Micellar microstructure of linear and guerbet micelles: Fluorescence probe study

Micellar Microstructure of Linear and Guerbet Micelles: Fluorescence Probe Study RAMESH VARADARAJ, 1 PAUL VALINT, 2 JAN BOCK, STEPHEN ZUSHMA, AND NElL BRONS Exxon Research and Engineering Company, CorporateResearch Science Laboratories, Route 22 East, Annandale, New Jersey 08801

ReceivedJuly 2, 1990;acceptedNovember29, 1990 Pyrene, 1-decylpyrene,and 1,10-bis(1-pyrene)decanewereusedas fluorescenceprobesto differentiate the microstructureof micellesformedfrom linearand Guerbetsurfactants.The pyreneprobesexperience a higher micropolarityand microfluidityin a Guerbet micelle compared with the linear counterpart. © 1991 Academic Press, Inc. INTRODUCTION

Guerbet surfactants are a unique class of amphiphiles in which the carbon atom beta to the head group carries an alkyl chain (Chart 1 ). The " Y " branch that characterizes the Guerbet hydrophobe imparts unique interfacial and micellar properties to surfactants derived from Guerbet alcohols (1, 2). Micelles formed from such Guerbet surfactants are of interest since the branched hydrophobe results in a micellar core that is expected to be more porous compared with that of the linear counterpart. In order to assess the effect of Guerbet branching on micellar properties three ethoxysulfate surfactants were studied, viz., CjrLEsS (linear), C16LGEsS (linear Guerbet), and C16BGEsS (branched Guerbet). This study concerns the use of fluorescence techniques to probe and differentiate the micellar microstructure (micropolarity and microviscosity/ microfluidity) of Guerbet surfactants from linear surfactants, The "microviscosity" or "microfluidity" addressed in this paper and many others reported in the literature on fluorescence techTo whom correspondence should be addressed. 2 Present address: Bausch & Lomb, Inc., 1400 North Goodman Street, Rochester, NY 14692.

niques is really a measure of the extent of hindrance experienced by a solubilizate molecule for translation motion within the micelle ( 3 5). "Micropolarity" is the effective polarity experienced by the solubilizate in the micelle and is a reflection of the porosity or extent of water penetration into the hydrocarbon portion of the micellar aggregate. Both the micropolarity and the microfluidity that a solubilizate experiences in a micelle are important in influencing its reactivity. Hence, such information on new and novel surfactants is critical for the choice of surfactants in micellar catalysis. Structural features of the micelle, viz., the nature of hydrocarbon chain packing at the probe solubilization site that influences these microscopic properties of the micelles, are examined in this communication. EXPERIMENTAL

Steady-state fluorescence and fluorescence lifetime experiments were conducted using pyrene (P), 1-decylpyrene ( D P ) , and 1,10bis (1-pyrene) decane (BP) fluorescence probes purchased from Polo Lambda. The C16 linear (C16LEsS) and Guerbet (C16LGEsS and C16BGEsS) ethoxysulfates were synthesized from the corresponding alcohols by procedures

340 0021-9797/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

LINEAR AND GUERBET SURFACTANT MICELLES

~

X LINEAR

U

LINEAR

ClsLEsS

X ClsLGEsS ~ X

GUERBET

C'~sBG EsS

BRANCHED GUERBE T

(CHART 1 )

reported earlier ( 1 ). Chart 1 depicts the structure of the linear and Guerbet surfactants. Micellar solutions were made in degassed water by stirring together surfactant and the corresponding pyrene probe. Solutions sealed in fluorescence cuvettes under nitrogen were used for all experiments. For all three surfactants, the surfactant concentration was above the critical micelle concentration (CMC) (0.01 M ) and probe concentration 5 × 10-6 M. At these concentrations the probe-to-micelle ratio was calculated to be about 0.022. These concentrations were deliberately chosen to avoid multiple occupancy of the micelle. Steady-state fluorescence spectra were measured on a Model 650-40 Perkin-Elmer spectrofluorometer. A Hitachi Model 650-0265 ordinate data processor attachment was used for correction of emission spectra. The excitation and emission bandwidths were 5 and 2 nm, respectively. A PRA System 9000 fluorescence lifetime instrument was used to determine fluorescence lifetimes using procedures reported in the literature (6). In all cases the excitation wavelength was 335 nm. Fluorescence lifetimes for DP and BP in micellar solutions were monitored at 375 nm, the m o n o m e r emission wavelength. Micellar aggregation numbers were obtained by the reported quenching of pyrene fluorescence by the cetylpyridinium chloride method (7). The critical micelle concentrations for the linear and Guerbet surfactants were determined by tensiometric techniques described earlier ( 1 ). RESULTS AND DISCUSSION The C M C and mean aggregation n u m b e r for the micellar aggregates formed from linear

