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Title of paper
Manuel Rodriguez
Miami Dade College
CHM1046: General Chemistry and Qualitative Analysis 2
Yahaira Reyes
March 20th, 2025
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Abstract
Global warming has been a widely researched topic in recent years. According to Roxanne
Greitz Miller from the California Journal of Science Education, it is estimated that the mean
temperature on Earth’s surface has risen an average of 1 ℉ in the last century. A common
approach among researchers for addressing these rising temperatures is limiting or controlling
the causes of global warming. There are many factors that lead to these temperature trends, but
heat-trapping gases generated by human activities have been the main contributor over the last
half century. Some of these gases include carbon dioxide (CO₂), methane (CH₄), nitrogen dioxide
(NO₂), and ozone (O₃), which build up in the atmosphere and trap the Sun’s rays, a phenomenon
known as the greenhouse effect. Recently, there has been a growing interest in how green
chemistry can help directly or indirectly slow down the production of these gases. As defined by
the United States Environmental Protection Agency, green chemistry is a principle that promotes
reducing or eliminating the production of harmful substances during chemical processes.
Eutectic systems have been observed to be a promising approach to green chemistry due to their
ability to manipulate melting points and their applicability in energy storage.¹˒⁴ The three most
commonly used are eutectic metals (or eutectic alloys), eutectic salts, and deep eutectic solvents
(DES),¹ which are the focus of this review. In this work we will observe and analyze their role in
research projects related to green chemistry and/or green energy solutions, specifically by
connecting general chemistry concepts, such as equilibrium and Gibbs free energy, into the
applications of eutectic systems.
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Introduction
The word “eutectic” originates from metallurgy, where it is interpreted as the reduction of
melting point after mixing two substances. In very simple terms, a eutectic system is composed
of two or more substances whose individual melting points are higher than the melting point of
the combined system when mixed in specific ratios.¹ As explained by Alhadid et al., eutectic
systems can be more specifically described as mixtures of compounds that, when in the solid
state, do not dissolve into each other effectively.² Because of this, the components are
thermodynamically more stable as a mixed liquid than as separate solids. This behavior is driven
by a combination of enthalpic and entropic factors: stabilizing interactions between unlike
molecules lower the system’s enthalpy, while increased molecular disorder in the liquid phase
raises its entropy, both of which contribute to a lower Gibbs free energy for the liquid phase.¹²⁵
There are many types of eutectic systems, classified according to the types of components
involved. Each relies on different intermolecular forces that determine how and why their
melting points are depressed. In the case of eutectic alloys, eutectic salts, and deep eutectic
solvents (DES), their compositions consist of metal + metal, salt + salt, and salt + molecular
compound, respectively.¹ Their respective compositions result in varying atomic-level behaviors,
often involving enthalpic and entropic factors that affect the system’s phase stability and melting
behavior.
Eutectic alloys are composed of metals, which exist as individual atoms. Eutectic alloys
consist of metals that, when combined in specific ratios, form a single liquid phase at a
temperature lower than the melting points of the individual components.¹ These systems are
governed by metallic bonding, where delocalized electrons maintain cohesion across the metal
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lattice. While most eutectic alloys are solid at room temperature and commonly used in structural
and thermal applications, certain compositions, particularly those based on gallium, remain
liquid under ambient conditions.¹ Their unusually low melting points allow them to disrupt
passivating oxide layers on metal surfaces, enabling reactions that are otherwise kinetically
hindered. This review will focus specifically on such reactive, liquid-phase eutectic alloys and
their potential roles in green energy applications.
Eutectic salts are composed of combinations of ionic compounds, typically salts, that
interact through ionic bonding between cations and anions.¹ These systems generally display
immiscibility in the solid phase and often undergo phase separation or incongruent melting,
reinforcing their classification as eutectic mixtures.¹ They are characterized by high thermal
stability and large latent heat, making them promising candidates for thermal energy storage,
particularly in solar power and heating systems.¹⁴ Their ability to operate at elevated
temperatures without decomposition supports greener energy systems by improving efficiency
and enabling energy reuse.
Deep eutectic solvents (DES) are typically formed from a mixture of a salt and a
molecular compound, such as a hydrogen bond donor, where hydrogen bonding and van der
Waals forces dominate their interactions.¹² DES are defined by a large depression of the
mixture’s melting point relative to that of the pure components, a result of strong non-ideal
behavior in the liquid phase.²³⁵ This non-ideality arises from specific interactions between the
components, such as hydrogen bonding, which lower the Gibbs free energy of the system.⁵ Their
low volatility, ease of preparation, and tunable solvent properties make DES attractive for
replacing hazardous organic solvents in chemical synthesis and separation processes.² Their
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flexibility and reduced environmental impact have made DES a widely researched alternative in
both laboratory and industrial settings.
