- Email: [email protected]
Study on the oscillating phenomena of electrical potential across a liquid membrane
Chinese Chemical Letters 18 (2007) 309–312 www.elsevier.com/locate/cclet
Study on the oscillating phenomena of electrical potential across a liquid membrane Jin Zhang Gao *, Hong Xia Dai, Hua Chen, Jie Ren, Wu Yang Chemistry & Chemical Engineering College, Northwest Normal University, Lanzhou 730070, China Received 29 June 2006
Abstract The electrical oscillations across a liquid membrane in water/oil/water system was studied with octanol as oil phase by introducing two opposite charged surfactants in oil and aqueous phase, respectively. The sustained and rhythmic oscillation was observed. To a certain extent, the features of the oscillation (e.g. induction time, frequency, life time and orientation of the pulse pikes) strongly depend on the property of surfactant, dissolved in octanol. The mechanism may be explained by the formation and destruction of dual-ion surfactant membrane accompanying with emulsification at the interface and considering the coupling effect of diffusion and associated reaction in the vicinity of the interface. # 2007 Jin Zhang Gao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Liquid membrane; Chemical oscillation; Electrical potential; Surfactant
In past decades, chemical oscillation as a part of the nonlinear phenomena across the artificial membrane have been studied extensively, which can be an useful approach to understand and simulate the intricate behaviors of biological organ (e.g., nerve impulses, heartbeat and brain rhythms). Up to now, various kinds of sustained and rhythmic oscillations were experimentally observed at an interface between two immiscible phases in the state far from equilibrium [1–4], and some of them have been applied in molecular recognition, such as taste and olfaction [5,6]. At present, two dominating theories have been introduced to explain those oscillations: Marangoni instability and the periodic construction and destruction of surfactant monolayer at the interface. No matter which model mentioned above was applied to elucidate the mechanism of the interfacial oscillation, they are almost common in using simplex surfactant in aqueous phase. In this paper, the potential change at the interface in a new water/oil/water system with n-octanol as the oil phase by introducing two opposite charged surfactants in the water and oil phase, respectively, was detected. Experiments were performed in a H-shaped cell (two glass tubes of 18 mm in diameter and 100 mm in length with a glass bridge of 4 mm in diameter and 10 mm in length). Take cetylpyridinium bromide (CPB) and cetyltrimethylammonium bromide (CTAB) as cationic surfactant while sodium dedecyl sulfate (SDS) and sodium dedecylbenzene sulphonate (SDBS) as anionic surfactant. The solutions were added as the following sequence: a pair of Pt electrodes was inserted into each aqueous phase to measure the electric potential. In the two sides of H-shaped cell, two kinds of aqueous solutions (8.0 mL each) were placed at first to form left side containing 0.5 mol/L NaCl and * Corresponding author. E-mail address: [email protected] (J.Z. Gao). 1001-8417/$ – see front matter # 2007 Jin Zhang Gao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2006.12.025
310
J.Z. Gao et al. / Chinese Chemical Letters 18 (2007) 309–312
Fig. 1. Potential oscillations across the liquid membrane at 293 K with cationic surfactant in oil phase. (a) 8 mmol L 1 SDS + 1.0 mol/L ethanol/ 6 mmol/L CTAB (octanol)/0.5 mol/L NaCl; (b) 8 mmol/L SDBS + 1.0 mol/L ethanol/6 mmol/L CPB (octanol)/0.5 mol/L NaCl; (c) 8 mmol/L SDBS + 1.0 mol/L ethanol/6 mmol/L CTAB(octanol)/0.5 mol/L NaCl; (d) 8 mmol/L SDS + 1.0 mol/L ethanol/6 mmol/L CPB(octanol)/0.5 mol/L NaCl.
