Physical Chemistry - American Chemical Society

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The Journal of

Registered i n

Physical Chemistry

US.P a t e n t Office 0 Copyright, 1981, by the American Chemical Society VOLUME 85, NUMBER 13

J U N E 25, 1981

LETTERS Charge-Transfer Complexation in Micellar Solutions. Water Penetrability of Micelles F. M. Martens and J. W. Verhoeven’ Laboratow for Organic Chemistw, University of Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Netherlands (Received: February 5, 198 1)

In anionic sodium dodecyl sulfate (SDS) micellar solutions a dramatic enhancement is observed in the degree of ground-state electron donor-acceptor (EDA) complexation between some hydrophobic electron donors and the hydrophilic electron acceptor N,N’-dimethyL4,4’-bipyridinium dichloride (paraquat, pq2+),when compared to the behavior of these EDA systems in homogeneous solutions or in cationic cetyltrimethylammonium bromide (CTAB) micellar solution. The occurrence of direct contact between the hydrophobic donor and the hydrophilic acceptor molecules, which is evident from the charge-transfer absorption of the EDA complexes formed, favors an open micellar structure such as provided by the porous cluster model recently advanced by Menger. Furthermore, the consequences of the enhanced ground-state complexation of the mechanism of quenching of fluorescent probe molecules are discussed. Thus while, for instance, the quenching of pyrene fluorescence by pq2+has formerly been discussed on the basis of a dynamic mechanism only, it is now demonstrated that static and dynamic mechanisms contribute almost equally to the overall fluorescence quenching observed in SDS solutions.

Introduction A~~~~ the most important properties of micellar sysare their ability to solubilize a variety of molecules insoluble in bulk (e.g., aqueous) solution and their substantial catalytic effect on many reactions.’ that howledge of the It needs no of the micelles and of the local microenvironment, of the local concentration, and of the relative orientation of the

solubilized molecules is of fundamental importance in understanding the nature of this solubilization and the physical and chemical behavior of the solubilized species.* Moreover, such data are relevant to biological sciences since reactions in micellar aggregates are popular (simple) model systems for chemical processes occurring at interfaces in the living cell. The classical picture of a micelle, first clearly stated by Hartley in the 1930’s; is an essentially spherical aggregate

(1) For recent reviews see (a) J. H. Fendler and E. J. Fendler, “Catalysis in Micellar and Macromolecular Systems”, Academic Press, New York, 1975. (b) K. L. Mittal, “Micellization, Solubilization and Microemulsions”, Vol. 1 and 2, Plenum Press, New York, 1977. (c) E. J. R. SudhBlter, G. B. van de Langkruis, and J. B. F. N. Engberts, Recl. Trau. Chim. Pays-Bas, 99, 73 (1980). (d) P. Mukerjee in “Solution Chemistry of Surfactants”, K. L. Mittal, Ed., Plenum Press, New York, 1979, pp 153-174.

(2) (a) P. Mukerjee, J. R. Cardinal, and N. S. Desai in ref lb, Vol. 1, J. Phys. Chem., 82, 1620 (1978). (3) (a) G. S. Hartley, Trans. Faraday SOC.,31, 31 (1935). (b) G. S. Hartley, “Aqueous Solutions of Paraffin Chain Salts”, Hermann, Paris, 1936. (c) For a recent review see H. Wennerstrom and B. Lindman, Phys. Rep., 62,1(1979). (d) H. Wennerstrom and B. Lindman, J.Phys. Chem., 83, 2931 (1979). p 241. (b) P. Mukerjee and J. R. Cardinal,

