Surface Electric Properties of the Na-Bentonite/Cetylpyridinium Chloride System as Studied by Reversing-Pulse Electric Birefringence

The adsorption of a cationic surfactant, cetylpyridinium chloride (CPC), on disklike Na-bentonite particles has been studied in very dilute suspensions by reversing-pulse electric birefringence (RPEB). The electric parameters of the particle/surfactant system were evaluated from signals taken at extremely low degrees of orientation by using the ion-fluctuation RPEB theory. The electric field orientation of the particles was found to result from the simultaneous action of the polarizability anisotropy Δα′ and the ion-atmosphere polarizability α3caused by ion fluctuation. Two different CPC concentration ([CPC]) ranges could be distinguished according to the magnitude of the induced dipole moment of the particles. When [CPC] was lower than 0.01 mM, the orientation of the particles was mainly due to the covalent polarizability anisotropy Δα′. When [CPC] was higher than 0.01 mM, the ionic polarizability α3was predominant over Δα′, the particles being oriented with the symmetric axis parallel to the direction of the electric field. Values of the electric parameters of the particles changed upon reversal of the sign of the applied reversing-pulse electric field. The change was probably due to the reordering and/or redistribution of the multilayer structure of adsorbed CPC molecules. Such effects were negligible below the critical [CPC] of 0.01 mM, where the adsorbed molecules may form a loose monolayer on the bentonite surface.