ELECTROCHEMISTRY SPECIAL CHEMPHYSCHEM 2003,4,179±185 ¹ 2003 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 1439-4235/03/04/02 $ 20.00+.50/0 179 Regular Irregularity in the Transfer of Anionic Surfactant across the Liquid/Liquid Interface Takashi Kakiuchi,* Naoya Nishi, Takuya Kasahara, and Minako Chiba [a] Irregular current spikes and other anomalies seen in voltammetry of the transfer of anionic surfactants, alkyl sulfonates, and alkyl sulfates across the 1,2-dichloroethane/water (DCE/W) interface are reproducible. The anomalies have a certain regularity that is predicted by a recently proposed concept, the electrochemical instability. Irregular current spikes follow after the augmentation of the current induced when the phase-boundary potential is brought close to the mid-point potential of the transferring surfactant ions. Potential-step chronoamperometry clearly demonstrates the pres- ence of the instability window, that is, the potential region where the interface becomes unstable only in the limited range of the phase-boundary potential. KEYWORDS: alkyl sulfonates ¥ alkyl sulfates ¥ electrochemistry ¥ instability window ¥ ionic surfactants ¥ liquid/liquid interfaces Advances in the electrochemistry of charge transfer in liquid/ liquid two-phase systems have enabled us to determine fundamental properties of ion transfer with great ease by using voltammetric techniques. [1±3] Voltammetric measurements have been used to determine the apparent values of the standard Gibbs energy of transfer of various ionic species. [4±15] There are,however,ionsthatdefysuchasimpleapproachforstudying ion-transfer properties. The transfer of anionic surfactants across the liquid/liquid interface almost always accompanies the chaotic oscillation of current after the potential is brought close to the half-wave potential region of the ions. Current± potential curves, frilled with irregular current spikes and other disordering features, are very different from those in the transfer of simple anions, which do not show any anomaly related to the adsorption at the interface, [7, 8, 16, 17] and in fact have rendered the accurate determination of the standard ion-transfer potential of anionic surfactants prohibitively diffi- cult. We recently showed that the condition of the thermodynamic stability of a liquid/liquid interface with respect to the phase- boundary potential Df may be violated in the presence of simultaneous equilibria of the partition and adsorption of ionic surfactants [18] and also reported that voltammetric behavior in the transfer of two anionic surfactants, decanesulfonate and dodecyl sulfate, at the 1,2-dichloroethane/water (DCE/W) inter- face supports the presence of the electrochemical instability as predicted. [19] Conversely,theirregularitiescommonlyobservedin the transfer of ionic surfactants may be generalized in terms of the electrochemical instability. Thepresentpaperreportsasystematicstudyofthetransferof, alkyl sulfonates and alkyl sulfates across the DCE/W interface, to show the regularity in the occurrence of irregular current spikes in voltammetry and potential-step chronoamperometry and to demonstrate the existence of the instability window in a certain range of Df. Experimental We used six alkanesulfonate sodium salts (C n SO 3  ) and six alkyl sulfate sodium salts (C n SO 4  ). C n SO 3  (n  8±13) were purchased from Nacalai Tesque (Kyoto), C n SO 4  (n  9±11, 13; 99%) from Lancaster Synthesis, C 8 SO 4  from ICN Biomedicals, and C 12 SO 4  (99%) from Sigma. Tetrapentylammonium tetraphenylborate (TPnATPB), used as the supporting electrolyte in the DCE phase, was prepared from tetrapentylammonium iodide (Tokyo Kasei Kogyo) and sodium tetraphenylborate (Dojindo Laboratories) as described elsewhere. [20] Tetrapentylammonium chloride (98%, Wako Chem.),LiCl¥H 2 O(99.9%,WakoChem.),MgCl 2 ¥6H 2 O(> 99%,Merck), and sorbitan monooleate (Nacalai Tesque, Extra Pure) were used without further purification. DCE (99.5%, Wako Chem.) was washed with water three times before use. The electrochemical cell employed in the present study is given in Scheme1, where W ref is the aqueous phase for the reference of the potentialinDCE,Wistheaqueousphasecontainingionicsurfactant, and TPnACl stands for tetrapentylammonium chloride. In the following, the potential of the right-hand side (RHS) of the Ag/AgCl electrodewithrespecttotheleftwillbedenotedas E andthecurrent carried by the positive charge from W to DCE will be taken to be positive. Occasional drifts of E was corrected, if necessary, using the mid-point potential of tetraethylammonium ion transfer as an internal standard. We note that E  Df  constant. Electrochemical measurements were made using a four-electrode configuration with a positive feedback for the Ohmic drop compen- sation. [21] A glass cell employed was similar to the one previously reported. [13] Thepolarizedinterface(area0.16cm 2 )wasformedatthe upper orifice of a glass tube in the cell. The part of the inner wall of [a] Prof. T. Kakiuchi, N. Nishi, T. Kasahara, M. Chiba Department of Energy and Hydrocarbon Chemistry Graduate School of Engineering Kyoto University Kyoto 606-8501 (Japan) Fax:( 81)75-753-3360 E-mail:kakiuchi@scl.kyoto-u.ac.jp