10 C h e m i c a l Reactions i n W a t e r - i n - O i l
Inorganic Reactions in Organized Media Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/02/18. For personal use only.
Microemulsions ALVORO GONZALEZ and JOHN MURPHY University of Georgia, Department of Chemistry, Athens, GA 30602 SMITH L. HOLT Oklahoma State University, Department of Chemistry, Stillwater, OK 74078 We have carried out a variety of chemical reactions in microemulsions. These include the metalation of meso-tetraphenylporphine, the base hydrolysis of long chain esters, the syntheses of macrocyclic lactones and the catalytic formation of ketones. In all instances there is a clear demonstration of the effect of microemulsification on reaction rate and pathway. Microemulsions as media for chemical reactions have only recently received close scrutiny. This neglect arose, in part, because of the limited number of carefully characterized microemulsion systems and, in part, because strong sentiment existed that microemulsions were in actuality merely swollen micelles. Current thinking suggests that there is indeed a difference between micellar solutions and microemulsion media, and that difference is such that reaction rates and pathway need not be similar in the two mediaCL). (For a current review of the literature on microemulsions see Ref. 1.) Micelles can exist as two component systems consisting of an amphiphile dissolved in either water or a hydrocarbon. When amphiphile sufficient to exceed the critical micelle concentration is dissolved in water, a "normal" micelle is formed, Figure la, i.e. the hydrophobic tails of the surfactant are directed inward while the polar head groups are in contact with the aqueous external phase. If a hydrocarbon is the bulk phase, the hydrophobic tails of the amphiphile will be directed outward, creating an "inverse" micelle, Figure lb. Water added to an inverted micellar solution, is not distributed evenly throughout the hydrocarbon continum, but is found associated with the amphiphilic head groups. This is termed a "swollen inverse" micelle, Figure 1c. The volume of water which can be taken up and stabilized in these swollen inverse micelles is limited, usually only a small fraction of a mole percent of the total liquid present in the system. 0097-6156/82/0177-0165$05.00/0 © 1982 American Chemical Society
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INORGANIC REACTIONS IN ORGANIZED MEDIA
Figure 1. Some organized assemblies: a, "normal" micelle; b, "inverse" micelle; c, "swollen inverse" micelle; d, water-in-oil microemulsion; and e, oil-in-water microemulsion. Key: ·*ΑΛΛ/ , surfactant; · , water; θ + » 2-propanol; and ΛΛΛ/VS , hexane.
10.
GONZALEZ
ET AL.
167
Water-in-Oil Microemulsions
Microemulsions a r e r e l a t e d to mic e l l e s CL). The most common, the f o u r component microemulsions, a r e constructed from a hydrocarbon, a s u r f a c t a n t , a short chain a l c o h o l ( c o s u r f a c t a n t ) and water. When the hydrocarbon component present i s s i g n i f i c a n t l y l a r g e r than the water component the microemulsion i s g e n e r a l l y a w a t e r - i n - o i l (w/o) microemulsion, F i g u r e Id. T h i s d e s i g n a t i o n a r i s e s by v i r t u e of the f a c t that the water i s present i n the form o f spheres, i n v i s i b l e to the naked eye (250& to lOOoX i n diameter), d i s p e r s e d throughout the hydrocarbon continuum. The s u r f a c t a n t and c o s u r f a c t a n t s t a b i l i z e these w a t e r - r i c h d r o p l e t s and help render them thermodynamically s t a b l e . These systems a r e o p t i c a l l y transparent and can c o n t a i n up to 0.3XH O — · consequence of the l a r g e mole f r a c t i o n of water present, w/o microemulsions d i s p l a y a much greater a b i l i t y to s o l u b i l i z e p o l a r r e a c t a n t s than swollen i n v e r t e d m i c e l l e s . S i m i l a r l y , o i l - i n - w a t e r microemulsions, o/w, F i g u r e l e . show an enhanced p r o p e n s i t y to d i s s o l v e non-polar r e a c t a n t s when compared to normal m i c e l l e s . A
S
A
9
