Mass transport of water vapor and ions from Å-scale graphene nanopores

Wan-Chi Lee
EPFL, 2022
EPFL thesis

Abstract

Hydrodynamics at the nanoscale involves both fundamental study and application of fluid and mass transport phenomena in nanometer-sized confinements. Nanopores in single-layer graphene can be highly attractive for exploring the molecular transport of gas and water molecules and hydrated ions at the ultimate scales of pore size and pore length. However, the experimental data is limited, and the state-of-the-art artificial nanopores still underperform compared to biological channels in cellular membranes. This dissertation focuses on developing ultimate graphene nanopore devices to study mass transport phenomena under controlled spatial confinement. We first investigated the kinetics of liquidâvapor transport from nanoscale confinements which is attractive for novel evaporation and separation applications; however, it is not explored at the ultimate confinement limit, i.e., at the atomic-thick and Ã-scale nanopore placed at the liquidâvapor interface. We show that the evaporation flux from such nanopores increases with decreasing pore size by up to one order of magnitude relative to the bare liquidâvapor interface. Molecular dynamics simulations reveal that oxygen-functionalized nanopores render rapid rotational and translational dynamics to water molecules by reducing and shortening the lifetime of waterâwater hydrogen bonds. Graphene nanopores also enable the study of ion transport across sub-nanometer-scale 2D confinements. We produce tailor-made nanopores approaching the size of hydrated ions by decoupling the pore nucleation and expansion. Monovalent metal ions are efficiently sieved from divalent ions, with K+/Mg2+ selectivity up to 70 and Li+/Mg2+ selectivity up to 50, corresponding to a sieving resolution of 1 Ã. Mitigating the non-selective pore formation further enhance the ion-sieving performance, reaching K+/Mg2+ selectivity up to 350 and Li+/Mg2+ selectivity up to 260. The pore size and structure allow adjusting the diffusion of ions across the nanopores, suggesting that the sterically induced partial dehydration process may play an important role in the observed cation selectivities. These selectivities were realized from centimeter-scale suspended graphene membranes, prepared in crack-free fashion by using dual layer reinforcement strategy where the first layer is 200-nm-thick nanoporous carbon (NPC) film hosting 20 nm pores which ensures a conformal contact and reinforcement of the graphene film and the second (top) layer is Nafion.Finally, a dual layer reinforcement is also demonstrated for preparing crack-free centimeter-scale gas separation membranes to utilize the full potential of graphene nanopores for energy-efficient applications. The bottom layer of the composite film is NPC film while the top layer is made of a 500-nm thick multi-walled carbon nanotube (MWNT) film with a pore size ranging from 200 to 300 nm. The obtained selectivities from crack-free centimeter-scale graphene membranes for H2/CH4 and H2/CO2 are 11â23 and 5â8, respectively, which is significantly higher than the corresponding Knudsen selectivities. Overall, this dissertation presents a graphene nanopore toolkit for studying fluid mechanics at the ultimate scales. The findings of enhanced water evaporation rate and ion selectivity using the nanopore platform could enrich our understanding of mass transport under extreme confinement and open new opportunities for a range of separation applications.

Official source

https://infoscience.epfl.ch/record/295115?ln=en

About this result

This page is automatically generated and may contain information that is not correct, complete, up-to-date, or relevant to your search query. The same applies to every other page on this website. Please make sure to verify the information with EPFL's official sources.

Related concepts

Related publications

Wan-Chi Lee
EPFL, 2022
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/295115?ln=en

About this result

Related concepts (36)

Related concepts

Related publications (32)

