This work combined gold colloid probe atomic force microscopy (AFM) with a quartz crystal microbalance (QCM) to accurately quantify the molecular interactions of fluorine-free phosphonium-based ionic liquids (ILs) with gold electrode surfaces. First, the interactions of ILs with the gold electrode per unit area (𝐹′A𝐹A′, N/m2) were obtained via the force–distance curves measured by gold probe AFM. Second, a QCM was employed to detect the IL amount to acquire the equilibrium number of IL molecules adsorbed onto the gold electrode per unit area (NIL, Num/m2). Finally, the quantified molecular interactions of ILs with the gold electrode (F0, nN/Num) were estimated. F0 is closely related to the IL composition, in which the IL with the same anion but a longer phosphonium cation exhibits a stronger molecular interaction. The changes in the quantified interactions of gold with different ILs are consistent with the interactions predicted by the extended Derjaguin–Landau–Verwey–Overbeek theory, and the van der Waals interaction was identified as the major contribution of the overall interaction. The quantified molecular interaction is expected to enable the direct experimental-derived interaction parameters for molecular simulations and provide the virtual design of novel ILs for energy storage applications.

The dehydration of water by dimethyl carbonate (DMC) is of great significance for its application in electrochemistry and oil industry. With the rapid development of nanomaterial, one-dimensional (e.g. carbon nanotube (CNT)) and two-dimensional (e.g. lamellar graphene) materials have been widely used for molecular sieving. In this work, the molecular behavior of dimethyl carbonate/water mixture confined in CNT with varying diameters was studied based on molecular dynamics simulation. Due to different van der Waals interactions for the components in the mixtures with the solid surface, DMC molecules are preferentially adsorbed on the inner surface of the pore wall and formed an adsorption layer. Comparing with the pure water molecules confined in CNT, the adsorption DMC layer shows notable effect on the local compositions and microstructures of water molecules under nanoconfinement, which may result in different water mobility. Our analysis shows that the surface-induced DMC molecules can destroy the hydrogen bonding network of water molecules and result in an uniform and dispersed distribution of water molecules in the tube. These clear molecular understandings can be useful in material design for membrane separation.

Choline chloride/urea (1:2) is the most widely used deep eutectic solvent, which has attracted much attention due to its excellent advantages of low cost, environment friendly and easy synthesis. In this work, nanofriction-based molecular dynamics simulations were performed to investigate the effect of interfacial hydrophilicity on the flow resistance of Choline chloride/urea (1:2) confined in ionic model nanoslits. Simulation results showed that the flow resistance of the choline chloride/urea system increases with the increasing interfacial hydrophilicity. Urea molecules form a preferential adsorption layer on the wall. As the interfacial hydrophilicity increases, the number of urea molecules in the interfacial adsorption layer increased, whereas the stability decreased. Unique confined spatial distributions of urea molecules greatly contribute to ionic association between choline cations and chloride anions. Furthermore, with the increase of interfacial hydrophilicity, orientation distributions of urea molecules in the adsorption layer are more orderly, then causing a decrease in the average hydrogen bond number (N_HB) of urea molecules. Moreover, the more the N_HB of urea molecules, the better is the stability in the interfacial adsorption layer, which in turn results in less flow resistance.

Residual Ca2+ decreases the efficiency and increases the power consumption of the chlor-alkali industry. However, Ca2+ and Na+ sieving is challenging due to the similar ionic radii of these cations. Inspired by the presence of carbonyl oxygens in key selective filters of biological Ca2+ and Na+ channels, we used molecular dynamics to investigate the effects of carbonyl oxygen atoms in modified graphene nanopores of various sizes (characteristic diameters: 0.57–1.50 nm) on Ca2+/Na+ sieving. The results demonstrated that selectivity is closely associated with the different roles of the carbonyl oxygen atoms. In small nanopores, Ca2+ sheds increased numbers of water molecules due to the predominant steric effect of carbonyl oxygen atoms. Thus, Ca2+ must overcome a higher energy barrier than Na+. This requirement prevents the passage of Ca2+. In large nanopores, carbonyl oxygen atoms do preferentially substitute water molecules outside the first hydration shell of Ca2+ compared with those outside the first hydration shell of Na+, thereby hindering Na+ departure from the nanopore. These findings provide useful guidance for the further development of Ca2+ separation materials as sensors and ion separators.

