Carbon membranes for efficient water-ethanol separation

Spontaneous sieving of water from ethanol using angstrom-sized nanopores

Ethanol is widely used as a precursor in products ranging from drugs to cosmetics. However, distillation of ethanol from aqueous solution is energy intensive and expensive. Here, we show that angstrom-sized nanopores with precisely controlled pore sizes can spontaneously remove water from ethanol-water mixtures through molecular sieving at room temperature and pressure. For small-diameter nanotubes, water-filling is observed, but ethanol is completely excluded, as evidenced by time-dependent density functional theory (TD-DFT) calculations and spectroscopy measurements. Potential of mean force calculations were performed to determine how the free energy barriers for water and ethanol-filling of the nanotubes change with increasing pore size. Water/ethanol selectivity ratio reaching as high as 6700 is observed with a (6,4) nanotube, which has a pore size of 0.204 nm. This selectivity vanishes as the pore size increases beyond 0.306 nm. These findings provide insights that may help realize energy efficient molecular sieving of ethanol and water.

Catalytic Reduction of Graphene Oxide Membranes and Water Selective Channel Formation in Water-Alcohol Separations

Graphene oxide (GO) is a promising membrane system for chemical separation applications due to its 2-D nanofluidics properties and an ability to control interplanar spacing for selectivity. The permeance of water, methanol (MeOH) and isopropyl alcohol (IPA) through 5 µm thick membranes was found to be 0.38 ± 0.15, 0.33 ± 0.16 and 0.42 ± 0.31 LMH/bar (liter/m²·h·bar), respectively. Interestingly, the permeance of a water-alcohol mixture was found to be dramatically lower (~0.01 LMH/bar) than any of its components. Upon removing the solvent mixture, the transmembrane flux of the pure solvent was recovered to near the original permeance. The interlayer space of a dried GO membrane was found to be 8.52 Å, which increased to 12.19 Å. 13.26 Å and 16.20 Å upon addition of water, MeOH and IPA. A decrease in d-space, about 2 Å, was consistently observed when adding alcohol to water wetted GO membrane and an optical color change and reduction in permeance. A newly proposed mechanism of a partial reduction of GO through a catalytic reaction with the water-alcohol mixture is consistent with experimental observations.

Optimization of the Production of 1,1-Diethoxybutane by Simulated Moving Bed Reactor

Simulated moving bed technology is applied in the field of pharmaceutical, petrochemical and fine chemistry. It shows capability in separating multicomponent mixtures up to high purities. In this work, an attempt was made to optimize the production of 1,1-diethoxybutane (DEB), using the simulated moving bed technology. A fixed bed model is made with good agreement with experimental results. This fixed bed model was expanded to a simulated moving bed model. This model was used to determine the optimum conditions regarding the switching time and flowrates in each section. From this model, the optimum switching time was found to be 2.4 min, and the ratio of liquid flowrate over the solid flowrate in Section 1Section 2Section 3 and Section 4 of the SMBR was found to be 4.24, 1.77, 3.03 and 1.35, respectively. Under those conditions, the productivity was 19.8 kg DEB per liter of adsorbent per day, and the desorbent consumption was 6.1 L of ethanol per kg of DEB. The results were obtained with a minimum purity of the extract and raffinate of 97%.

Formation of a Stable Bridge between Two Disjoint Nanotubes with Single-File Chains of Water

It was recently demonstrated that stable water bridges can form between two relatively large disjoint nanochannels, such as carbon nanotubes (CNTs), under an applied pressure drop. Such bridges are relevant to fabrication of nanostructured materials, drug delivery, water desalination devices, hydrogen fuel cells, dip-pen nanolithography, and several other applications. If the nanotubes are small enough, however, then one has only single-file hydrogen-bonded chains of water molecules. The distribution of water in such nanotubes manifests unusual physical properties that are attributed to the low number of hydrogen bonds (HBs) formed in the channel since, on average, each water molecule in a single-file chain forms only 1.7 HBs, almost half of the value for bulk water. Using extensive molecular dynamics simulations, we demonstrate that stable bridges can form even between two small disjoint CNTs that contain single-file chains of water. The structure, stability, and properties of such bridges and their dependence on the applied pressure drop and the length of the gap between the two CNTs are studied in detail, as is the distribution of the HBs. We demonstrate, in particular, that the efficiency of flow through the bridge is at maximum at a specific pressure difference.

