428 Reprinted from LANGMUIR, 1995,11.Copyright @ 1995 by the American Chemical Society and reprinted by permission of the copyright owner.

Aggregation Behavior of 12-(1-Pyrenyl)dodecanoic Acid inHomogeneous and Micellar Solutions

Joy T. Kunjappu and P. Somasundaran*

Langmuir Center for Colloids and Interfaces, Henry Krumb School of Mines,Columbia University, New York, New York 10027

Received January 18. 1994. In Final Form: November 4, 1994@

The steady-state fluorescence emission from 12-( I-pyrenyl>dodecanoic acid (PyDDA) in aqueous, organic,and micellar solutions has been studied as a function of pH and concentration of PyDDA and selectedsurfactants. The pH-dependent aggregation ofp)'DDA in aqueous solution shows strong aggregation ofthe acid molecules through H-bonding and hydrophobic interaction of alkyl chains under suitable conditions,manifested as blue shifts in the excimer emission. Emission studies in dodecanoate solution respondedto the premicellar aggregation phenomenon. Possible implications of this study to solid-liquid interfacialprobing are discussed.

12- (1- PYRENYLI DOOECANOIC ACID [PYDDAJ

Figure 1. Structure of 12-(1-pyrenyl)dodecanoate acid (Py-DDA).

IntroductionFatty acids and their derivatives have been the focus

on many recent studies mainly due to their film-formingability which has potential applications for biosensor andelectronic devices. 1.2 The physicochemical properties andstructure of the film have to be controlled to optimize thequality of cast films.3 The above acids form mono- andmultilayers at the aqueous solution -air interface and havebeen studied with their surface pressure-area (Jl-A)isotherms.. A new dimension was added to the Jl-Ainformation by coupling the fluorescence emission studies(steady-state and time-resolved) on films, using pyrene-labeled acids, with the available information on thecondensed nature of these films.5

Fatty acids have also found application as flotationcollectors for the enrichment of a variety of minerals.6.7Unravelling of the adsorption mechanism of these acidson minerals, a prime concern in the effective control of theflotation efficiency, may be achieved by combining theclassical adsorption data with spectroscopic informationon the same system.S-1. Fluorescence spectroscopicstudies with labeled fluorogen-bearing compounds in

. Author w whom correspondence should be addressed.

. Abstract published in Advance ACS Abstracts, February 1,1995.

(1) Loughran, T.; Hatlee, M. D.; Patterson, L. K; Kozak, J. J. J.Chern. Phys. 1980, 72, 5791.

(2) Grieser, F.; Thistlethwaite, P. J.; Urquhart, R. S. Chern. Phys.Len. 1987,141, 108.

(3) Swalen, J. D.; Allar&, D. L.; Andrade, J. D.; Chandross, E. A.;Gazoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.;Rabolt, J. F.; Wynne, K J.; Yu, H. Lan&7rtwr 1987,3,932.

(4JYamazaki, T.; Tamai, N.; Yamazaki, I. Chern. Phys. Leu. 1986,124, 326.

(5) Yamazaki, I.; Tamai, N.; Yamazaki, T.J. Phys. Chern. 1987,91,3572.

(6J Somasundaran, P.; Grieves, R. B. (Eds) inAdvances in InterfacialPhenomena of Particulate I Solution I Gas Systems; AIChE SymposiumSeries 71; AIChE: 19i5; p 1.

(7) Somasundaran, P.; Moudgil, B. M. (Eds) Reagents in MineralTechnology; Marcel Dekker: New York, 1987.

(8) Somasundaran, P.; Kunjappu. J. T.lnlnnovations in MaterialsProcessing Using AqueuoUB, Colloid and Surface Chemistry; TheMinerals, Metals and Materials Society: Warrendale, PA, 1988; P 31.

(9) Somasundaran, P.;Kunjappu,J. T. Miner. Metall.Process. 1988,6,67.

(10) Somasundaran, P.; Kunjappu, J. T.; Kumar, C. V.; Turro, N. J.;Barton, J. K Langmuir, 1989,5,215.

m) Somasundaran, P.; Turro, N. J.; Chandar, P. Colloids Surf. 1986,20, 145.

(12) Chandar, P.; Somasundaran, P.;Turro,N.J.J. Colloid InterfaceSci. 1987, 117, 31.

(13) Chandar, P.; Somasundaraw, P.; Waterman, K C.; Turro, N. J.J.Phys. Chem.I987,91, 150.

(14) Somasundaran, P.; Kunjappu,J. T.ColloidsSurf.I989,37,245.

