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Formation of a polymer layer from monomers adsorbed at a liquid ∣ liquid interface
Journal of Electroanalytical Chemistry 516 (2001) 103– 109 www.elsevier.com/locate/jelechem
Formation of a polymer layer from monomers adsorbed at a liquid liquid interface Kohji Maeda a, Hana Ja¨nchenova´ a, Alexandr Lhotsky´ a, Ivan Stibor b, Jan Budka b, Vladimı´r Marecˇek a,* a
J. Heyro6sky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇko6a 3, 182 23 Prague 8, Czech Republic b Department of Organic Chemistry, Institute of Chemical Technology Prague, Technicka´ 5, 166 28 Prague 6, Czech Republic Received 9 June 2001; received in revised form 18 September 2001; accepted 21 September 2001
Abstract Polymerization of adsorbed monomers at the water 1,2-dichloroethane interface was studied with a series of surface active derivatives of pyrrole. Polymerization was induced by an electron transfer reaction between a monomer dissolved in the organic phase and Ce(SO)4 dissolved in the aqueous phase. The formation of a polymer layer at the interface was monitored by surface tension measurement and by cyclic voltammetry in the presence of transferable ions. It was found that in contrast to a simple adsorbed layer, an interfacial tension in the presence of a polymer layer becomes independent of the applied potential and that a polymer layer strongly inhibits ion transfer reactions. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Immiscible electrolyte solutions; Adsorption; Polymerization; Polymer layer
1. Introduction The formation of monolayers at the liquid liquid interface has recently attracted increasing interest, because of its importance in modeling membrane processes. It also offers a tool to influence the selectivity of ion transfer reactions. Adsorption of e.g. lipids offers an easy and simple way to form an oriented monolayer. However, in contrast to the inhibition of an ion transfer in the presence of a high concentration of lecithin [1], it has been shown that a simple adsorption process can hardly form a compact layer able to influence an ion transfer reaction to an appreciable extent [2 – 4]. One way to increase the stability and density of an adsorbed layer is to form a chemical bond between the adsorbed molecules. This approach would open up the possibility to create an oriented monolayer of specific properties. First a monolayer of oriented molecules has to be formed by adsorption at the interface, followed by their polymerization. In the literature [5,6] electron * Corresponding author. Tel.: + 420-2-8582011; fax: +420-28582307. E-mail address: [email protected] (V. Marecˇek).
transfer reactions across the interface have been reported which result in the formation of oligomers dissolved in the organic phase. At high concentrations oligomers precipitate from solution which leads to polymer formation [6]. In this paper, we demonstrate that a polymer layer can be formed at the water 1,2-dichloroethane interface by electron transfer from an adsorbed monomer to Ce4 + in the aqueous phase. The layer formed is sufficiently compact to impede both cation and anion transfer reactions greatly.
2. Experimental
2.1. Chemicals Compounds 2 and 3 (Fig. 1) were prepared by direct alkylation of 4-(pyrrol-1-yl)phenol (I). Compound 1 was obtained by a two-step reaction, i.e. alkylation of I, followed by hydrolysis of the prepared ethyl ester II giving an acid, the monomer 1. All compounds were characterized by elemental analysis and 1H-NMR spectroscopy.
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2.1.1. Alkylation 4-(Pyrrol-1-yl)phenol (I) (200 mg, 1.25 mmol), potassium carbonate (350 mg, 2.50 mmol), sodium iodide (1 mg) and ethyl bromoacetate (0.28 ml, 2.50 mmol) were refluxed in 60 ml of dry acetone for 3 days. After cooling, acetone was evaporated and the oily residue was dissolved in 50 ml of 1 M HCl and extracted 3 times by 30 ml of chloroform. The organic extract was dried over magnesium sulfate, evaporated and dried at 1 Torr at 80 °C for 4 h to give 290 mg (94%) of pure ethyl 4-(pyrrol-1-yl)phenyloxyacetate (II). 2.1.2. Hydrolysis Compound II (250 mg, 1 mmol) and NaOH (240 mg, 6 mmol) were dissolved in a mixture of 7 ml of water and 10 ml of ethanol, and refluxed overnight. After cooling the mixture was neutralized by 1 M HCl to pH 3 and the free acid was filtered off and dried at 1 Torr at 80 °C for 4 h to give 208 mg (94 %) of pure 4-(pyrrol-1-yl)-phenyloxyacetic acid, the monomer 1. 2.1.3. Prepared compounds 2.1.3.1. 4 -(Pyrrol-1 -yl) -1 -propoxybenzene (monomer 2). Yield 54 %, 1H-NMR (CDCl3, 300 MHz, 25 °C, TMS). l (ppm): 7.30 (d, 2 H, H-arom, J =8.8 Hz), 7.00 (t, 2H, H-pyrrol, J=2.2 Hz), 6.95 (d, 2H, H-arom, J =8.8 Hz), 6.32 (t, 2H, H-pyrrol, J =2.2 Hz), 3.94 (t, 2H, OCH2, J = 6.6 Hz), 1.86–1.79 (m, 2H, OCH2CH2), 1.05 (t, 3H, CH3, J=7.2 Hz). 2.1.3.2. 4 -(Pyrrol-1 -yl) -1 -dodecyloxybenzene (monomer 3). Yield 29%, 1H-NMR (CDCl3, 300 MHz, 25 °C, TMS): l (ppm): 7.29 (d, 2H, H-arom, J = 8.8 Hz), 7.00 (t, 2H, H-pyrrol, J =2.2 Hz), 6.93 (d, 2H, H-arom, J= 9.2 Hz), 6.32 (t, 2H, H-pyrrol, J = 2.2 Hz), 3.96 (t, 2H, O-CH2, J= 6.6 Hz), 1.82– 1.77 (m, 2H,
OCH2CH2), 1.48-1.44 (m, 2H, OCH2CH2-CH2), 1.27 (brs, 16H, 8×CH2), 0.88 (t, 3H, CH3, J= 6.6 Hz).