Selective Permeation of Water through Angstrom-Channel Graphene Membranes for Bioethanol Concentration

Graphene-based laminate membranes have been theoretically predicted to selectively transport ethanol from ethanol-water solution while blocking water. Here, robust angstrom-channel graphene membranes (ACGMs) fabricated by intercalating carbon sheets derived from chitosan into thermally reduced graphene oxide (GO) sheets are reported. ACGMs with robust and continuous slit-shaped pores (an average pore size of 3.9 Å) are investigated for the dehydration of ethanol. Surprisingly, only water permeates through ACGMs in the presence of aqueous ethanol solution. For the water-ethanol mixture containing 90 wt% ethanol, water can selectively permeate through ACGMs with a water flux of 63.8 ± 3.2 kg m^-2 h^-1 at 20 °C and 389.1 ± 19.4 kg m^-2 h^-1 at 60 °C, which are over two orders of magnitude higher than those of conventional pervaporation membranes. This means that ACGMs can effectively operate at room temperature. Moreover, the ethanol can be fast concentrated to high purity (up to 99.9 wt%). Therefore, ACGMs are very promising for production of bioethanol with high efficiency, thus improving its process sustainability.

Universal and Nonuniversal Aspects of Electrostatics in Aqueous Nanoconfinement

Dielectric water properties, which significantly change in confinement, determine electrostatic interactions and thereby influence all molecular forces and chemical reactions. We present comparative simulations of water between graphene sheets, decanol monolayers, and phospholipid and glycolipid bilayers. Generally, dielectric profiles strongly differ in perpendicular and parallel surface directions and for large surface separation decay to the bulk value 1-2 nm away from the surface. Polar surface groups enhance the local interfacial dielectric response and for phospholipid bilayers induce a giant parallel contribution. A mapping on a box model with asymptotically determined effective water layer widths demonstrates that the perpendicular effective dielectric constant for all systems decreases for confinement below a nanometer, while the parallel one stays rather constant. The confinement-dependent perpendicular effective dielectric constant for graphene is in agreement with experimental data only if the effective water layer width is suitably adjusted. The interactions between two charges at small separation depend on the product of parallel and perpendicular effective water dielectric components; for large separation the interactions depend on the confining medium. For metallic confining media the interactions at large separation decay exponentially with a decay length that depends on the ratio of the effective parallel and perpendicular water dielectric components.

Entropy deepens loading chemical potentials of small alcohols by narrow carbon nanotubes

Small alcohol confinement within narrow carbon nanotubes has been extensively and systematically studied via rigorous free-energy calculations.

Separation of water-alcohol mixtures using carbon nanotubes under an electric field

Carbon nanotubes (CNTs) are a promising candidate for separation membranes because of their ability to transport substances at very high flow rates. However, there is a tradeoff between achieving a high selectivity using small pore sizes and the reduction of water flux. Here, using molecular dynamics simulations, we report that CNTs can effectively separate water-methanol mixtures under an electric field. Without an electric field and under piston pressure, both water and methanol flow through a CNT, resulting in no separation effect. In contrast, under an electric field and high piston pressure, CNTs allow selective water permeation while rejecting the permeation of methanol molecules. This separation effect is caused by the ordered structures of water molecules in the CNT. A high filtering effect is observed under the conditions of high methanol concentration in the solution or even with large-diameter CNTs up to 3.39 nm. As long as the ordered structure of water in the CNTs can be maintained, the strong filtering effect can be maintained.

Adsorption of water, methanol, and their mixtures in slit graphite pores

The behavior of water, methanol, and water-methanol mixtures confined in narrow slit graphite pores as a function of pore size was investigated by Monte Carlo, hybrid Monte Carlo, and Molecular Dynamics simulations. Interactions were described using TIP4P/2005 for water, OPLS/2016 for methanol, and cross interactions fitted to excess water/methanol properties over the whole range of concentrations, which provide a rather accurate description of water-methanol mixtures. As expected for hydrophobic pores, whereas pure methanol is adsorbed already from the gas phase, pure water only enters the pore at pressures well beyond bulk saturation for all pore sizes considered. When adsorbed from a mixture, however, water adsorbs at much lower pressures due to the formation of hydrogen bonds with previously adsorbed methanol molecules. For all studied compositions and pore sizes, methanol adsorbs preferentially over water at liquid-vapor equilibrium conditions. In pure components, both water and methanol are microscopically structured in layers, the number of layers increasing with pore size. This is also the case in adsorbed mixtures, in which methanol has a higher affinity for the walls. This becomes more evident as the pore widens. Diffusion of pure water is higher than that of pure methanol for all pore sizes due to the larger size of the methyl group. In mixtures, both components present similar diffusivities at all pore sizes, which is explained in terms of the coupling of molecular movements due to strong hydrogen bonding between methanol and water molecules. This is particularly evident in very narrow pores, in which pure methanol diffusion is completely impeded on the time scale of our simulations, but the presence of a small amount of water molecules facilitates alcohol diffusion following a single-file mechanism. Additionally, our results indicate that pure water diffusivities display a non-monotonous dependence of pore size, due to effects of confinement (proximity to a fluid-solid-fluid transition induced by confinement as reported in previous work) and the dynamic anomalies of water.

