Spectroscopic investigations in molecularly organized solvent media. Part 2: Examination of the nitromethane selective quenching rule at different ‘effective’ micellar surface

JOURNAL OF

LUMINESCENCE ELSEVIER Journal of Luminescence 71 (1997) 189-197

Spectroscopic investigations in molecularly organized solvent media. Part 2: Examination of the nitromethane selective

quenching rule at different ‘effective’ micellar surface charge densities

Siddharth Pandey”, William E. Acree, Jr.a,*, John C. Fetzerb a Department of Chemistry, University of North Texas, Denton. TX 76203-0068, USA

b Chevron Research and Technology Center, Richmond, CA 94802-0627, USA

Received 29 August 1996; revised and accepted 7 December 1996

Abstract

Applicability of nitromethane as a selective quenching agent for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for select solutes dissolved in micellar sodium dodecylsulfate (SDS) f 1-pentanol, sodium dodecylsulfate + 1-hexanol and cetyltrimethylammonium bromide (CTAB) + 1-pentanol solvent media. Results of measurements show that nitromethane quenched fluorescence emission of only the 6 alternant PAHs in the cationic CTAB + 1-pentanol solvent media as expected. Emission intensities of nonalternant PAHs, except for two exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar anionic SDS -I- 1-pentanol and micellar anionic SDS + 1-hexanol. Nitromethane quenched fluorescence emission of all nonalternant PAHs studied in these two anionic surfactant systems, which is contrary to the selective quenching rule. The reduced emission quenching that is observed at increasing alcohol cosolvent concentration is rationalized in terms of the decreased micellar surface charge density that occurs when more alcohol molecules are incorporated into the anionic micelle.

Keywords: Fluorescence quenching; Micellar solvent media; Nitromethane selective quenching rule; Alcohol cosolvents

1. Introduction

This study continues a systematic examination of the effect that solvent media and substituent func- tional group has on the ability of nitromethane to selectively quench fluorescence emission of alter-

*Corresponding author. Fax: (817) 565-4318; e-mail:

acree@casl .unt.edu.

nant polycyclic aromatic hydrocarbons (PAHs). Emission intensities of nonalternant PAHs are for the most part unaffected by nitromethane addition. Published studies [l-S] involving over 63 PAHs have identified dibenzo[hi, wxlheptacene, benzo- [klfluoranthene and naphtho[2,3b]fluoranthene as among the few exceptions to the so-called nitromethane selective quenching rule in the PAH6 benzenoid, fluorenoid, fluoranthenoid and “methylene- bridged” cyclopenta-PAH subclasses.

0022-2313/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved Pff SOO22-23 13(96)00303-l

190 S. Pandey et al. /Journal of Luminescence 71 (1997) 189-197

More recent measurements [9, lo] revealed that nitromethane quenched fluorescence emission of all eighteen acenaphthylene- and acephenanthrylene-derivatives studied thus far which is com- pletely contrary to what would be expected based upon the fact that the solutes contain a single five-membered ring. The unusual fluorescence quenching behavior of the acenaphthylene- and acephenanthrylene-derivatives results from the molecules’ fixed double bond in the five-membered ring. The double bond is alkenic in nature, rather than aromatic as one might initially believe. This observation is confirmed by independent NMR coupling measurements [l l-141.

Zander, Breymann and coworkers [7,15,16] attributed nitromethane’s selectivity to an electron/charge transfer reaction whereby an electron is transferred from the excited PAH fluorophore to nitromethane, which acts as an electron acceptor. The mechanism was established in part based upon a log k versus El,2(redj Rehm-Wel- ler correlation for the rate of electron transfer in acetonitrile. From a strictly thermodynamic point- of-view, it is conceivable that the extent of quenching could be altered simply by changing the elec- tronic nature of the surrounding solvent media in order to either stabilize or destabilize the positive charge (or partial positive charge) that is temporar- ily formed on the polycyclic aromatic hydrocarbon. Micellar solutions provide a very convenient means to introduce ionic character, and still have a solvent media capable of solubilizing the larger, hydrophobic PAH solutes. Anionic surfactants could perhaps stabilize a positive charge on the PAH ring system, whereas cationic surfactants would tend to inhibit fluorescence emission quenching by nitromethane.

