Cyclic voltammetry of the transfer of anionic surfactant across the liquid–liquid interface manifests electrochemical instability

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  Cyclic voltammetry of the transfer of anionic surfactant across the liquid–liquid interface manifests electrochemical instability
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  Cyclic voltammetry of the transfer of anionic surfactant acrossthe liquid–liquid interface manifests electrochemical instability Takashi Kakiuchi  * , Minako Chiba, Natsuyuki Sezaki, Masatoshi Nakagawa Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan Abstract Experimental evidence for the presence of the instability window in the polarized potential range of the phase-boundary potentialhas been obtained in cyclic voltammograms in the presence of the transfer of anionic surfactants across the 1,2-dichloroethane– water interface. Irregular current spikes and fluctuations appeared in the vicinity of the half-wave potential for the transfer of decylsulfonate and dodecyl sulfate ions. Chaotic current became more pronounced with increasing the concentration of the ionic surf-actant. This trend was in excellent agreement with the theoretical prediction based on the recently proposed concept of the elec-trochemical instability.    2002 Elsevier Science B.V. All rights reserved. Keywords:  Stability; Instability window; Liquid–liquid interface; Ionic surfactant; Decyl sulfonate; Dodecyl sulfate 1. Introduction Recently, we proposed a new concept, electrochemicalinstability, at the liquid–liquid interface [1]. The insta-bility is caused by the potential-dependent adsorptionand partition of an ionic surfactant, i, between the twophases. The partition and the adsorption at the interfacefrom both of the adjacent bulk phases are all dependenton the phase-boundary potential. A thermodynamic linkbetween these three processes predicts that the adsorp-tion becomes maximum at a potential close to the stan-dard ion-transfer potential of i,  D WO / > i  [2]. Thisadsorption in turn causes a decrease in the interfacialtension and may bring about the positive curvature in theelectrocapillary curve. In the polarized potential range of the interface, the existence of this thermodynamicallyforbidden state in the range of the phase-boundary po-tential means the emergence of the instability window inthe vicinity of   D WO / > i  . Cyclic voltammograms of anionicsurfactants across the 1,2-dichloroethane (DCE)–water(W) interface reported in the present communicationclearly attest the presence of this electrochemicalinstability. 2. Experimental Tetrapentylammonium tetraphenylborate (TPnATPB)used as the supporting electrolyte in the DCE phase wasprepared from tetrapentylammonium iodide and sodiumtetraphenylborate as described elsewhere [3]. Sodiumdecyl sulfonate (Nacalai tesque, a reagent grade for ionchromatography) and sodium dodecyl sulfate (Sigma,99%) were used without further purification. All otherchemicals were of reagent grade.The electrochemical cell employed in the presentstudy is Electrochemistry Communications 4 (2002) 701–704www.elsevier.com/locate/elecom 5 mM TPnACl 20 mM TPnATPB 50 mM LiClAg AgCl 10 mM MgCl 2  a  mM anionic AgCl Agsurfactant(W ref  ) (DCE) (W) * Corresponding author. Tel.: +81-75-753-5528; fax: +81-75-753-3360. E-mail address:  kakiuchi@scl.kyoto-u.ac.jp (T. Kakiuchi).1388-2481/02/$ - see front matter    2002 Elsevier Science B.V. All rights reserved.PII: S1388-2481(02)00427-7  where W ref   is the aqueous phase for the reference of thepotential in DCE and TPnACl stands for tetrapenty-lammonium chloride. In the following, the potential of the right-hand side of the Ag/AgCl electrode with re-spect to the left will be denoted as  E   and the currentcarried by the positive charge from W to DCE will betaken to be positive.Electrochemical measurements were made with afour-electrode configuration [4]. The area of the flatpolarized interface was 0.16 cm 2 . A platinum electrodewas used as a counter electrode in each of W and DCEphases. Ag/AgCl electrodes were used as referenceelectrodes in W ref   and W. Cyclic voltammograms wererecorded with a computer-controlled voltammetry sys-tem. The initial potential was selected to be 500 mVwhere no transfer of decyl sulfonate and dodecyl sulfateanions from W to DCE was induced. The solution re-sistance was compensated for with a positive-feedbackmethod. All measurements were made at 25   C. 3. Results and discussion Fig. 1 shows a series of cyclic voltammograms atdifferent scan rates when the W phase contained 0.5mmol dm  3 sodium decyl sulfonate. When the scan rateis relatively high, the