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Coacervation of Surface-Functionalized Polymerized Vesicles Derived from Ammonium Bromide Surfactants. Application to the Selective Speciation of Chromium in Environmental Samples Nikolaos I. Kapakoglou, Dimosthenis L. Giokas,* George Z. Tsogas, and Athanasios G. Vlessidis Department of Chemistry, University of Ioannina, 45110, Ioannina, Greece The potential of polymerized vesicle coacervates made up of ammonium bromide surfactants for the extraction of metallic ions from natural waters was examined for the first time. Linear linked polymerized vesicles prepared by UV excitation of (4-carboxybenzyl)bis[2-(10-undecenoyloxy)ethyl]methylammonium bromide monomer were characterized, and several factors affecting their phase behavior were investigated. Evidently, the permeation of metallic elements through the polymeric membrane was found to be sensitive to ionic radius excluding ions larger than the interbilayer space of the vesicle assembly. To this effect, Cr3+ ions could selectively diffuse through the polymeric membrane. Optimization of vesicle structure and surface charge were the regulating parameters in exploiting this unique feature toward the analytical speciation of Cr species in natural waters. Detection limits as low as 0.1 µg L-1 were achieved by preconcentrating only 10 mL of sample volume with recoveries in the range of 97.0105.5% and very good reproducibility (RSD ) 1.51%). Solventless extraction constitutes today one of the most challenging issues in contemporary chemical analysis with a growing number of publications dedicated to the development, modification, and optimization of systems that minimize or alleviate the use of organic solvents in extraction. Among the most popular liquid-phase microextraction and single-drop microextraction,1,2 coacervation has only recently attracted the interest of researchers as means for obtaining solvent-free extractions. Up to date, despite the plethora of molecules that can give rise to coacervate phases (lipids, proteins, polysaccharides, synthetic polymers, organized assemblies, etc.),3-6 the use of a dehydrating agent (salt, nonsolvent, pH, or temperature) that promotes the interactions among neutral or charged macromol* To whom correspondence should be addressed. Phone: +30-26510-98400. Fax: +30-26510-98781. E-mail:
[email protected]. (1) Lee, J.; Lee, H. K.; Rasmussen, K. E.; Pedersen-Bjergaard, S. Anal. Chim. Acta 2008, 624, 253–268. (2) Xu, L.; Basheer, C.; Lee, H. K. J. Chromatogr., A 2007, 1152, 184–192. (3) Ishii, F.; Takamura, A.; Ishigami, Y. Langmuir 1995, 11, 483–486. (4) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1998, 27, 113–158. (5) Saegusa, K.; Ishii, F. Langmuir 2002, 18, 5984–5988. (6) Gander, B.; Blanco-Prieto, M. J.; Thomasin, C.; Wandrey, Ch.; Hunkeler, D. Coacervation/Phase Separation. In Encyclopedia of Pharmaceutical Technology; Marcel Dekker: New York, 2002. 10.1021/ac802018w CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
ecules predominates the applications of coacervation in analytical extraction processes. A possible explanation can be sought in the difficulty of obtaining functional coacervate phases that offer both distinct phase separation and adequate partitioning of the analytes. In this context, alkyltrimethylammonium micelle-based coacervates in the presence of high electrolyte concentration (e.g., 400 g L-1 NaCl),7 acid-induced coacervation of alkyl sulfates, sulfonates, and sulfoccinates,8 lamellar phases between anionic surfactants and alkaline earth metals,9 alkylcarboxylic acids under the addition of tetrabutylammonium salt,10 coacervates based on alkanoic (C8-C16) and alkenoic (C18) acid reverse micelles and tetrahydrofuran11 have been reported as means for accomplishing solventfree extractions based on coacervation. Depending on the system used, analyte partition is controlled either by hydrophobic interactions in the nonpolar vesicular core or by electrostatic interactions with the hydrophilic head group or in some cases both. In the vastness of available literature regarding the coacervation phenomenon and vesicular media, the polymerized vesicle chemistry in analytical separations has been overlooked. Several studies have shown that many surfactants that have unsaturation in their fatty chains can be polymerized under a variety of conditions placing the unsaturated bonds of adjacent molecules in appropriate proximity for orderly polymerization.12 Free radical initiation causes a chain reaction that results in linking of the surfactant molecules at the locations of the unsaturation. In most cases, the macroscopic order of the system is not disrupted by polymerization, which yields a microscopic “plastic bag”.12 These molecular sacs have three basic features that render them highly attractive for analytical separations. The first is that polymerized membranes are less permeable to ions than those of unpolymerized membranes; therefore, diffusional input or loss of ions may be minimized. Second, the surface of a polymerized vesicle can be chemically modified (functionalized) to promote attachment of various compounds; and third, the surface charge of the vesicles is easily controlled by varying either the composition of the reaction mixture or the relative amounts of three types of (7) Jin, Z.; Zhu, M.; Conte, E. D. Anal. Chem. 1999, 71, 514–517. (8) Casero, I.; Sicilia, D.; Rubio, S.; Pe´rez-Bendito, D. Anal. Chem. 1999, 71, 4519–4526. (9) Giokas, D. L.; Tsogas, G. Z.; Vlessidis, A. G.; Karayannis, M. I. Anal. Chem. 2004, 76, 1302–1309. (10) Ruiz, F. J.; Rubio, S.; Per`ez-Bendito, D. Anal. Chem. 2006, 78, 7229–7239. (11) Ruiz, F. J.; Rubio, S.; Per`ez-Bendito, D. Anal. Chem. 2007, 79, 7473–7484. (12) Guo, C. Y.; Shankar, R. R.; Cai, S. H.; Wu, J. Q.; Abe, S.; Thomas, R. N.; Kup, J. E. Langmuir 1992, 8, 815–823.
