Interaction of Cationic Species with Polyelectrolytes - ACS Symposium

Chapter 32

Interaction of Cationic Species with Polyelectrolytes

Downloaded by UNIV OF TEXAS AT DALLAS on July 11, 2016 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch032

Deh-Ying Chu and J . K. Thomas Department of Chemistry, University of Notre Dame, Notre Dame, IN 46556

The interactions of various cationic species with polyacids, such as poly(methacrylic acid), PMA and poly(acrylic acid), PAA have been studied. In particular, the effect of polyacid conformation on the interaction is discussed in detail, and also the nature of the aggregation of PMA and cationic sur factants alkyltrimethylammonium bromide, C TAB. The effect of the intermediate conformation states of PMA around pH 4-6 is noted, where the photophysical properties of cationic probes bound to PMA dramat ically change, effects such as a large enhancement of the fluorescence intensity of Auramine O, Au Ο at pH 4.5, a blue shift of the luminescence spectra of tris(2,2'-bipyridine)ruthenium(II) complex, Ru(bpy) 2+ at pH 5, and a great increase of the excimer yield of 1-pyrenebutyltrimethyl ammonium bromide, C PN at pH 6. n

3

+

4

Water soluble synthetic polyelectrolytes have attracted increasing attention in recent years, mainly because of their wide u t i l i t y in industrial applications, and also because of their resemblance to biopolymers. Poly(methacrylic acid), PMA, a weak polyelectrolyte, exhibits a marked pH induced conformational transition. A wide variety of techniques have been employed to gain more information on the nature of the conformational transition of PMA, these techniques include: viscometry, potential titrimetry,(1-5) Raman spectrometry,(6) calorimetry,(7-9) electrical conductometry,(10) dilatometry,(11) H ΝMR linewidth,(12) viscoelastic studies,(13) kinetics of chemical reactions,(14) small-angle neutron scattering,(15) pH jump,(16,17) and fluorescent probing.(18-27) The data tend to support a two state model, i.e. at low pH, a compact globular conformation (A states) and at high pH, an extended rod-like form (B states) for the conformation transition of PMA. The conformational transition is considered to be highly cooperative and occurs in one step. Nevertheless, the 1

0097-6156/87/0358-0434$06.00/0 © 1987 American Chemical Society

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

32. CHU AND THOMAS

Interaction of Cationic Species

435

c o n f o r m a t i o n a l t r a n s i t i o n as o b s e r v e d i n Raman s p e c t r o s c o p y which indicates a m u l t i p l i c i t y of structures,(6) exhibits progressive s t a t e s r a t h e r than a c o o p e r a t i v e change. D i f f e r e n t arrangements o f the lumophore pyrene i n ΡMA i n d i c a t e d i f f e r e n t d e g r e e s o f o p e n i n g o f t h e polymer compact c o i l as the pH o f an aqueous s o l u t i o n increases,(28) The c o v a l e n t l y bound pyrene i n d i c a t e s a l a t e r t r a n s i t i o n than t h a t o f g u e s t m o l e c u l e s i n c l u d e d by s i m p l e solubilization. D e t a i l s o f the u n c o i l i n g p r o c e s s a r e , t h e r e f o r e , s t i l l open t o q u e s t i o n and o t h e r measurements a r e d e s i r a b l e . In o r d e r t o extend e a r l i e r work, s e v e r a l p o s i t i v e l y charged l u m i n e s c e n t p r o b e s , t r i s ( 2 , 2 ' - b i p y r i d i n e ) r u t h e n i u m ( I I ) complex, R u ( b p y ) , Auramine 0, Au Ο, 1 - p y r e n e b u t y l t r i m e t h y l ammonium bromide, C^PN " and 1 -pyreneundecyltrimethylammonium i o d i d e , C ^ P N * have been employed t o monitor the n a t u r e o f the c o n f o r m a t i o n a l t r a n s i t i o n o f PMA over t h e pH range o f 2 t o 8, from A s t a t e s i n t o Β states. The purpose o f t h e p r e s e n t work i s , t o i n v e s t i g a t e t h e i n t e r m e d i a t e s t a t e s i n t h e u n c o i l i n g p r o c e s s o f PMA polymer c o i l w i t h i n c r e a s i n g pH and t o s t u d y any e f f e c t s t h a t a r e i n d u c e d by p o l y e l e c t r o l y t e s on the p h o t o p h y s i c s and p h o t o c h e m i s t r y o f c a t i o n i c probes. R u C b p y ) ^ * , which has a l a r g e b i p y r i d i n e l i g a n d s t r u c t u r e , Au 0 which i s n o n - f l u o r e s c e n t i n water, C P N and C ^ P N * , which form e x c i m e r s , a l l r e p o r t on v a r i o u s f e a t u r e s o f t h e i r e n v i r o n m e n t . 2 +


