Ion–polymer affinities of PEDOT:PSS from open-circuit titrations in a multi-cation background
Year: 2026
Type: preprint
Abstract: Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a widely used solid contact in solid-state ion-selective electrodes. In an electrolyte, the fixed charge of the PSS − backbone can exchange cations in a manner that depends on the electrolyte composition. Whether the sulfonate group has an intrinsic affinity for particular cations remains open. We estimate the fixed PSS charge density (c f = 1.18 M) from an open-circuit potential (OCP) calibration in pure NaCl, then independently dose NaCl, KCl, CaCl2 and MgCl2 into a 0.1× HEPES-buffered saline baseline containing the four cations at extracellular proportions and record the OCP of a PEDOT:PSS film after each addition. By inverting the titrations through a Donnan partition model, we numerically calculate the ion–polymer affinities k i . Taking Na + as reference, we find k K = 1.05, k Ca = 0.65 and k Mg = 0.44: the polyelectrolyte phase shows no measurable preference between monovalent cations Na + and K + , while showing less affinity with the divalent cations Ca 2+ and Mg 2+ . However, the divalent uptake remains substantial, since the affinity enters the partition linearly while the valence of the ion sits in the exponential of the Donnan potential. Therefore, the ionic composition of the film depends strongly on the ionic strength: at the used baseline Ca 2+ and Mg 2+ together compensate about half of the fixed charge while carrying under 5% of the cation charge in the electrolyte, a phenomenon known as electroselectivity. At 10× our baseline (physiological ionic strength) Na + dominates the PSS compensation and the divalent compensating fraction amounts to 10%.
Source: ChemRxiv
Language: English
Subfield: Polymers and Plastics
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7205005265