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Adsorption of cetyltrimethylammonium bromide on parent and molybdenum-modified silica gels in the solid state
Therthochimice Acta, 217 (1993) 91-98 Elsevier Science Publishers B.V., A m s t e r d a m
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Adsorption of cetyltrimethylammonium bromide on parent and molybdenum-modified silica gels in the solid state M. M o k h t a r M o h a m e d ' , * a n d E t i e n n e F. V a n s a n t b "Chemistry Department, Faculty of Science, Al-zhar University, Cairo (Egypt) b Chemistry Department, Faculty of Science) Antwerp University, Universiteitsplein 1, B-2610 Wilrij!¢ (Belgium) (Re c e i ve d 17 August 1992)
Abstract
The adsorption of the cationic surfactant c e t y l t r i m e t h y l a m m o n i u m b r o m i d e ( C T A B ) on silica gel and on m o l y b d e n u m - m o d i f i e d silica gel substrate, in the solid state, was studied by differential scanning calorimetry and Fourier transform I R photoacoustic spectroscopy ( F T I R - P A ) techniques. Examination of the solid state IR spectra of C T A B absorbed onto m o l y b d e n u m - s i l i c a sut~strate, as a function of t e m p e r a t u r e , shows a n ew b a n d at 938 cm ~ shifted to 961 cm -~ (during the t e m p e r a t u r e flow), representing a chemisorption via the interaction of the head group of the suffactant (CH3).~N ÷ and Mo--O stretching vibration species. In the presence of NaCI, shifting of the high t e m p e r a t u r e e x o t h e r m i c peak at 411'C to 451"C was obtained, providing an e n h a n c e m e n t of c e t y l t r i m e t h y l a m m o n i u m chloride adsorption onto m o l y b d e n u m - s i l i c a substrate and a considerable reduction in the A H value is observed. This decrease in the AH value was o b t a i n e d as a result of the interference of the less reactive site Na ÷ ions in the adsorption, because of substitution of N a " for M e 6÷ ions. INTRODUCTION
The adsorption of cetyltrimethylammonium bromide (CTAB) on silica in aqueous and non,aqueous media has been extensively studied over the last two decades [1-3]. The b e h a v i o u r of t h e adsorption of C T A B on silica and on metal rod surfaces, from solutions, have been evaluated using Fourier transform IR ( F T I R ) and surface enhanced R a m a n spectroscopy techniques [4-5]. Calorimetric measurements concerning the adsorption of surfactants on silica gel from solutions have recently been described :[6, 7] . . . . . . Fewer studies have been reported on metal-surfactant complexes.
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1993 - Elsevier Science Publishers B.V. All rights reserved
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M. Mokhtar Moltamed, E,F. Vansant/Thermochim. Acta 217 (1993) 91-98
H o w e v e r , detailed i n f o r m a t i o n c o n c e r n i n g the a d s o r p t i o n of cationic surfactant on silica s u b s t r a t e modified by m e t a l ions has not yet b e e n c a r r i e d out. A s s t a t e d by I k e d a and c o - w o r k e r s [8], s u r f a c t a n t s in a q u e o u s solutions generally f o r m spherical micelles a b o v e certain critical c o n c e n t r a t i o n s , a n d s o m e o t h e r surfactants (i.e. ionic ones) can f o r m rod-like micelles o v e r a certain •r a n g e of c o n c e n t r a t i o n of s u r f a c t a n t or a d d e d electrolyte. T h e s p h e r e - r o d transition has b e e n d e d u c e d f r o m physical m e a s u r e m e n t s of viscosity, conductivity a n d flexibility of rod-like micelles [9, 10]. In this p a p e r , the a d s o r p t i o n of C T A B on m o l y b d e n u m - m o d i f i e d silica gel s u b s t r a t e with time and the influence of t e m p e r a t u r e using D S C a n d F T I R - P A h a v e b e e n investigated a n d discussed. M o r e o v e r , the effect of N a C ! a d d i t i o n on the p r o p e r t i e s of the modified s u b s t r a t e is also e x a m i n e d . EXPERIMENTAL
Materials Silica gel 60 ( M e r c k ) with a particle size of 0.040-0.063 m m ( 2 3 0 400 m e s h A S T M ) was used as a s u p p o r t . T h e m o l y b d e n u m - l o a d e d s a m p l e ( 1 0 w t . % with a surface a r e a of 2 5 0 m 2 g -~) was p r e p a r e d by i m p r e g n a t i o n with an a q u e o u s solution of a m m o n i u m h e p t a m o l y b d a t e (NH4)~, MO7024" 4 H 2 0 . T h e M o - m o d i f i e d s u b s t r a t e was dried at 393 K a n d finally calcined at 773 K in air for 16 h. 5 × 1 0 - ' ~ M C T A B ( A l d r i c h ) was u s e d to study its a d s o r p t i o n o n t o M o - m o d i f i e d silica gel substrate. T h e s u r f a c t a n t c o n c e n t r a t i o n was c h o s e n to be a b o v e the C T B A critical micelle c o n c e n t r a t i o n ( C M C ) , which is k n o w n to be 9 x 1 0 - 4 M [11], to p r e v e n t a n y c h a n g e in the a d s o r p t i o n as a result of the s u r f a c t a n t concentration. 0.5 M NaC! ( A R ) was also used to p r e p a r e 5 × 10 --~ M C T A B in o r d e r to e s t i m a t e the influence of the electrolyte on the micelle a n d c o n s e q u e n t l y on the s a m p l e s u n d e r investigation.