Applications of Eutectic Systems
The phase behavior and tunable properties of eutectic systems have led to their increasing use in
green chemistry and energy technologies. By combining components in specific ratios to reach a
depressed melting point, eutectic systems enable processing and reactivity under lower thermal
conditions than their individual constituents would allow. This characteristic can reduce energy
consumption, eliminate the need for volatile or toxic solvents, and support the development of
efficient, low-temperature energy conversion processes. In particular, the adaptability of eutectic
compositions makes them valuable for applications ranging from thermal energy storage to
catalytic media and hydrogen production.
While the general properties of eutectic alloys are well known, their ability to enable chemical
reactivity, particularly in the context of green energy, has become a focus of more recent
research. Certain liquid-phase eutectic alloys have shown promise in facilitating the release of
hydrogen gas from aluminum’s reaction with water, a reaction that is normally blocked by the
presence of a thin, stable oxide layer on aluminum’s surface.
In a study by Ziebarth et al., aluminum was combined with gallium, indium, and tin to form a
low-melting alloy that reacts with water to produce hydrogen.7 Normally, aluminum does not
react easily with water because it forms a protective oxide layer. But when aluminum is alloyed
with these metals, the mixture partially melts at low temperatures, creating a liquid phase that
allows aluminum atoms to reach the surface and react with the water. This results in the
production of hydrogen gas and aluminum hydroxide, a solid byproduct.7
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The presence of a liquid phase is what facilitates this reaction. It allows aluminum atoms to
move through the alloy and come into contact with water. This transport would not occur if the
material were fully solid. The study showed that in the Al-Ga binary system, hydrogen
production began only once the temperature rose to the eutectic melting point, around 26–27 °C.7
In the Al-Ga-In-Sn quaternary alloy, melting occurred near 9.4 °C, enabling the alloy to remain
reactive at room temperature.7
Building on this mechanism, Godart developed a reactor system using eutectic gallium–indium
to extract hydrogen from scrap aluminum.6 Because eGaIn is already liquid at room temperature,
it can maintain continuous contact with the aluminum and disrupt its oxide layer without the use
of corrosive chemicals. The hydrogen produced can then be used in fuel cells, offering a portable
and low-emission energy source. This system is being explored for clean energy storage, backup
power, and use in off-grid environments.
These examples show how liquid eutectic alloys can serve a dual function: they act both as
materials and as active agents in promoting clean chemical reactions. By enabling hydrogen
production under mild conditions, they support solutions in green energy without relying on high
heat or hazardous additives.
Eutectic salts are mixtures defined by their ionic bonding and are typically used in hightemperature applications due to their thermal stability.1 While most eutectic salts are solid at
room temperature, their relatively low melting points and high heat capacities make them useful
for storing and transferring energy in the form of heat. This has led to their integration into largescale renewable energy systems, particularly in solar power plants, where they serve as both heat
transfer fluids and thermal storage media.
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One of the most well-documented applications of eutectic salts in green energy is in the Solar
Two power tower project, which used a eutectic mixture of sodium nitrate and potassium nitrate
to store solar energy as heat.1 In this system, sunlight was concentrated using mirrors to heat the
molten salt, which could reach temperatures as high as 565 °C. The heated salt was then stored in
a large tank and used later to generate steam and drive turbines for electricity production—even
after sunset.8 This allowed for energy storage and dispatch, solving one of the key limitations of
solar energy systems: their inability to produce power at night or during cloudy conditions.
The success of the Solar Two project demonstrated that eutectic salts could provide a stable, nonvolatile, and recyclable medium for storing large amounts of thermal energy. Their high specific
heat capacity means they can absorb and retain significant amounts of energy with minimal
degradation over time. According to Yu et al., systems based on eutectic NaNO₃–KNO₃ have
been shown to maintain functionality after over 1,000 heating and cooling cycles, with minimal
change in performance.1 This makes them a promising solution for long-term, high-efficiency
thermal energy storage.
In addition to energy storage, eutectic salts have also been explored for solar-to-chemical energy
conversion. For example, a system using molten Na₂CO₃–K₂CO₃ was shown to facilitate the
Boudouard reaction—a process that converts carbon dioxide and carbon into carbon monoxide,
which can then be used to synthesize cleaner fuels such as methanol.1 In this application, the
eutectic salt serves as a heat medium that maintains the high temperatures needed for the
reaction, offering a method of recycling CO₂ using solar energy.
