Probing the Interaction Mechanism of Sodium Oleate and Dodecyl Amine with Quartz Surfaces in the Presence of Ca2+ Ions by AFM Force Measurement.

Quartz is a key raw material in high-tech fields (such as photovoltaics and semiconductor microelectronics), and the most efficient extraction method of quartz is mineral flotation. Quartz activation plays a crucial role in mineral flotation. However, the mechanism underlying the process remains unclear, and the role of additional metal ions is controversial. In this study, the interaction forces between the quartz surface, the dodecylamine (DDA) cation/sodium oleate (NaOL) anion mixed collectors, and Ca2+ were analyzed using atomic force microscopy in order to systematically explore the activation process of quartz flotation. The results confirmed that the activation process was initialized from NaOL, which was adsorbed on the surface of a calcium-covered quartz surface. The existence of DDA inhibited the binding of Ca2+ to NaOL so that more Ca2+ was adsorbed on the quartz surface to provide the adsorption site for NaOL. Moreover, the best adsorption condition of Ca2+ + NaOL + DDA mixed solution was analyzed by quartz crystal microbalance with dissipation, and it demonstrated that the most stable chemisorption environment on the quartz surface was at pH 11.0. In these circumstances, Ca2+ could first adsorb in a point-like manner on the quartz surface, which was then adsorbed with a mixture of NaOL and DDA. This result showed that, at a specific pH, Ca2+ could encourage the coadsorption of cationic/anionic mixed collectors on quartz. This work provides an important new understanding of the intermolecular interactions that take place during complex mineral flotation processes between chemical additives and mineral surfaces. The methodology used in this study can be easily adapted to different interfacial processes, including water treatment, membrane technology, bioengineering, and oil production.

2D Material Nanofiltration Membranes: From Fundamental Understandings to Rational Design.

Since the discovery of 2D materials, 2D material nanofiltration (NF) membranes have attracted great attention and are being developed with a tremendously fast pace, due to their energy efficiency and cost effectiveness for water purification. The most attractive aspect for 2D material NF membranes is that, anomalous water and ion permeation phenomena have been constantly observed because of the presence of the severely confined nanocapillaries (<2 nm) in the membrane, leading to its great potential in achieving superior overall performance, e.g., high water flux, high rejection rates of ions, and high resistance to swelling. Hence, fundamental understandings of such water and ion transport behaviors are of great significance for the continuous development of 2D material NF membranes. In this work, the microscopic understandings developed up to date on 2D material NF membranes regarding the abnormal transport phenomena are reviewed, including ultrafast water and ion permeation rates with the magnitude several orders higher than that predicted by conventional diffusion behavior, ion dehydration, ionic Coulomb blockade, ion-ion correlations, etc. The state-of-the-art structural designs for 2D material NF membranes are also reviewed. Discussion and future perspectives are provided highlighting the rational design of 2D material membrane structures in the future.

Surface interaction mechanisms in mineral flotation: Fundamentals, measurements, and perspectives.

As non-renewable natural resources, minerals are essential in a broad range of biological and technological applications. The surface interactions of mineral particles with other objects (e.g., solids, bubbles, reagents) in aqueous suspensions play a critical role in mediating many interfacial phenomena involved in mineral flotation. In this work, we have reviewed the fundamentals of surface forces and quantitative surface property-force relationship of minerals, and the advances in the quantitative measurements of interaction forces of mineral-mineral, bubble-mineral and mineral-reagent using nanomechanical tools such as surface forces apparatus (SFA) and atomic force microscope (AFM). The quantitative correlation between surface properties of minerals at the solid/water interface and their surface interaction mechanisms with other objects in complex aqueous media at the nanoscale has been established. The existing challenges in mineral flotation such as characterization of anisotropic crystal plane or heterogeneous surface, low recovery of fine particle flotation, and in-situ electrochemical characterization of collectorless flotation as well as the future work to resolve the challenges based on the understanding and modulation of surface forces of minerals have also been discussed. This review provides useful insights into the fundamental understanding of the intermolecular and surface interaction mechanisms involved in mineral processing, with implications for precisely modulating related interfacial interactions towards the development of highly efficient industrial processes and chemical additives.

Probing the Interaction Mechanism between Benzohydroxamic Acid and Mineral Surface in the Presence of Pb2+ Ions by AFM Force Measurements and First-Principles Calculations.