341

and Guerbet surfactants at 25°C in water are shown in Table I. Branching of the hydrocarbon chain increases the CMC. This is not unexpected since a branched hydrophobe offers greater steric hindrance to micellization than a linear one (1). However, the mean aggregation n u m b e r for the linear and branched surfactants does not vary very much. Based on the hydrophobe structure and aggregation number it appears that the Guerbet surfactants would tend to form oblate or rodlike aggregates rather than spherical micelles. The C M C and aggregation n u m b e r data together suggest that the micellar microstructure of Guerbet surfactants could be different from that formed from a comparable linear hydrophobe surfactant. In order to understand the differences in micellar microstructure between the linear and Guerbet micelles emission probes, pyrene, 1dodecylpyrene, and 1,10-bis (1-pyrene) decane were used. Results of the fluorescence study using these probes are tabulated in Table I. The fluorescence fine structure of pyrene and certain substituted pyrene derivatives has been used to obtain information on the micropolarity of micelles (6-10). Figure 1 is a typical spectrum of pyrene in aqueous 0.01 M sodium dodecyl sulfate solution. The five vibronic bands are denoted 1 through 5. Band 1 is very sensitive to solvent polarity, whereas band 3 displays minimal intensity variation

TABLE I Micellar Properties and Emission Characteristics of Pyrene (P), 1-Decylpyrene(DP), and 1,10-Bis(1-pyrene) decane (BP) in Linear and Guerbet Micelles Property

CI~LE~S

I~/I3 (P) IJ13 (DP)

1.10 2.18 101 0.42 117

1.18 2.52 81 0.73 57

1.20 2.70 61 1.08 56

43 _+4

45 _+ 1

39 _+ 1

0.86

1.85

~-(DP) (ns) Idlr, (BP)

r (BP) (ns) Aggregation number CMC ( 10-4) (mol/dm3)

0.25

CIrLGEsS

CjrBGEsS

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VARADARAJ ET AL. t

i

I

1.0

0.5

tv

/

0.0

J 340

i 390

I 440

Wavelength ( n m )

FIG. 1. Corrected emission spectrum of pyrene solubilized in 0.01 M SDS solution. with polarity. Hence, the ratio of fluorescence intensities of the first and third emission bands, i.e., 1]/13, is a sensitive monitor of the environment of pyrene. This unique photophysical feature has been utilized to determine the micropolarity of microheterogeneous systems like micellar aggregates. The I1/I3 value decreases with an increase in the hydrophobic environment that the pyrene probe experiences. With alkyl pyrenes, because of the loss of molecular symmetry, a partial loss of fine structure results. The emission spectra of DP in C16LGE5 micelles and n-octane solvent are shown in Figs. 2a and 2b, respectively. Although the second and third emission bands are not as distant as in pyrene it is still possible to detect and obtain 11/I3 values. As with pyrene, 11/13 in DP measures the micropolarity of the micellar aggregate that the probe experiences. Journal of Colloid and Interface Science, Vol. 144, No. 2, July 1991

The 11/13 ratio for pyrene increases from the linear to the Guerbet systems, indicating that the micropolarity of the Guerbet surfactants is higher than that of the linear counterpart. Increased water penetration due to the porosity of Guerbet micelles leads to this feature. I i / / 3 for DP is also observed to increase with Guerbet branching. The increase is more marked in l-decylpyrene than in pyrene. The fluorescence lifetime results for DP are in agreement with the I]/I3 observation. With increased micropolarity or water penetration the excited-state lifetime of the pyrene probe is expected to decrease. In the linear system the lifetime is 101 ns; this decreases to 81 and 61 ns in the linear Guerbet and branched Guerbet systems, respectively. The lifetime of 101 ns for DP reported here is comparable with a value of 139 ns reported by Johansson and Lindblom for 1-dodecylpyrene in lyotropic liquid crystalline aggregates ( 11 ). Turro and co-workers have shown that bichromophoric fluorescent probes can be used to monitor the microfluidity of micellar aggregates via a measure of the ratio of monomer to intramolecular excimer emission intensity (5). Lower Ie/Im values indicate higher microfluidity (4, 6, 8). Steady-state emission spectra for 1,10I

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I

I

a

E

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b

1.0

1.0

5

o= = O.S

5

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0.0 I

I

I

340

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3 0

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FIG. 2. Correctedemission spectra of 1-decylpyrenein (a) 0.01 M C~6LGEsSsolution and (b) n-octane solvent.