Wetting behavior of droplets made of choline chloride/urea (1:2), an archetypal deep eutectic solvent mixture, is studied using molecular dynamics simulations. The droplets are placed on a smooth model ionic substrate with positive and negative charges of the same magnitude q (0 e ≤ q ≤ 1.0 e), corresponding to a step-by-step change from a hydrophobic to hydrophilic surface. The molecular microstructure of the droplets and their spatial compositions are systematically studied in details on how they both change while gradually moving from hydrophobic to hydrophilic surface. It is observed that urea initially forms a monolayer on the surface with a planar orientation. This layer slowly shrinks while it becomes laterally more and more constrained. It becomes also molecularly more ordered when the surface becomes hydrophilic, at the same time as the contact angles become larger and larger. The anions (Cl-) are continuously pushed further away from the charged surface. While the contact angle increases and wetting decreases, and urea forms even a secondary stable layer where it changes its orientation and turns to have one of its amines facing up and carbonyl down. The average number of urea-urea H-bonds decreases linearly while the number of ion-pair contacts increases when the urea molecules are separating from the mixture. Our analysis gives a clear molecular understanding of the process and can be useful in many applications from membrane separation to catalysis.

With the emergence of membrane separation and heterogeneous catalysis applications that are associated with confined ethanol/water binary mixture in the pores of two-dimensional (2D) nanomaterials, understanding their confined microstructures is the first step for further relevant applications. In this work, molecular dynamics was performed to investigate the microstructure of ethanol/water binary mixture of 5% mole fraction confined within the four typical 2-nm width 2D-nanoslits (i.e. hBN, GO-0.2, GO-0.4 and Ti3C2(OH)2). Results demonstrated that different chemical properties of solid surfaces can induce distinctive microstructures of mixed fluid within the interfacial contact (adsorbed) layer and thus can result in different mobility of water molecules within the subcontact layer. The residence times of water molecules in the subcontact layer were found in the sequence of Ti3C2(OH)2 > hBN > GO-0.4 > GO-0.2, whereas their sequence of diffusion coefficient within the x-z plane was Ti3C2(OH)2 > hBN > GO-0.2 > GO-0.4. Detailed hydrogen bond (HB) microstructure analysis showed that a high average number of HBs (between fluid molecules of the interfacial contact layer and water molecules of the subcontact layer) induced by solid surfaces could facilitate water molecules to reside in the subcontact layer. Moreover, the small average number of HBs between the water molecules themselves in the subcontact layer could lead to high in-plane diffusion coefficients.

In modern chemical engineering, process intensification is realised by introducing a nano- or micro-interface. The introduction of such an interface breaks the homogeneous nature of the fluid and forms a unique nano- or micro-interfacial structure, which has a significant impact on the properties of fluids. However, the existing traditional chemical engineering theories cannot be used to describe this inhomogeneous behaviour and clarify the underlying intrinsic mechanisms, making it difficult to find control factors which enhance chemical processes. It is necessary to establish theoretical models suitable for liquid–solid systems which can describe the fluid behaviour at interfaces. The key is to accurately recognise the structural as well as thermodynamic and dynamic properties of the complex fluid mixtures at these nano- or micro-interfaces. Previous studies that have been conducted to study the behaviour of simple fluids at interfaces found that, because of the strongly asymmetric interactions, the fluid tends to form layered structures at or close to the interface, which further affects the behaviour of the fluid molecules in close vicinity. However, for complex fluids and/or fluid mixtures, the effects of the interface and the interactions of fluid molecules on the formation of the layered structure and the fluid behaviour above the formed layer have not been elucidated.

In this thesis, to conduct a systematic study, several typical liquid mixtures, which are also important in the modern chemical industry, i.e. immiscible dimethyl carbonate (DMC)/water mixtures with relatively weak van der Waals interactions, miscible aqueous alcohol solutions with strong interactions due to hydrogen bonding (H-bonding), and deep eutectic solvents (DESs) with strong electrostatic interactions, were selected as representatives to construct a complex fluid–solid interface. Molecular dynamics simulation was used as the main tool to quantitatively describe the formation and influence of the adsorption layer on the structures and properties of the fluids at the interface at the molecular level. Additionally, in the future, key parameters will be provided for establishing theoretical nano- or micro-interface models. The main results obtained are summarised as follows:

(1) The local composition and microstructure of DMC/water mixtures in carbon nanotubes (CNTs) were studied. It is found that DMC molecules preferentially get adsorbed on the inner surface of CNTs and form a “DMC tube”-like structure, i.e. a “secondary confinement” for the water molecules. Because of the formed DMC adsorption layer, the water molecules in the “DMC tube” are more widely dispersed owing to the disordered orientation structure and the destroyed H-bonding structure.

