Langmuir 2002, 18, 7753-7755
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Driving Force and Kinetic Studies of Iodide Ion Uptake by Triphenylpyrylium Gallate Microcapsules†,‡ Elena Y. Komarova, Kangtai Ren, and Douglas C. Neckers* Figure 1. Quinaldine Red (QR). Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 Received May 13, 2002. In Final Form: June 27, 2002
Introduction 2,4,6-Triphenylpyrylium tetrakis(pentafluorophenyl)gallate (TPPGa) has been found to be an effective visible light photoinitiator.1 To increase its poor thermal stability, TPPGa was microencapsulated in polystyrene. In the present work, the sorption of small molecules and ions by TPPGa microcapsules has been studied. During the last three decades, a particular interest has been focused on sorbent microparticles that remove various ions from dilute solutions. The adsorption of heavy metal ions such as mercury, lead, chromium, and uranium by chitosanbased beads (modified chitosan polymer) has been extensively investigated.2-8 In addition, chemically modified chitosan particles can effectively adsorb a variety of organic molecules including dyes and polychlorinated biphenyls.9 The driving force and kinetics of iodide ion sorption by TPPGa microparticles are addressed in this paper. Experimental Section Materials and Facilities. All reagents were purchased from Aldrich Chemical Co. and used without further purification unless noted otherwise. TPPGa and microencapsulated TPPGa were prepared according to a procedure reported elsewhere.1 Absorption spectra were recorded on a Hewlett-Packard 8452A diode array UV-vis spectrometer. Scanning electron microscopy (SEM) was performed with a Hitachi-S2700 microscope equipped with a LaB6 crystal as an electron emitter. Samples for SEM were coated with an 8-10 nm film of Au-Pd alloy in a Polaron sputtercoater. Laser scanning confocal microscopy (LSCM) was conducted on a Zeiss 310 LSCM equipped with three lasers: a 543 nm He-Ne, a 488 and 514 nm multiline argon ion, and a 350 nm ultraviolet argon ion laser (Wayne State University, Center for Molecular and Cellular Toxicology with Human Applications). Adsorption Studies. The adsorption of iodide ions was investigated using Quinaldine Red (QR) dye (Figure 1). TPPGa microcapsules were suspended in a solution of QR in methanol [concentrations vary from 1 × 10-5 to 8 × 10-5 M], and the suspension was stirred in the dark. The rates of adsorption were † This paper is dedicated to Professor Dr. J. W. Neckers, on the occasion of his 100th birthday. ‡ Contribution #472. * To whom correspondence should be addressed.
(1) Komarova, E. Y.; Ren, K.; Neckers, D. C. Langmuir 2002, 18, 4195-4197. (2) Piron, E.; Accominotti, M.; Domard, A. Langmuir 1997, 13, 16531658. (3) Guibal, E.; Jansson-Charrier, M.; Saucedo, I.; Le Cloirec, P. Langmuir 1995, 11, 591-598. (4) Lee, S.-T.; Mi, F.-L.; Shen, Y.-J.; Shyu, S.-S. Polymer 2001, 42, 1879-1892. (5) Coughlin, R. W.; Deshaires, M. R. Environ. Prog. 1990, 35-39. (6) Jansson-Charrier, M.; Guibal, E.; Roussy, J.; Delanhe, B.; Le Cloirec, P. Water Res. 1996, 30, 465-475. (7) Udaybhaskar, P.; Iyengar, L.; Prabhakara, A. V. S. J. Appl. Polym. Sci. 1990, 39, 739-747. (8) Volesky, B.; Holan, Z. R. Biotechnol. Prog. 1996, 11, 235-250. (9) Inoue, K.; Baba, Y.; Yoshizuka, K.; Noguchi, H.; Yoshizaki, M. Chem. Lett. 1988, 1281-1284.
obtained by monitoring the disappearance of the absorption band of QR at 530 nm in methanol with UV/vis spectroscopy. The adsorption of tetrabutylammonium iodide (5 × 10-5 M) from methanol solution was detected after the TPPGa microcapsules were dissolved in dichloromethane.