0022-3654/81/2085-1773$01.25/00 1981 American Chemical Society

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The Journal of Physical Chemistry, Vol. 85,No. 13, 1981

of a number of surfactant molecules (typically on the order of 30-150) to minimize contact between the apolar chain and water. In this model a well-defined shell (Stern layer) separates the apolar paraffinic inner core from the polar aqueous bulk solution. This two-state model has recently been questioned by Menger,4 who prefers to describe micelles in terms of porous clusters with a roughly defined Stern layer (Stern “region”) and in which water is able to penetrate deeply into the aggregates. As argued especially by Wennerstrom and Lindman,3d however, the average hydration numbers of micellar aggregates mitigate against extensive water penetrability. The present controversy concerning micellar structure is rooted mainly in the inconclusive results obtained from studies on the location and orientation of the solubilized species1c*2and on the micellar microenvironment^^*^ of solubilized spectroscopic sensors. The experimental techniques used in the determination of the site of solubilization have been summarized in ref la. Among these are the use of luminescent probe molecules (allowing studies in a very large dynamic range, 10-1-10-12s ) and ~ UV absorption spectroscopy. The latter is usually applied to determine the polarity of the microand the shape of environment by comparison of the ,A, the spectrum of molecules solubilized in the micelle with those in homogeneous solvent media.“” In this communication we report experimental findings which not only lead to a direct estimate of the relative orientation of highly hydrophilic and hydrophobic molecules solubilized in the same micellar medium, but which also bear consequences for the interpretation of data obtained by the use of fluorescent probe molecules, particularly where fluorescence intensity measurements correlated with kinetic parameters of, e.g., quenchers are involved.

Results and Discussion Absorption Spectroscopy. During our investigation of photoredox processes between 1,4-dihydropyridines as dielectron donors and N,N’-dimethyl-4,4’-bipyridinium chloride (paraquat, pq2+) as an electron acceptor12 we observed that these molecules form relatively weak ground-state complexes of the electron donor-acceptor (EDA) type, characterized by a broad long wavelength (-400-500 nm) charge-transfer (CT) absorption band.12 We now unexpectedly found that the degree of ground-state association between the hydrophobic 1benzyl-l,4-dihydronicotinamide(BNAH) and the hydrophilic pq2+is strongly enhanced in sodium dodecyl sulfate (SDS) anionic micellar solutions when compared to cetyltrimethylammonium bromide (CTAB) cationic micellar or homogeneous methanol solution^.'^ (4)(a) F. M. Menger, Acc. Chem. Res., 12, 111 (1979); (b) F. M. 102,5936 (1980). Menger and B. J. Boyer, J . Am. Chem. SOC., (5)N.J. Turro, M. W. Geiger, R. R. Mantala, and N. E. Schore in ref lb, Vol. 1, p 75. (6)M. Gratzel and J. K. Thomas in “Modern Fluorescence Spectrscopy”, E. L. Wehry, Ed., Plenum Press, New York, 1976. (7) J. K. Thomas, Acc. Chem. Res., 10, 133 (1977). Reu., 7,453 (1978). (8)K.Kalyanasundarum, Chem. SOC. (9)(a) S.J. Rehfeld, J. Phys. Chem., 74,117(1970);(b) ibid., 75,3905 (1971). (10)J. H.Fendler, E. J. Fendler, G. A. Infante, P. S. Shih, and L. K. 97,89 (1975). Patterson, J . Am. Chem. SOC., (11)If pyridinium iodide head groups are used as polarity reporters some of the disadvantagesof using probe molecules can be circumvented, see E. J. R. Sudholter and J. B. F. N. Engberts, J. Phys. Chem., 83,1854 (1979),and references cited therein. (12)F. M. Martens and J. W. Verhoeven, Recl. Trau. Chim. Pays-Bas, in press.

Letters

OD

I

wavelength P

Flgure 1. CT absorption spectra of some electron donors with pq2+ in 0.1 M SDS solution at 20 O C : (a) [BNAH] = 3.4 X lo3 M, [pq2+] = 5.9 X M, full scale (OD)= 1.0; (b) [3-methylindole] = 4.7 X lo4 M, [pq2+J = 1.6 X lo-’ M, full scale (OD)= 0.5; (c) [pyrene] = 8.1 X 10 M, [pq2+] = 2.8 X M, full scale (OD)= 0.2.

Emaxl 016012-

X

x

x

X

X

X

I 5

10

15

10’1 [SDS]

CMClit

Figure 2. Observed optical density (E-) of a solution of 3-methylindole (5.6X lo4 M) and pq* (2.4 X 1W2M) In H,O SDS. Foro < [SDS] I1X M the solution Is cloudy, thus hampering exact determination of E,.