(2) The molecular behaviour of aqueous solutions containing alcohol (i.e. methanol, ethanol, n-propanol, and n-butanol, respectively), confined in a two-dimensional graphene slit, was studied. A distinctly layered structure is formed at the interface, and the alcohol molecules are preferentially adsorbed on the graphene wall. Among the four studied systems, for the n-propanol system, the water molecules on the interfacial adsorption layer have the longest residence time because of the least distortion to the H-bonding network of the water molecules, restricting their motion. The nanophase separation (e.g. separation from water at the interface) of aqueous methanol solutions with stronger intermolecular interactions is less prominent compared to that of DMC/water and other aqueous alcohol solutions.

(3) The wetting behaviour of the deep eutectic choline chloride (ChCl)/urea (1:2) droplets on the ionic model substrate was studied, in which the substrate continuously and linearly changed its state from hydrophobic to hydrophilic. Due to the strong electrostatic interactions between the anions and cations, the neutral urea molecules are stripped out and adsorbed on the interface, forming a stable “new interface”. The orientation and H-bonding structure between the urea molecules in the adsorption layer lead to a difference in hydrophilicity of the “new interface”, further affecting the wetting behaviour of the upper molecules.

A further comparison of the results for the different systems that were studied shows that for a weaker intermolecular interaction in the bulk phase, a clearer separation at the interface of the nanophase is observed. Therefore, for more complex systems (such as aqueous ionic liquids/DESs), it is essential to study their microscopic mechanism in the bulk phase before investigating their interfacial behaviour. Subsequently, the microstructure of ChCl/urea/water (i.e. one type of aqueous DES) was studied. The investigation shows that the hydration strength of chloride ions is higher than that of choline ions, indicating that the anions have a greater impact on the non-ideal behaviour of the mixture, which was further proven by analysing the interaction energy. In addition, the competition between the ion pairs and ionic hydration was suggested as the underlying mechanism for the non-ideal changes in the thermodynamic properties of complex systems with strong electrostatic interactions.

In modern chemical engineering processes, the involvement of solid/fluid interface is the most important component of process intensification techniques, such as confined membrane separation and catalysis. In the review, we summarized the research progress of the latest theoretical and experimental works to elucidate the contribution of interface to the fluid properties and structures at nano- and micro-scale. We mainly focused on water, alcohol aqueous solution, and ionic liquids, because they are classical systems in interfacial science and/or widely involved in the industrialization process. Surface-induced fluids were observed in all reviewed systems and played a critical role in physicochemical properties and structures of outside fluid. It can even be regarded as a new interface, when the adsorption layer has a strong interaction with the solid surface. Finally, we proposed a perspective on scientific challenges in the modern chemical engineering processes and outlined future prospects.

The confined mass transfer separation membrane is mainly for the high-precision separation process at the molecular/ion level, which is of great significance to solve the application needs of CO2 separation, azeotrope separation, lithium extraction from salt lake, desalination of seawater and so on. However, at present, the research of the confined mass transfer mechanism of this kind of membrane is lagging behind, and the theoretical models of confined mass transfer are lacking, which can no longer meet the needs of the rapid development of materials and chemical engineering. From the perspective of meso-science, the abnormal phenomenon of high flux and high selectivity of the confined mass transfer separation membrane is considered, that is, breaking through the trade-off effect, which is governed by compromise-in-competition between the selectivity mechanism and the flux mechanism. It is found that the fluid molecules will preferentially adsorb at the interface and form a stable adsorption layer. Based on this, the hypothesis of "secondary confinement" is put forward, that is, the surface induced new solid-like interface will have confinement effect on the intermediate fluid again. By comparing the pore size and the secondary confined size of the confined mass transfer separation membrane, the selective mechanism of the secondary confinement is further confirmed, and the quantitative prediction of the membrane flux and selectivity is preliminarily explored by combining the selective mechanism and the flux model, which may provide a theoretical basis for the precise construction of the limited area mass transfer membrane. © 2020, Chemical Industry Press Co., Ltd. All right reserved.