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Review

A Review of the State of the Art on Ionic Liquids and Their Physical Properties During Heat Transfer

by
Krzysztof Dutkowski
1,
Marcin Kruzel
1,*,
Małgorzata Smuga-Kogut
1 and
Marcin Walczak
2
1
Department of Mechanical and Power Engineering, Koszalin University of Technology, 75-453 Koszalin, Poland
2
Department of Electronics and Computer Science, Koszalin University of Technology, 75-453 Koszalin, Poland
*
Author to whom correspondence should be addressed.
Energies 2025, 18(15), 4053; https://doi.org/10.3390/en18154053
Submission received: 9 June 2025 / Revised: 17 July 2025 / Accepted: 25 July 2025 / Published: 30 July 2025
(This article belongs to the Special Issue Heat Transfer in Heat Exchangers: 2nd Edition)
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Abstract

This paper presents information on ionic liquids (ILs) and explores their potential applications in heat exchange systems. Basic information on ionic liquids and their selected thermophysical properties is presented in a manner that facilitates their use in future research. The physical properties of IL that are important in the area of heat exchange are described in detail, with particular emphasis on heat exchange in flow. Issues related to the melting point, specific heat, thermal conductivity coefficient, and viscosity of selected ionic liquids, as well as the effect of temperature on their changes, are discussed. The physical properties of IL are compared with the physical properties of water treated in heat exchange as a reference substance. The issues of creating aqueous solutions of ionic liquids and the effect of the amount of water on the physical properties of the resulting solution are discussed. It is demonstrated that selected ionic liquids can be considered an alternative to traditional working liquids commonly used in heat exchange systems.

1. Introduction

Ionic liquids are one of the most promising categories of liquids that have emerged in recent decades. Ionic liquids are chemical compounds or salts that consist of at least one cation and one anion [1,2].
The discovery of ionic liquids took place independently in different scientific centers. However, the earliest discovery is considered to have been made in 1914 by Paul Walden [3]. He was looking for molten salts that were liquids at room temperature, where there would be a relationship between the size of the molecules and the conductivity of the resulting liquid. He found that such properties were possessed by the chemical compound [EtNH3][NO3], which melted at 12 °C. This was the first example of a protic ionic liquid (PIL) [4], which later became an essential subclass of ionic liquids [5].
There are many reasons for the interest in ionic liquids. One of them is their attractive physical properties [6], such as low phase change temperature at which they shift from solid to liquid, non-flammability, high thermal and chemical stability [7], and low vapor pressure [8,9]. These properties favor their use in various fields, such as [10,11,12] chemistry (solvents in many chemical reactions or for the extraction and separation of rare earth metals, aromatics and plant dyes, used in liquid and gas chromatography or as phase modifiers in capillary electrophoresis); electrochemistry (electrolytes in lithium-ion batteries, fuel cells), as they improve the efficiency and selectivity of many catalytic reactions; in biotechnology (solvents: biomolecules, enzymes, DNA, proteins); in environmental protection (absorption and removal of pollutants in the form of heavy metals); in the production of advanced materials, where they are often the environment for the production of polymers; and in the synthesis of nanomaterials or membranes.
The use of ionic liquids for transporting or storing thermal energy is a potential area of their application. These places additional requirements on ionic liquids in terms of their physical properties [13,14,15]. An ionic liquid, as a working fluid in heat exchange systems, should be indicated by a high specific mass (density), low viscosity, high specific heat value, stable physical and chemical properties in a wide temperature range, and should not cause corrosive changes in installation materials. As a thermal storage energy material in an explicit or latent manner, an ionic liquid should additionally be indicated by low vapor pressure, high thermal conductivity coefficient, and high enthalpy of phase change [16,17,18]. In general, an ionic liquid should be a cheap and non-toxic substance. Currently available data in the literature on the properties of ionic liquids are limited and often divergent. This results from the measurement method used, but above all from the purity of the sample or the source of the ionic liquid [19].
The study presents a current review of the state of knowledge on ionic liquids, with particular emphasis on their thermophysical properties that are important from the perspective of transporting or storing thermal energy.

2. Ionic Liquids

2.1. General Characteristics of Ionic Liquids

Ionic liquids (ILs) are chemical compounds defined as organic or organic–inorganic salts that are liquid at temperatures below 100 °C [20,21,22,23]. This is such a large group of compounds with a broad spectrum of structures and sizes, and thus intermolecular interactions, that it is impossible to describe them using generalizations. Very often, individual ionic liquids must be treated individually [5].
Understanding the behavior of ionic liquids and their properties is crucial for practical applications [20,24,25]. To design technological processes or to search for new applications of ionic liquids, it is necessary to know reliable relationships between physicochemical properties and the structure of ionic liquids, both at atmospheric pressure and at high pressures [26,27,28,29]. In addition, phase transitions of ionic liquids can occur in several stages, depending on the structure of the cation and anion and the rate of temperature change [5]. The mechanisms accompanying the phase transition of ILs are complex, and different lengths of carbon chains or substitutes affect the balance of internal forces, which can lead to the melting of ILs at different temperatures [21].