0743-7463/95/2411-0428$09.00/0

solution assisted by emission information from a Lang-muir-Blodgett (L-B) film can be a starting point forcorrelating the properties of adsorbed layers with infor-mation on emission from the solid-liquid interface. Thisis particularly so since the complex mechanistic factorsof flotation depend to a great extent on the solid -liquidand liquid-air interfaces.7

12-(I-pyrenyl)dodecanoic acid (PyDDA), for which thestructure is represented in Figure 1, has been studied bymany workers as a model L-B film compound.l.2 Ag-gregation of fatty acid molecules through H-bonding andother interactive forces was reflected in the monomer-excimer forming efficiencies of the pyrene moiety. Inaddition to L- B films and films on the water surface,these labeled acids have been studied also as vacuum-deposited thin films15.16 with pulsed and continuous lightto explore their excimer-forming ability under time-resolved and steady-state conditions. The results indi-cated the importance of the ground state association ofpyrene and the resultant variation in the excimer speciesformed. A brief description of the emission properties ofpyrene-substituted dodecanoic acid at three pH valueshas been given by Itaya et al.l6b The present study aimstoward exploring the emission behavior of PyDDA inmicelles as well as in aqueous and organic media by s~ady-state fluorescence measurements. Such information isdeemed to be essential for using this molecule as a probefor studying adsorbed surfactants at the solid -liquidinterface. In the present work, the nature of emission ofPyDDA has been studied in aqueous solution at differentpH values and in cyclohexane, as well as in dodecanoateand sodium dodecyl sulfate (SDS) micelles.

(15) Itaya, A.; Kawamura, T.;Masuhara,H.;Taniguchi, Y.;Mitsuya,M. Chern. Phys. Lett. 198'7,133,235.

(16) (a) Taniguchi, T.; Mitsuya, M.; Tarnai, N.; Yamazaki, I.;Masuhara,H.Chem.Phys.Lett.I986,132,516. (b) Itaya, A.; Kawamura,T.; Masuhara, H.; Taniguchi, Y. Chern. Lett. 1986, 1541. (c) Gatt, S.;Fibach, E. Biochim. Biophys. Acta 1988,943,447. (d) Katusin-Razem,B.; Wong, M.; Thomas, J. K. J. Am. Chern. Soc. 1978, 100, 1679.

@ 1995 American Chemical Society

Langmuir, Vol. 11, No.2, 1995 429Aggregation Behavior of PyDDA

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ranging from 2.4 to 10.5. The broad excimer peak witha maximum at 445 nm appearing at low pH diminishesin intensity as the pH is increased; a peak at 425 nm iswell-resolved at higher pH. Also, the monomer peaksappearing between 375 and 400 nm are red-shifted athigher pH values. A peak at ~360 nm is due to Ramanscattering from the solvent. In addition, the spectrum atpH 10.5 exhibits a shoulder at ~472 nm. To throw lighton the origin of these peaks, excitation spectra of the abovesamples were recorded at emission wavelengths of 377,422,445, and 472 nm at two extreme pH values of2.5 and10.5, and they are shown in Figure 3. The excitationspectra at all emission wavelengths under these conditionsare seen to be different at the two pH values.

Figure 4 depicts the variation in the ratio of the excimer(445 and 425 nm) and monomer (377 nm) intensities (IFl1M) ofPyDA as a function of pH. TheIFlIMvalues decreasedmarkedly from pH 4, with very little change at high andlow pH values.

Figure 5 shows the fluorescence emission spectra ofPyDDA in cyclohexane along with that in water at naturalpH. The spectrum in water corresponds to the one givenearlier at low pH values. The spectrum in cyclohexaneexhibits the characteristic fluorescence features of acovalently labeled pyrene moiety in a hydrophobic envi-ronment under conditions when the excimer is not formed.8The emission spectrum ofPyDDA in cyclohexane remainedthe same irrespective of its concentration up to 15 ,uM(not shown in the figure).

Pyrene emission from PyDDA displayed strong depen-dence on the pH of the solution at a given concentration.At very low concentrations, 10-6 M, a negligible amountof excimer is formed. The pH-dependent excimer forma-tion points to the possibility of different types of interactionbetween dodecanoic acid molecules in solution resultingin different types of excimers. The hydrophobic associationbetween the alkyl chains has been recognized as animportant factor in determining the overall interactionbetween these acid molecules.17,18 These effects added tothe electrostatic requirements arising from the charge onthe carboxylate group and the associative H-bondingbetween them lead to various structural responsibilitiesas represented in Figure 6.188£08 ~ . . . . . /1' .

