Water molecules can be easily adsorbed on the surface or in the interlayer space of clay minerals. This process is named hydration which plays an important role in various application fields. In order to find how water interacted with surface through minimizing the interaction among multiple waters, the adsorptions of single water molecule on external surfaces of sodium-montmorillonite (Na-MMT), including (001) basal and (010) edge surface, were theoretically investigated based on periodic density functional theory (DFT) method. The adsorption energies and geometries as well as electronic properties were studied in the work. It was found that water molecule was adsorbed on Na-MMT (001) basal surface mainly through electrostatic interaction between water molecule and Na+ cation, and was adsorbed on (010) edge surface through hydrogen bonding between water and surface -OH or -OH2 groups. The adsorption energy E-ads value of water molecule on (010) edge surface was larger than that on (001) basal surface. After adsorption, a part of electron density was transferred from both Na-MMT (001) and (010) surfaces to water molecule. Based on the PDOS, there was the bonding between Na 3s and O-w 2p orbitals on (001) basal surface and between H 1s and O 2p orbitals for hydrogen bonds on (010) edge surface. (C) 2016 Elsevier B.V. All rights reserved.