This work presents results, aimed at identifying the crystallization mechanism of surfactant-stabilized hexadecane-in-water droplet with 15 nm diameter at the molecular level. The question is addressed by atomistic molecular dynamics simulations of models consisting of ca. 2 million atoms. To represent as closely as possible real-world systems, a procedure for constructing droplets of different sizes is developed, allowing control of the surfactant surface coverage. Two crystallization protocols are applied: slow freezing, in which the surface solidifies first, followed by relaxation and subsequent cooling to induce crystallization in the bulk, and fast freezing, in which the droplet is directly cooled to the final temperature, leading to simultaneous surface and core crystallization. A few key results may be outlined. The droplet surface always freezes first. Nucleation occurs stochastically at multiple independent sites, gradually propagating over the surface. Both shell and core undergo heterogeneous nucleation initiated by surfactant molecules. The nuclei in the core typically form close to the surface. The cooling procedure affects the spatial freezing pathway and molecular ordering in the drop core. The preferred orientation of bulk crystallites relative to the originating surface is nearly perpendicular. Once solidified, the surface alone is sufficient to induce droplet deformation to a triangular prism-like shape, additionally stabilized by the bulk crystallites. The deformation is a clear indication of a rotator phase, stable over hundreds of nanoseconds, further confirmed by the fraction of gauche conformations, P2 order parameters, and radial distribution functions. These findings agree with and complement experimental data and provide molecular-level verification of the experimentally observed fundamental difference in the stability of hexadecane rotator phase: transient in bulk and stable under micro- and nanoconfinement. This is the fundamental knowledge on microscopic freezing mechanisms of hexadecane and similar even-parity alkane-based materials at interfaces.

Introduction: The use of intravenous fat emulsions for treatment of drug intoxication still follows
a trial-and-error approach, due to insufficient understanding of the mechanisms of the process
and lack of predictive in vitro tools.

Molecular dynamics simulations are employed to investigate the crystallization of pentadecane-containing systems. Reference crystalline and rotator phase constructed from crystallographic data benchmark the structures formed upon cooling. Phase identification is achieved through global and local structural descriptors, with the fraction of gauche conformations and angular P₂ profiles outlined as the most sensitive indicators.
Pentadecane exhibits a markedly more stable rotator phase than hexadecane. This is confirmed by simulations spanning more than 30 K for the reference rotator phase. In this temperature range, a model regular rotator phase of pentadecane remains stable without undergoing significant structural changes, showing high reproducibility across independent trajectories. This contrasts hexadecane, for which a rotator phase rapidly transforms toward a triclinic structure [Iliev et al. 2023]. The presence of a surfactant in the system promotes heterogeneous nucleation, shifts crystallization to higher temperatures, and stabilizes the rotator phase, in agreement with experiments.
A computationally efficient protocol for simulating SAXS spectra from MD trajectories is proposed. The simulated spectra align very well with experimental data for both crystalline and rotator phases. Crystallographic lattice parameters can be extracted from the most intense SAXS peaks, even for experimentally unknown structures, demonstrating the general applicability of the approach to solid-state phase analysis.

Phase behavior of lipids is of primary importance for the manufacturing and applications of foods, cosmetics and pharmaceuticals, as well as for the functions of biological membranes. Upon cooling, the molten bulk lipids crystallize into ordered domains which determine their rheological properties. While storage and loss moduli are typically used to describe these properties, their direct connection to the underlying molecular rearrangement remains poorly understood. In the current study, we performed a detailed rheological characterization of the rotator phases (intermediate phases between fully ordered crystalline and completely disordered liquid phases) formed in bulk linear alkanes. Large series of stress-relaxation and creep-recovery experiments were performed and interpreted, using generalized Kelvin-Voigt model with one spring, connected in series with three combined elements of a spring and a dashpot. We determined the elasticities and viscosities of all these rheological elements, along with the respective three relaxation times of the combined elements. These relaxation times are governed by different molecular processes in the sheared samples and differ by three orders of magnitude: t1 ≈ 0.45 s and t2 ≈ 8–9 s are related to local molecular rearrangements at the domain boundaries, while t3 ≈ 140–200 s most probably describes the rearrangement of disordered lipid molecules entrapped between the ordered domains. The storage and loss moduli, calculated from the constants of the generalized Kelvin-Voigt model, were in a very good agreement with those measured directly in amplitude sweep and temperature ramps oscillatory tests, thus supporting the self-consistency of data interpretation. The methodology presented here is applicable to other polycrystalline lipid materials with 2D or 3D domain structures, providing a valuable framework for interpreting their rheological behavior.

Hydroxypropyl cellulose (HPC) is a non-digestible water-soluble polysaccharide used in various food, cosmetic, and pharmaceutical applications. In the current study, the aqueous solutions of six HPC grades, with molecular mass ranging from 40 to 870 kDa, were characterized with respect to their precipitation temperatures, interfacial tensions (IFTs), rheological properties and emulsifying and stabilization ability in palm (PO) and sunflower (SFO) oil emulsions. The main conclusions from the obtained results are as follows: (1) Emulsion drop size follows a master curve as a function of HPC concentration for all studied polymers, indicating that polymer molecular mass and solution viscosity have a secondary effect, while the primary effect is the fraction of surface-active molecules, estimated to be around 1–2% for all polymers. (2) Stable emulsions were obtained only with HPC polymers with Mw ≥ 400 kDa at concentrations approximately 3.5 times higher than the critical overlap concentration, c*. At PO concentrations beyond 40 wt. % or when the temperature was 25 °C, these emulsions appeared as highly viscous liquids or non-flowing gels. (3) HPC polymers with Mw < 90 kDa were unable to form stable emulsions, as the surface-active molecules cannot provide steric stabilization even at c ≳ 4–5 c*, resulting in drop creaming and coalescence during storage.