shape of the voltammograms is of typical reversible charge transfer of anions: the currentwas proportional to a square root of the scan rate andthe peak separation was 65 mV irrespective of the scanrates, apparently assuring the reversible transfer of decylsulfonate anions across the interface whose mid-pointpotential was 215 mV. However, when the scan rate was10 mV s  1 , irregular pulse-like current spikes appearedon the reverse-scan. Two traces of cyclic voltammo-grams at this scan rate in Fig. 1 (curves 1 and 1 0 ) indicatethat the appearance of this irregularity is reproducible.Aside from the irregular currents in the middle of thevoltammograms, one can discern small irregularities atthe negative end of the polarized potential.When the concentration of sodium decyl sulfonate inW was increased to 1 mmol dm  3 , the irregular currentsbecame more pronounced as shown in Fig. 2 at threedifferent scan rates, 10, 50, and 200 mV s  1 . Even athigher scan rates, the shape of the voltammograms wasanomalous and the degree of the irregularity in thecurrent obviously increased with the lowering of thescan rate. An interesting feature common to three tracesin Fig. 2 is that the current was smooth on the forwardscan from  E  ¼ 500 to 200 mV, the latter of which isabout 15 mV more negative than the mid-point potentialof the transfer of decyl sulfonates shown as a verticaldashed line in the figure.The addition of a non-ionic surfactant may be effec-tive to suppress the irregular current spikes. In nonionic Fig. 1. Cyclic voltammograms for the transfer of decyl sulfonate acrossthe 1,2-dichloroethane–water interface at 25   C, when the aqueousphase contained 0.5 mmol dm  3 sodium decyl sulfonate. Scan rates: 10(curves 1 and 1 0 ), 50 (curve 2), 200 (curve 3), 500 (curve 4), 1000 (curve5) mV s  1 .Fig. 2. Effect of scan rate on cyclic voltammograms when the aqueousphase contained 1 mmol dm  3 sodium decyl sulfonate. Scan rate in mVs  1 is indicated by each line. Dashed line superimposed shows the effectof 1 mmol dm  3 sorbitan monooleate in 1,2-dichloroethane. Verticalline indicates location of mid-point potential.702  T. Kakiuchi et al. / Electrochemistry Communications 4 (2002) 701–704  surfactants having a polyoxyethylene unit as a hydro-philic group can facilitate the transfer of hydrophiliccations [5,6]. However, nonionic surfactants havingsorbitan or sugar moiety as an hydrophilic group do nothave any appreciable ability of complexation with hy-drophilic ions but strongly adsorb in the entire range of the polarized potential of the liquid–liquid interface [7].A dashed line in Fig. 2 shows a cyclic voltammogram atthe scan rate of 50 mV s  1 for the transfer of decylsulfonate (1 mmol dm  3 in W) in the presence of 1 mmoldm  3 sorbitan monooleate in DCE. The irregular cur-rent spikes disappeared and the shape of the voltam-mograms was similar to a reversible transfer of monovalent anions with no surface activity. Comparingthis curve with the solid line recorded at the same scanrate, 50 mV s  1 , one can see that the current in theabsence of sorbitan monooleate (solid line) was normalin the forward scan up to  E  ¼ 250 mV and increased butslightly in the vicinity of the mid-point potential. Thecurrent started to increase appreciably beyond the nor-mal current values expected from the reversible transferwhen  E   was more negative than 200 mV. Marked ir-regular oscillations started at about 170 mV.The observed normal behavior of the voltammo-grams in the forward scan up to the mid-point potentialis in good agreement with the theory of the electro-chemical instability in the presence of potential-depen-dent adsorption and partition of ionic surfactants in theliquid–liquid two-phase systems [1] which predicts thatthe instability may exist in the potential region aroundthe mid-point potential, 215 mV in the present case of decyl sulfonate transfer. After the accumulation of acertain amount of the surfactant at the interface, thedecrease in the interfacial tension is large enough to giverise to a positive curvature in the electrocapillary curve.In this context, the bulk concentration of the ionicsurfactant should affect the stability of the interface.Furthermore, in cyclic voltammetry, the scan rateshould also vary the observability of the electrochemicalstability, as the adsorbed amount must be supplied tothe interface by the mass transfer, which is generallytime-dependent. In addition, the irregularity in currentin Figs. 1 and 2, which seems to be associated with theconvective motion of the solution, is a macroscopicconsequence of numerous incipient events at a micro-scopic level that are induced by the instability