The above ideas were borne out in three prelimi- nary studies [ 17-191 involving micellar sodium dodecylsulfate (SDS), sodium octanoate (SO), sodium dodecanoate (SDD), dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTACl) solutions. Nitromethane quenching selectivity was lost in only the anionic SDS, SO and SDD solutions. Except for benzo[k] fluoranthene, dibenzo[b, klfluoranthene, dibenz [a, elacephenan- thrylene and naphtho[2,3b] fluoranthene, emission

intensities of the select nonalternant PAHs studied were for the most part not significantly affected by nitromethane addition in the case of the cationic surfactant solvent media. These four PAHs are known exceptions to the nitromethane selective quenching rule. Earlier studies further documented that the presence of micellar aggregates was a necessary condition for the anionic head-group charge to affect the quenching mechanism. At SDS and SDD molar concentrations below the cmc, nitromethane quenching selectivity was again observed.

While our earlier studies did suggest that the anionic versus cationic headgroup plays an impor- tant role in the observed loss of quenching selectivity, we did not fully explore the possibility of modifying the micellar size and head-group’s ‘effective’ surface charge density through addition of alcohol cosolvents and/or electrolyte salts. In this communication, the fluorescence behavior of select alternant and nonalternant polycyclic aromatic hydrocarbons in several micellar SDS + 1-pentanol, SDS + I-hexanol and CTAB + 1-pentanol solvent media is reported in both the absence and presence of added nitromethane. Varela et al. [20] estimated micellar aggregation number, alcohol cosolvent aggregation number and surface charge density from steady-state fluorescence measurements using pyrene as the fluorescent probe and cetylpyridin- ium chloride as the quenching agent. For the three alcohol cosolvents (1-butanol, 1-pentanol and I-hexanol) studied the authors observed that the surface charge density decreased with increasing alcohol mole fraction in the micellar phase. Cal- culated hydrophobic radius for the presumed spherical micelle, however, exhibited a minimum value in the 0.4-0.6 alcohol mole fraction region. Micellar size is governed by both the repulsive forces between charged head groups, which tend to increase the interfacial area per monomer, and by the attractive forces of the hydrocarbon-like core, which tend to decrease the interfacial area. Intro- duction of a polar alcohol cosolvent alters both the attractive and repulsive forces, hence, the un- predictable change in micellar size. Published mole-fraction based partition coefficients of K, = 190 for 1-pentanol, K, = 690 for 1-hexanol and K, = 1750 for I-heptanol in SDS solutions

S. Pandey et al. /Journal of Luminescence 71 (1997) 1X9-197 191

[21] show that a significant amount of alcohol respectively. The fluorescence spectra represent

cosolvent is incorporated into the micellar phase, a single scan which was then solvent blank cor-

even at relatively low alcohol concentrations. rected and verified by repetitive measurements.

2. Experimental

1-Pentanol (Aldrich, 99 + %), 1-hexanol (Al- drich, 99 + %), sodium dodecylsulfate (Aldrich, 98%) and cetyltrimethylammonium bromide (Al- drich) were used as received. The various surfactant + alcohol cosolvent solvent mixtures were prepared volumetrically from the neat alcohols and

micellar SDS and CTAB stock solutions of known concentrations. Synthetic references and/or com-

mercial suppliers for the PAH solutes contained in Tables l-3 are listed in our earlier papers (for a single source listing see Tucker [22]). PAH stock solutions were prepared by dissolving the solutes in

dichloromethane, and were stored in closed amber glass bottles in the dark to retard any photochemical reactions between the PAH solutes and dichloromethane solvent. Carbon tetrachloride and chloroform (to a much lesser extent) are reported to

react with polycyclic aromatic hydrocarbons uiu

a hypothesized concerted trans-annular addition with free radical formation [23-281. Small aliquots of each stock solutions were transferred into test

tubes. allowed to evaporate, and diluted with the micellar solvent media of interest. Solute concentrations were sufficiently dilute (10m6 molar) so as to prevent excimer formation. All solutions were

ultrasonicated, vortexed and allowed to equilibrate for a minimum of 24 h before any spectrofluoro- metric measurements were made. Experimental results were unaffected by longer equilibration times.