3

1

+

4

P o l y m e r - s u r f a c t a n t systems have a l s o been t h e s u b j e c t o f many r e c e n t s t u d i e s . However, e a r l y s t u d i e s have m a i n l y f o c u s s e d on t h e i n t e r a c t i o n between n o n i o n i c polymers and a n i o n i c s u r f a c t a n t s , sodium d o d e c y l s u l f a t e , SDS.(29-36) A g r e a t v a r i e t y o f e x p e r i m e n t a l d a t a v i a d i f f e r e n t t e c h n i q u e s e x i s t s , b u t the n a t u r e of t h e s u r f a c t a n t - p o l y m e r a s s o c i a t i o n i n these systems i s r e l a t i v e l y weak. F o r i n s t a n c e , the s t u d i e s have shown t h a t i n t h e P o l y ( e t h y l e n e o x i d e ) , PEO - SDS system, t h e r e i s no i n t e r a c t i o n between SDS and PEO f o r SDS c o n c e n t r a t i o n s below t h e CMC c r i t i c a l m i c e l l e c o n c e n t r a t i o n , o f SDS. There a r e few r e p o r t s c o n c e r n i n g the i n t e r a c t i o n o f p o l y e l e c t r o l y t e s w i t h s u r f a c t a n t s o f o p p o s i t e charge,(37-40) e s p e c i a l l y t h e i n t e r a c t i o n o f weak p o l y e l e c t r o l y t e s with c a t i o n i c surfactants.(41) However, t h e r e i s no r e p o r t on t h e e f f e c t o f c h a i n l e n g t h o f a c a t i o n i c s u r f a c t a n t on t h e s t a t e o f a g g r e g a t i o n and no d e t a i l e d s t u d i e s on t h e n a t u r e o f a g g r e g a t e s . T h e r e f o r e , t h e e f f e c t o f c a t i o n i c s u r f a c t a n t s on t h e c o n f o r m a t i o n a l t r a n s i t i o n o f PMA has been i n v e s t i g a t e d i n the p r e s e n t s t u d y . The i n t e r a c t i o n between c a t i o n i c s u r f a c t a n t s w i t h PMA p e r m i t s a s t u d y o f t h e e f f e c t o f charge d e n s i t y and c o n f o r m a t i o n o f p o l y e l e c t r o l y t e on t h e a g g r e g a t i o n p r o c e s s , and a l s o the e f f e c t o f c h a i n l e n g t h c a t i o n i c s u r f a c t a n t s on t h e c o n f o r m a t i o n a l t r a n s i t i o n o f PMA. EXPERIMENTAL P o l y ( m e t h a c r y l i c a c i d ) , PMA, and p o l y ( a c r y l i c a c i d ) , PAA, used i n t h i s s t u d y were o b t a i n e d from P o l y s c i e n c e and A l d r i c h C h e m i c a l s Inc., r e s p e c t i v e l y . The m o l e c u l a r w e i g h t o f PMA measured by s t a n d a r d v i s c o s i t y methods was 1.1 χ 1 0 · The m o l e c u l a r w e i g h t o f PAA was g i v e n as 2.5 χ 10 by t h e A l d r i c h C h e m i c a l I n c . PMA samples o f d i f f e r e n t m o l e c u l a r weight (3.9 χ 10^, 1.6 χ 10 and 6.4 χ 1 0 ) used f o r s t u d y i n g e f f e c t s o f m o l e c u l a r weight, were s y n t h e s i z e d by u s i n g d i f f e r e n t amounts o f AIBN and monomer. Free 4

5


436

PHOTOPHYSICS OF POLYMERS

r a d i c a l polymerization were carried out as reported earlier.(42) The concentrations of polymer solutions are expressed as weight/volume r a t i o , i . e . grams per l i t e r . Unless stated to the contrary, a l l polymer samples are used i n 1 g/L. Cationic surfactants, C T A B , such as decyltrimethylammonium bromide, C T A B (Kodak), dodecyltrimethylammonium bromide, C , (Kodak) and cetyltrimethylammonium bromide, gTAB (Sigma) were purchased as indicated and then p u r i f i e d by r e c r y s t a l l i z a t i o n from ethanol. However, hexyltrimethylammonium bromide ( C ^ T A B ) and octyltrimethylammonium bromide ( C Q T A B ) were synthesized by refluxing either 1-bromohexane or 1-bromooctane (Aldrich) with trimethylamine methanol solution and f i n a l l y r e c r y s t a l l i z e d twice from benzene.(5) An anionic surfactant, sodium dodecyl sulfate, SDS (BDH) was used as received. The cationic probes used i n this study are displayed i n Figure 1. 1-Pyrenebutyltrimethyl ammonium bromide (C^PN*), 1-pyreneundecyl trimethyl ammonium iodide (Cj-jPN*), were used as received from Molecular Probes. Tris(2,2'-bipyridine)ruthenium(II) chloride, Ru(bpy)2^ (G» Fredrick Smith) was p u r i f i e d by double r e c r y s t a l l i z a t i o n from the deionized water. Auramine 0, Au 0 (Aldrich) and Pyrene (Kodak) were p u r i f i e d by t r i p l e and double r e c r y s t a l l i z a t i o n from ethanol, respectively. The pH of the sample was adjusted with concentrated HC1 or NaOH aqueous solution and measured with a Sargent-Welch combination electrode at room temperature, 20° C, using a Model LS pH meter. Before taking measurements, the pH meter was calibrated with standard buffer solutions of pH 4, pH 7 and pH 10. Steady state absorption spectra and emission spectra were recorded on a Perkin-Elmer 552 UV-Vis and MPF-44B fluorescence spectrophotometer respectively. The r a t i o of I / I the r a t i o of the intensity of excimer (λ 480 nm) to monomer fluorescence (λ 377 nm)· The r a t i o of I3/I1 i s the r a t i o of the i n t e n s i t y of the pyrene monomer fluorescence intensity of peak 3 (λ 384 nm) to peak 1 (λ 373 nm). Fluorescence decay curves were determined by a PRA LN-1000 nitrogen laser system with a response of less than 10" seconds.(42) The monomer fluorescence of pyrene and pyrene derivatives was monitored at 400 nm, and that of the excimer fluorescence at 480 nm. N

