What Is Selectivity and What Happens at an Interface Spin-Offs of a Collaboration with R.P. Buck
The original purpose of the collaboration with Dick was to study the interfacial electrode kinetics at plasticized PVC ISEs. The results were exciting but quite different from the original goal: we could explain why the apparently ion-free membranes conducted electricity and we could also detect and explain the large surface impedances [1,2]. Both phenomena could be traced back to contaminations in the membrane components. The results with the membrane bulk impedances encouraged us to revive the DC experiments of the Simon group with the PVC membranes. This work led to a full explanation of this complex phenomenon .
Later on this work was extended to making the first amperometric and cyclic voltammetric measurements with PVC ISEs [4-6]. This was probably the prelude (this word is used here to remember Dick, the musician) to many novel potentiometric methods developed by colleagues. Another direction that followed from our collaboration was to study the ion exchange properties of ISEs [7,8]. This radiotracer study proved that the membranes were indeed selective ion exchangers and their exchange selectivity correlated well with their potentiometric selectivity. Yet another work, this time with small angle neutron scattering  helped to understand the structure of plasticized PVC while surface spectroscopies and AFM allowed a detailed analysis of the membrane composition in the immediate vicinity of the interface .
The distribution of ions in the first few nanometers of the interfacial layer is essential for understanding potentiometric sensors. Lacking suitable experimental tools for such studies our attention turned to computer simulations. We could indeed simulate the interface and correctly predict emf values in complex situations [11, 12]. At this point we left potentiometry and devoted much effort to discovering the structure of the first molecular layer at liquid-liquid and liquid-air interfaces. We have developed a method to identify the truly interfacial molecules and showed that many interfacial phenomena are concentrated to this single layer .
A promising area of selective analysis similar to ISE potentiometry has emerged recently with molecularly imprinted polymers. Our experience with ISEs has helped to shed light on the less than ideal selectivity of these materials . On the go we have recognized that selectivity, a fundamental concept of analytical chemistry, is very little understood. We have developed a selectivity concept which appears to be sufficiently general and sheds also new light on potentiometric selectivities.
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