Non-Newtonian Dynamics in Water-in-Salt Electrolytes

Water-in-salt electrolytes have attracted considerable interest in the past decade for advanced lithium-ion batteries, possessing important advantages over the non-aqueous electrolytes currently in use. A battery with a LiTFSI-water electrolyte was demonstrated in which an operating window of 3 V is made possible by a solid-electrolyte interface. Viscosity is an important property for such electrolytes, because high viscosity is normally associated with low ionic conductivity. Here, we investigate shear and longitudinal viscosities using shear stress and compressional longitudinal stress measurements as functions of frequency and concentration. We find that both viscosities are frequency-dependent and exhibit almost identical frequency and concentration dependences in the high-concentration region. A comparison to quasielastic neutron scattering experiments suggests that both are governed by structural relaxation of the TFSI- network. Thus, LiFTSI-water electrolytes appear to be an unusual case of a non-Newtonian fluid, where shear and longitudinal viscosities are determined by the same relaxation mechanism.

Structure of water-in-salt and water-in-bisalt electrolytes

We report a systematic diffraction study of two "water-in-salt" electrolytes and a "water-in-bisalt" electrolyte combining high-energy X-ray diffraction (HEXRD) with polarized and unpolarized neutron diffraction (ND) on both H₂O and D₂O solutions. The measurements provide three independent combinations of correlations between the different pairs of atom types that reveal the short- and intermediate-range order in considerable detail. The ND interference functions show pronounced peaks around a scattering vector Q ∼ 0.5 Å^-1 that change dramatically with composition, indicating significant rearrangements of the water network on a length scale around 12 Å. The experimental results are compared with two sets of Molecular Dynamics (MD) simulations, one including polarization effects and the other based on a non-polarizable force field. The two simulations reproduce the general shapes of the experimental structure factors and their changes with concentration, but differ in many detailed respects, suggesting ways in which their force fields might be modified to better represent the actual systems.

Nanoscale Relaxation in "Water-in-Salt" and "Water-in-Bisalt" Electrolytes

"Water-in-salt" (WIS) and "water-in-bisalt" (WIBS) electrolytes have recently been developed for Li-ion batteries, combining the safety and environmental friendliness of aqueous electrolytes with a larger operating window made possible by a solid-electrolyte interphase. We report quasielastic neutron scattering (QENS) measurements on solutions of a WIS electrolyte at two concentrations, 13.9 and 21 m (molal) lithium bis(trifluoromethane)sulfonimide LiTFSI in H₂O/D₂O and a WIBS electrolyte at (21 m LiTFSI + 7 m lithium triflate (LiOTf)) in H₂O/D₂O. The data were Fourier transformed to obtain experimental intermediate scattering functions (ISFs) and compared with corresponding quantities obtained from molecular dynamics (MD) simulations. Both QENS and MD ISFs could be fitted well by a single stretched exponential function to obtain apparent translational diffusion coefficients for the water molecules. The QENS values agree well with the MD simulations for the 13.9 and 21 m solutions, but MD simulations predict a slower relaxation of water compared to QENS for the WIBS electrolyte. Comparison of the incoherent and coherent scattering reveals much faster water dynamics compared with structural relaxation of the ionic framework, consistent with the nanodomain picture where the lithium diffusion occurs through the tortuous water domain around the slower relaxing ionic matrix, leading to highly non-Gaussian water motion.

Molecular Dynamics and Neutron Scattering Studies of Potassium Chloride in Aqueous Solution

Neutron diffraction with isotopic substitution (NDIS) experiments were done on both natural abundance potassium and isotopically labeled ⁴¹KCl heavy water solutions to characterize the solvent structuring around the potassium ion in water. Preliminary measurements suggested that the literature value for the coherent neutron scattering length (2.69 fm) for ⁴¹K was significantly in error. This value was remeasured using a neutron powder diffractometer and found to be 2.40 fm. This revision increases significantly the contrast between the natural abundance K and ⁴¹K by about 30% (from 1.0 to 1.3 fm). The experimentally determined structure factor of the potassium ion was then compared to that calculated from molecular dynamics (MD) simulations. Previous neutron scattering measurements of potassium gave a solvation number of 5.5 (see below). In this study, the NDIS and MD results are in good agreement and allowed us to derive a coordination number of 6.1 for water molecules and 0.8 for chloride ions around each K⁺ ion in 4 molal aqueous KCl solution.

