Biomedical Engineering Reference
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across the membrane-water interface. The subsequent charge recombination results in a
reverse transfer of protons from the membrane phase back to the aqueous phase. We
named this model the interfacial charge transfer (ICT) mechanism [52]. A similar analysis of
the latter mechanism leads to a slightly different equivalent circuit (Figure 15.5). From the
point of view of an external observer who conducts an electrical measurement, these two
equivalent circuits are equivalent to the same irreducible circuit shown at the bottom of
Figure 15.5. This irreducible circuit is part of the circuit shown in Figure 15.1. The differ-
entiation of these two mechanisms may be made by chemical means. For example, since
the AC signals shown in Figure 15.3A is generated by interfacial electron transfers at
the oxidant interface, the pseudofirst-order relaxation-time constants are expected to
be proportional to the ferricyanide concentration at the oxidant side. Qualitatively, the
ICT mechanism
OD mechanism
P
h
A +
P*
h
A
P +
+
+
A +
P
Aqueous
Membrane
Aqueous
Aqueous
Membrane
Aqueous
E
p R p
E p R p
+
+
+
+
−−
+
+
C d
C g
C d
C d
C g
C d
E
p
R p
C p
+
Common irreducible
equivalent circuit
FIGURE 15.5
Interfacial charge transfer (ICT) mechanism and oriented dipole (OD) mechanism. The symbols P, P * , and P have
the same meanings as in Figure 15.1. In the ICT mechanism, the symbol A stands for either proton acceptor or elec-
tron donor. The symbol A stands for either proton donor or electron acceptor. Symbols A and A can be replaced
by B - and B, if necessary. The reverse reaction is also shown. The OD mechanism is self-explanatory. The space
charge profiles are shown just above the two microscopic equivalent circuits. In the two models, the photoemf,
(with an internal resistance, R p ) is located at different places with respect to the geometric capacitance, C m , and the
two double-layer capacitances, C d . The irreducible equivalent circuit is shown below them. (From Hong, F. T.
(1978). Mechanisms of generation of the early receptor potential revisited. Bioelectrochem. Bioenerg. 5:425-455.)
 
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