Emission intensities associated with the quenching measurements were corrected for primary inner-filtering artifacts and self-absorption arising from the absorption of excitation radiation by nit-

romethane and the PAH solute, respectively, ac- cording to the following expression [29-311:

which differs slightly from the approximate form

~321

fprim ~ 100’5A. (2)

In the above equations F”“” and Fobs refer to the corrected and observed fluorescence emission signal, respectively, A is the absorbance per centimeter

of pathlength at the excitation wavelength, and x and y denote distances from the boundaries of the interrogation zone to the excitation plane. Several

of the PAHs have excitation wavelengths in the 300-320 nm spectral region and a few drops of nitromethane gave solutions having appreciable

absorbances. Computational procedures and interrogation zone dimensions are discussed in greater detail elsewhere [3-5,33,34]. Every effort was made to work at solution absorbances of A cm-’ < 0.95 Ubrim < 3.0) where Eqs. (1) and (2) are valid. Secondary inner-filtering corrections were not necessary as nitromethane is ‘optically transparent’ in most of these PAH emission ranges.

Results and discussion

Absorption spectra were recorded on a Milton Roy Spectronic 1001 Plus and a Hewlett-Packard

8450A photodiode-array spectrophotometer in the usual manner. The fluorescence spectra were measured on a Shimadzu RF-SOOOU spectrofluorimeter with the detector set at high sensitivity. Solutions were excited and fluorescence measured at the wavelengths listed in Tables l-3. Fluorescence data were accumulated in a 1 cm* quartz cuvette at 23°C (ambient room temperature) with excitation and emission slit width settings of 15 and 3 nm,

Tables l-3 summarize our fluorescence quenching measurements for 6 alternant and 10 nonalter-

nant polycyclic aromatic hydrocarbons dissolved in micellar SDS + 1-pentanol, micellar SDS + l- hexanol and micellar CTAB + I-pentanol solvent media. Experimental results are reported as the stoichiometric mole fraction ratios of the surfactant and alcohol cosolvent, and the corresponding value of F,/F - 1 after the addition of nitromethane. Our spectrofluorometer is not capable of measuring fluorescence lifetimes (rrluor), and thus it is impossible for us to report separately the fluorescence lifetimes


and quenching rate constants (knuor) for the various polycyclic aromatic hydrocarbons. The values of Fe/F - 1 reported in Tables l-3 correspond to the

product of rrluor kfluor [Quencher] in the Stern-Vol- mer equation

where Fe and F refer to the observed emission intensities in the absence and presence of nitromethane, respectively, and [Quencher] is the molar concentration of nitromethane around the solubilized PAH fluorophore. As indicated in the table footnotes, numerical entries correspond to

Fe/F - 1 = rrluor kruor [QuencherI, (3) either 25 , 50 or 100 ul of neat quenching agent

Table 1 Summary of nitromethane quenching results for select altemant polycyclic aromatic hydrocarbons dissolved in various micellar SDS + I-pentanol and micellar CTAB + l-pentanol solvent media

Benzo[e]pyrene (360-480 nm)

Benzo[ghi]perylene (400-520 nm)

Chemical name/(Emission range) l_ SDS + 1-pentanol CTAB + 1-pentanolb

(nm) XSDSKYO” (F,/F) - lc*d &4El&J”

(F,/F) - lc*d

Coronene 334 l.O/O.O 11.5 (420-540 nm) 0.9JO.l 5.3

0.810.2 5.1 OS/O.5 4.6 0.210.8 1.9 0.1/0.9 0.8

335 1 .o/o.o 11.5 0.9/o. 1 11.5 0.810.2 10.1 OS/O.5 9.0 0.210.8 5.3 0.1/0.9 4.9

380 l.O/O.O 15.7 l.O/O.O 3.0 0.9lO.l 15.7 0.9lO.l 3.2 0.8/0.2 13.3 0.810.2 3.2 0.5/0.5 10.1 0.5/0.5 3.4 0.210.8 6.7 0.2/0.8 3.5 O.llO.9 4.6 0.1/0.9 3.2