Drying colloidal systems: Laboratory models for a wide range of applications

The drying of complex fluids provides a powerful insight into phenomena that take place on time and length scales not normally accessible. An important feature of complex fluids, colloidal dispersions and polymer solutions is their high sensitivity to weak external actions. Thus, the drying of complex fluids involves a large number of physical and chemical processes. The scope of this review is the capacity to tune such systems to reproduce and explore specific properties in a physics laboratory. A wide variety of systems are presented, ranging from functional coatings, food science, cosmetology, medical diagnostics and forensics to geophysics and art.

Molecular Dynamics and Neutron Scattering Studies of Mixed Solutions of Caffeine and Pyridine in Water

Insight into the molecular interactions of homotactic and heterotactic association of caffeine and pyridine in aqueous solution is given on the basis of both experimental and simulation studies. Caffeine is about 5 times more soluble in a 3 m aqueous pyridine solution than it is in pure water (an increase from ∼0.1 m to 0.5 m). At this elevated concentration the system becomes suitable for neutron scattering study. Caffeine-pyridine interactions were studied by neutron scattering and molecular dynamics simulations, allowing a detailed characterization of the spatial and orientational structure of the solution. It was found that while pyridine-caffeine interactions are not as strong as caffeine-caffeine interactions, the pyridine-caffeine interactions still significantly disrupted caffeine-caffeine stacking. The alteration of the caffeine-caffeine stacking, occasioned by the presence of pyridine molecules in solution and the consequent formation of heterotactic interactions, leads to the experimentally detected increase in caffeine solubility.

Synthesis of metastable hard-magnetic ε-Fe2O3 nanoparticles from silica-coated akaganeite nanorods

We present a simple preparation route to obtain a nanoscale metastable hard-magnetic ε-Fe₂O₃ phase, using silica coated β-FeOOH nanorods as a precursor and an annealing process. The synthesized ε-Fe₂O₃ nanoparticles exhibit large coercivity (H_C ∼ 20 kOe at 300 K and H_C ∼ 1.6 kOe at 400 K), confirming their high potential for practical applications.

Relaxation in a Prototype Ionic Liquid: Influence of Water on the Dynamics

The influence of water on the relaxation of a prototype ionic liquid (IL) C₈mimBF₄ is examined in the IL-rich regime combining quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations. The QENS and MD simulations results for relaxation of IL and the equimolar mixture with water probed by the dynamics of the C₈mim hydrogen atoms in the time range of 2 ps to 1 ns are in excellent agreement. The QENS data show that translational relaxation increases by a factor of 7 on the addition of water, while rotational relaxation involving multiple processes fitted by a KWW function with low β values is speeded up by a factor of 3 on the time scale of QENS measurements. The MD simulations show that the cation diffusion coefficient, inverse viscosity, and ionic conductivity increase on the addition of water, consistent with the very small change in ionicity. The difficulties in obtaining rotational and translational diffusion coefficients from fits to QENS experiments of pure ILs and IL-water mixtures are discussed.

Folic acid-capped PEGylated magnetic nanoparticles enter cancer cells mostly via clathrin-dependent endocytosis

BACKGROUND

This work is focused on mechanisms of uptake in cancer cells of rationally designed, covalently assembled nanoparticles, made of superparamagnetic iron oxide nanoparticles (SPIONs), fluorophores (doxorubicin or Nile Blue), polyethylene glycol (PEG) and folic acid (FA), referred hereinafter as SFP-FA.

METHODS

SFP-FA were characterized by DLS, zetametry and fluorescence spectroscopy. The SFP-FA uptake in cancer cells was monitored using fluorescence-based methods like fluorescence-assisted cell sorting, CLSM with single-photon and two-photon excitation. The SFP-FA endocytosis was also analyzed with electron microscopy approaches: TEM, HAADF-STEM and EELS.

RESULTS

The SFP-FA have zeta potential below -6mW and stable hydrodynamic diameter close to 100nm in aqueous suspensions of pH range from 5 to 8. They contain ca. 109 PEG-FA, 480 PEG-OCH₃ and 22-27 fluorophore molecules per SPION. The fluorophores protected under the PEG shell allows a reliable detection of intracellular NPs. SFP-FA readily enter into all the cancer cell lines studied and accumulate in lysosomes, mostly via clathrin-dependent endocytosis, whatever the FR status on the cells.

CONCLUSIONS

The present study highlights the advantages of rational design of nanosystems as well as the possible involvement of direct molecular interactions of PEG and FA with cellular membranes, not limited to FA-FR recognition, in the mechanisms of their endocytosis.

