Solutions confined within a pore size of approximately 5-1000 Å are referred to as confined solutions. Confined solutions play an important role in various fields such as separation science, catalysis science, mineral leaching, mineral deposition, biofilms, etc., and their structural and dynamic behaviors have received widespread attention. For instance, in the field of salt lake resource separation, understanding the structure and diffusion behavior of confined solutions can guide the precise control of the transport behavior of ions and water molecules by optimizing the pore structure and surface characteristics of the channels, and develop new media with efficient mass transfer and separation performance.
The Solution Structure and Interface Research Group of Qinghai Institute of Salt Lakes, Chinese Academy of Sciences used experimental methods such as X-ray scattering (XRS) and quasi-elastic neutron scattering (QENS) to study the structure and dynamics of capillary condensed water in ordered microporous carbon (OMC) with a pore size of 18.7 Å in the temperature range of 200-300 K. The study shows that the tetrahedral network structure of water is perturbed by the volumetric water structure, and as the temperature decreases, it tends towards a tetrahedral hydrogen bond network structure at sub-zero temperatures under confinement. Below 230 K, a fraction of hexagonal ice Ih formed in the OMC pores. The translational diffusion of water molecules in OMC pores at 300 K was comparable to that of bulk water by analysis, and it slowed down with decreasing temperature. QENS data analysis suggested that the diffusion of water in hydrophobic micropores did not slow down due to spatial confinement, which is in marked contrast to the diffusion coefficients in hydrophilic micropores. Because of the confinement effect, the activation energy of water confined in OMC pores (Ea) was smaller than that of the bulk water, indicating that the confinement diffused with a small energy as the hydrophobicity of the confining wall increased. It was concluded that water molecules confined in a hydrophilic interface or an amphiphilic interface interacted strongly with the hydrophilic groups, while those confined in a hydrophobic interface tended to aggregate in the central part of the pore. The analysis of the elastic incoherent structure factor (EISF) found evidence for the existence of immobile and mobile components of confined water. Water molecules exhibit a hopping diffusion behavior, with a hopping length consistent with the constraint radius of a sphere with radius a. Through QENS analysis of the dynamics of amphiphilic molecules (MeOH) with hydrophobic methyl and hydrophilic hydroxyl groups in organic-inorganic pores (Ph-PMO) and completely hydrophobic pores (OMC), further investigation was conducted from structural and dynamic perspectives on the interaction rules between confined liquid molecules and the confining interface. This work was published in the Journal of Molecular Liquids 2024, 415, 126316. DOI: 10.1016/j.molliq.2024.126316.
Figure 1. Structure and Dynamics of Water Molecules Confined in Ordered Microporous Carbon
In recent years, the Solution Structure and Interface Research Group of Qinghai Institute of Salt Lakes, Chinese Academy of Sciences has achieved a series of advancements with the support of the National Natural Science Foundation, the Young Team in the Basic Research Field of the Chinese Academy of Sciences, and the Basic Conditions Platform of Qinghai Province. Facing the fundamental scientific issues in salt lake resource separation, they have continuously tackled problems starting from the structure of concentrated salt solutions, dynamics, and interfaces. They have systematically defined the hydration behavior of alkali metals and halogen ions based on the orientation of ion-hydrated water molecules and their average retention time (The Journal of Physical Chemistry Letters 2023, 14 (27), 6270-6277. DOI: 10.1021/acs.jpclett.3c01302). Using crown ether as a model compound, the chemical essence of crown ether selectively recognizing alkali metal ions is revealed from the microscopic structural level (The Journal of Physical Chemistry B 2023, 127 (21), 4858-4869. DOI: 10.1021/acs.jpcb.3c01875). To achieve microscale experimental observation of solution interfacial interactions, a multi-data (X-ray scattering + neutron scattering data) driven all-atom modeling method for solid-liquid interface experimental observation was constructed using the nano-confined solutions as the research object (Journal of Molecular Liquids 2023, 388, 122746. DOI: 10.1016/j.molliq.2023.122746). This method effectively solves the difficult problem of inelastic correction of neutron scattering data in hydrogen-containing systems (The Journal of Physical Chemistry B 2024, 128 (30), 7445-7456. DOI: 10.1021/acs.jpcb.4c02826). Taking microdroplets as the research object, a microfocus X-ray scattering measurement method for ultrasonic levitation of single droplets has been developed, which providing a new method for atomic scale observation of liquid-gas interfaces (Journal of Solution Chemistry 2023, 53, 610-625. DOI: 10.1007/s10953-023-01309-9).