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Purification of water through nanoporous carbon membranes: a molecular simulation viewpoint
Erich A Mullera
a Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK
Summary
It has been reported that a 1μm thick supported membrane with graphene oxide showed permeability for water but not for helium. In this paper, the transport mechanisms of water have been reviewed. Water has unrealistic features like its adsorption on hydrophobic looking carbon surfaces. Sometimes, the adsorption is preferential over hydrocarbons. The nanoscale studies can elucidate such properties of water.
Unlike other chemicals, water can make four hydrogen bonds with its neighbors, it is strongly polar, and it adsorbs on activated carbon surfaces in a different manner. Unlike other chemicals, that make layers on adsorbate (Langmuir adsorption), water forms three dimensional clusters as shown in the following Figure 1. Experimental problems include characterization of carbon surfaces, activation of the surfaces, and macroscopic behaviors in terms of molecular level phenomena. Muller et al., showed the adsorption mechanism of water in contrast to conventional Langmuir model which gives a clear picture of three dimensional adsorption of water over surfaces. The process is called fluid-fluid process.
Figure 1
Water confinement gives extraordinary properties like uneasy desorption and depression of freezing point of water in the confinements. Simulations of water over 1D and 2D carbon surfaces show different behavior of water. For example, unlike the four hydrogen bonded neighbors, the water makes a nanowire in the carbon nanotubes.
The generalization of conventional water behavior is difficult. But, the molecular simulations have shown that diffusion is several orders of magnitude larger than the experimental observations. High diffusion of water through bacterial potassium channels prove the results of simulations. Moreover, the substituents at entrance and exit improve the diffusion of water. Based on these kinds of simulations, several groups are focusing on the fabrication of aligned carbon nanotubes and have shown several orders of magnitude high diffusion of water. The water moves in axial direction. But recently, the focus has been diverted to graphene nanopores where water flows orthogonal to the nanopores. If graphene holes allow single file of water, the transport of water is less than in nanotubes. For larger pores, the transport becomes convective but the pores cannot be left unfunctionalized. Functionalized pores show higher diffusion of water with rejection of large molecules and single small sized ions. Since, the simulations of simple structures have no theoretical restrictions. Moreover, the transport properties of water are governed by the local pore morphologies rather than the bulk structures, so, the geometries like nanotubes and self-assemblying nano-flakes have been proposed for water purification.
The heart of a molecular simulation is force field that can be obtained with ab initio calculations. After lengthy calculations a semi-empirical model is required to fit the modeled data. This is very complex and so a simple first principle study of adsorption energy of single water molecule on graphite surface with different electronic structures has reported the adsorption energies between -5 to ? 151 meV. There are several water models and are consistent with each other. But, each of them fails to explain one or more thermo-physical properties of water. Therefore, no generalization can be made. The simulations cannot involve some properties like the electronic quadrupoles of sp2-hybridized carbons. So, a clear phenomena cannot be obtained. So, the selection of force field is crucial in molecular simulations. An example that predictions are sensitive to force field is the adsorption of water over graphite. That's why there is scope of improvement in the modeling approaches.
Reviewer: Aamir Alaud Din