Photochemistry on Colloidal Silica Solutions - ACS Symposium Series

6

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Photochemistry on C o l l o i d a l S i l i c a Solutions J. WHEELER and J. K. THOMAS University of Notre Dame, Department of Chemistry, Notre Dame, IN 46556 Two probe molecules, Ruthenium tris-bipyridyl, Ru(II), and4-(1-pyrenyl)butyltrimethylammonium bromide, PN have been used to investigate the nature of colloidal silica particles in water. The fluorescence spectra of the two probes show that the silica surface is very polar and similar to water. Quenching studies of the excited state of RuII and PN+ by anionic quenching molecules show that the particles are negatively charged but that the charge is not as effective as that on sodium lauryl sulfate micelles. Quenching studies with cationic quenchers show that the cations are bound strongly to the silica particles but do not move as readily around the surface as on anionic micelles. A small steric effect is observed with neutral quenchers. Several charge transfer reactions, including photo-ionization are strongly affected by the silica particles. The studies show many similarities to anionic micelles; they differ from micelles in two important aspects: (a) they do not solubilize neutral organic molecules and (b) cationic organic molecules such as PN , hexadecyltrimethylammonium bromide, and hexadecylpyridinium chloride, tend to cluster on the silica surface rather than disperse uniformly around it as with ionic micelles. +

+

The past decade has seen great strides in the utilization of organized assemblies, such as micelles, microemulsions, etc., to promote desirable features of photochemical reactions. (2,3) The most prominent feature of these systems is the use of an ionic surfactant, such as sodium lauryl sulfate, NaLS, or hexadecyltrimethylammonium bromide, CTAB, to create a charged barrier between the lipid and aqueous phases of a small region of the system. Reactants located 0097-6156/82/0177-O097$05.00/O © 1982 American Chemical Society Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

98

INORGANIC REACTIONS IN ORGANIZED MEDIA

at t h i s i n t e r f a c e are s t r o n g l y i n f l u e n c e d i n t h e i r subsequent r e a c t i o n s by: (a) c l o s e p r o x i m i t y f o r r a p i d r e a c t i o n (b) the strong e l e c t r i c f i e l d o f the surface which i n f l u e n c e s 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 , r e p e a l s ions of s i m i l a r charge t o the s u r f a c e , and a t t r a c t s ions of opposite charge (c) O r g a n i z a t i o n o f reactants t o produce s p e c i f i c d e s i r e d effects (d) s o l u b i l i z e hydrophobic molecules i n c l o s e proximity t o a h y d r o p h i l l i c surface. Future developments i n u t i l i z a t i o n o f organized assem b l i e s c o u l d l i e i n the use o f c o l l o i d a l s e m i c o n d u c t o r s , ^ ) and the use o f i n o r g a n i c c o l l o i d s i n p l a c e of the organic s u r f a c t a n t s i n d i c a t e d above. One p a r t i c u l a r system of i n t e r est i s t o use c o l l o i d Bentonite c l a y s , which as they s t r o n g l y promote thermal r e a c t i o n s , should a l s o promote photochemical r e a c t i o n s . ^ ' However, a c l a y system i s q u i t e d i f f e r e n t than the simple m i c e l l e s which have been s t u d i e d , being a s t a t i c very p o l a r s t r u c t u r e which can absorb c a t i o n s by ex changing the present c l a y c a t i o n s . The exact nature of ad s o r p t i o n o f uncharged organic molecules i s u n c e r t a i n , but c a t i o n s are expected t o be s t r o n g l y absorbed. To i n i t i a t e our s t u d i e s we r e p o r t data on photochemistry i n aqueous s o l u t i o n s o f s i l i c a . These s o l u t i o n s are mainly water and the s i l i c a p a r t i c l e s possess a negative charge which i s counteracted by an i n v i s i b l e sodium i o n . The systems bear some resemblance of a n i o n i c m i c e l l e s such as NaLS. Experimental Absorption s p e c t r a were recorded i n a Perkin-Elmer spec trophotometer, and f l u o r e s c e n c e s p e c t r a were recorded on a Perkin-Elmer kkB s p e c t r o f l u o r i m e t e r . Flash photolysis s t u d i e s were c a r r i e d out u s i n g an excimer l a s e r , λ e x c i t a t i o n = 3080A°, a ruby l a s e r , λ e x c i t a t i o n = 3^T1A°, or a n i t r o g e n l a s e r , λ e x c i t a t i o n = 3391A . The system has been d e s c r i b e d previously. The c o l l o i d a l s i l i c a s o l u t i o n s were obtained from NALCOAG Chemicals #1115, pH 1 0 Λ , r = kOA° r a d i u s ; #103^-A, pH 3.2, r = 200A°; #1050, pH 9.0, r = 200A°. These s o l u t i o n s were run at 20$ d i l u t i o n i n water. A new probe molecule h-(1-pyrenyl)butyltrimethylammonium bromide was made from pyrene b u t y r i c a c i d as f o l l o w s : - pyrene b u t y r i c a c i d was r e f l u x e d w i t h methanol and converted t o the methyl e s t e r , ,0 Q