T A B

2


1 Q

+

i s

e

m

9

RESULTS AND DISCUSSION Studies with Au Ο and Ru(bpy) 3^"**· Studies of binding of cationic dyes to polyelectrolytes has attracted interest f o r some time. Auramine 0, Au 0, a cationic diphenylmethane dye i s non-fluroescent in water, but fluoresces strongly i n r i g i d media or i n the bound states of a compact PMA coil.(18-23) Other cationic dyes such as C r y s t a l V i o l e t (CV)(24-25), Acridine Orange ( A O ) and Rhodamine Β (RB)(27) are also found to bind strongly to PMA. These dyes are reported to bind to both the open and coiled states of the polymer, and i t i s concluded that binding i s stronger i n the A states than i n the Β states. However, the present photophysical studies on the binding of Au Ο to PMA show some additional features compared to other previous studies, i . e . , the dye binding i s stronger i n the intermediate states rather than i n the A states. 26



C H U AND THOMAS

xx xr >

2 +

ο

N(CtU


c

H

N-.-.tfA I

\

Ru(bpy) 2+

Auramine Ο ( Au O)

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η

3

3

n=4

n=11

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4

P N

CnPN

+

+

The c a t i o n i c f l u o r e s c e n t probes used i n t h i s s t u d y .



438

The maximum wavelength of absorption of Au Ο i n water i s not dependent on pH. However, i n aqueous solutions of PMA, the spectrum i s s i g n i f i c a n t l y dependent on pH (shown i n Figure 2A). At pH 4-5, the spectra move to longer wavelengths (the X of two peaks are 375 nm and 442 nm at pH 4.5), while the spectra i n the solutions at pH 2 and 8 are i d e n t i c a l to that i n water, where the X of two peaks are 368 nm and 430 nm. Absorption spectra of Au Ο i n glycerol, i n water and i n SDS are displayed i n Figure 2B, for the sake of comparison. Again a bathochromic s h i f t of maximum wavelength i s observed i n glycerol, λ of 372 nm and 438 nm, while i n SDS the λmax ^ are 370 nm and 437 nm. The data indicate a m a x


m a x

s p e c i a l interaction between Au Ο and PMA polymer c o i l s (pH 4-5) which i s similar to the r e s t r i c t i o n placed by g l y c e r o l on mobility of Au Ο. This r e s t r i c t i o n i s stronger at pH 4-5 than at other pH and also stronger than that i n SDS. The r e s t r i c t i o n increases the conjugation of molecular electrons, i . e . , increases the coplanarity of Au Ο as required for the most effective overlap of the arene π o r b i t a l s and non-bounding electrons of nitrogen. I t can be seen that i n the Au Ο - PMA system at pH 8, e l e c t r o s t a t i c binding does not cause a bathochromic s h i f t , while p a r t i a l l y ionised PMA (pH 45), s h i f t s the absorption spectrum about 10 nm toward the longer wavelength region. The polymer conformation i n aqueous solutions at pH 4-5 e f f e c t i v e l y immobilizes Au 0 leading to the above spectral e f f e c t s . A wavelength of 388 nm was chosen for e x c i t a t i o n i n the studies of Au Ο fluorescence, as the absorbance i s invariant or nearly invariant over the pH 2-8. The r e l a t i v e fluorescence intensity of Au Ο i n aqueous solutions as a function of pH i s shown i n Figure 3A. The r e l a t i v e fluorescence intensity exhibits a marked enhancement on increasing the pH from 2 to 4.5, followed by a marked decrease, almost to zero, on increasing the pH above 5; i t can be noted that the r e l a t i v e intensity of Au Ο fluorescence reaches a maximum at pH 4.5. The fluorescence i n t e n s i t y at pH 4.5 i s four hundred times greater than that i n water, one hundred times larger than that i n SDS anionic surfactants solution (5 χ 10~ M) and i n p o l y ( a c r y l i c acid) at pH 3-4, and even larger than that i n g l y c e r o l . The steady state fluorescence data correspond well to the data obtained v i a absorption spectra studies. A simple explanation of the observed data i s as follows: anionic sites are formed on the PMA as the compact PMA c o i l opens with increasing pH,leading to binding of Au 0 to these s i t e s . On binding to these s i t e s , the Au 0 causes a r e s t r i c t i o n which tends to p u l l the polymer chain around the probe molecule, thus decreasing the i n t e r n a l rotation or other motion of bonds of the probe molecule, leading to an increase i n the fluorescence y i e l d . Structural r i g i d i t y enhances the fluorescence by i n h i b i t i n g radiationless processes that compete with fluorescence, and by preventing a large Frank-Condon geometric difference between the excited s i n g l e t state S^ and the ground state S . The above data indicate that the environment of Au Ο i n PMA at pH 4.5 i s much more r i g i d than at other pH, on SDS micellar surfaces, and even i n g l y c e r o l . At higher pH, Au Ο i s completely bound to an extended highly negatively charged polymer, and close to the aqueous 2

Q



32. CHU AND THOMAS

2.


Absorption spectra of 2 χ 10 M Auramine Ο. A. i n aqueous solutions of PMA at various pH. B. i n water, i n SDS and i n g l y c e r o l .


