Eutectic Research Articles

Eutectic mixtures of nevirapine: Phase diagrams, solid-state characterization, and dissolution studies

Solid-state modifications can improve drug performance. Studies have shown that multicomponent crystals, such as cocrystals and eutectic compositions, have successfully improved the performance of certain drugs, including their solubility. Nevirapine (NEV) is an antiretroviral drug with low aqueous solubility, impacting its bioavailability. This work aimed to study different nevirapine/co-former solid eutectic systems, define their phase diagrams and evaluate their dissolution properties. Caffeine (CAF), Theobromine (TEOB), and Theophylline (THEO) were chosen as co-formers due to their functional groups that can interact with NEV. Aiming to determine both temperature and eutectic composition, phase and Tamman diagrams were obtained using the differential scanning calorimetry (DSC) analysis of the mixtures indifferent NEV-co-former compositions (%w/w). Powder X-ray diffraction (PXRD) and DSC were used to characterize the eutectic materials. To assess the influence of eutectic systems on dissolution properties, we determined the powder dissolution profiles and intrinsic dissolution rates of anhydrous NEV and eutectic systems NEV-CAF and NEV-THEO using different dissolution media (pH 1.2 and pH 6.8). The eutectic compositions were calculated through the phase diagrams, interpolating the curves obtained by linear regression. Thus, the eutectic composition of the NEV-CAF system was determined to be 36.55 % NEV and eutectic temperature at 201.1 °C, while in the NEV-THEO system, the eutectic was obtained in a composition of 71.04 % NEV and eutectic temperature at 217.8 °C. Tamman diagrams were generated using the enthalpy values obtained from the DSC curves, and eutectic compositions were calculated using linear regression. The analysis revealed a eutectic composition of 36.51 % of NEV for the NEV-CAF system, and a eutectic composition of 70.94 % of NEV for the NEV-THEO system. It was not possible to determine the eutectic mass fraction of NEV-TEOB. A comparison of dissolution profiles and intrinsic dissolution rates in different dissolution media showed a significant improvement in the NEV dissolution rate in both eutectic systems. In an acidic medium, NEV dissolved 16 times faster in the NEV-CAF sample and 4 times faster in the NEV-THEO sample compared to pure anhydrous NEV. In a neutral medium, the dissolution profile of NEV was even more favorable in the same eutectic systems, showing that the increase in dissolution is relevant in a wide range of pH. In addition, the intrinsic dissolution rate in the two eutectic systems was higher than that of anhydrous NEV in all employed dissolution mediums. These eutectic systems improve the dissolution rate compared to pure NEV, offering the potential for enhancing the dissolution of poorly water-soluble drugs in the future.

Activation of muscle amine functional groups using eutectic mixture to enhance tissue adhesiveness of injectable, conductive and therapeutic granular hydrogel for diabetic ulcer regeneration

Herein, Polydopamine-modified microgels and microgels incorporated with superficial epoxy groups were synthesized and applied as precursors for the fabrication of four granular hydrogels. To enhance the tissue adhesiveness, a ternary deep eutectic solvent was synthesized to activate the muscle amine functional groups facilitating the formation of robust NC bonds at ambient conditions. At a certain shear rate of 10 s−1, hydrogel DMG displayed a viscosity of 9×103 Pa/s, representing the highest complex viscosity among the tested hydrogels primarily driven by quinone groups in PDA which enhanced reversible interactions, thereby increasing particle cohesion. Moreover, the intersection point escalating from about 4×103 to approximately 9×104 as the concentration of DMG increased from 0 % (for MG) to 70% (7D3MG) by weight. There was a decrease in adhesion strength from 0.45 ± 0.08 N in MG to 0.39 ± 0.16 N, 0.35± 0.18 N, and 0.33 ± 0.15 N for 3D7MG, 7D3MG, and DMG respectively, suggesting that MG was capable of forming numerous covalent bonds, thereby enhancing its adhesion to the substrate. The type of eutectic mixture affected the electrical conductivity and a very important point was the changes in resistance value with time. For MG catalyzed by [DES]AZG, the resistance increased only by 1.3 % (from 3.37 to 3.81 kΩ) at day 3 and 37.09 % (from 3.37 to 4.62 kΩ) at day 5. The 3D7MG hydrogel exhibited superior therapeutic efficacy toward diabetic wound regeneration. The proliferation index value for 3D7MG-[DES]AZG and 3D7MG-[DES]AG were calculated 42.3 % and 58.6 %, respectively, while the control group exhibited a lower value of 37.8 %.

