Specific surface area of clay minerals: Comparison between atomic force microscopy measurements and bulk-gas (N2) and -liquid (EGME) adsorption methods

The specific surface area of natural particles is an important parameter to quantify processes such as mineral dissolution and sorptive interactions in soils and sediments. In this study, the external specific surface area (SSA), specific edge surface area (ESA) and specific basal surface area (BSA) of an illite (Inter-ILI) and a montmorillonite (Ceratosil) were determined by atomic force microscopy (AFM) and compared with the SSA obtained by N2 gas adsorption (BET) and by liquid adsorption using ethylene glycol monomethyl ether (EGME). For the illite we found an SSA of 41 ± 3 m2 g− 1 by BET and of 83 ± 5 m2 g− 1 by analysing 54 particles by AFM. For the montmorillonite BET we estimated a SSA of 61 ± 2 m2 g− 1, whereas the analysis of 62 particles by AFM images gave a much larger mean SSA of 346 ± 37 m2 g− 1. We assume that the sample treatment prior to AFM imaging (involving dispersion by NaOH in a dilute dispersion and sonication for 2 min) resulted in delamination of the clay mineral particles. The ESA was 5.6 ± 0.4 m2 g− 1 for the illite, and 15 ± 2 m2 g− 1 for the montmorillonite. This leads to an ESA/BSA ratio of 0.07 for the illite and 0.05 for the montmorillonite for the delaminated particles. For the untreated, non-delaminated particles we calculated an ESA/BSA ratio of 0.16 for the illite and of 0.27 for the montmorillonite. The specific surface area as estimated by EGME was 112 m2 g− 1 for the illite and 475 m2 g− 1 for the montmorillonite, i.e. about 30–40% larger than the respective AFM values. However, this difference in specific surface area was not in agreement with the expected interlayer surface area of both minerals.