Methylene Blue Absorption by Montmorillonites. Determinations of Surface Areas and Exchange Capacities with Different Initial Cation Saturations (Clay‐Organic Studies XIX)

Absorption isotherms have been determined for methylene blue absorbed from aqueous solutions on montmorillonites saturated with various cations. Surface areas are determined from observations of the amount of methylene blue absorbed on the clay required to produce optimum flocculation; this condition corresponds to an effective coverage of the surfaces with methylene blue ions. Cation exchange capacities are determined from saturation absorptions. With Li‐ and Na‐saturated clays, the surface areas are approximately 750 m2/g and the exchange capacities agree with values obtained by titration techniques. With K‐montmorillonites, values about 15% smaller are obtained and this is probably due to a small fraction of the clay not expanding beyond about 15Å in water. With M2+‐montmorillonites, a marked difference in behavior is observed between a clay from Wyoming and one from Texas. The Wyoming clay gives much smaller effective flocculation areas and clearly defined saturation absorptions are not obtained. These results are attributed to relatively large particle sizes, so that external surfaces are covered when internal surfaces are only partially covered, and ultimate saturation is hindered by the large particles. With finer particles, the Texas clay gives flocculation areas nearer to those obtained with Li‐ and Na‐clays and the apparent exchange capacities also are nearer to those obtained for the monovalent ion clays. A limitation on surface area measurements for montmorillonites with exchange capacities less than about 100 meq/100 g is discussed.