These examples show how eutectic salts are being used to enable renewable energy systems by
storing heat, extending solar energy usability, and even driving chemical reactions. Their
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reliability over repeated use, resistance to thermal decomposition, and relatively low cost make
them well suited for practical, scalable energy solutions.
Deep eutectic solvents are mixtures formed by combining a salt—typically a quaternary
ammonium compound such as choline chloride—with a hydrogen bond donor like urea or an
organic acid. When mixed in certain ratios, these components create a stable liquid at a much
lower temperature than either substance alone.3 This melting point depression occurs because of
strong hydrogen bonding between the components, which disrupts the formation of a solid
crystal structure.2 DES are gaining attention in green chemistry due to their low volatility, low
toxicity, biodegradability, and ease of preparation.
One of the most established uses for DES is as a solvent for organic synthesis. Unlike many
traditional solvents that are volatile or hazardous, DES can carry out reactions safely at lower
temperatures. For example, a thiophene–aryl coupling reaction was performed in a DES system
without the need for an inert atmosphere, yielding high product quantities under mild
conditions.1 DES have also proven effective in catalytic reactions, where they often act both as a
medium and a co-catalyst, helping reduce the use of harmful reagents and minimizing waste.
DES are also widely used in materials chemistry. They have been employed in the preparation of
nanostructured metals, porous polymers, and electrode materials for batteries and
supercapacitors. Because DES can solubilize metal salts and oxides, they allow for the
electrodeposition or synthesis of complex materials under less harsh conditions.1 In some cases,
DES even take part in the chemical transformation, influencing the structure or morphology of
the final product.
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Beyond synthesis, DES are being applied to metal recovery and recycling, particularly in the
extraction of lithium, cobalt, and other valuable elements from used batteries. Their ability to
dissolve metal oxides without corrosive acids makes them a cleaner alternative for industrialscale recycling.1 These properties are supported by modeling studies that link DES performance
to their enthalpic and entropic characteristics.2
In gas separation and environmental cleanup, DES have shown selective absorption of gases like
CO₂ and SO₂, making them potential candidates for carbon capture and pollution control.4 The
molecular design of DES can be tuned to enhance gas solubility and selectivity, and some
systems have demonstrated reversible gas uptake for regeneration and reuse.
Finally, DES are being used in biomass processing and bio-based extractions. Their hydrogenbonding capacity allows them to break down tough biological materials like lignin or chitin
without the need for strong acids or high heat. This makes them especially useful for recovering
natural products from plant matter or processing agricultural waste into useful chemicals.1
Together, these examples highlight DES as one of the most versatile categories of eutectic
systems. Their tunable properties, chemical stability, and environmental compatibility allow
them to replace traditional solvents and reagents across a wide range of green chemistry and
energy applications.
Conclusion
Eutectic systems are being studied for their usefulness in green chemistry and energy. By
lowering melting points, they allow certain chemical processes to happen at lower temperatures.
This helps reduce energy use, avoid harmful solvents, and support cleaner ways to store and
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produce energy. The three main types of eutectic systems—eutectic alloys, eutectic salts, and
deep eutectic solvents—each have their own role in these areas.
Eutectic alloys, like those made from aluminum, gallium, indium, and tin, help produce
hydrogen gas from aluminum and water. Normally, this reaction is blocked by a thin oxide layer
on aluminum. But when part of the alloy is in liquid form, aluminum atoms can move to the
surface and react with water. This method can be used to make clean hydrogen fuel under simple
conditions. In one project, eutectic gallium–indium was used to extract hydrogen from scrap
aluminum without using strong acids. The hydrogen could then be used in fuel cells for energy.
Eutectic salts are mostly used for storing thermal energy. A well-known example is the
Solar Two project, where a sodium nitrate and potassium nitrate mixture was used to store heat
from sunlight. The stored heat was later used to make electricity. These salts stay stable through
many heating and cooling cycles, and they work at high temperatures without breaking down.
Some eutectic salts have also been used in solar-driven chemical reactions, like converting
carbon dioxide into carbon monoxide, which can then be turned into fuel.
Deep eutectic solvents are used in a wider range of applications. They are made by
mixing a salt with a molecular compound, which creates a liquid through hydrogen bonding.
DES are safer and easier to handle than many traditional solvents. They are used in organic
synthesis, metal recycling, gas separation, and biomass processing. For example, DES can
remove valuable metals from used batteries or absorb gases like CO₂. They can also help break
down plant materials without using strong chemicals.
In each case, the way eutectic systems behave can be explained using general chemistry
ideas like equilibrium and Gibbs free energy. The types of forces between particles play a big
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role in how these systems melt and function. These examples show how chemistry learned in the
classroom can be used to solve real problems in energy and the environment.