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The interfacial charge on spin-coated films of poly(tetrafluoroethylene-co-2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole) (Teflon AF) was studied by streaming potential and streaming current measurements in diluted aqueous solutions of potassium chloride, potassium hydroxide, and hydrochloric acid. ζ potential and surface conductivity were derived from electrokinetic data determined at varied concentrations of the electrolytes by means of the novel microslit electrokinetic setup (ref :  J. Colloid Interface Sci. 1998, 208, 329). The results obtained revealed the high relevance of unsymmetrical (preferential) adsorption of ions as the origin of charge formation at unpolar polymer materials in aqueous environments. The preferential adsorption of hydroxide ions (OH-) was found to predominate as compared to the adsorption of hydronium ions (H3O+) at similar concentrations, i.e., in solutions of neutral pH. No effect of preferential adsorption was induced by chloride (Cl-) and potassium (K+) ions. For the first time ζ potential and surface conductivity data were evaluated to quantify the charge density of the inner layer at the polymer−water interface. The results indicate the presence of both cations and anions in the stagnant layer in all analyzed cases. The charge density and the total ion concentration in this inner part of the electrical double layer were found to increase with increasing ionic strength of the solution. However, the total ion densities of the stagnant layer remained rather low. This suggests that the ions are localized in one interfacial plane. With regard to the origin of the characterized charging process we conclude that the OH- and H3O+ ions capability to form hydrogen bonds and the difference in the structures of the hydration shells of these two ions are of highest importance to explain the observed phenomena.

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Streaming current measurements were performed on poly(N-isopropylacrylamide)-co-N-(1-phenylethyl) acrylamide [P(NIPAAm-co-PEAAm)] thermoresponsive thin films above and below the transition temperature of the polymer (i.e., at 22 and 4 degrees C, respectively). Electrokinetic measurements (ionic strength 0.01-10 mM KCl, pH 2.5-9.5 in 1 mM KCl) revealed that the charging of the polymer/aqueous solution interface is determined by unsymmetrical adsorption of hydroxide and hydronium ions onto the Teflon AF substrate that supports the hydrogel film. The magnitude of the streaming current significantly decreased with decreasing temperature, that is, when the hydrogel was swelling. The pH- and ionic strength-dependent data for unswollen and swollen films were interpreted on the basis of the here-reported general theory for the electrokinetics of diffuse soft gel layers. The formalism based on the Debye-Brinkman equation for hydrodynamics and the nonlinear Poisson-Boltzmann equation for electrostatics extends previous theoretical studies by considering the most general situation of a charged gel layer supported by a charged rigid surface. Full analytical expression is provided for the streaming current in the limit of homogeneous distribution of segments under low potential conditions. Numerical analysis of the governing transport and electrostatic equations allows for the computation of streaming current for cases where analytical developments are not possible. The theory successfully reproduces the electrokinetic data for the P(NIPAAm-co-PEAAm) copolymer film at 22 and 4 degrees C over the whole range of pH and ionic strength examined. It is found that the 3-fold increase of the hydrogel film thickness with decreasing temperature from 22 to 4 degrees C (i.e., from 23 to 70 nm as measured by ellipsometry), is in line with homogeneous swelling and an increase of the hydrodynamic penetration length (1/lambdao) by a factor of approximately 1.6. Additionally, the hydrodynamic thicknesses (deltaH) of the swollen and unswollen hydrogels are evaluated in terms of their respective hydrodynamic penetration length and electrosurface characteristics of the supporting Teflon AF surface.

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Streaming current measurements were performed on poly(N-isopropylacrylamid-co-carboxyacrylamid) (PNiPAAM-co-carboxyAAM) soft thin films over a broad range of pH and salt concentration (pH 2.5-10, 0.1-10 mM KCl) at a constant temperature of 22 °C. The films are negatively charged because of the ionization of the carboxylic acid groups in the repeat unit of the copolymer. For a given salt concentration, the absolute value of the streaming current exhibits an unconventional, nonmonotonous dependence on pH with the presence of a maximum at pH ∼6.4. This maximum is most pronounced at low electrolyte concentration and gradually disappears with increasing salinity. Complementary ellipsometry data further reveal that the average film thickness increases by a factor of ∼2.2 with increasing pH over the whole range of salt concentration examined. The larger the solution salt concentration, the lower the pH value where expansion of the hydrogel layer starts to take place. The dependence of film thickness on pH and electrolyte concentration remarkably follows that obtained for surface conductivity. The streaming current and surface conductivity results could be consistently interpreted on a quantitative basis using the theory we previously derived for the electrokinetics of charged diffuse (heterogeneous) soft thin films complemented here by the derivation of a general expression for the surface conductivity of such systems. In particular, the maximum in streaming current versus pH is unambiguously attributed to the presence of an interphasial gradient in polymer segment density following the heterogeneous expansion of the chains within the film upon swelling with increasing pH. A quantitative inspection of the data further suggests that pK values of ionogenic groups in the film as derived from the streaming current and surface conductivity data differ by ∼0.9 pH unit. Such a difference is attributed to the impact of position-dependent hydrophobicity across the film on the degree of ionization of carboxylic sites.