2.1.3.3. Ethyl 4 -(pyrrol-1 -yl)phenyloxyacetate (II). Yield 94 %, 1H-NMR (CDCl3, 300 MHz, 25 °C, TMS): l (ppm): 7.31 (d, 2H, H-arom, J =8.8 Hz), 7.00 (t, 2H, H-pyrrol, J= 2.2 Hz), 6.96 (d, 2H, H-arom, J= 9.4 Hz), 6.32 (t, 2H, H-pyrrol, J= 2.2 Hz), 4.66 (s, 2H, OCH2CO), 4.29 (q, 2H, OCH2CH3, J= 7.2 Hz), 1.31 (t, 3H, CH3, J= 7.2 Hz). 2.1.3.4. 4 -(Pyrrol-1 -yl)phenyloxyacetic acid (monomer 1). Yield 94 %, 1H-NMR (CDCl3, 300 MHz, 25 °C, TMS): l (ppm): 7.33 (d, 2H, H-arom, J=8.8 Hz), 7.01 (t, 2H, H-pyrrol, J=2.2 Hz), 7.00 (d, 2H, H-arom, J=9.0 Hz), 6.33 (t, 2H, H-pyrrol, J= 2.2 Hz), 4.71 (s, 2H, OCH2CO). 2.1.4. Other chemicals LiCl, KCl, KPF6, tetrabutylammonium chloride (TBACl), tetramethylammonium chloride (TMACl) and dibenzo-18-crown-6 (DB18C6) were purchased from Fluka as reagent grade chemicals, Ce(SO4)2 from Aldrich RDH and H2SO4 from Lachema, Czech Republic. The organic base electrolyte TBACBB [CBB− is a hexabromo derivative of monocarborane: (7,8,9,10,11,12-Br61-CB11H6)−] was a generous gift of Dr S. Herˇma´ nek, Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic. Highly purified and deionized water (GORO system, Czech Republic) was used to prepare the aqueous solutions. 1,2-Dichloroethane (1,2-DCE, Fluka) was purified by passing it twice through columns of basic alumina. 2.2. Apparatus and cells The interfacial tension at water 1,2-DCE was measured using drop shape analysis in a four-electrode arrangement with an aqueous pendant drop electrode [7]. Cyclic voltammetry measurements were carried out in a four-electrode cell [8] with a flat water 1,2-DCE interface having an area of 23.5 mm2. An EG&G 263A potentiostat (Princeton Applied Research, USA) equipped with a home made four-electrode adapter was used for cyclic voltammetry measurements. The potential E of the cell was related to the formal potential difference for tetrabutylammonium ion 0 Dw o TBA + = − 0.226 V [8]. The surface tension measurements were carried out at 2989 0.1 K, all other experiments at 29692 K.
3. Results and discussion
Fig. 1. Structure of the monomers.
Oxidation potentials of monomers 1–3 were determined by cyclic voltammetry at a 25 mm gold mi-
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Fig. 2. Cyclic voltammograms of 0.01 M TBACBB in 1,2-DCE (a) without and (b) with 1 mM monomer 1. Reference electrode Ag – AgCl in a 0.01 M aqueous solution of TBACl. Polarization rate 0.05 V s − 1. Cyclic voltammograms of 0.01 M LiCl, 1.5 M H2SO4 in water (c) without and (d) with 0.01 M Ce(SO4)2. Reference electrode Ag –AgCl, polarization rate 0.01 V s − 1. Working electrode: 25 mm gold microelectrode.
croelectrode in 1,2-DCE. The oxidation potential of all monomers is relatively high (see curve b in Fig. 2, the cyclic voltammogram of monomer 1). Curve d in Fig. 2 corresponds to the reduction wave of Ce4 + in 1.5 M H2SO4 aqueous solution. Because the reference electrodes in these experiments were the same as those in the cell for liquid liquid experiments, the feasibility of an electron transfer reaction between this redox species separated by a liquid liquid interface can be estimated directly from Fig. 2. The difference between the standard potentials of the two redox reactions appears to be less than 0.5 V and falls inside the potential range available in the given liquid liquid system. The organic base electrolyte TBACBB was proved to be inactive in the potential range under study (curve a in Fig. 2).