2.2. Classification of Ionic Liquids

Ionic liquids used in heat exchange systems can be classified according to their chemical composition. They are then divided according to the type of cation (Table 1) and anion (Table 2) used for the synthesis of the IL.
Cations and anions are characterized by different symmetry and structure (including the number of carbon atoms in the alkyl substituent, the presence of branched structures and functional groups), charge density and different ability to form hydrogen bonds between themselves [44].
The most common anionic components of ILs are polyatomic inorganic compounds, i.e., [PF6]− and [BF4]−. In contrast, the most common cations are compounds of pyridinium and imidazolium rings with one or more alkyl groups attached to nitrogen or carbon atoms [45].
Due to the enormous number of combinations of different ions, the number of possible ionic liquids is unlimited. According to [40], the number of theoretically possible ionic liquids is 1018. The properties of these compounds can be designed at the stage of their synthesis by selecting a cation and an anion with an appropriate structure. Depending on the selection made, they can exhibit various desirable features. They can be thermally stable in the range of high and/or low temperatures. In addition, they are usually characterized by good electrical conductivity, high heat capacity and good solubility in both polar and nonpolar organic and inorganic compounds [46,47]. Among all known ionic liquids, imidazole-based liquids are among the most studied. Examples of ionic liquids that can be used in thermal power engineering are presented in Table 3. The most frequently recurring substances were selected. There are over 80 of them. A literature review shows that the most frequently tested and described ionic liquids in the thermal engineering literature are those composed of a 1-alkyl (mainly ethyl, butyl) 3-methylimidazolium cation. These cations were paired with virtually every anionic group used to create ionic liquids.
Ionic liquids, which can act as a working fluid in a heat exchange system or be an alternative to current phase change materials (PCMs), can be divided into [23,40]:
  • cationic: imidazolium (i.e., [BmIm][BF4], [BmIm][PF6]), pyridinium, ammonium, phosphonium, sulfonium;
  • anionic: fluorinated (np.: [BF4]−, [PF6]−); organic (i.e., [Tf2N]−, [OTf]−); inorganic (i.e., Cl−, NO3−, SCN−).
To design an IL that meets the assumed requirements, organic cations with different substituent groups (i.e., ammonium, sulfonium, imidazolium, triazolium, pyridinium, phosphonium, pyrazolium, guanidinium, etc.) can be combined with other anions. The asymmetric and bulky structure of the component ions contributes to the fact that their properties are very different from those of conventional salts such as sodium chloride [51,52].
Ionic liquids can be further divided into two categories: protic and aprotic. Protic ionic liquids have the advantage of being relatively easy to synthesize, since simple proton transfer reactions between a Brønsted acid and a Brønsted base form them [22]. They are used as electrolytes in fuel cells and supercapacitors, catalysts in chemical syntheses or as a medium for storing hydrogen in a chemically bound form. Despite their advantages, such as higher ionic conductivity and low viscosity, protic ionic liquids have significant limitations that affect their practical applications. Their lower thermal and chemical stability, tendency to degrade and variability of properties depending on operating conditions make them not always the best choice for long-term industrial applications [53,54].
Aprotic ionic liquids are those whose cation does not contain an easily exchangeable proton. Most often, these are imidazolium, pyridinium, phosphonium or ammonium derivatives [55,56,57]. They are characterized by the lack of proton transfer, which is why they exhibit greater chemical and thermal stability. They usually have higher viscosity and lower ionic conductivity than protic ILs. Their advantage is the possibility of adjusting their properties by selecting appropriate ions [23]. Therefore, they are widely used as working fluids in heat exchange systems, for the extraction of rare earth metals, as solvents in catalytic processes and organic chemistry, as well as a reaction medium in electrochemical syntheses [14].
Ionic liquids can also be categorised by their operating temperature into high-temperature (>300 °C), medium-temperature (100–300 °C), and low-temperature (<100 °C) [5].
The diversity of ionic liquids, because they can be composed by combining different cations and anions, means that their informed selection and use in specific applications requires knowledge of their physical properties. When planning the use of ionic liquids in heat transfer systems, it is crucial to consider parameters such as melting point, density, heat conduction coefficient, specific heat, and viscosity, as well as how these properties are affected by different combinations of ions. Furthermore, it is crucial to know how these parameters change as a function of temperature and when diluted with water. This information is included in the next section.

3. Selected Physical, Thermal and Hydrodynamic Properties of Ionic Liquids

Knowledge of the physical, thermal, and hydrodynamic properties of ionic liquids ensures that they are selected appropriately for existing needs, and in terms of thermal and flow conditions, enable optimal operation of the heat exchange system. The most critical thermophysical and hydrodynamic properties of ionic liquids acting as a working fluid in the heat exchange system include melting point, density, thermal conductivity coefficient, specific heat, and viscosity.

3.1. Melting Point (Tm) of Ionic Liquids

Ionic liquids (ILs) are salts that are supposed to be an alternative to conventional fluids used in heat exchange systems. In this aspect, the interest of researchers is directed towards liquids with a low melting point (Tm < 30 °C). These liquids are known as room temperature ionic liquids (RTILs) [58,59]. They are referred to as the so-called “green solvents” due to their low vapor pressure, ability to be designed with specific properties, non-volatile and non-flammable in ambient conditions, and due to the possibility of their recycling [36,60]. However, certain conditions can cause RTILs to become volatile and flammable. The flammability of RTILs can be influenced by factors such as the presence of a voltage bias, the type of anion, and high temperatures [3,5]. Figure 1 graphically illustrates the melting point of exemplary RTILs, while Figure 2 presents the melting point of other exemplary, low-temperature (Tm < 100 °C) ionic liquids.
Figure 1, which shows ionic liquids from the RTIL group, shows that ionic liquids with a negative melting point (Tm) are available. These liquids can be used to transport “cold” materials and can serve as substitutes for ice slurries. Their advantage may be that the liquid does not contain ice crystals, which in ice slurries accelerate the degradation process of the internal surfaces of pipelines used for their transfer. Furthermore, it is noted that ionic liquids can operate at temperatures as low as −80 °C. Figure 2 shows ILs with melting points ranging from 31 °C to 99 °C. These substances are used both for storing thermal energy using latent heat (phase change heat) and for “high-temperature” thermal energy transfer (e.g., in solar installation circuits that focus solar radiation linearly or pointwise).
The appropriate selection of anions and cations can modify the melting point of ionic liquids. This is of particular importance in the synthesis of low-temperature ionic liquids, especially RTILs. The usually selected cations are imidazolium, triazolium and tetrazolium [5,21,23,61,62,63,64,65,66,67,68]. Such a selection of cations causes nitrogen-rich heterocycles to disrupt the packing of the crystal lattice. In the case of anions, lower melting points are obtained for substances based on nitrate (NO3−), azide, dicyanamide and nitrocyanamide. The inclusion of functional groups, such as allyl and vinyl, in cations can also lower the melting points of ILs [69].
The analysis of the list presented in Figure 1 and Figure 2 shows that ionic liquids containing the anionic group [BF4]-, [PF6]- have a substantially lower melting point. It was found that the melting point of ionic liquids is higher, the shorter the alkyl chain in the imidazolium cation [70]. In addition, the melting point of ionic liquids is lower the more the volume of anions in the IL, the smaller the packing of molecules in the IL structure, and the ionic liquid has unsaturated alkyl tails [70].
There are two ways to lower the melting point of ionic liquids. One way is to increase the steric effect of the cation and/or anion. This can be performed by introducing bulk substitutes to the cation or by using bulk anions. Another way is to break the symmetry of the cation [71].