Adsorption o f C T A B on the samples • T w e n t y milliliters of C T A B was mixed with 0.16 g of the s a m p l e (silica or Mo-silica) in a s t o p p e r e d centrifuge tube, and the m i x t u r e was gently stirred by a m a g n e t i c chip for the r e q u i r e d time at 25°C. T h e s a m p l e s u s p e n s i o n s were c e n t r i f u g e d a t 3 0 0 0 g to s e d i m e n t the s a m p l e , and t h e s u p e r n a t a n t was carefully w i t h d r a w n . T h e s e d i m e n t e d s a m p l e s w e r e dried at 393 K for 4 h a n d p l a c e d in a dry n i t r o g e n - p u r g e d g!ovebox. .
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Characterization o f the samples F o r D S C m e a s u r e m e n t s , 12 m g o f t h e s a m p l e w a s t a k e n in AI c r u c i b l e s a n d t r e a t e d in a P e r k i n - E l m e r D S C 7 u n i t a n d s c a n n e d f r o m 30 to 500°C at a r a t e o f 10°C m i n - ' , ml m e t a l w a s u s e d as a r e f e r e n c e a n d a n i t r o g e n p u r g e o f 50 ml m i n -I w a s u s e d d u r i n g t h e e x p e r i m e n t . IR spectra were r e c o r d e d with a Nieolet 5 - D X B F T I R s p e c t r o m e t e r , using a METC-100 photoacoustic detector. In this w o r k , a s c a n s p e e d o f 0 . 1 6 c m s -t w a s u s e d . T h i s gives t h e s t r o n g e s t p h o t o a c o u s t i c signals. T h e s p e c t r a w e r e t a k e n with 4 c m - ' r e s o l u t i o n a n d 3 0 0 - 5 0 0 s c a n s w e r e a v e r a g e d , d e p e n d i n g on t h e i n t e n s i t y o f t h e signal.
AND
RESULTS
DISCUSSION
T h e D S C t h e r m o g r a m s o f t h e p a r e n t silica gel 60 c a l c i n e d at 773 K, silica gel 60 m o d i f i e d with 10 w t . % M o a n d silica gel 60 with 0.005 M C T A B a r e s h o w n in Fig. 1.
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Fig. I. D S C t h e r m o g r a m s : c u r v e a, silica gel 60, c a l c i n e d at 773 K; c u r v e b, silica gel 60 m o d i f i e d with 1 0 w t . % M e , c a l c i n e d a t 7 7 3 K : c u r v e c, silica g e l 6 0 + 0 . 0 0 0 5 M C T A B , h e a t e d at 373 K.
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M. Mokhtar Moliamed, E.F. Vansant/Thermochim. Acta 217 (1993) 91-98
A n e x o t h e r m i e p e a k with a m a x i m u m at 89°C for the p a r e n t s a m p l e is o b s e r v e d . T h e s a m e p e a k position is also o b s e r v e d for the M o - m o d i f i e d silica gel s a m p l e , b u t with a n o t h e r e x o t h e r m i c p e a k at 242°C. In the case of the silica gel 60 plus C T A B system, a large e x o t h e r m i c p e a k a r o u n d 87°C is o b t a i n e d . T h e r e f o r e , it is n o t e w o r t h y t h a t the first e x o t h e r m i e p e a k s (low t e m p e r a t u r e p e a k ) in all the s a m p l e s h a v e the s a m e p e a k position b u t differ only in A H values. T h e increase in A H intensity of t h e low t e m p e r a t u r e p e a k for the silica gel 60 plus C r A B system indicates a value of 1 0 2 J g - l , w h e r e a s silica gel 60 modified with 10 w t . % M o and the p a r e n t s a m p l e give values of 99 a n d 66 J g-t respectively. H o w e v e r , we m a y a t t r i b u t e this difference to the i n v o l v e m e n t of the w a t e r which could h a v e b e e n o b t a i n e d d u r i n g the p r e p a r a t i o n . T h e s e c o n d p e a k o b s e r v e d for the Mo-silica s a m p l e r e p r e s e n t s possible i n t e r a c t i o n s b e t w e e n m o l y b d e n u m a n d silica, M o replacing a c o n s i d e r a b l e n u m b e r of hydroxyl groups, indicating the high probability of oxygen vacancies; p o l y m o l y b d a t e species could be o b t a i n e d . This p e a k gives a A H value of 16.6 J g- t. A c c o r d i n g to the findings of Cirillo et al. [12], large areas of a l u m i n a surface r e m a i n u n c o v e r e d at 8 w t . % M o loading, a n d becau,~e we n e e d to r e a c h to a fairly u n i f o r m distribution of M o o v e r a large surface a r e a of silica gel, we tried to c o v e r o u r surface with a 10 w t . % M o loading. F u r t h e r m o r e , Fig. 1 also indicates t h a t t h e r e is no interaction b e t w e e n the p a r e n t s a m p l e and the s u r f a c t a n t C T A B , r e v e a l i n g that no b o n d f o r m a t i o n could be o b t a i n e d d u r i n g the t e m p e r a t u r e flow. F i g u r e 2 shows the D S C