curves a and c. The surface tension decrease in the presence of monomer 1 is greater (curve c) than in the presence of monomer 2 (curve a). The surface activity of monomer 1 is enhanced in alkaline solution due to the dissociation of proton from the carboxylic group displaying the effect of charge on the adsorption [9]. The surface tension decrease from 28 to 16 mN m − 1 at E= 0.3 V is observed when the pH of the aqueous
3.1. Surface tension measurements In a strongly acidic medium of the aqueous phase which is neceseary to dissolve Ce4 + , the nitrogen atom in the pyrrole group of the monomers can be protonised. The presence of a positive charge can influence the adsorption of monomers at the interface. To examine the role of protonization of pyrrole in this adsorption process three different derivatives of pyrrole were synthesized. While adsorption of the monomer 1 was anticipated due to the presence of a carboxylic group in the molecule (Fig. 1), adsorption of the other two monomers was observed only if protonization of the pyrrole group produced partial hydrophilicity of the molecule. The dependence of surface tension on the applied potential in the absence of Ce4 + is shown in Fig. 3,
Fig. 3. Surface tension at the water 1,2-DCE interface in the presence of 1 mM: (a, b) monomer 2, (c, d) monomer 1 in the organic phase. Curve b was recorded after 35 min electrolysis at 0.55 V. Curve d shows five measurements each after 17 min of the polymerization reaction with a new drop. Aqueous phase: 0.01 M LiCl, 1.5 M H2SO4, with or without 0.01 M Ce(SO4)2. Organic phase: 5 mM TBACBB, 1 mM monomer.
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Fig. 4. Time dependence of surface tension at the water 1,2-DCE interface in the presence of 1 mM monomer 1 in the organic phase. Composition of phases is the same as in Fig. 3.
quently, the surface layer formed in the presence of Ce4 + is very stable and does not desorb at low potentials. Also the surface tension decrease is much faster in the presence of Ce4 + (Fig. 4) due to inhibition of the desorption of monomer 1 in the presence of Ce4 + . Compression of the drop with the layer formed gives a typical, time independent drop shape distortion, ‘a wrinkled skin’. These results indicate that the surface layer formed in the presence of Ce4 + is not a simple adsorbed layer. Such behavior can be anticipated only when adsorbed molecules are bound together to some extent by formation of a polymer layer. The polymerization reaction is in this case induced by electron transfer from adsorbed monomer 1 to Ce4 + . In contrast to monomer 1, the surface tension in the presence of monomers 2 and 3 did not change significantly after addition of Ce4 + to the aqueous phase (Fig. 3, curve b). This indicates that the polymerization reaction does not proceed, probably due to low surface concentrations of monomers 2 and 3 in the adsorbed layer. Enhancement of the adsorption of monomers 2 and 3 induced by protonization of the nitrogen atom in pyrrole, and formation of a polymer layer by a homogeneous electron transfer reaction in the aqueous phase was not observed.
3.2. Voltammetry
Fig. 5. Cyclic voltammograms of TMA+ transfer across the water 1,2-DCE interface in the presence of monomer 1 in the organic phase and Ce4 + in the aqueous phase. Aqueous phase: 0.01 M LiCl+1.5 M H2SO4 +0.01 M Ce(SO4)2 + 0.5 mM TMACl. Organic phase: 5 mM TBACBB+ 0.94 mM monomer 1. Delay between scans 1 min at 0.35 V. Polarization rate 0.1 V s − 1.
phase is increased from 0.4 to 12 in the presence of 50 mM of monomer 1 in the organic phase. The limiting surface concentration Ym of monomer 1 in acidic solution was calculated from adsorption data using the state equation based on a Langmuir-type expression. At E=0.15 V, Ym = 1.4 × 10 − 6 mol m − 2. A large effect on surface tension is observed after addition of Ce4 + to the aqueous phase in the presence of monomer 1 (Fig. 3, curve d). The surface tension is low and independent of the applied potential. Conse-
The transfer reaction of TMA+, PF− 6 and the facilitated transfer of K+ and H+ by DB18C6 were used to monitor formation of a polymer layer at the water 1,2DCE interface. Cyclic voltammetry measurements were performed both with and without Ce4 + ions in the aqueous phase. In the absence of Ce4 + no polymerization reaction can proceed and only an adsorbed layer of the monomer dissolved in the organic phase is formed. In this case, no effect on the charge transfer reactions was found for any of the monomers under study. In contrast, with Ce4 + in the aqueous phase and monomer 1 in the organic phase, the kinetics of the charge transfer reactions, except the facilitated H+ transfer, were strongly influenced. This indicates that a surface polymer layer is created, which is evidently more compact than a simple adsorbed layer. The charge transfer reactions were not influenced in the presence of monomers 2 and 3, evidently because no compact layer at the interface was formed. Cyclic voltammograms of TMA+ transfer across the water 1,2-DCE interface in the presence of Ce4 + in the aqueous phase are shown in Fig. 5. The first scan, recorded immediately after filling the cell is the same as a reversible voltammogram in the absence of Ce4 + . With increasing number of scans recorded always after 1 min, the potential difference of the forward and the reverse peaks increases and the peak currents decreases.
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Finally, almost no peak is observable on the 18th voltammetric cycle. Similar results were obtained for the transfer of PF6− from water to the organic phase (Fig. 6). The same influence of the compact layer on the transfer of cation and anion indicates that the layer provides a barrier which is of a mechanical rather than of an electrostatic nature. The decrease of current at the peak potential related to the initial current of TMA+ or PF6− transfer is shown in Fig. 7. With a higher concentration of monomer 1 the current decrease is faster (curves a and b). This demonstrates that the compact layer formation
Fig. 8. Shift of the reverse peak potential of TMA+ transfer with the number of scans. Concentration of monomer 1: (a) 0.94 mM, cf. Fig. 5; (b) 1.4 mM.
Fig. 6. Cyclic voltammograms of PF− 6 transfer across the water 1,2DCE interface in the presence of monomer 1 in the organic phase and Ce4 + in the aqueous phase. Aqueous phase: 0.01 M LiCl + 1.5 M H2SO4 +0.01 M Ce(SO4)2 + 0.5 mM KPF6. Organic phase: 5 mM TBACBB+0.94 mM monomer 1. Delay between scans 1 min at 0.35 V. Polarization rate 0.1 V s − 1.
