Concentration dependent effects of urea binding to poly(N-isopropylacrylamide) brushes: a combined experimental and numerical study

The binding effects of osmolytes on the conformational behavior of grafted polymers are studied in this work. In particular, we focus on the interactions between urea and poly(N-isopropylacrylamide) (PNIPAM) brushes by monitoring the ellipsometric brush thickness for varying urea concentrations over a broad temperature range. The interpretation of the obtained data is supported by atomistic molecular dynamics simulations, which provide detailed insights into the experimentally observed concentration-dependent effects on PNIPAM-urea interaction. In particular, in the low concentration regime (c(u) = 2 mol L-1, the lower T-tr is explained by the favorable replacement of water molecules by urea, which can be regarded as a cross-linker between adjacent PNIPAM chains. Significant effects of the concentration-dependent urea binding on the brush conformation are noticed: at c(u) <= 0.5 mol L-1, although urea is loosely embedded between the hydrated polymer chains, it enhances the brush swelling by excluded volume effects. Beyond 0.5 mol L-1, the stronger interaction between PNIPAM and urea reduces the chain hydration, which in combination with cross-linking of monomer units induces the shrinkage of the polymer brush.DFG, EXC 310, SimulationstechnikDFG, SFB 716, Dynamische Simulation von Systemen mit großen TeilchenzahlenDFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface

A polarizable MARTINI model for monovalent ions in aqueous solution

We present a new polarizable coarse-grained martini force field for monovalent ions, called refIon, which is developed mainly for the accurate reproduction of electrostatic properties in aqueous electrolyte solutions. The ion model relies on full long-range Coulomb interactions and introduces satellite charges around the central interaction site in order to model molecular polarization effects. All force field parameters are matched to reproduce the mass density and the static dielectric permittivity of aqueous NaCl solutions, such that experimental values are well-reproduced up to moderate salt concentrations of 2 mol/l. In addition, an improved agreement with experimentally measured ionic conductivities is observed. Our model is validated with regard to analytic solutions for the ion distribution around highly charged rod-like polyelectrolytes in combination with atomistic simulations and experimental results concerning structural properties of lipid bilayers in the presence of distinct salt concentrations. Further results regarding the coordination numbers of counterions around dilute poly(styrene sulfonate) and poly(diallyldimethylammonium) polyelectrolyte chains also highlight the applicability of our approach. The introduction of our force field allows us to eliminate heuristic scaling factors, as reported for previous martini ion models in terms of effective salt concentrations, and in consequence provides a better agreement between simulation and experimental results. The presented approach is specifically useful for recent martini attempts that focus on highly charged systems—such as models of DNA, polyelectrolytes or polyelectrolyte complexes—where precise studies of electrostatic effects and charge transport processes are essential

Stabilizing effect of TMAO on globular PNIPAM states: preferential attraction induces preferential hydration

We study the effect of the organic co-solute trimethylamine N-oxide (TMAO) on the volume phase transition of microgel particles made from poly(N-isopropylacrylamide) (PNIPAM) using dynamic light scattering (DLS) and all-atom molecular dynamics (MD) simulations. The DLS measurements reveal a continuous TMAO-induced shrinking process from a coil to a globular state of PNIPAM microgel particles. Analyzing the DLS data by the phenomenological Flory–Rehner theory verifies the stabilization of the globular state of the particles in the presence of TMAO. Complementary atomistic MD simulations highlight a pronounced accumulation of TMAO molecules around PNIPAM chains. We observe a significant preferential attraction between TMAO and the globular state of PNIPAM, which is additionally stabilized by a larger number of hydrating water molecules compared to pure aqueous solutions. Further DLS measurements were also conducted on PNIPAM suspensions with the co-solute urea added. The observed differences compared with the results obtained for TMAO support the proposed mechanism

Tetrahydrothiophene 1-oxide as highly effective co-solvent for propylene carbonate-based electrolytes

Propylene carbonate (PC) together with cyclic sulfur compounds such as tetrahydrothiophene 1-oxide (THT1oxide) as co-solvent and lithium hexafluorophosphate (LiPF6) as conducting salt are introduced as new aprotic liquid electrolytes for lithium-ion batteries. Starting with the single solvent electrolyte LiPF6 in PC, by addition of THT1oxide, the ion transport properties even at temperatures down to −20 °C are improved by the different solvation behavior of Li+ ions due to the high Li+ ion affinity of the sulfinyl (-S=O) group and by the resulting decrease of the Li+ ion complex size. Electrolytes that contain Li+ ion complexes with both PC and THT1oxide molecules in the solvation shell are able to form protective interphase layers on graphite and NCM111 (LiNi1/3Co1/3Mn1/3O2) electrodes that are both permeable for Li+ ions while ensuring good electronic insulation, thus enabling stable cycling in lithium-ion cells with only minor capacity fading. THT1oxide/PC-based electrolytes afford better long-term as well as low temperature cycling behavior compared to established state-of-the-art (SOTA) organic carbonate-based electrolytes. The obtained results allow for the design of new co-solvents for PC and comparable cyclic organic carbonates, and provide a non-toxic and cheap alternative to crown ethers without affecting the Li+ ion transference/transport numbers

From the Atomistic to the Macromolecular Scale: Distinct Simulation Approaches for Polyelectrolyte Solutions

Polyelectrolytes reveal interesting properties in solution. At short length scales,the dissociation of counterions is heavily affected by the chemical structure of thepolyelectrolyte, the properties of the solution, and specific ion effects. At largerlength scales, the structure of polyelectrolyte solutions is dominated by long-range interactions. In the special case of dissolved polyanions and polycations,polyelectrolyte complexes or multilayers can form. In this review we presentdistinct simulation approaches to study the corresponding effects at differentlength scales in more detail. Whereas at short length scales, atomistic moleculardynamics simulation is often the method of choice, semi-coarse-grained and coarse-grained models with a lower level of details reveal their benefits at largerlength scales