Multifractal Analysis of Nitrogen Adsorption Isotherms Obtained from Organoclays Exposed to Different Temperatures

Abstract

Multifractal analysis was used to characterize changes in N2 adsorption isotherms of organoclays prepared with different cations and exposed to various thermal treatments. Wyoming bentonite was exchanged with three organic cations: benzyltrimethylammonium (BTMA‐clay), tetraethylammonium (TEA‐clay), and n‐hexadecyltrimethylammonium exchanged at 41 and 90% of the cation exchange capacity of the clay (HDTMA41‐ and HDTMA90‐clay). Freeze‐dried organoclays were exposed to temperatures of 25, 150, 250, 360 or 420°C for 2 h, and N2 adsorption isotherms were measured at 77 K. The isotherms behaved like a multifractal, and parameters derived from Rényi and singularity spectra varied with changes in the type of exchanged cation and the treatment temperature. The main factor responsible for changes in the spectra was the type of cation, HDTMA90‐clay being the organoclay that showed more differences among the sorbents. There was an interaction between the exchanged cation and the temperature. A significant correlation was found between organic C content and multifractal parameters, probably related to changes in N2 sorption as a result of thermal transformations of the organic cations. Significant correlations were also found between some multifractal parameters and the heterogeneity exponent of the Freundlich model fitted to nitrobenzene sorption isotherms measured in those organoclays. Multifractal analysis of N2 adsorption isotherms could be useful to characterize differences in the heterogeneity of sorption sites in sorption determinations.