439


440

environment, the environment of Au Ο i s non-rigid and the polymer has l i t t l e effect, the I i s close to that i n water. At pH 2, no e l e c t r o s t a t i c binding exists, but Au 0 molecules may be p a r t i a l l y s o l u b i l i z e d i n compact polymer c o i l , so that I at pH 2 i s higher than i n water. A similar unique e f f e c t of PMA on the photophysics of Ru(bpy) i s observed at pH 5, for example both the l i f e t i m e and luminescence intensity of R u ( b p y ) show maxima at pH of about 5. The luminescence of the probe also exhibits a blue spectral s h i f t at this p a r t i c u l a r pH compared to other pH. The change i n the photophysical properties are due to binding of Ru(bpy) into a p a r t i a l l y c o i l e d or swollen polymer PMA at pH 5. The binding i s e l e c t r o s t a t i c i n nature and the ligands of the organometallic complex probe are quite r e s t r i c t e d i n a hydrophobic environment, so that unlike more mobile systems such as water or a stretched polymer, complete relaxation of the excited state i s not achieved. Hence, the l i f e t i m e and the y i e l d of luminescence increase accordingly and the emission spectra show a blue shift.(42) Photophysical studies of Au 0 and R u ( b p y ) i l l u s t r a t e common features, i . e . the conformational t r a n s i t i o n induced by pH i s a progressive continuous process over several pH units, and at pH 45, the compact polymer c o i l i s partly swollen. This p a r t i c u l a r pH region allows some cationic species to bind into the swollen polymer c o i l s of PMA. Binding i s e l e c t r o s t a t i c i n nature, however the mobility of cationic probe i s quite r e s t r i c t e d i n a hydrophobic environment. With the probes used, this e f f e c t increases the I of the probe which shows a maximum at pH 4.5 or pH 5. The pH at which photophysical effects are maximized i s dependent on the probe properties such as s i z e , water s o l u b i l i t i e s , e t c . Au 0 possesses a smaller size than R u ( b p y ) , at least i n one dimension, and i t may be p a r t i a l l y s o l u b i l i z e d or intercalated into a compact polymer c o i l at low pH, as well as binding into a swollen polymer c o i l at pH 4.5. The larger size of R u ( b p y ) may deter i t s s o l u b i l i z a t i o n i n compact polymer c o i l s , without preventing i t s binding into a swollen polymer at higher pH, i.e., pH 5. where the polymer i s further expanded to contain the Ru(bpy) molecule. In accordance with this picture, Au Ο shows a maximum fluorescence y i e l d i n PMA at pH 4.5, and R u ( b p y ) at pH 5. The e f f e c t of the intermediate polymer states on the photophysics of C^PN , a more hydrophobic cationic probe than Au 0 and R u ( b p y ) , i s not as marked, as this probe i s s o l u b i l i z e d i n polymer A states and i n intermediate states. A comparison can be seen i n Figure 3A, which shows the v a r i a t i o n of I vs. pH f o r 2 χ 10" M C PN i n PMA. The data are similar to the case of pyrene, which shows l i t t l e variation i n photophysical properties on increasing pH from 2 to 4, quite unlike Au Ο and R u ( b p y ) . Similar studies i n aqueous solutions of p o l y ( a c r y l i c acid), PAA, i.e., variation of I vs. pH are shown i n Figure 3B. A smaller increase i n I with Au Ο fluorescence was observed around pH 3-4 than i n the case of PMA, however, i t i s s t i l l 6-7 times larger than that i n water. The data again indicate that due to the lack of methyl side groups i.e. lack of hydrophobic interaction PAA forms much looser c o i l s at low pH than PMA, with smaller effects of the conformational t r a n s i t i o n on the photophysics of Au 0. r