More r e c e n t l y i t has been demonstrated that microemulsions can be formed u s i n g only water, hydrocarbon and 2-propanol, o m i t t i n g the a d d i t i o n of a conventional s u r f a c t a n t . These " d e t e r g e n t l e s s " microemulsions have been constructed using e i t h e r hexane(2) o r toluene(2) as the hydrocarbon phase. The p r o p e r t i e s of these systems have been shown to be s i m i l a r t o those which c o n t a i n long chain amphiphiles(2-5). The a b i l i t y of m i c e l l a r s o l u t i o n s and microemulsions to d i s s o l v e and compartmentalize both p o l a r and non-polar r e a c t a n t s has a s i g n i f i c a n t e f f e c t on chemical r e a c t i v i t y . An i d e a l i z e d r e p r e s e n t a t i o n o f a t y p i c a l m i c e l l e c a t a l y z e d r e a c t i o n i s depicted i n F i g u r e 2. Here the non-polar reactant i s s o l u b i l i z e d w i t h i n the m i c e l l e while the i o n i c r e a c t a n t i s a t the s u r f a c e . The p o l a r head groups of the s u r f a c t a n t s generate a charge a t the m i c e l l e surface which serves to a t t r a c t an o p p o s i t e l y charged water s o l u b l e r e a c t a n t i n c r e a s i n g the c o n c e n t r a t i o n o f that r e a c t a n t near the m i c e l l e . The r e s u l t i s an enhanced r e a c t i o n r a t e . Microemulsions work much i n the same way; i n an o/w microemulsion, the non-polar r e a c t a n t i s d i s s o l v e d i n the o i l d r o p l e t , with the p o l a r reactant i n the water continuum. Chemical r e a c t i o n occurs when there i s an encounter i n the interphase, F i g u r e 3a, or one r e a c t a n t i s transported across the interphase, F i g u r e 3b. The mechanism i s much the same when d e a l i n g with a w a t e r - i n - o i l microemulsion, the only d i f f e r e n c e being that here the p o l a r r e a c t a n t i s d i s s o l v e d i n the dispersed phase while the non-polar reactant i s i n the continuous phase. Because the interphase volume i s so l a r g e , up t o 40% of the t o t a l volume, one can expect r a p i d r e a c t i o n due to the high p r o b a b i l i t y of reagent encounter. Further m o d i f i c a t i o n o f r e a c t i o n r a t e s and pathways can be achieved by 1) v a r y i n g the amphiphile i n such a way as to change the charge gradient across the interphase, 2) adjustment of s t e r i c bulk of the interphase through v a r y i n g of the s u r f a c t a n t c o n c e n t r a t i o n or molecular complexity, or 3) through the i n t r o d u c t i o n o f a phase t r a n s f e r c a t a l y s t . These a r e exemplified below.
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Figure 2. Micellar catalysis: a, reaction of a water-soluble ion with a nonpolar organic compound; and b, reaction of a water-soluble ion with a polar organic compound. Key: φ, ZOW/AWNA, nonpolar organic reactant; and ΟΛΛΛΛΛ> polar organic reactant.
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ET
169
Water-in-Oil Microemulsions
AL.
Interfacial Effects The r e a c t i o n of Cu(II) with meso-tetraphenylporphine, 2
Cu *
+
+TPPH (aq) 2(oil)« N
TPP^,
CuTPP, + 2 H , (oil) (aq)
0/
4 1 N
N
i s an i d e a l system with which to probe the nature of micro emulsions and to a s c e r t a i n the u t i l i t y of microemulsions i n modifying r e a c t i o n r a t e and pathway. The C u ( H 0 ) 2 i o n i s s o l u b l e only i n the aqueous phase while TPPH i s i n s o l u b l e i n water. As a consequence, r e a c t i o n must occur i n the interphase or some mechanism must be invoked which permits movement of a reactant from one phase to the other. Studies conducted i n our l a b o r a t o r i e s on the m e t a l a t i o n reactionCl.,j6) have involved two types of microemulsions: d e t e r gentless microemulsions composed of water, toluene and 2-propanol and microemulsions of the same formulation but w i t h small amounts ( 1 0 ~ % ) of added s u r f a c t a n t . Using these systems both the r o l e of s u r f a c t a n t gegenion and the e f f e c t of medium composition on r a t e and mechanism have been i n v e s t i g a t e d . The r e s u l t s are tabulated i n Table I. +
2
Table I Observed Pseudo F i r s t - Order Rate Constants i n " S u r f a c t a n t " Containing and "Surfactant"-Free Microemulsions
