Langmuir 1997, 13, 4219-4222
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Effect of Urea on the Dimerization Equilibrium of Nickel Tetrasulfonated Phthalocyanine in Bulk and in the Hydrophilic Compartment of AOT Reversed Micelles Carmen Lu´cia Costa Amaral and Ma´rio Jose´ Politi* Departamento de Bioquı´mica, Laborato´ rio Interdepartamental de Cine´ tica Ra´ pida, Instituto de Quı´mica da Universidade de Sa˜ o Paulo, Caixa Postal 26077, Sa˜ o Paulo, S.P., 05599-97 Brazil Received November 4, 1996. In Final Form: May 12, 1997X Dimerization equilibrium of the title compound is investigated in bulk and in an AOT aqueous pool in the presence and absence of urea by simple spectrophotometric determinations. The association in bulk is decreased by the addition of urea and strongly favored in reversed micelles with or without urea. The decrease in bulk media is assigned to a direct involvement (enthalpic) of urea in bridging the two phthalocyanine moieties whereas in the micelle electrostatic and volume confinements drive the equilibrium to the dimer species.
Introduction There is a considerable interest in the study of phthalocyanines (PCs) and metallophthalocyanines (MPCs) because of their variety of technological and biomedical applications as sensitizers for solar energy conversion1 or for photodynamic therapy of tumors.2 MPCs and PCs also exhibit unique electrical properties that have led to their use as active ingredients in electronic devices.3 One notable aspect of these compounds is the aggregation of the mono-, di-, tri-, and tetra-substituted sulfonated derivatives in which dimers and higher aggregates are formed in aqueous solution. This is an important aspect for biomedical applications since dimers and larger n-mers are much more inactive than monomers as sensitizers.4 Many applications of PCs and MPCs require water-soluble derivatives as the sulfonates. These derivatives undergo extensive aggregation in aqueous solution, owing to the presence of a water cluster linking the sulfonate groups and the azo atoms of adjacent macrocycles.5 The disassembling to the monomer was shown to occur by the addition of organic solvents such as methanol, ethanol, and dimethyl sulfoxide.6 Much of the work has been consequently directed to the understanding of the aggregation effects of organic solvents in water. Recently, emphasis has been placed on the incorporation and aggregation of PCs into aqueous and reversed micelles. For negatively charged MPCs, such as zinc tetrasulfophthalocyanine (ZnPCS4), monomerization is facilitated by the use of cationic hexadecyltrimethylammonium bromide and chloride or even anionic sodium dodecyl sulfate aqueous micelles.7 For sulfonated aluminum phthalocyanine (AlPCSn with n ) 1-4) in benzyldimethyln-hexadecylammonium chloride (BHDC) reversed micelles, only the di- and trisulfonated derivatives are monomeric and the extent of aggregation for S1 and S4 increased with water concentration.8 However, the effect of negatively charged reversed micelles, like AOT (sodium * Author to whom correspondence should be addressed: e-mail:
[email protected]. Fax: (55)(011) 815 5579. X Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Darwent, J. R.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, M. C. Coord. Chem. Rev. 1982, 44, 83. (2) Valduga, G.; Reddi, E.; Jori, G. J. Inorg. Biochem. 1987, 29, 59. (3) Rikukawa, M.; Rubner, M. F. Langmuir 1994, 10, 519. (4) Spikes, D. J. Photochem. Photobiol. 1986, 43, 691. (5) Reddi, E.; Jori, G. Rev. Chem. Intermed. 1988, 10, 241. (6) Yang, Y. C.; Ward, J. R.; Seiders, R. Inorg. Chem. 1985, 24, 1765. (7) Daraio, M. E.; Aramendia, P. F.; San Roma´n, E. A.; Braslavsk, S. E. Phtotochem. Photobiol. 1991, 54, 367.
