The Binding of Short-Chain N-Alkylpyridinium Ions to Sodium Dodecyl Sulfate Micelles

Journal of Colloid and Interface Science 243, 463–468 (2001) doi:10.1006/jcis.2001.7856, available online at http://www.idealibrary.com on

The Binding of Short-Chain N-Alkylpyridinium Ions to Sodium Dodecyl Sulfate Micelles Ana Paula Romani,∗,1 Fl´avia Cristina Bonilha Vena,∗ Patr´ıcia Maria Nassar,† Antonio Claudio Tedesco,∗ and Jo˜ao Baptista Sargi Bonilha∗,2 ∗ Departamento de Qu´ımica, Faculdade de Filosofia, Ciˆencias e Letras de Ribeir˜ao Preto, Universidade de S˜ao Paulo, CEP 14040-901 Ribeir˜ao Preto, Brazil; and †Faculdade de Ciˆencias, Fundac¸a˜ o Educacional de Barretos, CEP 14783-226 Barretos, Brazil Received August 28, 2000; accepted July 21, 2001; published online October 5, 2001

Steady-state and time-resolved fluorescence quenching techniques were used to study the interaction of short-chain Nalkylpyridinium ions with sodium dodecyl sulfate (SDS) micelles using pyrene as a probe. The rate constants of the quenching of pyrene and binding constants of the cations to SDS micelles were obtained in different conditions of surfactant and salt concentrations. The increase in the number of carbon atoms in the alkyl chain (ethyl, propyl, butyl, and hexyl) of these counterions leads to a reduction of both intramicellar mobility and exchange rate with the aqueous phase. The intramicellar quenching rate constant value is in the range of (3–6) × 107 s−1 while the exit rate constant of the organic cations decreased and is in the range of (1–8) × 105 s−1 . The fluorescence decay parameters of pyrene in the presence of ethyl, NEP+ , and propyl, NPP+ , pyridinium cations were treated based on the ion-exchange formalism. The selectivity constants obtained, 13 ± 1 and 47 ± 1 for NEP+ /Na+ and NPP+ /Na+ , respectively, suggest that the highest value for NPP+ cation is ascribed to electrostatic field contribution in addition to a specific adsorption potential due to the size of the hydrophobic chain, leading to a reduction of the alkyl chain–water contacts of both organic cation and surfactant monomer. These results show that the selectivity was determined by the nature of the counterions present and it is less dependent on the characteristics of the micellar aggregates. °C 2001 Academic Press Key Words: micelle; fluorescence; counterion exchange; quenching.

INTRODUCTION

Micelles are aggregates formed by self-association of surfactant molecules. Due to the chemical structure of these molecules, the aggregates formed provide different regions to solubilize substrates, depending on the nature of the solutes, and may catalyze chemical reactions (1–4). Micelles may be classified as anionic, cationic, zwitterionic, or nonionic according to the charge of the hydrophilic headgroup (1–4) of the surfactant. Organic and inorganic counterions and additives can affect the properties of micelles (1–4). 1 2

To whom correspondence should be addressed. E-mail: [email protected]. In memoriam.

Ionic micelles can attract ions or exclude them depending on the electrical charge of the ion and the surfactant headgroup (2). The presence of organic and inorganic ions may affect the composition of the micellar surface due to the competition between counterions to bind to the surface of the ionic micelles (4–9). Many studies have used photophysical techniques to analyze the ion-exchange process in the presence of two or more types of counterions (7–15). Selectivity coefficients for monovalent–monovalent, monovalent–divalent, and divalent– divalent counterions have been determined using the pseudophase ion-exchange formalism, and the values obtained describe a process influenced by the hydrophilic–hydrophobic characteristics of the ions present in micellar solutions (7–15). Quina and co-workers employed time-resolved fluorescence decay measurements with pyrene as a probe to study the dynamics of entry and exit of the N -alkylpyridinium ions (N -ethyl-, N -nonyl-, N -decyl-, and N -dodecylpyridinium chlorides) from ionic micelles (16, 17). Data analysis provided a simple pseudophase model of the quenching of immobile probes by mobile and immobile quenchers, assuming that the counterion exit from the micelle surface is governed by the micellar surface potential (15–18). A relationship between dynamics and selectivity of ion binding to ionic micelles was suggested (17). In the present paper, a study of the ion exchange between sodium and short-chain N -alkylpyridinium counterions (N -ethyl-, N -propyl-, N -butyl-, and N -hexylpyridinium) at the surface of sodium dodecyl sulfate micelles is reported. Steady-state and time-resolved fluorescence quenching of pyrene by these pyridinium counterions is used to investigate the counterion micellar kinetics. Analysis of quenching data allow the determination of the selectivity constants and, hence, evaluation of the binding affinities of N -alkylpyridinium counterions. MATERIALS AND METHODS

