Ion Coupling, Bonding, and Transfer in Narrow Carbon Nanotubes.

Narrow carbon nanotubes (nCNT) are unique mimics of biological channels with water-ion selectivity attractive for applications such as water purification and osmotic energy harvesting, yet their understanding is still incomplete. Here, an ab initio computation is employed to develop the full picture of ion transfer in nCNT including specificity and coupling between ions. The thermodynamic costs of ion transfer are computed for single ions and ion pairs and used to evaluate different local coupling scenarios including strong (pairing) and weak (free-ion) coupling as well as "electroneutrality breakdown" (EB), possible for cations only due to their chemisorption-like interaction with nCNT. The results also indicate that nCNT behaves as a highly polarizable metal-like shell, which eliminates the dielectric energy when CNT accommodates coupled cation and anion. This allows facile computation and comparison of the full transfer costs, including translation entropy, for different ions in different coupling modes to identify the dominant regime. EB transfer appears most favorable for K+, while anions strongly favor transfer as pairs, except for chloride which favors weak coupling and, at neutral pH, transfers as a trace ion coupled to both cation and OH-. The results demonstrate that, in general, observed ion permeation and conduction in nCNT, especially for anions, reflect a complex ion-specific and composition-dependent interplay between different ions.

Pseudo-bottle-brush decorated thin-film composite desalination membranes with ultrahigh mineral scale resistance.

High water recovery is crucial to inland desalination but is impeded by mineral scaling of the membrane. This work presents a two-step modification approach for grafting high-density zwitterionic pseudo-bottle-brushes to polyamide reverse osmosis membranes to prevent scaling during high-recovery desalination of brackish water. Increasing brush density, induced by increasing reaction time, correlated with reduced scaling. High-density grafting eliminated gypsum scaling and almost completely prevented silica scaling during desalination of synthetic brackish water at a recovery ratio of 80%. Moreover, scaling was effectively mitigated during long-term desalination of real brackish water at a recovery ratio of 90% without pretreatment or antiscalants. Molecular dynamics simulations reveal the critical dependence of the membrane's silica antiscaling ability on the degree to which the coating screens the membrane surface from readily forming silica aggregates. This finding highlights the importance of maximizing grafting density for optimal performance and advanced antiscaling properties to allow high-recovery desalination of complex salt solutions.

Why Does Ethaline Apparently Behave as an Ideal Binary Mixture?

Phase diagram mixtures of the salt choline chloride (ChCl) with ethylene glycol (EG) surprisingly seem to behave as ideal binary nonelectrolyte mixtures [Agieienko, V.; Buchner, R. Phys. Chem. Chem. Phys.2022, 24, 5265]. To shed some light on this conundrum, results of broad-band dielectric relaxation spectroscopy (DRS) and quantum-chemical calculations are reported for solutions of ChCl, choline iodide (ChI), and chlorocholine chloride (ClChCl), in EG up to saturation at 298.15 K. The data revealed that all three solutes are only weakly solvated in the sense that on average per equivalent of solute only one EG OH-group is dynamically affected. While contact ion pairs are significant for solute concentrations of ≲1 M, free cation concentrations are rather low. Instead, over the entire concentration range a large fraction of the dipolar cations could not be detected by DRS. We argue that the latter are embedded in large solute aggregates, explaining thus the phase diagram of ChCl + EG and the very low ionicity of all systems.

Putting together the puzzle of ion transfer in single-digit carbon nanotubes: mean-field meets ab initio.

Nature employs channel proteins to selectively pass water across cell membranes, which inspires the search for bio-mimetic analogues. Carbon nanotube porins (CNTPs) are intriguing mimics of water channels, yet ion transport in CNTPs still poses questions. As an alternative to continuum models, here we present a molecular mean-field model that transparently describes ion coupling, yet unlike continuum models, computes ab initio all required thermodynamic quantities for the KCl salt and H+ and OH- ions present in water. Starting from water transfer, the model considers the transfer of free ions, along with ion-pair formation as a proxy of non-mean-field ion-ion interactions. High affinity to hydroxide, suggested by experiments, making it a dominant charge carrier in CNTPs, is revealed as an exceptionally favorable transfer of KOH pairs. Nevertheless, free ions, coexisting with less mobile ion-pairs, apparently control ion transport. The model well explains the observed effects of salt concentration and pH on conductivity, transport numbers, anion permeation and its activation energies, and current rectification. The proposed approach is extendable to other sub-nanochannels and helps design novel osmotic materials and devices.