scans of the influence of time on the a d s o r p t i o n of C T A B on silica gel 60 c o n t a i n i n g 10 w t . % Mo. T w o e x o t h e r m i c p e a k s with. m a x i m a at 7 8 - 8 0 a n d 4110C w e r e o b s e r v e d t h r o u g h o u t t h e r r n o g r a m s a - e in Fig. 2. It is o b s e r v e d t h a t the first t e m p e r a t u r e p e a k is shifted to a l o w e r t e m p e r a t u r e than fer silica gel modified with 1 0 w t . % M o (Fig. 1). M o r e o v e r , the intensity of tb.e p e a k b e c o m e s smaller after C T A B a d s o r p t i o n t a k e s place. Its o b v i o u s that C T A B has a c o n s i d e r a b l e influence on the low t e m p e r a t u r e p e a k , r e p r e s e n t i n g the i n t e r a c t i o n with the rest of the n o n - r e a c t e d hydroxyl g r o u p s a n d indicating that the p r e s e n c e of m o l y b d e n u m helps C T A B adsorFtion. F u r t h e r m o r e , a d e c r e a s e of the A H values for the low t e m p e r a t u r e p e a k s was o b s e r v e d w i t h time. . . . . . . . . . . . . It w a s f o u n d also that the A H values for the e x o t h e r m i c p e a k s at 4110C increase c o n t i n u o u s l y with time to give values of 46.3, 131.9 a n d 149.9 J g - t a t 10, 20 a n d 60 min respectively, a n d t h e n d e c r e a s e after 180 a n d 400 min t o g i v e A H values of 111.3 and 112.0J g-Z respectively. This can be due to such a high M o loading: a n excess of M o O 3 can a g g r e g a t e to f o r m a bulk p h a s e on the surface, blocking off the pores of the silica with Mo. A t such a
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Fig. 2. D S C t h e r m o g r a m s o f C T A B a d s o r b e d o n silica gel 60 m o d i f i e d with 10 w t . % M o as a f u n c t i o n o f time: curve a, 10 min; curve b, 20 rain; curve c, 60 min; curve d, 180 min; curve e, 400 min. Curve f s h o w s the influence o f 0.5 M NaC! on the m o d i f i e d s a m p l e .
high c o v e r a g e of C T A B (correlated with time) the less reactive sites are o c c u p i e d and c o n s e q u e n t l y A H decreases. T h e activa~.ion e n e r g y values (which are valuable data attributed to the relative values of various properties of the various active sites present in thc solid materials u n d e r study and the strength of their interactions) have a l m o s t identical E^ values. It is clear from T a b l e 1 that after 2 0 m i n TABLE
1
P a r a m e t e r s o f the high t e m p e r a t u r e p e a k s (411 and 451°C) o b t a i n e d f r o m C T A B a d s o r b e d o n t o Mo-silica substrate by D S C as a f u n c t i o n o f time Time (min)
T e m p . range (°C)
P e a k max. (°C)
A H (J g - ' )
10 20 60 180 400 20 ~
365-455 370-471 • 368-470 370-461 372-460 426-478
411 411 411 411 411 451
46.3 131.9 149.9 111.3 112.0 34.1
E,., (kJ m o l - ' ) 44.7 .... 53.9 53.7 51.4 50.1 40.3
" D a t a based on the result o f interaction o f Mo-silica s a m p l e with C T A B 0.5 M NaCI.
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Fig. 3. Low frequency FrFR-PA spectra of CTAB adsorbed onto 10 wt.% Mo-silica sample, as a function of the temperature flow. Samples represented by curves a, b, c, d and e were desorbed at 413, 473, 543, 643 and 723 K respectively. a d s o r p t i o n is b e i n g l i m i t e d and n o m o r e r o o m is a v a i l a b l e on t h e a d s o r b e n t for f u r t h e r m o l e c u l e s to b e a d s o r b e d . T h e r e f o r e , a d e c r e a s e in EA with t i m e is o b t a i n e d . T h e p e a k at 2 4 2 ° C ( f r o m M o - s i l i c a i n t e r a c t i o n ) c o m p l e t e l y d i s a p p e a r s u p o n C T A B a d s o r p t i o n and t h e p r o m i n e n c e o f the n e w p e a k at 4 1 1 ° C reflects a m a j o r i n t e r a c t i o n o f M o with C T A B . T h e a b o v e result is c o n s i s t e n t with the I R s p e c t r a t a k e n for the M o - m o d i f i e d silica s a m p l e at v a r i o u s t e m p e r a t u r e s (Fig. 3). F i g u r e 3 s h o w s (in the r a n g e 1 7 0 0 - 4 0 0 c m -1) a n e w b a n d at 938 c m -t and t h r e e o t h e r b a n d s at 986, 867 a n d 694 c m -1, r e p r e s e n t i n g t h e silica structure. A t t h e b e g i n n i n g , t h e b a n d at 938 c m - ' is a t t r i b u t a b l e to the i n v o l v e m e n t o f w a t e r , but o n h e a t i n g t h e s a m p l e to 723 K the p r o m i n e n c e o f this b a n d suggests chemical bond formation. T h i s i s s u p p o r t e d b y t h e shifting o f the b a n d at 9 3 8 c m -t with a c o n t i n u o u s i n c r e a s e in t e m p e r a t u r e (to 723 K ) to 961 c m -~. F r o m the a b o v e d i s c u s s i o n , it is o b v i o u s that this b a n d at 938 c m "i is a result o f a p r e f e r e n t i a l i n t e r a c t i o n o f M o with C T A B . T h e r e f o r e , it is •
