Rheological properties of rotator and crystalline phases of alkanes

Ionic Liquid Crystals as Chromogenic Materials

Ionic liquid crystals (ILCs), a class of soft matter materials whose properties can be tuned by the wise pairing of the cation and anion, have recently emerged as promising candidates for different applications, combining the characteristics of ionic liquids and liquid crystals. Among those potential uses, this review aims to cover chromogenic ILCs. In this context, examples of photo-, electro- and thermochromism based on ILCs are provided. Furthermore, thermotropic and lyotropic ionic liquid crystals are also summarised, including the most common chemical and phase structures, as well as the advantages of confining these materials. This manuscript also comprises the following main experimental techniques used to characterise ILCs: Differential Scanning Calorimetry (DSC), Polarised Optical Microscopy (POM) and X-Ray Powder Diffraction (XRD). Chromogenic ILCs can be interesting smart materials for energy and health purposes.

Thermodynamic Analysis of n-Nonadecane (C19H40)/1-Octadecanol (C18H37OH) Blends

The article delves into the intricate phase transitions of 1-Octadecanol and n-Nonadecane within a binary system, unveiling dynamic structural changes under varying conditions. Through Fourier transform infrared (FTIR) analysis, specific molecular vibrations were identified, shedding light on the molecular composition and interactions. The study highlights the challenges in detecting subtle phase transitions and emphasises the individuality of molecular behaviours in closely related compounds. The findings underscore the complexity of phase transitions in binary systems and advocate for a nuanced approach to studying molecular structures and behaviours.

Morphologies of Comb-like Polyacrylic Acid/Polyacrylate Copolymers as Functions of the Degree of Derivatization with n-C22H45 Side Chains

Polymers with crystallizable side chains have numerous applications, and their properties depend on their crystal morphologies and phase separation. Structural analysis on a wide spatial scale plays an important role in controlling the thermal properties and higher-order structures of these polymers. In this study, we elucidated the melting and crystallization processes of copolymers with varying crystallizable side-chain fractions over a wide spatial range. Differential scanning calorimetry revealed that the enthalpies of melting and crystallization increased linearly with increasing crystallizable side-chain fraction. The results of wide-angle X-ray scattering indicated that the crystal lattice was hexagonal. Conversely, spherulite-like higher-order architectures with linear structures and radial spreading were observed in the highly crystallizable components, but no micrometer-scale structures were observed in the less crystallizable components. In situ small-angle X-ray scattering was used to elucidate the phase separation and mixing processes. Lamellar crystallites were observed at crystallizable side-chain fractions of >55 wt.%, whereas small crystallites were observed at fractions of <45 wt.%. At temperatures above the order-disorder transition temperature, density fluctuations caused by correlation holes were observed. These properties have a strong effect on the crystallizable side-chain fraction.

Computational assessment of hexadecane freezing by equilibrium atomistic molecular dynamics simulations

HYPOTHESIS

Upon cooling, alkanes can form intermediate phases between liquid and crystal. They are called "rotator" or "plastic" phases and have long-range positional order with rotational freedom around the long molecular axis which gives them non-trivial and useful visco-plastic properties. We expect that the formation and structure of rotator phases formed in freezing alkanes can be understood much deeper by tracking the process at molecular level with atomistic molecular dynamics.

SIMULATIONS

We defined an appropriate CHARMM36-based computational protocol for simulating the freezing of hexadecane, which contained a sufficiently long (500 ns) equilibrium sampling of the frozen states. We employed it to simulate successfully the freezing of bulk and interface-contacting hexadecane and to provide a pioneering clarification of the effect of surfactant on the crystallization mechanism and on the type of intermolecular ordering in the crystallites.

FINDINGS

The devised computational protocol was able to reproduce the experimentally observed polycrystalline structure formed upon cooling. However, different crystallization mechanisms were established for the two types of models. Crystallites nucleate at random locations in the bulk and start growing rapidly within tens of nanoseconds. In contrast, the surfactants freeze first during the fast cooling (<1 ns), followed by rapid hexadecane freezing, with nucleation starting along the entire surfactant adsorption layer. Thereby, the hexadecane molecules form rotator phases which transition into a more stable ordered phase. This collective transition is first-time visualized directly. The developed robust computational protocol creates a foundation for future in-depth modelling and analysis of solid-state alkane-containing, incl. lipid, structures.

Computational Procedure for Analysis of Crystallites in Polycrystalline Solids of Quasilinear Molecules

In the current work, a comprehensive procedure for structural analysis of quasilinear organic molecules arranged in a polycrystalline sample generated by molecular dynamics is developed. A linear alkane, hexadecane, is used as a test case because of its interesting behavior upon cooling. Instead of a direct transition from isotropic liquid to the solid crystalline phase, this compound forms first a short-lived intermediate state known as a "rotator phase". The rotator phase and the crystalline one are distinguished by a set of structural parameters. We propose a robust methodology to evaluate the type of ordered phase obtained after a liquid-to-solid phase transition in a polycrystalline assembly. The analysis starts with the identification and separation of the individual crystallites. Then, the eigenplane of each of them is fit and the tilt angle of the molecules relative to it is computed. The average area per molecule and the distance to the nearest neighbors are estimated by a 2D Voronoi tessellation. The orientation of the molecules with respect to each other is quantified by visualization of the second molecular principal axis. The suggested procedure may be applied to different quasilinear organic compounds in the solid state and to various data compiled in a trajectory.