OCH3

Holt; Inorganic Reactions in Organized Media ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

6.

W H E E L E R AND THOMAS

Photochemistry on Colloidal Silica Solutions

99

The e s t e r "was reduced w i t h l i t h i u m aluminum hydride t o t h e alcohol, P y ( C H ) - CH 0H 2

2

3

LIAI Hj, ^ * P y ( C H ) - CH 0H h

2

mp 8l-83°C

2

3

The a l c o h o l was converted t o the bromide by r e f l u x i n g w i t h CBrl| and triphenylphosphine: CBn P y ( C H ) - CH 0H 2

mp 76-77°C

Py(CH ) -CH Br

2

3

2

3

2

The bromide d e r i v a t i v e was r e f l u x e d w i t h trimethylamine t o produce the quaternary ammonium s a l t , Ρ Ν · +

Py(CH ) -CH -Br 2

3

2

me

3p

[Py(CH )i^-N(CH ) ]+ Br~ 2

3

3

Experimental Data and D i s c u s s i o n Spectroscopy. I t has been shown previously(j) t h a t the f l u o r e s c e n c e spectrum o f Ruthenium t r i s - b i p y r i d i n e , RuII, i s solvent dependent, showing a r e d s h i f t w i t h i n c r e a s i n g s o l vent p o l a r i t y . The f l u o r e s c e n c e spectrum o f RuII on s i l i c a p a r t i c l e s i s i d e n t i c a l t o t h a t o f t h e e x c i t e d molecule i n water. I t w i l l be shown subsequently t h a t the RuII i s essen t i a l l y a l l bound t o the s i l i c a p a r t i c l e , hence the data show t h a t the environment o f a probe molecule such as RuII on s i l i c a p a r t i c l e s i s very p o l a r and s i m i l a r t o water. E s s e n t i a l l y , the same data i s obtained f o r t h e organic probe molecule k-( 1-pyrenyl)butyltrimethylammonium bromide, PN+. The f l u o r e s c e n c e s p e c t r a o f t h i s molecule i n s e v e r a l environments i n c l u d i n g s i l i c a and NaLS m i c e l l e s are shown i n F i g . 1. The water and s i l i c a s p e c t r a are i d e n t i c a l , thus confirming the RuII probe data. K i n e t i c Data: RuII System F i g . 2 shows the r a t e o f formation and subsequent decay of e x c i t e d RuII f o l l o w i n g l a s e r e x c i t a t i o n . The f i r s t order p l o t o f the data i s a l s o shown, and the slope o f t h i s l i n e a r p l o t gives t h e r a t e constant f o r t h e process and t h e h a l f l i n e o f r e a c t i o n . A d d i t i o n o f a quenching molecule t o t h e s o l u t i o n increases the r a t e o f decay o f (RuII)*: RuII

RuII +

(RuII)*

hv

ko* ^products

The c o n c e n t r a t i o n o f quencher [Q], i s much l a r g e r than [(RuII)*] SO r e a c t i o n 2 i s pseudo f i r s t order. The o v e r a l l r a t e constant k i f o r decay o f (RuII)* from data, such as those


100



6.

W H E E L E R A N D THOMAS



101

102


Figure 2. Fluorescence decay of [Ru(II)]* in water where Aem = 610 nm. Key: 1,fluorescencevs. time in arbitrary units; and 2, natural log offluorescencevs. time; rate = 1.71 χ 10 s' ; half-life = 414 ns. 6

1


6.

