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Aggregation of Azo Dyes with Cationic Amphiphiles at LowConcentrations in Aqueous Solution

Rixt T. B uwa lda, J essica M. J onker, an d J a n B . F. N. En gberts*

Dep a r tm en t of O r g a n i c a n d M ol ecu l a r I n o r g a n i c Ch emi str y, U n i ver si ty o f G r on i n g en ,

N i j e n b or g h 4 , 9 7 47 A G G r o n i n g en , T h e Neth er l a n d s

Received Jul y 7, 1998. I n F in al F orm: December 3, 1998

The aggregation of a series of n-alkyltrimeth ylamm onium bromide (CnTAB , n) 10, 12, 14, 16, an d 18)a n d 4-n-alkyl-1-methylpyridinium iodide amphiphiles (C mpyI, m ) 8, 10, 12, and 14) induced by lowc onc entr a t ions of azo d y es in aq ueous solution has been investigated by means of ul tr aviolet (U V)spectroscopy. It w as observed tha t a ggregation ta kes place at surfa ctant concentra tions far below t he cmcof C 12TAB, C 14TAB, C 16TAB, and C 18TAB with methyl orange (MO), ethyl orange (EO), and p a r a -methylred (pMR). Aggregation below the cmc of C 10TAB wa s a lso induced by EO. In the case of MO and pMR,however, higher dye concentra tions were necessary t o induce aggregat ion. Interactions a t low surfa ctantconcentra tions ha ve also been observed in a queous solutions of C 10pyI, C 12pyI, and C 14pyI with MO. Bindingof methyl red (MR), methyl yellow (MY), and azobenzene sulfonate (ABS) with cationic surfactants belowtheir cmc did not occur. Aggregation w as reflected by th e appeara nce of a blue-shifted a bsorption ba ndin th e spectra of th e dyes. Precipitat es formed in a queous solutions from t he cationic surfacta nts a nd MOan d pMR are C nTA-MO, C nTA-pMR, and C mpy-MO salts a nd consist of an equimolar r at io of surfacta ntand d y e. In similar exper iments with EO, M R, M Y , or A BS as the solvatoc hr omic d y e molec ules, no

precipitat ion occurred,a lthough surfacta nt-

EO salt s precipitat ed at h igh EO concentra tion. Dye-

surfactantsal ts f or med f r om C nTAB and M O w er e f ound to f or m my elins in phase penetr at ion exper iments.Temperat ures at wh ich myelins sta rt t o form increase upon increasing alkyl chain length of the surfa ctantas revealed by optical microscopy. The formation of vesicles from C 10TA-MO and from C 12TA-MO crysta lswas indicated by electron microscopy. The presence and position of the ionic group in the dye moleculeis importan t in determining the a ssociat ion. The importance of hydrophobic interactions is revealed bythe chain length dependence on the a ggregation process and t he observation th at interactions are a bsentin ethanol. Electrostatic interactions also play an important role, as shown by the effect of NaCl on thebinding process.

Introduction

The binding of dye molecules to proteins has beenextensively st udied by spectra l t echniques. The da taprovide insight into small molecule-ma cromolecule in-teractions, which are ofma jor importance in biochemistry.1

Particular ly the interactions between cationic dye mol-ecules and anionic proteins 2 as well as those of anionicdyes with cationic proteins 3 h a v e b ee n t h e s u b je ct ofnumerous studies. Changes in the absorption or fluores-ce n ce s p ec t r um of t h e d y e u p on b in d i n g h a v e b e e nat tr ibuted ma inly to electrosta tic interactions.4 Markedchan ges also occur in th e spectra of organ ic a nions whens y n t h e t i c p o l y m e r s a r e a d d e d . C h a r g e d 5-7 a s w ell a snonionic8-10 polymers have been shown to interact w ithazo dyes such as methyl orange (MO). It has been shown

t h a t i n t er a c t i on s b e t w e en c a t i on i c p ol y m er s a n d M Oinduce marked reductions in reduced viscosity. 11,12 Notonly polymers are able to induce spectral changes butalso surfactants have been shown to aggregate with dyemolecules in aqueous solution. Cationicsurfactants induce

changes in the spectra of anionic dyes such as 2-naph-tholate, 13 bromophenol blue,14 dansylglycine,15 and MO.11,16

Also, anionic surfactants interact with cationic dyes.17

Interestingly, the la rgest spectra l changes a re observeda t surfa cta nt concentra tions far below the cmc. The origino f t h e s p e c t r a l s h i f t s i s s t i l l a m a t t e r o f d e b a t e i n t h eli terature. D ye a ggregation wa s held responsible for theoccurrence of the new ba nd in t he case of MO in a queoussolutions of cationicpolymers.6,7 Upon increasing polymerconcentration, dilution of the dye bound to the polymeroccurs and the MO spectrum shifts from tha t of ag gregat es

(1) (a) Stone, A. L.; Bra dley, D. F. J. Am. Chem. Soc. 1961, 83, 3627.(b)von Hippel, P. H.; McGhee, J . D. Annu. Rev. Biochem. 1972, 41, 231.(c) Blake, A.; Peacocke, A. R. Biopolymers1968, 6, 1225. (d) Wa ring,M. J . J . M o l . Bi o l . 1965, 13, 269.