Sodium dodecyl sulfate, SDS (Aldrich), was purified by repeated extraction in a Soxhlet (48 h) using hexane, followed by recrystallization three times from ethanol. SDS purity was confirmed by surface tension measurements and a of comparison of the cmc with the reported value from the literature (19).

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Pyrene (Aldrich) was recrystallized three times from ethanol. Sodium chloride (Merck, p.a.) was used as received. All the solutions were prepared in ultra-pure water from a Millipore system and other solvents were of analytical grade. The fluorescence quenchers N -ethyl (NEP+ ), N -propyl (NPP+ ), N -butyl (NBP+ ), and N -hexylpyridinium (NHP+ ) chlorides were prepared from additions of the corresponding alkyl bromides to a freshly distilled pyridine after being stirred for 72 h at room temperature (20). The corresponding chlorides were obtained by ion exchange from the bromide salt with silver chloride under stirring (72 h) in the dark. The solutions obtained were lyophilized and the products were analyzed. Absorption, 1 H-NMR spectral data, and microanalyses were in agreement with the proposed structures (20). Absorption spectra of N -alkylpyridinium ions were registered on a Hitachi U-3000 spectrophotometer. Pyrene fluorescence measurements were performed on a Hitachi F4500 spectrofluorimeter (337-nm excitation, 374-nm emission). Fluorescence decay of pyrene was obtained by the single-photon-counting technique using a Edinburgh Analytical Instrument FL-900 Lifetime spectrometer (H2 flash lamp, 337-nm excitation, 390-nm emission). Fluorescence decays of samples in the absence of quenchers were analyzed by the standard single-exponential decay routines of the FL-900 operating software. The decay curves in the presence of quenchers were analyzed using the TachiyaInfelta equation, Eq. [1] (21–23), I (t) = I (0) exp{−1/τ1 − C[1 − exp(−1/τ2 )]},

[1]

where I (0) is the fluorescence initial intensity of pyrene, and C, τ1 , and τ2 are parameters of the intramicellar quenching model. The values of C, τ1 , and τ2 are related to the rate constants for exit (k− ) and entry (k+ ) of the quencher from the micelle, the pseudounimolecular rate constant (kq ) for intramicellar quenching, and the average number hni of bound quenchers per micelle, by the following expressions: kq = C(1/τ2 )2 /(1/τ1 − 1/τ0 + C/τ2 )

[2]

k− = 1/τ2 − kq

[3]

hni = C/τ2 2 kq 2

[4]

k+ = k− hni/[Q t ] − hni[M].

[5]

τ0 is the pyrene fluorescence lifetime in the absence of quencher, [Q t ] is the total concentration of added quencher, and [M] is the micelle concentration (CD − cmc). In all fluorescence measurements, pyrene stock solutions were prepared in methanol. Aliquots of these stocks were added to volumetric flasks containing SDS, and the solutions were stirred for 30 min. The concentration of N -alkylpyridinium ions were calculated based on the absorption measurements (λmax 258 nm). The solutions that contained pyrene (1.6 × 10−6 M), SDS (0.04– 0.08 M), and NaCl (0.0–0.08 M) were air-equilibrated. Aliquots of concentrated solutions of the quenchers (0.15 M) were added

directly to the cuvette via a calibrated Hamilton microsyringe, and the emission intensities (or probe fluorescence decay) were registered after each addition. All measurements were performed at 30◦ C. RESULTS AND DISCUSSION

