The surface thermodynamic properties, expressed by the Lifshitz-van der Waals apolar surface tension component γLW, and the polar Lewis acid/base parameters γ+ and γ- of several illites and mixed-layer I/S clay minerals were determined by thin layer wicking. The surface tension components indicate that most illites are monopolar materials with moderate to high γLW (37-42 mJ/m2), moderate Lewis base γ- (14-19 mJ/m2) and zero Lewis acid γ+. NH4+-exchanged illites have the same magnitude of γLW and γ+, slightly small but more consistent γ- (11-14 mJ/m2). The Lewis basicity originates from the lone pair electrons of oxygen composing the outermost surfaces of the mineral particles and possibly from the seat of charges originated from substitution in tetrahedral and/or octahedral sheets. The surface tension parameters of mixed layer I/S clay minerals are different from those of illites and are almost the same after exchanged with NH4+, indicating that the intrinsic structures rather than interlayer cations of these mixed-layer clay minerals play the key role in the surface thermodynamics. For natural illites both γLW and γ- decrease as their cation exchange capacity increases, but for NH4+-exchanged illites the γLW decreases while γ- increases with an increase in cation exchange capacity. Also, there is a general trend that surface tension of these clay minerals increases with respect to the increase in their specific surface area.
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