(2) (a)Klotz, I . M.; Maria n Walker, F.; P ivan, R. B . J. A m. Chem. Soc.1946, 68, 1486. (b)Klotz , I. M.; Mar ian Walker , F. J. Phys. Chem. 1947,55, 666. (c) Klotz, I. M. J. Am. Ch em. Soc. 1946, 68, 2299. (d) Klotz, I .M.; Burkhard, R. K. ; U rq uha rt , J . M. J. Am. Chem. Soc. 1952, 74, 202.(e) Forbes, W. F.; Milligan, B. Aust. J. Chem. 1962, 15, 841. (f) Miller,J . A.; Sa pp, R. W.; Miller, E. C. J. Am. Chem. Soc. 1948, 70, 3458.

(3) Michaelis, L.; G ra nick, S. J. Am. Chem. Soc. 1946, 68, 1486.(4) Vitagliano, V. In Aggregation Processes in Solut ion; Wyn-J ones,

E . , Gormal l y , J . , E ds. ; E l se vi er: Amste rdam, 1 98 3; p 2 7 1-308 andreferences therein.

(5) Klotz, I . M.; Royer, G. P .; Sloniewsky, A. R. Biochemistry1969,8, 4752.

(6 ) (a) Yamamoto, H.; Naka z awa , A.; Ha y aka wa , T. J. Polym. Sci.,Polym. Lett. Ed. 1983, 21, 131. (b) Ta ka gishi, T.; Uen o, T.; Ku roki, N.;Shi ma, S. ; Sakai , H. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 1281.

(7 ) Yamamoto, H.; Nakaz a wa , A. Biopolymers1984, 23, 1367.

(8) (a)H am ada , K.; Fujita , M.; Mitsuishi, M. J. Chem. Soc., FaradayT r a n s . 1990, 86, 4031. (b) Ta ka gishi, T.; Fujii, S.; Kur oki, N. J. ColloidIn terface Sci. 1983, 94, 114. (c) Frank, H. P.; Barkin, S.; Eirich, F. R.J. Phys. Chem. 1957, 61, 1375.

(9) Kim, W.-S.; Lee, S. -K.; Seo, K .-H. M acromol. Ch em. Ph ys. 1994,195, 449.(10) Kobaya shi, K.; Sumimoto, H. M acromolecul es 1980, 13, 234.(11) Wan g, G .-J .; E ngberts, J . B . F . N. L a n g m u i r 1994, 10, 2583.(12) Malik, W. U.; Pal Verma, S. J. Phys. Chem. 1966, 70, 26.(13) Amire, O. A.; B urr ows, H . D. In Surfactants in Solution; Mi t tal ,

K. L., B othorel, P ., Eds.; P lenum P ress: New York, 1986; Vol. 4.(14) Colichman , E . L. J. Am. Chem. Soc. 1951, 73, 3385.(15) Da vis, G . A. J. Am. Chem. Soc. 1972, 94, 5089.(16) Quadr ifoglio, F.; Crezenzi, V. J. Colloid InterfaceSci. 1971, 35,

447.(1 7) (a) Ne umann, M. G . ; G e hle n, M. H. J . C ol l o i d I n t e r f a ce Sci .

1990, 135, 209. (b)Mu kerjee, P.; Mys els, K. J . J. Am. Chem. Soc. 1955,77, 2 93 7. ( c) Sato, H.; K aw asa ki , K. ; Nakash i ma, N. Bul l. Ch em. Soc.J p n . 1983, 56, 3588. (d)H am a i, S. Bul l. Chem. Soc. Jpn . 1985, 58, 2099.(e) Sheppard, S. E.; Geddes, A. L. J. Chem. Phys. 1945, 13, 63.

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to that of monomers. Different polymers, however, giverise to dif ferent shif ts in the wavelength of maximumabsorption of MO. It was therefore postulated that thelarger shift was caused by the formation of higher-orderdye a ggrega tes. The format ion of MO dimers ha s errone-ously been excluded on the basis of circular dichroismexperiments.18 Another interpretation was provided byQuadrifoglio et al . 16 w h o p os t u l a t e d a c on f or m a t i o n a lchan ge from the tr a ns t o the cis form of the dy e moleculein order to explain the a ppear a nce of the short-wa velengtha b s or pt i on b a n d . H ow e v er , u s in g r es on a n ce R a m a nspectroscopy i t has been shown that the dye moleculereta ins its tra ns form upon binding to proteins and cationics u r f a c t a n t s .19 More recently, D utta and Bhat 20 proposedthe formation of water-structure-enforced ion pairs inwhich the headgroup ofthe surfactant moleculeis at ta chedto the sulfonat e group of MO in order to expla in the largeblue shift in the spectrum of MO upon addition of smallamounts of a cationic amphiphile. A short-wavelengtha bsorption ba nd ha s also been observed upon a ddition ofinorga nic salt s (e.g., Ca Cl2) t o M O ,18,21 but simulta neously,a precipita te is formed, which wa s thought t o be the originof the blue-shifted absorption band.