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Figure 1. Schematic representation of the polymer structure formed from linear linked 4-CBUAB monomers tethered in the aliphatic chain via CHdCH* reactive groups and potential conformation of head groups.
monofunctional monomers (of negative, neutral, or positive charge) thus enabling a better control of analyte/polymerized vesicle interactions. Despite their high versatility, there is a dearth of information regarding the analytical potential of polymerized vesicles. Several studies have demonstrated the ability of metal-sorbing vesicles (MSVs) that mimic most closely the functional and structural properties of cell membranes, for metal uptake by chemical modification of polymerized vesicles. This is accomplished by carrier-mediated transport of metal ions across the polymerized vesicle bilayer in the presence of a lipophilic metal carrier, which serves as the metal ion carrier through the lipid bilayer.13,14 However, MSVs are limited by the fact that they will not take up metal ions for which the carrier has no affinity and will have no capacity for metal ions that the chelator does not bind. Nevertheless, yet although some systems have been shown to meet both criteria, no analytical data regarding these applications have been reported so far. In view of the lack of published work on the analytical utility of surface functionalized polymerized vesicles, this work describes the first analytical application of polymerized vesicular phases. Parameters related to the formation of the vesicular coacervate phase and its suitability for the extraction of metal ions from environmental samples is evaluated. Evidently, the limited permeability of the polymerized membranes to metallic ions with ionic (13) Shamsai, B. M.; Monbouquette, H. G. J. Membr. Sci. 1997, 130, 173–181. (14) Stanish, I.; Monbouquette, H. G. J. Membr. Sci. 2000, 179, 127–136.
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radius higher than the interbilayer space offers a unique and stable microenvironment for accomplishing the selective and reagentless isolation of Cr3+ species. The interaction of the polymeric membrane with water-soluble and sparingly soluble chelating agents is also assessed and discussed. From an analytical point of view, this is the first study reporting on the application of polymerized vesicles for the coacervate-based extraction of metal ions from aqueous matrixes. EXPERIMENTAL SECTION Reagents. All reagents were of analytical grade and were employed as purchased, unless otherwise stated. Standard metal solutions were prepared by dissolving appropriate amounts of Cr(NO3)3 · 9H2O (Sigma-Aldrich, Athens, Greece, 99.99%) and K2Cr2O7 (Sigma-Aldrich, min 99.5%) in doubly distilled water. 10Undecenoyl chloride (Sigma-Aldrich) N-methyliminobis(ethanol) (Fluka Chemie AG, Switzerland), dimethylformamide (Merck, Darmstadt, Germany), ethyl acetate (Merck), diethyl ether (Merck), NaOH (Merck), CH2Cl2 (Merck), and R-bromo-p-toluic acid (Fluka) used for the synthesis of (4-carboxybenzyl)bis[2-(10undecenoyloxy)ethyl]methylammonium bromide monomer, were used without any further purification. Ammonium pyrrolidinedithiocarbamate (APDC) obtained from Sigma (99.0%) was prepared in doubly distilled water (10%, w/v) while 8-hydroxyquinoline (8HQ) was dissolved in 99.8% spectroscopic grade methanol (Carlo Erba) due to its restricted solubility in water. Humic acid was obtained from Fluka and used as purchased.
