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In most of natural montmorillonites, Na + and Ca 2+ ions commonly coexist in the interlayer space as compensation ions. Molecular dynamics simulations have been performed to investigate the swelling properties, hydration behaviors, and mobility of the interlayer species of (Na x , Ca y )-montmorillonites with different water contents. Nine montmorillonites with different Na + /Ca 2+ ratio were selected as model clay frameworks, and the content of interlayer water was set within a range from 0 to 486 mg water /g clay . The results show that the montmorillonites with coexisting of Na + and Ca 2+ present slightly different swelling curves, hydration energies, and immersion energies from Na-or Camontmorillonite. The double-layered hydrates are the thermodynamically stable states for all montmorillonites in the regime of crystalline swelling. A total of 170 mg water /g clay is found as the threshold water content for the complexing modes of interlayer Ca 2+ and Na + ions switching from inner-sphere complexes to outer-sphere ones. The self-confusion coefficient of interlayer species obviously reveals the confining effects of clay surfaces. In all montmorillonites, the mobility of Na + is always much greater than that of Ca 2+ due to their different hydration shells. According to the water residence time in typical Na + and Ca 2+ hydration complexes, Ca 2+ hydration complexes is pronounced more stable than those of Na + , and in montmorillonites with high Ca 2+ /Na + ratio, the inhibitory effects of Ca 2+ hydration complexes on the mobility of Na + is clearly revealed.

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A non-conjugated polymeric poly(acrylic acid) (PAA) is shown to efficiently form a stable composite with multi-walled carbon nanotubes (MWNTs). The prepared PAA-wrapped MWNTs composite is readily soluble and stable in water. The PAA–MWNTs composite exhibits interesting optical properties. FT-IR spectra show that the characteristic peaks for MWNTs are unchanged, and new chemical bonds are not formed in PAA–MWNTs, indicating that the electronic structures of the MWNTs are still intact after polymer wrapping. The obvious blue-shift of the peak at 266 nm for the C = C double bonds of pristine MWNTs upon polymer-wrapping, the systematic upshift in peak position and the enhancement in the band intensities of characteristic Raman bands of MWNTs after winding with PAA as well as the disappearance of 1H NMR spectra for the wrapped polymer in PAA-MWNTs composite were observed, which indicate that there is strong binding of PAA to MWNTs surface via the hydropholic attraction between PAA and MWNTs. The as-prepared PAA-MWNTs dispersions could facilitate the processing of the nanotubes into composites with high nanotube loading for bioelectronic devices and biological applications where a water-based environment is needed.

Understanding the wettability of clay mineral surfaces is crucial for enhancing oil recovery, investigating primary migration of hydrocarbon, and evaluating the performance of sealing rocks in a petroleum system. On the basis of molecular dynamics simulations, we investigated the interactions between four typical clay minerals (i.e., pyrophyllite, montmorillonite, illite, and kaolinite) and confined pore fluids (i.e., water/alkane/salts). The influences of surface group, layer charge, and salts on the wettability of clay surfaces were revealed. As the layer charge increases, the hydrophilicity of the montmorillonite basal surface gradually increases. The basal surface of 2:1-type pyrophyllite is completely alkane-wet independent of salts. However, for 1:1-type kaolinite, the presence of salts makes the siloxane surface completely water-wet, whereas it is partially alkane-wet at the absence of salts. In general, the salt ions adsorbed onto clay surfaces promote the surface hydrophilicity. By using nonequilibrium molecular dynamics, we explored the hydrodynamics of the water/alkane/salts fluid confined in slit nanopores with pore walls made up of montmorillonite and kaolinite. Both montmorillonite and kaolinite surfaces remarkably restrain the movement of the water confined in nanopores. Decane molecules tend to aggregate together and transport as a cluster. Moreover, the migration of the decane cluster is faster than that of water molecules. These findings are helpful for understanding the primary migration of hydrocarbon in clayey source rocks and the geological sealing of oil by clayey cap rocks in petroleum systems.