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r e a s o n a b l e to assign t h e 9 3 8 c m -~ b a n d to t h e s t r e t c h i n g v i b r a t i o n s o f Mo---O • • • N + C m o d e . T h e M o O 3 s p e c i e s is f o r m e d p r e f e r e n t i a l l y at s u c h a high w t . % o f m o l y b d e n u m a n d t h e r e f o r e t h e t e r m i n a l Mo---O ~pecies is o b t a i n e d . M o r e o v e r , I R d a t a s h o w s t h a t n o b a n d is f o r m e d in t h e p l a c e o f t h a t a t 938 c m -z b e t w e e n silica gel a n d C T A B , w h i c h s u p p o r t s o u r a s s i g n m e n t . T h e r e f o r e , it is r e a s o n a b l e to c o r r e l a t e t h e t h e r m o g r a m p e a k at 411°C to t h e c h e m i c a l b o n d Mo---O • • • N + C o b t a i n e d f r o m I R d a t a . •Figure 2f s h o w s t h e effect o f NaC1 a d d i t i o n o n C T A B a d s o r p t i o n . T w o e x o t h e r m i c p e a k s a t 74 a n d 451°C w e r e o b s e r v e d . T h i s t h e r m o g r a m c l e a r l y reflects a m a j o r c h a n g e if it is c o m p a r e d to t h e o t h e r s . T h e shift of t h e high t e m p e r a t u r e p e a k f r o m 411 to 4510C reflects t h e s t r e n g t h o f t h e Mo----O • • • N ÷ C b o n d upon electrolyte addition. I k e d a a n d c o - w o r k e r s [8] s h o w e d t h a t for a s e r i e s of c a t i o n i c s u r f a c t a n t s in N a C I s o l u t i o n s a s h a r p b r e a k in t h e a p p a r e n t m i c e l l e m o l e c u l a r w e i g h t is o b s e r v e d w h e n t h e salt c o n c e n t r a t i o n r e a c h e s a v a l u e o f 0.45 M, w h i c h c o r r e s p o n d s to t h e s p h e r e - r o d t r a n s i t i o n . T h e r e f o r e , in o u r case t h e d r i v i n g f o r c e c o u l d b e r e f e r r e d to t h e c h a n g e o f t h e s t r u c t u r e w h e n t h e s m a l l C1 + ions r e p l a c e t h e l a r g e r B r - i o n s c a u s e s an e n h a n c e m e n t o f C T A + affinity t o w a r d s Mo---O s p e c i e s . A c c o r d i n g l y , t h e i n c r e a s e o f i n t e r a c t i o n of c e t y l t r i m e t h y l a m m o n i u m chloride (CARC) and Mo causes a decrease of the AH value. It is likely to a s s u m e t h a t t h e s t r o n g i n t e r a c t i o n b e t w e e n M o a n d N + C will i n i t i a t e f u r t h e r r e a c t i o n o f s u c c e s s i v e a m o u n t s o f C T A C , so t h e less r e a c t i v e site N a + ions ( o r i g i n a t i n g f r o m t h e high c o n c e n t r a t i o n o f t h e N a C ! e l e c t r o l y t e u s e d ) will i n t e r f e r e with such i n t e r a c t i o n a n d c o n s e q u e n t l y A H will d e c r e a s e . M o i o n s i m p r e g n a t e d o n silica c a n e x p o s e Mo---O s p e c i e s o n t h e s u r f a c e to e n h a n c e t h e A H v a l u e u p o n a d s o r p t i o n . H o w e v e r , t h e h e t e r o g e n o u s s u r f a c e site d i s t r i b u t i o n o b t a i n e d as a r e s u l t o f t h e r e p l a c e m e n t o f M o i o n s b y less a c t i v e N a ÷ ions c a u s e s t h e significant d e c r e a s e o f b o t h A H a n d EA v a l u e s . T h e low t e m p e r a t u r e p e a k o f t h e s a m e t h e r m o g r a m (Fig. 2f) also s h o w s t h e i n v o l v e m e n t o f N a + i o n s in t h e h y d r o x y l g r o u p s r e g i o n b y shifting t h e p e a k to a l o w e r t e m p e r a t u r e o f 74°C c o m p a r e d to t h e o t h e r s . REFERENCES 1 2 3 4 5 6 7 8
H. Rupprecht, Koiloid Z. Z. Polym., 249 (I97I) 1127. B.H. Bijsterboseh, J. Colloid Interface Sci., 47 (1974) 186. H. Rupprecht, J. P h a r m . Sci., 61 (1972) 700. R. Sides; J. Y a r w o o d and K. Fox, Microehirn. Acta, 2 (1988) 93. A . L Dendramis, E.W. Schwinn and R.P. Sperline, Surf. Sci., 134 (1983) 675. G.W. W o o d b u r y and L. NolI, Colloids Surf., 28 (1987) 233. M. Lind Heimer, R. Keh, S. Zaini and S. Partyka, J. Colloid Interface Sci., I38 (1990) 83. T . Irnae, R. Kamiya and S. lkeda, J. Colloid Interface Sci., 108 (1985) 215.
