Authors Zimmermann, R. ; Gunkel-Grabole, G. ; Bünsow, J. ; Werner, C. ; Huck, W. T. S. ; Duval, J. F. L.
Title Evidence of ion-pairing in cationic brushes from evaluation of brush charging and structure by combined electrokinetic and surface conductivity analysis
Date 01.03.2017
Number 52426
Abstract Strong cationic poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) (PMETAC) brushes are widely employed as platform for studying fundamental physicochemical properties of permanently charged polymers at interfaces. We report here a detailed analysis of the structure, interfacial and bulk charging of PMETAC brushes over a broad range of pH values and salt concentrations (pH 2.5–9.5, 0.01–10 mM KCl electrolyte). Streaming current and surface conductivity measurements were quantitatively analyzed on the basis of a recently developed theory for electrohydrodynamics at diffuse soft interfaces under lateral flow conditions taking into account ion pairing at low salt concentrations. In addition, the effects of different chaotropic anions on brush charge and thickness were deciphered from interpretation of surface conductivity data collected in 1 mM KNO3, KI, and KClO4 electrolytes. In combination, confrontation between theory and experiments reveals the existence of a PMETAC segment density gradient at the brush/solution interphase that is independent of pH and KCl concentrations above 0.1 mM. With decreasing salt content at pH 6 from 0.1 mM to 0.01 mM, the corresponding nonmonotonous variation of streaming current indicates electrostatically driven interfacial swelling of the brush, while the accompanied variation in surface conductivity reveals a dramatic decrease in the net density of structural charges within the bulk material. The latter feature is assigned to the occurrence of pairing between chloride anions from background electrolyte and quaternary ammonium groups supported by PMETAC chains. The decrease of the net charge density in the bulk of the brush is shown to be more pronounced with increasing anion hydrophobicity, i.e., upon promoting the ion-pairing process that subsequently leads to a shrinking of the brush volume. The study illustrates the complementary information derived from surface conductivity and streaming current analysis: the former reflects electrohydrodynamic processes in the bulk of the brush, whereas the latter primarily mirrors processes at the polymer/solution interphase. It further demonstrates that electrokinetic and surface conductivity are valuable tools for demonstrating and monitoring ion-pairing processes useful to reversibly tune macroscopic properties such as brush swelling or stiffness.
Publisher Physical Chemistry, Chemical Physics
Citation Physical Chemistry, Chemical Physics 121 (2017) 2915-2922

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