Type
1
(hr" ) k , obs
Surfactant
0.00696+.00070
None anionic cationic
Sodium Hexadecylsulfate Hexadecyltrimethylammonium Hexadecyltrimethylammonium Hexadecyltrimethylammonium
Perchlorate Chloride Bromide
0.00413+.00035 0.00612+.00001 0.213 +.0066 0.744 +.0603
Pseudo f i r s t - o r d e r constants i n Table I were obtained i n a micro emulsion composed of Ο . Α Ι Ι Χ ^ , 0.186^ > ° · · law f o r the r e a c t i o n i n the absence ^ of detergent i s : 4 0 3 Χ
0
T
h
e
r
a
t
e
Ι Ρ Α
2 +
r
a
t
e
m
k [ C u ] [TPPH ] 2
3
+
[H 0 ] In the presence of hexadecyltrimethylammonium bromide, HTAB, t h i s r a t e law can be w r i t t e n :
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INORGANIC REACTIONS IN ORGANIZED MEDIA
r a t e = k [Cu
] [TPPH ] [HTAB] 2
The pseudo f i r s t - o r d e r r a t e constants, Table I, f o r the d e t e r g e n t l e s s system, with added sodium h e x a d e c y l s u l f a t e , SHS, and hexadecyltrimethylammonium p e r c h l o r a t e , HTAP, are much the same, i . e . 0.004-0.007 h r " . A d d i t i o n of h e x a d e c y l t r i m e t h y l ammonium c h l o r i d e , HTAC, or HTAB d r a s t i c a l l y e f f e c t s the r a t e however: k ( H T A C ) *30 k ( H T A P ) and k ( H T A B ) «100 k ( H T A P ) . T h i s can be r a t i o n a l i z e d based on the mechanism diagramed i n F i g u r e 4a and 4b. In a d e t e r g e n t l e s s system c o n t a i n i n g only aqueous copper p e r c h l o r a t e and TPPH r e a c t i o n much occur i n the interphase s i n c e n e i t h e r reagent shows a p p r e c i a b l e s o l u b i l i t y i n the other r e a c t a n t s host system. The a d d i t i o n of HTAP does l i t t l e to a l t e r these c o n d i t i o n s . While there undoubtably e x i s t s a S t e r n - l i k e l a y e r , i t appears that the c o n c e n t r a t i o n of HTAP i s i n s u f f i c i e n t i n any given microemulsion d r o p l e t f o r t h i s to be a f a c t o r . A d d i t i o n of SHS causes a s l i g h t r e t a r d a t i o n i n k . T h i s can be r a t i o n a l i z e d by n o t i n g that the s u l f a t e head groups have some a f f i n i t y f o r Cu(II) and may be decreasing the copper m o b i l i t y through complexation. In any case the a f f e c t i s not l a r g e . When HTAC or HTAB are added i t i s c l e a r from the r a t e i n c r e a s e s that a mechanism which r e l y s only on a random encounter i n the interphase i s no longer a p p l i c a b l e . I n s i g h t i n t o the r a t e enhancement process can be obtained i f we compare the s t a b i l i t y f o r the formation of CuBr|~ and CuCl|". Log 34 f o r the r e a c t i o n : 1
obs
obs
obs
obs
2
o b s
Cu
2+
+ 4Br
-
± CuBr.
2-
i s 8.92 and, f o r the analogous r e a c t i o n i n v o l v i n g the c h l o r i d e i o n i t i s 5.62. These data are c o n s i s t e n t w i t h a mechanism whereby the format i o n of C u X ~ species f a c i l i t a t e the m e t a l a t i o n r e a c t i o n . T h i s could be e f f e c t e d i n two ways. F i r s t , CuX|~ i s formed, a t t r a c t e d to the c a t i o n head groups (but not "bound"), F i g u r e 4b. T h i s would then i n c r e a s e the c o n c e n t r a t i o n of Cu(II) i n the interphase enhancing the p r o b a b i l i t y of an encounter with a TPPH^ molecule. An a l t e r n a t e pathway r e q u i r e s that CuXo be the dominant s p e c i e s . This molecule i s l e s s p o l a r than C u ( H 0 ) £ or Cux|" and as a consequence can more r e a d i l y penetrate the toluene continuum. T h i s l a t t e r mechanism i s phase t r a n s f e r i n nature. A d d i t i o n ôf f i r s t NaBr then NaBr + HTAB to the d e t e r g e n t l e s s system suggests that both pathways are important. When the Br*" i s 5.8x 10" M k i s found to be 0.0951+0.0126 h r " , c o n s i d e r a b l y higher than f o r the r e a c t i o n i n the d e t e r g e n t l e s s system sans NaBr. I f 3.5xlO~ M HTAB and 2.2xlO"*M NaBr are used ( t o t a l c o n c e n t r a t i o n [Br"] = 5.7x10-4) k i s 0.172 + 0.025, a f a c t o r of 2 greater than that observed with NaBr alone. Since there i s no s u r f a c t a n t head group when only NaBr i s used i t i s l i k e l y that transport i s 2
n
n
+
2
4
1
o b s
4
Q b s
GONZALEZ ET A L .
