Methylene Blue Dimerization does not Interfere in Surface-Area Measurements of Kaolinite and Soils

Marcelo J. Avena, Laura E. Valenti, Valeria Pfaffen and Carlos P. De Pauli
INFIQC, Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina
E-mail of corresponding author: depauli@fisquim.fcq.unc.edu.ar

Abstract: Methylene blue (MB) was adsorbed from aqueous solutions onto a kaolinite and four soil samples to determine the effects of MB dimerization on the measured surface area. Adsorption isotherms were prepared using four adsorbing solutions containing, respectively, 9, 46, 71, and 83% of MB molecules in the dimeric state. Langmuir-type isotherms were obtained in each case. The results indicate that equilibration occurs quickly. The aggregation state of MB molecules at the surface does not depend on the aggregation state in the initial adsorbing solutions, but on the final equilibrium concentration of MB. A comparison with the specific surface area measured by adsorption of ethylene glycol monoethyl ether indicates that MB adsorbs as a monomer, regardless of the aggregation number in solution. This result occurs owing to the strength of monomer-surface and monomer-monomer interactions. If monomer-surface interactions are favored, the MB dimer adsorbs in the monomeric form. If monomer-monomer interactions are favored, dimer adsorption may occur. The visible spectra of adsorbed molecules indicated that MB was present at the surface as a mixture of monomeric and dimeric species. These results suggest that dimers are formed in the contact region between two aggregating particles.

Key Words: Dye Adsorption • Ethylene Glycol Adsorption • Kaolinite Surface • Methylene Blue Dimerization • Specific Surface Area

Clays and Clay Minerals; April 2001 v. 49; no. 2; p. 168-173; DOI: 10.1346/CCMN.2001.0490206
© 2001, The Clay Minerals Society
Clay Minerals Society (www.clays.org)