right side consisting of the surfactant and 1.0 mol/L ethanol. And then, 8.0 mL of octanol containing the contrary surfactant was laid gently over the tops of two aqueous sides to pass through the bridge to form a whole oil phase. Two Pt-electrodes were connected to a CHI-832 electroanalytical instrument to monitor the electrical potential with prolonging time. Each experiment was repeated at least three times at room temperature (293 2 K) in order to ascertain the reproducibility. Both two type oscillations may occur in the following conditions: C(anionic surfactant)/ C(cationic surfactant) = 0.5–2.0(C(each surfactant) 10 mmol/L), CNaCl = 0.2–0.8 mol/L, Cethanol = 0.8–1.2 mol/L. The optimization was obtained by orthogonal experiment. Two types of system were designed to show oscillations across the liquid member, namely type-I and type-II for convenience. Type-I denoted the system composed of an aqueous solution of sodium chloride/octanol solution of cationic surfactant/an aqueous solution of anionic surfactant with ethanol. On the other hand, interchanging cationic and anionic surfactant in type-I composed type-II systems for studying these influences. As shown in Fig. 1, rhythmic and sustained potential oscillations were easily obtained in type-I system, where the cationic surfactants were dissolved in the octanol and anionic surfactants in aqueous phase. The common character lasted a long induction period (ca. longer than 1000 s) with small amplitude (ca. 40 mV). In addition, the turbidity, accompanied by the synchronous and feeble self-motion, gradually appeared above the interface following contact of the octanol phase with the aqueous phase. It means the precondition to the oscillation was that the cationic and anionic surfactant attracted with each other near the oil/water interface to form the dual-ion membrane or complexes. Compared with type-I, the proposed type-II system displayed different peculiarities (Fig. 2). It oscillated not only without induction time but also the direction of pulse was opposite to the type-I. Moreover, the latter exhibited larger amplitude (ca. 400 mV), shorter life time (ca. 1600 s) and higher frequency than type-I. However, the type-II system also oscillated synchronously with the self-motion, except for a little stronger contrary to the former and with little emulsion, produced in the vicinity of the interface. No oscillating phenomenon was observed if surfactants holding the same charge (e.g. positive charge or negative charge) were used in the two immiscible phases.
J.Z. Gao et al. / Chinese Chemical Letters 18 (2007) 309–312
311
Fig. 2. Potential oscillations across the liquid membrane at 293 K with anionic surfactant in oil phase. (a) 5 mmol/L CTAB + 1.0 mol/L ethanol/ 5 mmol/L SDS(octanol)/0.5 mol/L NaCl; (b) 5 mmol/L CPB + 1.0 mol/L ethanol/5 mmol/L SDS (octanol)/0.5 mol/L NaCl; (c) 5 mmol/L CPB + 1.0 mol/L ethanol/5 mmol/L SDBS (octanol)/0.5 mol/L NaCl; (d) 5 mmol/L CTAB + 1.0 mol/L ethanol/5 mmol/L SDBS (octanol)/ 0.5 mol/L NaCl.
On the basis of the experimental results, the mechanism for the sustained oscillations in the proposed system can be explained as follows: State I: Due to the strong electrostatic attraction, anionic and cationic surfactants moved toward the interface. The surfactant dissolved in oil phase diffused more quickly toward the interface, owing to its smaller assembled number of reversed-micelles. So the interface is temporarily positively (or negatively) charged by the absorption of the surfactant from the oil phase, corresponded to the initial oscillating potential. State II: Accompanied with the evolution of the state I, the surfactant in aqueous phase incorporated with alcohol moved toward the interface. More important, two reverse-charged surfactants bind with each other quite closely at the interface to form dual-ion membrane, which was different with the other oscillatory system previously studied. When the concentration of the surfactant at the interface reached a critical value, the associated reaction took place abruptly with the formation of associated matter, which is corresponding to the emulsion phenomenon and the abrupt potential change for each oscillating circle (also shown in Figs. 1 and 2). State III: The dual-ion layer was destroyed going with the formation of indissoluble associated matter at the interface. And then, the associated matter took some liquid near the interface and diffused to the bulk solutions (mostly to the oil phase). When the concentration of two opposite surfactants on the interface decreased to the lower critical value, the diffusion process became dominant again, resulting in the re-construction of dual-ion layer at the liquid membrane. State IV: When the concentration of anionic and cationic surfactant increased to the upper critical value, abrupt associated reaction between these reverse-charged surfactants accompanying the transformation of the associated matter from the interface to the bulk phase. To continuously increase emulsion changed the primary single-phase to multiphase not only from the microcosmic point but also from the macroscopical point.
312
J.Z. Gao et al. / Chinese Chemical Letters 18 (2007) 309–312
Acknowledgments This work was supported by the International Cooperation Project between China and Ukraine (043–05) and the project of KJCXGC-01 of Northwest Normal University and Gansu Key Lab. of Polymer Materials, China. References [1] [2] [3] [4] [5] [6]
S. Nakata, M. Ukitsu, M. Morimoto, K. Yoshikawa, Mater. Sci. Eng. C 3 (1995) 191. A. Shioi, Y. Sugiura, R. Nagaoka, Langmuir 16 (2000) 8383. V. Pimieta, D. Lavabre, T. Buhse, J.C. Micheau, J. Phys. Chem. B 108 (2004) 7331. J.Z. Gao, L. Wang, W. Yang, F.W. Yang, J. Iran. Chem. Soc. 2 (2005) 71. Y.K. Oshikawa, Y. Matsubara, Langmuir 2 (1985) 230. K. Yoshikawa, M. Shoji, S. Nakata, S. Maeda, Langmuir 4 (1988) 759.