Enabling Gradient-Structured Solid Electrolyte Interphase by a Hydrated Eutectic Electrolyte for High-Performance Zn Metal Batteries.

Aqueous Zn metal batteries are attracting tremendous interest as promising energy storage systems due to their intrinsic safety and cost-effectiveness. Nevertheless, the reversibility of Zn metal anodes (ZMAs) is hindered by water-induced parasitic reactions and dendrite growth. Herein, a novel hydrated eutectic electrolyte (HEE) consisting of Zn(BF4)2·xH2O and sulfolane (SL) is developed to prevent the side reactions and achieve the outstanding cyclability of ZMAs. The strong coordination between Zn2+ and SL triggers the eutectic feature, enabling the low-temperature availability of HEEs. The restriction of BF4 - hydrolysis in the eutectic system can realize favorable compatibility between Zn(BF4)2-based electrolyte and ZMAs. Besides, the newly-established solvation structure with the participation of SL, H2O, and BF4 -, can induce in situ formation of desirable SEI with gradient structure consisting of B,O-rich species, ZnS, and ZnF2, to offer satisfactory protection toward ZMAs. Consequently, the HEE allows the Zn||Zn symmetric cell to cycle over 1650h at 2mAcm-2 and 1mAhcm-2. Moreover, the Zn||NH4V4O10 full batteries can deliver a prolonged lifespan for 1000 cycles with a high capacity retention of 83.4%. This work represents a feasible approach toward the elaborate design of advanced electrolyte systems for next-generation batteries.

Carbonisation of lignin in the presence of a eutectic salt mixture: Identifying the lignin properties that govern the characteristics of the resulting carbon material

Numerous studies have explored the behaviour of various lignin types during carbonisation. Yet, misconceptions persist regarding the effects arising from both the source plant and production method of lignin. Lignin is often referenced merely by type, lacking detailed characterisation. Our research examines the properties of three lignin types, elucidates their behaviour during pyrolysis, and establishes the structure-property correlation and interaction of lignin with deep eutectic solvents. By combining several analytical techniques — including NMR, FTIR, XPS, TGA-MS for functional group detection, and HPLC, EA, and ICP for compositional analysis, alongside particle size distribution and SEC for morphology — we can conduct a thorough analysis that facilitates a meaningful comparison across lignin types. This approach allows for better control over the desired carbon properties. Furthermore, we demonstrate how modification with deep eutectic solvents enables the production of biochar from different lignin types, exhibiting properties conducive to large-scale, one-step sustainable production of biochar.

Controlling the dielectric constant values of nematic liquid crystals using ferroelectric and multiferroic nanoparticles

Notably, the simultaneous manifestation of ferromagnetic and ferroelectric properties at ambient temperature has spurred significant interest in incorporating multiferroic bismuth ferrite nanoparticles (BiFeO3) into liquid crystal matrices. In this study, the parallel and perpendicular dielectric components ( and ) and dielectric anisotropy () of two different nanoparticles, barium titanate (BaTiO3) and BiFeO3, doped into E7, were measured at various temperatures and frequencies. Nematic mixture E7, a widely utilized nematic liquid crystal in research and display technologies, is a eutectic mixture composed of four cyanobiphenyl derivatives i.e. 5CB, 7CB, 8OCB, and 5CT. The findings indicate a substantial reduction in dielectric anisotropy upon introducing nanoparticles into the nematic liquid crystal environment. This research highlights the significant influence of barium titanate nanoparticles, which are smaller (approximately 12 nm), on the dielectric constant. This effect was analyzed based on varying concentrations and compared to a nanoparticle with ferroic properties. Moreover, the dielectric constant can be modulated through an alternative approach. Specifically, applying an electric field during the filling process of liquid crystal cells leads to changes in the dielectric constant. These changes, observed for both nanoparticles, were particularly evident at a precise concentration of 0.1 wt%, resulting in a decreased dielectric constant.

