Catanionic Surfactant-Assisted Mineralization and Structural Properties of Single-Crystal-like Vaterite Hexagonal Bifrustums

Crystalline vaterite is the most thermodynamically unstable polymorph of anhydrous calcium carbonate (CaCO₃), and various morphologies can be controlled in the presence of organic additives. Constructing vaterite with minimal defects, determining its distinctive properties, and understanding the formation mechanism behind a biomimetic process are the main challenges in this field. In this paper, a unique single-crystal-like vaterite hexagonal bifrustum with two hexagonal and 12 trapezoidal faces has been fabricated through a catanionic surfactant-assisted mineralization approach for the first time. Compared with the polycrystalline vaterite aggregates, these bifrustums clearly present a doublet for Raman <i>v</i>₁ symmetric stretching mode, a low depolarizaiton ratio for carbonate molecular symmetry, and a high structural stability. These indicate a dominant position of hexagonal phase in each crystallite and confirm the Raman <i>v</i>₁ doublet characteristics of synthetic and biomineral-based vaterites. Our finding may provide evidence to distinguish vaterite with different structures and shed light on a possible formation mechanism of vaterite single crystals

Two-Step Freezing in Alkane Monolayers on Colloidal Silica Nanoparticles: From a Stretched-Liquid to an Interface-Frozen State

The crystallization behavior of an archetypical soft/hard hybrid nanocomposite, that is, an <i>n-</i>octadecane C₁₈/SiO₂-nanoparticle composite, was investigated by a combination of differential scanning calorimetry (DSC) and variable-temperature solid-state ¹³C nuclear magnetic resonance (VT solid-state ¹³C NMR) as a function of silica nanoparticles loading. Two latent heat peaks prior to bulk freezing, observed for composites with high silica loading, indicate that a sizable fraction of C₁₈ molecules involve two phase transitions unknown from the bulk C₁₈. Combined with the NMR measurements as well as experiments on alkanes and alkanols at planar amorphous silica surfaces reported in the literature, this phase behavior can be attributed to a transition toward a 2D liquid-like monolayer and subsequently a disorder-to-order transition upon cooling. The second transition results in the formation of a interface-frozen monolayer of alkane molecules with their molecular long axis parallel to the nanoparticles’ surface normal. Upon heating, the inverse phase sequence was observed, however, with a sizable thermal hysteresis in accord with the characteristics of the first-order phase transition. A thermodynamic model considering a balance of interfacial bonding, chain stretching elasticity, and entropic effects quantitatively accounts for the observed behavior. Complementary synchrotron-based wide-angle X-ray diffraction (WAXD) experiments allow us to document the strong influence of this peculiar interfacial freezing behavior on the surrounding alkane melts and in particular the nucleation of a rotator phase absent in the bulk C₁₈

Confined Crystallization of <i>n</i>‑Hexadecane Located inside Microcapsules or outside Submicrometer Silica Nanospheres: A Comparison Study

Crystallization and phase transition behaviors of <i>n</i>-hexadecane (<i>n</i>-C₁₆H₃₄, abbreviated as C₁₆) confined in microcapsules and <i>n</i>-alkane/SiO₂ nanosphere composites have been investigated by the combination of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). As evident from the DSC measurement, the surface freezing phenomenon of C₁₆ is enhanced in both the microcapsules and SiO₂ nanosphere composites because the surface-to-volume ratio is dramatically enlarged in both kinds of confinement. It is revealed from the XRD results that the novel solid–solid phase transition is observed only in the microencapsulated C₁₆, which crystallizes into a stable triclinic phase via a mestastable rotator phase (RI). For the C₁₆/SiO₂ composite, however, no novel rotator phase emerges during the cooling process, and C₁₆ crystallizes into a stable triclinic phase directly from the liquid state. Heterogeneous nucleation induced by the surface freezing phase is dominant in the microencapsulated sample and contributes to the emergence of the novel rotator phase, whereas heterogeneous nucleation induced by foreign crystallization nuclei dominates the C₁₆/SiO₂ composite, leading to phase transition behaviors similar to those of bulk C₁₆

Unusual Interfacial Freezing Phenomena in Hexacontane/Silica Composites

The crystallization behaviors of <i>n-</i>hexacontane (C₆₀H₁₂₂)/Stöber silica (SiO₂) composites with various compositions were investigated by a combination of differential scanning calorimetry (DSC), solid-state ¹³C nuclear magnetic resonance (solid-state ¹³C NMR), and proton NMR relaxation experiments. By means of DSC, C₆₀H₁₂₂ molecules in C₆₀H₁₂₂/silica composites were observed to be involved in the interfacial freezing not present in the free bulk C₆₀H₁₂₂. The orientation of C₆₀H₁₂₂ molecules, being preferentially normal to silica surface, was confirmed by grazing incidence X-ray diffraction experiments on thin <i>n-</i>hexacontane film adsorbed on the silicon wafer with a native SiO₂ layer. Inferred from the solid ¹³C NMR data, the interfacial monolayer is in orthorhombic phase with certain chain disorders. It is speculated that the “interfacial freezing” of C₆₀H₁₂₂ formed in the presence of silica particles is driven by the combination of the strong attraction between the molecules and the enhanced number of interfacial molecules on the silica surface

Oil-in-Water Emulsion Templated and Crystallization-Driven Self-Assembly Formation of Poly(l‑lactide)–Polyoxyethylene–Poly(l‑lactide) Fibers

A molecular solution of an amphiphilic block copolymer may act as an oil phase by dispersing into an aqueous micellar system of small-molecular surfactant, forming oil-in-water (O/W) emulsion droplets. In this paper, an as-synthesized triblock copolymer poly­(l-lactide)–polyoxyethylene–poly­(l-lactide) (PLLA–PEO–PLLA) was dissolved in tetrahydrofuran (THF) and then added to an aqueous micellar solution of nonaethylene glycol monododecyl ether (AEO-9), forming initially coalescent O/W emulsion droplets in the size range of 35 nm–1.3 μm. Along with gradual volatilization of THF and simultaneous concentration of PLLA–PEO–PLLA molecules, the amphiphilic copolymer backbones themselves experience solution-based self-assembly, forming inverted core–corona aggregates within an oil-phase domain. Anisotropic coalescence of adjacent O/W emulsion droplets occurs, accompanied by further volatilization of THF. The hydrophilic block crystallization of core-forming PEOs and the hydrophobic chain stretch of corona-forming PLLAs together induce the intermediate formation of rod-like architectures with an average diameter of 300–800 nm, and this leads to a large-scale deposition of the triblock copolymer fibers with an average diameter of ∼2.0 μm. Consequently, this strategy could be of general interest in the self-assembly formation of amphiphilic block copolymer fibers and could also provide access to aqueous solution crystallization of hydrophilic segments of these copolymers

Controlled Mineralization of Calcium Carbonate on the Surface of Nonpolar Organic Fibers

Isotactic polypropylene (iPP) fiber, the surface of which is hydrophobic, can modulate the crystallization polymorphs of calcium carbonate (CaCO₃) at the air/solution interface under mild conditions. The present results provide a novel perspective on controlling the crystallization of biominerals by an insoluble matrix, and they can shed new light on understanding the biomineralization process of CaCO₃ as it occurs in nature