In addition to proteins and polyelectrolytes, dyes have

also been employed as probes for the micropolari ty ofcyclodextrines, 22 reverse micelles,23 microemulsions, 24

l iquid crysta ls,25 bilayers,26 an d monolayers.27 The short -wavelength absorption band of MO has been reported instud ies of monola yers 27 a nd microemulsions 24 where MOwa s used a s a solvat ochromic reporter m olecule. In t hel a t t e r ca s e , t h e a p p ea r a n ce of t h e s h or t -w a v el en g t ha b s o r p t i o n b a n d w a s a t t r i b u t e d t o d y e a g g r e g a t i o n . I na nother st udy, t he incorporat ion of alcohols into cationicmicelles has been investigated using MO as a reportermolecule.28

I n t h e p r e se n t s t u d y , w e h a v e i n ve s t ig a t e d t h e i n t e r -a ctions of a s eries of n-alkyltrimethylammonium bromidea n d 4 -n-a lkyl-1-meth ylpyridinium iodide a mphiphiles

with methylorange,ethylorange,methylred, par a-methylred, methyl yellow, and azobenzene sulfonat e (Cha rt 1)in aq ueous solution. Aggrega tion wa s monitored by UV-vis spectroscopy, and the effect of alkyl chain length onthe aggregation process was examined. Also, ef fects ofstructural variat ions of the azo dyes on the aggregationprocess have been investigat ed. Dye-surfactant sal ts werestud ied by optical microscopy, conductometr y, a nd electronmicroscopy with th e aim of chara cterizing the morphologyof the a ggregates formed in aq ueous solution.

Experimental Section

Materials. C 10TAB was purchased from Lancaster, C 12TABf r om S i gm a C h e mi ca l C o ., C 14TAB an d dodecylamine from

Aldrich, C 16TAB from Merck, and C 18TAB from Fluka. Dode-cylamine hydrochloride (DAHCl) was prepared by dissolvingd od e cy l am i n e i n w at e r a n d a d d i n g 1 e q u i v of HC l . Th e p H ofDAHCl solutions w as adjusted to 6. The purity of all surfacta ntswas checked by 1H NMR. C 10TAB and C 12TAB were crysta llizedfrom acetone. All surfacta nts, except C 18TAB , were dried in vacuobefore use. C 8pyI, C 10p y I , C 12p y I , an d C 14pyI were synthesizedaccording t o a l i terat ure procedure.29

Methy l oran ge (MO), eth yl ora nge (EO), meth yl yellow (MY),a n d p a r a -meth yl red (pMR) w ere obta ined from Acros Organ ics.M e t h y l r e d (M R) an d d i s od i u m t e t r a b or at e d e cah y d r a t e w e r epurchased from Merck. Azobenzene was obtained from Aldrich.MO wa s crysta ll ized from doubly disti l led wa ter. EO w as driedi n vac u o d u r i n g on e n i g h t . Az ob en z e n e s u l fon at e (AB S ) wa ssynthesized according to a l i terature procedure. 30 Wa t e r w a sdemineralized and disti l led twice in an all-quartz disti l lation

u n i t .1H NMR spectra w erem easured at 200or 300MHz on a VarianGemini-200 or a Varian VXR-300 spectrophotometer, respec-tively.

UV-vis Spectroscopy. U V-vi s a b s or p t i on s p ec t r a we r er e cor de d on a P h i l i ps P U 8740 U V-vis spectrophotometer, aPerkin-Elmer 5, or a P e r k i n -E l m e r 12 spectrophotometer,equipped with a thermostated cell compartment. MO, MR, pMR,an d ABS concentra tions were 25 M. The EO concentration was23 M , an d t h a t of M Y wa s 12 M. All solutions were preparedin 0.02 M sodium borat e buffers a djusted t o pH 9.4.

Optical Microscopy. Melting points were determined on aKof l e r h ot s t ag e or a M e t t l e r F P 2 m e l t i n g p oi n t ap p ar at u sequipped with a Mettler FP 21 microscope. Phase penetrationexperiments were performed on an Olympus BX 60 polarizationmicroscope equipped with a Linkam THMS 600 hot s ta ge.

Conductivity Experiments. Critical aggregation concentra-tions were determ ined by conductivity experiments. Conductivi-ties were determined on a Wayn e-Kerr Autobalan ceB ridgeB 642fitted w ith a P hilips electrode P W 9512101 with a cell consta ntof 0.71 cm-1. S ol u t i on s i n t h e c on d u c t i vi t y c e l l we r e s t i r r e dm ag n e t i cal l y a n d t h e r m os t at e d at t h e d e s i r ed t e m pe r at u r e .Concentra tions w ere corrected for volume changes.

Electron Microscopy. Transmission electron micrographswe r e obt ai n e d u s i n g a J E M 1200 E X e le ct r on m i cr osc op eoperating at 80 kV. Samples were prepared on carbon-coatedcollodion grids. Samples were stained with 1%uranyl acetate.

C16TA-MO. The precipita te formed from a solution of C 16-TA B an d M O was an al y z e d . A n al . C al c d f or t h e 1:1 ad d u c t ,

(1 8) Daw be r, J . G . ; Fi she r, D . T.; Warhurst , P. R. J. Chem. Soc.,F a r a d a y T r a n s. 1 1986, 82, 119.

(19) Kim, B.-K.; Ka gaya ma, A.; Sa ito, Y.; Machida, K.; U no, T. B u l l .Chem. Soc. J pn. 1975, 48, 1394.

(2 0) (a) Dutt a, R. K. ; B hat , S . N. Bu l l . C h e m . Soc. J p n . 1993, 66,2 45 7. ( b) Dutta , R. K . ; Bha t , S . N. C ol l o i d s Su r f . , A 1996, 106, 127.(21) De Vijlder, M. J. Chem. Soc., Faraday Tr ans. 11983, 79, 155.( 2 2 ) Suz uki , M.; Takai , H. ; Tanaka, K. ; Nari ta, K. ; Fuji wara, H.;

Ohmori, H. Carbohydr. Res. 1996, 288, 75.(23) Zhu, D.-M.; Schelly, Z. A. J. Ph ys. Chem. 1992, 96, 7121. (b)Zhu,

D.-M.; Schelly, Z. A. L a n g m u i r 1992, 8, 48.(24) (a )Or tona , O.; Vita glian o, V.; Robinson, B. H . J.Colloid Interface

Sci. 1988, 12 5, 271. (b)Fujieda, T.; Ohta, K.; Wakabayashi, N.; Higuchi,S . J. Colloid Interface Sci. 1997, 185, 332.

