Some considerations on the triple layer model framework

There are several recent investigations on the triple layer model (TLM) of the oxide-electrolyte interface which assess its general applicability as well as its self-consistency. Thus, some workers show that in their implementation of the TLM the point of zero net proton charge (PZNPC) and the isoelectric point (IEP) have to coincide. It is shown in this paper that in the general implementation of the TLM this is not the case. The TLM may then be used to describe differences in the points of zero charge and it is not necessary to involve more complicated models. For the cases where PZNPC = IEP, which is then also the pristine point of zero charge (PPZC), a peculiar situation has been obtained by other authors. They derived a quadratic equation valid in the vicinity of the PPZC and argued that both solutions to this equation have to coincide. This results in a serious incompatibility within the TLM, because a capacitance must assume a negative value in the vicinity of the PPZC. Our calculations show that the previously published equations are, in principle, obeyed. However, the speciation code only uses one of the two theoretical solutions to the quadratic equation. This suggests that the solution causing the incompatibility should have been rejected. (C) 1998 Elsevier Science B.V.