GENERAL SIGNIFICANCE

Composition, magnetic and optical properties of the SFP-FA as well their ability to enter cancer cells are promising for their applications in cancer theranosis. Combination of complementary analytical approaches is relevant to understand the nanoparticles behavior in suspension and in contact with cells.

Effect of water on the structure of a prototype ionic liquid

The influence of water on the structure of a prototype ionic liquid (IL) 1-octyl-3-methylimidazolium tetrafluoroborate (C₈mimBF₄) is examined in the IL-rich regime using high-energy X-ray diffraction (HEXRD) and molecular dynamics (MD) simulations.

The conformation of a ribose derivative in aqueous solution: a neutron-scattering and molecular dynamics study

The structure of aqueous solutions of methyl β-D-ribofuranoside was investigated by coupling molecular dynamics (MD) simulations and neutron scattering measurements with isotopic substitution. Using a sample of the sugar isotopically-labeled at a single unique position, neutron scattering structure factors and radial distribution functions can be compared with MD simulations constrained to different conformations to determine which conformer best fits the experimental results. Three different simulations were performed with the methyl ether group of the sugar unconstrained and constrained in each of its staggered orientations. The results of the unconstrained simulation showed that the methyl ester group occupied predominantly the 300° position, which is in agreement with the diffraction experimental results. This result suggests that the molecular mechanics force field used in the simulation adequately describes the conformation of the 1-methyl ether group in the methyl β-D-ribofuranoside.

Patterned surfaces in the drying of films composed of water, polymer, and alcohol

A study of the complex drying dynamics of polymeric mixtures with optical microscopy and gravimetric measurement is presented. Droplet formation is observed, followed by a collapse that leads to the residual craters in the dried film. The process is followed in situ under well-defined temperature and hygrometric conditions to determine the origin and nature of these droplets and craters. The drying process is usually completed within 1 h. The observations are explained using a simple diffusion model based on experimental results collected from mass and optical measurements as well as Raman confocal microspectrometry. Although the specific polymeric mixtures used here are of interest to the cosmetic industry, the general conclusions reached can apply to other polymeric aqueous solutions with applications to commercial and artistic painting.

Caffeine and sugars interact in aqueous solutions: a simulation and NMR study

Molecular dynamics simulations were carried out on several systems of caffeine interacting with simple sugars. These included a single caffeine molecule in a 3 m solution of α-D-glucopyranose, at a caffeine concentration of 0.083 m, a single caffeine in a 3 m solution of β-D-glucopyranose, and a single caffeine molecule in a 1.08 m solution of sucrose (table sugar). Parallel nuclear magnetic resonance titration experiments were carried out on the same solutions under similar conditions. Consistent with previous thermodynamic experiments, the sugars were found to have an affinity for the caffeine molecules in both the simulations and experiments, and the binding in these complexes occurs by face-to-face stacking of the hydrophobic triad of protons of the pyranose rings against the caffeine face, rather than by hydrogen bonding. For the disaccharide, the binding occurs via stacking of the glucose ring against the caffeine, with a lesser affinity for the fructose observed. These findings are consistent with the association being driven by hydrophobic hydration and are similar to the previously observed binding of glucose rings to various other planar molecules, including indole, serotonin, and phenol.

Magnetic nanocarriers of doxorubicin coated with poly(ethylene glycol) and folic acid: relation between coating structure, surface properties, colloidal stability, and cancer cell targeting

We report the efficient one-step synthesis and detailed physicochemical evaluation of novel biocompatible nanosystems useful for cancer therapeutics and diagnostics (theranostics). These systems are the superparamagnetic iron oxide nanoparticles (SPIONs) carrying the anticancer drug doxorubicin and coated with the covalently bonded biocompatible polymer poly(ethylene glycol) (PEG), native and modified with the biological cancer targeting ligand folic acid (PEG-FA). These multifunctional nanoparticles (SPION-DOX-PEG-FA) are designed to rationally combine multilevel mechanisms of cancer cell targeting (magnetic and biological), bimodal cancer cell imaging (by means of MRI and fluorescence), and bimodal cancer treatment (by targeted drug delivery and by hyperthermia effect). Nevertheless, for these concepts to work together, the choice of ingredients and particle structure are critically important. Therefore, in the present work, a detailed physicochemical characterization of the organic coating of the hybrid nanoparticles is performed by several surface-specific instrumental methods, including surface-enhanced Raman scattering (SERS) spectroscopy, X-ray photoelectron spectrometry (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). We demonstrate that the anticancer drug doxorubicin is attached to the iron oxide surface and buried under the polymer layers, while folic acid is located on the extreme surface of the organic coating. Interestingly, the moderate presence of folic acid on the particle surface does not increase the particle surface potential, while it is sufficient to increase the particle uptake by MCF-7 cancer cells. All of these original results contribute to the better understanding of the structure-activity relationship for hybrid biocompatible nanosystems and are encouraging for the applications in cancer theranostics.