S0743-7463(96)01065-7 CCC: $14.00
bis(2-ethylhexyl) sulfosuccinate), in the aggregation of MPCs is unknown. AOT reversed micelles are the most studied ones since they can solubilize large amounts of water and do not require cosurfactants.9,10 AOT reversed micelles consist of spherical water droplets surrounded by a monomolecular layer of surfactant molecules dispersed in a continuous oil phase (n-hexane, isooctane, CCl4, etc.). The size of the reversed micelles is determined by the ratio of water over surfactant concentration (W ) [water]/[AOT]). Several properties of water in the core (polarity, viscosity, three-dimensional structure, etc.), depending on W, are very distinct from bulk liquid water.11-13 An effect that can be easily observed by monitoring, for example, the photophysical properties of organic dyes solubilized in the aqueous pool.14-16 The knowledge of these properties is important for understanding the effects on chemical and photochemical reactivities17-19 and on enzymatic activity,20-23 that is to set foundations for studies aiming to exploit and to take advantage of the properties of reduced water activity and reactants confinement. The properties of the core of reversed micelles can be investigated by using various probes that can access different regions of the water pool. The preferential solubilization site of probes can be deduced with the help of spectroscopic data. For instance, from the absence of effects on the photodissociation and H+ recombination of (8) Dhami, S.; Cosa, J. J.; Bishop, S. M.; Phyllips, D. Langmuir 1996, 12, 293. (9) Fendler, J. H. Acc. Chem. Res. 1976, 9, 153. (10) De, K. T.; Maitra, A. Adv. Colloid Interface Sci. 1995, 59, 95. (11) Christopher, D. J.; Yarwood, J.; Belton, P. S.; Hills, B. P. J. Colloid Interface Sci. 1992, 152, 465. (12) Belleteˆte, M.; Lachapelle, M.; Durocher, G. J. Phys. Chem. 1990, 94, 5337. (13) Valeur, B.; Keh, E. J. Phys. Chem. 1979, 83, 3305. (14) Rodgers, M. A. J. In Reverse Micelles; Luisi, P. L., Straus, B. E., Eds.; Plenum Press: New York, 1984; p 165. (15) Belleteˆte, M.; Durocher, G. J. Phys. Chem. 1989, 93, 1793. (16) Terpko, A. T.; Serafin, R. J.; Bucholtz, M. L. J. Colloid Interface Sci. 1981, 84, 202. (17) Gehlen, M. H.; De Schryever, F. C. Chem. Rev. 1993, 93, 199. (18) Fletcher, P. D. I.; Robinson, B. H. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 863. (19) Kalyanasundaran, K. In Photochemistry in Microheterogeneous Systems; Academic Press, Inc.: New York, 1987. (20) Luisi, P. L. Angew Chem. 1985, 97, 44. (21) Luisi, P. L.; Giomini, M.; Pilene, M. P.; Robinson, B. L. H. Biochim. Biophys. Acta 1988, 947, 209. (22) Khmelmitsky, Y. L.; Kabanov, N. L.; Levashov, A. V.; Martinek, K. In Structure and Reactivity in Reverse Micelles; Pilene, M. P., Ed.; Elsevier: New York, 1989. (23) Florenzano, F. H.; Santos, L. G. C.; Cuccovia, I. M.; Scarpa, M. V.; Chaimovich, H.; Politi, M. J. Langmuir 1996, 12, 1166.
© 1997 American Chemical Society
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8-hydroxy-1,3,6-pyrenetrisulfonate upon addition of urea in AOT reversed micelles it was concluded that the probe localizes mainly in the center of the pool and that water and water/urea in the core of reversed micelles have similar properties.24 In the present study we examined the effect of urea on the aggregation of MPCs in aqueous solutions and on the properties of the AOT reversed micelles in the presence of urea. The aim of the present investigation is to establish the effect of urea on the dimerization of nickel tetrasulfophthalocyanine (NiPCS4) and to understand the properties of AOT reversed micelles containing urea from the viewpoint of NiPCS4 dimerization equilibrium. Materials AOT (sodium bis(2-ethylhexyl) sulfosuccinate) 99% purity, obtained from Aldrich Chemical Co., was used as received. Urea (Merck) was triply recrystallized from hot ethanol. n-Hexane (spectroscopy grade) was purchased from Merck. Water was bidistilled in an all glass aparatus. Nickel tetrasulfophthalocyanine (NiPCS4) was obtained from Aldrich and used as received. Solutions of NiPCS4 were prepared by adding appropriated weight fractions in aqueous solutions of 3 and 5 M urea, respectively. Reversed micelles were prepared by adding solutions of NiPCS4 in water, urea (3 M and 5 M) to a solution of AOT in n-hexane. For aqueous solutions of NiPCS4, optically clear reversed micelle solutions were obtained with gentle hand shaking, whereas in the presence of urea optically clear solutions were obtained after bath sonication (thermolab) for at least 5 min. Electronic absorption spectra were measured either on a DU-7 single-beam spectrophotometer (Beckman) with a thermostated cell compartment using 10 mm optical path length quartz cuvettes or with a double-beam Hitachi spectrophotometer (model U2000) using 100 mm optical path length. The dimerization constant, KD, were calculated assuming that only monomer (M) and dimer (D) species are present in the concentration range studied (4 × 10-7-1 × 10-5 M). For the analytical wavelength region where Lambert-Beer law holds for M and D, the absorbance, A, is given by
A ) M[M] + D[D]
Figure 1. Absorption spectra of NiPCS4 in water at temperatures ranging from 30 to 85 °C (5 °C steps a-k). Inset spectra of NiPCS4 in water/ETOH (50%) at room temperature (s) and in 5 M urea solution at 75 °C (- - -). [NiPCS4] ) 1 × 10-5 M.
