Partial blocking of ion transport at the interface of an ion-selective liquid membrane electrode by neutral surfactants. Experiments and computer simulation Qingshan Ye a , A. Vincze b , G. Horvai a, *, F.A.M. Leermakers c a Division of Chemical Information Technology, Technical University of Budapest, Gelle Ârt teÂr 4, H-1111 Budapest, Hungary b Department of Physical Chemistry, Technical University of Budapest, P.O. Box 91, H-1521 Budapest, Hungary c Department of Physical and Colloid Chemistry, Wageningen Agricultural University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands Received 15 November 1997; received in revised form 15 February 1998 Abstract Lipophilic neutral surfactants of the sugar alkyl ester type are experimentally observed to substantially increase the ion transfer impedance between an aqueous solution and ion-selective electrode membranes made from plasticized PVC with neutral carrier. The magnitude of the eect depends considerably on the molecular structure and on the concentration of the surfactant. Statistical thermodynamic simulation of the experimental system agrees qualitatively with the experimental observations. # 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: Ion-selective electrode; Neutral surfactant; Ion transfer; Impedance 1. Introduction Ion-selective electrodes (ISEs) are widely used ana- lytical devices. Their most important part is a mem- brane in contact with the sample solution. A potential drop develops at the interface membrane/sample. This potential drop depends in a predictable manner on sample composition and this is why ISEs are useful in chemical analysis. Examples of ISEs are hydrogen and sodium selective glass electrodes, solid crystalline mem- brane electrodes like the lanthanum ¯uoride or the sil- ver halide membrane electrodes and dierent polymer based membrane electrodes. Many studies have shown that the establishment of the interfacial potential drop is connected to an equilibrium or stationary ion exchange process between the membrane material and the sample. The rate of this ion exchange process in equilibrium has been estimated to be very high in almost all cases. This observation is con®rmed for example by noting the negligible ion transfer impe- dance at such interfaces (when mass transfer eects are avoided or accounted for). It was observed some time ago [1±5] that ISE mem- branes made from plasticized PVC with addition of a neutral carrier like valinomycin showed unusual impe- dance behavior under certain experimental conditions. Impedance diagrams of such membranes typically con- sisted of a single semicircle, which could be attributed to the resistance and capacitance of the membrane bulk. In some cases, however, a second semicircle could be observed at lower frequencies than the bulk semicircle. The magnitude of this semicircle depended strongly on the composition of the sample solution. Cammann [4] considered this phenomenon an inherent property of the interface and he explained ISE selectiv- ity by it. He was associating the low frequency impe- dance with the equilibrium exchange current density and showed that ions causing a lower interfacial impe- dance (at identical concentrations) were measured by the ISE with higher selectivity. Another interpretation of the phenomenon was de- rived from the observation that occasionally the low Electrochimica Acta 44 (1998) 125±132 0013-4686/98/$19.00 # 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S0013-4686(98)00160-1 PERGAMON * Corresponding author.