STREAMS papers

4. Molecular Jamming in Tortuous Nanochannels
P. Dementyev, Y. Yang, M. Rezvova, A. Gölzhäuser
J. Phys. Chem. Lett. 2020, 11, 238

Ultrathin nanostructured membranes are widely pursued to apply into different processes ranging from air separation to seawater desalination. Here, freestanding carbon nanomembranes (CNMs) are employed to dehydrate vaporous alcohols at room temperature. The structure of the microporous material is addressed by measuring permeation rates of homologous n-alkanols. To examine the separation performance, we introduce a model heavy water/n-propanol azeotrope. While ordinary
nanomembranes show moderate selectivity of around 300, complete rejection of organic molecules is achieved upon stacking two CNM layers. Furthermore, the mixture experiments with the stacks demonstrate a 10-fold slowdown in the transmembrane diffusion of water as compared to both the single-layer material and pure vapor. We discuss the observed effect as a “molecular jam” in the interlayer spacing, which effectively disrupts the collective flow of liquefied water. Our work sheds light on molecular transport under nanoconfinement.

3. Water-assisted permeation of gases in carbon nanomembranes
D. Naberezhnyi, A. Gölzhäuser, P. Dementyev
J. Phys. Chem. Lett. 2019, 10, 5598

Planar nanomaterials finished with transverse ducts represent an intriguing avenue for exploring interfacial phenomena. Due to their small thickness, the kinetics of molecular diffusion across the channels is likely to be dominated by entrance events. Therefore, measuring transport rates in freestanding films can yield valuable information on surface processes. In this work, we study permeation of gases in carbon nanomembranes (CNMs) when accompanied by saturated water vapor. The experimental data reveal a manifold increase in transmembrane fluxes compared to dry conditions. Gas molecules are found to be trapped in adsorbed water, which enhances their translocation likelihood. We demonstrate that the permeance correlates with the vapor relative pressure and discuss the
observed crossing mechanism in terms of water condensation and Henry’s law. Our findings provide guidance for designing gas separation membranes upon two-dimensional materials.

2. Vapour permeation measurements with free-standing nanomembranes
P. Dementyev, T. Wilke, D. Naberezhnyi, D. Emmrich, A. Gölzhäuser
Phys. Chem. Chem. Phys. 2019, 21, 15471

Mass transfer across porous materials with nanoscale thickness is of great interest in terms of both fundamentals of fluid dynamics and practical challenges of membrane separation. In particular, few-atom thick sieves are viewed as attractive candidates to achieve ultimate permeability without compromising membrane selectivity. In this work, we introduce a vacuum system for studying vapour and gas permeation in micrometre-sized samples of suspended nanometre-thick films. Steady-state permeation rates are measured with a mass-spectrometer directly connected to the downstream side of a membrane cell. A built-in nanoaperture is used as a reference to calibrate the detector in situ. A feed compartment is designed in a way that allows for preparing gaseous mixtures of variable composition, including vapours of volatile liquids. Room-temperature measurements with carbon nanomembranes confirm that this material is selective to water vapour and can efficiently separate it from mixtures with a variety of gases and organic compounds. We demonstrate that a high permeance for water is maintained regardless of the molar fraction and discuss its strong pressure dependence by invoking adsorption-related formalism.

1. Rapid water permeation through carbon nanomembranes with sub-nanometer channels
Y. Yang, P. Dementyev, N. Biere, D. Emmrich, P. Stohmann,  R. Korzetz, X. Zhang, A. Beyer, S. Koch, D. Anselmetti, A. Gölzhäuser
ACS Nano 2018, 12, 4695

The provision of clean water is a global challenge, and membrane filtration is a key technology to address it. Conventional filtration membranes are constrained by a trade-off between permeance and selectivity. Recently, some nanostructured membranes demonstrated the ability to overcome this limitation by utilizing well-defined carbon nanoconduits that allow a coordinated passage of water molecules. The fabrication of these materials is still very challenging, but their performance inspires research toward nanofabricated membranes. This study reports on molecularly thin membranes with sub-nanometer channels that combine high water selectivity with an exceptionally high permeance. Carbon nanomembranes (CNMs) of ∼1.2 nm thickness are fabricated from terphenylthiol (TPT) monolayers. Scanning probe microscopy and transport measurements reveal that TPT CNMs consist of a dense network of sub-nanometer channels that efficiently block the passage of most gases and liquids. However, water passes through with an extremely high permeance of ∼1.1 × 10^(−4) mol·m^(−2)·s^(−1)·Pa^(−1), as does helium, but with a ∼ 2500 times lower flux. Assuming all channels in a TPT CNM are active in mass transport, we find a single-channel permeation of ∼66 water molecules·s^(−1)·Pa^(−1). This suggests that water molecules translocate fast and cooperatively through the sub-nanometer channels, similar to carbon nanotubes and membrane proteins (aquaporins). CNMs are thus scalable two-dimensional sieves that can be utilized toward energy-efficient water purification.