Probing the interaction mechanism between organic molecules and material surfaces in the presence of metal ions is of great importance in many fields, such as mineral flotation. The collectability of benzohydroxamic acid (BHA) to a spodumene (LiAl(SiO3)2) mineral surface during mineral flotation could be enhanced with the addition of metal ion activators-Pb2+ ions. Pb2+ ions could be added as either Pb-BHA complex formed by premixing Pb2+ ions and BHA molecules at a given ratio or sequential addition of Pb2+ ions and BHA molecules. However, the complete understanding of the interaction mechanisms (e.g., adhesion) between BHA and the spodumene mineral surface in the presence of Pb2+ ions remains very limited. In this study, atomic force microscopy (AFM) was used to measure the intermolecular forces between BHA and the spodumene mineral surface in aqueous solutions. A BHA model molecule, that is, N-hydroxy-4-mercaptobenzamide (MBHA), was synthesized to prepare a BHA-functionalized AFM probe for force measurements. Two model systems (i.e., a Pb-BHA complex interacting with the spodumene mineral surface (model I) and BHA with a Pb2+-activated spodumene surface (model II)) were investigated for comparing the role of Pb2+ in BHA-mineral adhesion. The adhesion measured for model I (23.7 mN/m) is much higher than that of model II (12.5 mN/m), as further supported by the adsorption energies obtained from density functional theory (DFT) calculations. The calculation results showed a higher adsorption energy for model I (∼188.58 kJ/mol) than model II (∼128.16 kJ/mol), which is due to the better spodumene flotation recovery for the Pb-BHA complex as a collector than the sequential addition of Pb2+ and BHA. This work provides useful information on the intermolecular interactions between chemical additives and mineral surfaces in complex mineral flotation processes, and the methodology can be readily extended to other related interfacial processes such as membrane technology, water treatment, oil production, and bioengineering processes.

Novel sodium alginate-assisted MXene nanosheets for ultrahigh rejection of multiple cations and dyes.

Energy-efficient process for water desalination and organic dye separation is urgently needed to meet the dramatically increasing demand for fresh water globally. Nb2CTx MXene, as one type of 2D transition metal carbides (TMDCs) family, has attracted tremendous research interest in the fields of separation technology over the past decade. However, it has been challenging to fabricate surface-modified Nb2CTx MXene nanosheet films with superior performance for desalination and separation applications. Herein, we report a novel, facile and scalable sodium alginate (SA)-assisted surface termination method to fabricate SA-modified Nb2CTx MXene (NbSA) nanosheets. It is found that films of the NbSA nanosheets demonstrate overall better performance than bare Nb2CTx MXene nanosheet film. The NbSA film with a thickness of 5 µm shows >95% rejection towards various cations under forward osmosis process. The film also shows a fast water flux of 2200 ± 100 L m-2 h-1 bar-1 (LMHB) with almost 100% rejection rate towards multiple dyes under vacuum-driven filtration mode. Moreover, the NbSA film has exhibited selective separation performance on Li+/Mg2+ mixture solution under forward osmosis process. This work reports a novel NbSA film with excellent performances for desalination and separation, with useful implications for developing 2D material films in separation processes and environmental engineering.

Tannic acid modified MoS2 nanosheet membranes with superior water flux and ion/dye rejection.

Energy-efficient membranes are urgently needed for water desalination and separation due to ever-increasing demand for fresh water. However, it is extremely challenging to increase membrane water flux and simultaneously achieve high rejection rates of cations or organic dyes. Herein, we report a tannic acid (TA) assisted exfoliation method to fabricate TA-modified MoS2 (TAMoS2) nanosheets with high production yield (90 ±â€¯5%). The TAMoS2 nanosheets membranes show excellent non-swelling stability in water. It is found that a hybrid membrane with 1 wt% of TAMoS2 in MoS2 nanosheets demonstrates overall better performance than pure MoS2 and TAMoS2 membrane. Such a hybrid membrane with a thickness of 5 µm shows fast water flux at around 32 L m-2 h-1 (LMH) and >97% rejection of various cations under static diffusion mode. Under vacuum-driven filtration condition, the as-prepared hybrid membrane demonstrates ultrafast water flux of 15,000 ±â€¯100 L/(m2 h bar) and 99.87 ±â€¯0.1% rejection of multiple model organic dyes. To the best of our knowledge, the above performances are superior to those of all MoS2-based membranes reported previously in terms of water flux and ion/dye rejection. This work represents a leap forward towards the practical applications of 2D TAMoS2 membranes in various engineering and environmental areas.

Probing molecular interactions of PEGylated chitosan in aqueous solutions using a surface force apparatus.

PEGylation can modify the physicochemical properties of native chitosan and improves its water solubility. PEGylated chitosan has been widely used as a gene/drug delivery vector by forming a polyelectrolyte complex (PEC) in biomedical engineering. The molecular interactions of PEGylated chitosan play a critical role in forming the core-shell structure of the complexes. In this work, we systematically investigated the cohesive interaction between PEGylated chitosan films using a surface forces apparatus (SFA) under different solution conditions, and the corresponding morphology change was characterized using atomic force microscopy (AFM). The force measurements demonstrated that the cohesion could be enhanced by increasing the contact time and the PEGylation degree, but could be weakened by increasing the solution pH, which is closely related to the morphology change of the PEGylated chitosan films. The strong cohesion of PEGylated chitosan, as compared to that of native chitosan, is primarily attributed to improved polymer solubility and flexibility, and enhanced formation of hydrogen bonds between the polymer chains. In addition, continuously increasing the PEGylation degree was found to be less effective in further strengthening the cohesion at relatively high pH (e.g., pH ∼ 8.5), which is most likely due to the repulsion originating from the formation of dense hydration PEG shells. Our results provide useful nanomechanical insights into the fundamental understanding of the interaction mechanism of PEGylated chitosan, with implications for the development of novel and effective gene/drug carriers in bioengineering.