403 1 .o/o.o 1.3 0.9/o. 1 1.4 0.810.2 1.3 0.5/0.5 1.1 0.2/0.8 0.7 0.1/0.9 0.6

Chrysene 320 l.O/O.O 13.3 (340-460 nm) 0.9/o. 1 10.1

0.8jO.2 9.0 0.5/0.5 9.0 0.2/0.8 5.3 O.liO.9 3.5

a The stoichiometric molarity of SDS plus 1-pentanol totalled 0.05 molar. bThe stoichiometric molarity of CTAB plus 1-pentanol totalled 0.05 molar. ‘Unless otherwise indicated, only 1 small 50 ul drop of ‘neat’ nitromethane was added to 10 ml of dissolved PAH solution. ‘F, refers to the corrected PAH emission intensity prior to nitromethane addition and F is the corrected emission intensity after nitromethane addition.

Perylene (420-540 nm)

Table 2

S. Pandey et al. J Journal of Luminescence 71 (1997, 189-197 193

Summary of nitromethane quenching results for select nonaiternant polycyclic aromatic hydrocarbons dissolved in various micellar SDS + I-pentanol and micellar CTAB + l-pentanol solvent media

Chemical Name/(Emission range) lex SDS + 1 pentanol” CTAB + 1-pentanolb

(nm) -GDS/XP,OH (F,/F) - 1C.d XCTABlXP,oH (F”/F) - 1C.d

Benz[defjindeno[1,2,3hi]chrysene (450- 600 nm)

Benzo[a]fluoranthene (4300600 nm)

Naphtho[1,2b&roranthene (37OO540 nm)

Naphtho[2, lalfluoranthene (430-570 nm)

Dibenzo[a,e]Auoranthene (44OO 570 nm)

Naphtho[2,3b]fluoranthene” (4000520 nm)

Benzo[k]fluoranthene’ (3900570 nm)

406

406

350

400

390

316

306

1 .o/o.o 0.6 1 .o/o.o 0.9/o. 1 0.6 0.4iO.l 0.8/0.2 0.6 0.8/0.2 0.5/0.5 0.4 OS/O.5 0.210.8 0.2 0.2/0.8 0.1/0.9 0.1 O.ljO.9

1 .o/o.o 0.3 l.OJO.0 0,9/o. 1 0.3 0.9/o. ’ O.S/O.Z 0.2 0.8/0.2 0.5/0.5 0.2 OS/O.5 0.2/0.8 0 0.2/0.8 O.ljO.9 0 O.ljO.9

l.OjO.0 2.1 0.9/o. 1 1.9 0.810.2 1.8 0.5J0.5 1.3 0.2/0.8 0.6 0.1/0.9 0.3

1 .o/o,o 0.9/o. 1 0.8,‘0.2 o.r/os 0.2JO.8 0.1/0.9

0.3 0.2 0.2 0.2 0 0

l.OjO.0 0.9/o. 1 0.8j’O.Z o.sio.5 0210.8 O.ljO.9

0.2 0.2 0.2 0.2 0.1 0

l.OjO.0 1.4 l.OjO.0 0.9/o. 1 1.4 0.9/o. 1 O.SiO.2 1.4 0.8fO.2 0.5/0.5 1.1 0.5/‘0.s

0.2jO.8 0.6 0.210.8 O.ljO.9 0.4 O.lfO.9

l.OjO.0 1.5 1 .o/o.o 0.9/o. 1 1.3 0.9/O. 1 0.8/0.2 1.2 0.8fO.2 O.S!O.S 1.0 0.5/0.5 t&2/0.8 0.6 0.2,JO.S 0.110.9 0.3 0.1/0.9

0 0 0 0 0 0

0

0

0 0

0

0

0.1 0.1 0.1 0.1 0.1 0.2

0.2 0.2 0.2 0.1 0.2 0.2

“The stoichiometric molarity of SDS plus 1-pentanol totaled 0.05 molar. bThe stoichiometric molarity of CTAB plus 1-pentanol totaled 0.05 molar. ’ Unless otherwise indicated, only 1 small 50 ul drop of ‘neat’ nitromethane was added to 10 ml of dissolved PAH solution. d F, refers to the corrected PAH emission intensity prior to nitromethane addition and F is the corrected emission intensity after nitromethane addition. e Because of the large primary inner-filtering correction only 1 smali 25 ut drop of ‘neat’ nitromethane was added to 10 ml of dissolved PAH solution.