Related publications

The unique structure and dynamics of the water hydrogen (H)-bond network enable a multitude of structures and chemical reactions in both bulk solutions and at interfaces. The underlying molecular interactions between water and dissolved electrolytes, organic molecules, and nanoscale interfaces are difficult to study and hence not fully understood, especially when it involves interactions of length scales larger than one nanometer. In this thesis, we perform second order nonlinear light scattering experiments to investigate extended hydration shells (over one nanometer) and interfacial structures of buried aqueous interfaces that surround nanodroplets. We investigate the spatial range of ion-water interactions by probing the molecular structure of the water network in dilute electrolyte solutions. We find that electrolytes induce long-range, non-ion-specific orientational order in the H-bond network of water (over distances up to 70 hydration shells or 20 nm). These modifications in the water network start at electrolyte concentrations as low as 10 micromolar. The amount of induced orientational order and the concentration at which it occurs is different between light and heavy water. We link the observed ion induced changes in the bulk H-bond network of water to ion induced surface tension anomalies that occur in the same concentration range. We also study the same interactions in solutions with higher electrolyte concentrations focusing on cations. We observe specific ion effects in both the local charge distribution and water ordering in extended hydration shells of ions. Cations with larger charge densities induce larger changes in these two observables that follow a direct Hofmeister trend. The ion-induced structural changes in the water network are strongly correlated with the viscosity B-coefficient and hydration free energy of cations. These correlations facilitate constructing a better molecular model for specific ion effects in aqueous solutions. Next, we study the nanoscale hydrophobe/water interface and related interactions by probing the molecular structure of the interface formed with amphiphilic alkanols. We find that alkanols form a fluid film at the oil nanodroplet surface. Long chain alkanols show similar molecular structures and number densities at the interface. With increasing alkanol density, interfacial water loses its initial orientational alignment. The structure of interfacial oil molecules is more distorted for shorter alkanols. This interfacial structure differs significantly from those of macroscopic planar alkanol/water and alkanol/air interfaces and charged surfactant/oil/water interface, which can be explained by the balance between dispersive and H-bonding interactions. Finally, we develop a new membrane model system: 3D-lipid monolayers with tunable molecular structure on oil nanodroplets in water. This in-situ prepared system mimics the structure of lipid droplets in cells. The interfacial structure of lipids can be tuned from a tightly packed monolayer (liquid condensed phase) to a dilute one (liquid condensed/liquid expanded coexistence phase) by varying the lipid/oil/water ratio. The tunability of the chemical structure, the high surface-to-volume ratio, and the small sample volume make this system ideal for studies of molecular interactions of lipids in membranes.

