THE CHARACTERIZATION OF THE ELECTRICAL DOUBLE-LAYER OF MONTMORILLONITE (ZETA-POTENTIAL, DISSOCIATION, NEGATIVE-ADSORPTION)

STEVEN ERIC MILLER, Purdue University

Abstract

Uncertainty regarding the relative magnitudes of (psi)(,o) and (psi)(,(delta)), the electric potentials at the clay-water interface and outer Helmholtz plane (OHP), respectively, and of (sigma)(,o) and (sigma)(,(delta)), the corresponding surface charge densities, prompted the determination of (psi)(,(delta)) and (sigma)(,(delta)) as a function of electrolyte concentration by as many methods as possible. To determine (psi)(,(delta)) for Li-Upton, Na-Upton, K-Upton, Na-Texas and Na-Otay montmorillonite, the mobilities of the respective particles were determined by microelectrophoresis and a moving-boundary method. Also, the negative adsorption of Cl('-) was determined by analyzing the associated equilibrium dialyzates. Then the experimental data were substituted into the relevant equations of electrical double-layer theory to make the necessary calculations. It was found that: (1) the OHP and the plane of shear are coincident and so (psi)(,(delta)) equals (zeta), the potential at the latter plane, (2) (psi)(,(delta)) remains constant as the electrolyte concentration changes from 10('-4)N to 10('-2)N, (3) (psi)(,(delta))/(psi)(,o) << 1.0 and, (4) representative values of (psi)(,(delta)) are -71.6, -58.1, -49.5, -48.2 and -60.2 mv for the Li-Upton, Na-Upton, Na-Texas and Na-Otay montmorillonite. By using these values of (psi)(,(delta)) in the appropriate equations, the respective values of (sigma)(,(delta)) were determined at different electrolyte concentrations. Also, values of (sigma)(,(delta)) for Na-Upton montmorillonite were determined at different electrolyte concentrations by two additional methods, both involving membrane equilibrium. In the first, the concentration of Cl('-) and NaCl were measured in the suspension and solution, respectively, and utilized in the equations of Donnan theory. In the second, the membrane potential was measured and utilized in an equation relating this potential to the charge of the particles. The results of the different studies were mutually consistent and showed that (1) (sigma)(,(delta)) increased from (TURN)500 to (TURN)5000 esu/cm('2) as the electrolyte concentration increased from 10('-4)N to 10('-2)N and (2) (sigma)(,(delta))/(sigma)(,o) increased correspondingly from (TURN)0.015 to (TURN)0.15. From the results of these studies, it was concluded that most of the exchangeable cations of montmorillonite are inside the OHP, i.e., within the Stern layer, so that (psi)(,(delta))/(psi)(,o) and (sigma)(,(delta))/(sigma)(,o) are both << 1.0. Also, it was concluded that (psi)(,(delta)) remains constant but (sigma)(,(delta)) varies with electrolyte concentration.

Degree

Ph.D.

Subject Area

Agronomy

COinS