194 S. Pandey et al. 1 Journal of Luminescence 71 (1997) 189-197

Table 3 Summary of nitromethane quenching results for select alternant and nonalternant polycychc aromatic hydrocarbons dissolved in various micellar SDS + 1-hexanol solvent media

Chemical name/(emission range) I ex SDS + 1-hexanol”

(nm) xSDS/X”xOH (F,/F) - lb.E

Benzo[ghi]perylened (400-520 nm)

Perylened (420-540 nm)

Benzo[e]pyrened (360-480 nm)

Coronened (420-540 nm)

380 1 .o/o.o 13.3 0.9/o. 1 13.3 0.710.3 11.5 0.5/0.5 9.0 0.310.7 7.3

403 l.O/O.O 0.9/o. 1 0.7/0.3 0.5/0.5 0.310.7

1.4 1.4 1.3 1.1 0.9

335 l.O/O.O 10.1 0.9/o. 1 10.1 0.7/0.3 9.0 0.5/0.5 9.0 0.3/0.7 6.7

334 l.O/O.O 6.7 0.9/o. 1 6.1 0.7JO.3 5.6 0.5jo.5 4.0 0.310.7 2.7

Alternant Polycyclic Aromatic Hydrocarbons (PAH6 Benzenoids)

Pyrened 338 1 .o/o.o (360-480 nm) 0.9/o. 1

0.7lO.3 0.510.5 0.3/0.7

Nonalternant Fluoranthenoids and Fluorenoids Benz[defJindeno[1,2,3hi]chrysene

(450-600 nm)

Benzo[a]fluoranthene (430-600 nm)

Naphtho[2,lk]benzo[ghi]fluoranthene (390-550 nm)

Naphtho[1,2k]benzo[ghi]fluoranthene (400-550 nm)

406

406

368

366

l.O/O.O 0.9/o. 1 0.710.3 0.5/0.5 0.310.7

l.O/O.O 0.9/o. 1 0.710.3 0.5/0.5 0.310.7

LO/O.0 0.9/o. 1 0.710.3 0.5/0.5 0.3lO.7

l.O/O.O 0.9/o. 1 0.710.3 0.5/0.5 0.310.7

49.0

49.0

49.0

24.0 9.0

0.7 0.6 0.5 0.4 0.2

0.3 0.3 0.2 0.1 0.1

0.9 0.8 0.6 0.4 0.2

0.8 0.7 0.6 0.4 0.2

S. Pandey et al. / Journal of Luminescence 71 (1997) 189- 197 195

Table 3 Continued

Chemical name/(Emission range) I ei SDS + l-hexanol”

Mm) xsnJx”xo” (F,/F) - 1 b,c

Naphtho[2,la]fluoranthene

(430-570 nm)

Naphtho[ I, Zblfluoranthene

(370-540 nm)

Benzo[b]fluoranthene

(370-570 nm)

Benzo[k]fluoranthened

(390-570 nm)

Naphtho[Z, 3b]fluoranthened

(400-520 nm)

400 l.OjO.0

0.9/o. 1

0.710.3

0.510.5

0.310.7

0.3

0.4

0.2

0.1

0.1

350 1 .o/o.o 2.2

0.9/o. 1 2.1

0.710.3 1.8

0.5/0.5 1.6

0.3/0.7 1.1

346 l.OjO.0 4.3

0.9/o. 1 4.0

0.710.3 3.2

0.510.5 2.6

0.310.7 1.6

306 1 .o/o.o 1.4

0.9/o. 1 1.4

0.710.3 1.2

0.510.5 1.0

0.310.7 0.7

316 1 .o/o.o 1.6

0.9/o. 1 1.5

0.7lO.3 1.4

0.5jo.5 1.2

0.3/0.7 0.8

Dibenzo[a,e]fluoranthene’ 390 1 .o/o.o

(440-570 nm) 0.9/o. 1

0.710.3

0.5,‘0.5

0.310.7

0.4

0.3

0.3

0.2

0.1

a The stoichiometric molarity of SDS plus 1-hexanol totaled 0.02 molar.

b Unless otherwise indicated, only 1 small 50 ul drop of ‘neat’ nitromethane was added to 10 ml of dissolved PAH solution.