Fig. 7. Relative decrease of current of TMA+ and PF− 6 transfer at a constant potential with the number of scan. Concentration of monomer 1: (a) 1.4 mM; (b, c) 0.94 mM, cf. Figs. 5 and 6.
is faster with a higher concentration of monomer 1. The decrease of the PF6− current is somewhat slower than that of TMA+ (curves b and c in Fig. 7) which could result from the size of the ions. The change of the rate constant of TMA+ transfer with the number of scans can be estimated from the shift of the reverse peak potential, since the forward peak potential is hardly detectable. Fig. 8 shows the shift of the reverse peak with the number of scans for the polarization rate 6= 0.1 V s − 1. It can be deduced [10] that a shift of 0.13 V of the reverse peak, obtained at the fifth scan for 1.4 mM concentration of monomer 1, corresponds to more than a sixfold decrease of the rate constant. The effect of a polymer layer on the facilitated transfer of H+ by DB18C6 is negligible. The current peaks of this facilitated transfer do not depend on the presence of Ce4 + in the aqueous phase (Fig. 9). When potassium ions are added to the aqueous phase in the absence of Ce4 + , a new wave appears at a less positive potential (Fig. 10), which corresponds to a facilitated transfer of K+ by DB18C6. Because the formation of a complex of DB18C6 with K+ is thermodynamically favorable, the facilitated H+ transfer reaction is suppressed and its voltammetric peak is missing. The comparison of voltammograms in Figs. 10 and 11 indicates that in the presence of the polymer layer the facilitated transfer of K+ becomes slower and a potential shift of the current peaks occurs. At a polarization rate 6\0.2 V s − 1, the potential shift of the forward current peak exceeds the peak potential difference of 0.057 V measured for the reversible facilitated transfer, cf. Figs. 9 and 10. A wave corresponding to facilitated H+ transfer is then partially recovered (Fig. 11). This demonstrates that not all DB18C6 ligands were consumed in the reaction with K+ when the potential difference E
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reached the value at which the facilitated H+ transfer reaction proceeds. Direct measurement of the electron transfer reaction between monomer 1 in the organic phase and Ce4 + in the aqueous phase was not successful. No difference was observed in the current transient of a potentiostatic pulse from 0.2 to 0.5 V or in voltammetric curves in the freshly prepared cell in the presence and absence of Ce4 + . This is understandable when we take into ac-
Fig. 11. Cyclic voltammograms of a facilitated transfer of K+ and H+ by DB18C6 across a modified water 1,2-DCE interface. Aqueous phase: 0.01 M KCl + 1.5 M H2SO4 +0.01 M Ce(SO4)2. Organic phase: 5 mM TBACBB +0.5 mM DB18C6+ 1.4 mM monomer 1. Polarization rates from inner to outer curve 0.02, 0.05, 0.1, 0.2, 0.5 and 1 V s − 1.
Fig. 9. Cyclic voltammograms of a facilitated transfer of H+ by DB18C6 across a modified water 1,2-DCE interface. Aqueous phase: 0.01 M LiCl+ 1.5 M H2SO4 + 0.01 M Ce(SO4)2. Organic phase: 5 mM TBACBB+ 0.5 mM DB18C6+1.4 mM monomer 1. Polarization rates from inner to outer curve 0.02, 0.05, 0.1, 0.2 and 0.5 V s − 1.
count that the polymerization reaction is very slow (Figs. 7 and 8) and the charge passed is small. If the surface concentration of the monomer 1 is of the order of 10 − 6 mol m − 2, the area of the interface A =23.5 mm2 and the.reaction time #5 min, the current will be in the range of 10 − 9 A. This current can hardly be detected when the background base electrolyte current is about three orders of magnitude larger. 4. Conclusions The above results demonstrate that a compact surface layer can be formed at the liquid liquid interface and that its properties differ significantly from a simple adsorbed layer. Even though a direct proof of the heterogeneous electron transfer reaction failed, it is probable that the polymerization reaction at the interface proceeds. This approach opens a new way to create surface layers with properties tailored by choosing a suitable monomer or its mixture with other surface active compounds. Acknowledgements This work was supported by the Grant Agency of the Czech Republic (grant no. 203/00/0636).
Fig. 10. Cyclic voltammograms of a facilitated transfer of K+ by DB18C6 across the water 1,2-DCE interface. Aqueous phase: 0.01 M KCl+ 1.5 M H2SO4. Organic phase: 5 mM TBACBB + 0.5 mM DB18C6+1.4 mM monomer 1. Polarization rates from inner to outer curve 0.02, 0.05, 0.1, 0.2 and 0.5 V s − 1.
References [1] J. Koryta, L.Q. Hung, A. Hofmanova´ , Stud. Biophys. 25 (1982) 90.