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Figure 2. Fluorescence spectra ofPyDDA in aqueous solutionat different pH values; [PyDDA] = 3.3 x 10~ M; Au = 320 nm;pH Values: (1) '2.45; (2) 3.74, (3) 7.31, (4) 8.89, and (5) 10.50.

Experimental Section

PyDDA was purchased from Molecular Probes and used asreceived. Lauric acid (dodecanoic acid) was from Alfa chemicalsand sodium dodecyi sulfate (SDS) from Bio-Rad (electrophoresisgrade). The pH of the solutions was adjusted from 1 M solutionsof sodium hydroxide and hydrochloric acid (Fisher certified).Methanol and cyclohexanol were of HPLC grade (Aldrich). Allthe aqueous solutions were made in triple-distilled water. Lauricacid was dissolved in sodium hydroxide at a pH of ~12. Stocksolutions of PyDDA were prepared in methanol (~1 mM). Theaqueous solutions of PyDDA were prepared initially at alkiDepH and acidified to obtain a lower pH. To ensure that PyDDAis not precipitated in homogeneous solution, excimer efficiencywas monitored by changing the pH for several cycles, and constantvalues were observed at each pH. PyDDA is known to have anaqueous solubility of > 3.3 x 10~ M, the concentration used inthe present study. ISo

The fluorescence emission measurements were performed witha SPEX FLUOROLOG spectrofluorometer at an excitationwavelength of 320 Dm. The emission spectra were corrected bymeans of a built-in program; the excitation spectra were notcorrected.

Results and Discussion

Homogeneous Solutions. Figure 2 shows the fluo-rescence emission spectra ofPyDDA at various pH values

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.U')AI1antba~dmall~bhan, K. P.; SoD1asundaran. P. J. CoUoid..~ Sci. 1188. 122, 10;4. ..:~) Goddard. E. D.. Smith, S. R.; Kao. O. J. Colloid Interface Sa.

C .21.320.is 19) Yakhot,Q.; ~ M. D.;Ludmer, z.Advan«S in photochem-"'-;-" b)I; Jnh...ueY &. Sona: New York. 1979,. VoL II , P 489.

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and 377 nm.The shifts in the maximum ofpyrene excimer emisSlorare reported to be strongly dependent on the degree 0overlap between the two pyrene groupS involved in thtexcimer formation. Similar dependence is reported in thoexcimer emission maximum of crystalline pyrene witJtemperature,~ where the pyrene-pyrene distance changewith temperature. The pH-dependent variation in thexcimer peak ofPyDDA may also be attributed to changEin the proximity of the two pyrene groups responsible fcthe excimer formation.2 It is possible that the excimer ;low pH may be formed from ground-state pyrene agregates since the strong H-bonding and hydrophotinteraction between the acid molecules may favor such :aggregate formation. That the monomer intensity at lcpH almost vanishes as observed with thin films (vacuu

deposited16 or L-B2) also suggests the possibility

ground-state aggregation at low pH.The spectrum of PyDDA in cyclohexane shows 1characteristic emission features of pyrene moiet)'pyrene-labeled compounds in a predominantly nonpcenvironment, even though linear relations may not e~between the vibronic band intensities and solvent polarIn this regard, it may be mentioned that Thomas et alon the basis of their electron-transfer reaction stutwith pyrene-labeled decanoic acid in micelles, 11suggested a methanol-like environment for pyrene ra'

Aggregation Behavior of PyDDA Langmuir, Vol. 11, No.2, 1995 431

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Figure 7. Emission spectra of PyDDA in SDS solution:[PyDDA] = 2.1 x 10-6 M; ;- = 320 nm; [SDS]: (1) 0.0, (2) 5x 10-6, (3) 1 x 10-., (4) 1.5 x 10-4, (5) 5.0 x 10-4, (6) 1.0 x 10-3(7) 1.3 x 10-2, (8) 3.8 x 10-2 M. 0--