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9 S. Kato, H. Normura, H. Honda, R. Zielinski and S. Ikeda, J. Phys. Chem., 92 (1988) 2305. 10 Z.-J. Yu and G. Xu, J. Phys. Chem., 93 (1989) 7441. 11 A.E. Westwell, M.O. Gunsch, P.T. Jacobs and E.W. Anacker, J. Colloid Interface Sci., 137 (1990) 92. 12 A.C. Cirillo, Jr., J.M. Dereppe and W.K. Hall, J. Catal., 61 (1980) 170.
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Adsorption of cetyltrimethylammonium bromide on parent and molybdenum-modified silica gels in the solid state M. M o k h t a r M o h a m e d ' , * a n d E t i e n n e F. V a n s a n t b "Chemistry Department, Faculty of Science, Al-zhar University, Cairo (Egypt) b Chemistry Department, Faculty of Science) Antwerp University, Universiteitsplein 1, B-2610 Wilrij!¢ (Belgium) (Re c e i ve d 17 August 1992)
Abstract
The adsorption of the cationic surfactant c e t y l t r i m e t h y l a m m o n i u m b r o m i d e ( C T A B ) on silica gel and on m o l y b d e n u m - m o d i f i e d silica gel substrate, in the solid state, was studied by differential scanning calorimetry and Fourier transform I R photoacoustic spectroscopy ( F T I R - P A ) techniques. Examination of the solid state IR spectra of C T A B absorbed onto m o l y b d e n u m - s i l i c a sut~strate, as a function of t e m p e r a t u r e , shows a n ew b a n d at 938 cm ~ shifted to 961 cm -~ (during the t e m p e r a t u r e flow), representing a chemisorption via the interaction of the head group of the suffactant (CH3).~N ÷ and Mo--O stretching vibration species. In the presence of NaCI, shifting of the high t e m p e r a t u r e e x o t h e r m i c peak at 411'C to 451"C was obtained, providing an e n h a n c e m e n t of c e t y l t r i m e t h y l a m m o n i u m chloride adsorption onto m o l y b d e n u m - s i l i c a substrate and a considerable reduction in the A H value is observed. This decrease in the AH value was o b t a i n e d as a result of the interference of the less reactive site Na ÷ ions in the adsorption, because of substitution of N a " for M e 6÷ ions. INTRODUCTION
The adsorption of cetyltrimethylammonium bromide (CTAB) on silica in aqueous and non,aqueous media has been extensively studied over the last two decades [1-3]. The b e h a v i o u r of t h e adsorption of C T A B on silica and on metal rod surfaces, from solutions, have been evaluated using Fourier transform IR ( F T I R ) and surface enhanced R a m a n spectroscopy techniques [4-5]. Calorimetric measurements concerning the adsorption of surfactants on silica gel from solutions have recently been described :[6, 7] . . . . . . Fewer studies have been reported on metal-surfactant complexes.
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M. Mokhtar Moltamed, E,F. Vansant/Thermochim. Acta 217 (1993) 91-98
H o w e v e r , detailed i n f o r m a t i o n c o n c e r n i n g the a d s o r p t i o n of cationic surfactant on silica s u b s t r a t e modified by m e t a l ions has not yet b e e n c a r r i e d out. A s s t a t e d by I k e d a and c o - w o r k e r s [8], s u r f a c t a n t s in a q u e o u s solutions generally f o r m spherical micelles a b o v e certain critical c o n c e n t r a t i o n s , a n d s o m e o t h e r surfactants (i.e. ionic ones) can f o r m rod-like micelles o v e r a certain •r a n g e of c o n c e n t r a t i o n of s u r f a c t a n t or a d d e d electrolyte. T h e s p h e r e - r o d transition has b e e n d e d u c e d f r o m physical m e a s u r e m e n t s of viscosity, conductivity a n d flexibility of rod-like micelles [9, 10]. In this p a p e r , the a d s o r p t i o n of C T A B on m o l y b d e n u m - m o d i f i e d silica gel s u b s t r a t e with time and the influence of t e m p e r a t u r e using D S C a n d F T I R - P A h a v e b e e n investigated a n d discussed. M o r e o v e r , the effect of N a C ! a d d i t i o n on the p r o p e r t i e s of the modified s u b s t r a t e is also e x a m i n e d . EXPERIMENTAL
Materials Silica gel 60 ( M e r c k ) with a particle size of 0.040-0.063 m m ( 2 3 0 400 m e s h A S T M ) was used as a s u p p o r t . T h e m o l y b d e n u m - l o a d e d s a m p l e ( 1 0 w t . % with a surface a r e a of 2 5 0 m 2 g -~) was p r e p a r e d by i m p r e g n a t i o n with an a q u e o u s solution of a m m o n i u m h e p t a m o l y b d a t e (NH4)~, MO7024" 4 H 2 0 . T h e M o - m o d i f i e d s u b s t r a t e was dried at 393 K a n d finally calcined at 773 K in air for 16 h. 5 × 1 0 - ' ~ M C T A B ( A l d r i c h ) was u s e d to study its a d s o r p t i o n o n t o M o - m o d i f i e d silica gel substrate. T h e s u r f a c t a n t c o n c e n t r a t i o n was c h o s e n to be a b o v e the C T B A critical micelle c o n c e n t r a t i o n ( C M C ) , which is k n o w n to be 9 x 1 0 - 4 M [11], to p r e v e n t a n y c h a n g e in the a d s o r p t i o n as a result of the s u r f a c t a n t concentration. 0.5 M NaC! ( A R ) was also used to p r e p a r e 5 × 10 --~ M C T A B in o r d e r to e s t i m a t e the influence of the electrolyte on the micelle a n d c o n s e q u e n t l y on the s a m p l e s u n d e r investigation.
