Using Excimeric Fluorescence to Study How the Cooling Rate Determines the Behavior of Naphthalenes in Freeze-Concentrated Solutions: Vitrification and Crystallization

Spectroscopic characterization of phenol in frozen aqueous solution and on the ice surface

Phenol is one of the omnipresent pollutants in the environment, frequently detected in ambient air, water, soil, snow, and ice. Due to its low aqueous reactivity and inability to undergo direct photolysis under typical tropospheric conditions, phenol can be widely distributed and accumulated in the environment for an extended period of time. However, the reactivity of phenol can be influenced by a number of factors, including temperature, pH, and phase transitions. The present study examines the impact of the ice matrix and ice surface on the photophysical properties of phenol via UV-VIS, fluorescence, and Raman spectroscopies. We demonstrate that the freezing of an aqueous solution results in the vitrification or crystallization of the freeze-concentrated solution. The latter case is accompanied by a bathochromic shift of the absorption spectrum above 290 nm. The most pronounced red-shifts were obtained for deprotonated and crystalline samples, which suggests that direct photolysis under tropospheric conditions would be significantly enhanced in these cases. The study further demonstrates the difference in the molecular arrangement in freeze-concentrated solutions as compared to the surface of Ih ice.

Inorganic substrates in frozen solutions: Transformation mechanisms and interactions with organic compounds - A review

In cold environments, such as polar regions and high latitudes, the freezing of aqueous solutions plays a crucial role in releasing and transforming nutrients, organic compounds, and trace gases. Freezing processes typically affect biogeochemical cycles and environmental processes by reducing the rate of chemical reactions. However, substantial studies have found that some chemical reactions may accelerate unexpectedly under freezing conditions. These reactions include oxidation of nitrite, dissolution of metals/metal oxides, transformation of halogen species, etc. Although freezing process significantly affects the interaction between the inorganic substrate and coexisting organic compounds, there are few review articles on the behavior of the inorganic compound. Therefore, this review examines the transformation behavior of inorganic substrates and their interactions with organic compounds during freezing. The transformation behavior of inorganic substrates during freezing was comprehensively discussed, their underlying mechanisms were elucidated, and the interactions between inorganic substrates and coexisting organic compounds were highlighted. Meanwhile, key factors influencing the freeze-induced chemical processes were articulated. Furthermore, the potential application of freezing reactions in engineering processes is explored. This article aims to improve understanding of the important role of freezing processes in the recycling of substrates in the natural environment and supplement knowledge in the field of ice chemistry.

Impact of secondary ice in a frozen NaCl freeze-concentrated solution on the extent of methylene blue aggregation

Freezing and lyophilization have been utilized for decades to stabilize pharmaceutical and food products. Freezing a solution that contains dissolved salt and/or organic matter produces pure primary ice crystal grains separated by freeze-concentrated solutions (FCS). The microscopic size of the primary ice crystals depends on the cooling conditions and the concentration of the solutes. It is generally accepted that primary ice crystals size influences the rate of sublimation and also can impact physico-chemical behaviour of the species in the FCS. This article, however, presents a case where the secondary ice formed inside the FCS plays a critical role. We microscoped the structures of ice-cast FCS with an environmental scanning electron microscope and applied the aggregation-sensitive spectroscopic probe methylene blue to determine how the microstructure affects the molecular arrangement. We show that slow cooling at -50 °C produces large salt crystals with a small specific surface, resulting in a high degree of molecular aggregation within the FCS. In contrast, fast liquid nitrogen cooling yields an ultrafine structure of salt crystals having a large specific surface area and, therefore, inducing smaller aggregation. The study highlights a critical role of secondary ice in solute aggregation and introduces methylene blue as a molecular probe to investigate freezing behaviour of aqueous systems with crystalline solute.

Fucoxanthin Loaded in Palm Stearin- and Cholesterol-Based Solid Lipid Nanoparticle-Microcapsules, with Improved Stability and Bioavailability In Vivo

Fucoxanthin (FX) is a marine carotenoid that has proven to be a promising marine drug due to the multiple bioactivities it possesses. However, the instability and poor bioavailability of FX greatly limit its application in pharmaceuticals or functional foods. In this study, the creative construction of a solid lipid nanoparticle-microcapsule delivery system using mixed lipids of palm stearin and cholesterol wrapped with gelatin/Arabic gum to load lipophilic FX was fabricated, aiming to improve the stability and bioavailability of FX. The results showed that the encapsulated efficiency (EE) and drug loading capacity (LC) of optimized FX microcapsules (FX-MCs) obtained were as high as 96.24 ± 4.60% and 0.85 ± 0.04%, respectively, after single-factor experiments. The average particle size was 1154 ± 54 nm with negative Zeta potential (−20.71 ± 0.93 mV) as depicted with size-zeta potential spectrometer. The differential scanning calorimeter (DSC) and thermogravimetric analyzer (TG) results indicated that FX-MC has a higher Tg and slower weight loss than FX monomers (FX crystal) and blank MCs. Besides, The Fourier transform infrared spectrometer (FTIR) confirmed the good double encapsulation of FX into the solid lipid and composite coacervate. Moreover, the encapsulated FX showed higher storage stability, sustained release (55.02 ± 2.80% release in 8 h), and significantly improved bioavailability (712.33%) when compared to free FX. The research results can provide a principle theoretical basis for the development and application of FX in pharmaceuticals or functional foods.