2+

3

2+

3

+


3

2+

3

f

2+

3

2+

3

+

3

2+

3

+

2+

3

r

6

+

4

2+

3

r

r



32. CHU AND THOMAS

441

S t u d i e s w i t h C^PN* and C ^ P N * . I n t r a m o l e c u l a r pyrene e x c i m e r f o r m a t i o n was used t o s t u d y the c y c l i z a t i o n dynamics o f p o l y m e r s , such as pyrene end capped p o l y ( e t h y l e n e o x i d e ) , ( 4 3 ) p o l y s t y r e n e s , ( 44) and o t h e r p o l y m e r s . ( 4 5 ) However, t h e r e i s no r e p o r t on t h e e f f e c t o f any c o n f o r m a t i o n a l t r a n s i t i o n o f p o l y e l e c t r o l y t e s i n d u c e d by pH on t h e excimer f o r m a t i o n . In t h i s study, a c a t i o n i c pyrene d e r i v a t i v e s , t h e probe C P N e x h i b i t s a maximum i n excimer f o r m a t i o n w i t h pH, a f e a t u r e n o t a v a i l a b l e w i t h o t h e r c a t i o n i c p r o b e s , w h i l e t h e l o n g c h a i n c a t i o n i c probe, C ^ P N * , f a i l s t o e x h i b i t such a pH e f f e c t on excimer f o r m a t i o n . The f l u o r e s c e n c e s p e c t r a o f 6 χ 10""^ M C P N i n aqueous s o l u t i o n s o f PMA a t pH 4, 5, 6 and 7 a r e d i s p l a y e d i n F i g u r e 4. I t can be seen t h a t excimer f o r m a t i o n i s u n f a v o r a b l e i n aqueous s o l u t i o n s o f PMA a t pH 4 and 5, where a l a r g e monomer f l u o r e s c e n c e spectrum w i t h f i n e s t r u c t u r e ( X 377 nm) i s o b s e r v e d . A t pH 6, a g r e a t l y enhanced excimer f l u o r e s c e n c e spectrum i s o b s e r v e d a t l o n g e r wavelengths U 480 nm), which i s s t r u c t u r e l e s s and b r o a d , w h i l e the monomer f l u o r e s c e n c e and i t s f i n e s t r u c t u r e a r e d r a m a t i c a l l y r e d u c e d . On i n c r e a s i n g pH another u n i t , from 6 t o 7, a g r e a t l y reduced excimer f l u o r e s c e n c e i s observed w i t h a s m a l l i n c r e a s e i n monomer f l u o r e s c e n c e . L i t t l e e x c i m e r f o r m a t i o n i s observed i n aqueous s o l u t i o n s w i t h o u t added PMA, even up t o c o n c e n t r a t i o n s o f C^PN o f 4 χ 1 0 ~ M. However, i n aqueous s o l u t i o n s o f PMA a t pH 6, t h e excimer y i e l d of 4 χ 1 0 ~ M C P N i s a l r e a d y s i g n i f i c a n t , a l t h o u g h a t o t h e r pH the excimer i s s t i l l n o t o b s e r v e d . The d a t a i n d i c a t e t h a t t h e c o n f o r m a t i o n o f PMA p l a y s an i m p o r t a n t r o l e i n e x c i m e r f o r m a t i o n o f C PN . F i g u r e 5A, r e p r e s e n t s t h e e f f e c t o f pH on t h e r a t i o o f the r e l a t i v e i n t e n s i t y o f e x c i m e r t o monomer f l u o r e s c e n c e , I / I i n aqueous s o l u t i o n s o f PMA w i t h v a r i o u s c o n c e n t r a t i o n s o f C P N . The d a t a c l e a r l y demonstrate t h a t t h e maximum e x c i m e r f o r m a t i o n o c c u r s a t pH 6, w h i l e i n s o l u t i o n s a t pH s m a l l e r than 5 o r l a r g e r than 7, t h a t t h e e x c i m e r y i e l d i s much lower than a t pH 6. E a r l i e r s t u d i e s have shown t h a t t h e u n c o i l i n g p r o c e s s o f compact PMA c o i l s i s a c o n t i n u o u s p r o c e s s over s e v e r a l pH u n i t s . ( 2 8 , 4 2 ) In s o l u t i o n s of pH 4-5, a l t h o u g h t h e PMA c o i l s a r e a l r e a d y s w o l l e n due t o p a r t i a l i o n i z a t i o n , f r e e pyrene c a n n o t be s o l u b i l i z e d by t h e polymer, w h i l e a c a t i o n i c probe i s bound i n t o polymer c o i l . However, t h e r i g i d environment h i n d e r s any r e a d j u s t m e n t o f t h e pyrene lumophores, a s i t u a t i o n t h a t i s u n f a v o r a b l e f o r excimer f o r m a t i o n . On i n c r e a s i n g pH from 5 t o 6, s w o l l e n PMA c o i l s a r e l o o s e r and f u r t h e r expanded, and where more than one c a t i o n i c probe i s bound t o a polymer c o i l , excimer formation i s p o s s i b l e v i a readjustment o f the probes i n the l o o s e polymer c o i l . However, i n t h e case o f polymer s o l u t i o n s o f Β s t a t e s , s h o r t c h a i n pyrene lumophores a r e n o t bound i n c l o s e p r o x i m i t y and excimer f o r m a t i o n i s n o t f a v o r e d . Higher c o n c e n t r a t i o n s o f C P N l e a d t o enhanced I / I as t h e p r o b e s a r e c o n c e n t r a t e d on t h e polymer c h a i n . +

4

+


4

m a x

m

a

x

+

6

4

+

4

+

4

e

4

+

4

e

m

Experimental s t u d i e s u s i n g a long c h a i n c a t i o n i c pyrene d e r i v a t i v e , C ^ P N * a r e g i v e n i n F i g u r e 5B. C^PN " forms excimer a t low c o n c e n t r a t i o n , 2 χ 1 0 ~ M, i n aqueous s o l u t i o n s o f PMA a t pH > 7, n e v e r t h e l e s s , a b e l l shaped curve w i t h a maximum a t pH 6 i s n o t shown. T h i s can be e x p l a i n e d i f t h e l o n g c a r b o n c h a i n s o f t h e -1

6




Relative X

1

0

5

fluorescence intensity of cationic probes: 2

M

U

3 8 8

2

x

M 5

, " , 2+ ,° e x c i t a t i o n ™>' lg" Ru(bpy) U iio nm), 2 χ 10 M C PN ^excitation ' 2 χ 10 M pyrene fluorescence ^excitation ^ f i ° °f P i - aqueous solutions of PMA. -5 Relative fluorescence of 2 χ 10 M Au Ο (λexcitation 388 nm) i nintensity PAA. +

2+

3

e x c i

3 4 0

3

B.