‘F, refers to the corrected PAH emission intensity prior to nitromethane addition and F is the corrected emission intensity after

nitromethane addition.

d Because of the large primary inner-filtering correction only 1 small 25 ul drop of ‘neat’ nitromethane was added to 10 ml of dissolved

nonalternant PAH solution. For alternant PAHs 25 ul of nitromethane was sufficient to give a sizeable decrease in emission intensity.

e A 100 ul drop of ‘neat’ nitromethane was added to 10 ml of dissolved PAH solution.

added to circa 10 ml of solution. All emission inten- of 0.0270 g obtained by weighing 25 individual sities used in the computations were corrected for Eppendorf pipette droplets of nitromethane, which primary inner-filtering and solute self-absorption ranged in size from 0.0267-0.0275 g. Uncertainties as discussed in the last paragraph of Section 2. in the percent reductions [i.e., 100 - lOO(F/F,)] are Stoichiometric nitromethane molar concentrations believed to be f 2% (or better) based upon repli- are estimated as follows: 0.044 M (25 ul), 0.088 M cate measurements for select solutes. Lack of (50 ~1) and 0.176 M (100 ~1). Molar concentrations micelle/water partition coefficient data prevents the are calculated using an average droplet mass computation of [Quencher] in the micelle from the


stoichiometric molar concentrations of nitromethane. Solutions do contain identical stoichiometric molar concentrations of nitromethane, and the F,,/F - 1 entries given in Tables l-3 do permit relative, semi-quantitative comparisons regarding the ability of nitromethane to quench the fluorescence emission of the various alternant and nonalternant PAH fluorophores.

Careful examination of Tables l-3 reveals that nitromethane quenches the fluorescence emission of all 6 alternant PAH6 benzenoids studied, in accordance with the nitromethane selective quenching rule. Percent reduction in the emission signal is greater in the two micellar SDS + l-al- kanol solvent media than in the micellar CTAB + 1-pentanol solvent media. At increasing alcohol mole fraction concentrations nitromethane’s effectiveness in quenching fluorescence emission is reduced, particularly in solutions con- taining the anionic sodium dodecylsulfate surfactant. Calculated I/III band emission intensities of I/III = 1.03 (Xsns = l.O), I/III = 1.03 (Xsns = 0.9), I/III = 0.99 (Xsns = 0.7), I/III = 0.95 (Xsns = 0.5) and I/III = 0.89 (Xsns = 0.3) for the solvent polarity probe pyrene molecule at different SDS/l-hexa- no1 mole fraction ratios indicate that the molecular environment around the PAH molecule is moder- ately polar, and that the environment is becoming less polar with increasing alcohol cosolvent concentration. A much smaller ratio of I/III z 0.58 for pyrene [35] would be expected if the fluorophore were solubilized in an entirely hydrocarbon-like micellar region. Similar trends were observed in the experimental I/III band intensity ratios for the probe molecules coronene and benzo[ghi]perylene in the three surfactant + alcohol cosolvent systems investigated. The above argu- ment seems reasonable as a search of the published chemical literature [36-421 reveals that most authors conclude that the site of solubilization of aromatic probes is close to the micelle surface in the so-called palisade layer.

More interesting quenching behavior is observed in the case of the 10 nonalternant fluoranthenoid and fluorenoid derivatives. Two of the nonalternant PAHs, benzo[k]fluoranthene and naphtho[2,3b]fluoranthene, were quenched by nitromethane in micellar CTAB + 1-pentanol solvent

media. Observed reduction in emission intensities [i.e., 100 (F, - F)/F,] ranged between 7-20% depending upon the stoichiometric CTAB/l-pentanol mole fraction ratio. Both compounds have been previously [3,6,43] noted to be exceptions to the nitromethane selective quenching rule, and their behavior here is not totally unexpected. What is unexpected, however, is the large number of nonalternant PAHs that are quenched in the SDS + 1-pentanol and SDS + 1-hexanol solvent media, even at the higher alcohol mole fractions. Readers should note that nitromethane’s effectiveness in quenching fluorescence emission of nonalternant PAHs decreases significantly with increasing alcohol concentration.