(1)
where M, D, [M], and [D] are the molar absorption coefficients (M-1 cm-1) and concentrations of monomer and dimer species, respectively. Considering the mass balance, the total concentration of the NiPCS4, CT, can be expressed as
CT ) [M] + 2[D]
(2)
and the dimerization constant, KD, as
KD ) [D]/[M]2
(3)
Combining eqs 1-3 results in the following relation as expressed by Yang et al.:6
A ) [M[M] + D(CT - [M])/2]
(4)
[M] ) [(1 + 8KDCT)1/2 - 1]/4KD
(5)
where
The parameters M, D, and KD for NiPCS4 in water were determined as follows. Since there is an appreciable amount of dimers in bulk water in the concentration range investigated, M was determined initially in the presence of 50% ETOH (M(λ)658nm) ) 1.03 × 105 M-1 cm-1 ) where only monomer species exist (Figure 1 inset) as suggested in ref 5. Spectral shape and calculated M at 662 nm for 3 and 5 M urea solutions at 75 °C, where mainly monomers species are present (see Results and Discussion Section (24) Amaral, C. L. C.; Brino, O.; Chaimovich, H.; Politi, M. J. Langmuir 1992, 8, 2417.
Figure 2. (a) Absorption spectra of NiPCS4 in aqueous solution of urea (3 M) at temperatures ranging from 35 to 75 °C (10 °C steps a-e) and (b) urea (5 M) at temperatures ranging from 35 to 65 °C (10 °C steps a-d). [NiPCS4] ) 1 × 10-5 M. and Figures 1 and 2), are much closer to the spectra in pure aqueous media at high T. The mean M for urea solutions (see below) was therefore assigned for bulk water. D and KD were then adjusted by data-fitting minimization with eq 4 and 5. Initially KD values were varied to minimize D, that is to obtain the lowest standard deviation. From M and D, KD was calculated for each concentration. For solutions in the presence of urea D and KD were determined as those in bulk water.
Effect of Urea on the Equilibrium of NiPCS4
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Table 1. Dimerization Constants, KD (×105) M-1, for NiPCS4 in Water. Calculated with EM and ED Fixeda [NiPCS4]
temp 35
temp 45
temp 55
4.94 × 10-7 1.48 × 10-6 2.46 × 10-6 3.44 × 10-6 4.41 × 10-6 5.38 × 10-6 6.34 × 10-6 7.30 × 10-6 8.25 × 10-6 9.20 × 10-6 1.02 × 10-5 1.11 × 10-5 1.20 × 10-5 1.30 × 10-5
8.10 6.52 5.80 5.28 5.15 5.01 5.01 4.95 5.00 5.05 5.08 5.20 5.17 5.29
4.18 2.07 2.17 1.99 1.98 1.95 1.95 1.96 1.94 1.94 1.91 1.94 1.94 1.99
0.53 0.44 0.80 0.86 0.91 0.91 0.93 0.95 0.96 0.95 0.94 0.95 0.95 0.95
a
Table 2. Dimerization Constant, KD (×105), for NiPCS4 in Water and Urea at Various Temperatures
temp 65
temp 75
[urea]
temp 35
temp 45
temp 55
temp 65
temp 75
0M 3M 5M
5.47 0.43 0.0084
2.14 0.41 0.0070
0.86 0.22 0.0064
0.79 0.18 0.0067
0.51
0.56 0.92 1.00 1.00 1.00 1.00 1.00 0.90 0.98 0.96 0.93 0.91 0.89
0.26 0.89 0.78 0.69 0.61 0.59 0.57 0.57 0.56 0.55 0.24 0.26 0.28 0.30
M ) 7.59 × 104 M-1 cm-1. D ) 3.93 × 104 M-1 cm-1.