W H E E L E R A N D THOMAS


103

shown i n F i g u r e 2 i s thus: k = k^ + k [Q]. S e v e r a l r a t e con s t a n t s f o r r e a c t i o n o f (RuII)* with quenchers are shown i n Table I . The data may be d i v i d e d i n t o t h r e e s e c t i o n s , quench i n g by uncharged s p e c i e s , quenching by anions, and quenching by c a t i o n s . The r a t e constants f o r r e a c t i o n i n s i l i c a systems are compared t o data f o r these systems i n homogeneous s o l u t i o n i . e . water, and i n a n i o n i c NaLS m i c e l l e s . I t i s noted t h a t the quenching r a t e s f o r 0 and nitrobenzene i s about 25$ s m a l l e r than those i n water, but s i m i l a r t o those i n NaLS. This i s a t t r i b u t e d t o a s t e r i c f a c t o r imposed by t h e s i l i c a background on the approach o f 0 t o (RuII)*. Both negative ions Fe(CN)g" and 3-5 d i n i t r o b e n z o a t e are much slower i n s i l i c a compared t o water but not as slow as i n NaLS where the r a t e s were t o o slow t o measure a c c u r a t e l y . The decreased r e a c t i o n r a t e s are due t o r e p u l s i o n o f the a n i o n i c quencher by t h e a n i o n i c s i l i c a p a r t i c l e s on NaLS micelles. (8) I t i s p o s s i b l e t o use the Debye m o d i f i c a t i o n of t h e Smoluckowski equation t o e x p l a i n these data. (_>2J The equation i n d i c a t e s t h a t the d i f f u s i o n c o n t r o l l e d r a t e constant k^ f o r r e a c t i o n o f two ions i s given by k = kJlr D Ν l 2 / f expj l 2 I -1 ] 1000 rEkT / I * rEkT where Ν i s an Avagadro's number, r i s t h e i n t e r a c t i o n r a d i u s , Ε the d i e l e c t r i c constant o f t h e medium and Z-^e and Z e are the charges o f the two r e a c t a n t s and D i s the t o t a l d i f f u s i o n constant. The RuII data can be explained q u a n t i t a t i v e l y i f charges o f - 8 t o -10 u n i t s / p a r t i c l e are used f o r s i l i c a ; a much l a r g e r charge (>20), i s necessary t o e x p l a i n t h e NaLS data. The exact p o s i t i o n o f t h e probe i n the surface i s important i n t h i s c a l c u l a t i o n , and the data c o u l d i n d i c a t e t h a t f o r r e a c t i o n t o occur the a n i o n i c quenchers do not have t o penetrate as c l o s e t o S i 0 as t o NaLS. The c a t i o n C u i s s t r o n g l y bound t o the s i l i c a p a r t i c l e s and t o NaLS m i c e l l e s . \ i 2 ' P a r t i c l e s or m i c e l l e s c o n t a i n i n g (RuII)* and C u should show quenching o f (RuII)* t h a t i s more r a p i d than t h a t i n water. T h i s i s t h e case f o r NaLS m i c e l l e s but t h e r a t e s on s i l i c a are a c t u a l l y lower than those observed i n water. T h i s i s due t o a lower m o b i l i t y o f C u around a s i l i c a p a r t i c l e compared t o an NaLS m i c e l l e , a f a c t already i n d i c a t e d by the strong b i n d i n g o f ions such as RuII t o s i l i c a particles. 2

2

Z

Z

e

2

Z

Z

e

2

f

J

2

2

+ +

+ +

+ +

K i n e t i c S t u d i e s , PN+ System Table I I shows t h e quenching r a t e constants f o r e x c i t e d PN w i t h s e v e r a l quencher molecules on s i l i c a and on NaLS m i c e l l e s . The patterns shown by n e u t r a l quenchers e.g. 0 , CH N0 and d i m e t h y l a n i l i n e , a n i o n i c quenchers e.g. 3 - n i t r o p r o +

