Hexacyanoferrate(III) Transport in Coated Montmorillonite Clay Films. Effects of Water-Soluble Polymers

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Hexacyanoferrate(III) Fe(CN)63-, transport through coated montmorillonite clay films at a platinum electrode is studied with the cyclic voltammetric method. Experimental conditions are first established guaranteeing voltammetric detection based on a linear diffusion in the clay film. The square of the ratio (R2) of current intensities obtained at the clay modifed electrode (CME) and the bare Pt electrode measures the relative variation of the Fe(CN)63-, diffusion coefficient in the clay film. Thus, the effects of bathing electrolyte concentrations on R2 are investigated and related to the swelling properties of the montmorillonite clay characterized by X-ray diffraction and sedimentation volume results. According to an applied model for the electric conductivity of montmorillonite clay solution, a relationship between the formation factor F = 1/R2, the shape factor (α), and the clay film effective porosity (Φe) is established. The shape factor (α) includes orientation and axial ratio (the ratio (A/B) of the major axis (A) to the minor axis (B)) parameters which characterize the arrangement of montmorillonite particles in the clay film. The determination of Φe is based on relative variations of the sedimentation volumes. In a salt concentration range from 0.02 to 1 M NaCl, it can be thus proposed that an interparticle Fe(CN)63- transport in the clay film is limited by occluded montmorillonite particles with a constant axial ratio. The effects of water-soluble polymers on the shape factor (α), of montmorillonite particles are investigated. At polymer concentrations ≤5% (polymer to montmorillonite, w/w), nonionic poly(vinylpyrrolidone) (PVP) decreases Fe(CN)63- transport in CME while interactions with a low-molecular-weight poly(acrylic acid) (PAA) increases it. The polymer modifications of the clay film can be thus modeled. An aggregation with PVP favors a lateral formation of a band-type network with an increase of the particle axial ratio, i.e., lengthening of the interparticle transport pathway in the clay film. In the case of low-molecular-weight PAA, a specific adsorption at the edge of montmorillonite platelets induces an apparent decrease of the particle axial ratio. This can be related to an electrostatic stabilization of clay platelets stacked in a column model, which shortens the interparticle transport pathway in the clay film.