Synthesis of the (4-Carboxybenzyl)bis[2-(10-undecenoyloxy)ethyl]methylammonium Bromide Monomer (4-CBUAB) (Figure 1). Polymerized vesicles were synthesized following the procedure of Guo et al.12 Briefly, 10-undecenoyl chloride (44.7 g, 0.22 mol) was added to a solution of 11.9 g (0.10 mol) of N-methyliminobis(ethanol) in 80 mL of DMF. The solution was left to stand for 1 h and the product spontaneously crystallized. Subsequently, 250 mL of diethyl ether was added and the mixture was cooled at -10 °C and filtered. The product was recrystallized with ethyl acetate, affording 43.8 g (90% yield) of the bis[2-(10undecenoyloxycarbonylethyl]methylamine hydrochloride (7HCl). The hydrochloride salt (2.44 g, 5 mmol) was treated with 1 N sodium hydroxide (30 mL) in methylene chloride (40 mL). After solvent removal, the liquid amine that was produced was mixed with R-bromo-p-toluic acid at a ratio of 1.00/1.25 mol. The reaction system was then refluxed at 130 °C overnight under argon flow. The product was purified using methylene chloride. The final product was a yellowish viscous solid with a melting point between 55 and 62 °C, with a yield of ∼55%. Formation of Vesicles. The formation of the vesicles was performed in a 20-mL tube using a heat system ultrasonic processor in continuous mode. The ultrasonication took 15 min to transform the heterogeneous monomer into a clear, homogenized colloidal solution, colored weak smoky blue. After ultrasonication, the vesicle solution was transferred into a quartz tube. Polymerization was carried out under UV irradiation at 450 W for 8-10 h. Instrumentation. A Shimandzu AA-6800 graphite furnace atomic absorption spectrophotometer (GFAAS) with hollow cathode lamp operating at 10 mA was used throughout the measurements, which were made at 357.90 nm. An adjustable-capillary nebulizer and supplies of argon were used for the generation of aerosols and atomization. The output signals were collected and processed in the continuous peak height mode. Infrared (IR) spectra of the polymerized vesicular coacervates dispersed in KBr pellets were recorded on a Perkin-Elmer Spectrum GX FT-IR spectrometer. Absorbance measurements were performed with matched quartz cells of 1-cm path length in a Jenway 6405 UV/ vis spectrophotometer. A pH meter, WTW 552 model glass electrode was employed for pH adjustment of the solutions. Samples. Water samples were collected in glass bottles from four rivers (Loudias, Aliakmon, Axios, and Edessaios) in central Macedonia, Greece. The samples were filtered through a Whatman No. 40 (0.45 µm) filter to remove suspended solids and stored in dark glass containers at 4 °C. Analytical Procedure. In a typical extraction experiment, 10 mL of aqueous solution containing Cr species (Cr3+ and/or CrO42-) in the range of 1-50 µg L-1 was spiked with KCl to adjust the ionic strength of the solution. An appropriate amount of 4-CBUAB polymerized vesicle aqueous solution was added, followed by the addition of 50 µL of HCl to adjust the pH of the solution to the value of 4. The mixture was shaken and left to stand for 15 min at 50 °C in a thermostatic water bath. Separation of the phases was accomplished by centrifugation for 20 min at 4000 rpm. The bulk aqueous phase was decanted and the vesicular phase was treated with a methanolic solution of 1 M HNO3 in order to dissociate the vesicular structure. Twenty microliters of the resulting solution was injected into the atomizer.
Characterization of the Polymerized Vesicular Phase. Surfactant polymerization can occur when the unsaturated bonds present in their fatty chains are linked together at the locations of the unsaturation. The manner in which molecules link together in polymer chains largely depends on the number and location of reactive groups per monomer, as well as the mode of initiation. In our study, surfactant monomers were photochemically excited under UV radiation causing the formation of an activated CHdCH* radical group, which is involved in the polymerization (Figure 1). Since the examined monomers have their reactive groups at the hydrophobic terminus of the amphiphilic tails, which renders insufficient the cross-linking process, we assumed that polymerization proceeds in a linear manner. The linkages among polymers in the vesicle structure play an important role since linear polymerizations cause modest changes in bilayer properties, whereas cross-linking polymerizations can significantly decrease the membrane bilayer fluidity and permeability.15 The polymerization-induced decrease in membrane permeability is associated with a parallel increase in the chemical stability of vesicles, which is observed by resistance to dissolution by either surfactants or organic solvents.16 Experimental evidence on the manner in which monomers are linked together in the polymer chains (linear or cross-linked) was pursued by the addition of Triton X-114 (0-2.5%, w/v) and methanol (0-50%, v/v) in an aqueous solution of polymerized vesicles (0.6%, w/v). The absorbance of the solutions was recorded at 525 nm. Evidently, the presence of