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emits the characteristic spectrum of pyrene monomers ina hydrocarbon environment. This indicates that PyDDAis solubilized in SDS micelles, the interior of the micellesproviding the highly hydrophobic environment. However,no excimer is formed as the molar ratio ofPyDDA to SDSis very small; in other words, individual pyrene groupsare screened by alkyl chain ends of the unlabeledmolecules. The situation is quite different in the sodiumdodecanoate case. Here, the dodecanoate being preparedfrom dodecanoic acid at alkaline pH, PyDD~ in solutionprefers to be in the ionized form. At its natural pH,PyDDAshows the characteristic blue shift (Figure 8, scan 1). Indodecanoate solutions at low concentrations (pH> 1.1),PyDDA shows a spectrum similar to that in A1kA1inesolutions (Figure 8, scan 2). As the concentration ofdodecanoate is increased, the excimer is red-shifted toyield the usual dynamic excimer, i.e. the two-centersandwich-type excimer.20 These results indicate thatPyDDA is in a flexible state in the above concentrationrange. 111is trend is especially maintained for concentra-tions immediately preceding the cmc of the surfactant.Such an observation suggests the importance of theexistence of premicellar aggregates in surfactant solutions.Figures 9 and 10 show clear evidence for this phenomenonin the dodecanoate case. In the case of SDS, even ifpremicellar aggregates are formed, it is not registered byPyDDA as it may not be associated in these aggregates.This might be due to the possibility of stronger H -bondingbetween carboxylate groups of dodecanoate and ofPyDDA,probably through a water molecule. The premicellaraggregate formation is highlighted in Figure 10 by brokenlines. Similar type of bonding may not be favored betweensulfate and carboxylate acid groups inhibiting the inclu-sion of PyDDA in SDS premicellar aggregates. The

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than a hydrocarbon one. As discussed earlier, theexistence of dimeric cyclic H-bonded stnlctures in hy-drocarbons makes the formation of an excimer difficult.Naturally, an increase in the concentration ofPyDDA inthis type of solvent cannot be expected to contribute towardexcimer formation, since intermolecular excimer formationfrom such dimeric species will not be significant atconcentration levels used here.

In Surfactant Solutions. Figure 7 shows the emissionspectra of PyDDA in sodium dodecylsuIfate solutions atconcentrations ranging from 5 x 10-5 to 3.8 X 10-2 M ata constant PyDDA concentration of 2.1 x 10-6 M. Thespectrum above the CMC ofSDS (8.3 x 10-3 M) is similarto the one obtained in the cyclohexane solution (Figure 5).

Figure 8 shows the emission spectra of PyDDA indodecanoate solutions at alkaline pH. In the premicellarregion, the spectrum changes from a typical alkalineaqueous spectrum (as in Figure 2) into one in a hydro-carbon environment (as in Figure 5) through an inter-mediate range where dynamic excimers with emissionmaximum of -472 om are formed. Figure 9 shows thevariation of 144fJiI377 and Figure 10 the variation of 13ge/l 377for the two surfactants.

In dodecyl sulfate solution, the un-ionized form ofPyDDA does not have any specific interaction with SDSbelow its cmc. But, in the postmicellar region, PyDDA

432 Langmuir, Vol. 11, No.2, 1995 Kunjappu and Somasundaran

acids, carboxylic acid being a commonly used surfactantin many processes. PyDDA may find application as afluorescent probe sensitive to solloidal surfactant ag-gregates (surfactant clusters existing as adsorbed layerson a solid). The results obtained in homogeneous andmicellar solutions suggest the implications of the presenceof small premicellar surfactant aggregates and of pH-dependent changes in the analysis of excimer emission ofpyrene-labeled hydrolyzable surfactants. Furthermore,the present system may serve to yield new insights intothe question of interfacial polarities from acid-dissociationconstant values.23

possibility that premicellar aggregates may not be fonnedat all in the SDS case is not excluded here. The excimerpeak at 445 nm is observed only when PyDDA isincorporated into the relatively small premicellar ag-gregates. In bigger micellar aggregates, the typicalmonomer spectrum is observed.

The parameter 139si1377 as plotted in Figure 10 may nothave much significance as an index of polarity. A similarratio (138311372) for free pyrene has been widely used tofigure out the microscopic polarity of its environment,Sbut such a correlation was not found with pyrene-labeledcompounds21 even though sporadic attempts to exploitthis ratio may be found in recent literature.22

As mentioned earlier, the interfacial phenomena in-volved in various processes like emulsification, detergency,and flotation may be understood by resorting to fluores-cence emission studies with pyrene-labeled carboxylic

Acknowledgment. The authors wish to express theirthanks to the National Science Foundation, the Depart-ment of Energy, and IBM for their support.

LA9400621(21) Kunjappu, J. T.; Somasundaran, P., unpublished results.(22) Viaene, K.; Schoonheydt, R. A; Crutzen, M.; Kunyima, B.; De

Schryver, F. C. Langmuir 1988,4,749.(23) lAIvelock, B.; Grieser, F.; Healy, T. W. J. Phys. Chem.1985, 89,

SOl.