Results and Discussion QR was chosen to investigate the sorption dynamics of TPPGa microcapsules due to strong absorption at 530 nm of the cationic portion of this compound (Figure 1) and the presence of iodide anion. The disappearance of the absorption band of QR at 530 nm where TPPGa does not absorb in methanol solutions has been monitored to follow the kinetics of the process. Dye disappearance has monoexponential character. The rate constant of the process strongly depends on the quantity of the microparticles in the mixture. There is no effect of the concentration of QR on the rate constant of the process. Empty polystyrene beads do not adsorb the dye under identical conditions. The examination of the basic nature of the sorption started with the assumption that complex formation between iodide ion and triphenylpyrylium cation is the initial step of the process. The next step, sorption of the QR cation, happens due to electrostatic forces, that is, to compensate negative charge accumulated in the microparticles because of the adsorbed iodide ions. Driving Force. The triphenylpyrylium cation (TPP+) forms charge-transfer complexes with electron-rich aromatic molecules such as naphthalene.10 In addition, intramolecular charge-transfer complexes are formed by TPP+ with counterions with low ionization potential such as iodide, selenocyanate, 1,1,3,3-tetracyanopropenide, tricyanomethanide, and 1,2,3,4,5-pentacarbomethoxycyclopentadiene.11 Intramolecular charge-transfer complexes with halide anions, especially with iodide, are known for many aromatic cations. Tropylium, pyrylium, and pyridinium iodides are the most well studied.12,13 The iodide salts of the triphenylpyrylium cation possess a distinctive longer wavelength absorption band (540 nm in dichloromethane) assigned to the transfer of an electron from iodide (n-type donor) to a pyrylium vacant orbital (π-type acceptor). Moreover, the pyranyl radical (reduced form of the triphenylpyrylium cation) can be detected in solutions of halide salts of TPP+ in nonpolar solvents.14 In general, pyrylium iodides are prepared by ion exchange reactions.15 The equilibrium constant of the complex formation can be determined by the Benesi and Hildebrand method if the following equilibrium is involved: (10) Miranda, M. A.; Garcı´a, H. Chem. Rev. 1994, 94, 1063-1089. (11) Tamamura, T.; Yasuba, H.; Okamoto, K.; Imai, T.; Kusabayashi, S.; Mikawa, H. Bull. Chem. Soc. Jpn. 1974, 47 (2), 448-454. (12) Feldman, M.; Winstein, S. Tetrahedron Lett. 1962, 19, 853857. (13) Kosower, E. M.; Lindqvist, L. Tetrahedron Lett. 1965, 50, 44814485. (14) Balaban, A. T.; Mocanu, M.; Simon, Z. Tetrahedron 1964, 20, 119-130. (15) Ba˜dilescu, S.; Balaban, A. T. Electrochim. Acta 1976, 32A, 13111318.
10.1021/la025944b CCC: $22.00 © 2002 American Chemical Society Published on Web 08/24/2002
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Langmuir, Vol. 18, No. 20, 2002
Notes
K
TPP+ + Ga- + I- + QR+ {\} TPPI + Ga- + QR+ K)
[TPPI] [TPP+][I-]
where K is the constant of the complex formation (association constant). The Benesi and Hildebrand equation for the complex with one-to-one stoichiometry is16
K[TPP+]c - KAc )
Ac [I-]
where Ac and c are the absorption and extinction coefficients of the complex, and [TPP+] and [I-] are molar concentrations of acceptor and donor. The linear plot of Ac/[I-] versus Ac gives the equilibrium constant as an intercept. K ) 220 L/mol (Figure 2). The adsorption of tetrabutylammonium iodide by TPPGa microcapsules was examined under similar conditions. The UV/vis spectrum of the TPP+ and I- complex was observed after the adsorption step when the microparticles were subsequently dissolved in dichloromethane. The assumption that the driving force of the QR sorption process is the complex formation between the triphenylpyrylium cation and iodide anion was confirmed. The TPPI salt does not fluoresce since intramolecular electron transfer occurs immediately upon excitation.13 QR has a strong fluorescence band (600 nm) when excited at 530 nm. LSCM allows one to determine the distribution of QR in microcapsules separately from TPPGa since TPPGa does not absorb in that region of the spectrum. The LSCM cross section obtained upon 514 nm laser excitation shows that QR is evenly spread in the microparticles (Figure 3, right). A similar distribution was observed for TPPGa in the microparticles (Figure 3, left). Figure 4 shows SEM images of TPPGa microcapsules before (left) and after (right) the adsorption of QR. Most of the microparticles lose spherical shape and become discotic. The microparticles have many cracks indicating that they have been swollen to a significant extent. Similar results (exploded microparticles) were observed for the adsorption of metal ions by chelating resin particles.4 Kinetics of Adsorption. We adopted a strategy for the study of kinetics using models from several studies of biosorbents such as chelating resin microbeads. The nature of the adsorption process of the TPPGa microparticles is similar to that of chelating resins. The kinetics of the ion adsorption process typically includes four steps:2,3,16 (1) transfer of the dissolved ion (molecule) from the whole solution to the boundary film, (2) transport of the ion from the boundary film to the surface of the sorbent particles, (3) transfer of the ion from the surface to the intraparticular active sites, and (4) interaction of the solute molecules with the active sites by complexation or intraparticular precipitation. The first two steps can be neglected in well-stirred suspensions. The fourth step is generally fast. In most of the cases in solution, the intraparticular diffusion is the slowest, rate-limiting step of the sorption process. This phenomenon is described by Fick’s laws.17,18 (16) Guibal, E.; Saucedo, I.; Roussy, J.; Roulph, C.; Le Cloirec, P. Water SA 1993, 19, 119-126. (17) Vieth, W. R. Diffusion In and Through Polymers: Principles and Applications; Hanser Publishers: Munich, 1991.
