e s . Vacancies i n the l a t t i c e may play a s i m i l a r r o l e . I t must be recognized that the d e s c r i p t i o n of the average chemical formulae by the chemical a n a l y s i s of c l a y minerals i s not a simple matter. Sheets of the same clay from the same deposit are l i k e l y to be heterogeneous with respect to composition and d i s t r i b u t i o n of l a t t i c e elements. The charge d i s t r i b u t i o n , as a consequence, i s heterogeneous as w e l l . The c o l l o i d a l chemistry of very d i l u t e d c l a y suspensions i s u s u a l l y described by the double layer theory (3). In more concentrated suspensions or t h i c k pastes, where the average distance between sheets i s shorter, the Van der Waals forces have to be considered. The combination of a l l these e f f e c t s influences the d i s t r i b u t i o n of the i n d i v i d u a l sheets and, thus, the v i s c o s i t y o f the s l u r r y . From the viewpoint of t h i s review, these s p a t i a l d i s t r i b u t i o n s of the i n d i v i d u a l sheets a f f e c t the a v a i l a b i l i t y o f the surface area with the important consequence that the surface area i s no longer an extensive property of the m a t e r i a l . Disordered

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

362

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES

materials of t h i s nature can, eventually, be described i n terms of f r a c t a l s . The fundamentals of the f r a c t a l theory and of i t s applications to disordered s o l i d s are developed i n (4). Reviewing t h i s i n t e r e s t i n g aspect here would be outside the scope of t h i s review.

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The Tactoid Concept It^has been taught i n the past that smectites, and e s p e c i a l l y Na* or L i smectites, could be dispersed completely into i n d i v i d u a l elementary sheets i n very d i l u t e suspensions and i n media of very low i o n i c strength. Under these r e s t r i c t e d conditions this complete dispersion may occur, but i n most cases, and, for instance, i n a suspension containing more than 0.01% w/w s o l i d i n water, i t seems highly probable that some aggregation "face to face" occurs, forming tactoids (5). The aggregation process does not produce perfect stacks, but i r r e g u l a r l y shaped p a r t i c l e s as represented i n Figure 1, from (6). I f such an image i s magnified, the sheets represented i n close contact are, i n fact, separated by a few monolayers of water corresponding to what we s h a l l c a l l an irreducible distance d. I f the sheets are i d e a l l y p a r a l l e l when at t h i s distance, X ray d i f f r a c t i o n w i l l y i e l d a r e f l e c t i o n with basal spacing (d + D) where D i s the sheet thickness. In the tactoid shown i n Figure 1, the f r a c t i o n of the surface area of the layers at distance d i s 50% of the t o t a l area. Let us c a l l this f r a c t i o n a; i n Figure 1, a - 0.5. The (1-a) f r a c t i o n of the surface area encloses columns of voids, eventually f i l l e d with water, with heights larger than d. Therefore, r e f l e c t i o n s at larger spacings can be detected by small angle X-ray scattering (SAXS), i f s u f f i c i e n t ordering i s achieved. In addition, a tactoid containing η sheets i n such a disordered state i s separated from an adjacent t a c t o i d containing n' sheets by i n t e r - t a c t o i d pores. I f two adjacent tactoids are t i l t e d with respect to one another, very complicated architecture may e x i s t within the suspension. The description of such architectures should be performed i n terms of c o r r e l a t i o n functions r e l a t i n g the spaces occupied by s o l i d s or by voids. Such an analysis i s d i f f i c u l t f o r anisotropic objects. Any information on these architectures would be, however, worthwhile because the a v a i l a b i l i t y of the surface i s ruled to a large extent by the size of the tactoids, and by t h e i r mutual arrangements. Water Associated with the External Contour of Tactoids. The existence of tactoids can be evidenced by studying the longitudinal relaxation time of the nuclear magnetic resonance of Η or Η i n aqueous s l u r r i e s (7-8). In simple terms, the relaxation rate, e.g. the inverse of the relaxation time, i s the sum of three terms Τ-" 1

1

- χ Τ " a la Ί

X

+ χ, T-, " D lb

1

+ x.T,," ι 11

1

1

(1)

Τ , Τ ^ and Τ ^ are the proton (or deuteron) relaxation rates of: the Yntertacto'id water (a) , of the water at the external surface of the t a c t o i d (b), and of the few layers (