3.2. Density

Density, also known as specific mass, is one of the most important physical properties of a substance and plays a crucial role in heat exchange. This applies to both the storage of thermal energy and heat transfer in fluids. The density of a substance determines its heat conduction coefficient, the nature of the fluid flow, and hence, among other things, heat exchange coefficients or flow resistance. The density of selected ionic liquids at 20 °C, 25 °C and 30 °C [22,45,72,73,74,75,76,77,78,79,80,81,82,83,84] is presented in Table 4.
From Table 4, it is noted that the density of ionic liquids is greater than the density of water. In the case of ionic liquids such as [EmIm][Tf2N], [C2mIm][Tf2N], [EMmIm][Tf2N], their density is even 50% greater than the density of water. The higher the density of the substance, the smaller the volume required to transport or store the same amount of thermal energy. This helps to reduce the volume of the installation, the diameters of the pipe channels, etc.
The effect of temperature on the density of other, exemplary ionic liquids in relation to the density of water [85] is shown in Figure 3.
The characteristics presented in Figure 3 show that in almost every case (except for ILs based on cations from the phosphonium group, e.g., trihexyltetradecylphosphonium—[P6,6,6,14]+), the density of the ionic liquid is higher than the density of water. It can be observed that it can be even 70% higher (e.g.: [C4elm][SbF6], [C5elm][SbF6]) than the density of water at the same temperature. Moreover, a linear decrease in the density of the IL was observed as the temperature increased. It should be noted that the rate of change of the density of the IL with increasing temperature is the same for each of the substances presented. Figure 3 shows that the density of 1-alkyl-3-methylimidazolium saccharinate ionic liquids decreased with increasing the total number of carbon atoms (butyl → hexyl → octyl → decyl) [19]. Moreover, it was found that the density of the tested ILs decreased with the increase in their molar mass, which is typical for chemical compounds that are characterized by a very similar chemical structure, regardless of the cationic group or the anion used [19].
The effect of water on the density of the IL solution is shown in Figure 4. In the studies presented in [30], water was added to two ionic liquids ([EmIm][OAc] and [DEMA][OMs]) in amounts from about 30 to 70% molar fraction.
It was observed that when the density of the aqueous solution of [DEMA][OMs] decreased with the increasing molar fraction of water, in the case of the aqueous solution of [EMIM][OAc], the density increased to reach a maximum. Further addition of water caused a decrease in the density of the aqueous solution of [EMIM][OAc]. The authors [36] note that a similar behavior was described in the publication of other authors and concerned aqueous acidic acid.

3.3. The Thermal Conductivity Coefficient

The thermal conductivity coefficient is a physical quantity describing the ability of a substance to conduct heat. This property is one of the most important parameters determining the intensity of heat exchange. Figure 5 shows the value of the thermal conductivity coefficient of selected ionic liquids at a temperature of 25 °C.
Figure 5 shows that none of the presented ionic liquids had such a high thermal conductivity coefficient as water (the thermal conductivity coefficient of water, according to NIST [85], varies from 0.57878 [W/(mK)] at 10 °C to 0.67279 [W/(mK)] at 90 °C). For most of the presented ILs, the thermal conductivity coefficient was below 0.2 [W/(mK)], i.e., it was approximately 30% of the thermal conductivity coefficient of water. Figure 5 shows, among others, the thermal conductivity coefficient of the ionic liquid based on dioctylammonium [DOA] with organic and inorganic anions. The thermal conductivity coefficient of the IL based on [DOA] varied, depending on the type of anion, from 0.18 [W/(mK)] to 0.373 [W/(mK)]. Therefore, it is possible to increase the conductivity coefficient of some ionic liquids by changing the anion by more than 100%.
Another way to improve (increase) the heat transfer coefficient of fluids used for energy storage, transport and conversion is to add solid particles made of a material with a high heat transfer coefficient. These particles are in the nanometer range, and when added to the base fluid, they create a stable nanosuspension. Ionic liquid-based nanofluids—ionanofluids (INF) are a new, intensively studied type of heat transfer fluid (HTF) [29,86,87,88].
The influence of temperature on the value of the thermal conductivity coefficient of ionic liquids is shown in Figure 6.
The characteristics shown in Figure 6 indicate that the thermal conductivity coefficient of the ionic liquid decreases with an increase in its temperature. The decrease in this value, in the temperature range between 10 °C and 70 °C, is of the order of several percent, and the trend of changes is reflected by a linear relationship [89,90,91].
Energies 18 04053 g006aEnergies 18 04053 g006b
Figure 6. The influence of temperature on the thermal conductivity coefficient of sample ionic liquids [15,35,37,38,49,61,62,75,79,84,89,91].
Figure 6. The influence of temperature on the thermal conductivity coefficient of sample ionic liquids [15,35,37,38,49,61,62,75,79,84,89,91].
Energies 18 04053 g006aEnergies 18 04053 g006b

