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

Confined Crystallization and Melting Behaviors of 3-Pentadecylphenol in Anodic Alumina Oxide Nanopores

To explore the effects of end groups on the confined crystallization of an alkyl chain, 3-pentadecylphenol (PDP) was infiltrated into the anodic aluminum oxide template (AAO) to investigate the melting and crystallization behaviors of PDP in a nanoconfined environment. Wide-angle X-ray diffraction (WAXD) found that the solid-solid phase transition of PDP occurred under confined conditions, and the absence of the (00L) reflections indicated that the stacking of the end groups of the alkyl chain layered structure was seriously disturbed. Thermal analysis (TG) showed that the thermal stability of the confined samples decreased due to the confinement effect, and the introduction of end groups made the confinement effect more obvious. Differential scanning calorimeter (DSC) results well reflected the space-time equivalence in the PDP crystallization processes, i.e., the solid-solid phase transition can be achieved by reducing the cooling rate or confining PDP in the nanometer space. Compared with C15, the introduction of the end groups with a phenol ring led to the disappearance of the solid-solid phase transition of an alkyl chain at high cooling rates. In the confined environment, the introduction of the end groups with a phenol ring caused the melting double peaks of the alkyl chain to become a single melting peak, and it also caused the disappearance of the surface freezing monolayer for alkyl chains. Through the analysis of crystallinity, it was found that AAO-PDP was more sensitive to AAO pore size changes than AAO-C15, the X c of AAO-PDP had a good linear relationship with the pore size d, but the X c of the AAO-C15 had a nonlinear relationship with the pore size d. Attenuated total reflection (ATR)-IR proved that in the confined environment, the order of the alkyl chain decreased and the degree of chain distortion increased.

Unusual Interfacial Freezing Phenomena in Hexacontane/Silica Composites

The crystallization behaviors of n-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 n-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.