Atomic scale structures of interfaces between kaolinite edges and water

This paper reports the atomic scale structures of kaolinite edge surfaces in contact with water. The commonly occurring edge surfaces are investigated (i.e. (0 1 0) and (1 1 0)) by using first principles molecular dynamics (FPMD) technique. For (1 1 0)-type edge surface, there are two different surface topologies (denoted as (1 1 0)-1 and (1 1 0)-2) and they are considered separately.

By using constrained FPMD technique, the free-energy changes for the leaving processes of water ligands of O-sheet Al cations have been calculated and thus, the coordination states of those Al cations are determined. The results show that for (0 1 0) and (1 1 0)-2 edges, both the 5 and 6-fold coordination states are possible whereas for (1 1 0)-1 edges, only the 6-fold states are stable. Based on the analyses of H-bonding structures at the interfaces, the surface acid/base reactive sites are illustrated. (1) T-sheet groups are Si-OH, behaving as both proton donors and acceptors. (2) Bridging oxygen At (0 1 0) and (1 1 0)-2 edges, these sites are inaccessible from the water and thus, they are not effective reactive sites. At (1 1 0)-1 edges, the bridging oxygen atoms are proton accepting sites. (3) O-sheet groups At (0 1 0) and (1 1 0)-2 edges, for 6-fold Al cases, the active surface groups are Al-(OH)(OH2) and for the 5-fold Al cases, the active surface groups are Al-(OH). At (1 1 0)-1 edges, the active site is Al-OH2. This study provides fundamental structural properties for understanding the interface chemistry of widely occurring 1:1 type phyllosilicates.