3.4. Viscosity

Fluid viscosity is one of the most important hydrodynamic parameters related to heat transfer fluid. The higher the fluid viscosity, the higher the power required to drive the pump [92,93,94]. Figure 7 presents a comparison of the viscosities of selected ionic liquids. The presented characteristics show the effect of fluid temperature on the dynamic viscosity coefficient. The viscosity of sample ILs is compared with the viscosity of water according to [1]. Based on the comparison presented in Figure 7, it is possible to compare the viscosity of ionic liquids containing the same cationic group (e.g., [BmIm]+, [P14,6,6,6]+ or [EmIm]+) and different anionic groups.
Figure 7 illustrates that the viscosity of IL can be several orders of magnitude higher than that of water. The increase in the temperature of IL caused a significant decrease in its viscosity. This decrease can be an order of magnitude. For example, for [EmIm][OTf], the viscosity coefficient changed from approximately 100 [mPa·s] at 5 °C to 10 [mPa·s] at 75 °C. It is known that the viscosity coefficient of IL depends mainly on the combination of hydrogen bonds and van der Waals forces [42]. The viscosity of a liquid results from intermolecular interactions and the forces that must be applied to break the bonds between the moving “layers” of the fluid. On the one hand, an increase in fluid temperature causes thermal expansion of the material; on the other hand, an increase in the distance between molecules leads to a decrease in the strength of intermolecular interactions. Research [42] has shown that in ionic liquids, the forces between the cation and the anion are also sensitive to temperature, resulting in a decrease in the viscosity of the ionic liquid with an increase in temperature.
For ionic liquids to be widely used in heat exchange as a working fluid, they should be characterized by low viscosity. The introduction of anions [(NC)2N]−, [Tf2N]− and [BF4]− results in the formation of an ionic liquid with the lowest viscosity [95]. Dicationic ionic liquids have a much higher viscosity than ionic liquids with a single charged cation [71]. The introduction of a second ion pair to the structure of a monocationic ionic liquid led to a significant increase in its viscosity. On the other hand, the introduction of an ion pair resulted in an increased probability of IL crystallization [71]. Van der Waals forces between ions forming a specific ionic liquid, as well as Coulomb interactions, affect both the viscosity and volatility of ionic liquids. Therefore, the improvement of one property (e.g., viscosity) could lead to the deterioration of other properties (e.g., volatility) [71]. As a result, there is a need to optimize the physicochemical structure of ionic liquids to provide them with the required physical properties.
Figure 8 presents characteristics showing the effect of temperature on the viscosity coefficient of an aqueous solution of an ionic liquid. Water of different molar fractions was added to [EmIm][OAc] and [DEMA][OMs]. Figure 8 shows that regardless of the type of ionic liquid, the higher the water concentration in the solution, the lower its viscosity. Therefore, it is possible to reduce the viscosity of IL by creating an aqueous solution on its basis and, as a result, lowering its viscosity, approaching the value of water.

3.5. Specific Heat

The specific heat of a substance is a measure of the amount of energy that must be introduced into a unit of mass to change its temperature by one degree. The higher the value of the specific heat, the more energy must be supplied to the substance, but also removed, in the same place (storage of thermal energy) or another place (transport of thermal energy).
Figure 9 shows the effect of temperature on the specific heat of exemplary ionic liquids. It is noticeable that it is much lower than the value of the specific heat of water, which changes from 4.1952 [kJ/(kgK)] at 10 °C to 4.2052 [kJ/(kgK)] at 90 °C [85]. Similarly to water, the increase in the temperature of IL caused an increase in the value of its specific heat. The presented characteristics show that the rate of change of this value depended on the type of ionic liquid. It was greater in the case of choline ionic liquids—[Chol], or protic ionic liquid based on 2-hydroxyethylammonium lactate—[HEA][La].
The specific heat of ionic liquids depends on the type of anion, the hydrogen bond network or the alkyl length. The formation of metastable crystals was often observed in ionic liquids, which affected the measurement of the specific heat of IL [21,96].
The effect of the molar fraction of water on the specific heat of the solution based on [EmIm][OAc] and [DEMA][OMs] is shown in Figure 10. It is noted that for both types of ionic liquids, the specific heat of the solution was closer to the value of the specific heat of water, the higher its fraction in the tested solution. The specific heat of the aqueous solution of IL with about 30% molar fraction of water was about 2 [kJ/(kgK)] and was doubled in the case of the solution with 96% molar fraction of water in the ionic liquid solution.
In the context of thermal energy storage, the so-called energy storage density is a crucial parameter. This is the amount of energy that can be stored in the volume of the tank, resulting from the product of the specific heat of the substance and its density. The larger the volumetric heat capacity of the substance, the smaller the dimensions that the thermal energy storage can take. This means that the physical properties of the IL require optimization, because with the increase in temperature, on the one hand, the specific heat of the ionic liquid increases, while on the other hand, it leads to a decrease in its density [87].
The scope of thermal ILs examined is still expanding. It is also beginning to include IL mixtures. Their properties, previously unrecognized, may indicate directions for further research by selecting substances with high potential for use in the transport and storage of thermal energy [4].

