(474b) Selective Flow through Membrane Pores with in Situ Change of Wettability

This report demonstrates in situ change of wettability of water on the gold surfaces adsorbed with various molecules responding to relatively low electric current and voltage. This tunable wettability is achieved by the conformational change of linear molecules adsorbed on gold surfaces when the functional groups with dipoles at the end of the molecules are attracted to the electric potential at the gold surface. While the water is in contact with the end groups before the conformation change without voltage, the water would be in contact with the backbone of the linear molecules after conformation change with voltage, resulting in the change in wettability. While there are many other ways of achieving the wettability of molecule-adsorbed surfaces, utilizing the conformation change with small voltage (< 2V) is a more practical way of achieving in situ change because the liquid should neither have to be water as in manipulating pH, conductive as in electrowetting also requiring high voltage ( > 100 V), nor it requires high energy or long wait time as in achieving such change with temperature.

The realization of in situ wettability change can lead to the development of new kinds of membranes that can be used as selective control valves that can separate two-liquid mixture that permeate one liquid with no voltage while permeating the other liquid with voltage by adsorbing long linear molecules with dipoles on metal-coated membranes with a specific geometry. As the first step in preparing such membrane and to show the possibility of such application with the in situ change of wettability, each of four thiol molecules with long backbones was adsorbed onto flat gold and nickel surfaces. While changing the voltage across those metal surfaces were changed from 0 to 2 V, the advancing contact angles of water on those molecules were measured. The space between the long molecules were also varied by mixing those long thiol molecules with a short thiol molecule, i.e., ethanethiol, to provide more space for the long molecules to bend toward the metal surfaces, exposing the backbones to water. We found that the water contact angle increased up to 30 ° when the ratio of long molecules to the short molecule was at 1:1, providing enough difference to realize the fabrication of membrane control valve controlling the flow of water through membranes with 200-µm openings.