Steady-State Measurements The fluorescence quenching experiments of pyrene in aqueous micellar solutions were monitored at different surfactant concentrations as well as in the presence of sodium chloride. Data were treated using the Stern–Volmer formalism and K SV values were obtained from the linear portion of the plot of I 0 /I , where I 0 and I are the fluorescence intensities in the absence and in the presence of the quencher, respectively, as a function of quencher concentration. Typical Stern–Volmer plots obtained are shown in Fig. 1. The upper curvature observed is related to the statistics of the quencher micellar occupancy. In the limit of low hni, K SV is defined in terms of the micellar parameter as K SV =

τ0 k q k − K , (kq + k− )(1 + K [M])

[6]

where K is the quencher binding constant given by the ratio of k+ /k− . The values of K SV estimated for NEP+ , NPP+ , NBP+ , and NHP+ cations are listed in Table 1. The N -alkylpyridinium ions employed in this study are considered mobile quenchers (23). In the absence of salt the K SV values increase in the following order: NHP+ < NBP+ < NPP+ < NEP+ , in agreement with a diffusion-controlled quenching process. An increase in the length of the alkyl chain of the quencher increases the hydrophobic character and reduces the mobility of the ion. K SV values decrease with an increase in SDS

FIG. 1. Fluorescence quenching of pyrene (1.6 × 10−6 M) by NPP+ ion in a micellar solution of SDS (j 0.02 M, d 0. 04 M, m 0.06 M, and . 0.08 M) in the presence of NaCl (0. 06 M) at 30◦ C.

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concentration due to the dilution of the probe and quencher in the micellar phase, leading to a monodistribution of the probe inside the micelle. The addition of salt changes the micellar structure and may affect the partition coefficients of solubilized species. Adding salt to an ionic micellar solution leads to a decrease in the cmc value and increases the aggregation number, owing to the screened electrostatic repulsion between the surfactant head groups (1–4, 24, 25). The Stern–Volmer constant (K SV ) data presented in Table 1 show that the values increase according to the salt concentration and the solubilization process of the probe and quencher becomes size-dependent. In the presence of salt, K SV values for NEP+ are lower than those found for NPP+ , NBP+ , and NHP+ . These results reflect changes in the specific micelle/counterion interactions. Time-Resolved Measurements The analysis of fluorescence time-resolved decay of micellebound probes allows parameters such as the average aggregation number, intramicellar quenching rate constant, and the rate constant related to the binding of quencher to the micelle (23) to be determined. A typical time-resolved profile of the quenching of pyrene by N -alkylpyridnium ion in SDS micelles is shown in Fig. 2. Table 2 summarizes the experimentally determined rate constants for NEP+ , NPP+ , NBP+ , and NHP+ . The kq results for all the examined quenchers in the SDS system change in the following order: NHP+ < NBP+ < NPP+ < NEP+ , suggesting that the mobility of counterions bound to the micellar surface

TABLE 1 Stern–Volmer Constants (K SV ) for the Fluorescence Quenching of Pyrene by AP+ Ions (NEP+ , NPP+ , NBP+ , and NHP+ ) in Different SDS Concentrations in the Absence and in the Presence of NaCl at 30◦ C [SDS] (M)

NEP+

NPP+

NBP+

NHP+

0.04 0.06 0.08

K SV (M−1 ) (in the absence of NaCl) 3850 ± 100 2720 ± 100 2500 ± 90 2330 ± 50 1610 ± 60 1600 ± 80 1610 ± 40 1520 ± 40 1470 ± 30

2400 ± 120 1550 ± 80 1420 ± 50

0.04 0.06 0.08

K SV (M−1 ) [NaCl] = 0.02 M 1720 ± 120 2290 ± 100 3260 ± 200 1420 ± 60 1860 ± 40 2380 ± 100 850 ± 30 1100 ± 30 1450 ± 30

