Modeling of the electrified interface of liquid membrane ion-selective electrodes

Computer modeling of the interface between two immiscible electrolyte solut ions confirms that non-Nernstian behavior of cation-sensitive ion-selective electrodes (ISEs) can occur by the following molecular mechanisms: (i) ext raction of weakly hydrophilic anions into the membrane, known as the anion effect or Donnan exclusion failure; (ii) leaching of the membrane's hydroph obic anion at low aqueous electrolyte concentrations; (iii) interference ef fects of competing cations. All three effects are known to occur under expe rimental conditions. Our theory employs a lattice-based self-consistent fie ld model extended to include ion-carrier complexation by means of a multist ate mechanism. The interface is created in the computer model by a self-ass embling mechanism controlled by an unfavorable interaction parameter betwee n the (PVC-lookalike) polymer of the membrane phase and the aqueous solutio n. Electrostatic potential profiles and charge-density profiles are availab le for the whole range of parameters considered in the computation of the c alibration lines. The width of the polar/apolar interface is only a few mol ecular diameters thick, but at low ionic strength conditions the electric d ouble layer extends far into both water and the apolar membrane phase.