Electrokinetics of Diffuse Soft Interfaces. IV. Analysis of Streaming Current Measurements at Thermoresponsive Thin Films

Abstract

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.

Hydroxide and hydronium ion adsorption — A survey

“…increasing the extension of the hydrogel, further reduced the streaming current vs. pressure gradient but did not shift the IEP. Combined with streaming current measurements at varied KCl solution concentrations [58], the observed characteristics can be attributed to the adsorption of hydroxide and hydronium ions onto the Teflon® AF substrate supporting the hydrogel film, i.e. the water ions apparently do not interact with the highly hydrated hydrogel [58,59].…”

“…As a consequence, the hydrodynamics in the interfacial region and the position of the shear plane within the double layer are well defined. Cellulose films exhibit a more diffuse nature of the interface, i.e., the hydrodynamic flow partially penetrates into the polymeric layer, which in turn causes increased friction [58]. Macroscopically one can treat this as a shift of the shear plane towards the liquid phase [58].…”

“…In order to further extend the discussion of the phenomenon of asymmetric water ion adsorption, we present streaming current measurements performed on poly(N-isopropylacrylamide-co-N-(1-phenylethyl)) acrylamide thermo-responsive thin films above and below the transition temperature of the polymer (i.e. at 22°C and 4°C, respectively) [58,59] (Fig. 3).…”

“…Combined with streaming current measurements at varied KCl solution concentrations [58], the observed characteristics can be attributed to the adsorption of hydroxide and hydronium ions onto the Teflon® AF substrate supporting the hydrogel film, i.e. the water ions apparently do not interact with the highly hydrated hydrogel [58,59]. Based on this finding, the temperature dependence of the streaming current vs. pressure gradient can be attributed to different hydrodynamic accessibilities of the counterions in the diffuse layer at 4°C and 22°C [58,59].…”

Controlling the adhesion of the diatom Navicula perminuta using poly(N-isopropylacrylamide-co–N-(1-phenylethyl) acrylamide) films

“…The temperature-dependent properties of the hydrophobically-modified PNIPAAm films were extensively characterized using a variety of surface sensitive techniques, providing insights on the interfacial properties of the generated films Duval et al 2009). In the present paper, the efficacy of P(NIPAAm-co-PEAAm) copolymer films to modulate initial attachment and adhesion strength of the diatom Navicula perminuta was investigated.…”