2520 ± 100 2300 ± 50 1700 ± 40

0.04 0.06 0.08

K SV (M−1 ) [NaCl] = 0.04 M 1980 ± 110 2960 ± 70 3750 ± 100 1770 ± 70 1990 ± 50 2360 ± 120 990 ± 30 1180 ± 30 1650 ± 70

3350 ± 60 2420 ± 50 1760 ± 50

0.04 0.06 0.08

K SV (M−1 ) [NaCl] = 0.06 M 3120 ± 40 4370 ± 80 4500 ± 180 1860 ± 60 2230 ± 80 3000 ± 110 1140 ± 50 1250 ± 60 1800 ± 40

4000 ± 80 2840 ± 40 2180 ± 70

0.04 0.06 0.08

K SV (M−1 ) [NaCl] = 0.08 M 2100 ± 100 5000 ± 200 5500 ± 90 2060 ± 100 2650 ± 150 2800 ± 100 1350 ± 70 1625 ± 30 2450 ± 200

5875 ± 240 3120 ± 60 2800 ± 80

increases in the same order, as expected on the basis of the diffusion-controlled quenching process in micelles. The increase of the micellar size, induced by added salt, reduces kq (1, 23, 26).

FIG. 2. Decay of fluorescence quenching of pyrene (1.6 × 10−6 M) in a micellar solution of SDS (0.040 M) containig NaCl (0.02 M) in the absence of quencher (upper curve) and in the presence of quencher NPP+ (1.57 × 10−4 M, 4.70 × 10−4 M, and 9.36 × 10−4 M).

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TABLE 2 Constants of Fluorescence Quenching of Pyrene by AP+ Ions in Micellar Solution of SDS (0.04 M) at 30◦ C [NaCl] (M)

NEP+

0 0.02 0.04 0.06 0.08

5.6 ± 0.06 4.8 ± 0.1 4.3 ± 0.1 3.9 ± 0.1 3.4 ± 0.2

0 0.02 0.04 0.06 0.08

NPP+

1.0 ± 0.3 1.5 ± 0.3 2.1 ± 0.4 2.4 ± 0.5 3.0 ± 0.6

hkq i (s−1 × 10−7 ) 5.1 ± 0.06 4.5 ± 0.06 3.7 ± 0.06 3.6 ± 0.2 3.3 ± 0.1 hk− i

(s−1

NBP+

NHP+

5.1 ± 0.2 4.4 ± 0.06 3.9 ± 0.06 3.6 ± 0.1 3.3 ± 0.06

4.4 ± 0.06 4.0 ± 0.06 3.5 ± 0.1 3.2 ± 0.06 3.0 ± 0.1

0.8 ± 0.6 2.5 ± 0.2 2.1 ± 1.5 3.0 ± 1.3 3.4 ± 1.1

2.2 ± 1.0 1.8 ± 0.2 1.2 ± 0.1 1.9 ± 0.3 2.9 ± 0.6

× 10−5 )

3.5 ± 0.9 4.4 ± 1.5 5.1 ± 1.8 7.6 ± 1.7 8.1 ± 3.1

The residence time of ionic quenchers in micelles depends on the balance between the electrostatic and hydrophobic contributions involved in the association process (23). The hydrocarbon segment in NEP+ is short for an effective hydrophobic interaction; the values of rate constant for the exit (k− ) of NEP+ from SDS micelles are higher than the corresponding values for NPP+ , NBP+ , and NHP+ . Increasing the sodium chloride concentration leads to changes in the electrical potential of the micellar surface (10), thus k− values increase with salt addition and the affinity for quencher ions is weakened. The rate constants for the entry (k+ ) of AP+ ions into the micelles were determined only for NEP+ (3.3 ± 1.0 × 1010 M−1 s−1 ) and NPP+ (4.0 ± 2.0 × 1010 M−1 s−1 ), both with large standard deviations. Short-chain pyridinium cations may bind to the anionic surface of SDS micelles as their counterions (16) k+x