K. Maeda et al. / Journal of Electroanalytical Chemistry 516 (2001) 103–109 [2] T. Kakiuchi, T. Kondo, M. Kotani, M. Senda, Langmuir 8 (1992) 169. [3] T. Kakiuchi, M. Kotani, J. Nogachi, M. Nakanishi, M. Senda, J. Colloid Interf. Sci. 149 (1992) 279. [4] A.K. Kontturi, K. Kontturi, L. Murtoma¨ ki, B. Quinn, V.J. Cunnane, J. Electroanal. Chem. 143 (1997) 69. [5] V.J. Cunnane, U. Evans, J. Chem. Soc. Chem. Commun. (1998) 2163. [6] V.J. Cunnane, U. Evans, Euroconference on Modern Trends in
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Electrochemistry of Molecular Interfaces, Majvik August 1999, Finland, Book of Abstracts L 13. V. Marecˇ ek, A. Lhotsky´ , K. Holub, I. Stibor, Electrochim. Acta 44 (1998) 155. V. Marecˇ ek, Z. Samec, J. Electroanal. Chem. 185 (1985) 263. V. Marecˇ ek, A. Lhotsky´ , H. Ja¨ nchenova´ , J. Electroanal. Chem. 483 (2000) 174. V. Marecˇ ek, J. Honz, Coll. Czech. Chem. Commun. 38 (1973) 965.
Formation of a polymer layer from monomers adsorbed at a liquid liquid interface Kohji Maeda a, Hana Ja¨nchenova´ a, Alexandr Lhotsky´ a, Ivan Stibor b, Jan Budka b, Vladimı´r Marecˇek a,* a
J. Heyro6sky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇko6a 3, 182 23 Prague 8, Czech Republic b Department of Organic Chemistry, Institute of Chemical Technology Prague, Technicka´ 5, 166 28 Prague 6, Czech Republic Received 9 June 2001; received in revised form 18 September 2001; accepted 21 September 2001
Abstract Polymerization of adsorbed monomers at the water 1,2-dichloroethane interface was studied with a series of surface active derivatives of pyrrole. Polymerization was induced by an electron transfer reaction between a monomer dissolved in the organic phase and Ce(SO)4 dissolved in the aqueous phase. The formation of a polymer layer at the interface was monitored by surface tension measurement and by cyclic voltammetry in the presence of transferable ions. It was found that in contrast to a simple adsorbed layer, an interfacial tension in the presence of a polymer layer becomes independent of the applied potential and that a polymer layer strongly inhibits ion transfer reactions. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Immiscible electrolyte solutions; Adsorption; Polymerization; Polymer layer
1. Introduction The formation of monolayers at the liquid liquid interface has recently attracted increasing interest, because of its importance in modeling membrane processes. It also offers a tool to influence the selectivity of ion transfer reactions. Adsorption of e.g. lipids offers an easy and simple way to form an oriented monolayer. However, in contrast to the inhibition of an ion transfer in the presence of a high concentration of lecithin [1], it has been shown that a simple adsorption process can hardly form a compact layer able to influence an ion transfer reaction to an appreciable extent [2 – 4]. One way to increase the stability and density of an adsorbed layer is to form a chemical bond between the adsorbed molecules. This approach would open up the possibility to create an oriented monolayer of specific properties. First a monolayer of oriented molecules has to be formed by adsorption at the interface, followed by their polymerization. In the literature [5,6] electron * Corresponding author. Tel.: + 420-2-8582011; fax: +420-28582307. E-mail address: [email protected] (V. Marecˇek).
transfer reactions across the interface have been reported which result in the formation of oligomers dissolved in the organic phase. At high concentrations oligomers precipitate from solution which leads to polymer formation [6]. In this paper, we demonstrate that a polymer layer can be formed at the water 1,2-dichloroethane interface by electron transfer from an adsorbed monomer to Ce4 + in the aqueous phase. The layer formed is sufficiently compact to impede both cation and anion transfer reactions greatly.
2. Experimental
2.1. Chemicals Compounds 2 and 3 (Fig. 1) were prepared by direct alkylation of 4-(pyrrol-1-yl)phenol (I). Compound 1 was obtained by a two-step reaction, i.e. alkylation of I, followed by hydrolysis of the prepared ethyl ester II giving an acid, the monomer 1. All compounds were characterized by elemental analysis and 1H-NMR spectroscopy.
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2.1.1. Alkylation 4-(Pyrrol-1-yl)phenol (I) (200 mg, 1.25 mmol), potassium carbonate (350 mg, 2.50 mmol), sodium iodide (1 mg) and ethyl bromoacetate (0.28 ml, 2.50 mmol) were refluxed in 60 ml of dry acetone for 3 days. After cooling, acetone was evaporated and the oily residue was dissolved in 50 ml of 1 M HCl and extracted 3 times by 30 ml of chloroform. The organic extract was dried over magnesium sulfate, evaporated and dried at 1 Torr at 80 °C for 4 h to give 290 mg (94%) of pure ethyl 4-(pyrrol-1-yl)phenyloxyacetate (II). 2.1.2. Hydrolysis Compound II (250 mg, 1 mmol) and NaOH (240 mg, 6 mmol) were dissolved in a mixture of 7 ml of water and 10 ml of ethanol, and refluxed overnight. After cooling the mixture was neutralized by 1 M HCl to pH 3 and the free acid was filtered off and dried at 1 Torr at 80 °C for 4 h to give 208 mg (94 %) of pure 4-(pyrrol-1-yl)-phenyloxyacetic acid, the monomer 1. 2.1.3. Prepared compounds 2.1.3.1. 4 -(Pyrrol-1 -yl) -1 -propoxybenzene (monomer 2). Yield 54 %, 1H-NMR (CDCl3, 300 MHz, 25 °C, TMS). l (ppm): 7.30 (d, 2 H, H-arom, J =8.8 Hz), 7.00 (t, 2H, H-pyrrol, J=2.2 Hz), 6.95 (d, 2H, H-arom, J =8.8 Hz), 6.32 (t, 2H, H-pyrrol, J =2.2 Hz), 3.94 (t, 2H, OCH2, J = 6.6 Hz), 1.86–1.79 (m, 2H, OCH2CH2), 1.05 (t, 3H, CH3, J=7.2 Hz). 2.1.3.2. 4 -(Pyrrol-1 -yl) -1 -dodecyloxybenzene (monomer 3). Yield 29%, 1H-NMR (CDCl3, 300 MHz, 25 °C, TMS): l (ppm): 7.29 (d, 2H, H-arom, J = 8.8 Hz), 7.00 (t, 2H, H-pyrrol, J =2.2 Hz), 6.93 (d, 2H, H-arom, J= 9.2 Hz), 6.32 (t, 2H, H-pyrrol, J = 2.2 Hz), 3.96 (t, 2H, O-CH2, J= 6.6 Hz), 1.82– 1.77 (m, 2H,
OCH2CH2), 1.48-1.44 (m, 2H, OCH2CH2-CH2), 1.27 (brs, 16H, 8×CH2), 0.88 (t, 3H, CH3, J= 6.6 Hz).
