Relaxations in the metastable rotator phase of n-eicosane

MUF-n-Octadecane Phase-Change Microcapsules: Effects of Core pH and Core-Wall Ratio on Morphology and Thermal Properties of Microcapsules

Phase change energy storage microcapsules were synthesized in situ by using melamine-formaldehyde-urea co-condensation resin (MUF) as wall material, n-octadecane (C18) as core material and styryl-maleic anhydride copolymer (SMA) as emulsifier. Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry and thermogravimetric analysis were used to study the effects of emulsifier type, emulsifier dosage, core-wall ratio and pH on the morphology and thermal properties of microcapsules. The results show that the pH of core material and the ratio of core to wall have a great influence on the performance of microcapsules. SMA emulsifiers and MUF are suitable for the encapsulation of C18. When the pH is 4.5 and the core-wall ratio is 2/1, the latent heat and encapsulation efficiency of phase transition reaches 207.3 J g-1 and 84.7%, respectively. The prepared phase-change microcapsules also have good shape stability and thermal stability.

Structure of the Hexadecane Rotator Phase: Combination of X-ray Spectra and Molecular Dynamics Simulation

Rotator phases are rotationally disordered plastic crystals, some of which can form upon freezing of alkane at alkane-water interfaces. Existing X-ray diffraction studies show only partial unit cell information for rotator phases of some alkanes. This includes the rotator phase of n-hexadecane, which is a transient metastable phase in pure alkane systems, but shows remarkable stability at interfaces when mediated by a surfactant. Here, we combine synchrotron X-ray diffraction data and molecular dynamics (MD) simulations, reporting clear evidence of the face-centered orthorhombic RI rotator phase from spectra for two hexadecane emulsions, one stabilized by Brij C10 and another by Tween 40 surfactants. MD simulations of pure hexadecane use the recently developed Williams 7B force field, which is capable of reproducing crystal-to-rotator phase transitions, and it also predicts the crystal structure of the RI phase. Full unit cell information is obtained by combining unit cell dimensions from synchrotron data and molecular orientations from MD simulations. A unit cell model of the RI phase is produced in the crystallographic information file (CIF) format, with each molecule represented by a superposition of four rotational positions, each with 25% occupancy. Powder diffraction spectra computed using this model are in good agreement with the experimental spectra.

Immobilizing diamine oxidase on electroactive phase-change microcapsules to construct thermoregulatory smart biosensor for enhancing detection of histamine in foods

Aiming at enhancing the biosensing detection of histamine in foods at high temperature, we developed a thermoregulatory biosensor based on diamine oxidase-immobilized phase-change microcapsules consisting of an n-docosane core, a TiO₂ shell, and an electroactive polyaniline/ZnO composite coating layer. The microcapsules exhibit a satisfactory latent heat capacity of over 112 J/g for thermo-temperature regulation. Through an innovative integration of electroactive phase-change microcapsules and biological enzyme in the working electrode, the biosensor obtained a thermoregulatory function through reversible phase transitions by the n-docosane core under high-temperature environments. This enables the biosensor to achieve a higher response sensitivity of 28.57 µA⋅mM^-1⋅cm^-2 and a lower detection limit of 0.473 µmol/L at the high assay temperatures compared to conventional histamine biosensors. With enhanced electrochemical biosensing performance through in-situ thermo-temperature regulation, the smart biosensor developed in this study has found practical applications for high-sensitive detection and high-accurate quantitive determination of histamine in foods across a broad temperature range.

Effect of Thermal History and Hydrocarbon Core Size on Perfluorocarbon Endoskeletal Droplet Vaporization