→ X mic , X aq + Mic ←

[7]

k−x

where X represents the N -alkylpyridinium ion. This equilibrium is governed by the rate constants for the entry and exit of quencher from the micelle. As a result of ionic competition for micellar headgroups, a displacement of micellar counterion (Y ) can occur (5): k−y

→ Yaq + Mic. Ymic ←

[8]

k+y

The rate constants for entry and exit of the counterions from the micelles are dependent on the counterion charge and the micellar surface potential (9 0 ) according to (10, 16–18) 0 k− = k− exp[βz x F9 0 /RT ]

k+ =

0 k+

[9]

exp[−(1 − β)z x F9 /RT ]. 0

[10]

F is the Faraday constant, R is the ideal gas constant, and z x rep-

resents the charge of the ion. Considering that AP+ ions may be partitioned between the aqueous and micellar phases, a fraction (β) of the overall electrostatic work is associated with the exit process, while the other (1 − β) is related to the entry process (16–18). The micellar surface potential is logarithmically related to the concentration of free micellar counterions ([Yaq ]) in the aqueous phase (27, 28), z y F9 0 /2.303RT = K y + log[Yaq ],

[11]

where K y is a constant. A combination of Eqs. [9] and [11] indicates that log k− is a linear function of log [Yaq ] with a slope equal to (z x /z y )β (16–18) log k− = K 0 + (z x /z y )β log[Yaq ],

[12]

where K 0 is a constant. The measured rate constants for micellar exit of NEP+ and NPP+ quenchers as a function of salt added can be analyzed with this treatment. The aqueous counterion concentrations ([Naaq ]) are calculated from the pseudophase ionexchange relationship (4, 5): [Naaq ] = αCD + cmc + [Naad ].

[13]

The value used for α (the apparent degree of counterion dissociation) was 0.25 for SDS micelles (16). Standard linear regression analysis using Eqs. [12] and [13] provides the following relationship for N -alkylpyridinium ions: log k− (NEP+ ) = 7.11 + 0.63 log[Naaq ] +

log k− (NPP ) = 6.37 + 0.48 log[Naaq ].

[14] [15]

β values obtained for NEP+ and NPP+ ions are 0.63 ± 0.08 and 0.48 ± 0.13, respectively. The result for NEP+ is in agreement with a previous result reported in the literature (16). The β value for NEP+ indicates that the electrical contribution to the Gibbs energy barrier for escape the ion from the micelle is higher than the contribution of NPP+ . This fact suggests that the association of NEP+ is modulated mainly by the attraction of the ion by the electrostatic field of SDS micelles while for NPP+ its binding to the micellar surface is in addition influenced by the contribution of the adsorption potential due to the larger size of its alkyl chain (17). In these experiments, the concentration of SDS employed was relatively low to avoid micelle-mediated transfer of counterions. The basic assumption of the method employed considers that the statistical distribution of the quenchers among the micelles obeys a Poisson distribution. Micelle–micelle collisions would result in an aggregate with more than one quencher; this situation would be a problem for the statistical distribution because the number of the quencher may be restricted, less than one per micelle. The quencher concentration used in this study (0–1×10−3 M) was

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low and does not cause an impact on the average aggregation number of SDS micelles.

θ AP = (|z x |/Nag )(A2 − 1/τ ◦ + A3 A4 )2 /[A3 (A4 )2 ].

Pseudophase Ion-Exchange Model Photophysical techniques have been used to determine counterion-exchange selectivities at the surface of ionic micelles (6–17). The binding of counterions to SDS micelles can be treated in terms of competitive ion exchange between the foreign counterion AP+ (NEP+ or NPP+ ) and the detergent counterion Na+ (4). The micelle is considered a distinct phase of the solution and the process is described by an equilibrium of the type K AP/Na

−−→ APaq + Namic ←− − APmic + Naaq .