4

n m

0

n m

i

a

a

o

n

s

n

+

4

d

a

u n c t

n

H

n

+

Fluorescence spectra of C PN i n aqueous solutions of PMA at pH 4, 5, 6 and 7 tt 356> nm. 4

excitation


32. CHU AND THOMAS


443

C^PN cause a l o c a l clustering of the probes thus enhancing s t a t i c excimer formation. If excimers are formed by migration together of two pyrene molecules, then excimer emission intensity w i l l increase with time, as exhibited by pyrene i n micelles.(46) However, i f pyrene i s stacked or clustered together on the assembly then the excimer emission at 480 nm w i l l be observed immediately after the laser pulse.(47) Figure 6A i l l u s t r a t e s an immediate formation and accelerated decay of excimer i n 1 χ 10~ M C PN i n aqueous solution of PMA at pH 6, showing that C^PN i s stacked or clustered i n PMA. The f i r s t order decay curves of, ln(Intensity of excimer C PN fluorescence) vs. time i n aqueous solutions of PMA at pH 6, 7 and 8 are given i n Figure 6B. The lifetime of the excimer i s larger at pH 6 than at pH 7 or pH 8. The longer l i f e t i m e of the excimer at pH 6 confirms that the environment of the probe at pH 6 i s less polar than at pH 7-8 and that at pH 6 PMA i s not f u l l y open or extended but s t i l l exists as a loose c o i l . 4

+

4

+

+


4

Conformational Transition of PMA Induced by C T A B . Aqueous solutions of PMA at pH 8, i n the absence and i n the presence of C J Q T A B were examined by transmission electron microscopy. No p a r t i c l e s were observed i n simple aqueous solutions of PMA at pH 8 either v i a the p o s i t i v e l y charged or the negatively charged stains (uranyl acetate or phospho-tungstic a c i d ) . However, i n the presence of C, T A B (8 χ 10~ M), spherical p a r t i c l e s of about 350 Â - 500 Â diameter can be c l e a r l y observed v i a the above process, the electron micrograph of aggregates of PMA - C-JQTAB i s shown i n Figure 7. This indicates that the PMA polymer chain i s extended and stretched at pH 8 as shown i n e a r l i e r studies(1-17), while addition of cationic surfactants such as, C T A B r e f o l d the polymer chain to form spherical p a r t i c l e s . Photophysical studies both steady state and pulsed, of pyrene and i t s positive and negatively charged derivatives,(48) confirm that a conformational t r a n s i t i o n of PMA i s induced by C TAB. The stretched PMA chain at pH 8 collapses on addition of the cationic surfactants. N

3

Q

1 Q

n

C r i t i c a l Aggregate Concentration - the CAC. Figure 8 shows a sharp increase both i n I and Ιβ/Ι^ of pyrene fluorescence over a narrow range of C^QTAB concentration, i n aqueous solution of PMA at pH 8. I t i s noted that the r a t i o of I / I i n PMA - C ^ T A B solutions reaches a steady value (~ 0.70) at a C^QTAB concentration, which i s c a l l e d a C r i t i c a l Aggregate Concentration, CAC. At the CAC, the hydrophobic aggregates of Cj T A B and PMA are formed which host hydrophobic molecules such as pyrene. The CAC corresponds to the midpoint of the t r a n s i t i o n i n a plot of I of pyrene fluorescence versus [ C T A B ] . The decay rate constants sharply decrease at a C J Q T A B concentration 3 χ 10~ M, which i s i n good agreement with the CAC determined v i a fluorescence r a t i o of I 3 / I 1 measurement. This indicates that techniques that have been successfully used for the CMC measurements i n micellar systems,(49) can also be used to investigate the aggregates of PMA and C T A B . The i n i t i a l addition of C T A B (< CAC), causes a decrease i n I , the pyrene fluorescence i n t e n s i t y and an increase i n the decay rate constants, due to r

3

1

Q

r

1 Q

3

1 Q

1 0

r



Β

6.0 1.0

0

JΕ 4.0 ι—ι

Α

I

I

1 4

6 ρΗ

\

10

8

u

\

\

1Λ


•

(C PN1M 4

4

\, 2 Χ 1 0 "

/ ' » /' »

/ Α •·

2.0

0.0

1X1 Ο*

4

_.4?-


\ 0.7

T k k

ι

A

x —5 i0.2

0.5 ^

0.3

1

A

L_

1,

>

[C ] ,10" M 3

1 0

Relative fluorescence intensity, I (o) and intensity r a t i o , I 3 / I 1 (Δ) of pyrene as functions of concentrations of C TAB, [ C , Q ] , i n aqueous solutions of PMA at pH 8 , [pyrene] = 2 χ 1 0 ~ M. Relationship between I of pyrene fluorescence and [CIQ] water. (Reproduced from Ref. 4 8 . Copyright 1 9 8 6 American Chemical Society.) R

1Q

6

B.

R

i

n

10

12

14

16

n in C T A B n

9.

Plot of Log(CAC) vs. n i n C TAB (A) and p l o t of log(CMC) vs. n i n C TAB(#). (Reproduced from Ref. 4 8 . Copyright 1 9 8 6 American Chemical Society.) n

n

American Chemical Society Library 1155 16th S t , N.W. Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987. Washington, D.C 20036

447

448


C TAB aggregates has l i t t l e charge and that the k i n e t i c data are similar to those found for non-ionic micelles. Analysis of the quenching data of pyrene fluorescence by 1dodecylpyridinium chloride, DPC v i a pulsed laser studies confirms the Poisson d i s t r i b u t i o n of DPC amongst the aggregates. Figure 10A shows the excellent f i t of the Poisson kinetics to the time dependent quenching of Pyrene fluorescence i n deaerated aqueous solution of PMA at pH 8, contained 8 χ 10"" M C TAB and quencher 2 χ 10"^ M DPC. The Poisson equation used i s 1Q