+

2+ 3

]

]* - Ru(bpy)

3+ 3

+ [Co(NH ) Cl] 3

+

5

> decomposition products H

+

The f a s t decomposition of the s a c r i f i c i a l electron acceptor prevents back electron transfer. The formation of molecular oxygen r e s u l t s from Ru(bpy)

3+

+ OH"

3

> Ru(bpy)

2+ 3

+ h^O

+

h0

2

cat. The c a t a l y s t was a c o l l o i d a l suspension of Ru0 (diameter -150Â). The same reaction has-been studied i n clay s l u r r y (32). The c o l l o i d a l system [Ru(bp^r) exchanged hectorite/Ru0 ] i n an aqueous suspension of Co(NH )Cl * i n large excess with respect to the clay CEC, d i d not photolyze water when illuminated with r a d i a t i o n whose wavelength was larger than 400 nm. Co(NH )C1 has access to the interlamellar space, but the catalyst (Ru0 ) has no access because of i t s size (Figure 7a). Note tha£ there i s no spectroscopic evidence for desorption of Ru(bpy) into the solution. I f hectorite was pretreated at a temperature high enough to collapse^the tactoid, and subsequently exchanged with a mixture of Ru(bpy) , Co(NH ) CI and Ru0 , then photooxidation i s observed i n the c o l l o i d a l suspension because the c a t a l y s t i s i n contact with the reagents, a l l components being on the external surface (Figure 7b). On the opposite, photooxidation should occur with a c a t i o n i c c a t a l y s t having access to the interlamellar space. This has been shown to be correct by using cisRu(bpy) (H 0) as catalyst; then a l l the components f o r the reaction are within the interlamellar space and photooxidation proceeds at a rate comparable with that observed i n solution (Figure 7c). These observations outline the importance of the textural arrangement of the clay sheets with respect to the a v a i l a b i l i t y of the surface s i t e s , i n l i n e with what has been previously discussed i n the paragraph dealing with the concept of tactoids. 2

2

3

2+

2

+

4

5

2

2

2

2

2

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

In Spectroscopic Characterization of Minerals and Their Surfaces; Coyne, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990. 2

3

2+

2

2

2+

Figure 7. Models suggested for explaining the observation of the photooxidation of water by a sensitizer S, (Ru(bpy) ), which in the excited state S transfer an electron to a sacrificial acceptor A , [ C o i N H ^ C l ] , in the presence of a colloidal catalyst (RuOj) or a molecular catalyst (cwRu(bpy) (H 0) ). See text for the significance of a, b, and c. (Reprinted with permission from ref. 32. Copyright 1983.)

