(175e) Wetting Behaviors of Water in Confined Graphite and Rutile Slits

The surface science enjoys an increasing interest, and many researchers develop attentions on wetting behaviors of water on hydrophobic and hydrophilic surfaces. Moreover, the wetting behaviors of nano-scale confinements draw attention of researchers who focus on the fields such as membrane separation, biological channel, heterogeneously porous catalysts and so on.

Rutile (110) is well accepted as the representative bulk model oxide and is used to discover the distinct phenomena on metal oxide surfaces which represent the hydrophilic ones. Meanwhile graphite (100) is the exact model for hydrophobic surfaces. Furthermore the surface effect will be enhanced in nano-scale pores and gaps, which are difficult to characterize in experiments. However, molecular simulation offers an alternative method to get insight on the molecular level information about the fluid around the surfaces. In this work, we study the static structure properties of water confined to slab geometry of graphite (100) or rutile (110) with molecular dynamics simulation, and then try to find out the distinction in these two kinds of slits.

We build a model that contains two slabs of graphite(100) or rutile(110) opposite to each other with water molecules between them, and their distance is ranging from 0.4 to 2.0 nm by NVT-ensemble (T=300K) MD simulations. The long-range electrostatic interaction plays an important role on the interactions between atoms of the rutile slabs and water molecules. Significant water layers were observed along the normal direction of theses rutile surface. The Water molecules in the closest layer are absorbed at the five-coordinated Ti and two-coordinated O (or bridging O) atoms on the surfaces. On the other hand, water molecules prefer to keep distance (about 0.2 nm) from the graphite surfaces. Further, the dipole of water molecules near the (graphite or rutile) surfaces was found to have considerable preferential orientation, mainly because of long-rang electrostatic interactions of the surface atoms for rutile surfaces and repulsion force for graphite surfaces. With the decrease of the distance between slabs, water molecules have smaller diffusion coefficient in rutile slits while approximate diffusion coefficients in graphite slits.