Vaporizable hydrocarbon-in-fluorocarbon endoskeletal droplets are a unique category of phase-change emulsions with interesting physical and thermodynamic features. Here, we show microfluidic fabrication of various morphologies, such as solid-in-liquid, liquid-in-solid, and Janus-type, of complex solid n-C₂₀H₄₂ or n-C₂₁H₄₄ and liquid n-C₅F₁₂ droplets. Furthermore, we investigated the vaporization behavior of these endoskeletal droplets, focusing on the effects of heat treatment and core size. Comparison of vaporization and differential scanning calorimetry results indicated that vaporization occurs prior to melting of the bulk hydrocarbon phase for C₂₀H₄₂/C₅F₁₀ droplets and near the rotator phase for C₂₁H₄₄/C₅F₁₀ droplets. We found that heat treatment of the droplets increased the fraction of droplets that vaporized and also increased the vaporization temperature of the droplets, although the effect was temporary. Furthermore, we found that changing the relative size of the solid hydrocarbon core compared to the surrounding liquid shell increased the vaporization temperature and the vaporizing fraction. Taken together, these data support the hypothesis that surface melting behavior exhibited by the linear alkane may trigger the fluorocarbon vaporization event. These results may aid in the understanding of the interfacial thermodynamics and transport and the engineering of novel vaporizable endoskeletal droplets for biomedical imaging and other applications.

Thermal self-regulatory intelligent biosensor based on carbon-nanotubes-decorated phase-change microcapsules for enhancement of glucose detection

Enzyme-based biosensors are sensitive to temperature due to their strong temperature dependency of catalytic activity. Aiming at enhancing biosensing detection for glucose assay over a wide range of applicable temperatures, we designed a thermal self-regulatory intelligent biosensor through an innovative integration of phase change material (PCM) and bioelectrocatalytic substances. An electroactive phase-change microcapsule system was firstly fabricated by microencapsulating n-docosane as a PCM core in the SiO₂ shell, followed by depositing polydopamine along with carbon nanotubes as an electroactive layer on the surface of SiO₂ shell. The resultant microcapsules showed a regularly spherical morphology and well-defined core-shell microstructure. They also exhibited a satisfactory latent heat capacity of around 137 J/g for implementing temperature regulation with a good working stability. An electrochemical biosensing system was constructed with the resultant electroactive microcapsules together with glucose oxidase as a redox enzyme, achieving a thermal self-regulation capability to enhance the biosensing detection of glucose under in-situ thermal management at higher temperatures. With a high sensitivity of 5.95 μA⋅mM^-1⋅cm^-2 and a lower detection limit of 13.11 μM at 60 °C, the intelligent biosensor developed by this study demonstrated a superior determination capability and better detection performance toward glucose than conventional biosensors in a high temperature region thanks to effective regulation of microenvironment temperature in the electrode system. This study provides a promising strategy for the development of thermal self-regulatory smart biosensors with an enhanced identification ability to detect various chemical substances over a wide range of applicable temperatures.

Morphology-controlled fabrication of magnetic phase-change microcapsules for synchronous efficient recovery of wastewater and waste heat

Contamination and waste heat are major issues in water pollution. Aiming at efficient synchronous recovery wastewater and waste heat, we designed a novel CaCO₃-based phase-change microcapsule system with an n-docosane core and a CaCO₃/Fe₃O₄ composite shell. The system was fabricated through an emulsion-templated in situ precipitation approach in a structure-directing mode, resulting in a controllable morphology for the resultant microcapsules, varying from a peanut hull through ellipsoid to dumbbell shapes. The system has a significantly enlarged specific surface area of approximately 55 m²·g^-1 with the CaCO₃ phase transition from vaterite to calcite. As a result, the microcapsule system exhibits improved adsorption capacities of 497.6 and 79.1 mg/g for Pb²⁺ and Rhodamine B removal, respectively, from wastewater. Moreover, increase in the specific surface area of the microcapsule system with a sufficient latent heat capacity of approximately 130 J·g^-1 also resulted in an enhanced heat energy-storage capability and thermal conductance for waste-heat recovery. The microcapsule system also exhibits a good leakage-prevention capability and good multicycle reusability owing to the tight magnetic CaCO₃/Fe₃O₄ composite shell. This study provides a promising approach for developing CaCO₃-based phase-change microcapsules with enhanced thermal energy storage and adsorption capabilities for efficient synchronous recovery of wastewater and waste heat.

