Estimating the fully developed diffuse double layer thickness from the bulk electrical conductivity in clay

The possibility of estimating the thickness (t) of the fully developed diffuse double layer (DDL) and the specific surface area (S) of clay from its bulk electrical conductivity (the composite electrical conductivity of the clay–water–air mixture), hereafter called the electrical conductivity and denoted by EC, was investigated. The assumption is that when a clay is wetted with distilled water, the fully developed DDLs around the clay particles create the maximum number of electrically conductive pathways that provide the highest electrical conductivity of the clay. This assumption was tested by measuring the electrical conductivity of 17 samples of different clays over a wide range of water content by time-domain reflectometry (TDR), and by using 2 relevant sets of data from the literature. Good agreement between the thicknesses of the DDLs estimated from the water content at fully developed DDLs, hereafter called the critical water content and denoted by w, and that calculated by the method of Schofield [Schofield, R.K., 1947. Calculation of surface areas from measurements of negative adsorption. Nature (London) 160, 408−410] for different samples proves the validity of our assumption. The critical water content being a direct function of the specific surface area of the clay and the thickness of the fully developed DDLs, provides a way of estimating any one of these two parameters when the other parameter is known. When the clay is wetted with salt solutions, the bulk electrical conductivity is governed by the electrical conductivity of both the DDLs as well as the salt solution outside the DDLs, and the above assumption does not hold true under such condition.