Macromolecules 1988,21,950-953950Fluorescence Pr_~be Investigation of Anionic Polymer-Cationic

Surfactant Interactions

PremChandar,' P. Somasundaran,' and N. J. Turro..School of Engineering and Applied Science and Department of Chemistry, ColumbiaUniversity, New York, New York 10027. Received April 20, 1987

py-PAA (-40K MW. 1.5 mol CYo pyrene). shownbelowwassy11~..thesized as described elsewhere.s. i:

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(1 g/L) containing 6 X 10-7 M pyrene and 0.03 M NaC) waS"adjusted to the desired pH value and titrated with DTAB. Af~r

each titration, ;excitation) and the third (384 DID) to first (374 nm)peak ratio, Ia/Il' was measured. This ratio is known to be

monitor of environment polarity.~Excimer Formation or pyrene-Labeled Polymer. The ti~tration experiments were repeated with the solution containing'!i2.5 mg/L of py-PAA instead of free pyrene. The ratio of eJ(cimer;~to monomer, 1.11 m' was determined at each titration point from ~:the emission spectrum (excitation at 332 nm, monomer 376 nm.~~'. ."-excuner 490-486 nm). - - --

fluorescence decay profiles of ihe excimer for each sample (320.nm

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IntroductionWhen polymers and surfactants are present together

either inherently or by design, there can be significantinteractions between them that can be important inlong-established areas such as mineral and materialsprocessing, enhanced oil recovery, detergency, and paintformulation.l More recent interest also stems from thepotential for utilizing the microstructural environment ofpolymer-surfactant complexes in applications such ascontrol of chemical reactivity, drug delivery, solar energyconversion, and isotope separation.2 Oppositely chargedpolymers and long-chain surfactants can interact stronglyowing to electrostatic factors and the self-aggregationtendency of the surfactant. Thus polymer can inducemicelle-tyJ>:e aggregation of the surfaCtant at concentrationswell below the surfactant critical micelle concentration(cmC).3 Such aggregation has been studied by viscometric,surface tension, dialysis, conductivity, and potentiometric

techniques..The microstructural characteristics of pol~er-surfac-

tant complexes are only beginning to be understood withthe advent of molecular probing techniques involvingphenomena such as fluorescence.5 In most of these studiesthe fluorescent species is used to probe the surfactantmicroenvironment in terms of molecular properties suchas micropolarity,5c aggregation numbers,5b etc. Almostnoinformation is available on the conformational aspects ofthe polymer upon interaction with surfactants althoughit is recognized that this factor can play an important rolein polymer-surfactant complexation. A priori, polymerconformations in polymer-surfactant complexes could be'studied by using labeled polymers. Indeed labeled poly-mers have been used to study chain conformation andflexibility for a number of polymer systems as well as tostudy interpolymer complexation. 6 In this paper we reportthe use of pyrene-labeled poly(acrylic acid) as a probe forstudying the interactions in polyacrylic acid-dodecyltri-methylammonium bromide (DTAB) system. Specifically,the extent of excimer formation was used as a measure ofpolymer strand coiling. Also, particular attention was paidto the influence of thepyrene label on the native polymerby comparing the results obtained by using the polymericprobe versus that obtained by using unlabeled polymer and

"free" pyrene probe.

Experimental SectionDodecyltrimethylammonium bromide, DTAB (Aldrich Chem-

i~), was recrystallized from ethanol. Sodium polyacrylate, PAA(40K MW monodispersed, molecular weight standard, PQ:lysci-ences) was used as received. pyrene-labeled poly(acrylic acid),

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Results and Discussion .~

Micropolarity Studies Using Pyrene. Figure 1 showstypical results for the change in theparameter, 13/11 in 1 g/L PAA as a function of -concentration obtained at pH 6.4, 0.03 M NaCL .- .

decrease in micropolarlty (increase in 13/1 J in the presence1of the polymer and at DT AB concentration well below the:,cmc (1.3 X 10-2 M, indicated by the arrows, Suggests the;,:association of polymer-bound surfactant.6a The onset of A;this polymer-induced association, referred to here as the;~..'critical aggregate concentration (CAC), was determined at'!various pH and NaCllevels and these results are shownfin Figure 2. It is noted that the addition of salt as e.s,:';pected leads to an increase in the CAC as a result ofsuppression - - ...

The data in Figure 2 also show that the CAC is relatively.,independent of pH with no NaCI. At 0.03 M NaCI, CAO~increases from 0.4 to 0.9 mM when the pH is' .from 5 to 8 and remains relatively constant above pH 80,

This variation of CAe with pH is somewhatsince as the degree of ionization, a, increases with pH,

increase in the extent of electrostatic!