+

A manifestation of this enhancement of EDA complexation is the immediate brown colorization of an SDS solution containing BNAH upon the addition of only small quantities of pq2+whereas the methanol and CTAB solutions require large amounts of the acceptor before a color can be observed; the typical UV/visible absorption spectrum of the CT transition is represented in Figure 1. We decided to investigate whether these observations could be extended to complexes of pq2+with other donors. Pyrene was choosen as a donor because this compound is widely used as a fluorescent probe m01ecule,6,~J~J~ while 3-methylindole was studied because of its (slight) water solubility, which allows for measurement of the “pure” effect of addition of SDS surfactant molecules to the aqueous solution. Thus the absorbance in the CT absorption maximum (-400 nm) was monitored for an aqueous solution containing fixed concentrations of pq2+ and 3-methylindole and increasing amounts of SDS. As shown in Figure 2 a sharp rise in the CT absorption occurs around the critical micelle concentration (cmc), which confirms the micelle-dependent nature of the increased (13)Enhanced C T interaction between the oxidized and reduced forms of nicotinamide implemented chemically in a micellar system has been observed S. Shinkai, K. Tamaki, and F. Kunitake,Bull. Chem. SOC. Jpn., 48, 1918 (1975). (14)R. C. Dorrance and T. F. Hunter, J. Chem. Soc., Faraday Trans. I , 68,1312 (1972). (15)T.Forster and B. Selingeer, 2.Naturforsch. A, 19, 39 (1964).

Letters

The Journal of Physical Chemistry, Vol. 85,No. 13, 198 1

1775

TABLE I: Stability Constants of CT Complexes of Some Donors with pq” in Various Solvents at 20 “C donor

BNAH

3-methylindole pyrene

medium

KCT, L mol-’

methanol 0.02 M CTAB 0.02 M SDS 0.1 M SDS water 0.1 M SDS methanol 0.1 M SDS

0 . 9 3 f 0.05 0.5 * 0.2b 316 f 5 485 * loc 7 580 f 10 3.4 704 * 20

a Most reported values are slightly dependent on [pql’]. [pq”], cf. Figure 3.

15

E

0

IO-‘

I-Methyl-indole

0 pyrrnr A

BNAH

10

5

300

L&

Figure 3. Scatchard plots‘’ of complexes of some electron donors with pq2+ in 0.1 M SDS micellar solution at 20 “C.

tendency for EDA complexation. The tendency for complexation can be expressed quantitatively by the equilibrium constant (KCT) for ground-state association (eq 1). In Table I values for K m D

+A

(DA)

in various solvents for the donor/pq2+systems are listed. The formation constant KF of the ground-state complexes was evaluated by monitoring the intensity of the CT absorption as a function of the pq2+concentration and fitting the data to the Scatchard formula (eq 2).16,17 In (2) E x

and represent the observed absorbance and the molar charge-transfer absorbance a t wavelength X under the conditions [pq2+]>> [D] and E A= 0 for [D] = 0; [D] represents the donor concentration. In general, good linear Scatchard plots were obtained (cf. Figure 3) leading to the KCTvalues listed in Table I. At low saturation fraction, however, strong deviations from linearity were observed for the BNAH/pq2+ complex in 0.1 M SDS solution. This is clearly due to the fact that the 1:l complex model16 is only a very crude approximation in micellar solutions, where a more sophisticated treatment taking the distribution of donor molecules over the micelles into account is probably required. The same conclusion may be drawn from the slight drop in charge-transfer absorbance at higher surfactant concentration notable in Figure 2. Irrespective of the model used, however, the present data unequivocally demonstrate the profound enhancement of (16)R. Foster, “Organic Charge Transfer Complexes”, Academic Preea, New York, 1969. (17) D. A. Deranleau, J. Am. Chem. Soc., 92,4044,4050 (1969).