Adsorption o f C T A B on the samples • T w e n t y milliliters of C T A B was mixed with 0.16 g of the s a m p l e (silica or Mo-silica) in a s t o p p e r e d centrifuge tube, and the m i x t u r e was gently stirred by a m a g n e t i c chip for the r e q u i r e d time at 25°C. T h e s a m p l e s u s p e n s i o n s were c e n t r i f u g e d a t 3 0 0 0 g to s e d i m e n t the s a m p l e , and t h e s u p e r n a t a n t was carefully w i t h d r a w n . T h e s e d i m e n t e d s a m p l e s w e r e dried at 393 K for 4 h a n d p l a c e d in a dry n i t r o g e n - p u r g e d g!ovebox. .
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M. Mokhtar Mohamed, E.F. Vansant/Thermochim. Acta 217 (1993) 91-98
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Characterization o f the samples F o r D S C m e a s u r e m e n t s , 12 m g o f t h e s a m p l e w a s t a k e n in AI c r u c i b l e s a n d t r e a t e d in a P e r k i n - E l m e r D S C 7 u n i t a n d s c a n n e d f r o m 30 to 500°C at a r a t e o f 10°C m i n - ' , ml m e t a l w a s u s e d as a r e f e r e n c e a n d a n i t r o g e n p u r g e o f 50 ml m i n -I w a s u s e d d u r i n g t h e e x p e r i m e n t . IR spectra were r e c o r d e d with a Nieolet 5 - D X B F T I R s p e c t r o m e t e r , using a METC-100 photoacoustic detector. In this w o r k , a s c a n s p e e d o f 0 . 1 6 c m s -t w a s u s e d . T h i s gives t h e s t r o n g e s t p h o t o a c o u s t i c signals. T h e s p e c t r a w e r e t a k e n with 4 c m - ' r e s o l u t i o n a n d 3 0 0 - 5 0 0 s c a n s w e r e a v e r a g e d , d e p e n d i n g on t h e i n t e n s i t y o f t h e signal.
AND
RESULTS
DISCUSSION
T h e D S C t h e r m o g r a m s o f t h e p a r e n t silica gel 60 c a l c i n e d at 773 K, silica gel 60 m o d i f i e d with 10 w t . % M o a n d silica gel 60 with 0.005 M C T A B a r e s h o w n in Fig. 1.
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Fig. I. D S C t h e r m o g r a m s : c u r v e a, silica gel 60, c a l c i n e d at 773 K; c u r v e b, silica gel 60 m o d i f i e d with 1 0 w t . % M e , c a l c i n e d a t 7 7 3 K : c u r v e c, silica g e l 6 0 + 0 . 0 0 0 5 M C T A B , h e a t e d at 373 K.
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M. Mokhtar Moliamed, E.F. Vansant/Thermochim. Acta 217 (1993) 91-98
A n e x o t h e r m i e p e a k with a m a x i m u m at 89°C for the p a r e n t s a m p l e is o b s e r v e d . T h e s a m e p e a k position is also o b s e r v e d for the M o - m o d i f i e d silica gel s a m p l e , b u t with a n o t h e r e x o t h e r m i c p e a k at 242°C. In the case of the silica gel 60 plus C T A B system, a large e x o t h e r m i c p e a k a r o u n d 87°C is o b t a i n e d . T h e r e f o r e , it is n o t e w o r t h y t h a t the first e x o t h e r m i e p e a k s (low t e m p e r a t u r e p e a k ) in all the s a m p l e s h a v e the s a m e p e a k position b u t differ only in A H values. T h e increase in A H intensity of t h e low t e m p e r a t u r e p e a k for the silica gel 60 plus C r A B system indicates a value of 1 0 2 J g - l , w h e r e a s silica gel 60 modified with 10 w t . % M o and the p a r e n t s a m p l e give values of 99 a n d 66 J g-t respectively. H o w e v e r , we m a y a t t r i b u t e this difference to the i n v o l v e m e n t of the w a t e r which could h a v e b e e n o b t a i n e d d u r i n g the p r e p a r a t i o n . T h e s e c o n d p e a k o b s e r v e d for the Mo-silica s a m p l e r e p r e s e n t s possible i n t e r a c t i o n s b e t w e e n m o l y b d e n u m a n d silica, M o replacing a c o n s i d e r a b l e n u m b e r of hydroxyl groups, indicating the high probability of oxygen vacancies; p o l y m o l y b d a t e species