3

1Q


I = I

Q

{Exp(-k t) - n[1-Exp(-k t)]} Q

(2)

g

where η i s the average number of DPC quencher molecules s o l u b i l i z e d in each aggregate, k^ and k^ are the f i r s t order rate constants f o r the decay of pyrene i n the absence and i n the presence of quencher^ respectively. In this figure, k = 2 χ 10^ s" , k = 2.0 χ 10 s~" calcd ° · · Figure 10B presents the plots of I n ( I ) of pyrene fluorescence versus time i n deaerated solutions of PMA contained 8 χ 10~ M C TAB (pH 8), with various concentrations of quencher DPC (0-1.4 χ 10 M). These are t y p i c a l plots of quenching data according to a Poisson distribution.(50,51) The aggregation numbers of C TAB calculated by the relationship Q

n

=

2 7

r

3

1Q

4

1Q

[C TAB]/N = [DPC]/n _ _ 10 calcd.

(3)

in

correspond well to the results from steady state experiments (N = 105 ± 10).(48) The above experimental data confirm that the model for aggregation of PMA - C Q T A B i s not v i a l o c a l small and random clusters, but v i a discrete structures,that are much larger than pure C T A B micelles (Ν ~ 36).(52) It i s concluded that the aggregates of PMA - C Q T A B are large structures consisting of about one hundred C - J Q T A B molecules and one c o i l e d polymer chain. The i n t e r i o r of the aggregate has hydro phobic domain that i s similar to that of a micelle. However, the bromide ions are only i n bulk aqueous phase, and not close to the surface of aggregate. The degree of p o l a r i z a t i o n of 2-methylanthracene fluorescence i n PMA - C Q T A B aggregate i s four f o l d smaller than i n PMA compact c o i l at pH 2, two f o l d smaller than i n C J Q T A B micelle, indicating that the aggregate i s a much looser structure than a compact PMA c o i l , or a C T A B micelle. The data of electron microscopy show that the aggregates are spherical p a r t i c l e s which are about 350-500 Â, rather than h e l i x forms. PMA samples of molecular weight 3.9 χ 10 , 1.6 χ 10 and 6.4 χ 10^ emphasize the e f f e c t of molecular weight of PMA on the aggregates of PMA-C TAB. The curves of ^/I-j f o r 2 χ 10" M pyrene fluorescence i n aqueous solutions of PMA at pH 8 show a sharp increase i n above PMA samples at pH 8, indicating that i n a l l cases the aggregates of PMA - C TAB are formed above the CAC. The aggregation numbers of C TAB i n each aggregate, measured by analysis of the quenching data of pyrene fluorescence by DPC, are 90 ~ 100 surfactant molecules, and also contain a portion of the c o i l e d PMA chain. 1

1 Q

1

1

1 Q

4

1Q

1Q

1Q



32. CHU AND THOMAS

Θ.ΘΙ

8

10.

A.


•

.

288

400

680 800 Time CnS)

449

.

1000

Poisson quenching f i t of the time-dependent fluorescence decay of pyrene i n deaerated PMA solutions at pH 8, containing 8 χ 10" M C TAB and quencher 2 χ 10~ M DPC. The smooth one i s from computer f i t t i n g and the other one i s r e a l data. In(Intensity) of pyrene fluorescence vs. time i n deaerated PMA solutions containing 8 x 1 0 ~ " M Cj Q T A B at pH 8, with various concentrations of quencher DPC, (10" M): (a) 0; (b) 2.0; (c) 4.0; (d) 6.0; (e) 8.0; (f) 10.0; (g) 12.0; (h) 14.0. 3

1 Q

5

B.

3

5


450



CONCLUSIONS This study shows that fluorescence probing techniques are useful and powerful tools for investigation of conformational transitions of polyelectrolytes as induced by c a t i o n i c surfactants, pH or other means. Studies on the interaction of cationic probes with polyelectrolytes provide useful information on the intermediates that l i e between A states and Β states. I t i s concluded that the conformational t r a n s i t i o n induced by pH i s a progressive process over several pH units. Studies on the interaction of cationic surfactants with PMA at pH 8 show that the aggregates formed are large loose structures, while the i n t e r i o r of the aggregate has a hydrophobicity that i s similar to that of a micelle. Acknowledgment. We thank the National Science Foundation for support of this work via Grant CHE-01226-02.