EPFL2017

Release kinetics of volatiles from smectite clay minerals

Pascal Clausen

The focus of this thesis was on the mechanisms of release of volatile molecules from smectite clay minerals. The weight loss was monitored by thermogravimetry, differential calorimetry, and mass spectrometry under isothermal conditions and under ramp temperature conditions. Modeling of the clay-volatile release systems was performed with the help of finite element method calculations at the macroscopic level and ab initio calculations at the molecular level. A first comparison between finite element method simulations and experimental data for the evaporation of bulk volatile liquids under a convective gas flow showed the calculations to be in good agreement with experiments. Results of the simulations were used to develop a semi-analytical model explaining the dependence of the evaporation on the total pressure, the carrier gas flow, the temperature, and material constants. In particular, its temperature dependence could be approximated to good accuracy by an Arrhenius-type equation derived from the semi-analytical model. Differential calorimetry measurements of the heats of vaporization showed equilibrium conditions at the surface of the liquids to be satisfied. The same approach was extended to the release of model volatiles (water, ethanol, ethyl acetate and toluene) from smectite clays. At high coverage, the release was found to be close to that for the bulk liquids. Its decrease with time followed the behaviour observed in the respective curves of the gas/condensed phase partition coefficients, the equilibrium desorption isotherms. Equilibrium condition at the surface of the sample was evidenced by a comparison between the measured heats of vaporization and the equilibrium desorption isotherms. The differences observed in the measured equilibrium desorption isotherms of the volatile on the smectite clays could be rationalized by the use of ab initio calculations of the binding of the volatiles on the surface of a sodium smectite clay. At low coverage, the differences could be attributed to their differences in binding energies with the clay counter ions, which could be explained in terms of the chemical nature of the interacting species. At high coverage, the differences could be related to the properties of the bulk liquids, because high coverage corresponds to high activities of the volatiles on the clay samples. The differences observed in the measured rates of release of the volatiles from the smectite clays could be rationalized by the use of finite element calculations and the semi-analytical model developed for the bulk liquids, taking as input parameters the measured equilibrium desorption isotherms, that determines the volatile gas phase concentration at the surface of the sample. The slower rates of release of ethanol and ethyl acetate from the smectite clay, compared to water, could be explained from their differences in their equilibrium desorption isotherms from the clay, though diffusion effects also played a minor role. However, the slower rate of release of toluene, compared to water, could only be explained by its slow diffusion in the smectite clay. Diffusion effects were found to be enhanced with an increase in the size of the aggregate, such that, in addition to the binding strength of the counter ions, the swelling capacity of the clay, allowing the clay to clump, was also found to be an important factor for the controlled release of volatiles from the clay. Ion-exchange of the clay with lithium cations was found to be optimal in terms of ionic strength of the cation and swelling capacity, as compared to other metallic cations and small organic cations. This clay modification was tested on the release of malodorous compounds determined in cat urine by mass spectrometry and sniffing experiments. The smectite clay modified by ion-exchange with lithium cations showed improved controlled release of volatile organic compounds found in cat urine, compared to the sodium smectite, and improved clumping capacity compared to the calcium smectite. The calcium smectite clay used by Nestlé Purina could be modified by column exchange with lithium cations in high quantities. However the potential toxicity of the lithium is unknown. Additional information was obtained by ab initio calculations on the interaction of water with the smectite. The calculations of the binding of one to six water molecules on the surface of a sodium clay showed the water molecules bind to the cation and form hydrogen bonds between each other and with the clay oxygens, leading to the formation of different configurations for the same number of water molecules on the clay surface. With an increase in the number of water molecules on the surface of the clay, intermolecular interaction between the water molecules became gradually more important than sodium-water interactions. The binding energy was also found to be dependent on the location of the charge substitution in the clay (tetrahedral or octahedral) and to the formation of hydrogen bonds between the water molecule and a basal oxygen linked to a tetrahedral replacement. With an increase in the number of water molecules bound to one counter ion, the sodium counter ion was found to move away from the clay surface. The dynamic calculations of water molecules in the clay interlayer showed both the water molecules and the cations to be mobile. The average diffusion coefficient of the water molecules was reduced compared to that of bulk water, due to restrictions imposed by both the clay platelets and the counter ions. The structure of the water molecules showed to be similar to that of bulk water with a preferential orientation of their hydroxyl group almost perpendicular to the clay surface. The difference in the state of order of the water molecules around the sodium cation was interpreted as a symptom of hysteresis as observed in the measured equilibrium sorption isotherm.