Water and Ion Transfer to Narrow Carbon Nanotubes: Roles of Exterior and Interior.

Narrow carbon nanotubes (CNTs) desalinate water, mimicking water channels of biological membranes, yet the physics behind selectivity, especially the effect of the membrane embedding CNTs on water and ion transfer, is still unclear. Here, we report ab initio analysis of the energies involved in transfer of water and K+ and Cl- ions from solution to empty and water-filled 0.68 nm CNTs for different dielectric constants (ϵ) of the surrounding matrix. The transfer energies computed for 1 ≤ ϵ < ∞ permit a transparent breakdown of the transfer energy to three main contributions: binding to CNT, intra-CNT hydration, and dielectric polarization of the matrix. The latter scales inversely with ϵ and is of the order 102/ϵ kJ/mol for both ions, which may change ion transfer from favorable to unfavorable, depending on ion, ϵ, and CNT diameter. This may have broad implications for designing and tuning selectivity of nanochannel-based devices.

Magneto-Optical Properties of the Magnetite-Graphene Oxide Composites in Organic Solvents.

Graphene oxide (GO) aqueous solutions are known to form liquid crystals that can switch in electric fields. Magnetic fields as external stimuli are inefficient toward GO because of its diamagnetic properties, and GO is known to be insoluble in most of the organic solvents. In this study, composites of GO with oleate-protected magnetite nanoparticles were prepared as stable colloid solutions in the mixed isopropanol-chloroform solvents. The structure of the composite particles and the optical properties of their solutions can be controlled by the ratio of the mixing parent components. The as-prepared solutions are highly responsive to external magnetic field. As the consequence, the optical transmission and the direction of light scattering can be efficiently manipulated. These systems pave the way for fabricating functional materials, such as magneto-optical switches, density-gradient materials, and micromotors. Solubility in nonpolar organic solvents broadens the scope of their potential applications.

On the Solvation Behavior of Graphene Oxide in Ethylene Glycol/Water Mixtures.

The self-association and solvation pattern of graphene oxide (GO) in water, ethylene glycol (EG), and their mixtures were analyzed by means of UV/Vis spectrophotometry. A careful analysis of the absorbance dependencies vs. the GO concentration shows that self-association of the GO sheets in EG occurs at higher concentration compared to that in water. It was established that depending on the mixed solvent composition, two different types of the GO solvates are formed. The results of quantum chemical calculations allow one to suggest that in the water-rich compositions, the GO oxygen-containing groups are in direct contact with water molecules while in the glycol-rich media the EG molecules fully substitute water in the GO's first solvation layer.

New insights into the solubility of graphene oxide in water and alcohols.

One of the main advantages of graphene oxide (GO) over its non-oxidized counterpart is its ability to form stable solutions in water and some organic solvents. At the same time, the nature of GO solutions is not completely understood; the existing data are scarce and controversial. Here, we demonstrate that the solubility of GO, and the stability of the as-formed solutions depend not just on the solute and solvent cohesion parameters, as commonly believed, but mostly on the chemical interactions at the GO/solvent interface. By the DFT and QTAIM calculations, we demonstrate that the solubility of GO is afforded by strong hydrogen bonding established between GO functional groups and solvent molecules. The main functional groups taking part in hydrogen bonding are tertiary alcohols; epoxides play only a minor role. The magnitude of the bond energy values is significantly higher than that for typical hydrogen bonding. The hydrogen bond energy between GO functional groups and solvent molecules decreases in the sequence: water > methanol > ethanol. We support our theoretical results by several experimental observations including solution calorimetry. The enthalpy of GO dissolution in water, methanol and ethanol is -0.1815 ± 0.0010, -0.1550 ± 0.0012 and -0.1040 ± 0.0010 kJ g-1, respectively, in full accordance with the calculated trend. Our findings provide an explanation for the well-known, but poorly understood solvent exchange phenomenon.