343

LINEAR AND GUERBET SURFACTANT MICELLES I

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~

f

l

r

i

f

1.0

c ~

0.5

0.5

°°1 0.0

-

r- ~

340

r

440 Wavelength

540

040

(nm)

FIG. 3. Corrected emission spectrum of 5 X 10-6 M BP solubilized in 0.01 M C~LEsS surfactant solution.

bis( 1-pyrene)decane in C16LEsS, C16LGEsS, and C16BGEsS are shown in Figs. 3, 4, and 5, respectively. These spectra were recorded on very dilute solutions of BP. Since the concentration of BP (5 X 10-6 M ) and the mean aggregation number and concentration ofsurfactants (0.01 M ) are known, assuming Poisson distribution for the BP probe, the probeto-micelle ratio is about 0.022. Multiple occupancy of a micelle is avoided and the broad structureless emission at about 460 nm corr

]

I

r 340

I I 440 540 Wavelength (nrn)

640

FIG. 5. Corrected emission spectrum of 5 X 10 -~ M BP solubilized in 0.01 M C16BGEsS surfactant solution.

responds to the intramolecular excimer emission. (In order to show that the emission spectra in Figs. 3, 4, and 5 are not due to aggregated probe molecules an emission spectrum of BP at 5 X 10-4 M BP concentration and 0.001 M surfactant concentration was run (Fig. 6). Under these conditions the probe-to-micelle ratio is about 4.5 and represents a condition of multiple occupancy. The spectrum shows an almost complete loss of monomer peaks and a broad emission peak. This spectrum is similar to 16-( 1-pyrenyl)hexanoic acid aggre-

r

I

I

I

r

J

I

1.0

-fi

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19

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?: 0.0

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Wavelength (nm)

FIG. 4. Corrected emission spectrum of 5 X l0 -6 M B P solubilized in 0.01 M C~6LGEsS surfactant solution.

0.0

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340

440 540 Wavelength (nm)

I

640

F I G . 6. C o r r e c t e d e m i s s i o n s p e c t r u m o f 5 × 1 0 - 4 M B P i n 0.001 M C ~ 6 L G E s S s u r f a c t a n t s o l u t i o n .

Journa! of Colloid and Interface Science, Vol. 144, No. 2, July 1991

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VARADARAJ ET AL.

gate spectra reported by L'Heureux and Fragata and is representative of BP aggregate spectra 9b. The ratio of emission intensities at 460 and 327 nm, i.e., Ie/Im, is higher for the Guerbet surfactants than for the linear countel-part, indicating that the two pyrene moieties of BP in Guerbet micelles can easily approach each other and form intramolecular excimers. This suggests increased microfluidity for the Guerbet micelles over the linear counterpart. Fluorescence quenching via intramolecular excimer formation introduces an additional excited-state deactivation pathway, and a decrease in the fluorescence lifetime of the monomer emission is expected. If the microfluidity of the micelle is high, excited pyrene deactivation via intramolecular excimer formation is facile and the observed lifetime is expected to be low. On the other hand, if micellar microfluidity is low, the excited-state lifetime of pyrene in 1,10-bis( 1-pyrene)decane would be high. Measurement of monomer excited-state lifetimes of pyrene in BP confirms the Ie/Im result. The excited-state lifetime of pyrene in BP decreases drastically from 117 to 56 ns upon Guerbet branching, indicative of increased microfluidity. The increase in microfluidity for the Guerbet systems is expected based on the highly branched hydrophobe structure. CONCLUSIONS

A combination of steady-state fluorescence and fluorescence lifetime techniques using carefully selected probe molecules can provide

Journalof Colloidand InterfaceScience,Vol.144,No. 2, July1991

useful information on the microstructure of micelles formed from surfactants with unique structural features. The fluorescence probe study discussed in this communication reveals that a Guerbet hydrophobe provides a micellar environment in which a solubilized guest molecule experiences an effective polarity and fuidity that is higher than what it would be if the molecule were in a micelle formed from a conventional linear hydrophobe. The Guerbet micelle may therefore provide a site for micellar catalysis that is quite different from a conventional micelle. REFERENCES 1. Varadaraj, R., Bock, J., Valint, P., Zushma, S., and Thomas, R., J. Phys. Chem. 95, 167l (1991). 2. Varadaraj, R., Schaffer, H., Bock, J., and Valint, P., Langmuir 6, 1372 (1990). 3. Zachariasse, K. A., Chem. Phys. Lett. 57, 429 (1978). 4. Emmert, J,, Behrena, C., and Goldenberg, M., ar. Am. Chem. Soc. 101, 771 (1979). 5. Turro, N. J., Aikawa, M., and Yekta, A., J. Am. Chem. Soc. 101, 772 (1979). 6. (a) Char, K., Gast, A. P., and Frank, C. W., Langmuir 4, 989 (1988); (b) Malliaris, A., and Paleos, C. M., J. Phys. Chem. 91, 1149 (1987). 7. Kalyanasundaram, K., "Photochemistry in Microheterogeneous Systems." Academic Press, London, 1987. 8. Chandar, P., Somasundaran, P., and Turro, N. J., J. Colloid Interface Sci. 117, 31 (1987). 9. (a)L'Heureux, G. P., and Fragata, M., ar. Colloid Interface Sci. 117, 513 (1987); (b) L'Heureux, G. P., and Fragata, J., Photochem. Photobiol. 3, 53 (1989). 10. Kalyanasundaram, K., Langmuir 4, 942 (1988). 11. Johansson, L. B., and Lindblom, G., Liq. Cryst. 1, 53

(1986).