2.1.3.3. Ethyl 4 -(pyrrol-1 -yl)phenyloxyacetate (II). Yield 94 %, 1H-NMR (CDCl3, 300 MHz, 25 °C, TMS): l (ppm): 7.31 (d, 2H, H-arom, J =8.8 Hz), 7.00 (t, 2H, H-pyrrol, J= 2.2 Hz), 6.96 (d, 2H, H-arom, J= 9.4 Hz), 6.32 (t, 2H, H-pyrrol, J= 2.2 Hz), 4.66 (s, 2H, OCH2CO), 4.29 (q, 2H, OCH2CH3, J= 7.2 Hz), 1.31 (t, 3H, CH3, J= 7.2 Hz). 2.1.3.4. 4 -(Pyrrol-1 -yl)phenyloxyacetic acid (monomer 1). Yield 94 %, 1H-NMR (CDCl3, 300 MHz, 25 °C, TMS): l (ppm): 7.33 (d, 2H, H-arom, J=8.8 Hz), 7.01 (t, 2H, H-pyrrol, J=2.2 Hz), 7.00 (d, 2H, H-arom, J=9.0 Hz), 6.33 (t, 2H, H-pyrrol, J= 2.2 Hz), 4.71 (s, 2H, OCH2CO). 2.1.4. Other chemicals LiCl, KCl, KPF6, tetrabutylammonium chloride (TBACl), tetramethylammonium chloride (TMACl) and dibenzo-18-crown-6 (DB18C6) were purchased from Fluka as reagent grade chemicals, Ce(SO4)2 from Aldrich RDH and H2SO4 from Lachema, Czech Republic. The organic base electrolyte TBACBB [CBB− is a hexabromo derivative of monocarborane: (7,8,9,10,11,12-Br61-CB11H6)−] was a generous gift of Dr S. Herˇma´ nek, Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic. Highly purified and deionized water (GORO system, Czech Republic) was used to prepare the aqueous solutions. 1,2-Dichloroethane (1,2-DCE, Fluka) was purified by passing it twice through columns of basic alumina. 2.2. Apparatus and cells The interfacial tension at water 1,2-DCE was measured using drop shape analysis in a four-electrode arrangement with an aqueous pendant drop electrode [7]. Cyclic voltammetry measurements were carried out in a four-electrode cell [8] with a flat water 1,2-DCE interface having an area of 23.5 mm2. An EG&G 263A potentiostat (Princeton Applied Research, USA) equipped with a home made four-electrode adapter was used for cyclic voltammetry measurements. The potential E of the cell was related to the formal potential difference for tetrabutylammonium ion 0 Dw o TBA + = − 0.226 V [8]. The surface tension measurements were carried out at 2989 0.1 K, all other experiments at 29692 K.
3. Results and discussion
Fig. 1. Structure of the monomers.
Oxidation potentials of monomers 1–3 were determined by cyclic voltammetry at a 25 mm gold mi-
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Fig. 2. Cyclic voltammograms of 0.01 M TBACBB in 1,2-DCE (a) without and (b) with 1 mM monomer 1. Reference electrode Ag – AgCl in a 0.01 M aqueous solution of TBACl. Polarization rate 0.05 V s − 1. Cyclic voltammograms of 0.01 M LiCl, 1.5 M H2SO4 in water (c) without and (d) with 0.01 M Ce(SO4)2. Reference electrode Ag –AgCl, polarization rate 0.01 V s − 1. Working electrode: 25 mm gold microelectrode.
croelectrode in 1,2-DCE. The oxidation potential of all monomers is relatively high (see curve b in Fig. 2, the cyclic voltammogram of monomer 1). Curve d in Fig. 2 corresponds to the reduction wave of Ce4 + in 1.5 M H2SO4 aqueous solution. Because the reference electrodes in these experiments were the same as those in the cell for liquid liquid experiments, the feasibility of an electron transfer reaction between this redox species separated by a liquid liquid interface can be estimated directly from Fig. 2. The difference between the standard potentials of the two redox reactions appears to be less than 0.5 V and falls inside the potential range available in the given liquid liquid system. The organic base electrolyte TBACBB was proved to be inactive in the potential range under study (curve a in Fig. 2).
