To quantitatively investigate the relationship between the
surface charge densities and the phase boundary potentials at poly(vinyl chloride)-based liquid membranes
of
ion-selective electrodes, lipophilic derivatives of photoresponsive azobis(benzo-15-crown-5) were used as the
probe molecule. The photoinduced change in the number
of primary cations permselectively extracted into the
membrane side of the interface was estimated from the
corresponding concentrations of the cis and trans isomers
of the photoswitchable ionophores and the ratio of their
complexation stability constants. To accurately
estimate
the changes in the phase boundary potential that arises
at the interface of the membrane and the aqueous sample
solution, an electrode system without an inner filling
solution was employed. The direction as well as the
magnitude of the photoinduced change in phase boundary
potential depended on the change in the number of uptake
of primary cations into the membrane side of the interface: Membranes containing sufficient amount of ionophores to give Nernstian slopes simply exhibited a parallel
shift in phase boundary potentials, keeping Nernstian
slopes upon UV irradiation. The membranes containing
relatively low concentrations of ionophores exhibited only
sub-Nernstian responses, in which case a photoinduced
increase in the response slope was observed. The photoinduced changes in the phase boundary potentials thus
observed were analyzed by a surface model based on
electrical diffuse layers at both membrane and aqueous
sides of the interface. In this model, the phase
boundary
potential is correlated to the number of surface charges
due to the formation of cationic complexes at the membrane side of the interface. The observed photoinduced
changes in the phase boundary potential and the response
slope were in good agreement with the values calculated
from the proposed surface model. It is therefore concluded that (1) the change in the phase boundary potential
can be quantitatively correlated to that in the number of
the primary cations permselectively extracted into the
membrane side of the interface, which determines the
surface charge density, and (2) the sub-Nernstian response slope is attributed to a low surface charge density
due to insufficient number of permselectively extracted
primary cations. It is estimated on the basis of the
present surface model that, to obtain a Nernstian potentiometric response, a sufficient amount of primary cation
to produce a net positive surface charge density of > 2
×
10-7 C cm-2 must be
uptaken into the membrane side of
the interface by complexation with the ionophore.
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