Low-melting mixture solvents as novel solubilizing agents: Improving tolvaptan formulation and therapeutic efficacy

Presently, a significant number of pharmaceutical drugs exhibit poor solubility in water and fall within categories II and IV of the biopharmaceutical classification system. This predicament has prompted the exploration for novel solvents or excipients to enhance the solubility and bioavailability of the active compounds in aqueous environments. Low-melting mixture solvents (LoMMSs) have gained recognition as potential solvents for active pharmaceutical ingredients to improve therapeutic efficacy and delivery. This study focuses on the preparation of a eutectic mixture composed of betaine, malic acid, proline, and water (BE-MA-PR-WA) to enhance the formulation options for tolvaptan, an FDA-approved medication used to treat acute congestive heart failure, which has limited solubility in aqueous solutions. The findings presented in this article represent the first reported results on the administration of LoMMS-solubilized tolvaptan. Furthermore, an examination of the solubility and bioavailability of tolvaptan utilizing this mixture was conducted, alongside an assessment of its toxicity. The solubility of tolvaptan in BE-MA-PR-WA (1:1:1:7) increased 483 times compared to solubility in water. Toxicity studies indicated that the LoMMS formulation BE-MA-PR-WA was safe for oral administration. The oral bioavailability of tolvaptan was significantly increased by 1.9-fold in the BE-MA-PR-WA than that in water. In addition, the 5-hour urine volume of rats indicated that BE-MA-PR-WA also enhanced the diuretic effects of tolvaptan in rats. The findings suggest that BE-MA-PR-WA enhance the solubility and bioavailability of tolvaptan, and are deemed non-toxic. Consequently, LoMMS may be deemed promising solubilizing agents, potentially facilitating the absorption of poorly bioavailable medications.

Assessment of oxidative stress biomarkers in Palaemon varians exposed to deep eutectic systems.

In recent years, there has been extensive research within the scientific community on deep eutectic systems due to their remarkable versatility in solubilizing diverse substances and serving as effective solvents in catalytic processes. While initially regarded as non-toxic, a comprehensive toxicological assessment is essential to comprehend their behavior within organisms. In this study, seven distinct systems, composed of N,N,N-triethyl-N-(2,3-dihydroxypropyl)ammonium chloride (N00Cl) and glycerol-derived ethers with alkyl chains of varying lengths (100, 200, 3F00, 300, 3i00, and 400), in a 1:2 molar ratio were investigated for their aquatic toxicity in shrimp (Palaemon varians). The assessment involved analyzing oxidative stress biomarkers such as glutathione S-transferase, glutathione peroxidase, catalase, superoxide dismutase, total antioxidant capacity (TAC), and lipoperoxidation (MDA content). Results show an odd-even effect for LC50 values being N00Cl-300, the system showing higher values. Regarding oxidative stress, an imbalance between reactive oxygen species (ROS) and antioxidant capacity in the organisms has been observed, suggesting significant toxicity to shrimps due to the changes in oxidative stress biomarkers at high concentrations. However, at 100mg/l all systems can be considered environmentally safe, and no negative impacts are expected on aquatic ecosystems.

Efficient synthesis of highly hydrophobic lignin nanoparticles and their application as nano fillers for multifunctional composite membranes

Controlling the hydrophobic behaviors of lignin nanoparticles (LNPs) is crucial and advantageous for their application as film Nano Fillers, yet it poses a significant challenge. Herein, we successfully developed a novel method for the preparation of LNPs with highly hydrophobic behaviors using ternary deep eutectic solution (DES)-H2O systems. The resulting LNPs exhibit a significantly reduced content of hydrophilic groups on their outer surface, leading to a zeta potential value of only −4.9 mV, and up to 140.0° of water contact angle (WCA). Afterwards, a “Dissolution-Restructuration” mechanism was proposed to explain the formation of the prepared LNPs. Firstly, DES demonstrates superior H-bonding capabilities, facilitating the complete disruption of inter- and intramolecular H-bonding in lignin, and resulting in the formation of a highly homogenized lignin network. Subsequently, the DES system undergoes compromise after adding water, triggering the reformation of both inter- and intramolecular H-bonding and π-π interactions. Consequently, lignin orderly self-assembles into LNPs with highly hydrophobic characteristics. Especially, using the as-prepared LNPs as Nano fillers, a series of LNPs-containing polyvinyl alcohol (PVA) films were successfully fabricated, exhibiting exceptional hydrophobic behaviors (with contact angles reaching up to 124.0°). Furthermore, the mechanical strength, UV-shielding capabilities, and biodegradability of the PVA films are significantly enhanced. This study introduces a sustainable and efficient approach for the synthesis of hydrophobic LNPs, thereby facilitating the broadening of applications for lignin-based functional composite film materials.

