Identification of endohedral water in single-walled carbon nanotubes by 1H NMR

Water confinement within single-walled carbon nanotubes (SWCNTs) has been a topic of current interest, due in part to their potential nanofiltration applications. Experiments have recently validated molecular dynamics predictions of flow enhancement within these channels, although few studies have probed the detailed structure and dynamics of water in these systems. Proton nuclear magnetic resonance (H-1 NMR) is a technique capable of providing some of these details, although care must be exercised in separating the confined water of interest from exterior water. By using controlled experiments with both sealed and opened SWCNTs and by providing a quantitative measure of water content through desorption experiments, a signature for confined water in SWCNTs has been positively identified. This endohedral or interior water is characterized by a relatively broad feature located at 0.0 ppm, shifted upfield relative to bulk water. With the identification of a signature for water inside SWCNTs, further studies aimed at probing water dynamics will be enabled.