Innovative Integration of Phase-Change Microcapsules with Metal-Organic Frameworks into an Intelligent Biosensing System for Enhancing Dopamine Detection

This work focuses on an interdisciplinary issue in energy management and biosensing techniques. Aiming at enhancing the biosensing detection of dopamine at high ambient temperatures, we developed an innovative integration of phase-change microcapsules with a metal-organic framework (MOF) based on zeolitic imidazolate framework-8 to develop an intelligent electrochemical biosensing system with a thermal self-regulation function. We first fabricated a type of electroactive microcapsules containing a MOF-anchored polypyrrole/SiO₂ double-layered shell and a phase-change material (PCM) core. The resultant microcapsules not only exhibit a regular spherical morphology with a layer-by-layer core-shell microstructure but also display an effective temperature-regulation capability to enhance enzymatic bioactivity under phase-change enthalpies of around 124.0 J·g^-1 along with good thermal impact resistance and excellent thermal cycling stability for long-term use in thermal energy management. These electroactive microcapsules were then used to modify a working electrode together with laccase as a biocatalyst to construct a thermal self-regulatory biosensor. With a high sensitivity of 3.541 μA·L·μmol^-1·cm^-2 and a low detection limit of 0.0069 μmol·L^-1 at 50 °C, this biosensor exhibits much better determination effectiveness toward dopamine at higher temperatures than conventional biosensors thanks to in situ thermal management derived from its PCM core in the electroactive microcapsules. This study offers a promising approach for development of intelligent thermal self-regulatory biosensors with an enhanced detection capability to identify various chemicals accurately in a wide range of applicable temperatures.

Fluorescent sensing system based on molecularly imprinted phase-change microcapsules and carbon quantum dots for high-efficient detection of tetracycline

Aiming at enhancing the detection efficiency and identification accuracy of tetracycline under a high-temperature condition, this study focuses on an innovative fluorescent sensing system (MIP@CQD-PCM) based on molecularly imprinted phase-change microcapsules along with the carbon quantum dots (CQDs) embedded in their shell. This system was fabricated by microencapsulating n-eicosane as a phase change material (PCM) core within a CQDs-embedded SiO₂ shell, followed by coating a tetracycline-templated molecularly imprinted polymer (MIP) layer onto the surface of the SiO₂ shell. The specific recognition sites to tetracycline molecules were finally achieved by removal of tetracycline template from the MIP layer. Comprehensive characterizations and investigations on the structure and performance of the fluorescent sensing system were given to confirm its successful fabrication in accordance to our design strategy. The resultant MIP@CQD-PCM exhibits a satisfactory thermal storage capacity and phase-change cycle stability for temperature regulation and thermal management applications under a phase-change enthalpy of over 162 J/g. Most of all, a typical fluorescence-quenching effect was obtained from the combination of the CQDs embedded in the SiO₂ shell and the tetracycline molecules adsorbed in the MIP layer. This makes the MIP@CQD-PCM achieve an enhanced capability for the fluorescence identification of tetracycline in a high-temperature environment through the in situ thermal management of its PCM core. The MIP@CQD-PCM also displays high selectivity and good reusability for tetracycline detection in industrial applications. This work provides a promising strategy for the design and development of fluorescent sensing systems with high recognition efficiency and identification accuracy in the detection of hazardous substances.

A theoretical investigation into the cooperativity effect on the TNT melting point under external electric field

External electric field has been regarded as an effective tool to induce the variation of melting points of molecular crystals. The melting point of 2,4,6-trinitrotoluene (TNT) was calculated by molecular dynamics simulations under external electric field, and the electric field effects on the cooperativity effects of the ternary (TNT)₃ were investigated at the M06-2X/6-311+G(d) and ωB97X-D/6-311++G(2d,p) levels. The results show that the melting points are decreased while the intermolecular interactions are strengthened under the external electric fields, suggesting that the intermolecular interactions cannot be used to explain the decreased melting points. A deduction based on the cooperativity effect is put forward: the enhanced cooperativity effects create the more serious defects in the melting process of the molecular crystal under the external electric fields, and simultaneously the local order parameters are decreased, leading to the decreased melting point. Thus, the cooperativity effect stemmed from the intermolecular C-H∙∙∙O H-bonding interactions controls the change of TNT melting point under the external electric field. Employing the information-theoretic approach (ITA), the origin of the cooperativity effects on the melting points of molecular crystal is revealed. This study opens a new way to challenge the problems involving the melting points for the molecular crystal under the external electric fields. However, note that above deduction needs to be improved; after all, the simple (TNT)₃ model cannot replace the crystal structure.