Literature Cited. 1. Katchalsky, Α.; Eisenberg, H. J. Polym. Sci., 6, 145, 1951. 2. Silberberg, Α.; Eliassaf, J.; Katchalsky, A. J. Polym. Sci., 7, 393, 1951. 3. Mandel, M.; Stadhouder, M. G. Makromol. Chem., 80, 141, 1964. 4. Anufrieva, Ε. V.; Birshtein, T. M. J. Polym. Sci. C, 16, 3519 1968. 5. Arnold, R. J. Coll. Sci., 12, 549, 1957. 6. Koenig, J. L.; Angood, A. C.; Semen, J.; Lando, J. B. J. Am. Chem.Soc.,91, 7250, 1969. 7. Crescenzi, V.; Quadrifoglio, F.; Delben, F. J. Polym. Sci., A 2, 10, 347, 1972. 8. Delben, F.; Crescenzi, V.; Quadrilfoglio, F. Eur. Polym. J., 8, 933, 1972. 9. Daoust, H.; Thanh, H. L; Ferland, P.; St-cyn, D. Can. J.Chem. 63, 1568, 1985. 10. Kern, Ε. E.; Anderson, D. K. J. Polym. Sci. A-1, 6, 2765, 1968. 11. Suzuki, K.; Taniguchi, Y. J. Polym. Sci. A-2, 8, 1679, 1970. 12. Kay, P. J.; Kelly, D. P.; Milgate, G. I.; Treloar, F. E. Makromol. Chemie., 177, 885, 1976. 13. Okamoto, H.; Wada, Y. J. Polym. Sci. Polym. Phys., 12, 2413, 1974. 14. Jager, J.; Engberts, J. B. F. N. J. Org. Chem., 50, 1474, 1985; J. Am. Chem.Soc.,106, 3331, 1984. 15. Moan, M.; Wolff, C.; Cotton, J. P.; Ober, R. J. Polym. Sci. Polym. Symp., 61, 1, 1977; Polym., 16, 781, 1975. 16. Irie, M. Makromol. Chem. Rapid Commun., 5, 413, 1984. 17. Bednar, B.; Marowetz, H.; Shafer, J. A. Macromolecules, 18, 1940, 1985. 18. Stork, W. J. H.; Van Boxsel, J. A. M.; DeGoeij, A. F. P. M.; Haseth, P. L. De.; Mandel, M. Biophys. Chem., 2, 127, 1974; Madel, M.; Stork, W. H. J. Biophys. Chem., 2, 137, 1974. 19. Anufrieva, Ε. V.; Birshtein, T. M.; Nekrasova, T. N.; Ptitsyn, O. B.; Sheveleva, T. V. J. Polym. Sci. C., 16, 3519, 1968. 20. Oster, G. J. Polym. Sci., 16, 235, 1955. 21. Baud, C. Eur. Polym. J., 13, 897, 1977.


32. CHU AND THOMAS 22. 23. 24. 25. 26.


27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.


Erny, B.; Muller, G. J. Polym. Sci. Polym. Chem., 17, 4011, 1979. Wang, Y.; Morawetz, H. Macromolecules, 19, 1925, 1986. Stork, W. H. J.; Hasseth, P. L. de.; Schippers, W. B.; Körmeling, C. M.; Mandel, M. J. Phys. Chem., 77, 1772, 1973. Stork, W. H. J.; Hasseth, P. L. de.; Lippits, G. J. M.; Madel, M. J. Phys. Chem., 77, 1778, 1973. Muller, G.; Fenyo, J. C. J. Polym. Sci. Polym. Chem., 16, 77, 1978. Snare, M. J.; Tan, K. L.; Treloar, F. E. J. Macro. Sci. Chem. Α., 17(2) 189, 1982. Chu, D. Y.; Thomas, J. K. Macromolecules, 17, 2142, 1984. Tokiwa, F.; Tsujii, K. Bull. Chem. Soc. Jpn., 46, 2684, 1973. Schwuger, M. J. J. Colloid & Interf. Sci., 43, 491, 1983. Shirahama, K. J. Colloid & Polym. Sci., 252, 978, 1974. Cabane, B. J. Phys. Chem., 81, 1639, 1977. Shirahama, K.; Tohdo, M.; Murahashi, M. J. Colloid & Interf. Sci., 86, 282, 1982. Turro, N. J.; Baretz, B. H.; Kuo, P. L. Macromolecules, 17, 1321, 1984. Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem., 89, 41, 1985. Kresheck, G. C.; Hargraves, W. J. Colloid & Interf. Sci., 95, 453, 1983; 105, 589, 1985. Dubin, P. L.; Davis, D. D. Macromolecules, 17, 1294, 1984. Leung, P. S.; Goddard, E. D. etc., Colloids Surf., 13, 47 & 63, 1985. Hayakawa, K.; Kwak, C. T. J. Phys. Chem., 86, 3866, 1982. Abuin, E. B.; Scaiano, J. C. J. Am. Chem.Soc.,106, 6274, 1984. Hayakawa, K.; Santerre, J. P.; Kwak, C. T. Macromolecules, 16, 1642, 1983. Chu, D. Y.; Thomas, J. L. J. Phys. Chem., 89, 4065, 1985. Cuniberti, C.; Perico, A. Eur. Polym. J., 13, 369, 1977; 16, 887,1980; Ann. N.Y. Acad. Sci., 366, 35, 1981. Winnik, M. A.l Redpeth, A. E. C.; Paton, K.; Danhelka, J. Polymer, 25, 91, 1984. Tazuke, S.; Ooki, H.; Sato, K.; Macromolecules, 15, 400, 1982 Suzuki, Y.; Tazuke, S. Macromolecules, 14, 1742, 1981. Thomas, J. K. Chem. Rev., 80, 283, 1980; Thomas, J. K. The Chemistry of Excitation at Interfaces, ACS Monograph, No. 181. Washington, D. C., 1984. DellaGardia, R. D.; Thomas, J. K. J. Phys. Chem., 87, 3550, 1983. Chu, D. Y.; Thomas, J. K. J. Am. Chem.Soc.,108, 6270, 1986. Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem.Soc.,99, 2039, 1977. Atik, S. S.; Singer, L. A. Chem. Phys. Lett., 59, 519, 1978. Hashimoto, S.; Thomas, J. K. J. Colloid & Interf. Sci., 102, 152, 1984. Tabtar, H. V. J. Colloid & Interf. Sci., 14, 115, 1959.

RECEIVED April 27, 1987


Interaction of Cationic Species with Polyelectrolytes - ACS Symposium

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