Photoswitchable Azobis(benzo-15-crown-5) Ionophores as a Molecular Probe for Phase Boundary Potentials at Ion-Selective Poly(vinyl chloride) Liquid Membranes

Abstract

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.

Threshold Ionic Site Concentrations Required for Nernstian Potentiometric Responses of Neutral Ionophore-Incorporated Ion-Selective Liquid Membranes

“…[23][24][25] It is shown on the basis of this theory that the ionic site concentrations that provide ISEs based on neutral ionophores with Nernstian potentiometric responses depend on the lipophilicity of primary and counterions, and on the concentration of ionophores. The validity of the prediction of our theory was experimentally confirmed with ion-selective liquid membranes based on typical neutral ionophores such as valinomycin, nonactin and bis(benzo-15-crown-5).…”

Studies on the Matched Potential Method for Determining the Selectivity Coefficients of Ion-Selective Electrodes Based on Neutral Ionophores: Experimental and Theoretical Verification

“…Unfortunately, the definition of the selectivity factor (k IJ Psel ) in the mixed potential model is different from that of the MPM-selectivity coefficient: the value of k IJ Psel is given by the ratio of primary and interfering ion activities in the sample solution, which give, when separately measured, the same membrane potential. 14,15 In this paper, a theory based on diffuse double layers 16,17,[25][26][27] is presented that describes directly the MPM-selectivity coefficient for two ions of any charge with the assumption that distribution equilibrium is reached both for the primary and interfering ions at the membrane surface. The validity of the proposed MPM theory were confirmed by comparing the absolute values of the MPM-selectivity coefficients determined from the EMF measurement and those calculated by the proposed theory with the measured complexation stability constants and single ion distribution coefficients for the same systems.…”

“…16 The membranes were cut into small disks of 7 mm diameter and mounted onto Philips electrode bodies (Model IS-561, Philips Electronic Instruments Co., Mahwah, NJ).…”

“…There are many cases where the response does not necessarily exhibit formal Nernstian slopes under the condition that the distribution of the primary and interfering ions at the membrane / sample solution interface is reached at equilibrium: contamination of the primary or interfering ions in the sample solution, interference from the ions in the sample solution such as proton, buffer and supporting electrolyte as interfering ions, insufficient number of permselectively extracted or released primary ions into or out of the membrane surface due to low concentrations of ionophores, 16 the presence of a locally elevated concentration of the primary ion at the sample-membrane interface due to the transmembrane diffusion of the primary ion, 5,6 and Donnan exclusion failure. In all of these cases, except for Donnan exclusion failure and the contribution of the diffusion potential change within the membrane, the primary ion response is supposed to exhibit Nernstian slopes; but it is superimposed with background ions other than the added primary ions into the sample solution.…”

Optimum Concentration of Anionic Sites in Lipophilic Macrocyclic Pentaamine-Based Liquid Membranes for Potentiometric Responses to Anionic Analytes

“…Equation Based on the ionic concentrations at the OHP on the membrane side of the interface, the surface charge density, σ mem , can be expressed as [17][18][19][20] (4) Finally, the phase boundary potential, Eb, can be described as a function of the salt concentration in the bulk of the membrane, C mem salt, tot , and the surface charge density at the membrane side of the interface, σ mem , as (5) where ε0 is the permittivity of the vacuum, εmem the relative permittivity of the membrane and R and T have their usual significance.…”