The twist-bend nematic liquid crystal phase is a three-dimensional fluid in which achiral bent molecules spontaneously form an orientationally ordered, macroscopically chiral, heliconical winding of a ten nanometer-scale pitch in the absence of positional ordering. Here, the structure of the twist-bend phase of the bent dimer CB7CB and its mixtures with 5CB is characterized, revealing a hidden invariance of the self-assembly of the twist-bend structure of CB7CB, such that over a wide range of concentrations and temperatures, the helix pitch and cone angle change as if the ground state for a pitch of the TB helix is an inextensible heliconical ribbon along the contour formed by following the local molecular long axis (the director). Remarkably, the distance along the length for a single turn of this helix is given by 2πRmol, where Rmol is the radius of bend curvature of a single all-trans CB7CB molecule. This relationship emerges from frustrated steric packing due to the bent molecular shape: space in the fluid that is hard to fill attracts the most flexible molecular subcomponents, a theme of nanosegregation that generates self-assembled, oligomer-like correlations of interlocking bent molecules in the form of a brickwork-like tiling of pairs of molecular strands into duplex double-helical chains. At higher temperatures in the twist-bend phase, the cone angle is small, the director contour is nearly along the helix axis z, and the duplex chains are sequences of biaxial elements formed by overlapping half-molecule pairs, with an approximately 45° rotation of the biaxis between each such element along the chain.

To study the effects of paraffin on viscosity of waxy crude oil and transport properties of small molecules, light and waxy crude oil models were investigated at atmospheric pressure and 293-323 K temperature range using atomistic molecular dynamics simulations. The optimized parameters for liquid simulations all-atom (OPLS-AA) and atomistic polarizable potential for liquids, electrolytes, and polymers (APPLE&P) force fields were employed. The self-diffusion coefficients, viscosity, and paraffin configurations were compared for two oil models and between the two employed force fields. However, the behavior of paraffin molecules predicted by two force fields was quite different. Simulations using the OPLS-AA force field showed crystallization of longer paraffin molecules below 323 K, while simulations with the APPLE&P force field demonstrated a homogeneous mixture down to 293 K. To provide additional validation of the employed force fields, the density, diffusion coefficient, and crystallization of pure alkanes were compared with experimental data. The density and diffusion coefficients of n-C-6 and n-C-14 simulated with the APPLE&P force field were found to be in much closer agreement with the experimental data. The OPLS-AA force field was found to overestimate the crystallization temperature of pure alkanes. Therefore, simulations with the APPLE&P provide more realistic description of the waxy oil structure and transport properties. In this temperature range, the paraffin molecules are homogeneously distributed in the mixture, and viscosity of the system increased by a factor of two compared to light oil. Crystallization of paraffins requires lower temperatures or/and the presence of other components such as nanoparticles or asphaltene molecules to facilitate nucleation.

We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.

Extensive atomistic molecular dynamics (MD) simulations employing a polarizable force field have been conducted to study hydrated anion exchange membranes comprised of a poly(p-phenylene oxide) (PPO) homopolymer functionalized with quaternary ammonium cationic side groups and hydroxide anions. Representative membranes with different cationic structures have been investigated to study correlations between polymer architecture, morphology and transport properties of hydrated membranes. Specifically, hydrated polymers with five different quaternary ammonium cationic groups (R1: -CH3, R2: -C2H5, R3: -C3H7, R4: -C6H13 and R5: -C4H8OCH3) and degree of functionalization of 50% were investigated at three hydration levels (λ = Nwater/Ncation = 5, 10 and 17). Effects of the polymer structure on the distribution of water-rich domains and dynamic relaxations were systematically investigated to uncover the complex interplay between the degree of hydrophobicity/hydrophilicity of the cationic groups, morphology, connectivity of water domains, and the hydroxide transport mechanisms. Structural and dynamical analysis indicates that the bottlenecks, formed between the water-rich domains, create a substantial free energy barrier for hydroxide transport associated with the partial loss of anion hydration structure. The energy penalty associated with the loss of the hydration structure hinders the vehicular transport of the hydroxide anion. The optimal structure of functionalized homopolymer chains should be sufficiently hydrophobic to create nanophase segregation and form an interconnected network of water channels with a minimal amount of narrow bottlenecks that inhibit the vehicular motion of hydrated anions. We demonstrate that utilization of asymmetrically modified cationic groups is a promising route to achieve the desired water channel morphology at low hydration levels.

Atomistic molecular dynamics simulations were conducted to study the wetting states of 1-ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl)-imide ionic liquid (IL) nanodroplets on surfaces with different strengths of van der Waals (VDW) interactions and in the presence of an electric field. By adjusting the depth of Lennard-Jones potential, the van der Waals interaction between the solid surface and ionic liquid was systematically varied. The shape of the droplets was analyzed to extract the corresponding contact angle utilized to characterize wetting states of the nanodroplets. The explored range of surface-IL interactions allowed contact angles ranging from complete IL spreading on the surface to poor wettability. The effect of the external electrical field was explored by adding point charges to the surface atoms. Systems with two charge densities (±0.002 e/atom and ±0.004 e/atom) that correspond to 1.36 V/nm and 2.72 V/nm electric fields were investigated. Asymmetrical wetting states were observed for both cases. At 1.36 V/nm electric field, contributions of IL-surface VDW interactions and Coulombic interactions to the wetting state were competitive. At 2.72 V/nm field, electrostatic interactions dominate the interaction between the nanodroplet and surface, leading to enhanced wettability on all surfaces.

The influence of force field details in all-atom molecular dynamics (MD) simulations on the predicted thermodynamic, structural, and dynamic properties of bulk 4-cyano-4'-pentylbiphenyl (5CB) systems have been investigated in the 292-368K temperature range. The effect of the molecular dipole moment and the details of dihedral potential for biphenyl unit were investigated using both polarisable (POL) and non-polarisable (NP) versions of the quantum chemistry-based force field. The predicted densities for the nematic and isotropic phases of bulk 5CB were found to be in excellent agreement with available experimental data. The nematic-isotropic transition temperature (T-NI) showed strong sensitivity to the force field details, MD simulations with partial atomic charge distributions and molecular dipole moment corresponding to high-level quantum chemistry calculations predicted an overestimation of the T-NI by about 30K. Rescaling the charges to allow the molecular dipole to be closer to experimentally reported values of 5CB dipole in condensed phases, significantly improved the prediction of T-NI as well as other thermodynamic and dynamic properties of 5CB. We also discuss how the structural, thermodynamic, and dynamic properties of bulk 5CB are affected by the flexibility of the central biphenyl dihedral and the inclusion of induced polarisation effects.[GRAPHICS].