EPFL2009

Heterogeneous kinetics of some atmospherically relevant reactions

A low pressure reactor (Knudsen cell) coupled to molecular beam modulated mass spectrometry was used to study the heterogeneous kinetics of the following three systems: (1) NO2 on amorphous carbon, (2) H2O vapor on ice and (3) gaseous HCl on ice. The first system was investigated at ambient temperature while the two last ones were investigated at temperatures typical of the stratosphere, that is roughly between 160 and 220K. The solid phase samples correspond to model surfaces whose physicochemical properties come very close to the ones of atmospheric particulates. Firstly, we studied the interaction of gaseous NO2 with three different commercial samples of amorphous carbon having widely differing physicochemical properties. Using in situ laser-induced fluorescence, the only major product was unambiguously determined to be NO with the oxidation product of carbon apparently remaining on the surface. Probing the gas phase with mass spectrometry (MS), both pulsed valve dosing and steady state experiments were performed and revealed a complex reaction mechanism for both the uptake as well as product formation. The initial uptake coefficient γo for NO2 was (6.4±2.0)·10-2 and proved to be identical within experimental uncertainty for all three types of amorphous carbon. The initial NO2 uptake rate was independent of the mass of the carbon sample and scaled with its external geometrical surface. Applying a simple chemical kinetic model to both pulsed dosing and steady state experiments resulted in an effective surface for uptake on amorphous carbon ranging between a factor of 2.8 to 8.4 larger than the geometrical surface area, but inversely proportional to the measured BET surface area of the three types of amorphous carbon. This means that the amorphous carbon with the highest BET surface (FW2) had the lowest external surface for NO2 adsorption. The chemical kinetic model included competing processes between Langmuir-type adsorption and inhibition controlling the adsorption kinetics of NO2 and revealed that the NO generation rate differed greatly between the three carbon samples examined. All samples showed saturation effects of differing degree that were partially reversible through prolonged pumping at 10-4 Torr and/or heating. Virgin amorphous carbon samples did not take up H2O vapor at 20 mTorr, and no HONO and/or HNO3 was detected in simultaneous NO2/H2O exposure experiments. CO and CO2 were detected when a sample previously exposed to NO2 was heated by an incandescent lamp. Moreover, upon heating such a sample, a MS signal m/e 62 originating from NO3 and/or N2O5 was detected. In addition to the experiments conducted on the three commercial carbon samples, we carried out uptake experiments of NO2 on acetylene soot. Originating from the flame of an acetylene burner, this soot was freshly deposited on glass dishes before each experiment. The initial uptake coefficient γo for NO2 was measured to be (3.0±1.1)·10-2 is smaller by a factor of two compared to γo observed in the uptake on commercial samples whose mass was a factor of 10 higher. In addition to NO, a net formation of HONO (m/e 47) was observed. This HONO formation is strongly dependent on the instantaneous water desorption from the sample as well as on the NO2 partial pressure in the reactor. On the other hand, it is not enhanced by an external water flow which did not show any interaction with the carbon sample. Upon heating the sample using an incandescent lamp, significant amounts of water desorbed, suggesting that the water vapor had condensed into the micropores of the soot during combustion. No quantitative results are presented yet. However, as no correlation between HONO formation and NO partial pressure has been observed so far, we suggest a reaction mechanism that only involves NO2 and H2O but no NO: 2 NO2 (g) + H2O HONO + HNO3 In a second study, the kinetics of condensation and evaporation of water on ice was determined in the temperature range of 170 to 220K. The rate of evaporation Fev corresponds to the evaporation of 70±10 monolayers of H2O per second at 200K. The condensation rate constant kc has a negative activation energy of -3.l±1.5kcal/mol which is significantly larger than the one recently measured by Haynes et al. We report the first real time kinetic measurements of water condensation on ice at stratospherically relevant temperatures. At temperatures above 160K, pulsed dosing experiments show a dose dependence of the condensation rate coefficient. For example, the condensation rate coefficient increases by a factor of five with a 1000 fold increase of the H2O dose from 1·1015 to 1·1018 molecules per pulse at 180K. This pressure dependence of the condensation rate constant may be explained by an autocatalytic mechanism involving the competition of two condensation channels. These two channels feed two different surface species, one of which leads to autocatalytic condensation. This mechanism gives insight into the manner in which a gas phase molecule is stepwise incorporated into tetrahedrally coordinated bulk ice. In a third and ongoing study, the uptake kinetics of HCl on ice has been investigated by pulsed valve experiments in the temperature range of 180 to 210K and are discussed as a function of the state of the surface. In fact, some of the pulsed valve experiments present a relaxation to a steady state level. It is shown that this level originates from the evaporation of a liquid solution corresponding to a temperature specific HCl concentration. The threshold of the injected dose leading to this solution layer is estimated to be 8·1014 molecules. This threshold corresponding to a fraction of a monolayer may be explained by the formation of small domains along grain boundaries or other lattice imperfections. Within detection limit, no difference in keff has been observed for pulses on the solution or on a previously exposed sample without formation of the solution. In the case of pulses forming a solution layer, a fraction of 20±20% of the injected molecules remains permanently adsorbed on the surface. In the temperature range from 210 to 180K, the pulsed valve experiments yield values of keff roughly between 5 and 25s-1 and an activation energy of -1.6±0.7kcal/mol which are consistent with the values observed by Flueckiger in steady state experiments using the 15mm escape aperture (kuni varying from 12 to 23s-1 and an activation energy of -1.9±0.25 kcal/mol).