Unraveling the exceptional kinetics of Zn||organic batteries in hydrated deep eutectic solution

Intuitively, the solvation structure featuring stronger interacted sheath in deep eutectic solution (DES) electrolyte would result in sluggish interfacial charge transfer and intense polarization, which obstructs its practical application in emerging Zn based batteries. Unexpectedly, here we discover a Zn||organic battery with exceptional kinetics properties enabled by a hydrated DES electrolyte, which can render higher discharge capacity, smaller voltage polarization, and faster kinetics of charge transfer in comparison with conventional aqueous 3 M ZnCl2 electrolyte, though its viscosity is two orders of magnitude higher than the latter. The improved kinetics of charge transfer and ion diffusion is demonstrated to originate from the local electron structure regulation of cathode in hydrated DES electrolyte. Furthermore, the DES electrolyte has also been shown to restrict parasitic reaction associated with active water by preferential urea-molecular adsorption on Zn surface and stronger water trapping in solvation structure, giving rise to long-term stable dendrite-free Zn plating/stripping. This work provides a new rationale for understanding electrochemical behaviors of organic cathodes in DES electrolyte, which is conducive to the development of high-performance Zn||organic batteries.

Phase-field modeling of peritectic coupled growth in the TRIS–NPG model system

Microstructures forming during the directional solidification of the hypo-peritectic TRIS–NPG model alloy were studied by phase-field simulations. Steady state growth forms could be obtained under conditions similar to those in recent space experiments. In two dimensions, the wavelength range of stable lamellar structures has been determined as function of the temperature gradient and the melt composition. Front temperatures extracted from simulations were compared to the predictions of the modified Jackson–Hunt theory. In three dimensions, structures and phenomena known from eutectic systems were reproduced. Transitions between lamellar and rod structures were induced by slowly changing the melt composition. The regularizing effect of tilting the temperature gradient, transforming a random labyrinth pattern to an ordered one, has also been demonstrated. These simulations show the universality of the basic governing mechanism — that is, the competition of solute diffusion with capillary effects — in the pattern formation of these multi-phase systems.

The feasibility of activated carbon derived from waste seaweed via molten salt activation in a eutectic mixture of ZnCl2-NaCl-KCl for adsorption of anionic dyes

The feasibility of activated carbon derived from waste seaweed via molten salt activation in a eutectic mixture of ZnCl2-NaCl-KCl for adsorption of anionic dyes

Phase and structural transformations during heating of multilayer Ti/Cu foils of eutectic composition obtained by the EBPVD method

Phase and structural transformations during heating of multilayer Ti/Cu foils of eutectic composition obtained by the EBPVD method

Determination of mometasone furoate solubility, using deep eutectic systems, and topical formulation- experimental and computational studies

The low solubility of some drugs in aqueous medium, is currently a challenge for the pharmaceutical industry, affecting drug bioavailability and treatment efficacy, so there is the need to find new solutions for this challenge. In this work, the solubility of a glucocorticoid drug, mometasone furoate (MF), in different deep eutectic systems (DES) is studied. DES present the advantage of being sustainable solvents, which can be used directly in the final formulation, and establish a synergistic effect with the drug. The solubility of MF in different DES is increased, when compared to the solubility of this drug in an aqueous medium, particularly in DES composed by levulinic acid and terpenes such as menthol and thymol, where the solubility of MF was of 24.1 and 103.3 mgMF/gDES, respectively. Solubility was also assessed using the computational determination of the Hansen solubility parameters of MF in these DES, in particular the total solubility parameter (δT) and Relative Energy Difference (RED). The obtained results correlate well with the MF measured experimentally, showing that this computational tool can be further used to predict MF solubility in DES. Furthermore, oil-in-water emulsions containing DES with MF were developed, allowing more versatility compared to the drug in powdered form, and were prepared containing 1 and 5 % of DES with MF. This work highlights that DES can have an important role in drug solubilization and in the development of new formulation strategies for the pharmaceutical industries.

Surface Reaction-Diffusion-Coupled Simulation of Ni–Fe–Cr Alloy under FLiNaK Molten Salt

A molten salt reactor is one of the fourth-generation reactors and is considered to be a feasible replacement for current reactors due to their many advantages. However, there are a number of issues that remain; one of which is the corrosion of the materials. Corrosion problems in molten salt reactors have been reported since The Molten Salt Reactor Experiment at Oak Ridge National Laboratory in the 1960s. There have been many attempts to mitigate the corrosion problem, but a fundamental solution has not yet been achived. In this study, surface reaction-diffusion-coupled simulations were performed to simulate the corrosion of a Ni–Cr–Fe material, a prototype of Hastelloy N, which is being promoted as a structural material for molten salt reactors in F–Li–Na–K eutectic salts. This surface reaction-diffusion-coupled simulation framework was developed to study which corrosion reactions are dominant in molten salt environment corrosion where a large number of oxidation–reduction reactions exist, the correlation between composition of alloy and corrosion rate, and the effect of Cr depletion on corrosion.