Results and Discussion Aqueous solutions of metallophthalocyanines (MPC) exhibit an electronic transition in the visible region (Q band at 600-750 nm) with the two major bands arising from π f π* transitions. The more intense band at the longer wavelength has been attributed to the monomeric MPC, whereas that at the shorter wavelength to the dimer.25,26 Figure 1 depicts the characteristic temperature dependent absorbance spectra of an aqueous solution of NiPCS4. The spectra reproduce previous reports for other MPC derivatives and demonstrate the equilibrium displacement toward the monomer with temperature.1 Accordingly with the decrease in temperature the absorbance intensity due to the monomer diminishes and a new band centered at 622 nm corresponding to the dimer NiPCS4 appears. An isosbestic point at 635 nm is clearly seen. A slight shift can also be observed in the absorption wavelength maximum (λmax) of the monomer with temperature (λmax’s ) 658 and 662 nm, below and above 75 °C, respectively). In the inset of Figure 1 spectra of NiPCS4 (1 × 10-5 M) in 50% volume aqueous ethanol and in 5 M urea (T ) 75 °C) are presented. In aqueous ethanol the spectrum consists of a strong π f π* absorption centered at λ ) 662 nm accompanied by a weaker vibrationally coupled satellite band at shorter λ (597 nm) , which disappears with the aggregation of the NiPCS4 (Figure 1). The spectrum in urea shows a broader transition peaking around 662 nm and the appearance of a small shoulder at the dimer absorption region (∼625 nm). The intersection between these spectra is slightly above (∼640 nm) the isosbestic point (∼635 nm). Since the spectrum in 50% aqueous ethanol is slightly distinct from that of bulk urea aqueous solutions and by taking the spectra of NiPCS4 in ethanol as the standard for ’s and KD calculations in homogeneous or micellar solutions resulted in negative KD values (data not shown), it was assumed that the spectrum of NiPCS4 in urea at high temperature corresponds to ∼99% monomeric form. Thus the value of M(λ)658nm) ) 7.59((0.01) × 104 M-1 cm-1 was used as that for the NiPCS4 monomer in bulk water. D and KD were calculated by eqs 4 and 5 (see Materials section). The mean value of D found is 3.934((0.005) × 104 M-1 cm-1. In Table 1 the minimization of KD for fixed M and D values in the temperature range investigated (35-75 °C) are presented. This table is included to demonstrate the goodness of the fit within the NiPCS4 concentration range and temperature studied. It is clear (25) Harriman, A.; Richoux, M. C. J. Photochem. 1980, 14, 253. (26) Martin, P. C.; Gouterman, M.; Pepich, B. V.; Renzoni, G. E. Inorg. Chem. 1991, 30, 3305.
Table 3. Thermodynamic Functions of NiPCS4 Dimerization in Water and Urea [urea]
∆H (kcal‚mol-1)
∆S (kcal‚mol-1)
0M 3M 5M
0.219 0.12 0.028
0.018 0.017 0.017
that larger deviations from the mean occur only at quite low probe concentration. Figure 2 depicts the temperature dependent spectra of NiPCS4 in aqueous solution in the presence of urea (3 M) (Figure 2a) and (5 M) (Figure 2b). It is clear that the addition of urea displaces the monomer-dimer equilibrium in favor of the monomer. An isosbestic point is observed at λ = 635 nm. From these spectra calculated D are 3.00((0.05) × 104 and 2.920((0.047) × 104 M-1 cm-1 for 3 M and 5 M urea, respectively. Calculated KD values for each composition and temperature are summarized in Table 2, and in Table 3 has derived ∆H and ∆S values. It becomes evident that the decrease in KD with urea is enthalpically driven. In other words the energy gain from monomer desolvation and dimer stabilization via H2O bridge linking the azo and sulfonate groups5 decrease with urea. The spectra of NiPCS4 in AOT/n-hexane/water and in AOT/n-hexane/water/urea (3 M) systems at various W’s (room temperature ≈23 °C) are shown in Figure 3a and 3b, respectively. It is observed in both cases that absorption maxima occur at ∼625 nm corresponding to the dimer transition. In the presence of urea the aggregation equilibrium was investigated at W’s varying from 2 to 16 since the maximum W which can be attained in this system is reduced24,27 (for these mixtures W is defined as {[H2O] + [urea]}/[AOT]). In contrast with the behavior in aqueous and aqueous/ urea solutions (Figures 1 and 2) the NiPCS4 dimer absorption maxima in AOT reversed micelles are slightly blue-shifted and displace from about 613 to 620 nm with the increase in W (Figure 3). These shifts in λmax added to the lack of isosbestic point evidence a change in the solvation effects on NiPCS4 in the interior of the reversed micelles, suggesting a displacement of the probe toward the hydrophilic region with the increase in W or equivalently to a change in the polarity of the micelle inner core with W where the probe resides. This result is in agreement with the electrostatic repulsion between the negatively charged micelle interface and the NiPCS4 dimer or monomer species. The appearance of the characteristic band (λmax ) 658 nm) due to the monomeric species occurs only above W’s ) 10 in AOT micelles (Figure 3). For these conditions KD were calculated assuming the ’s for the monomer and dimer obtained in aqueous/urea solutions (Table 4). In the absence of urea KD varies by 3 orders of magnitude from W ) 12 to W ) 55 and tends to the value observed in bulk water ∼2 × 10 6 M-1 (room temperature). A plot of the logarithm of KD against W shows two distinct regions below and above W ) 20 with a steep increase in KD with low W’s (Figure 4). This behavior reflects effects on the dimer solvation and on electrostatic volume confinements. (27) Garcia-Rı´o, L.; Leis, J. R.; Mejuto, J. C.; Pena, M. E. Langmuir 1994, 10, 1676.
