Pair correlations that link the hydrophobic and Hofmeister effects

Confinement effects on the solvation structure of solvated alkaline metal cations in a single-digit 1T-MoS2 nanochannel: A first-principles study

Confinement plays an important role in determining ion transport in porous materials, which, in turn, may influence the performance of many energy storage and desalination devices. In this work, we combined density functional theory (DFT) with an implicit solvation model and ab initio molecular dynamics (AIMD) to investigate the effects of nanoconfinement on several solvated alkaline metal cations in a single-digit 1T-MoS₂ nanochannel. Our DFT calculations with a solvation model indicated that cations with stronger hydration energy introduce a higher number of co-intercalated water molecules into the channel, consistent with early experimental observation obtained for MXene (2D transition metal carbide) channels. The predicted optimal water numbers for the cations were then used for AIMD simulations that explicitly include the effects of the solvent. When compared with the cations in bulk solution, our simulations showed that the hydration structure and coordination number (CN) of the solvated cations confined in the MoS₂ channel can be significantly altered. We found that larger cations with weaker hydration energy (K⁺, Rb⁺, and Cs⁺) exhibited a distinctive CN decrease under confinement, while smaller cations (Li⁺ and Na⁺) retained a similar hydration shell as in the bulk solution. More specifically, the hydration shell of large cations (K⁺, Rb⁺, and Cs⁺) in MoS₂ showed similar features of the coordination angle to the bulk, which suggests the partially broken hydration shell with no geometry change under confinement. Our simulations provided insights into the change of the hydration structure of alkaline metal cations under confinement, which may have important implications on their transport in the 1T-MoS₂ channel.

The Attraction of Water for Itself at Hydrophobic Quartz Interfaces

Structural forces within aqueous water at a solid interface can significantly change surface reactivity and the affinity of solutes toward it. We show using molecular dynamics simulations how hydrophilic and hydrophobic quartz surfaces perturb the orientational structure of aqueous water, ultimately strengthening dipolar forces between molecules in proximity to the interface. When derived as a function of distance from each surface, it was found that both surfaces indirectly enhance the long-range dipolar attraction of water for itself toward the interfacial region. This was found to be longer-ranged for water molecules solvating the hydrophobic surface than those solvating the hydrophilic surface, with a range of up to 2.5 nm from the hydrophobic surface. Our results give direct quantification of surface-induced changes in solvent-solvent attraction, ultimately providing a counterintuitive addition to the balance of hydrophobic forces at aqueous-solid interfaces.

Langmuir films of dipalmitoyl phosphatidylethanolamine grafted poly(ethylene glycol). In-situ evidence of surface aggregation at the air-water interface

The molecular packing-dependent interfacial organization of polyethylene glycol grafted dipalmitoylphosphatidylethanolamine (PE-PEGs) Langmuir films was studied. The PEG chains covered a wide molecular mass range (350, 1000 and 5000Da). In surface pressure-area (π-A), isotherms PE-PEG¹⁰⁰⁰ and PE-PEG⁵⁰⁰⁰ showed transitions (midpoints at π_m,t1∼11mN/m, "t1"), which appeared as a long non-horizontal line region. Thus, t1 cannot be considered a first-order phase transition but may reflect a transition within the polymer, comprising its desorption from the air-water interface and compaction upon compression. This is supported by the increase in the ν_s(C-O-C) PM-IRRAS signal intensity and the increasing surface potentials at maximal compression, which reflect thicker polymeric layers. Furthermore, changes in hydrocarbon chain (HC) packing and tilt with respect to the surface led to reorientation in the PO₂^- group upon compression, indicated by the inversion of the ν_asym(PO₂^-) PM-IRRAS signal around t1. The absence of a t1 in PE-PEG³⁵⁰ supports the requisite of a critical polymer chain length for this transition to occur. In-situ epifluorescence microscopy revealed 2D-domain-like structures in PE-PEG¹⁰⁰⁰ and PE-PEG⁵⁰⁰⁰ around t1, possibly associated with gelation/dehydration of the polymeric layer and appearing at decreasing π as the polymeric tail became longer. Another transition, t2, appearing in PE-PEG³⁵⁰ and PE-PEG¹⁰⁰⁰ at π_m,t2=29.4 and 34.8mN/m, respectively, was associated with HC condensation and was impaired in PE-PEG⁵⁰⁰⁰ due to steric hindrance imposed by the large size of its polymer moiety. Two critical lengths of polymer chains were found, one of which allowed the onset of polymeric-tail gelation and the other limited HC compaction.