2

3

2


104


Table I Ru I I , observed at λ = 610 run

1

Quencher

Water

None

1.67

°2 Nitrobenzene

System, k (LEH-S- ) NaLS, m i c e l l e x 10

6

1.3

x 10

l.k χ

6

2.8 χ 1 0

9

2.1 χ 1 0

9

3 x 10

9

1 χ 10

9

Silica

(1.5 [1.5

χ x

1.3 [1.2

x χ

io ) 6

^9

.8 7.6 χ (8.8 χ [7.5 x 1 0 ] 8

Fe(CN)g~

3.8 χ i o

1 0

2 ysec. I t i s suggested t h a t the photo-ionized e~ i s e j e c t e d i n t o the s i l i c a p a r t i c l e where i t i s s t a b i l i z e d and not observed over the s p e c t r a l range s t u d i e d , λ = 3000A° t o 6 5 O O A . D i m e t h y l a n i l i n e r a p i d l y quenches ( P N ) * on s i l i c a and p r o duces the anion o f PN , (PN+)~ and the DMA c a t i o n , DMA . The ions are short l i v e d as DMA+ and ( P N ) ~ do not escape from the p a r t i c l e r a p i d l y enough t o prevent back e l e c t r o n t r a n s f e r . T h i s has a l s o been observed i n a n i o n i c NaLS m i c e l l e s . (M.) Laser e x c i t a t i o n o f RuII leads t o (RuII)* and t o a b l e a c h i n g o f the RuII ground s t a t e absorption i n the r e g i o n λ~1*600Α°. Heptyl and methyl v i o l o g e n , HV++, MV , r a p i d l y quench the (RuII)* but the w e l l e s t a b l i s h e d r e l e a s e o f l o n g l i v e d i n t e r mediates such as reduced MV i s not observed; (RuII)* + MV++ — > (RuIIl) + MV+. T h i s i s s i m i l a r t o the PN+ - DMA system where the a n i o n i c s i l i c a s u r f a c e binds the c a t i o n i c products and pro motes back e" t r a n s f e r before the product ions can be separated. I t i s i n t e r e s t i n g t o note t h a t Ag+ r e a c t s w i t h (RuII)* on s i l i c a l e a d i n g t o a l o n g l i v e d , ( s e v e r a l seconds) b l e a c h i n g o f RuII and t o the formation o f c o l l o i d a l s i l v e r . The r e a c t i o n i s : + +

a

a(

a

0

+

+

+

+

++

+

(RuII)* + Ag+ — i >

(RuIIl) + Ag°

Colloidal

silver

The back r e a c t i o n o f Ag° + (RuIIl) i s r a p i d i n water, but i s s t r o n g l y r e t a r d e d i n s i l i c a where Ag° i s e j e c t e d from the v i c i n i t y o f (RuIIl) which i s s t r o n g l y bound t o the s i l i c a particle. Such a l o n g l i v e d s e p a r a t i o n o f products i s not observed i n homogeneous aqueous s o l u t i o n . Conclusion The data show t h a t i n many ways s i l i c a p a r t i c l e s behave i n a s i m i l a r f a s h i o n t o a n i o n i c m i c e l l e s , although the q u a n t i t a t i v e aspects o f the a n i o n i c surface are d i f f e r e n t . A major d i f f e r e n c e occurs i n the l o c a t i o n o f c a t i o n i c organic molecules on the p a r t i c l e s , as these molecules tend t o c l u s t e r together r a t h e r than d i s p e r s e uniformly as i n m i c e l l a r systems.


6. WHEELER AND THOMAS


Literature Cited 1. The authors wish to thank the Army Research Office via grant No. DAAG29-80-K-0007, P001, for support of this research. 2. Turro, N. J.; Grätzel, M.; Braun, A. M. Angewardte Chemi. 1980, 19, 675. 3. Thomas, J . K. Chem. Rev. 1980, 80, 283. 4. Harbour, J. R.; Hair, M. L. J . Phys. Chem. 1978, 82, 1397. 5. Grim, R. E.; Clay Mineralogy 1968, McGraw Hill, Ν. Y. 6. McNeil, R.; Richards, J . T.; Thomas, J. K. J . Phys. Chem. 1970, 74, 2290. Also Atik, S. S.; Thomas, J . K. JACS in Press. 7. Meisels, D.; Matheson, M.; Rabani, J . JACS 1978, 100, 117· 8. Thomas, J . K.; Accounts of Chem. Research 1977, 10, 133. 9. Matheson, M. S. Solvated Electron ACS Advances in Chem. 1965, No. 50, p 45. 10. Grätzel, M.; Thomas, J . K. J . Phys. Chem 1974, 78, 2248. 11. Atik, S. S.; Thomas, J . K. JACS in Press. 12. Richards, J . T.; West, G.; Thomas, J. K. J. Phys. Chem. 1970, 74, 4137. 13. Wallace, S. C.; Thomas, J . K. Rad. Res. 1972, 10, 76. 14. Katusin-Razem, B.; Wong, M.; Thomas, J . K. JACS 1978, 100, 19679. RECEIVED

August 4, 1981.


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Photochemistry on Colloidal Silica Solutions - ACS Symposium Series

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