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00

376

SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES

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Conclusions Hydrated clay surfaces are a c i d i c . When isomorphic s u b s t i t u t i o n occurs i n the tetrahedral layer, acid leaching or NH thermal decomposition may generate a c i d i c surface OH. For clays whose negative charges are produced by isomorphic substitutions i n the octahedral layer, mild dehydration removes the source of a c i d i t y , because of the r e v e r s i b i l i t y of reaction (3). Deamination of the ammonium exchanged clay with octahedral s u b s t i t u t i o n drives protons into the octahedral layer, as evidenced by the lowered temperature at s t r u c t u r a l dehydroxylation. Acid catalyzed reactions occur on the surface of clays as long as water molecules are present within the interlamellar space. Charge transfer reactions between adsorbed reducing reagents and s t r u c t u r a l i r o n are c l e a r l y established, but photo-triggered electron transfer has not been proved unless the donor and acceptor moieties are co-adsorbed. Irrespective of the nature of the reaction, the strong tendency of clay anisotropic p a r t i c l e s to agglomerate into tactoids, as well as the i n t r i c a t e s p a t i a l d i s t r i b u t i o n of tactoids, reduces the a v a i l a b i l i t y of surface s i t e s . Upon p i l l a r i n g a noticeable f r a c t i o n of the surface s i t e s remains available for reaction processes operating at high temperature. Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Grim, R. E. Clay Mineralogy. 2nd Edt., McGraw: New York, 1968. Bleam, W. F.; Hoffman, R. Inorgan. Chem,. 1988, 27, 3180. Van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd Ed., John Wiley and Sons, Inc., 1977. Avnir, D., Ed. The Fractal Approach to Heterogeneous Chemistry: Surface, Colloids, Polymers, J. Wiley Publisher, 1989. Banin, Α.; Lahav, N. Israel J. Chem., 1968, 6, 235. Van Damme, H.; Levitz, P.; Fripiat, J. J.; Alcover, J. F.; Gatineau, L.; Bergaya, F. In Physics of Finely Divided Matter; Boccara N.; Daoud, D., Ed.; Springer: Berlin, 1985; p. 24. Fripiat, J. J.; Cases, J.; Francois, M.; Letellier, M. J. Coll. and Interface Sci., 1982, 89, 378. Fripiat, J. J.; Letellier, M.; Levitz, P. Phil. Trans. Royal Soc., 1984, A311, 287. Tessier, D.; Pedro, G. In International Clay Conference, 1981; Developments in Sedimentology No. 35, Van Olphen, H.; Veniale, F. Eds.; Elsevier: Amsterdam, 1982; p. 165. Van Damme, H.; Crespin, M.; Cruz, M. I.; Fripiat, J. J. Clays and Clay Min., 1977, 25, 19. Hang P.T.; Brindley, G.W. Ididem, 1970, 18, 203. Cenens J.; Schoonheydt, R. A. Ibidem, 1988, 36, 214. Viane, K.; Caigui, J.; Schoonheydt, R. Α.; DeSchryver, F. C., Langmuir, 1987, 3, 107. Schoonheydt R. Α.; Cenens, J. This Symposium. .... Mortland, M. M.; Fripiat, J. J.; Chaussidon, J.; Uytterhoeven, J. B. J. Phys. Chem,. 1963, 67, 248. Thomas, J. M. In Intercalation Chemistry; Wittingham, M. S.; Jacobson, A. J., Eds.; Academic Press: New York, 1982; p. 56.

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

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17. Balantine, J. A. In Chemical Reaction in Organic and Inorganic Constrained Systems; Setton, R., Ed.; NATO, ASI Series C 165: Reidel, Dordrecht, 1986; p. 197. 18. Durand, Β.; Pelet, R.; Fripiat, J. J. Clays andClayMin., 1972, 20, 21. 19. Schutz, Α.; Stone, W. Ε. E.; Poncelet, G.; Fripiat, J. J. Ibidem, 1987, 35, 251. 20. Chourabi, B.; Fripiat, J. J. Ibidem. 1981, 29, 260. 21. Plee, D., Borg, F.; Gatineau, L.; Fripiat, J. J.; J. Amer. Chem. Soc., 1985, 107, 2362. 22. Pinnavaia, T. J.; Landau, S. D.; Tzou, Mingh-Shin; Johnson, I. D.; Lipsicas, M. J. Amer. Chem. Soc., 1985, 107, 2362. 23. Tennakoon, D. T. B.; Jones, W.; Thomas, J. M.; J. Chem. Soc. Faraday Trans I, 1986, 82, 3081. 24. Cloos, P.; Fripiat, J. J.; Vielvoye, L. Soil Sci., 1961, 91, 55. 25. Russel, J. D.; Goodman, Β. Α.; Fraser, A. R. Clays and Clay Min., 1979, 27, 63. 26. Stucki, J. W.; Roth, C. B.; J. Soil Science Society of America, 1977, 41, 808, and references therein. 27. Van Damme, H.; Obrecht, F.; Letellier, M. Nouveau J. de Chimie, 1984, 8, 681. 28. Krenske, D.; Abdo, S.; Van Damme, H.; Cruz, M.; Fripiat, J. J. J. Phys. Chem,. 1980, 8, 681. 29. Habti, Α.; Keravis, D.; Levitz, P.; Van Damme, H. J. Chem. Soc., Faraday Trans. 2, 1984, 80, 2447. 30. Van Damme, H.; Nys, H.; Fripiat, J. J. J. Molec. Cat., 1984, 27, 123, and references therein. 31. Lehn, J. M.; Sauvage, J. P.; Ziessel, R. Nouveau J. de Chimie, 1979, 3, 423. 32. Nys, H.; Van Damme, H.; Bergaya, F.; Habti, Α.; Fripiat, J. J. J. Molec. Catalys., 1983, 21, 223, and references therein. RECEIVED

July 13, 1989

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