curves a and c. The surface tension decrease in the presence of monomer 1 is greater (curve c) than in the presence of monomer 2 (curve a). The surface activity of monomer 1 is enhanced in alkaline solution due to the dissociation of proton from the carboxylic group displaying the effect of charge on the adsorption [9]. The surface tension decrease from 28 to 16 mN m − 1 at E= 0.3 V is observed when the pH of the aqueous
3.1. Surface tension measurements In a strongly acidic medium of the aqueous phase which is neceseary to dissolve Ce4 + , the nitrogen atom in the pyrrole group of the monomers can be protonised. The presence of a positive charge can influence the adsorption of monomers at the interface. To examine the role of protonization of pyrrole in this adsorption process three different derivatives of pyrrole were synthesized. While adsorption of the monomer 1 was anticipated due to the presence of a carboxylic group in the molecule (Fig. 1), adsorption of the other two monomers was observed only if protonization of the pyrrole group produced partial hydrophilicity of the molecule. The dependence of surface tension on the applied potential in the absence of Ce4 + is shown in Fig. 3,
Fig. 3. Surface tension at the water 1,2-DCE interface in the presence of 1 mM: (a, b) monomer 2, (c, d) monomer 1 in the organic phase. Curve b was recorded after 35 min electrolysis at 0.55 V. Curve d shows five measurements each after 17 min of the polymerization reaction with a new drop. Aqueous phase: 0.01 M LiCl, 1.5 M H2SO4, with or without 0.01 M Ce(SO4)2. Organic phase: 5 mM TBACBB, 1 mM monomer.
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Fig. 4. Time dependence of surface tension at the water 1,2-DCE interface in the presence of 1 mM monomer 1 in the organic phase. Composition of phases is the same as in Fig. 3.
quently, the surface layer formed in the presence of Ce4 + is very stable and does not desorb at low potentials. Also the surface tension decrease is much faster in the presence of Ce4 + (Fig. 4) due to inhibition of the desorption of monomer 1 in the presence of Ce4 + . Compression of the drop with the layer formed gives a typical, time independent drop shape distortion, ‘a wrinkled skin’. These results indicate that the surface layer formed in the presence of Ce4 + is not a simple adsorbed layer. Such behavior can be anticipated only when adsorbed molecules are bound together to some extent by formation of a polymer layer. The polymerization reaction is in this case induced by electron transfer from adsorbed monomer 1 to Ce4 + . In contrast to monomer 1, the surface tension in the presence of monomers 2 and 3 did not change significantly after addition of Ce4 + to the aqueous phase (Fig. 3, curve b). This indicates that the polymerization reaction does not proceed, probably due to low surface concentrations of monomers 2 and 3 in the adsorbed layer. Enhancement of the adsorption of monomers 2 and 3 induced by protonization of the nitrogen atom in pyrrole, and formation of a polymer layer by a homogeneous electron transfer reaction in the aqueous phase was not observed.
3.2. Voltammetry
Fig. 5. Cyclic voltammograms of TMA+ transfer across the water 1,2-DCE interface in the presence of monomer 1 in the organic phase and Ce4 + in the aqueous phase. Aqueous phase: 0.01 M LiCl+1.5 M H2SO4 +0.01 M Ce(SO4)2 + 0.5 mM TMACl. Organic phase: 5 mM TBACBB+ 0.94 mM monomer 1. Delay between scans 1 min at 0.35 V. Polarization rate 0.1 V s − 1.
phase is increased from 0.4 to 12 in the presence of 50 mM of monomer 1 in the organic phase. The limiting surface concentration Ym of monomer 1 in acidic solution was calculated from adsorption data using the state equation based on a Langmuir-type expression. At E=0.15 V, Ym = 1.4 × 10 − 6 mol m − 2. A large effect on surface tension is observed after addition of Ce4 + to the aqueous phase in the presence of monomer 1 (Fig. 3, curve d). The surface tension is low and independent of the applied potential. Conse-
The transfer reaction of TMA+, PF− 6 and the facilitated transfer of K+ and H+ by DB18C6 were used to monitor formation of a polymer layer at the water 1,2DCE interface. Cyclic voltammetry measurements were performed both with and without Ce4 + ions in the aqueous phase. In the absence of Ce4 + no polymerization reaction can proceed and only an adsorbed layer of the monomer dissolved in the organic phase is formed. In this case, no effect on the charge transfer reactions was found for any of the monomers under study. In contrast, with Ce4 + in the aqueous phase and monomer 1 in the organic phase, the kinetics of the charge transfer reactions, except the facilitated H+ transfer, were strongly influenced. This indicates that a surface polymer layer is created, which is evidently more compact than a simple adsorbed layer. The charge transfer reactions were not influenced in the presence of monomers 2 and 3, evidently because no compact layer at the interface was formed. Cyclic voltammograms of TMA+ transfer across the water 1,2-DCE interface in the presence of Ce4 + in the aqueous phase are shown in Fig. 5. The first scan, recorded immediately after filling the cell is the same as a reversible voltammogram in the absence of Ce4 + . With increasing number of scans recorded always after 1 min, the potential difference of the forward and the reverse peaks increases and the peak currents decreases.
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Finally, almost no peak is observable on the 18th voltammetric cycle. Similar results were obtained for the transfer of PF6− from water to the organic phase (Fig. 6). The same influence of the compact layer on the transfer of cation and anion indicates that the layer provides a barrier which is of a mechanical rather than of an electrostatic nature. The decrease of current at the peak potential related to the initial current of TMA+ or PF6− transfer is shown in Fig. 7. With a higher concentration of monomer 1 the current decrease is faster (curves a and b). This demonstrates that the compact layer formation
Fig. 8. Shift of the reverse peak potential of TMA+ transfer with the number of scans. Concentration of monomer 1: (a) 0.94 mM, cf. Fig. 5; (b) 1.4 mM.
