A DFT-D2 study of formic acid adsorption at smectite edges based on pyrophyllite models

[Display omitted] •Pyrophyllite is used as a model to study adsorption of formic acid to clay edges.•Formic acid is strongly adsorbed at all the edges both in monodentate and in bidentate fashions.•In wet conditions, these species give rise to inner sphere complexes stable at moderately acidic pH.•Monodentate species prevail at the (1 0 0) and (1 1 0) edges.•Strongly bound bidentate forms are most stable at the (0 1 0) and the (1 3 0) edges.•Results are in agreement with experimental data on the adsorption of dicarboxylic acids. The adsorption of formic acid at smectite edges is investigated by DFT calculations using a model system based on pyrophyllite. Two cases are considered: (I) only the Si atoms are saturated with water molecules and the edges are exposed at the vacuum; (II) all the exposed Al cations are also saturated, and the edges are in equilibrium with an aqueous environment at 298 K. In the former case, adsorption is strong, and involves the formation of monodentate e bidentate species. Most of complexes have a negative formation energy also in wet conditions, which indicates the occurrence of stable aqueous interfaces.