Water-in-Oil Microemulsions
2
Figure 4. Reaction of Cu * with TPPH»: a, in the absence of added halide; and b, in the presence of added halide.
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INORGANIC REACTIONS IN ORGANIZED MEDIA
e f f e c t e d by CuBr . On the other hand a d d i t i o n of HTAB does i n c r e a s e the r a t e so i t i s probable that the CuBr24- species are a l s o important. An i n v e s t i g a t i o n of the r a t e of metalation as a f u n c t i o n of s o l u t i o n composition i n a detergentless system i s a l s o very i n s t r u c t i v e . The r a t e of r e a c t i o n v a r i e s l i t t l e , Figure 5, along the Χ O = 0 . 2 i s o p l e t h while the s o l u t i o n composition l i e s i n the microemulsion r e g i o n . Once i n t o the "small aggregate" r e g i o n , however, k increases d r a m a t i c a l l y and continues to i n c r e a s e i n t o the ternary s o l u t i o n formulation r e g i o n . The p r i n c i p a l (and only discontinuous) change, which occurs i n l e a v i n g the micro emulsion r e g i o n , i s c o i n c i d e n t with a breakdown of the interphase. This r e s u l t i s a v i v i d demonstration of the e f f e c t of the i n t e r phase i n the c o n t r o l of the r a t e of r e a c t i o n . The e f f e c t of the presence of the microemulsion interphase has a l s o been demonstrated i n a study of the base h y d r o l y s i s of long chain e s t e r s i n a water d i s p e r s e hexane, water, 2-propanol microemulsion(7). Because of the hydrophobic nature of such e s t e r s as stéarate, l a u r a t e , and c a p r y l a t e , attempts have been made to enhance t h e i r r a t e of h y d r o l y s i s i n aqueous s o l u t i o n both through the a d d i t i o n of a phase t r a n s f e r c a t a l y s t to a two phase system and by the i n t r o d u c t i o n of m i c e l l e s . The maximum r a t e obtained i n m i c e l l a r s o l u t i o n when the reactant was the l a u r a t e e s t e r , was 0.26 min (8) while under normal c o n d i t i o n s f o r phase t r a n s f e r c a t a l y s i s the y i e l d of an e s t e r h y d r o l y s i s r e a c t i o n i s - 35%(9). In c o n t r a s t i f the same r e a c t i o n i s c a r r i e d out i n a hexane, water, 2-propanol microemulsion the y i e l d i s >98%, w i t h a r a t e as high as 0.4 m i n . When s t u d i e s are c a r r i e d out along an i s o p l e t h of constant mole f r a c t i o n water the r a t e of h y d r o l y s i s changes i n a r e g u l a r manner throughout the microemulsion r e g i o n , Figure 6, but a d i s c o n t i n u i t y occurs at s o l u t i o n compositions which correspond to the pseudo-phase boundry. Though not as dramatic an e f f e c t as was observed i n the s t u d i e s on the metala t i o n of meso-tetraphenylporphine t h i s behavior again demonstrates the importance of the microemulsion interphase on chemical reactivity. I n t e r e s t i n g l y enough, i n t e r f a c i a l environment has l i t t l e e f f e c t on the formation of t r a n s i t i o n metal complexes of Ν dodecanoylamino a l c o h o l s (10-12). In a s e r i e s of s t u d i e s on Cu(II) complexes with the surface a c t i v e N - d o d e c a n o y l h i s t i d i n o l , - l y s i n o l , - g l u t a m i n o l , - m e t h i o n i n o l and -tryptophanol i n hexane, water, 2-propanol microemulsions i t was found that the formation constants v a r i e d l i t t l e from those obtained i n aqueous s o l u t i o n . Further, where i t was p o s s i b l e to e l u c i d a t e s t r u c t u r e the c o o r d i n a t i o n geometry was the expected one based on analogy with s i m i l a r non surface a c t i v e l i g a n d s i n aqueous media. 2
H2
obs
-1
- 1
Microemulsions i n Chemical Detergentless
Synthesis
microemulsions would appear to have
considerable
10.
