Structural relaxation, viscosity, and network connectivity in a hydrogen bonding liquid.

In liquids, the ability of neighboring molecules to rearrange and jostle past each other is directly related to viscosity, the property which describes the propensity to flow. The presence of hydrogen bonds (H-bonds) complicates the molecular scale picture of viscosity. H-Bonds are attractive, directional interactions between molecules which, in some cases, result in transient network structures. In this work, we use experimental and computational methods to demonstrate that the timescale of H-bond network reorganization is the dominant dynamical timescale associated with viscosity for the case of the model H-bonding liquid n-methylacetamide (NMA). This molecule is a peptide analog which forms a transient linear H-bond network. Individual H-bond lifetimes and dynamical fluctuations were observed on the timescale of 1.5 ps, while collective motions and the longest lived population of H-bond partner lifetimes were observed on the order of 20 ps, in agreement with the Maxwell relaxation time. This identifies a mechanism which may aid in understanding the emergence of various complex phenomena arising from transient molecular structures, with implications ranging from the internal dynamics of proteins, to the glass transition, to better understanding the origins of the unique properties of H-bonding liquids.

Structure and Hydration of Highly-Branched, Monodisperse Phytoglycogen Nanoparticles.

Phytoglycogen is a naturally occurring polysaccharide nanoparticle made up of extensively branched glucose monomers. It has a number of unusual and advantageous properties, such as high water retention, low viscosity, and high stability in water, which make this biomaterial a promising candidate for a wide variety of applications. In this study, we have characterized the structure and hydration of aqueous dispersions of phytoglycogen nanoparticles using neutron scattering. Small angle neutron scattering results suggest that the phytoglycogen nanoparticles behave similar to hard sphere colloids and are hydrated by a large number of water molecules (each nanoparticle contains between 250% and 285% of its mass in water). This suggests that phytoglycogen is an ideal sample in which to study the dynamics of hydration water. To this end, we used quasielastic neutron scattering (QENS) to provide an independent and consistent measure of the hydration number, and to estimate the retardation factor (or degree of water slow-down) for hydration water translational motions. These data demonstrate a length-scale dependence in the measured retardation factors that clarifies the origin of discrepancies between retardation factor values reported for hydration water using different experimental techniques. The present approach can be generalized to other systems containing nanoconfined water.

Dynamics of Water Associated with Lithium Ions Distributed in Polyethylene Oxide.

The dynamics of water in polyethylene oxide (PEO)/LiCl solution has been studied with quasielastic neutron scattering experiments and molecular dynamics (MD) simulations. Two different time scales of water diffusion representing interfacial water and bulk water dynamics have been identified. The measured diffusion coefficient of interfacial water remained 5-10 times smaller than that of bulk water, but both were slowed by approximately 50% in the presence of Li(+). Detailed analysis of MD trajectories suggests that Li(+) is favorably found at the surface of the hydration layer, and the probability to find the caged Li(+) configuration formed by the PEO is lower than for the noncaged Li(+)-PEO configuration. In both configurations, however, the slowing down of water molecules is driven by reorienting water molecules and creating water-Li(+) hydration complexes. Performing the MD simulation with different ions (Na(+) and K(+)) revealed that smaller ionic radius of the ions is a key factor in disrupting the formation of PEO cages by allowing spaces for water molecules to come in between the ion and PEO.

Charge-dependent dynamics of a polyelectrolyte dendrimer and its correlation with invasive water.

Atomistic molecular dynamics (MD) simulations were carried out to investigate the local dynamics of polyelectrolyte dendrimers dissolved in deuterium oxide (D2O) and its dependence on molecular charge. Enhanced segmental dynamics upon increase in molecular charge is observed, consistent with quasielastic neutron scattering (QENS) measurements. A strong coupling between the intradendrimer local hydration level and segmental dynamics is also revealed. Compelling evidence shows these findings originate from the electrostatic interaction between the hydrocarbon components of a dendrimer and the invasive water. This combined study provides fundamental insight into the dynamics of charged polyelectrolytes and the solvating water molecules.

Consequences of Convex Nanopore Chemistry on Confined Water Dynamics.

A fundamental understanding of confined water is crucial for developing selective ion transport and water purification membranes, yet the roles of nanopore geometry and functionality on confined water dynamics remain unresolved. We report the synthesis of perdeuterated ionic alkylsulfonate amphiphiles and their water-induced self-assembly into lyotropic liquid crystal (LLC) mesophases with well-defined, convex, sulfonate-lined nanopores. Quasielastic neutron scattering (QENS) measurements demonstrate that the water self-diffusion coefficients within these sulfonate-lined convex nanopores depend on the hydration level and amphiphile counterion identity (H+, K+, NMe4+). The consistency of the observed counterion-dependent water dynamics trends with those of carboxylate LLCs is rationalized on the basis of similarities in the counterion spatial distributions in the water-filled channels, which we deduce from electron density maps derived from small-angle X-ray scattering (SAXS) analyses. These findings indicate that water diffusion is systematically faster in sulfonate-lined nanopores as compared to carboxylate-lined pores due to weaker water interactions with the softer and more hydrophobic-SO3- functionalities. These molecular-level insights into the relationships between convex pore wall chemical functionalities, hydrated counterions, and confined water diffusion may inform future development of new nanoporous media.

Ion-Specific Confined Water Dynamics in Convex Nanopores of Gemini Surfactant Lyotropic Liquid Crystals.

The impact of pore geometry and functionality on the dynamics of water nanoconfined in porous media are the subject of some debate. We report the synthesis and small-angle X-ray scattering (SAXS) characterization of a series of perdeuterated gemini surfactant lyotropic liquid crystals (LLCs), in which convex, water-filled nanopores of well-defined dimensions are lined with carboxylate functionalities. Quasielastic neutron scattering (QENS) measurements of the translational water dynamics in these dicarboxylate LLC nanopores as functions of the surfactant hydration state and the charge compensating counterion (Na+, K+, NMe4+) reveal that the measured dynamics depend primarily on surfactant hydration, with an unexpected counterion dependence that varies with hydration number. We rationalize these trends in terms of a balance between counterion-water attractions and the nanopore volume excluded by the counterions. On the basis of electron density maps derived from SAXS analyses of these LLCs, we directly show that the volume excluded by the counterions depends on both their size and spatial distribution in the water-filled channels. The translational water dynamics in the convex pores of these LLCs are also slower than those reported in the concave pores of AOT reverse micelles, implying that water dynamics also depend on the nanopore curvature.

Nanoscale Atomic Displacements Ordering for Enhanced Piezoelectric Properties in Lead-Free ABO3 Ferroelectrics.

In situ synchrotron X-ray diffuse scattering and inelastic neutron scattering measurements from a prototype ABO3 ferroelectric single-crystal are used to elucidate how electric fields along a nonpolar direction can enhance its piezoelectric properties. The central mechanism is found to be a nanoscale ordering of B atom displacements, which induces increased lattice instability and therefore a greater susceptibility to electric-field-induced mechanical deformation.