~ 1988 American Chemical Society

Macromolecules, Vol. 21, No.4, 1988. ,C

Anionic Polymer-cationic Surfactant Interactions 951~1

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centration (1 g/L PAA, 0.0025 g/L py-PAA) in 0.03 MNaCI and 3 pH values is shown in Figure 3. I./Im firstincreases and reaches a maximum and then decreases.This general trend was observed at all pH values above5 and at both NaCllevels tested (0 and 0.03 M). Theinterpretation of this behavior requires an understandingof the nature of the excimer formation process. 1\vo casesmay be distinguished:6c.d "Dynamic" excimer formationoccurs through the excitation of isolatOO pyrene groups andsubsequent diffusional encounter with ground-state pyrene.On the other hand "static" excimer formation can occurif the pyrene groups on the polymer are in juxtapositioneither due to the statistical conformation of the chain ordue to ground-state aggregation of pyrene groups. Staticand dynamic excimer formation can be distinguished byexamination of the fluorescence spectra as well as time-resolved behavior as discussed below.6d.a

The emission and excitation spectra and the excimerfluorescence decay profiles obtained at the DTAB con-centrations before the maximum in I.IIm and after themaximum showed the following features:

1. In water as well as at prema.'Cimum DTAB concen-trations, the excimer emission was centered around 490nm. Above the maximum, the emission wavelength shiftsgradually to about 486 nm.

2. The premaximum excimer excitation spectra (emis-sion at 490 om) were red-shifted by about 4 nm comparedto the monomer excitation spectra (emission at 376 nm)as shown in Figure 4a, In the postmaximum region, thered shift was progressively less with addition of DTAB(compare Figure 4b).

3. Whereas the fluorescence decay profiles of the ex-cimer in the premaXimum region showed no initial growth(rise time) those in the postmaximum region showed aninitial growth characteristic of a diffusion-controlled ex-cimer fonnation process (nanosecond timescale) (compareFigure 5, curves A and B).

The above observations are consistent with the existenceof ground-s~te pyrene dimers or aggregates at the pre- 0

maximum DTAB concentrations (as well as in water) sothat the excimer formation is a "static" process. Withaddition of DTAB beyond the maximum the ground-stateaggregates dissociate and the absorbing species are theisolated pyrene groups. Thus at high DTAB concentra-tions, the excimer formation is dynamic in nature with acharacteristic rise time. It is noteworthy that similarfindings were reported by Winnik et a1. 6d for the interac-tion of 8 polymeric pyrene~labeled probe, (hydroxy~;propyl)ceUulose, with SDS and HTAC. These authors alsoconcluded that at low surfactant concentrations, excim~r

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and a decrease in CAC would have been expected a priori.It Should be noted, however, that CAC, in addition to beinga measure of the extent of electrostatic binding, is ameasure of the efficiency of association of the surfactantwith the latter depending upon the conformational flexi-bility of the polymer which in turn depends upon a. Thatis, when the polymer is highly charged (large a), the as-sociation of alkyl chains of the surfactant and accompa-nying collapse of the polymer will be opposed by seg-ment-segment repulsive forces. As a decreases, thepolymer adopts a more coiled conformation and the as-sociation becomes easier to achieve.

Considering these factors, we suggest the following ex-planation for the pH dependence of CAC: At low NaCIlevels, the electrostatic binding efficiency and the seg-rnent-segment repulsive contribution offset each other sothat the net driving force for association is the hydrophobicchain interaction which can be considered to be inde-pendent of pH. On the other hand, at high salt levels theel~tatic binding of the surfactant is suppressed so thatthe pH dependence of the CAC observed is ~ntially dueto the conformational differences of the polymer.

The above results suggested that the chain flexibility /c-,', , conformation of the polymer plays a key role in governing

Mf'; the surfactant aggregation process as well. We therefore- ~c;:' attempted to get information on the conformational as-

1';. P{!cts of the polymer-surfactant interaction process using'::'j: a pyrene-labeled poly(acrylic acid) as a probe, and these;c.~c; results are discussed in the next section.

I," "":;,, Excbner Formation ofPyrene-Labeled Poly(acrylic. :'"~~ acid). 'l.'he variation in excimer to mo.norner ratio of the~ ;-;\, polymerIC probe, py-PAA, as a functIon of DTAB con-,', .,i;.,~::-:

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; Summary, Pyrene-labeled poly(acrylic acid) was used to investigate

interactions between poly(acrylic acid) and dodecyltri-methyIammonium bromide, and a schematic representa-tion of these interactions is shown in Figure 7. In par-ticular it was possible to show that binding of surfactantleads to increased excimer formation as a result of polymercoiling. A decrease in excimer formati~n was obseniedwhen the critical ~gregate concentration was reachedwhich was attributed to a decrease in the extent ofground-state pyrene ~gregation and resulting static ex-cimer formation asa result of solubilization of the pyrenemoieties within polymer-bound surfactant aggregates. Thecritic~ aggre;gate concen~ration determined by the maxi-mum m eXClDler formatIon was always lower than thatdetermined by using the micropolarity response of freepyrene. This difference was attributed to the hydrophobiccontribution to the binding process resulting from thepresence of the pyrene label so th8t~urfactant aggregationstarts at lower bTAB concentrations for the labeledpolymer then for the unlabeled one: The variation of CACwith pH indicated that the efficiency of surfactant ag-. gregation depended significantly upon the conformatio~al

flexibility of the system. Indeed with decrease in pH, t~e

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extent of coiling of the polymer-surfactant compiex wasgreater, as indicated by the higher excimer to monomerratio, resulting in a lowered CAC. '

Acknowled~ent. We are grateful to Dr. K Arora forthe synthesis of the pyrene-labeled polymer Used in,thisstudy. Financial assistance from NSF, ffiM, and ARO isgratefully acknowledged. . . , "':

Registry No... DTAB, 1119-94-4; PAA.Na, 9003-04-7; py,129-00-0; (acrylic acld)(2-( (4-(I-pyrene) butanoyl)amino )propenoicacid) (copolymer, 1~315-59-9. ','

References and Notes-(1) (a) Taber; J. J. Pure Appl. Chem. 1980,52,1323. (b) Soma-

sundaran, P.; Chandar, P. In Solid J-iquid Interactiont inPorous Media; Cases, J. P., Ed.;.Technip: P~ris, 1985. (,,)Somasund~ P.; Lee. L. T. Sep. Sci. TechnoL 1981, 16; ~475.

(2) (a) Fendler, J. K; Fendler, E. J. Catalysis in Micellar and.Macromolecular SYltems; Academic: New York, 1975; (b)Fendler, J. H. Membrane Mimetic Chemistry; Wiley: NewYork, 1982; pp 293-514. (c) Turro, N. J.; Kraeutler, B.1nIsotopes in Chemistry; Buncel, E., Lee, C. C" Eds.; ElSevier:Amste~ 1984; VoL 6. (d) Interfaci,al Proceuel: EnergyConverlian and Synthesis; Wrighton, M. J., Ed.;Advancea inChemistry Series 184; American Chemical Society: ,Was~-ton, DC, 1980. (e) Seki, K; Tirrell, D. A Macromolecules1984, 17, 1692-1698. .; ,

(3) See, for ~xample: (a) Goddard, E. D. Colloids Surf. 1986, 19,

301-329. (b) Malovikova, A.; Hayakowa, K; Kwak, J. C. T.1nStructure Performance Relationships in Surfactantlj Rosen,M. J...Ed.; ACS Symposium Series; American Cheol:ical SOci-ety: Washil)gton, DC, 1984. .

(4) See citations in: (a) Leung, P. S.; Goddard, E. D. ColloidsSurf. 1985, 13, 47-62. (b) Reference 3a. .

(5) (a) Chu, D.-Y.; Thomas,J. K. J. Am. Chem. Soc. 1986,.108,627(}-6276. (b) Abuin, E. B.; Sciano, J. C. J. Am. Chern. SQC.1984,106,6274. (c) Zana, R.; Yiu, S.; Strazielle; Lian08;P. J.Col~id Interface &i. 1981, SO, 208. (d) Anantbapadmans-bhan, K P.; Leung, P. S.; Goddard, E. D. Colloidl Sui-f. 1985,13,63. (e) Turro, N. J.; Bareu, B. H.; Kuo, P.-L. Macromol-eculel 1984, 17, 1321-1324. '

(6) See citations in: (a) Turro, N. J.; Arora, K Polymer 1986,27,783. (b) Chandar, P.; Somasund~, P.; Waterman, K ,C.;Turro, N. J. Langmuir, 1987,3,298-300. (c) Wang, Y.; Lab-sky, J.; Morawtz, H., submitted for publication in M~romol-ecules. (d) Winnik, F.M.; Winnik, M. A; Tazuke, S. J. Phys.Chem. 1987, ~1, 492.

(7) Kalyanasundaram, K; Thomas, J. K J. Am. Chem. Soc. 1977,

99,2039.(8) Viaene, K.; Caigui, J.; Schoonheydl:; De Schryver, F. C. Lang-

muir 1987,3,107-111.(9) The exc;jtation spectra of the monomer and excimer in etcbanol

and propanol as well as dioxane coincided which is in agree-ment with the idea that the ground-state aggregation and re-sultant static excimer formation in aqueous solution are dueto the hydrophobic interactions between pyrenes.

(10) We suggest that the initial coiling is not due only to chargeneutralization since addition of equivalent amounts of tetra-methylammonium brQmide did not result in an increase in1./1...; the presence of the hydrophobic chain is a requirementfor the coiling to occur.

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