ErnaXl

nm

ref

2 53 2 80 177 186 1050 4 20 360 230

12 12 12

468 475 485 496 403 412 465 473

Calculated with an estimated

L

mol-’ cm-’

=

200.

36 37 At high values of

the ground-state association of pq2+not only with BNAH but also with other neutral hydrophobic molecules such as pyrene and 3-methylindole in SDS micellar solution when compared to methanol and aqueous solution, respectively. The extremely high values of Km in SDS solutions serve to testify that the hydrophobic donor molecules and the highly hydrophilic pq2+are bound to the SDS micelles a t sufficiently close distance to provide a (electrooptical) radiative transition probability, manifested in a CT absorption with a molar absorbance and wavelength fully comparable to that of complexes formed in homogeneous solution. From these facts it is concluded that the enhanced CT interaction observed in SDS micellar solution cannot be ascribed to an increased donor-acceptor affmity but results purely from an increased local concentration of donor and acceptor molecules a t some site within the micellar solution, since a change in redox properties of D and A due to the micellar environment would be expected to result in substantial shifts in wavelength and molar absorbance of the CT transition. Such large local concentrations of reactants are also considered to be one of the more important factors determining the profound catalytic effects sometimes encountered in micellar systems.lg20 It seems rather certain that the hydrophilic pq2+ is bound at the SDS micelle surface in the Stern “region”.21 Not only is it well-documented that 5&80% of the counterions is bound, due to electrostatical forces, in the Stern region22but furthermore the double cation pq2+is likely to (as has been shown for sulfate and chloride ions in various alkylammonium b r o m i d e ~ ~ expel ? ~ ~ *a ~number ~) of the Na+ counterions from this Stern region of the anionic SDS micelles. The very low KcT observed for BNAH/pq2+ in cationic CTAB micelles gives a further indication of the electrostatic nature of the binding of pq2+to SDS micelles and, furthermore, shows that solubilization of BNAH (which is insoluble in pure water) alone is not sufficient for the observation of a strong CT complex. Since equilibrium distances between the donor and acceptor in CT complexes are typically on the order of 300-350 pm,16 we have to conclude that the highly hy~

~

(18)(a) A. K.Yatsimirski, K. Martinek, and I. V. Berezin, Tetrohedron, 27,2855 (1971);(b)K. Martinek, A.K. Yatsimirski, A. V. Leveshov, and I. V. Beresin in ref lb, Vol. 2, p 489. (19)L. Romsted in ref l b , Vol. 2,p 509. (20)C. A. Bunton, Catal. Reo. Sci. Eng., 20, l(1979). (21)R. M. Schmehl and D.G. Whitten, J. Am. Chem. Soc., 102,1938 (1980). (22)(a) B. Lindman, G. Lindblom, H. Wennerstrom, and H. Gustavson in ref lb, Vol. 1, p 195. (b) D.Stigter and K. J. Mysels, J. Phys. Chem., 59,45 (1955). (c) D.Stigter, ibid.,68,3603 (1964). (d) P.Mukerjee, K. J. Mysels, and P. Kapanan, ibid.,71, 4166 (1967). (23)M. Griitzel, K. Kalyanasundaram, and J. K. Thomas, J . Am. Chem. SOC., 96,7869 (1974). (24)R.R.Hautala, N. E. Schore, and N. J. Turro, J. Am. Chem. Soc., 95,5809 (1973).

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The Journal of Physical Chemistty, Vol. 85, No. 13, 1987