(25) (a) Ram esh, V.; Chien, H.-S.; Labes, M. M. J. Phys. Chem. 1987,91, 5937. (b) P ar tha sar at hy, R.; Labes, M. M. J. Phys. Chem. 1989, 93,5874.

(26) Nakash ima, N.; Fukush ima, H .; Kunita ke, T. Chem. Lett. 1981,1555.

(2 7) Takaha shi ,M.;Kobay ashi ,K .;Takaoka, K.;Taji ma,K. L a n g m u i r 1997, 13, 338.

(28) Karukst is, K. K.; D Angelo, N. D .; Loftus, C . T. J. Phys. Chem.B . 1997, 10 1, 1968.

(29) Sudholter, E. J . R.; E ngberts, J . B. F. N. J. Phys. Chem. 1979,83, 1854.

(30) Griess, P . An n. 1870, 154, 208.

Chart 1. Structures of the Azo Dyes

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C 33H 56N 4S O3 (588.89): C, 67.31; H, 9.58; N, 9.51; S, 5.44. Foun d:C, 66.91; H, 9.44; N, 9.42; S, 5.11.

Results and Discussion

UV-Vis Spectroscopy. The posit ion of t he long-wavelength absorption band of azo dyes is sensit ive tomedium ef fects; therefore, they can be used a s solvato-chromic micropolar ity reporter molecules. F or exam ple,the wavelength of maximum absorption of MO is posi-tioned at 463 nm in water , whereas i t is si tuated at 417nm in ethanol .2e Similar ly, upon binding of azo dyes tohydrophobic aggregat es, a shif t in a bsorption ma ximumocc ur s t o s h or t e r w a v el en g t h s . Al t h ou g h s e ve r a l d y emolecules are known to sometimes spontaneously ag-grega te in aq ueous solution, the dyes used in the presentstudy obey B eer s la w in t he employed concentra tionranges. Although the absorption of MO shows a break at0.5 mM, th e a bsorption spectrum of MO fai ls t o show adistin ct dimer band upon increasin g dye concentra tion.16,31

Increa sing t he concentra tion of MO does, however, resultin a blue shif t of the long w avelength a bsorption ba nd.16

On the other hand, dyes l ike thionine32 a n d m e t h y le n eblue33 a r e k n ow n t o a g g r eg a t e i n a q u e ou s s ol ut i on ,result ing in the appearance of a blue-shif ted aggregateb a n d .

Absorption spectra of the a zo dyes ha ve been recordedat different concentrations of surfactant. The spectra ofMO upon ad dition of different concentra tions of C 12TABa r e s h o w n i n F i g u r e 1 . S u c c e s s i v e a d d i t i o n s o f s m a l lconcentra tions of surfactant decrease t he a bsorption ofthe band at 463 nm. Further addit ion of surfactant givesrise to a new ba nd at ca . 380n m. The absorption spectru mshows the new band as a shoulder at a concentration of4 mM of C 12TAB, which is more than 3 times lower thanits cmc (13.3 mM).34 The intensity of the new band f irstincreases upon further increasing the surfactant concen-t r a t i o n a n d t h e n d e c r e a s e s u n t i l a b a n d a t c a . 4 3 0 n mappears, chara cteristic for MO bound to cationicmicelles.35

The wavelengths of maximum absorption of MO uponaddition of C 18TAB, C 16TAB, C 14TAB, C 12TAB, and C 10-TAB in a queous solution a t a MO concentra tion of 25 M

a re present ed in Figur e 2a . The effect of C18TAB , C 16TAB ,and C 14TAB is simila r to th a t of C 12TAB; at concentrationsconsiderably below the cmc of the surfa ctant s, a stronginteraction occurs t hat is ref lected by t he a ppeara nce oft h e s h or t -w a v el en g t h a b s o rp t i on b a n d . Ag a i n , u p onincreasing surfa ctant concentra tion this short-wa velengthabsorption ba nd gra dually disappears a t t he expense ofthe micellar band. The interactions are absent for C 10-TAB. However, upon increasing the dye concentra tion,the short-wavelength absorption band indeed appears in

the absorption spectrum of MO. Table 1 compares thecmc of C nTAB with the aggregation concentration of C n-TAB an d MO a t a MO concentra tion of 25 M.

Similar results were obtained from experiments using4-n-alkyl-1-methylpyridinium iodidesurfactants as shownin Figure 2b. In aqueous solutions of low concentrationsof C 14pyI, C 12pyI, and C 10pyI, agg regat ion occurs with MO,whereas interactions a re absent a t low concentra tions ofC 8pyI. The length of th e tota l a polar moiety of C 8pyI isc o m p a r a b l e t o t h a t o f C 10T A B , a n d i n b o t h c a s e s , a g -gregation at low surfacta nt concentra tion is absent (at aMO concentra tion of 25 M). The dependence of t hea g g r e g a t i o n p r o c e s s o n t h e a l k y l c h a i n l e n g t h o f t h es u r fa c t a n t i n d ic a t e s t h a t h y d r op h ob i c i n t er a c t i on s a r eimporta nt in determining the intermolecular int eractions.

