<p>Thermal stability is a crucial factor in selecting compounds for high-temperature applications, such as, lubricants, thermal fluids, and solvents for organic reactions. Ionic liquids (ILs) are a type of liquid salt made up solely of ions, usually consisting of organic cations and inorganic or organic anions. The thermal stability of ILs is governed primarily by the intrinsic characteristics of their constituent ions rather than by cation–anion interactions alone. Despite extensive studies, a comprehensive understanding of the thermal stability of both protic ionic liquids (PILs) and aprotic ionic liquids (AILs) remains elusive. Since many ILs have vanishingly low vapor pressures, the upper limit of their liquid state is often used to define their degradation temperature. This temperature is commonly determined through thermogravimetric analysis (TGA), which provides insights into their thermal stability. This review provides a critical assessment of the short-term thermal stability of ILs based on key parameters obtained from thermogravimetric analysis (TGA), including the decomposition temperature <InlineEquation ID="IEq1"> <EquationSource Format="TEX">$(T_{{\text{d}}} )$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mtext>d</mtext> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> and onset temperature <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$(T_{{{\text{onset}}}} )$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mtext>onset</mtext> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. In addition, phase-transition temperature, such as glass transition <InlineEquation ID="IEq3"> <EquationSource Format="TEX">$(T_{{\text{g}}} )$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mtext>g</mtext> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, crystallization <InlineEquation ID="IEq4"> <EquationSource Format="TEX">$(T_{{\text{c}}} )$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mtext>c</mtext> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, and melting <InlineEquation ID="IEq5"> <EquationSource Format="TEX">$(T_{{\text{m}}} )$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>T</mi> <mtext>m</mtext> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, determined through differential scanning calorimetry (DSC), are systematically discussed to define the operational thermal window of ILs. Aprotic ionic liquids have been extensively investigated in the literature and are therefore discussed only briefly in this review. In contrast, systematic studies on the thermal stability of protic ionic liquids remain comparatively limited. Accordingly, greater emphasis is placed on a detailed discussion of PILs, drawing on recent literature as well as contributions from our research group. Overall, this review highlights the important role of cation and anion structural modulation in shaping the stability of ILs, which is vitally important for their effectiveness in high-temperature applications.</p> Graphical abstract <p></p>

Thermal behavior of protic and aprotic ionic liquids: perspectives and insights

期刊论文
发表日期：

2026年2月6日

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Bignya Rani Dash,
K. K. Athira,
Ashok Kumar Mishra,
Ramesh L. Gardas

摘要

Thermal stability is a crucial factor in selecting compounds for high-temperature applications, such as, lubricants, thermal fluids, and solvents for organic reactions. Ionic liquids (ILs) are a type of liquid salt made up solely of ions, usually consisting of organic cations and inorganic or organic anions. The thermal stability of ILs is governed primarily by the intrinsic characteristics of their constituent ions rather than by cation–anion interactions alone. Despite extensive studies, a comprehensive understanding of the thermal stability of both protic ionic liquids (PILs) and aprotic ionic liquids (AILs) remains elusive. Since many ILs have vanishingly low vapor pressures, the upper limit of their liquid state is often used to define their degradation temperature. This temperature is commonly determined through thermogravimetric analysis (TGA), which provides insights into their thermal stability. This review provides a critical assessment of the short-term thermal stability of ILs based on key parameters obtained from thermogravimetric analysis (TGA), including the decomposition temperature $(T_{{\text{d}}} )$ $(T_{d})$ and onset temperature $(T_{{{\text{onset}}}} )$ $(T_{onset})$ . In addition, phase-transition temperature, such as glass transition $(T_{{\text{g}}} )$ $(T_{g})$ , crystallization $(T_{{\text{c}}} )$ $(T_{c})$ , and melting $(T_{{\text{m}}} )$ $(T_{m})$ , determined through differential scanning calorimetry (DSC), are systematically discussed to define the operational thermal window of ILs. Aprotic ionic liquids have been extensively investigated in the literature and are therefore discussed only briefly in this review. In contrast, systematic studies on the thermal stability of protic ionic liquids remain comparatively limited. Accordingly, greater emphasis is placed on a detailed discussion of PILs, drawing on recent literature as well as contributions from our research group. Overall, this review highlights the important role of cation and anion structural modulation in shaping the stability of ILs, which is vitally important for their effectiveness in high-temperature applications.

Graphical abstract