Fig. 6. Cyclic voltammograms of PF− 6 transfer across the water 1,2DCE interface in the presence of monomer 1 in the organic phase and Ce4 + in the aqueous phase. Aqueous phase: 0.01 M LiCl + 1.5 M H2SO4 +0.01 M Ce(SO4)2 + 0.5 mM KPF6. Organic phase: 5 mM TBACBB+0.94 mM monomer 1. Delay between scans 1 min at 0.35 V. Polarization rate 0.1 V s − 1.
Fig. 7. Relative decrease of current of TMA+ and PF− 6 transfer at a constant potential with the number of scan. Concentration of monomer 1: (a) 1.4 mM; (b, c) 0.94 mM, cf. Figs. 5 and 6.
is faster with a higher concentration of monomer 1. The decrease of the PF6− current is somewhat slower than that of TMA+ (curves b and c in Fig. 7) which could result from the size of the ions. The change of the rate constant of TMA+ transfer with the number of scans can be estimated from the shift of the reverse peak potential, since the forward peak potential is hardly detectable. Fig. 8 shows the shift of the reverse peak with the number of scans for the polarization rate 6= 0.1 V s − 1. It can be deduced [10] that a shift of 0.13 V of the reverse peak, obtained at the fifth scan for 1.4 mM concentration of monomer 1, corresponds to more than a sixfold decrease of the rate constant. The effect of a polymer layer on the facilitated transfer of H+ by DB18C6 is negligible. The current peaks of this facilitated transfer do not depend on the presence of Ce4 + in the aqueous phase (Fig. 9). When potassium ions are added to the aqueous phase in the absence of Ce4 + , a new wave appears at a less positive potential (Fig. 10), which corresponds to a facilitated transfer of K+ by DB18C6. Because the formation of a complex of DB18C6 with K+ is thermodynamically favorable, the facilitated H+ transfer reaction is suppressed and its voltammetric peak is missing. The comparison of voltammograms in Figs. 10 and 11 indicates that in the presence of the polymer layer the facilitated transfer of K+ becomes slower and a potential shift of the current peaks occurs. At a polarization rate 6\0.2 V s − 1, the potential shift of the forward current peak exceeds the peak potential difference of 0.057 V measured for the reversible facilitated transfer, cf. Figs. 9 and 10. A wave corresponding to facilitated H+ transfer is then partially recovered (Fig. 11). This demonstrates that not all DB18C6 ligands were consumed in the reaction with K+ when the potential difference E
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reached the value at which the facilitated H+ transfer reaction proceeds. Direct measurement of the electron transfer reaction between monomer 1 in the organic phase and Ce4 + in the aqueous phase was not successful. No difference was observed in the current transient of a potentiostatic pulse from 0.2 to 0.5 V or in voltammetric curves in the freshly prepared cell in the presence and absence of Ce4 + . This is understandable when we take into ac-
Fig. 11. Cyclic voltammograms of a facilitated transfer of K+ and H+ by DB18C6 across a modified water 1,2-DCE interface. Aqueous phase: 0.01 M KCl + 1.5 M H2SO4 +0.01 M Ce(SO4)2. Organic phase: 5 mM TBACBB +0.5 mM DB18C6+ 1.4 mM monomer 1. Polarization rates from inner to outer curve 0.02, 0.05, 0.1, 0.2, 0.5 and 1 V s − 1.
Fig. 9. Cyclic voltammograms of a facilitated transfer of H+ by DB18C6 across a modified water 1,2-DCE interface. Aqueous phase: 0.01 M LiCl+ 1.5 M H2SO4 + 0.01 M Ce(SO4)2. Organic phase: 5 mM TBACBB+ 0.5 mM DB18C6+1.4 mM monomer 1. Polarization rates from inner to outer curve 0.02, 0.05, 0.1, 0.2 and 0.5 V s − 1.
count that the polymerization reaction is very slow (Figs. 7 and 8) and the charge passed is small. If the surface concentration of the monomer 1 is of the order of 10 − 6 mol m − 2, the area of the interface A =23.5 mm2 and the.reaction time #5 min, the current will be in the range of 10 − 9 A. This current can hardly be detected when the background base electrolyte current is about three orders of magnitude larger. 4. Conclusions The above results demonstrate that a compact surface layer can be formed at the liquid liquid interface and that its properties differ significantly from a simple adsorbed layer. Even though a direct proof of the heterogeneous electron transfer reaction failed, it is probable that the polymerization reaction at the interface proceeds. This approach opens a new way to create surface layers with properties tailored by choosing a suitable monomer or its mixture with other surface active compounds. Acknowledgements This work was supported by the Grant Agency of the Czech Republic (grant no. 203/00/0636).
Fig. 10. Cyclic voltammograms of a facilitated transfer of K+ by DB18C6 across the water 1,2-DCE interface. Aqueous phase: 0.01 M KCl+ 1.5 M H2SO4. Organic phase: 5 mM TBACBB + 0.5 mM DB18C6+1.4 mM monomer 1. Polarization rates from inner to outer curve 0.02, 0.05, 0.1, 0.2 and 0.5 V s − 1.
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