could be o b t a i n e d . This p e a k gives a A H value of 16.6 J g- t. A c c o r d i n g to the findings of Cirillo et al. [12], large areas of a l u m i n a surface r e m a i n u n c o v e r e d at 8 w t . % M o loading, a n d becau,~e we n e e d to r e a c h to a fairly u n i f o r m distribution of M o o v e r a large surface a r e a of silica gel, we tried to c o v e r o u r surface with a 10 w t . % M o loading. F u r t h e r m o r e , Fig. 1 also indicates t h a t t h e r e is no interaction b e t w e e n the p a r e n t s a m p l e and the s u r f a c t a n t C T A B , r e v e a l i n g that no b o n d f o r m a t i o n could be o b t a i n e d d u r i n g the t e m p e r a t u r e flow. F i g u r e 2 shows the D S C scans of the influence of time on the a d s o r p t i o n of C T A B on silica gel 60 c o n t a i n i n g 10 w t . % Mo. T w o e x o t h e r m i c p e a k s with. m a x i m a at 7 8 - 8 0 a n d 4110C w e r e o b s e r v e d t h r o u g h o u t t h e r r n o g r a m s a - e in Fig. 2. It is o b s e r v e d t h a t the first t e m p e r a t u r e p e a k is shifted to a l o w e r t e m p e r a t u r e than fer silica gel modified with 1 0 w t . % M o (Fig. 1). M o r e o v e r , the intensity of tb.e p e a k b e c o m e s smaller after C T A B a d s o r p t i o n t a k e s place. Its o b v i o u s that C T A B has a c o n s i d e r a b l e influence on the low t e m p e r a t u r e p e a k , r e p r e s e n t i n g the i n t e r a c t i o n with the rest of the n o n - r e a c t e d hydroxyl g r o u p s a n d indicating that the p r e s e n c e of m o l y b d e n u m helps C T A B adsorFtion. F u r t h e r m o r e , a d e c r e a s e of the A H values for the low t e m p e r a t u r e p e a k s was o b s e r v e d w i t h time. . . . . . . . . . . . . It w a s f o u n d also that the A H values for the e x o t h e r m i c p e a k s at 4110C increase c o n t i n u o u s l y with time to give values of 46.3, 131.9 a n d 149.9 J g - t a t 10, 20 a n d 60 min respectively, a n d t h e n d e c r e a s e after 180 a n d 400 min t o g i v e A H values of 111.3 and 112.0J g-Z respectively. This can be due to such a high M o loading: a n excess of M o O 3 can a g g r e g a t e to f o r m a bulk p h a s e on the surface, blocking off the pores of the silica with Mo. A t such a
M. Mokhtar Mohamed, E.F. Vansant/Thermochim. Acta 217 (1993) 91-98
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Fig. 2. D S C t h e r m o g r a m s o f C T A B a d s o r b e d o n silica gel 60 m o d i f i e d with 10 w t . % M o as a f u n c t i o n o f time: curve a, 10 min; curve b, 20 rain; curve c, 60 min; curve d, 180 min; curve e, 400 min. Curve f s h o w s the influence o f 0.5 M NaC! on the m o d i f i e d s a m p l e .
high c o v e r a g e of C T A B (correlated with time) the less reactive sites are o c c u p i e d and c o n s e q u e n t l y A H decreases. T h e activa~.ion e n e r g y values (which are valuable data attributed to the relative values of various properties of the various active sites present in thc solid materials u n d e r study and the strength of their interactions) have a l m o s t identical E^ values. It is clear from T a b l e 1 that after 2 0 m i n TABLE
1
P a r a m e t e r s o f the high t e m p e r a t u r e p e a k s (411 and 451°C) o b t a i n e d f r o m C T A B a d s o r b e d o n t o Mo-silica substrate by D S C as a f u n c t i o n o f time Time (min)
T e m p . range (°C)
P e a k max. (°C)
A H (J g - ' )
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365-455 370-471 • 368-470 370-461 372-460 426-478
411 411 411 411 411 451
46.3 131.9 149.9 111.3 112.0 34.1
E,., (kJ m o l - ' ) 44.7 .... 53.9 53.7 51.4 50.1 40.3
" D a t a based on the result o f interaction o f Mo-silica s a m p l e with C T A B 0.5 M NaCI.