[16]

K AP/Na is the counterion-exchange selectivity coefficient, defined as ¶µ µ ¶ [Naaq ] θ AP , [17] K AP/Na = [APaq ] θNa where the subscript “aq” refers to the analytical concentration of the free counterions in the aqueous phase and θAP and θNa are the fractions of the micellar surface coveraged by AP+ and Na+ defined by (4, 5, 8) θ AP = [APmic ]/CD

[18]

θNa = [Namic ]/CD ,

[19]

where CD = [SDS]-cmc is the concentration of micellized surfactant, subject to the condition θNa + θ AP = (1 − α).

[20]

The micellar fractional coverages and the bound and free micelle counterion concentrations are related to the pseudophase ion-exchange mass balance by (4, 5, 8) [Namic ] = (1 − α)CD − θ AP CD

equation by the relationship (13, 17)

[21]

[Naaq ] = αCD + cmc + [Naad ] + θ AP CD

[22]

[APT ] = [APmic ] + [APaq ].

[23]

In these equations, [ APT ] and [Naad ] are the total concentrations of added foreign and common counterion salt, respectively. The time-resolved fluorescence decay measurements with pyrene as a probe and the monovalent N -alkylpyridinium ions as quenchers were employed to determine the selectivity coefficients for exchange of these two organic counterions at the surface of SDS micelles. The quenching of pyrene depends on the local concentration of AP+ ions at micellar surfaces (23). Quina and co-workers demonstrated that the analysis of the fluorescence decay of pyrene in micellar solutions allows the determination of θAP from the coefficients of the Tachiya–Infelta

[24]

τ ◦ is the pyrene fluorescence lifetime in the absence of AP+ ions and Nag is the micelle aggregation number. Using Eq. [24] and the Ai fitting parameters, K AP/Na values obtained are 13 ± 1 and 47 ± 1 for NEP+ /Na+ and NPP+ /Na+ , respectively. The lack of appreciable selectivity constants for the changes between NBP+ /Na+ and NHP+ /Na+ are attributed to differences in the specific adsorption potentials of these counterions due to the size of their alkyl chains. These results indicate that the N alkylpyridinium ions bind more strongly to the micelle than the sodium ion as would be expected. The differences observed in the selectivity coefficients for the exchange between Na+ and AP+ are similar to those found for the exchange between sodium and n-alkylammonium ions at the surface of SDS micelles (12), suggesting that the hydrophobicity of the alkyl chain is the main factor that influences K AP/Na values. The binding of a short alkyl chain counterion to the micellar surface reduces the alkyl chain– water contacts of both organic cation and surfactant monomer, leading to stabilization of the micelle by entropy or hydrophobic contribution (12). SUMMARY

Steady-state and time-resolved fluorescence quenching techniques were used to study the interaction of short-chain N alkylpyridinium ions with SDS micelles. The rate constants of the quenching of pyrene and binding of the cations to SDS micelles were obtained under different conditions of surfactant and salt concentrations. The results suggest that an increase in the number of carbon atoms in the alkyl chain of these counterions leads to the reduction of both intramicellar mobility and exchange rate with the aqueous phase. The fluorescence decay parameters of pyrene in the presence of NEP+ and NPP+ ions were treated within the framework of the ion-exchange formalism. The selectivity constants obtained, 13 ± 1 and 47 ± 1 for NEP+ /Na+ and NPP+ /Na+ , respectively, suggest that the higher value for NPP+ cation is ascribed to an electrostatic field contribution in addition to a specific adsorption potential due to the size of the hydrophobic chain (17), reducing the alkyl chain–water contacts of both the organic cation and surfactant monomer. These results show that the selectivity is determined by the nature of the counterions present and it is less dependent on the characteristics of the micellar aggregates (10, 11). ACKNOWLEDGMENTS ` Pesquisa This work was supported by grants from the Funda¸ca˜ o de Amparo A do Estado de S˜ao Paulo (FAPESP Thematic Projects 91/0480-1 and 94/3505-3) and by a fellowship from FAPESP (A. P. R., Proc. No. 95/9770-3). The authors are pleased to thank to Prof. Dr. Marcelo Henrique Gehlen for the important suggestions that contributed to the completion of this work.

