5 Characterization of Solid Surfaces by Wet Chemical Techniques FREDERICK M. FOWKES
Downloaded by UNIV LAVAL on October 22, 2015 | http://pubs.acs.org Publication Date: September 22, 1982 | doi: 10.1021/bk-1982-0199.ch005
Lehigh University, Department of Chemistry, Bethlehem, P A
18015
Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characteriza tion of the surface forces involved (the energy of dispersion forces, γd; the strength of acidic or basic sites; and the surface density of coulombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). Measurements of the strength of surface acids and bases can now be used to determine their Drago Ε and C constants. The surface analysis of solids has benefited greatly from the introduction of various high vacuum techniques, from electron microscopy through LEED, AES, ESCA, ISS, SIMS, etc. These tech niques have given detailed information on atomic composition and structure, but rather limited information on the capability for chemical reactivity of surface groups. This latter type of i n formation has much practical value, and is often more easily ob tained by wet chemical techniques than high vacuum techniques. The assessment of surface reactivity of solids involves determination of the number of each kind of sites and ability to interact with reactive molecules. The interactions are of two kinds; the general interactions of the London dispersion forces (1) and the specific interactions. Most of the specific inter actions are some kind of acid-base phenomenon; this category 0097-6156/82/0199-0069$06.00/0 © 1982 American Chemical Society
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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SURFACE ANALYSIS
i n c l u d e s c h e l a t i o n , c o o r d i n a t i o n , h y d r o g e n - b o n d i n g , and t h e c o m p l e x e s b e t w e e n n u c l e o p h i l e s and e l e c t r o p h i l i c s i t e s w h i c h a r e so i m p o r t a n t i n c a t a l y s i s . Most of t h e s e complexes w i t h o r d i n a r y o r g a n i c m o l e c u l e s h a v e e n t h a l p i e s o f f o r m a t i o n o f 1-15 K c a l / m o l e and c o n s e q u e n t l y d i s a s s o c i a t e a p p r e c i a b l y w i t h r i s e o f t e m p e r a t u r e or d i s s o c i a t e upon r e d u c t i o n o f p r e s s u r e . Thus a c i d i c s i t e s on an i n o r g a n i c c a t a l y s t may w e l l a d s o r b a m i n e s s t r o n g l y f r o m o r g a n i c s o l v e n t s b u t i n a vacuum s y s t e m t h e amines e v a p o r a t e o f f and a r e g e n e r a l l y n o t o b s e r v e d by vacuum t e c h n i q u e s . The s u r f a c e a c i d s i t e s c a n n o t be r e a d i l y o b s e r v e d by t h e v a r i o u s s u r f a c e a n a l y t i c a l vacuum d e v i c e s . F o r i n s t a n c e ESCA g i v e s c h e m i c a l r e a c t i v i t y i n f o r m a t i o n but c a n t observe hydrogen. On t h e o t h e r hand a number o f "wet" c h e m i c a l t e c h n i q u e s c a n be u s e d t o g a i n i n f o r m a t i o n a b o u t t h e c o m p l e x e s formed b e t w e e n r e a c t i v e s u r f a c e s i t e s and t e s t m o l e c u l e s . Some o f t h e s e t e c h n i q u e s d e t e r m i n e e n e r g i e s o f i n t e r a c t i o n by s p e c t r o s c o p i c s h i f t s i n t h e U V , v i s i b l e , o r IR ( i n c l u d i n g R a m a n ) . Others determine enthalpies of i n t e r a c t i o n by c a l o r i m e t r y o f t e m p e r a t u r e - d e p e n d e n c e o f f o r m a t i o n c o n s t a n t s , or a d s o r p t i o n i s o t h e r m s . Energies of complexation can a l s o be d e t e r m i n e d f r o m a v a r i e t y o f o t h e r t e c h n i q u e s s u c h as gas c h r o m a t o g r a p h y .
Downloaded by UNIV LAVAL on October 22, 2015 | http://pubs.acs.org Publication Date: September 22, 1982 | doi: 10.1021/bk-1982-0199.ch005
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The s u r f a c e p r o p e r t i e s o f i m p o r t a n c e f o r a d s o r b e n t s , c a t a l y s t s , a d h e r e n t s u r f a c e s , and c o r r o d a b l e s u r f a c e s a r e t h o s e p r o p e r t i e s which c o n t r o l i n t e r a c t i o n s w i t h adsorbable s p e c i e s . These i n t e r a c t i o n s always i n v o l v e d i s p e r s i o n f o r c e i n t e r a c t i o n s and may o r may n o t i n v o l v e s p e c i f i c i n t e r a c t i o n s . The a b i l i t y o f a s u r f a c e t o i n t e r a c t w i t h a n o t h e r m a t e r i a l c a n be d e t e r m i n e d at p r e s e n t b e s t by o b s e r v i n g i t s i n t e r a c t i o n s w i t h t e s t m a t e r i a l s , and t h e s e o b s e r v a t i o n s a r e n e v e r done i n h i g h vacuum and g e n e r a l l y i n v o l v e wet c h e m i c a l t e c h n i q u e s . London D i s p e r s i o n F o r c e
Interaction
The L o n d o n d i s p e r s i o n f o r c e s a r e p r e s e n t and i m p o r t a n t i n most a d s o r p t i o n p r o c e s s e s and i n a d h e s i v e i n t e r a c t i o n s b e t w e e n dissimilar materials. The f r e e e n e r g y o f i n t e r a c t i o n p e r u n i t a r e a b e t w e e n m a t e r i a l s 1 and 2 i n c o n t a c t i s - W - ^ w h e r e W^2 ^ t h e "work o f a d h e s i o n " . A t an i n t e r f a c e where a c i d - b a s e i n t e r a c t i o n s o c c u r , t h e t o t a l work o f a d h e s i o n i s t h e sum o f t h e c o n t r i b u t i o n s of d i s p e r s i o n f o r c e s , and a c i d - b a s e i n t e r a c t i o n s , W|b. I t has b e e n shown t h a t W ^ c a n be p r e c i s e l y c a l c u l a t e d f r o m s
the d i s p e r s i o n f o r c e c o n t r i b u t i o n s t h e two m a t e r i a l s (YJ- and γ ^ ) :
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