The short-w a velengt h a bsorption band a lso a ppears inabsorption spectra of solutions of MO and dodecylaminehydr ochloride (DAHC l), th e lat ter differing from C 12TABin i ts head group being a mmonium instea d of tr imethyl-am monium. The short-wa velength absorption band ap-p ea r s a l r e a d y a t a D A H C l c on ce n t r a t i on of 0 . 4 m M ,wh ereas in solutions of C12TAB, it a ppear ed at 4 mM. Thelowering of the a ggregation concentra tion in the case ofD AHCl can be at tr ibuted to an increase in electrostaticinteractions relat ive to C 12TAB but might , in pa rt, a lso beexplained by a contribution of hydrogen-bonding inter-a c t ion s b et w e en t h e a m m on iu m h ea d g r ou p a n d t h esulfonate group in the dye molecule.

Pa rts c a nd d of Figure 2 show similar results for th ea d d i t i on of n-alkyltr imethylamm onium bromide a m-phiphiles to pMR a nd EO, respectively, in a queous solutiona t pH 9.4. Note tha t low concentra tions of C 10TAB inducea short-wa velength a bsorption ba nd in the spectrum ofEO, wh ereas i t is a bsent in the case of MO and pMR (atsimilar dye concentration). In the case of C 10TAB, theshort-wa velength a bsorption ba nd in the spectru m of EOis positioned at longer wavelengths than those observedwith am phiphiles possessing a longer hydrocarbon ta i l .Again, increasing the EO concentration decreases theshort-wavelength absorption band until 395 nm, similarto tha t observed for th e other a mphiphiles studied. Theser e su l t s a r e a g a i n f u ll y r e con ci la b l e w i t h t h e r ol e o fhydr ophobicint eractions in the a ggrega tion process. Thisis confirmed by the fact th a t th e intera ctions betw een MO

and C 12TAB a re absent in etha nolic solutions. Solvophobicinteractions between solute molecules are decreased innona queous solvents such a s dimeth ylforma mide, toluene,a n d e t h a n o l ,36 although recently vesicle formation hasbeen reported for ethanol-water solutions and for pureethanol .37 On the other hand , electrost a tic intera ctions inethanol are stronger than those in water on the basis ofthe d ielectric consta nt s of both solvents (24.3 for eth a nola n d 7 8 . 5 f o r w a t e r ) ,38 b u t a p p a r e n t l y n o a g g r e g a t i o nresults f rom this ef fect . Moreover, the wavelength of(31) De Vijlder, M. J. Ch em. Soc., Faraday T rans 1 1986, 82, 2377.

(32) Lai, W. C.; Dixit, N. S.; Mackay, R. A. J. Phys. Chem. 1984, 88,5364.

(33) Herz, A. H. Adv. Colloid Interface Sci. 1977, 8, 237.(34) Berr, S. S .; Ca ponetti , E.; J ohnsson, J . S., J r.; J ones, R. R. M.;

Mag i d, L . D. J. Phys. Chem. 1986, 90, 5766.(3 5) Re eve s, R. L . ; Ha rkaw ay , S. I n M i cel l i z at i o n , Sol u b i l i z a t i on ,

and M icroemul sions; Mi t tal , K. , E d. ; P l enum P re ss: Ne w York 1 97 7.

(36) Ray, A. N a t u r e 1971, 231, 313.(37) Huan g, J .-B.; Zhu, B.-Y.; Zhao, G.-X.; Zhan g, Z.-Y. L a n g m u i r

1997, 13, 5759.(38) Br un o, T. J .; Svoronos, P . D. N. In CRC Han dbook of Basic Tabl es

for Chemical Analysis; C RC P re ss: Boc a Raton, FL ; p 2 12 .

Figure1. Effect of C 12TAB on th e absorpt ion spectru m of MOin a queous solution at 30 C, pH 9.4, [MO]) 25 M. Numberscorrespond to surfacta nt concentra tions (mM).

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m a x im u m a b s or pt i on of M O i n w a t e r on l y s li gh t l yincreases upon addit ion of tetramethylammonium bro-mide (TMAB). A red shif t of 4 nm was observed upona d d i t i on of 3 .1 M TM AB . Th i s a g a i n co nf ir m s t h a thydrophobic interactions ar e importa nt for aggrega tion.

To exa mine th e importa nce of th e presence and positionof the ionic group in the dye molecule, the ef fects ofsurfactants on the absorption spectrum of MR and MY

were st udied. In th e MR molecule, the carboxylat e groupis positioned orthowith respect tothe azobridge, whereas

in MY an ionicg roupis la cking. Although the MR moleculei s s t i ll f u ll y co nju g a t e d , i t s w a v el en g t h o f m a x im u mabsorption in aqueous solution is smaller than that forp M R . P r e s u m a b l y , f o r c i n g t h e a r o m a t e s y s t e m o u t o fplana rity results in a hypsochromic shift of the a bsorptionb a n d .39 Both dyes do not undergo spectral shif ts at lowsurfa cta nt concent ra tions. The position of th e absorptionm a x i m u m g r a d u a l l y s h if t s f r om t h a t i n w a t e r t o t h a t i nmicellar solution. Figure 2e presents results of experi-ments using MR. Geometric reasons probably preventassociation in this case. It might be suggested that in thecase of MY, the electrostat iccomponent in the ag gregat ionprocess is missing, thereby preventing efficient associa-

(39) Gore, P. H.; Wheeler, O. W. J. Am. Ch em. Soc. 1956, 78, 2160.

