Spontaneous Dipole Reorientation in Confined Water and Its Effect on Wetting/Dewetting of Hydrophobic Nanopores

Effects of Size and Porosity on the Hydrophobicity of Hierarchical Nanoparticles

Hierarchical nanoporous particles combine properties of microporous and mesoporous materials that are widely exploited for energy storage and conversion, separation of gases and liquids, water purification and desalination, fabrication of nanodevices, etc. Hierarchical meso/microporous level-2 and level-3 Menger sponge particles immersed in water were investigated using computer simulation methods to demonstrate a synergetic effect of additional porosity on the wettability of materials. The Menger sponge is an object with a fractal dimension. At each level, the particles are composed of the same structural blocks. The hydrophobicity of the blocks was shown to depend on their size and position in the nanoparticles. The additional porosity decreases the hydrophobicity of the particles due to the partial breaking of hydrogen bonds between water molecules in the pores. This effect can be used to tune and modify the hydrophobicity and wettability of bulky porous materials, nanoparticles, and nanostructured surfaces.

Energetic Characteristics of Hydrophobic Porous Materials as Candidates for Manufacturing of Nanorockets

A new propulsion mechanism for nano- and microrocket engines is hypothesized. It is based on the instantaneous expulsion from hydrophobic nanopores triggered by irradiation from electromagnetic microwaves, ultrasound, or sudden pressure release. A large energy output is needed for the propulsion of a nanoparticle, and the value can be determined experimentally and by means of atomistic simulations. As such, we measured the heat of intrusion of water into ITQ-29 (LTA) pure silica zeolite with cage structure of pores. The heat effect is exothermic and equal to -7.3 ± 0.8 J/g of zeolite. Similar values were reported for chabazite, ZIF-8, and grafted mesoporous silica EVA. All these materials have cage structures of pores. In contrast, silicalite-1 (MFI) zeolite with a channel structure of pores exhibits endothermic intrusion. Molecular dynamics simulations of pure silica zeolites with LTA, CHA, and MFI topologies at a broad range of water loadings show that water becomes thermodynamically stable in cage-shaped pores while it is unstable in channel-shaped pores. A large energy release is expected during water expulsion from channel-type pores.

Calorimetric Heats of Intrusion of LiCl Aqueous Solutions in Hydrophobic MFI-Type Zeosil: Influence of the Concentration

For the first time, we report calorimetric measurements of intrusion of aqueous LiCl solutions in a hydrophobic pure siliceous MFI zeolite (silicalite-1) under high pressure. Our results show that the intrusion heats are strongly dependent on the LiCl concentration. The intrusion process is endothermic for diluted solutions (molar H2O/LiCl = 12) as well as for water, but it becomes exothermic for a concentration close to saturation (molar H2O/LiCl = 4). Analysis of the data in the framework of wetting thermodynamics shows that besides surface wetting, other phenomena occur during intrusion, such as hydrogen-bond weakening and composition change. In all cases, water is preferentially intruded so that the intruded phase becomes more diluted than the bulk solution. In the case of the most diluted solution, only water molecules seemed to be intruded. Furthermore, silicalite-1 is shown to be very stable in the presence of LiCl solution, with no noticeable structural and textural modifications observed after intrusion.