Study on the oscillating phenomena of electrical potential across a liquid membrane Jin Zhang Gao *, Hong Xia Dai, Hua Chen, Jie Ren, Wu Yang Chemistry & Chemical Engineering College, Northwest Normal University, Lanzhou 730070, China Received 29 June 2006
Abstract The electrical oscillations across a liquid membrane in water/oil/water system was studied with octanol as oil phase by introducing two opposite charged surfactants in oil and aqueous phase, respectively. The sustained and rhythmic oscillation was observed. To a certain extent, the features of the oscillation (e.g. induction time, frequency, life time and orientation of the pulse pikes) strongly depend on the property of surfactant, dissolved in octanol. The mechanism may be explained by the formation and destruction of dual-ion surfactant membrane accompanying with emulsification at the interface and considering the coupling effect of diffusion and associated reaction in the vicinity of the interface. # 2007 Jin Zhang Gao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Liquid membrane; Chemical oscillation; Electrical potential; Surfactant
In past decades, chemical oscillation as a part of the nonlinear phenomena across the artificial membrane have been studied extensively, which can be an useful approach to understand and simulate the intricate behaviors of biological organ (e.g., nerve impulses, heartbeat and brain rhythms). Up to now, various kinds of sustained and rhythmic oscillations were experimentally observed at an interface between two immiscible phases in the state far from equilibrium [1–4], and some of them have been applied in molecular recognition, such as taste and olfaction [5,6]. At present, two dominating theories have been introduced to explain those oscillations: Marangoni instability and the periodic construction and destruction of surfactant monolayer at the interface. No matter which model mentioned above was applied to elucidate the mechanism of the interfacial oscillation, they are almost common in using simplex surfactant in aqueous phase. In this paper, the potential change at the interface in a new water/oil/water system with n-octanol as the oil phase by introducing two opposite charged surfactants in the water and oil phase, respectively, was detected. Experiments were performed in a H-shaped cell (two glass tubes of 18 mm in diameter and 100 mm in length with a glass bridge of 4 mm in diameter and 10 mm in length). Take cetylpyridinium bromide (CPB) and cetyltrimethylammonium bromide (CTAB) as cationic surfactant while sodium dedecyl sulfate (SDS) and sodium dedecylbenzene sulphonate (SDBS) as anionic surfactant. The solutions were added as the following sequence: a pair of Pt electrodes was inserted into each aqueous phase to measure the electric potential. In the two sides of H-shaped cell, two kinds of aqueous solutions (8.0 mL each) were placed at first to form left side containing 0.5 mol/L NaCl and * Corresponding author. E-mail address: [email protected] (J.Z. Gao). 1001-8417/$ – see front matter # 2007 Jin Zhang Gao. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2006.12.025
310
J.Z. Gao et al. / Chinese Chemical Letters 18 (2007) 309–312
Fig. 1. Potential oscillations across the liquid membrane at 293 K with cationic surfactant in oil phase. (a) 8 mmol L 1 SDS + 1.0 mol/L ethanol/ 6 mmol/L CTAB (octanol)/0.5 mol/L NaCl; (b) 8 mmol/L SDBS + 1.0 mol/L ethanol/6 mmol/L CPB (octanol)/0.5 mol/L NaCl; (c) 8 mmol/L SDBS + 1.0 mol/L ethanol/6 mmol/L CTAB(octanol)/0.5 mol/L NaCl; (d) 8 mmol/L SDS + 1.0 mol/L ethanol/6 mmol/L CPB(octanol)/0.5 mol/L NaCl.