Figure2. Effect of ca tionic amphiphiles on the position of the wa velength of maximu m abs orption of az o dyes in a queous solutionat 30 C, pH 9.4, [MO] ) [pMR] ) [MR] ) 25 M, [EO] ) 23 M: (a) effect of C nTAB on the absorption spectrum of MO, (]) n) 10, (b) n ) 12, (O) n ) 14, ([) n ) 16; (3) n ) 18, t he sy m b ol s i n par t s c , d , and e ha ve t he sam e m e ani ng as t hose in par t a ;(b)effect of C mpyI on th e absorption spectrum of MO, (1) m)8, (]) n)10, (b) n)12; (O) n)14; (c)effect of C nTAB on the absorptionspectrum of pMR; (d) EO; (e) MR. Measurements on solutions conta ining C 18TAB were performed a t 35 C.

Table 1. Aggregation Concentration of CnTAB with MOin 25 M MO Solution and Critical Micelle

Concentrations of CnTAB

C nTAB ca c (C nTA-M O) (m M) cm c (C nTAB) (mM )

n) 12 2.09 13.3n) 14 0.159 4.41n) 16 0.024 1.0n) 18 0.014 2.92

1086 L an gm u i r , V ol . 15, N o. 4, 1999 B u w al d a et al .


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tion. However, in the case of AB S (with a sulfonat e groupb u t w i t h o ut t h e d i m e t h y la m i n o s u b s t i t u en t ), t h e a g -gregation at low surfacta nt concentra tion wa s also absent.This indicat es that both the dialkylamino substi tuent andan ionic group are prerequisites for efficient binding. Ina study of the interactions of bovine and human seruma l b u m i n w i t h s e v e r a l a z o b e n z e n e a n i o n s , i t h a s b e e nobserved that t he isomericposit ion rat her than th e nat ure

of the anionic substi tuent is important .2d

Effect of Ionic Strength on Aggregation. Theaggrega tion process is strongly inf luenced by the ionics t r e n g t h . F i g u r e 3 s h o w s t h e e f f e c t o f C 16TAB on theposition of th e a bsorption maxim um of MO in the presenceof different concentrations of sodium chloride. The short-wa velength a bsorption ba nd observed at low concentra -tions of C 16TAB is diminished u pon increa sing t he ionicstrength, probably through an increase in hal ide coun-t e r ion b in d i n g. Th e s e n si t i vi t y t o N a C l r e fl ect s t h eimporta nce of electrostat icint eractions in the a ggregat ionprocess.40,41 I n a d d i t i on , i n t er a c t i on s b et w e e n a n i o n ics u r f a c t a n t s ( e . g . , S D S ) a n d M O a r e a b s e n t ,11 w h e r e a scationic surfacta nts w ith compara ble alkyl cha in length(e.g., C 12TAB ) d o i n d u ce ch a n g e s i n t h e a b s o rp t i on

spectrum of MO.Table 2 presents the w a velengths of maximum a bsorp-

tion of the dyes in a queous solutions of t he used surfa c-t a n t s .

Precipitation. In aq ueous solutions of MO an d pMRand surfactan ts (at concentra tions below t he cmc), pre-cipitation occurs.42 U s i n g 1H N M R i t w a s s h o w n t h a tcrysta ls formed from a queous solutions of C nTAB and MOand pMR and from solutions of C mpyI and MO consist of

a n e q ui mol a r r a t i o o f s u r fa c t a n t a n d d y e. E l em en t a lanalysis (see Experimental Section) performed on thematerial that precipitated from an aqueous solution ofC 16TAB a nd MO confirmed th is result. In contra st t o MOa n d p M R, c ry s t a l l iz a t i on d oe s n o t o ccu r i n a q u e ou ssolutions of EO a t low concentra tions of surfacta nt an ds o l u t i o n s a r e s t a b l e f o r m o r e t h a n 1 m o n t h . P r o b a b l yordering into dye-surfactant crystals is less efficient forEO with its diethyla mino group compa red to MO a nd pMRwith a less bulky dimethyl amino group. However, uponincreasing the concentration of EO (above ca. 0.7 mM),precipta tion does occur. Ana lysis of crysta ls precipita tedfrom a solution of C 16TAB and EO revealed a 1:1 molarr a t i o o f s u rf a c t a n t a n d d y e . C r y s t a l l iz a t i on h a s b ee nobserved before in aq ueous solutions of cationic am-phiphiles and methyl orange.16,43 P recipita t ion of dye-surfactant sal ts in a 1:1 molar rat io has been observedbefore in aqueous solutions of cationic surfactants anda nionic dyes a s well as in solutions of a nionic surfa cta ntsan d cat ionic dyes.17b

Aggregation Behavior of CnTA-MO. The aggrega-tion behavior of C nTA-M O i n w a t e r w a s s t u d i ed u s i n goptical microscopy and electron microscopy. The criticalaggregation concentration was determined by conductivityexperiments . Un der a pola rizing microscope, the cha ngefrom solid ma terial t o closed bilayer structures in w at ercan be followed. The precipita te wa s isolat ed from aqueoussolutions of MO a nd ca tionic surfacta nts. B efore precipi-