Osmosis, from molecular insights to large-scale applications

Osmosis is a universal phenomenon occurring in a broad variety of processes and fields. It is the archetype of entropic forces, both trivial in its fundamental expression - the van 't Hoff perfect gas law - and highly subtle in its physical roots. While osmosis is intimately linked with transport across membranes, it also manifests itself as an interfacial transport phenomenon: the so-called diffusio-osmosis and -phoresis, whose consequences are presently actively explored for example for the manipulation of colloidal suspensions or the development of active colloidal swimmers. Here we give a global and unifying view of the phenomenon of osmosis and its consequences with a multi-disciplinary perspective. Pushing the fundamental understanding of osmosis allows one to propose new perspectives for different fields and we highlight a number of examples along these lines, for example introducing the concepts of osmotic diodes, active separation and far from equilibrium osmosis, raising in turn fundamental questions in the thermodynamics of separation. The applications of osmosis are also obviously considerable and span very diverse fields. Here we discuss a selection of phenomena and applications where osmosis shows great promises: osmotic phenomena in membrane science (with recent developments in separation, desalination, reverse osmosis for water purification thanks in particular to the emergence of new nanomaterials); applications in biology and health (in particular discussing the kidney filtration process); osmosis and energy harvesting (in particular, osmotic power and blue energy as well as capacitive mixing); applications in detergency and cleaning, as well as for oil recovery in porous media.

Molecular Dynamics Simulation Study of Polyamide Membrane Structures and RO/FO Water Permeation Properties

Polyamide (PA) membranes possess properties that allow for selective water permeation and salt rejection, and these are widely used for reverse osmotic (RO) desalination of sea water to produce drinking water. In order to design high-performance RO membranes with high levels of water permeability and salt rejection, an understanding of microscopic PA membrane structures is indispensable, and this includes water transport and ion rejection mechanisms on a molecular scale. In this study, two types of virtual PA membranes with different structures and densities were constructed on a computer, and water molecular transport properties through PA membranes were examined on a molecular level via direct reverse/forward osmosis (RO/FO) filtration molecular dynamics (MD) simulations. A quasi-non-equilibrium MD simulation technique that uses applied (RO mode) or osmotic (FO mode) pressure differences of several MPa was conducted to estimate water permeability through PA membranes. A simple NVT (Number, Volume, and Temperature constant ensemble)-RO MD simulation method was presented and verified. The simulations of RO and FO water permeability for a dense PA membrane model without a support layer agreed with the experimental value in the RO mode. This PA membrane completely rejected Na⁺ and Cl⁻ ions during a simulation time of several nano-seconds. The naturally dense PA structure showed excellent ion rejection. The effect that the void size of PA structure exerted on water permeability was also examined.

Breakdown of Linear Dielectric Theory for the Interaction between Hydrated Ions and Graphene

Many vital processes taking place in electrolytes, such as nanoparticle self-assembly, water purification, and the operation of aqueous supercapacitors, rely on the precise many-body interactions between surfaces and ions in water. Here we study the interaction between a hydrated ion and a charge-neutral graphene layer using atomistic molecular dynamics simulations. For small separations, the ion-graphene repulsion is of nonelectrostatic nature, and for intermediate separations, van der Waals attraction becomes important. Contrary to prevailing theory, we show that nonlinear and tensorial dielectric effects become non-negligible close to surfaces, even for monovalent ions. This breakdown of standard isotropic linear dielectric theory has important consequences for the understanding and modeling of charged objects at surfaces.

Carboxylated carbon nanotubes corked with tetraalkylammonium cations: A concept of nanocarriers in aqueous solutions

An explicit water molecular dynamics simulations were used to probe (6,6) and (9,9) single-walled carbon nanotubes, functionalized with three carboxylate ion groups at each of the two openings, as potential nanocarriers in aqueous solutions. Three tetraalkylammonium cations (i.e., tetraethyl-, tetrapropyl-, and tetrabuthylammonium) were tested as corks to cap the nanotube openings. The variation of the sizes of the nanotubes (diameter) and of the cork cations (bulkiness) allowed us to select the proper corks that fit the nanotube openings best. Smaller tetraalkylammonium ions could easily fit the openings, but since they are less hydrophobic compared to their larger analogues they showed less affinity for the interior of the nanotubes. On the other hand, the hydrophobicity (and thus the affinity for the nanotubes) can be adjusted through the increase of tetraalkylammonium cation size, providing that the cork still fits the opening. Additionally, an external electric field was tested as a means of nanotube uncorking. The field is capable of disjoining corked ions from the functionalized nanotube openings, triggering in this way a potential cargo release stored inside the nanotubes.

