Nanoporous ices: an emerging class in the water/ice family

Assisted Self-Assembly of Nanoporous Ices via Carbon Nanomaterial Templates

Self-assembly is a widely used synthetic method in nanoscience to assemble well-organized structures. Self-assembly processes usually occur in a water solvent environment. However, the self-assembly of water molecules is rarely studied. Herein, we show a strategy to fabricate porous ice via carbon nanomaterial-assisted self-assembly. Diverse frameworks of nanoporous ice are formed by using orthorhombic and tetragonal arrays of carbon nanotubes or carbon-atom chains as templates. In contrast to many bulk ices discovered in nature, nanoporous ices are shown to be stable only under negative pressure. Hence, nanoporous ices cannot be produced through the direct nucleation of water at negative pressure. The template-assisted self-assembly method is shown to be the most effective method to fabricate nanoporous ice in quantity. Several key factors for the self-assembly of nanoporous ices are identified, including proper gap spacings in the carbon nanomaterial template and suitable interactions between water and the carbon nanomaterials.

Evidence of Spontaneous Formation of Two-Dimensional Amorphous Clathrates on Superhydrophilic Surfaces

Clathrate hydrates reserved in the seabed are often dispersed in the pores of coarse-grained sediments; hence, their formation typically occurs under nanoconfinement. Herein, we show the first molecular dynamics (MD) simulation evidence of the spontaneous formation of two-dimensional (2D) clathrate hydrates on crystal surfaces without conventional nanoconfinement. The kinetic process of 2D clathrate formation is illustrated via simulated single-molecule deposition. 2D amorphous patterns are observed on various superhydrophilic face-centered cubic surfaces. Notably, the formation of 2D amorphous clathrate can occur over a wide range of temperatures, even at room temperature. The strong water-surface interaction, the characteristic properties of guest-gas molecules, and the underlying surface structure dictate the formation of 2D amorphous clathrates. Semiquantitative phase diagrams of 2D clathrates are constructed where representative patterns of 2D clathrates for characteristic gas molecules on prototypical Pd(111) and Pt(111) surfaces are confirmed by independent MD simulations. A tunable pattern of 2D amorphous clathrates is demonstrated by changing the lattice strain of the underlying substrate. Moreover, ab initio MD simulations confirm the stability of 2D amorphous clathrate. The underlining physical mechanism for 2D clathrate formation on superhydrophilic surfaces is elucidated, which offers deeper insight into the crucial role of water-surface interaction.

Computational investigations of stable multiple-cage-occupancy He clathrate-like hydrostructures

One of the several possibilities offered by the interesting clathrate hydrates is the opportunity to encapsulate several atoms or molecules, in such a way that more efficient storage materials could be explored or new molecules that otherwise do not exist could be created. These types of applications are receiving growing attention from technologists and chemists, given the future positive implications that they entail. In this context, we investigated the multiple cage occupancy of helium clathrate hydrates, to establish stable novel hydrate structures or ones similar to those predicted previously by experimental and theoretical studies. To this purpose, we analyzed the feasibility of including an increased number of He atoms inside the small (D) and large (H) cages of the sII structure through first-principles properly assessed density functional approaches. On the one hand, we have computed energetic and structural properties, in which we examined the guest-host and guest-guest interactions in both individual and two-adjacent clathrate-like sII cages by means of binding and evaporation energies. On the other hand, we have carried out a thermodynamical analysis on the stability of such He-containing hydrostructures in terms of changes in enthalpy, ΔH, Gibbs free energy, ΔG, and entropy, ΔS, during their formation process at various temperature and pressure values. In this way, we have been able to make a comparison with experiments, reaffirming the ability of computational DFT approaches to describe such weak guest-host interactions. In principle, the most stable structure involves the encapsulation of one and four He atoms inside the D and H sII cages, respectively; however, more He atoms could be entrapped under lower temperature and/or higher pressure thermodynamic conditions. We foresee such accurate computational quantum chemistry approaches contributing to the current emerging machine-learning model development.