Water transport in nanochannels has become an area of significant scientific interest due to its potential applications in water purification, desalination, and energy generation. Over the past decades, various nanomaterials have been explored for their ability to transport water faster than conventional materials. Among them, two-dimensional (2D) nanomaterials have shown exceptional promise, given their high surface area and unique nanochannel structures. MXenes, a family of 2D transition metal carbides, nitrides, and carbonitrides, are hydrophilic and electrically conductive and have surface functional groups such as hydroxyl, oxygen, chlorine, and fluorine. Since their discovery, they have demonstrated promising properties in various applications such as energy storage and conversion, water purification, electromagnetic interference shielding, and humidity sensors. Their hydrophilic nature, combined with a high metallic conductivity, sets them apart from most other 2D materials. From a potential application point of view, so far experimental studies have mainly focused on how ionic transport occurs in MXene nanochannels. However, a comprehensive study on water transport in these nanochannels, particularly concerning the interaction of water molecules with cations of varying hydrated diameters, remains elusive. Therefore, manipulating the affinity of water molecules towards the nanochannel walls through the intercalation of cations with different hydrated diameters could provide a novel approach to tailor and elucidate the water friction in the unique water transport phenomena in MXene-based nanochannels.
Here, we investigate the influence of cation intercalation on water transport through Ti3C2Tx MXene membranes. To this end, we intercalate different types of ions (using NaCl, KCl, LiCl, MgCl2, and CaCl2) with varying water affinities, allowing us to manipulate the number of hydrogen bonds attracted to the intercalated cation. This adjustment has led to the finding that water flux increases with the enlargement of cation-controlled interlayer space. Through water flux measurements, we demonstrate a direct correlation between water flux and the cation-controlled interlayer space, which is also related to the size of the hydration shell of the respective cation. Utilizing cations with different water affinities has allowed us to manipulate the number of hydrogen bonds attracted to cations on the wall of a MXene-based nanochannel and to experimentally observe its effect on water transport through these nanochannels. Furthermore, our findings provide important insights into the water transport mechanism in MXene-based nanochannels in general and support the application of the Hagen-Poiseuille equation to describe water transport in these nanochannels.