Rapid Dewatering and Consolidation of Concentrated Colloidal Suspensions: Mature Fine Tailings via Self-Healing Composite Hydrogel.

Billions of tonnes of thick waste streams with highly concentrated colloidal suspensions from different origins have accumulated worldwide, exampled as over 220 km2 mature fine tailings (MFT) from oil sands production in north Alberta. Current treatment technologies are limited by slow yet insufficient water release and sludge consolidation. Herein, a self-healing composite hydrogel system is designed to convert concentrated aqueous colloidal suspensions (e.g., MFT with colloidal solid content >30 wt %) into a dynamic double cross-linked network for rapid dewatering and consolidation. The resultant composite hydrogel demonstrates an excellent dewatering performance so that over 50% of water could be rapidly released within 30 min by vacuum filtration. Furthermore, the formed infinite cross-linked network with self-healing ability can effectively trap fine particles of all sizes and capture small flocs during mechanical mixing, thereby enabling a low solid content at the ppm level in the released water. This new strategy outperforms all the previously reported treatment methods; under mechanical compression, over 80% of water is removed from the MFT, thereby generating a stackable material with >70 wt % solids within an hour. These results demonstrate a highly effective approach and provide insight into the development of advanced materials to tackle the challenging environmental slurry issues.

Cost-Effective Strategy for Surface Modification via Complexation of Disassembled Polydopamine with Fe(III) Ions.

Mussel-inspired polydopamine (PDA) deposition provides a prominent approach for constructing functional coatings, which has received much research interest over the past decade. However, large PDA aggregates often formed and precipitated from the solution during the deposition process, significantly lowering the utilization efficiency of dopamine for surface modification. It is of both fundamental and practical importance to "reactivate" and reuse the precipitated aggregates to achieve higher usage efficiency of PDA in surface modifications. In this work, we report a facile, substrate-independent, and cost-effective coating strategy, by disassembling the precipitated PDA aggregates, to achieve the coating deposition through the complexation of disassembled polydopamine (d-PDA) species with Fe(III) ions on various substrates. Adsorption tests determined by a quartz crystal microbalance with dissipation (QCM-D) monitoring technique indicated that the pH of the solution and the ratio of d-PDA to Fe(III) significantly influence the deposition behavior of d-PDA/Fe(III). Force measurements using a surface force apparatus demonstrated that the coordination interaction between d-PDA and Fe(III) was the major force leading to the formation of coatings. The deposited d-PDA/Fe(III) coatings featured controllable nanoscale thickness, uniform surface morphologies, and light color. Furthermore, the d-PDA/Fe(III) coating could act as an intermediate layer in the preparation of hydrophobic polyurethane sponge for highly efficient oil/water separation. This work provides a useful strategy to realize the reusability of PDA aggregates for versatile surface functionalization, with implications for the fundamental understanding of the formation mechanism in the metal-phenolic complexation systems and development of new coating approaches in various engineering applications.

Scalable polyzwitterion-polydopamine coating for regenerable oil/water separation and underwater self-cleaning of stubborn heavy oil fouling without pre-hydration.

A facile and scalable polyzwitterion-polydopamine coating strategy has been developed to functionalize substrates and sponges. This approach, for the first time, achieves superior regenerable underwater self-cleaning of stubborn asphaltenes-containing heavy oil fouling without pre-hydration and removal of water residues from heavy oil, with significant implications in many engineering and environmental processes.

Deposition and Adhesion of Polydopamine on the Surfaces of Varying Wettability.

Mussel-inspired chemistry, particularly the versatile coating capability of polydopamine (PDA), has received much research interest as a promising strategy for fabricating functional coatings in numerous fields. However, the understanding of deposition mechanisms and adhesion behaviors of PDA on different substrates still remains incomplete, significantly limiting the related fundamental research and its practical applications. In this work, a colloidal probe atomic force spectroscopy technique was employed to quantify the interaction forces and adhesion between the PDA coatings and the substrate surfaces with different wettabilities. The surface force measurements and thermodynamic analysis of interaction energy indicate that the surface wettability has a significant influence on the adhesion, deposition behaviors, and morphologies of PDA coatings. Compared with the hydrophilic surfaces, the hydrophobic surfaces exhibit stronger adhesion with the PDA coatings. Furthermore, for the first time, this work demonstrates that ethanol has the capability of effectively displacing the trapped air/vapor layer or the so-called "hydrophobic depletion layer" on the hydrophobic substrate to allow the intimate contact between PDA and the substrate, thus enhancing the adhesion and facilitating the PDA deposition. This work provides new insights into the fundamental PDA deposition mechanism as well as the design and development of versatile mussel-inspired coatings on the substrates of varying hydrophobicity.