ta tion occurred, the a bsorption spectrum of the solutionsshowed th e short-wa velength a bsorption ba nd. The alkyl-tr imethylammonium-MO crystals were subjected to aso-called phase penetra tion experiment.44 Wa t e r w a sbrought into conta ct with t he crysta ls, and upon heatingof the sa mple, the forma tion of myelin structures emerginga t t h e b ou n da r y w i t h w a t e r w a s ex a m in ed u si ng apolar izing microscope. Ta ble 3 presents tempera tur es a twhich myelins appeared (Tmyelin) and melting points. Asexpected, Tmyelin increases upon increasing th e alkyl chainlength of the surfacta nt in the surfacta nt-MO salt . Notet h a t t h e m y e l in s a r e s t a b l e u p t o 1 00 C . M y e l in s h a v ebeen proposed to consist of w ormlike vesicles in which th ebilay ers al ternat ing with w at er layers are concentr ical lyarranged around a central core axis of water . 45 Melting

points (Ta ble 3) decrease upon increa sing th e alkyl cha inlength of the amphiphiles; that is , increasing the alkylchain length decreases the packing ef f iciency and con-sequently the stabil i ty of the surfacta nt-MO salts in thesolid.

The format ion of myelin st ructures is common to ma nycompounds forming lamellar phases. Therefore, themorphology of aggregates formed from C 10TA-M O a n dC 12TA-M O c r y s t a l s i n w a t e r w a s s t u d i e d b y e l e c t r o nmicroscopy. The ma teria l wa s dissolved in wa ter a t 50 C

(4 0) Shi raha ma, K. ; Taka shi ma, K. ; Taki saw a, N. Bul l. Ch em. Soc.J pn. 1987, 60, 43.

(4 1) Hay a kawa , K. ; Sa nat e rre, J . P. ; Kw ak, J . C . T. Biophys. Chem.1983, 17, 175.

(42) Attempts to obtain suita ble crysta ls for X-ra y diffra ction ha venot yet been successful.

(43) Hiskey, C. F.; Downey, T. A. J. Phys. Chem. 1954, 58, 835.(44) Law rence, A. S. C. In L i q u i d C r y st a l s 2 ; B r o w n , G . H . , E d . ; G o r d o n

and Bre ac h: L ondon, 1 96 9; P art 1 , p 1.(45) Sa kura i, I .; Suzuki, T.; Sa kura i, S. Bi o c h i m . Bi o p h y s . Ac t a 1989,

985, 101.

Figure 3. E ffec t of i oni c st r e ng t h on t he posi t ion of t hewa velength of ma ximum absorpt ion of MO in a queous solution(pH 9.4) a t 30 C of C 16TAB: (1) 0.01 M NaC l, (9) 0.1 M NaC l,(O) 0.3 M Na Cl, (2) 0.75 M Na Cl.

Table 2. Wavelengths of Maximum Absorption of theDyes in Different Media

dy ew a t e r

ma x (nm)micellesma x (nm)

low wavelengthabsorption ba nd (nm)

MO 463 431 ca . 380

E O 472 452 ca . 395pMR 463 428 ca . 375MR 429 416 aMY 440 420 aAB S 318 325 a

a Not observed.

Table 3. Temperature of Myelin Formation (TMyelin) andMelting Points (mp) ofn-Alkyltrimethylammonium-MO

Salts

s a l t Tmyelin ( C)a mp (C)

C 10TA-MO 43 237-239C 12TA-MO 62 236-239C 14TA-MO 68 230-232C 16TA-MO 72 227-229C 18TA-MO 74 220-222

a Myelins are stable up to 100 C.



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in th e case of C10TA-MO and a t 70 C in t he case of C12-TA-M O u n d e r s t i r r in g . F i g u r e 4 s h ow s a n e le ct r o nmicrogra ph of a 5.2 mM aq ueous solution of C10TA-MO .Vesicles with a diameter of 400-1000 nm a re observed.Vesicles formed fr om C 12TA-MO a re of similar size.Vesiclesolutions are sta blefor more tha n 1 week when kept aboveTmyelin but readily flocculate upon cooling.

It is tempting toview surfacta nt-MO sa lts as cat an ionicsurfactants. Characterist ic for catanionic surfactants isthe forma tion of vesicles in mixtures of the tw o, wherea smicelles are formed in aqueous solutions of the separates u r f a c t a n t s .46 However, MO is not a surfactant molecules i n ce i t l a c ks s u r fa c t a n t p r op er t i es s u ch a s e ff ect i v elow ering of the sur face tension an d a well-defined criticalaggr egation concentrat ion.47 Neverth eless, it forms vesiclesin combination with an oppositely charged surfactantana logous to cata nionic surfacta nts. Moreover, vesiclesprepared in t he present st udy form without t he input ofsignificant mechan ical a gita tion, a property chara cteristicof cat a nionic vesicles.46 We contend tha t t he dye moleculeshould ra th er be viewed a s a hydr ophobic counterion. Asa result, the packing para meter of the surfa ctan t molecule(P), wh ich usually determines the type of aggr egate formedby a surfactant molecule,48 increases by increasing thev o l u m e o f t h e h y d r o c a r b o n p a r t o f t h e s u r f a c t a n t (V).Furt hermore, the mean cross-sectional hea dgroup surfacearea (a0)decreases considerably 49 (eq 1; l is the alkyl chainlength). As a result of the increase in P, t h e n o v e l s u r f a c t a n tdisplays vesicle instead of micelle formation. A value ofP< 1/3 is indica tive of th e forma tion of micelles, 1/3 < P