drophobic donor molecules investigated are situated a t a site in direct contact with the binding site of the hydrophilic pq2+molecules. Fluorescence Quenching. At least one of the donor molecules, pyrene, has been depicted as occupying a central position within a spherical m i ~ e l l e . ~ , ’ ?The ~ ~ tquenching ~~ of its fluorescence by molecules held at the lipid-water interface has been explained from the rapid diffusion of excited pyrene molecules toward that interface. That pyrene resides in the hydrocarbon core of the micelles was also concluded from observations6 of exciplex emission in SDS solubilized pyrene/ triethylamine systems, since this exciplex emission is known to require a rather apolar medium if studied in homogeneous solution. From the rate of pyrene fluorescence quenching by CH212a kinetic model has been advanced in which the micelle exit and entry rates were determinedz5 and, moreover, the viscosity and permeability of the micellar interior were estimated from similar experiment^.^' The assumed site of pyrene solubilization in the fluorescence studies cited above clearly contrasts with our observations via absorption measurements reported in the previous section. In this context it should be noted that others have also questioned the hydrocarbon-enclosed nature of micellar pyrene solubilization. Thus Mukerjeeld already indicated that hydrophobic aromatic molecules are most likely bound at “surface sites” of the micellar hydrocarbon core. More specifically, Rodgers and Da Silva E. Wheelera have pointed out that solubilization models requiring the rapid translation of pyrene through a viscous hydrocarbon matrix ought to be replaced by a “channel” model of a more disordered micellar structure which allows water (and water dissolved solutes, e.g., quenchers) to penetrate into the lipid regions (a model indeed quite similar to the porous cluster model of Menger4). These authors arrived at such a model from data on the quenching of pyrene fluorescence by inorganic ions2” and by the electron acceptor pq2+,28b also investigated in the present study. The time-resolved decay of pyrene fluorescence in the presence of, e.g., pq2+was foundBb to be nonexponential, indicating that pq2+ is not homogeneously distributed in the water layer but is very probably associated, to a large extent, in the Stern region. It is obvious that our results on the ground-state association of pq2+with pyrene (and with the other donors) are more in line with the model advanced by Rodgers et al. than with a model where the donor and pq2+are contained in well-separated regions of the micellar structure. It should be realized, however, that the pyrene and quencher molecules depicted in a time-resolved fluorescence quenching experiment cannot be the same molecules visualized in a ground state association study as performed by us. This is because the fluorescence of donor molecules in a ground-state complex will be quenched in a static w a ~ , ~ i.e., , ~ Othese molecules show no fluorescence at all. Thus the decay curve obtained in time-resolved fluorescence studies contains only information concerning molecules engaged in dynamic (diffusional) quenching processes. As shown before,12 the contribution of static (25) P. P. Infelta, M. Gratzel, and J. K. Thomas, J. Phys. Chem., 78,

1-R- n” 11R7A\ ~ - --,..

(26) K. Kalyanasundaram, M. Gratzel, and J. K. Thomas, J . Am. Chem. SOC.,97, 3915 (1975). (27) M. Gratzel and J. K. Thomas, J. Am. Chem. SOC.,95,6885 (1973). (28) (a) M. A. J. Rodgers and M. E. Da Silva E. Wheeler, Chem. Phys. Lett., 43, 587 (1976); (b) ibid., 53, 165 (1978). (29) Th. Forster, “Fluoreszenz Organischer Verbindungen”, Vandenhoek und Ruprecht, Gottingen, 19511. (30) A. Weller, Prog. React. Kinet., 1, 189 (1961).

Letters

5

25

20

10 [pq’+]

x 104

*

Figure 4. Pyrene fluorescence quenching by pq2+ in 0.1 M SDS solution (nondeoxygenated) at 20 ‘C: (dashed line) observed quenching; (dashed areas) calculated contributions of static and dynamic quenching (cf. text).

quenching, of course, shows up in fluorescence quenching studies if not only the decay time but also the absolute fluorescence intensity is considered. In fact the contribution of static quenching to the overall reduction of fluorescence intensity may become quite large for the high Km values found in the donor/pq2+/SDS micellar systems. In the case of pyrene as a donor we will discuss this matter in a little more detail. In Figure 4 we have presented the pyrene fluorescence intensity ratio Ill0 (intensities in the presence (I) and of pq2+)as a function of the pq2+concentration absence (lo) in 0.1 M SDS solution. In Figure 4 we have also represented the calculated contributions of static and dynamic fluorescence quenching. For the static process the contribution is equal to the fraction of donor molecules that is complexed with pq2+ (eq 3). Since only uncomplexed Xatatic

=

KCT[Pq2’l

+ KCT[Pq2’l

for [pyrene]