4. Application of Ionic Liquids in Thermal Technology

Ionic liquids have several advantageous thermophysical properties, thanks to which they can effectively replace traditional materials used in thermal technology or those that have limitations resulting from, for example, the range of operating parameters or instability in cyclic operating conditions [53].
The most promising ionic liquids (IL) that can be used as heat transfer fluids include [95]:
  • 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BmIm][NTf2]) and N-Butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPyrr][NTf2]). These substances are considered to be among the most suitable for use as heat transfer fluids. Their thermophysical properties are similar to those of commonly used organic and organosilicon fluids;
  • ionic liquids with anions such as [NTf2]−, [OTf]− or [PF6]−. These liquids are characterized by high thermal stability and low viscosity;
  • Ionic liquids have low vapour pressure and high thermal stability. These liquids can be used at higher temperatures (>200 °C) without the risk of explosion or high volatility, which is particularly important in industrial applications.
ILs are distinguished by the possibility of precisely adjusting their properties through the appropriate selection of cations and anions, which makes them optimized for specific industrial processes, including heat transfer. The melting point can be adjusted by changing the combination of anions and cations, alkyl chain length and functional groups and can be varied from −96 °C to 359 °C [95].
In the study by [97], the synthesis and characterization of six different alkyl-alkane ionic liquids were presented, which showed favorable thermophysical properties and good cyclicity. They are promising carriers requiring further basic and applied research in the field of thermal energy. The use of ionic liquids in heat exchange systems contributes to the increased efficiency of energy systems. It can respond to the growing demand for effective technologies for transporting and storing thermal energy, especially in the context of renewable energy sources.
Ionic liquids, especially those based on imidazolium, show promising properties for thermal energy storage applications. High heat capacity and resistance to corrosion during phase transitions enhance their usefulness in thermal management systems, particularly in extreme environments such as space. These liquids can be used as a heat storage medium in solar installations, as well as in cooling and heating systems for energy-efficient buildings [21].
The structure of ionic liquids and the type of bonds between their ions have a key impact on their ability to accumulate and release heat. Depending on the type of interactions between ions, electrostatic forces occur in ionic liquids—dominant cation–anion interactions, which affect viscosity, thermal conductivity and thermal stability. The presence of hydrogen bonds (e.g., in protic ionic liquids) can increase the ability to store heat. On the other hand, van der Waals interactions affect physical properties, such as the enthalpy of phase change and solubility in different media [31].
The use of ionic liquids in heat transfer systems also poses challenges related to their toxicity and biodegradability. Research works emphasize the need to conduct life cycle analyses of ILs to determine their impact on the environment, including issues related to their synthesis, durability and recyclability [32]. Research focuses on optimizing their thermal properties, improving stability and developing more ecological methods of their synthesis, which is crucial for their further use in industry [98,99].
Some studies have shown that ionic liquids, such as choline-based ionic liquids, can be attractive phase change material (PCM) candidates due to their low toxicity, biodegradability, and environmental compatibility. For example, the study conducted by [43] showed the thermophysical properties of three different choline-based ionic liquids, such as cholinium dihydrogen phosphate ([CH][DHP]), cholinium tosylate ([CH][TOS]) and cholinium acetate ([CH][AC]), with different melting points and melting enthalpy, making them potential candidates as PCMs in certain temperature ranges [22]. Ionic liquids are superior to PCMs mainly in terms of flammability, volatility, and corrosiveness of PCMs [34,100]. Unlike traditional PCMs, ILs are chemically stable, do not degrade or delaminate. They do not exhibit supercooling and retain their properties after repeated heating and cooling cycles.

