Credit: © 2007 ACS

Liquids confined to nanocavities have diverse applications in drug delivery or catalysis. So far, most theoretical studies of liquids filling nanopores assume a 'vacuum' environment and only consider the interactions between the liquid and the surface of the pores. Now, molecular dynamics simulations carried out by Xi Chen and colleagues1 at the University of California and Columbia University show that the presence of gas can dramatically affect nanofluidics behaviour.

The team investigated how the presence of CO2 gas affects the uptake of water by carbon nanotubes of different sizes. In their model, they vary the pressure to induce uptake or outflow phenomena. In relatively large nanopores, they find that the CO2 is easily dissolved in the water as it enters the nanotube and, therefore, does not have a major influence on the behaviour of the system. Infiltration was observed above a threshold pressure, and virtually no outflow occurred. In nanotubes with diameters of only a few nanometres, however, the gas molecules form a cluster, which assists both the uptake and outflow of water. The presence of even a single CO2 molecule has a considerable effect during the infiltration process, increasing the volume of the tube filled by water by more than a factor of two.

Preliminary experiments monitoring the uptake of water by porous silica gel seem to be in good agreement with the results of the simulations.