Polyethylene glycol (PEG) is a structurally simple and nontoxic water-soluble polymer that is widely used in medical and pharmaceutical applications as molecular linker and spacer. In such applications, PEG's elastic response against conformational deformations is key to its function. According to text-book knowledge, a polymer reacts to the stretching of its end-to-end separation by a decrease in entropy that is due to the reduction of available conformations, which is why polymers are commonly called entropic springs. By a combination of single-molecule force spectroscopy experiments with molecular dynamics simulations in explicit water, we show that entropic hydration effects almost exactly compensate the chain conformational entropy loss at high stretching. Our simulations reveal that this entropic compensation is due to the stretching-induced release of water molecules that in the relaxed state form double hydrogen bonds with PEG. As a consequence, the stretching response of PEG is predominantly of energetic, not of entropic, origin at high forces and caused by hydration effects, while PEG backbone deformations only play a minor role. These findings demonstrate the importance of hydration for the mechanics of macromolecules and constitute a case example that sheds light on the antagonistic interplay of conformational and hydration degrees of freedom.
Room temperature ionic liquids (RTILs) are solvent-free liquids comprised of densely packed cations and anions. Properties of Py13Cl–AlCl3 ILs were studied and compared with EMIC-AlCl3 ILs for use as electrolyte in Al–graphite battery.
Donor-acceptor materials with small HOMO-LUMO gaps are important in molecular electronics, but are often difficult to synthesise. A simple and efficient way to position tetrathiafulvalene (TTF) as the donor and naphthalene diamide (NDI) as the acceptor in close proximity to each other in a divalent crown/ammonium pseudo[2]rotaxane is presented. The divalent design provides high chelate cooperativity and much stronger binding compared with a monovalent analogue. The pseudo[2]rotaxane was then doubly interlocked by stoppering it in a catalyst-free 1,3-dipolar cycloaddition. UV/Vis and cyclic voltammetry experiments with the resulting [2]rotaxane revealed the optoelectronic properties of an intramolecular charge transfer with a small HOMO-LUMO energy gap. Redox-switching experiments showed the rotaxane to be pentastable. DFT calculations provided insights into the electronic structures of the five redox states.
Due to its high specific and volumetric capacity and relatively low operation potential, silicon (Si) has attracted much attention to be utilized as a high-capacity anode material for lithium-ion batteries (LIBs) with increased energy density. However, the application of Si within commercial LIBs is still hindered by its poor cycling stability related to the huge volume changes of Si upon lithiation/delithiation, followed by continuous electrolyte decomposition and active lithium loss at the anode side. In this work, we present the application of pentafluorophenyl isocyanate (PFPI) as an effective electrolyte additive for lithium-ion full cells, containing a pure, magnetron-sputtered Si anode and a LiNiMnCoO (NMC-111) cathode. The performance of the Si/NMC-111 full cells is significantly improved in terms of capacity retention and Coulombic efficiency by the addition of 2 wt % PFPI to the baseline electrolyte and is compared to the well-known additives vinylene carbonate and fluoroethylene carbonate. Furthermore, it is revealed that the additive is able to reduce the active lithium losses by forming an effective solid-electrolyte interphase (SEI) on the Si anode. X-ray photoelectron spectroscopy investigations unveil that PFPI is a main part of the SEI layer, leading to less active lithium immobilized within the interphase. Overall, our results pave the path for a broad range of different isocyanate compounds, which have not been studied for Si-based anodes in lithium-ion full cells so far. These compounds can be easily adjusted by modifying the chemical structure and/or functional groups incorporated within the molecule, to specifically tailor the SEI layer for Si-based anodes in LIBs.
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