7/7

M O , E O , a n d p M R u p on a d d i t i on of s m a l l a m o u n t s o fc a t i o n i c s u r f a c t a n t i s c a u s e d b y t h e f o r m a t i o n o f d y ea g g r e g a t e s16,53 t h a t a r e b o un d t o s u r fa c t a n t a g g r e g a t e s .The a bsence of isosbestic points in overla y spectra (e.g.,Figure 1) suggests the formation of higher-order dyea g g r e g a t e s . Al t h ou g h t h e f or m a t i o n o f s e v er a l d y e a g -gregates (except for MO) is w ell-known an d is of tenref lected by a n absorption band blue-shif ted relat ive t othe absorption band of the monomeric dye molecule, 32,33

the absorption spectrum of MO does not show a dimer

band upon increasing dye concentration. However, theasym metric long-wa velength absorption band of MO inpure wa ter might hide an aggregat e band indicat ing thatdye aggregates are already present at low MO concentra-tions.16 Th e f or m a t i o n of d y e a g g r e g a t e s i n d u ce d b ysurfactants is supported by the fact that the binding ofMO to cationic polymers in water (here also the short-wa velengt h a bsorption ba nd is observed) is coopera tive.5

An MO molecule tha t binds to the cationic polymer createsa morehydrophobicbinding siteand facilitates thebindingof a n o t h e r d y e m ol ec ul e. Th i s i m p li es t h a t n e xt t oelectrostat ic interactions hyd rophobic st acking of thea r o m a t i c p a r t s o f t h e a z o d y es i s a l s o im p or t a n t i n t h eaggregation process. The change in wavelength of maxi-mum a bsorption in the spectrum of MO from ca. 380 nm

t o c a . 4 30 n m ca n b e e x pl a i n ed b y d i lu t i on of t h e d y eaggregates over surfactant micelles, and consequently,the spectrum of monomericMO in a h ydrophobicm ediumis observed. I t a ppears l ikely tha t th e dye aggregat es areH-aggregates in which the chromophores are aggregatedin a parallel fashion.54 Cha ra cterist ic for H -aggrega tes isthe blue shift of the a bsorption band of th e chromophoricu n i t d u e t o f a v o r a b le - stacking interactions of thechromophores. Moreover, the new absorption band isn a r r ow e r a n d h a s l ow e r i n t en s it y t h a n t h a t of p u r emonomeric MO, analogous to l i terature data. 55 Similarresults have been obtained in studies on amphiphilesca r r y i n g a n a z o be n ze n e c hr om op h or e u n i t .55,56 Here,aggregation is ref lected by a hypsochromic shif t of theabsorption maximum of the chromophore. Although the

formation of H-aggregates provides an explanation fort h e b l ue s h i ft ob s er v ed i n t h e s p ec t r um of M O u p onaddition of small amounts of cat ionic surfacta nt , furtherstudies are necessary for a more deta i led understa ndingof the na ture of th e blue-shif ted a bsorption band.

Conclusions

I n t h e p r e s e n t s t u d y , t h e i n t e r a c t i o n s o f s e v e r a l n-alkyltr imethylammonium bromide and 4-n-alkyl-1-me-thylpyridinium iodide amphiphiles with a number of azodyes ha ve been investiga ted. Aggrega tion of C12TAB , C 14-TAB, C 16TAB, C 18TAB, C 10pyI, C 12p y I , a n d C 14p y I w i t hMO, EO, and pMR occurs at concentr a tions far below thecmc a nd is reflected by a ca. 80 nm blue shift of the -*a bsorption band of the chromophores. In th e case of C10-TAB and C 8pyI, higher dye concentrations are necessaryto induce aggrega tion. Interactions of MR, MY, and ABSwith cat ionicsurfactant s are absent below the surfactantscmc.

Upon increasing surfa cta nt concentr a tion, solubilizat ionof the aggregates occurs into surfactant micelles. Theimporta nce of hydr ophobicint era ctions is revea led by th echain length dependence of the aggregation process andby the observation that binding is absent in ethanol. Theimportance of electrostatic interactions is evident fromthe inf luence of NaC l on th e a ggregation process.

P recipita tes isolated from a queous solutions of CnTAB(n)10, 12, 14, 16, 18)a nd MO a nd pMR a nd from solut ionsof C mpyI (m ) 10, 12, 14) and MO ha ve been sh own t oconsist of a n equimolar ra tio of surfa cta nt a nd dye. Usingoptical microscopy, the format ion ofm yelin stru ctures fromC nTA-MO salts has been demonstrated. Myelin temper-at ures ha ve been determined by optical microscopy. Kra ffttemperatures increase upon increasing the alkyl chainlength of the surfactant . Melting points decrease uponincreasing t he alkyl cha in length of the amphiphiles. Theformation of vesicles from C 10TA-MO crysta ls and fromC 12TA-MO crystals has been demonstrated using electronmicroscopy. Vesicles a re st able a bove th eir Kra f f t t em-

perature but precipitate upon cooling.

Acknowledgment. The investigat ions were supportedby The Netherlands Foundation for Ch emical Resear ch(SON) with financial aid from The Netherlands Founda-tion for Scientific Research (NWO).

LA980824I

(5 3) I tay a , T.; O c hi ai , H. ; U e da, K. ; I mamura , A. M acromolecul es1993, 26, 6021.

(5 4) McRae , E . G . ; Ka sha, M. J. Phys. Chem. 1958, 28, 721.(55) Everaars, M. D.; Marcelis, A. T. M.; Kuijpers, A. J .; Laverdure,

E.; K orona , J .; Koudijs, A.; Su dholter, E . J . R. L a n g m u i r 1995, 11, 3705.(56) Song, X.; P erlstein, J .; Whitten, D. G . J. Am. Chem. Soc. 1997,

119, 9144.


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