GONZALEZ
ET
AL.
Water~in-Oil Microemulsions
173
Figure 5. Rate of metalation as a junction of a reaction medium composition: microemulsion region is stippled.
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INORGANIC REACTIONS IN ORGANIZED MEDIA
10.
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ET AL.
Watet'An-Oil Microemulsions
175
p o t e n t i a l f o r u t i l i z a t i o n as media f o r chemical s y n t h e s i s . As noted e a r l i e r not only does the presence o f a very l a r g e i n t e r f a c i a l area enhance the p r o b a b i l i t y of reagent encounter but p u r i f i c a t i o n i s s i m p l i f i e d when compared t o a m i c e l l a r system or when phase t r a n s f e r c a t a l y s t s a r e employed. The e f f i c i e n c y of d e t e r g e n t l e s s microemulsions i n promoting the formation o f macrocylic lactones has been studied i n our l a b o r a t o r i e s C1D, The two most d i r e c t routes t o the formation of l a r g e macrocyclic lactones a r e the a c i d " c a t a l y z e d " e s t e r i f i c a t i o n of ω-hydroxyalkanoic a c i d s and the c y c l i z a t i o n o f potassium s a l t s o f ω-bromoalkanoic a c i d s . In both r e a c t i o n s a competitive pathway y i e l d s polymeric m a t e r i a l and as a consequence h i g h d i l u t i o n s a r e employed with the attendant extended r e a c t i o n times. U t i l i z a t i o n of microemulsions would appear t o be one method by which the p o l y m e r i z a t i o n problem might be reduced o r even eliminated. In a w a t e r - i n - o i l microemulsion we would expect the ω-hydroxy- and ω-bromoacids t o be compartmentalized on a molecular b a s i s i . e . an average of one molecule per drop up t o some c o n c e n t r a t i o n , then two per drop, e t c . , and movement between drops i n h i b i t e d . As a consequence s i n c e the base i s s o l u b l e i n water and the a c i d l i k e l y l o c a t e d i n the interphase we would expect that the chance o f r i n g c l o s u r e before d i m e r i z a t i o n , t r i m e r i z a t i o n , e t c . , would be g r e a t l y enhanced over that e x i s t i n g i n homogeneous media. Using a toluene based d e t e r g e n t l e s s microemulsion t o i n v e s t i g a t e the c y c l i z a t i o n o f 12-hydroxyoctadecanoic, 15hydroxypentadecanoic and 16-hydroxyhexadecanoic a c i d s i t was found p o s s i b l e t o i n c r e a s e the c o n c e n t r a t i o n 4 0 - f o l d and reduce the r e a c t i o n time to 14 hours (as opposed to days) w h i l e o b t a i n i n g 20% y i e l d o f l a c t o n e . The biggest d e t e r r e n t t o higher y i e l d s i n a d e t e r g e n t l e s s system appears to be formation o f the 2-propyl e s t e r which appeared as 40% o f the f i n a l product. ( I f the analogous r e a c t i o n i s r u n i n a mixture o f water and 2-propanol the r e s u l t i s 50/50 ester/polymer but no l a c t o n e ) . U t i l i z a t i o n of 11-bromoundecanoic and 15-bromopentadecanoic a c i d e l i m i n a t e d the problem o f e s t e r formation. After reacting a 5xlO~ M s o l u t i o n of the 11-bromo a c i d with KOH f o r a p e r i o d of one day, 25% l a c t o n e and 18% polymer were i s o l a t e d . The remaining m a t e r i a l was recovered as unreacted bromoacid. While n e i t h e r of the r e s u l t s i s s p e c t a c u l a r they do demonstrate the p o t e n t i a l u t i l i t y of microemulsions i n h e l p i n g to minimize the e f f e c t s of an unwanted competing r e a c t i o n . The palladium c a t a l y z e d formation of ketones from long chain«