Figure 2. Association constant determination (Benesi-Hildebrand model).
The steady-state solution of Fick’s law gives the equation2
(C0 - Ct)V ) Dxt m C0 and Ct are concentration of the sorbate at t ) 0 and after time t, D is the diffusion coefficient, V is the volume of the solvent, and m is the mass of the sorbent microparticles. A linear dependence of (C0 - Ct)V/m on the square root of time (t) in QR adsorption experiments indicates that intraparticular diffusion limits the rate of adsorption (Fickian behavior) (Figure 5). The diffusion coefficient can be evaluated. D is equal to 0.12 mg/(g min-0.5) indicating that diffusion is very slow. The partition coefficient (Kd) is determined by the following formula:2
Kd )
V(C0 - Ct) Ctm
Kd is equal to 3375 mL/g at the equilibrium (t f ∞). Another approach to estimate the intraparticular diffusion was described by Nakai and Tachikawa.20,21 The mass transfer for intraparticular diffusion in the case of this model is governed by the differential equation below: 19-21
(
)
∂2q 2 ∂q ∂q ) Di′ 2 + ∂t r ∂r ∂r
where r is the radial variable, q is the sorption capacity, and Di′ is the global diffusion coefficient. The solution of this differential equation if the adsorption rate is independent of the stirring speed and external mass transfer is not the limiting step of the sorption is given by the following equation:19,20
f
( ) [ ( ( )] qt qt ) - log 1 qm qm
2
)
4π2Di′t 2.3d2
where qt is the concentration of the sorbate in the particles at time t, qm is the concentration of the sorbate in the particles at equilibrium (t f ∞), and d is the mean particle diameter. Plotting f(qt/qm) versus time allows one to (18) Polymer Permeability; Comyn, J., Ed.; Elsevier Applied Science Publishers LTD: London, 1985. (19) Hand, D. W.; Crittenden, J. C.; Thacker, W. E. J. Environ. Eng. Div. 1983, 109, 82-101. (20) Urano, K.; Nakai, T. Nippon Kagaku Kaishi 1976, 9, 14861491. (21) Urano, K.; Tachikawa, H. Ind. Eng. Chem. Res. 1991, 30, 18971899.
Notes
Langmuir, Vol. 18, No. 20, 2002 7755
Figure 3. LSCM cross sections of a TPPGa microcapsule before (left) and after (right) the sorption of QR.
Figure 4. SEM images of the microparticles before (left) and after (right) the sorption of QR.
Figure 5. Intraparticular diffusion (Fickian model).
Figure 6. Intraparticular diffusion (Nakai and Tachikawa model).
determine the global diffusion coefficient if intraparticular diffusion is the limiting step of the adsorption process. Di′ is equal to 3.3 × 10-14 (m2/min) (Figure 6). A linear Nakai and Tachikawa relationship confirms that intraparticular diffusion is the limiting step of the QR adsorption by TPPGa microparticles.
process. Intraparticular diffusion is the limiting step of the sorption. The microparticular diffusion coefficients determined by two different kinetics models are much smaller if compared to analogous numbers for chelating resin microbeads, indicating that the diffusion process is much slower in TPPGa microcapsules.
Conclusions Adsorption of iodide anions by TPPGa microcapsules was discovered and studied. Intramolecular chargetransfer complex formation between the triphenylpyrylium cation and the iodide anion is the driving force of the
Acknowledgment. We thank the National Science Foundation Division of Materials Research (DMR 9803006) for financial support of this work. LA025944B