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REFERENCES 1. Tanford, C., “The Hydrophobic Effect: Formation of Micelles and Biological Membranes,” 2nd ed. Wiley, New York, 1980. 2. Tascioglu, S., Tetrahedron 52, 11113 (1996). 3. Fendler, J. H., and Fendler, E. J., “Catalysis in Micellar & Macromolecular Systems.” Academic Press, New York, 1975. 4. Bunton, C. A., Nome, F. J., and Quina, F. H., and Romsted, L. S., Acc. Chem. Res. 24, 357 (1991). 5. Quina, F. H., and Chaimovich, H., J. Phys. Chem. 83, 1844 (1979). 6. Chaimovich, H., Bonilha, J. B. S., Politi, M. J., and Quina, F. H., J. Phys. Chem. 83, 1851 (1979). 7. Abuin, E., Lissi, E., Bianchi, N., Miola, L., and Quina, F. H. J. Phys. Chem. 87, 5166 (1983). 8. Lissi, E., Abuin, E., Sepulveda, L., and Quina, F. H., J. Phys. Chem. 88, 81 (1984). 9. Abuin, E., and Lissi, E., J. Colloid Interface Sci. 112, 178 (1986). 10. Abuin, E., and Lissi, E., J. Colloid Interface Sci. 143, 97 (1991). 11. Abuin, E., and Lissi, E., J. Colloid Interface Sci. 151, 594 (1992). 12. Bonilha, J. B. S., Georgetto, R. M. Z., Abuin, E., Lissi, E. A., and Quina, F. H., J. Colloid Interface Sci. 135, 238 (1990). 13. Nassar, P. M., Naal, R. M. Z. G., Pauli, S. H., Bonilha, J. B. S., Okano, L. T., and Quina, F. H., J. Colloid Interface Sci. 190, 461 (1997). 14. Bonilha, J. B. S., Tedesco, A. C. T., Navarro, D. M. A. F., and Nassar, P. M., J. Lumin. 82, 315 (1999).

15. Okano, L. T., Alonso, E. O., Waissbluth, O. L., and Quina, F. H., Photochem. Photobiol. 63, 746 (1996). 16. Alonso, E. O., and Quina, F. H., Langmuir 11, 2459 (1995). 17. Ranganathan, R., Okano, L. T., Yihwa, C., Alonso, E. A., and Quina, F. H., J. Phys. Chem. B 103, 1977 (1999). 18. Alonso, E. O., and Quina, F. H., J. Braz. Chem. Soc. 6, 155 (1995). 19. Romsted, L. S., in “Surfactants in Solutions” (K. L. Mittal and B. Lindman, Eds.), p. 1018. Plenum, New York, 1984. 20. Soldi, V., Erismann, N. M., and Quina, F. H., J. Am. Chem. Soc. 110, 5137 (1988). 21. Tachiya, M., Chem Phys. Lett. 33, 289 (1975). 22. Infelta, P., Gratzel, M., and Thomas, J. K., J. Phys. Chem. 78, 190 (1974). 23. Gehlen, M. H., and De Schryver, F. C., Chem. Rev. 93, 199 (1993). 24. Bales, B. L., Messina, L., Vidal, A., Peric, M., and Nascimento, O. R., J. Phys. Chem. B 102, 10347 (1998). 25. Quina, F. H., Nassar, P. M., Bonilha, J. B. S., and Bales, B. L., J. Phys. Chem. 99, 17028 (1995). 26. Almgren, M., in “Kinetics and Catalysis in Microheterogeneous Systems” (M. Gratzel and K. Kalyanasundaram, Eds.), Surf. Sci. Ser., Vol. 38, Chap. 4, pp. 66–67. Dekker, New York, 1991. 27. Miola, L., Abarkeli, R. B., Ginani, M. F., Berci Filho, P., Toscano, V. G., and Quina, F. H., J. Phys. Chem. 87, 4417 (1983). 28. Quina, F. H., Contrib. Cient. Technol. (Santiago) 13, 115 (1983).