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Water-Filled Single-Wall Carbon Nanotubes as Molecular Nanovalves


Materials Explorer was used to carry out molecular dynamics simulations on stability of water inside single-wall carbon nanotubes (water-SWNTs) in various gas atmospheres. It is found that the resistivity of water-SWNTs exhibits a significant increase in gas atmospheres below a critical temperature Tc, at which a particular type of atmospheric gas molecule enters the SWNTs in an on-off fashion. On the basis of this phenomenon, it is proposed that water-SWNTs can be used to fabricate a new type of molecular nanovalve.



Figure 1. MD simulations for the coexistent systems of water and methane molecules. Initially the water clusters are located inside the SWNT. After 300 ps, methane molecules enter the SWNT at 200 K.

The process of the water ejection from the inside of the SWNTs triggered by the entering gas molecules is observed below Tc. The water ejected from the inside of the SWNTs is located around the open ends of the SWNTs, where it is able to reversibly re-enter the SWNTs. Such process is possible when the gas-molecule-SWNT interactions have a comparable strength to that of the water-SWNT interactions. Then, water and gases are competing with each other to fill the SWNTs. Evidently, gases with stronger attractive interactions with the SWNT walls are more stable inside the SWNTs, and therefore have higher Tc.



Figure 2. MD simulations of filling-ejecting phenomena for the coexistent systems of water and Ne molecules.

The filling-ejecting transition mentioned above were carried out on a system of an open SWNT, water, methane and Ne using NTV ensemble at T = 200 K. The interaction potentials were based on the TIP3P model for H2O and Lennard-Jones (LJ) sphere models for carbon, CH4 and Ne.
The coexistent cases are given in Figures 1 and 2. It is shown that the water cluster was gradually forced out from the SWNT by the entering CH4 molecules (Fig. 1). In contrast, for Ne with a weaker gas-SWNT interaction potential (about 1/2 of that of CH4), such replacement was not observed (Fig. 2), confirming qualitatively the observed phenomena in the present experiments.



References: Maniwa et al., Nature Materials (2007), 6, 135.