5. Summary and Conclusions

Ionic liquids (ILs) are salts that melt at <100 °C, often at room temperature (RTIL). They consist exclusively of ions, and through their proper selection, they can be characterized by an almost unlimited number of different structures. Studies indicate that the anion in the ionic liquid has a greater influence on its physical properties than the cation. The possibility of modeling the structure of ILs means that their physicochemical properties can be controlled and adjusted to one’s own needs. In many ways, ionic liquids are better than commercial heat transfer fluids, mainly due to their high stability, low vapor pressure and lack of harmful impact on the environment.
The use of ionic liquids as substances for storing thermal energy via phase change requires consideration of the fact that ILs exhibit hysteresis. This means that the liquid-to-solid phase transition temperature does not coincide with the solid-to-liquid phase transition temperature. Supercooling of the IL is often required to initiate crystal formation. According to [101], the freezing/melting temperature and the area of the thermal hysteresis loop depend on the size ratio of the anions to the cations that constitute the ionic liquid.
The specific heat of IL is high enough to be considered a heat transfer fluid, but due to its high viscosity, it is advisable to use IL in the form of a solution (aqueous or solvent-based). The main disadvantage of IL is its high price, usually anion. It may decrease over time when improved processes of ionic liquid synthesis are introduced or other previously unknown substances are created (from cheaper reagents).
Another way to reduce the price of IL is to start its serial production. However, this stage requires popularization and demonstration of the benefits resulting from the use of IL in thermal cycles.
In conclusion:
  • it is necessary to systematize the current state of knowledge in the field of thermodynamic properties of ionic liquids, taking into account the largest possible number of substances produced so far;
  • due to the scarcity of knowledge and in the context of the synthesis of new, previously unknown ionic liquids, there is a need to carry out extensive research on the fundamental physical properties of ILs in the field of thermal engineering;
  • it is necessary to conduct applied research using ionic liquids, including the determination of heat transfer coefficients and flow resistances;
  • numerical study is required to predict the behavior of ionic liquids in heat exchange systems, validated by reliable results of experimental studies.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Melting point of selected room temperature ionic liquids (RTIL) [5,21,33,61,62,63,64,65,66,67,68].
Figure 1. Melting point of selected room temperature ionic liquids (RTIL) [5,21,33,61,62,63,64,65,66,67,68].
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Figure 2. Melting point of selected low-temperature ionic liquids [5,21,23,33,43,45,53,65,66].
Figure 2. Melting point of selected low-temperature ionic liquids [5,21,23,33,43,45,53,65,66].
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Figure 3. The influence of temperature on the density of selected ionic liquids and water [19,36,37,38,54,62,72,75,79,80,81,82,83,84,85].
Figure 3. The influence of temperature on the density of selected ionic liquids and water [19,36,37,38,54,62,72,75,79,80,81,82,83,84,85].
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Figure 4. The influence of temperature and molar fraction of water on the density of [EmIm][OAc] and [DEMA][OMs] [36].
Figure 4. The influence of temperature and molar fraction of water on the density of [EmIm][OAc] and [DEMA][OMs] [36].
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Figure 5. Thermal conductivity coefficient of sample ionic liquids at 25 °C [22,54,66].
Figure 5. Thermal conductivity coefficient of sample ionic liquids at 25 °C [22,54,66].
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Figure 7. The influence of temperature on the dynamic viscosity coefficient of ionic liquids and water [15,61,79,81,82,83,84,85].
Figure 7. The influence of temperature on the dynamic viscosity coefficient of ionic liquids and water [15,61,79,81,82,83,84,85].
Energies 18 04053 g007
Energies 18 04053 g008
Figure 8. Effect of temperature and molar water fraction on the viscosity coefficient of the solution [EmIm][OAc] and [DEMA][OMs] [36].
Figure 8. Effect of temperature and molar water fraction on the viscosity coefficient of the solution [EmIm][OAc] and [DEMA][OMs] [36].
Energies 18 04053 g008
Energies 18 04053 g009aEnergies 18 04053 g009b
Figure 9. The effect of temperature on the specific heat of ionic liquids [15,37,38,43,54,65,74,79,84,96].
Figure 9. The effect of temperature on the specific heat of ionic liquids [15,37,38,43,54,65,74,79,84,96].
Energies 18 04053 g009aEnergies 18 04053 g009b
Energies 18 04053 g010
Figure 10. Effect of temperature and molar fraction of water on the specific heat of [EmIm][OAc] and [DEMA][OMs] solutions [36].
Figure 10. Effect of temperature and molar fraction of water on the specific heat of [EmIm][OAc] and [DEMA][OMs] solutions [36].
Energies 18 04053 g010
Table 1. Examples of cations commonly used for the synthesis of ionic liquids.
Table 1. Examples of cations commonly used for the synthesis of ionic liquids.
Full NameAbbreviationScheme
ammonium