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Figure 4. (a) Logarithm of the dimerization constants (KD) for NiPCS4 in AOT/n-hexane/water for W ranging from 12 to 60 (b) and in AOT/n-hexane/water/3 M urea for W ranging from 10 to 16 (9). [NiPCS4] ) 4 × 10-7 M and [AOT] ) 0.1 M.
Figure 3. (a) Absorption spectra of NiPCS4 of AOT/n-hexane/ water for W ranging from 10 to 60 (a-n W’s ) 10, 12, 14, 16, 18, 20, 25, 30, 35, 40, 45, 50, 55, and 60) and (b) AOT/n-hexane/ water/3 M urea for W ranging from 2 to 16 (a-h W’s ) 2, 4, 6, 8, 10, 12, 14, and 16). [NiPCS4] ) 4 × 10-7 M. [AOT] ) 0.1 M. Table 4. Dimerization Constant for NiPCS4 in AOT-Reversed Micelles at Various W’s KD (×1010) M-1 W
water
10 12 14 16 18 20 25 30 35 40 45 50 55 60
1.24 0.31 0.14 0.075 0.048 0.033 0.024 0.011 0.005 0.003 0.002 0.001 0.001
urea 3 M 0.047 0.011 0.0047 0.0025
At W < 20 dimer species are highly favored given the low water volume fraction and the probe-AOT electrostatic repulsion which decreases further the probe diffusible pool volume. Interestingly in the presence of urea the decrease in KD up to W ) 16 follows that in its absence, that is KD decreases
steeply to ∼50% (Figure 4). Since in both cases the magnitude of KD are very high, the dynamics of reverse micelles in promoting pool content exchange or in enhancing the probe distribution statistics does not need to be taken in account. Thus it is clear that the position of the NiPCS4 dimer/monomer equilibrium in AOT reversed micelles with or without urea is governed by KD, water content, and hydrophilic pool volume electrostatic restrictions. Furthermore the magnitude of KD is reflected directly in the solvation properties of the pool. In the presence of urea KD diminishes by approximately 2 orders of magnitude for the same W’s reflecting the direct effect on the water-bridging molecule which stabilizes NiPCS4 dimer species. Conclusions Addition of urea to bulk aqueous solutions of NiPCS4 decreases KD by a few orders of magnitude. An effect assigned to the enhanced monomer solubilization and also to a direct effect on the water bridge molecule which stabilizes dimer species is clearly evidenced by the enthalpic nature of the association equilibrium. NiPCS4 association equilibrium is largely displaced toward dimer species in AOT reversed micelles with or without urea. Since the magnitude of KD is quite high even with 3 M urea in the hydrophilic pool, the probe association behaves like a potential well and the micellar dynamics does not need to be taken in account. The simple spectrophotometric assay with NiPCS4 dimerization equilibrium added to the large molar absorbances of both monomer and dimer species renders a powerful tool to investigate selected microdomains and a possible route to modulate the NiPCS4 equilibrium for technical applications. Acknowledgment. We express our gratitude to the Brazilian granting agencies CNPq, CAPES, FINEP, PADCT, and FAPESP for the financial support of this work. LA961065B