We attribute this loss of quenching effectiveness to the reduced micellar surface charge density that occurs with increased alcohol cosolvent incorpor- ation into the micelle. Secondary effects might inc- lude changes in solvent polarity, dielectric constant, micellar size and/or water content in the polar region of the micelle. The established quenching mechanism [7,15,16] involves an electron/charge transfer reaction whereby an electron is transferred from the excited PAH fluorophore to nitromethane, which acts as an electron acceptor. From simple electrostatic considerations, the developing positive charge on the PAH would be stabilized to a much lesser extent by the smaller surface charge density. Varela et al. [ZO] observed that the micellar surface charge density in SDS + 1-butanol, SDS + I-pentanol and SDS + 1-hexanol solvent media decreased linearly with increasing alcohol mole fraction in the micellar phase. Examination of Table 2 reveals that, although not linear with alcohol cosolvent mole fraction, the values of Fe/F - 1 do follow a similar trend. The change in micelle size cannot account for the significant reduction in nitromethane quenching effectiveness because the hydrophobic radius exhibits a minimum value in the 0.4-0.6 micellar alcohol mole fraction region for these three surfactant + alcohol cosolvent systems. If micellar size were the only factor operable, then one would expect to see an increased nitromethane quenching effectiveness as the PAH and quencher molecules are brought closer together in the smaller micelles.

S. Pandey et al. J Journal of Luminescence 71 (1997) 189-197 191

Acknowledgements Cl91

This work was supported in part by the University of North Texas Research Council.

References

[1] G.-P. Bltimer and M. Zander, Fresenius Z. Anal. Chem.

296 (1979) 409.

[2] F.K. Ogasawara, Y. Wang and V.L. McGuffin, Appl. Spec-

trosc. 49 (1995).

[3] S.A. Tucker, W.E. Acree, Jr., B.P. Cho, R.G. Harvey and

J.C. Fe&r, Appl. Spectrosc. 45 (1991) 1699.

[4] V.L. Amszi, Y. Cordero, B. Smith, S.A. Tucker, W.E. Acree,

Jr., C. Yang, E. Abu-Shaqara and R.G. Harvey, Appl.

Spectrosc. 46 (1992) 1156.

[S] S.A. Tucker, H. Darmodjo, W.E. Acree, Jr., J.C. Fetzer and

M. Zander, Appl. Spectrosc. 46 (1992) 1260.

[6] H. Dreeskamp, E. Koch and M. Zander, Z. Naturforsch.

30A (1975) 1311.

[7] U. Breymann, H. Dreeskamp, E. Koch and M. Zander,

Chem. Phys. Lett. 59 (1978) 68.

[S] S.H. Chen, C.E. Evans and V.L. McGuffin, Anal. Chim.

Acta 246 (1991) 65.

[9] S.A. Tucker, H.C. Bates, V.L. Amszi, W.E. Acree, Jr.,

H. Lee, P.D. Raddo, R.G. Harvey, J.C. Fetzer and G.

Dyker, Anal. Chim. Acta 278 (1993) 269.

[lo] S.A. Tucker, J.M. Griffin, W.E.Acree, Jr., P.P.J. Mulder, J.

Lugtenburg and J. Cornelisse, Analyst 119 (1994) 2129.

[11] P.P.J. Mulder, B.B. Boere, A. Baart, J. Cornelisse and

J. Lugtenburg, Reel. Trav. Chim. Pays-Bas 112 (1993) 22.

[12] R. Sangaiah, A. Gold and G.E. Toney, J. Org. Chem. 48

(1983) 1632.

[13] P.P.J. Mulder, J. Olde Boerrigter, B.B. Boere, H. Zuihof, C.

Erkelens, J. Cornelisse and J. Lugtenburg, Reel. Trav.

Chim. Pays-Bas 112 (1993) 287.

[14] A.W.H. Jans, C. Tintel, J. Cornelisse and J. Lugtenburg,

Magn. Reson. Chem. 24 (1986) 101.

