## 2 Molten Salt Mixtures  
Thanks to their unique combination of properties, such as high thermal stability, good heat transfer capabilities, and wide operational temperature ranges, MSs are being considered for application across a spectrum of industrial and energy applications. In the chemical industry, they catalyze a variety of processes, enabling high-temperature reactions that are otherwise challenging to manage. Within nuclear reactors, MSs have been studied as coolant systems and as the medium for nuclear fuel in advanced reactor designs, promising safer and more efficient energy production. Their use extends significantly to RES, where they are now employed both as a heat transfer fluid and as a storage medium. Notably, in CSP plants, they facilitate electricity generation even when solar irradiance is absent. Beyond energy production, molten salts offer innovative solutions in waste treatment processes, where their chemical properties enable the detoxification, recovery, and recycling of valuable materials from industrial residues [10].  
The choice of salt type hinges on an understanding of the chemical and thermodynamic properties required by the different applications. Salts must exhibit high chemical and radiolytic stability at elevated temperatures, favorable freezing and boiling points, a significant specific heat capacity, and thermal conductivity and must maintain low vapor pressure to ensure operational integrity [11].  
Thermal exchange and storage materials are fundamental components of a CSP plant, affecting both the efficiency and the costs of the generated electric power [12]. Currently, MSs are the most used materials in CSP for temperatures above 100 \({}^{\circ}\)C as a heat storage medium (HSM) and a heat transfer fluid (HTF). They are attractive candidates, having high heat capacity, high density, high thermal stability, relatively low costs, no flammability, and very low vapor pressure, which leads to a storage design without pressurized vessels [12]. In general, MSs are defined as ionic liquids and classified according to anions and cations. In practice, the minimum operating temperature is mainly defined by liquidus temperature, that is, the solidification onset. Clearly, it is necessary to avoid freezing inside the piping, the heat exchanger, and the storage tanks. For this reason, an auxiliary back-up heating system is required in an MS-CSP plant.  
Nitrate/nitrite mixtures are studied and developed for the low and medium temperature of Parabolic Trough Collector (PTC) CSP. Chloride salts and their mixtures are particularly relevant as working fluids for high temperature (>400 \({}^{\circ}\)C) applications in solar power tower plants [13]. CSP with other anhydrous oxyanion salts (e.g., carbonates) and halide salts (e.g., fluorides and chlorides) is currently strongly limited: it is present only in theoretical studies and thermal analysis measurements, not in real commercial applications [14]. Indeed, its main shortcomings are a relatively very high freezing point presented by the salts and their mixtures and compatibility with containment materials, the latter problem being especially relevant for molten carbonates. However, a few molten mixtures of inorganic salts are presented in the literature as thermal energy storage (TES) for high temperatures, such as sodium hydroxide, which operates at a temperature between 320 \({}^{\circ}\)C and 800 \({}^{\circ}\)C.