Unsaturated Mn(II)-Centered [Mn(BDC)] n Metal-Organic Framework with Strong Water Binding Ability and Its Potential for Dehydration of an Ethanol/Water Mixture

An unsaturated Mn(II)-centered metal-organic framework was synthesized. The presence of an unsaturated Mn(II) center, together with a guest-responsive structural changing feature, plays a crucial role for strong binding with water, leading to its potential application for water/ethanol separation. In addition, the present framework is thermally stable up to 400 °C, which is beneficial for the regeneration process after adsorption.

Dripplons as localized and superfast ripples of water confined between graphene sheets

Carbon materials have unveiled outstanding properties as membranes for water transport, both in 1D carbon nanotube and between 2D graphene layers. In the ultimate confinement, water properties however strongly deviate from the continuum, showing exotic properties with numerous counterparts in fields ranging from nanotribology to biology. Here, by means of molecular dynamics, we show a self-organized inhomogeneous structure of water confined between graphene sheets, whereby the very strong localization of water defeats the energy cost for bending the graphene sheets. This leads to a two-dimensional water droplet accompanied by localized graphene ripples, which we call "dripplon." Additional osmotic effects originating in dissolved impurities are shown to further stabilize the dripplon. Our analysis also reveals a counterintuitive superfast dynamics of the dripplons, comparable to that of individual water molecules. They move like a (nano-) ruck in a rug, with water molecules and carbon atoms exchanging rapidly across the dripplon interface.

Evidence for confinement induced phase separation in ethanol-water mixture: a positron annihilation study

We report an experimental evidence for the phase separation of ethanol-water mixture confined in mesoporous silica with different pore size using positron annihilation lifetime spectroscopy (PALS). A bulk-like liquid in the core of the pore and a distinct interfacial region near the pore surface have been identified based on ortho-positronium lifetime components. The lifetime corresponding to the core liquid shows similar behavior to the bulk liquid mixture while the interfacial lifetime shows an abrupt rise within a particular range of ethanol concentration depending on the pore size. This abrupt increase is attributed to the appearance of excess free-volume near the interfacial region. The excess free-volume is originated due to microphase separation of confined ethanol-water primarily at the vicinity of the pore wall. We envisage that probing free-volume changes at the interface using PALS is a sensitive way to investigate microphase separation under nanoconfinement.

Solvent Transport Behavior of Shear Aligned Graphene Oxide Membranes and Implications in Organic Solvent Nanofiltration

Solvent transport in membranes composed of stacked sheets of graphene oxide (GO) with molecular scale channels and a complex arrangement of hydrophobic and hydrophilic domains is not well understood. Here, we observe that the interlayer space between GO sheets expands in different solvents without disturbing the membrane integrity and is typically larger in aqueous media compared to nonaqueous media. However, the membranes have a tighter molecule sieving feature in aqueous media as demonstrated by lower permeance and higher solute rejection arising from interfacial water layers "sticking" to charged polar groups. As a result of this polar interaction, the permeance of polar solvents in GO membrane scales inversely to the polarity of the solvent, which is contrary to other polymeric and ceramic hydrophilic membranes and also scales inversely to the viscosity of solvents as per continuum expectations. We highlight the extended solvent-handling space of GO membranes, such as in polar protic, polar aprotic, and nonpolar solvents, demonstrating versatility over a commercial nanofiltration membrane, and we predict exciting new applications in advanced separation engineering.

Electric field mediated separation of water–ethanol mixtures in carbon-nanotubes integrated in nanoporous graphene membranes

The tunable separation of water–ethanol mixtures inside CNTs by varying the electric field orientation angle θ.

Interplay Between Adsorption and Hydrodynamics in Nanochannels: Towards Tunable Membranes

We study how the adsorption of a near-critical binary mixture in a nanopore is modified by flow inside the pore. We identify three types of steady states upon variation of the pore Péclet number (Pe_{p}), which can be reversibly accessed by the application of an external pressure. Interestingly, for small Pe_{p} the pore acts as a weakly selective membrane which separates the mixture. For intermediate Pe_{p}, the flow effectively shifts the adsorption in the pore, thereby opening possibilities for enhanced and tunable solute transport through the pore. For large Pe_{p}, the adsorption is progressively reduced inside the pore, accompanied by a long-ranged dispersion of the mixture far from the pore.

Interaction between a water drop and holey graphene: retarded imbibition and generation of novel water–graphene wetting states

Water nanodrop imbibition in holey graphene is studied unraveling novel fiber-like wetting state that enhances water–accessible graphene surface area.