trimethylpropylammonium [30][N1,1,1,3]+Energies 18 04053 i001
triethylammonium [31][N2,2,2,H]+Energies 18 04053 i002
tributylammonium [31][N4,4,4,H]+Energies 18 04053 i003
tetrabutylammonium [31][N4,4,4,4]+ Energies 18 04053 i004
guanidinium
1,1,3,3-tetramethylguanidinium [32]HTBDH+Energies 18 04053 i005
imidazolium
1-methylimidazolium [3][mIm]+Energies 18 04053 i006
1-ethylimidazolium [3][C2Im]+ Energies 18 04053 i007
1-Alkyl-3-methylimidazolium
1-allyl-3-methylimidazolium [3][AmIm]+Energies 18 04053 i008
1-butyl-3-methylimidazolium [3][BmIm]+Energies 18 04053 i009
1-methyl-3-methylimidazolium [3][DmIm]+Energies 18 04053 i010
1-ethyl-3-methylimidazolium [3][EmIm]+ Energies 18 04053 i011
1-hexyl-3-methylimidazolium [3][HmIm]+Energies 18 04053 i012
1-octyl-3-methylimidazolium [3][OmIm]+Energies 18 04053 i013
1-propyl-3-methylimidazolium [3][PmIm]+Energies 18 04053 i014
1-hexadecyl-3-methylimidazolium [3][C16mIm]+Energies 18 04053 i015
1-Alkyl-2,3-dimethylimidazolium
1-butyl-2,3-dimethylimidazolium [33][BdmIm]+Energies 18 04053 i016
1-ethyl-2,3-dimethylimidazolium [34][EdmIm]+Energies 18 04053 i017
1-propyl-2,3-dimethylimidazolium [34][PdmIm]+Energies 18 04053 i018
1,1′-(butane-1,4-diyl)-bis(3-methylimidazolium) [34]C4(mim)2+Energies 18 04053 i019
1,1′-(pentane-1,5-diyl)-bis(3-methylimidazolium) [34]C5(mim)2+Energies 18 04053 i020
1,1′-(hexane-1,6-diyl)-bis(3-methylimidazolium) [34]C6(mim)2+Energies 18 04053 i021
morpholinium
4-methylmorpholinium [31]MmMor+Energies 18 04053 i022
4-ethyl-4-methylmorpholinium [31]EmMor+Energies 18 04053 i023
4-(2-methoxyethyl)-4-methylmorpholinium [31]MoemMor+Energies 18 04053 i024
phosphonium
trihexyltetradecylphosphonium [15][P6,6,6,14]+Energies 18 04053 i025
Triphenylphosphonium [15]Ph3PH+Energies 18 04053 i026
pyridinium
1-ethylpyridinium [31][Epy]+Energies 18 04053 i027
1-butylpyridinium [31][Bpy]+ Energies 18 04053 i028
piperidinium
1-methylpiperidinium [31][MPip]+Energies 18 04053 i029
1-ethylpiperidinium [31][EPip]+Energies 18 04053 i030
1-methyl-1-propylpiperidinium [31][MPPip]+Energies 18 04053 i031
1-methyl-1-butylpiperidinium [31][MBPip]+Energies 18 04053 i032
pyrrolidinium
1-butyl-1-methylpyrrolidinium [35][BMPyrr]+Energies 18 04053 i033
1-n-nonyl-1-butylpyrrolidinium [35][C9(Bpyrr)2]+Energies 18 04053 i034
1-n-nonyl-1-methylpyrrolidinium [35][C9(Mpyrr)2]+Energies 18 04053 i035
trialkylsulfonium
Trimetylsulfonium [31][S111]+Energies 18 04053 i036
Triethylsulfonium [31][S222]+Energies 18 04053 i037
Table 2. Examples of anions commonly used for the synthesis of ionic liquids.
Table 2. Examples of anions commonly used for the synthesis of ionic liquids.
Full NameAbbreviationScheme
acetate [36,37,38][OAc]−, [Ac]−, [AcO]−Energies 18 04053 i038
bis(fluorosulfonyl)imide [39][fsi]− or [N(SO2F)2]−Energies 18 04053 i039
bis(trifluromethylsulfonyl)imide [40][NTf2]− or [N(SO2CF3)2]−Energies 18 04053 i040
bromide [39][Br]−
chloride [39][Cl]−
dicyanamide [41][DCA]− Energies 18 04053 i041
dihydrogenphosphate [4][DHP]− or [H2PO4]−Energies 18 04053 i042
dimethylphosphate [31][DMP]−Energies 18 04053 i043
ethyl sulfate [31][EtSO4]−Energies 18 04053 i044
fluoride [42][F]−
fluoroalkylphosphates [4] [fap]−, [efap]−, etc.Energies 18 04053 i045
fluorohydrogenate [34][F(HF)2] −Energies 18 04053 i046
glutamate [3][Glu]−Energies 18 04053 i047
glycinate [3][Gly]−Energies 18 04053 i048
hexafluorophosphate [39][PF6]−Energies 18 04053 i049
hydrogen carbonate [3][HCO3]−Energies 18 04053 i050
hydrogensulfate [4][HSO4]−Energies 18 04053 i051
hydroxide [4][OH]−Energies 18 04053 i052
iodide [40][I]−
nitrate [40][NO3]−Energies 18 04053 i053
octenylsuccinate/organosulfonate [3][OSA]−Energies 18 04053 i054
p-toluenesulfonate or tosylate [43][Tos]−Energies 18 04053 i055
perchlorate [31][ClO4]−Energies 18 04053 i056
phenylalaninate [3][Phe]−Energies 18 04053 i057
prolinate [3][Pro]−Energies 18 04053 i058
serinate [3][Ser]−Energies 18 04053 i059
tetrachloroaluminate [39][AlCl4]−Energies 18 04053 i060
tetracyanoborate [4][B(CN)4]−Energies 18 04053 i061
tetrafluoroborate [39][BF4]−Energies 18 04053 i062
tetrahydroborate (borohydride) [4][BH4]−Energies 18 04053 i063
trifluoroacetate [39]CF3COO−, [tfa]−Energies 18 04053 i064
trifluoromethylsulfate [40][CF3SO3]−Energies 18 04053 i065
trifluoromethanesulfonate [39][OTf]− or [CF3SO3]−Energies 18 04053 i066
trifluoromethylsulfate [4][TfO]−Energies 18 04053 i067
triocyanate [31][SCN]−Energies 18 04053 i068
trifluoroethanoate (acetate) [39][tfa]−Energies 18 04053 i069
Table 3. Examples of ionic liquids used in the field of thermal energy (abbreviation and full name) [30,48,49,50].
Table 3. Examples of ionic liquids used in the field of thermal energy (abbreviation and full name) [30,48,49,50].
Ionic LiquidFull Name
[BmIm] [BF4] (or [C4mIm] [BF4])1-butyl-3-methylimidazolium tetrafluoroborate [30,48]
[BmIm] [Br]1-butyl-3-methylimidazolium bromide [48]
[BmIm] [DBP]1-butyl-3-methylimidazolium dibutylphosphate [48]
[C10mIm] [B(CN)4]1-decyl-3-methylimidazolium tetracyanoborate [49]
[C10mIm] [C(CN)3]1-decyl-3-methylimidazolium tricyanomethanide [49]
[C10mIm] [TF2N]1-decyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [49]
[C2mIm] [Ac]
(or: [C2mIm] [CH3COO], [EmIm] [Ac])		1-ethyl-3-methylimidazolium acetate [48,49,50]
[C2mIm] [BF4]1-ethyl-3-methylimidazolium tetrafluoroborate [49]
[C2mIm] [C(CN)3]1-ethyl-3-methylimidazolium tricyanomethanide [50]
[C2mIm] [C2SO4]1-ethyl-3-methylimidazolium ethylsulfate [50]
