Determination of the specific surface area of layered silicates by methylene blue adsorption: The role of structure, pH and layer charge

Abstract

The specific surface area of three layered silicates was determined by three independent methods; it was estimated from the average dimensions of individual silicate layers, determined by nitrogen adsorption using the BET model and calculated from the adsorption of methylene blue on their surface in aqueous sol. The BET model gave smaller surface areas than expected, because nitrogen molecules cannot penetrate freely into the interlayer space of the silicates. Geometric calculations and the methylene blue approach yielded very similar values for two different types of Laponite when the pH of the dispersion was adjusted to 6.5 or the edges of the silicate were modified with tetrasodium pyrophosphate dispersing agent. The measurement of surface area in water without the control of pH yielded smaller surface area, because methylene blue decreased the pH of the solution resulting in the competitive adsorption of methylene blue cations and protons at the basal surface. The methylene blue approach resulted in larger surface area than expected for the silicate with large ion exchange capacity, because of the tilted orientation of the adsorbed methylene blue molecules. All these factors must be considered during the use of the methylene blue method for the determination of the specific surface area of smectites.