No abstract

Complexes between organic cations and clays provide a research tool and have ecological applications. The purpose of this study was to elucidate details of interactions between monovalent organic cationic dyes and montmorillonite. Interactions were studied by x-ray diffraction and ultraviolet and infrared (IR) spectroscopies, IR linear dichroism, and adsorption isotherm measurements with model calculations. The adsorption model combined electrostatic equations with specific binding and considered neutral and positively charged complexes between surface sites and organic cations in a closed system. The model was extended to account for dye aggregation in solution. The adsorption of the dyes to montmorillonite was described by binding coefficients that were at least six orders of magnitude larger than those of inorganic cations such as Na + and Cd + + . The maximal amounts of crystal violet (CV), methylene blue (MB), and acriflavin adsorbed were 200, 150, and 175%, respectively, of the cation-exchange capacity (CEC) of the clay mineral. The model also simulated the competition between dyes for adsorption sites. The c-spacing of montmorillonite increased by the adsorption of CV. With MB at loadings of up to 40% of the CEC, the spacing was reduced, indicating desorption of water from the interlayer space. We conclude that MB lies preferentially parallel to the clay mineral plates, whereas CV lies at a slight inclination relative to the plates.

This study aimed to optimize organo-clay formulations for reduction of leaching of the herbicides alachlor, metolachlor, and norflurazon, which include a phenyl ring in the structure. The adsorbed amounts of herbicides increased severalfold when montmorillonite was preadsorbed by an organic cation; benzyltrimethylammonium (BTMA) was more effective than benzyltriethylammonium (BTEA). Fourier transform infrared studies indicated interactions between alachlor molecules and adsorbed BTMA. The adsorption affinity of the herbicides increased with BTEA loading up to the cation exchange capacity (CEC) of montmorillonite but reached a maximum at a BTMA loading of 5/8 of the CEC. The enhanced adsorbed amounts of herbicides are mainly due to interactions between the phenyl rings of herbicide molecules and organic cations, which are favored with the smaller cation, BTMA. BTMA preadsorbed on the clay up to the CEC forms a fraction (14-18%) of charged dimers so that less phenyl rings are available for interacting with herbicide molecules. This effect is small for preloading by BTEA, so that the amounts adsorbed increase with the degree of preloading. Thus, optimization of claybased herbicide formulations requires a selection of structurally compatible organic cations preadsorbed on the clay at optimal coverage.