[C2mIm] [CF3SO3] (or [EmIm] [TfO])1-ethyl-3-methylimidazolium trifluoromethanesulfonate [48,49]
[C2mIm] [CH3SO3]1-ethyl-3-methylimidazolium methanesulfonate [50]
[C2mIm] [DCA]1-ethyl-3-methylimidazolium dicyanamide [50]
[C2mIm] [DEP] (or [EmIm] [DEP])1-ethyl-3-methylimidazolium diethylphosphate [48,50]
[C2mIm] [EtSO4] (or [EmIm] [EtSO4])1-ethyl-3-methylimidazolium ethyl sulfate [48,49]
[C2mIm] [MeOHPO2]1-ethyl-3-methylimidazolium methyl phosphonate [49]
[C2mIm] [SCN]1-ethyl-3-methylimidazolium thiocyanate [50]
[C2mIm] [TF2N]1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [49]
[C4mIm] [C(CN)3]1-butyl-3-methylimidazolium tricyanomethane [36]
[C4mIm] [CF3SO3]1-butyl-3-methylimidazolium trifluoromethanesulfonate [36]
[C4mIm] [DCA]1-butyl-3-methylimidazolium dicyanamide [49]
[C4mIm] [PF6]1-butyl-3-methylimidazolium hexafluorophosphate [49]
[C4mIm] [SCN]1-butyl-3-methylimidazolium thiocyanate [49]
[C4mIm] [TF2N] (or [C4mIm] [NTf2])1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [49,50]
[C4mPyr] [DCA]1-butyl-1-methylpyrrolidinium dicyanamide [49]
[C4mPyr] [FAP]1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate [49]
[C4mPyr] [TF2N]1-butyl-1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide [49]
[C6mIm] [B(CN)4]1-hexyl-3-methylimidazolium tetracyanoborate [49]
[C6mIm] [BF4]1-hexyl-3-methyl imidazolium tetrafluoroborate [50]
[C6mIm] [PF6]1-hexyl-3-methylimidazolium hexafluorophosphate [30]
[C6mIm] [TF2N]1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [49]
[C8mIm] [C(CN)3]1-octyl-3-methylimidazolium tricyanomethanide [49]
[C8mIm] [PF6]1-octyl-3-methylimidazolium hexafluorophosphate [49]
[C8mIm] [TF2N]1-octyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [49]
[DEMA] [OMs]diethylmethylammonium methane sulfonate [48]
[DmIm] [BF4]1,3-dimethylimidazolium tetrafluoroborate [48]
[DmIm] [Cl]1,3-dimethylimidazolium chloride [48]
[DmIm] [DMP]1,3-dimethylimidazolium dimethylphosphate [48]
[EEIM] [DEP]1-ethyl-3-ethylimidazolium diethylphosphate [48]
[EmIm] [DMP]1-ethyl-3-methylimidazolium dimethylphosphate [48]
[EmIm] [TFA]1-ethyl-3-methylimidazolium trifluoroacetate [48]
[N4,1,1,1] [NTf2]butyltrimethylammonium bis(trifluoromethylsulfonyl)imide [50]
[P14,6,6,6] [AcO]trihexyl(tetradecyphosphonium) acetate [50]
[P14,6,6,6] [ButO]trihexyl(tetradecyphosphonium) butanoate [50]
[P14,6,6,6] [DecO]trihexyl(tetradecyphosphonium) decanoate [50]
[P14,6,6,6] [HexO]trihexyl(tetradecyphosphonium) hexanoate [50]
[P14,6,6,6] [OctO]trihexyl(tetradecyphosphonium) octanoate [50]
[P6,6,6,14] [Cl]trihexyltetradecylphosphonium chloride [50]
[P6,6,6,14] [NTf2]trihexyl(tetradecylphosphonium) bis((trifluoromethyl)sulfonyl)imide [50]
[P6,6,6,14] [Phosph]trihexyl(tetradecylphosphonium) phosphinate [50]
[TBPh] [CYS]tetrabutylphosphonium L-cysteinate [49]
[TBPh] [LYS]tetrabutylphosphonium L-lysinate [49]
[TBPh] [PRO]tetrabutylphosphonium L-prolinate [49]
[TBPh] [SER]etrabutylphosphonium L-serinate [49]
[TBPh] [TAU]tetrabutylphosphonium 2-aminoethanesulfonate [49]
[TBPh] [THR]tetrabutylphosphonium L-threoninate [49]
[TBPh] [VAL]tetrabutylphosphonium L-valinate [49]
[THTDPh] [Cl]trihexyl(tetradecyl)phosphonium chloride [49]
[THTDPh] [TF2N]trihexyl(tetradecyl)phosphonium bis[(trifluoromethyl)sulfonyl]imide [49]
Table 4. Density of selected ionic liquids at 20 °C, 25 °C and 30 °C.
Table 4. Density of selected ionic liquids at 20 °C, 25 °C and 30 °C.
Ionic LiquidDensity [kg/m3] at 20 °CIonic LiquidDensity [kg/m3] at 25 °CIonic LiquidDensity [kg/m3] at 30 °C
[C2mIm][CH3SO3]—[76]1243[AmIm][BF4]—[45]1231[bdmIm][BF4]—[73]1094
[DmIm][MPh]—[78]1180[BmIm][(CF3SO2)2N]—[45]1420[C2mIm][OTf]—[73]1370
[EmIm][EPh]—[78]1130[BmIm][BF4]—[73]1201[C2mIm][SCN]—[73]1114
[HmIm][BF4]—[74]1200[BmIm][BF4]—[45]1208[C2mIm][Tf2N]—[73]1514
[HmIm][PF6]—[74]1304[BmIm][BF4]—[77]1120
[HmIm][Tf2N]—[74]1370[BmIm][CF3SO3]—[45]1290
[HmIm][TfO]—[74]1240[BmIm][dca]—[73]1058
[BmIm][NTfO2]—[45]1404
[BmIm][PF6]—[82]1373
[BmIm][Tf2N]—[73]1439
[BmIm]Br—[45]1134
[BmIm]Cl—[83]1120
[BMmIm][PF6]—[73]1242
[BMmIm][PF6]—[77]1360
[BMPyrrol][NTfO2]—[45]1400
[bpy][BF4]—[73]1214
[C2mIm][EtSO4]—[72]1241
[C4mIm][BF4]—[75]1210
[C4mIm]Br—[22]1293
[C4mIm]Cl—[79]1086
[C4mIm]I—[22]1489
[EmIm][BF4]—[45]1248
[EmIm][ETSO4]—[77]1242
[EmIm][MeSO4]—[73]1280
[EmIm][PF6]—[45]1373
[EmIm][Tf2N]—[73]1521
[EmmIm][Tf2N]—[73]1491
[HmIm][BF4]—[80]1075
[HmIm][PF6]—[45]1304
[MPPyr][NTfO2]—[45]1440
[OmIm][BF4]—[81]1110
[OmIm]Cl—[84]1000
BAF—[45]990
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Dutkowski, K.; Kruzel, M.; Smuga-Kogut, M.; Walczak, M. A Review of the State of the Art on Ionic Liquids and Their Physical Properties During Heat Transfer. Energies 2025, 18, 4053. https://doi.org/10.3390/en18154053

AMA Style

Dutkowski K, Kruzel M, Smuga-Kogut M, Walczak M. A Review of the State of the Art on Ionic Liquids and Their Physical Properties During Heat Transfer. Energies. 2025; 18(15):4053. https://doi.org/10.3390/en18154053

Chicago/Turabian Style

Dutkowski, Krzysztof, Marcin Kruzel, Małgorzata Smuga-Kogut, and Marcin Walczak. 2025. "A Review of the State of the Art on Ionic Liquids and Their Physical Properties During Heat Transfer" Energies 18, no. 15: 4053. https://doi.org/10.3390/en18154053

APA Style

Dutkowski, K., Kruzel, M., Smuga-Kogut, M., & Walczak, M. (2025). A Review of the State of the Art on Ionic Liquids and Their Physical Properties During Heat Transfer. Energies, 18(15), 4053. https://doi.org/10.3390/en18154053

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