Valorization of Waste Wool to Keratin with a Green Solvent Based on a Deep Eutectic Mixture of Choline Chloride and Lactic Acid

Abstract Waste wool, a by-product of sheep husbandry, is primarily composed of keratin, which has potential use in cosmetic products, biotechnology and medical applications. Traditional chemical methods for keratin extraction face limitations due to health and environmental concerns, and thus, green alternatives such as deep eutectic solvents (DESs) have been developed. The aim of the present study was to determine whether a natural DES composed of choline chloride and lactic acid can be used to extract keratin from sheep wool. The dissolution and keratin yield were investigated under different reaction times, temperatures, and wool-to-DES ratios. Fractions of water-soluble keratin and water-insoluble keratin aggregates were obtained by dialysis, centrifugation, and drying. The results showed that both the dissolution of wool and the yield of recovered keratin increased as the reaction time and temperature increased from 1 to 8 h and from 80 to 110 $$^{\circ }$$ ∘ C, respectively. With the longest reaction time of 8 hours and the highest temperature of 110 $$^{\circ }$$ ∘ C, the yield of water-soluble keratin increased to 23 %. Characterizations revealed that DES detached first the cuticular scales from the surface of wool fibres and subsequently degraded wool cortex layer. The extracted water-soluble keratin retained the characteristic amide structure of wool, but the secondary structure of keratin was converted to cross-$$\beta$$ β sheet form. Moreover, it had a narrow size distribution and a molecular weight of 10 kDa. The results of this study indicate that the proposed DES can be used as a green solvent for recovering keratin from sheep wool. Graphical Abstract

Strong and stable ultrafine-grained Al-Mg alloys via polymer-derived in situ nanoparticles

Ultrafine-grained Al-Mg alloys provide immense promise as lightweight metallic materials for load-bearing applications, contributing significantly to energy efficiency and CO2 emission reduction. However, for prevalent bulk ultrafine-grained Al-Mg alloys fabricated by a combined mechanical alloying and thermomechanical consolidation, the presence of remaining polymers, which are used as a process control agent during milling, jeopardizes the mechanical performance of the consolidated materials and thus significantly limit their broad applications. An ideal solution to overcome this critical issue is to turn these polymers into useful nanoparticles, enabling simultaneous improvements in strength and grain size stability. In this study, we present an approach to currently achieve strength enhancement and grain size control by incorporating polymer-derived in situ nanoparticles. Through a combined process involving polymer-assisted mechanical alloying, extrusion at the eutectic temperature, and annealing-driven solid-state phase transformation, we achieved a strong and stable ultrafine-grained Al-Mg alloy. After post-heat treatment at 440 °C (i.e. 0.76 Tm; Tm is the absolute melting temperature of pure Al) for 24 h, the compressive yield strength exhibited minimal reduction from ∼1.3 GPa for the as-extruded alloy to ∼1 GPa for the as-annealed alloy. Similarly, the average grain size only experienced a slight increase, from 106 to 145 nm after post-annealing. These findings offer practical insights into the design of mechanically robust and thermodynamically stable ultrafine-grained metallic materials via a new strategy of polymer-derived in situ nanoparticles.

Theoretical insights into the diverse stabilizing effects of fenchyl alcohol/thymol eutectic solvent on carotenoids

Carotenoids, a class of vital natural pigments, are known for their antioxidant properties and are integral to various industrial applications. However, their instability during processing and storage has limited their broader utilization. This study investigated the stabilizing effects of a fenchyl alcohol/thymol-based deep eutectic solvent (DES) on a spectrum of carotenoids, including lutein, zeaxanthin, astaxanthin, and β-carotene. Through a combination of ab initio molecular dynamics (AIMD) simulations and quantum chemical calculations, we elucidated the molecular interactions and stabilization mechanisms within the DES environment (methanol as the benchmark). Our findings reveal that the DES could exert varying degrees of influence on the stability of different types of carotenoids. The temperature-dependent behavior of carotenoids in the DES suggested a complex interplay between hydrogen bonding, dispersion forces, and the inherent structural properties of carotenoids. AIMD simulations provided a detailed picture of the structural organization and hydrogen bond network topology within the DES, highlighting the role of temperature on the stability and orientation of solvent molecules around carotenoids. Energy decomposition analysis (EDA), Hirshfeld surface analysis, and visual study of interactions further dissected the non-covalent interactions, emphasizing the significance of electrostatic and dispersion forces-driven non-covalent interactions (van der Waals dispersion interactions) in stabilizing carotenoids within the DES matrix. In comparison to methanol, the interactions between components of the eutectic system and the target molecules demonstrate enhanced dynamism, increased complementarity, and a greater variety of interaction patterns. This research presented a novel perspective on the use of DESs for the stabilization of carotenoids, offering potential solutions to enhance their industrial applicability and performance. The insights gained from this study are crucial for the development of sustainable and eco-friendly solvent systems, aligning with the growing demand for green chemistry in the pharmaceutical, cosmetic, and food industries.

