Position of Shear Plane at the Clay–Water Interface: Strong Polarization Effects of Counterions

Abstract

Core Ideas A nonlinear Poisson–Boltzmann equation including ionic polarization effects was developed. A new approach was developed to estimate the shear plane position at solid–liquid interface. The strong polarization is an important factor in the change of shear plane position. The zeta potential at the shear plane is an important electrochemical parameter at solid–liquid interfaces. However, it is difficult to accurately determine the position of the shear plane because a rather important effect— strong ion polarization in the electrical double layer—is neglected. In the present study, a model for estimation of the shear plane position considering ion polarization was developed based on the modified Poisson–Boltzmann equation, and the intensity of polarization on a constantly charged surface was quantified through an ion exchange equilibrium experiment. The polarization effects increased the effective ionic charges of Na+ from 1 to 1.082 and that of K+ from 1 to 1.498 at the montmorillonite–water interface, which resulted in decreasing the thickness of the slipping layer. The theoretical prediction of the swelling pressure was in excellent agreement with the experimental measurements. The theoretical standpoint also sheds new light on one of the oldest puzzles in physical chemistry—specific ion effects.