Molecular dynamics simulation studies of caffeine aggregation in aqueous solution

Molecular dynamics simulations were carried out on a system of eight independent caffeine molecules in a periodic box of water at 300 K, representing a solution near the solubility limit for caffeine at room temperature, using a newly developed CHARMM-type force field for caffeine in water. Simulations were also conducted for single caffeine molecules in water using two different water models (TIP3P and TIP4P). Water was found to structure in a complex fashion around the planar caffeine molecules, which was not sensitive to the water model used. As expected, extensive aggregation of the caffeine molecules was observed, with the molecules stacking their flat faces against one another like coins, with their methylene groups staggered to avoid steric clashes. A dynamic equilibrum was observed between large n-mers, including stacks with all eight solute molecules, and smaller clusters, with the calculated osmotic coefficient being in acceptable agreement with the experimental value. The insensitivity of the results to water model and the congruence with experimental thermodynamic data suggest that the observed stacking interactions are a realistic representation of the actual association mechanism in aqueous caffeine solutions.

Water Confined in Cylindrical Pores: A Molecular Dynamics Study

Molecular dynamics simulations of water confined in two hydrophilic cylindrical pores-PH and PL-that differ in their silanol surface concentration (7.6 and 3.0 nm(-2), respectively) have been performed at 300 K. A strong interaction of interfacial water molecules with the pore was systematically found and gives rise to a layering effect, a significant distortion of both the hydrogen bond network (HBN) and the tetrahedral structure of these water molecules, and a corresponding subdiffusive mean square displacement along the main axis of the pores. By contrast, water molecules in the inner part of both pores were found to behave similarly to bulk water. The HBN and the tetrahedral configuration of water were more gradually distorted in the PL pore given the larger heterogeneity and rugosity of the surface, and the number of water-pore hydrogen bonds did not scale linearly with the silanol surface concentration of the pores, in part because of the close proximity between silanols in the PH pore. With the PL pore, the dynamic slowing down of water was found consistent with the experiment, suggesting that it provides a better model for the cylindrical MCM-41 mesopores. The structural and dynamical properties of water vary little with the silica force field.

Glucose interactions with a model peptide

Molecular dynamics simulations have been conducted of the helical polypeptide melittin, in concentrated aqueous solutions of the alpha and beta anomers of D-glucopyranose. Glucose is an osmolyte, and it is expected to be preferentially excluded from the surfaces of proteins. This was indeed found to be the case in the simulations. The results indicate that the observed exclusion may have a contribution from an under-representation of hydrogen bonding interactions between glucose groups and exposed side chains, compared to water. However, glucose was found to bind quite specifically to melittin by stacking its hydrophobic face, consisting of aliphatic protons, against the flat hydrophobic face of the indole group of the tryptophan-19 side chain. Although the binding site for this interaction is localized, the binding is weak for both anomers, with a binding free energy estimated as only ∼0.5 kcal/mol (i.e. near k(B)T). The face of the sugar stacked against the Trp indole ring is different for the two anomers of glucose, due to the disruption of the H1-H3-H5 hydrophobic triad of the beta anomer by the axial C1 hydroxyl group in the alpha anomer. The measurable affinity of the sugar for the Trp side chain is consistent with the very frequent occurrence of this group in the binding sites of proteins that complex with sugars.

Molecular dynamics and neutron scattering study of glucose solutions confined in MCM-41

Glucose aqueous solutions confined in MCM-41 cylindrical pores of diameter 3.2 nm have been studied by molecular dynamics (MD) simulations and quasielastic neutron scattering (QENS). MD simulations reveal a strong preferential interaction of glucose molecules with the silica walls, which induces significant concentration gradients within the pore. The influence of glucose on the structural and dynamical properties of water strongly depends on the region of the pore considered. The distortion of the hydrogen bond network (HBN) and of the tetrahedral organization of interfacial water molecules induced by silica is much stronger than that induced by glucose molecules. The interfacial glucose molecules diffuse about 1 order of magnitude slower than those in the core region. Differences in affinities for silica of the different species in confined hydrogen-bonded mixtures induce significant structural and dynamical heterogeneities not present in bulk solutions.