right side consisting of the surfactant and 1.0 mol/L ethanol. And then, 8.0 mL of octanol containing the contrary surfactant was laid gently over the tops of two aqueous sides to pass through the bridge to form a whole oil phase. Two Pt-electrodes were connected to a CHI-832 electroanalytical instrument to monitor the electrical potential with prolonging time. Each experiment was repeated at least three times at room temperature (293 2 K) in order to ascertain the reproducibility. Both two type oscillations may occur in the following conditions: C(anionic surfactant)/ C(cationic surfactant) = 0.5–2.0(C(each surfactant) 10 mmol/L), CNaCl = 0.2–0.8 mol/L, Cethanol = 0.8–1.2 mol/L. The optimization was obtained by orthogonal experiment. Two types of system were designed to show oscillations across the liquid member, namely type-I and type-II for convenience. Type-I denoted the system composed of an aqueous solution of sodium chloride/octanol solution of cationic surfactant/an aqueous solution of anionic surfactant with ethanol. On the other hand, interchanging cationic and anionic surfactant in type-I composed type-II systems for studying these influences. As shown in Fig. 1, rhythmic and sustained potential oscillations were easily obtained in type-I system, where the cationic surfactants were dissolved in the octanol and anionic surfactants in aqueous phase. The common character lasted a long induction period (ca. longer than 1000 s) with small amplitude (ca. 40 mV). In addition, the turbidity, accompanied by the synchronous and feeble self-motion, gradually appeared above the interface following contact of the octanol phase with the aqueous phase. It means the precondition to the oscillation was that the cationic and anionic surfactant attracted with each other near the oil/water interface to form the dual-ion membrane or complexes. Compared with type-I, the proposed type-II system displayed different peculiarities (Fig. 2). It oscillated not only without induction time but also the direction of pulse was opposite to the type-I. Moreover, the latter exhibited larger amplitude (ca. 400 mV), shorter life time (ca. 1600 s) and higher frequency than type-I. However, the type-II system also oscillated synchronously with the self-motion, except for a little stronger contrary to the former and with little emulsion, produced in the vicinity of the interface. No oscillating phenomenon was observed if surfactants holding the same charge (e.g. positive charge or negative charge) were used in the two immiscible phases.
J.Z. Gao et al. / Chinese Chemical Letters 18 (2007) 309–312
311
Fig. 2. Potential oscillations across the liquid membrane at 293 K with anionic surfactant in oil phase. (a) 5 mmol/L CTAB + 1.0 mol/L ethanol/ 5 mmol/L SDS(octanol)/0.5 mol/L NaCl; (b) 5 mmol/L CPB + 1.0 mol/L ethanol/5 mmol/L SDS (octanol)/0.5 mol/L NaCl; (c) 5 mmol/L CPB + 1.0 mol/L ethanol/5 mmol/L SDBS (octanol)/0.5 mol/L NaCl; (d) 5 mmol/L CTAB + 1.0 mol/L ethanol/5 mmol/L SDBS (octanol)/ 0.5 mol/L NaCl.
On the basis of the experimental results, the mechanism for the sustained oscillations in the proposed system can be explained as follows: State I: Due to the strong electrostatic attraction, anionic and cationic surfactants moved toward the interface. The surfactant dissolved in oil phase diffused more quickly toward the interface, owing to its smaller assembled number of reversed-micelles. So the interface is temporarily positively (or negatively) charged by the absorption of the surfactant from the oil phase, corresponded to the initial oscillating potential. State II: Accompanied with the evolution of the state I, the surfactant in aqueous phase incorporated with alcohol moved toward the interface. More important, two reverse-charged surfactants bind with each other quite closely at the interface to form dual-ion membrane, which was different with the other oscillatory system previously studied. When the concentration of the surfactant at the interface reached a critical value, the associated reaction took place abruptly with the formation of associated matter, which is corresponding to the emulsion phenomenon and the abrupt potential change for each oscillating circle (also shown in Figs. 1 and 2). State III: The dual-ion layer was destroyed going with the formation of indissoluble associated matter at the interface. And then, the associated matter took some liquid near the interface and diffused to the bulk solutions (mostly to the oil phase). When the concentration of two opposite surfactants on the interface decreased to the lower critical value, the diffusion process became dominant again, resulting in the re-construction of dual-ion layer at the liquid membrane. State IV: When the concentration of anionic and cationic surfactant increased to the upper critical value, abrupt associated reaction between these reverse-charged surfactants accompanying the transformation of the associated matter from the interface to the bulk phase. To continuously increase emulsion changed the primary single-phase to multiphase not only from the microcosmic point but also from the macroscopical point.
312
J.Z. Gao et al. / Chinese Chemical Letters 18 (2007) 309–312
Acknowledgments This work was supported by the International Cooperation Project between China and Ukraine (043–05) and the project of KJCXGC-01 of Northwest Normal University and Gansu Key Lab. of Polymer Materials, China. References [1] [2] [3] [4] [5] [6]
S. Nakata, M. Ukitsu, M. Morimoto, K. Yoshikawa, Mater. Sci. Eng. C 3 (1995) 191. A. Shioi, Y. Sugiura, R. Nagaoka, Langmuir 16 (2000) 8383. V. Pimieta, D. Lavabre, T. Buhse, J.C. Micheau, J. Phys. Chem. B 108 (2004) 7331. J.Z. Gao, L. Wang, W. Yang, F.W. Yang, J. Iran. Chem. Soc. 2 (2005) 71. Y.K. Oshikawa, Y. Matsubara, Langmuir 2 (1985) 230. K. Yoshikawa, M. Shoji, S. Nakata, S. Maeda, Langmuir 4 (1988) 759.