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Fig. 3. Low frequency FrFR-PA spectra of CTAB adsorbed onto 10 wt.% Mo-silica sample, as a function of the temperature flow. Samples represented by curves a, b, c, d and e were desorbed at 413, 473, 543, 643 and 723 K respectively. a d s o r p t i o n is b e i n g l i m i t e d and n o m o r e r o o m is a v a i l a b l e on t h e a d s o r b e n t for f u r t h e r m o l e c u l e s to b e a d s o r b e d . T h e r e f o r e , a d e c r e a s e in EA with t i m e is o b t a i n e d . T h e p e a k at 2 4 2 ° C ( f r o m M o - s i l i c a i n t e r a c t i o n ) c o m p l e t e l y d i s a p p e a r s u p o n C T A B a d s o r p t i o n and t h e p r o m i n e n c e o f the n e w p e a k at 4 1 1 ° C reflects a m a j o r i n t e r a c t i o n o f M o with C T A B . T h e a b o v e result is c o n s i s t e n t with the I R s p e c t r a t a k e n for the M o - m o d i f i e d silica s a m p l e at v a r i o u s t e m p e r a t u r e s (Fig. 3). F i g u r e 3 s h o w s (in the r a n g e 1 7 0 0 - 4 0 0 c m -1) a n e w b a n d at 938 c m -t and t h r e e o t h e r b a n d s at 986, 867 a n d 694 c m -1, r e p r e s e n t i n g t h e silica structure. A t t h e b e g i n n i n g , t h e b a n d at 938 c m - ' is a t t r i b u t a b l e to the i n v o l v e m e n t o f w a t e r , but o n h e a t i n g t h e s a m p l e to 723 K the p r o m i n e n c e o f this b a n d suggests chemical bond formation. T h i s i s s u p p o r t e d b y t h e shifting o f the b a n d at 9 3 8 c m -t with a c o n t i n u o u s i n c r e a s e in t e m p e r a t u r e (to 723 K ) to 961 c m -~. F r o m the a b o v e d i s c u s s i o n , it is o b v i o u s that this b a n d at 938 c m "i is a result o f a p r e f e r e n t i a l i n t e r a c t i o n o f M o with C T A B . T h e r e f o r e , it is •
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r e a s o n a b l e to assign t h e 9 3 8 c m -~ b a n d to t h e s t r e t c h i n g v i b r a t i o n s o f Mo---O • • • N + C m o d e . T h e M o O 3 s p e c i e s is f o r m e d p r e f e r e n t i a l l y at s u c h a high w t . % o f m o l y b d e n u m a n d t h e r e f o r e t h e t e r m i n a l Mo---O ~pecies is o b t a i n e d . M o r e o v e r , I R d a t a s h o w s t h a t n o b a n d is f o r m e d in t h e p l a c e o f t h a t a t 938 c m -z b e t w e e n silica gel a n d C T A B , w h i c h s u p p o r t s o u r a s s i g n m e n t . T h e r e f o r e , it is r e a s o n a b l e to c o r r e l a t e t h e t h e r m o g r a m p e a k at 411°C to t h e c h e m i c a l b o n d Mo---O • • • N + C o b t a i n e d f r o m I R d a t a . •Figure 2f s h o w s t h e effect o f NaC1 a d d i t i o n o n C T A B a d s o r p t i o n . T w o e x o t h e r m i c p e a k s a t 74 a n d 451°C w e r e o b s e r v e d . T h i s t h e r m o g r a m c l e a r l y reflects a m a j o r c h a n g e if it is c o m p a r e d to t h e o t h e r s . T h e shift of t h e high t e m p e r a t u r e p e a k f r o m 411 to 4510C reflects t h e s t r e n g t h o f t h e Mo----O • • • N ÷ C b o n d upon electrolyte addition. I k e d a a n d c o - w o r k e r s [8] s h o w e d t h a t for a s e r i e s of c a t i o n i c s u r f a c t a n t s in N a C I s o l u t i o n s a s h a r p b r e a k in t h e a p p a r e n t m i c e l l e m o l e c u l a r w e i g h t is o b s e r v e d w h e n t h e salt c o n c e n t r a t i o n r e a c h e s a v a l u e o f 0.45 M, w h i c h c o r r e s p o n d s to t h e s p h e r e - r o d t r a n s i t i o n . T h e r e f o r e , in o u r case t h e d r i v i n g f o r c e c o u l d b e r e f e r r e d to t h e c h a n g e o f t h e s t r u c t u r e w h e n t h e s m a l l C1 + ions r e p l a c e t h e l a r g e r B r - i o n s c a u s e s an e n h a n c e m e n t o f C T A + affinity t o w a r d s Mo---O s p e c i e s . A c c o r d i n g l y , t h e i n c r e a s e o f i n t e r a c t i o n of c e t y l t r i m e t h y l a m m o n i u m chloride (CARC) and Mo causes a decrease of the AH value. It is likely to a s s u m e t h a t t h e s t r o n g i n t e r a c t i o n b e t w e e n M o a n d N + C will i n i t i a t e f u r t h e r r e a c t i o n o f s u c c e s s i v e a m o u n t s o f C T A C , so t h e less r e a c t i v e site N a + ions ( o r i g i n a t i n g f r o m t h e high c o n c e n t r a t i o n o f t h e N a C ! e l e c t r o l y t e u s e d ) will i n t e r f e r e with such i n t e r a c t i o n a n d c o n s e q u e n t l y A H will d e c r e a s e . M o i o n s i m p r e g n a t e d o n silica c a n e x p o s e Mo---O s p e c i e s o n t h e s u r f a c e to e n h a n c e t h e A H v a l u e u p o n a d s o r p t i o n . H o w e v e r , t h e h e t e r o g e n o u s s u r f a c e site d i s t r i b u t i o n o b t a i n e d as a r e s u l t o f t h e r e p l a c e m e n t o f M o i o n s b y less a c t i v e N a ÷ ions c a u s e s t h e significant d e c r e a s e o f b o t h A H a n d EA v a l u e s . T h e low t e m p e r a t u r e p e a k o f t h e s a m e t h e r m o g r a m (Fig. 2f) also s h o w s t h e i n v o l v e m e n t o f N a + i o n s in t h e h y d r o x y l g r o u p s r e g i o n b y shifting t h e p e a k to a l o w e r t e m p e r a t u r e o f 74°C c o m p a r e d to t h e o t h e r s . REFERENCES 1 2 3 4 5 6 7 8
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