[15] M. Zander, U. Breymann, H. Dreeskamp and E. Koch,

Z. Naturforsch. 32A (1977) 1561.

[16] J. Malkin, Photophysical and Photochemical Properties of

Polycyclic Aromatic Compounds (CRC Press, Baco

Raton, FL, 1992) pp. 19-23.

[17] S. Pandey, W.E. Acree, Jr. and J.C. Fetzer, Anal. Chim.

Acta 324 (1996) 175.

[18] S. Pandey, J.R. Powell, W.E. Acree, Jr., B.P. Cho, J. Kum,

C.Yang and R.G. Harvey, Polycyclic Aromat. Compds., in

press.

cm

c-211

WI

~231

~241

cl51

IX1

c271

WI

P91 c301

c311

~321

c331

c341

c351

C361

c371

C381

c391

c401

c411

c421

c431

S. Pandey, K.A. Fletcher, J.R. Powell, M.E.R. McHale,

A.-S. Kauppila, W.E. Acree, Jr., J.C. Fetzer, W. Dai and

R.G. Harvey, Spectrochim. Acta, in press.

A. Suarez Varela, M.I. Sandez Macho and A. Gil Gon-

zalez, Colloid Polym. Sci. 273 (1995) 876.

H. Hoiland, E. Ljosland and S. Backlund, J. Colloid Inter-

face Sci. 101 (1984) 467.

S.A. Tucker, Ph.D. Dissertation. University of North

Texas, Denton, Texas (1994).

N. Selvarajan, N.M. Panicker, S. Vaidyanathan and V.

Ramakrishnan, Indian J. Chem. 18A (1979) 23.

M.V. Encinas, M.A. Rubio and E.A. Lissi, Photochem.

Photobiol. 37 (1983) 125.

M.V. Encinas, M.A. Rubio and E.A. Lissi, J. Photochem.

18 (1982) 137.

W.M. Wiczk and T. Latowski, Z. Naturforsch. A 42 (1987)

1290.

E.J. Bowen and K.K. Rohatgi, Disc. Faraday Sot. 14

(1953) 146.

S.A. Tucker, L.E. Cretella, R. Waris, K.W. Street, Jr., W.E.

Acree, Jr. and J.C. Fetzer, Appl. Spectrosc. 44 (1990) 269.

CA. Parker and W.J. Barnes, Analyst 82 (1957) 606.

J.F. Holland, R.E. Teets, P.M. Kelly and A. Timnick, Anal.

Chem. 49 (1977) 706.

M.C. Yappert and J.D. Ingle. Appl. Spectrosc. 43 (1989)

759.

J.R. Lakowicz, Principles of Fluorescence Spectroscopy

(Plenum Press. New York, 1983).

S.A. Tucker, W.E. Acree, Jr., J.C. Fetzer and J. Jacob,

Polycyclic Aromat. Compounds. 3 (1992) 1.

S.A. Tucker, V.L. Amszi and W.E. Acree. Jr., J. Chem.

Educ. 69 (1992) A8.

D.C. Dong and M.A. Winnik, Can. J. Chem. 62 (1984)

2560.

J.K. Thomas, Chem. Rev. 80 (1980) 283 and references

therein.

J.K. Thomas, Act. Chem. Res. 10 (1977) 133 and references

therein.

P. Mukerjee and J.R. Cardinal, J. Phys. Chem. 82 (1978)

1620.

R.A. Pyter, C. Ramachandran and P. Mukerjee, J. Phys.

Chem. 86 (1982) 3206.

C. Ramachandran, P. Pyter and P. Mukerjee, J. Phys.

Chem. 86 (1982) 3198.

M. Almgren, F. Grieser and J.K. Thomas, J. Am. Chem.

sot. 101 (1979) 279.

P. Lianos, M.-L. Viriot and R. Zana, J. Phys. Chem. 88

(1984) 1098.

S.A. Tucker, H.C. Bates, W.E. Acree, Jr. and J.C. Fetzer,

Appl. Spcctrosc. 47 (1993) 1775.

Documents

Spectroscopic investigations in molecularly organized solvent media. Part 2: Examination of the nitromethane selective quenching rule at different ‘effective’ micellar surface