Thermophysical Properties' Enhancement of LiNO3-NaNO3-KNO3-NaNO2-KNO2 Mixed with SiO2/MgO Nanoparticles.

The aim of this study is to further enhance the thermal storage and heat transfer performances of a low-melting-point quinary salt. The eutectic salt was prepared using LiNO3, NaNO3, KNO3, NaNO2, and KNO2 as raw materials, followed by the doping of nano-SiO2 and nano-MgO into the base salt using a microwave-assisted method. The thermal properties of the samples were analyzed using a Synchronous Thermal Analyzer and a Laser Flash Apparatus. The co-doping of two types of nanoparticles was found to significantly enhance the specific heat capacity of the base salt. The maximum specific heat reached 2.36 J/(g·K), showing a 50.4% increase compared to the base salt. The thermal conductivity of molten salts can be affected by nanoparticles. An observed sample demonstrated a thermal diffusivity of 0.286 mm2/s, indicating a 19.2% improvement over the base salt, which may be attributed to enhanced phonon thermal efficiency. In addition, this study revealed that while interfacial thermal resistance can enhance specific heat capacity, it can also lead to a decrease in the thermal conductivity efficiency of materials. This work can offer insights and references for the enhancement of molten salt properties.

Phase equilibrium investigations and thermodynamic modelling of the ZrO2-HfO2-Ta2O5 system

The phase equilibria in the ZrO2-HfO2-Ta2O5 system were studied at 1673 K and 1873 K by equilibration technique and in the temperature range up to 2473 K by differential thermal analysis (DTA). The phase and chemical composition of the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX). Melting relations were studied by DTA followed by microstructural investigation by SEM/EDX. The composition dependence of the eutectic and peritectic temperatures was shown. The heat capacity of the (Zr,Hf)6Ta2O17 compound with different compositions was measured by differential scanning calorimetry (DSC) in three temperature ranges from 100 K to 230 K, from 220 K to 570 K, and from 330 K to 1295 K. Based on the obtained results and our previous thermodynamic descriptions, a database for the ternary ZrO2-HfO2-Ta2O5 system using the CALPHAD approach was created.

Heterostructured fluoride-based solid electrolytes engineered by grain boundary softening and bonding for sustainable Na metal batteries

Fluoride solid electrolytes stand out as promising candidates for high-energy-density solid-state batteries owing to their inherent structural and air stability. However, their application in sodium based batteries has been hindered by their low ionic conductivity. Here, we propose a heterostructured fluoride-based solid electrolyte engineered by grain boundary softening and bonding for sustainable Na metal batteries. By leveraging a low-melting-point chloride eutectic mixture NaCl-1.1AlCl3 (NA), we achieve the in situ activation of sodium ion transport at the grain boundaries of Na3GaF6 (NGF), with the formation of a molten salt interphase primarily composed of NaAlCl4. This process effectively softens the edges of NGF and heals the intergranular voids. The NA decorated NGF electrolyte exhibits the significantly enhanced ionic conductivities of 2.7 × 10–4 S/cm at 60 °C and 2.45 × 10–5 S/cm at room temperature, representing two orders of magnitude improvement over pure NGF. The chloride shell does not degrade the wide electrochemical window and air stability of NGF. This electrolyte relies on a unique "self-protection" mechanism when cycling with sodium metal anode. The molten salt phase can evolve into a compact NaCl protective shell and an Na-Al conductive inner layer with tight contact with Na anode. This dual-layer structure mitigates the side reaction with NGF host, homogenizes the current flow, and suppresses the growth of sodium dendrites. The corresponding Na//Na symmetric cells display the small voltage polarization and stable cycling performance for over 1000 h. The solid-state Na//FeF3 batteries deliver a high capacity of 308 mAh/g and sustain for at least 100 cycles with high capacity retention based on conversion reaction.