of theinterface. A similar current spikes at the liquid–liquidinterface has been ascribed to the avalanche-typetransfer of charged particles [8,9]. In this case, the for-mation of sizable emulsion particles in the vicinity of theinterface seems to precede the avalanche-type transfer. Itis therefore natural to expect that the occurrence of theelectrochemical instability is time-dependent, that is, thescan-rate-dependent in addition to concentration-de-pendent. Experimental facts in Figs. 1 and 2 all agreewith these expectations.The potential of zero charge in the present system islocated around 310 mV in the absence of surfactantadsorption [10]. In the theory of the electrochemicalinstability, ions having the standard ion-transfer po-tential that is close to the potential of zero charge is themost powerful in destabilizing the interface. We alsomeasured cyclic voltammograms for the transfer of dodecyl sulfate. Fig. 3 shows a cyclic voltammogramrecorded at 10 mV s  1 when the aqueous phase con-tained 0.5 mmol dm  3 sodium dodecyl sulfate. Here,again, the irregular increase in current was observed inthe potential range more negative to the mid-pointpotential for the transfer of dodecyl sulfate ions shownas a vertical dashed line. The potential where the in-stability develops shifted to a positive potential by 160mV, which fact indicates that the instability becomesstrongest around the mid-point potential of surfactantions [1]. In the voltammogram in Fig. 3, after thecurrent increased in the negative direction, the furtherscan of the potential caused the recovery of the currentto an almost zero level. This suggests the existence of the instability window, that is, the system may escapeout of the instability in the limiting current region [1].On the reverse scan, again, the current oscillationstarted at  E  ¼ 230 mV but deceased at about 300 mV.In fact, we noticed that this recovery after the oscilla-tion was ill-reproducible, probably because in cyclic orlinear-scan voltammetry, once the oscillation takesplace, the system can be brought far away from theequilibrium or a steady state, which complicates therecovery process. 4. Conclusions A nuisance in voltammetry of ion transfer acrossthe interface, that is, chaotic behavior of the current Fig. 3. Cyclic voltammogram at the scan rate of 10 mV s  1 for thetransfer of dodecyl sulfate across the 1,2-dichloroethane–water inter-face at 25   C. Vertical line indicates location of mid-point potential. T. Kakiuchi et al. / Electrochemistry Communications 4 (2002) 701–704  703  in the interfacial transfer of ionic surfactants, hasturned out to provide ample evidence for the presenceof the electrochemical instability. Trends in the occur-rence of irregularity in the voltammograms are in goodagreement with the theoretical predictions that arebased on the thermodynamic reasoning of the insta-bility window in a certain range of the values of thephase-boundary potential. In view of the wide rele-vance of the electrochemical instability to differentbranches of science and technology [1], there are manyconceivable experiments as well as theoretical studiesrelated to this subject. A study for more elaboratetreatments of the link between specific and non-specificadsorption is underway. More systematic experimentaldata for the interfacial transfer of ionic surfactants willbe published elsewhere [11]. Acknowledgements Authors thank Naoya Nishi for helpful commentsand suggestions. This work was partially supported by aGrant-in-Aid for Scientific Research (No. 14205120), aGrant-in-Aid for Priority Research (No. 13022232), anda Grant-in-Aid for Exploratory Research (No.13875163) from the Ministry of Education, Science,Sports, and Culture, Japan, and CREST of JST (JapanScience and Technology). References [1] T. Kakiuchi, J. Electroanal. Chem., submitted.[2] T. Kakiuchi, J. Electroanal. Chem. 496 (2001) 137–142.[3] N. Nishi, K.I.M. Yamamoto, T. Kakiuchi, J. Phys. Chem. B 105(2001) 8162–8169.[4] Z. Samec, V. Mare  ccek, J. Koryta, M.W. Khalil, J. Electroanal.Chem. 83 (1977) 393–397.[5] Z. Yoshida, S. Kihara, J. Electroanal. Chem. 227 (1987) 171–181.[6] T. Kakiuchi, J. Electroanal. Chem. 345 (1993) 191–203.[7] T. Kakiuchi, Y. Teranishi, K. Niki, Electrochim. Acta 40 (1995)2869–2874.[8] T. Kakiuchi, Electrochem. Commun. 2 (2000) 317–321.[9] M. Nakawaga, N. Sezaki, T. Kakiuchi, J. Electroanal. Chem. 501(2001) 260–264.[10] N. Nishi, T. Kakiuchi, Elektrokhimiya, in press.[11] T. Kakiuchi, N. Nishi, T. Kasahara, M. Chiba, Chem. Phys.Chem., in press.704  T. Kakiuchi et al. / Electrochemistry Communications 4 (2002) 701–704
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