Theory and experiment of reversing-pulse electric birefringence: The case of bentonite suspensions in the absence and presence of cetylpyridinium chloride

Abstract

Reversing-pulse electric birefringence (RPEB) of a fractionated bentonite sample (0.005–0.029 g/l) was measured at 20°C and at a wavelength of 633 nm in aqueous media in the absence and in the presence of cetylpyridinium chloride (CPC), a surfactant, at various concentrations (0.001–1.3 mM). RPEB signals at weak electric fields showed either a hump each, or contrarily a dip each, in the build-up and reverse portions, depending on the concentrations of bentonite and CPC in suspension. The profile change of RPEB signals was interpreted as being due to the shift of the orientation axis from the symmetry axis to the plane of the disk-like particle. Observed RPEB signals were analyzed with the Y–S–K theory, which is based on an ion-fluctuation model and considers the contribution of three electric dipole moments to field orientation, and both electric and hydrodynamic parameters characteristic of RPEB profiles were evaluated by curve fitting with theoretical equations. The results are as follows: (1) the bentonite particle in the absence or in the presence of CPC possesses no permanent dipole moment; (2) it is oriented by applied field with the root-mean-square dipole moment due to ion-atmosphere fluctuation and the covalent dipole moment due to the polarizability anisotropy intrinsic to the particle; (3) the ratios of these two dipole moments either remain nearly constant or vary in a complex manner, depending on bentonite concentration, amount of added CPC, and field strength; and (4) the ratios of the relaxation time due to ion-fluctuation to the relaxation time due to overall particle rotation remain nearly constant for the bentonite suspension in the absence of CPC with increasing field strengths, though the system is polydisperse.