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Binary mixtures of hydrocarbons and a thermally robust ionic liquid (IL) incorporating a perarylphosphonium-based cation are investigated experimentally and computationally. Experimentally, it is seen that excess toluene added to the IL forms two distinct liquid phases, an “ion-rich” phase of fixed composition and a phase that is nearly pure toluene. Conversely, n -heptane is observed to be essentially immiscible in the neat IL. Molecular dynamics simulations capture both of these behaviours. Furthermore, the simulated composition of the toluene-rich IL phase is within 10% of the experimentally determined composition. Additional simulations are performed on the binary mixtures of the IL and ten other small hydrocarbons having mixed aromatic/aliphatic character. It is found that hydrocarbons with a predominant aliphatic character are largely immiscible with the IL, while those with a predominant aromatic character readily mix with the IL. A detailed analysis of the structure and energetic changes that occur on mixing reveals the nature of the ion-rich phase. The simulations show a bicontinuous phase with hydrocarbon uptake akin to absorption and swelling by a porous absorbent. Aromatic hydrocarbons are driven into the neat IL via dispersion forces with the IL cations and, to a lesser extent, the IL anions. The ion–ion network expands to accommodate the hydrocarbons, yet maintains a core connective structure. At a certain loading, this network becomes stretched to its limit. The energetic penalty associated with breaking the core connective network outweighs the gain from new hydrocarbon–IL interactions, leaving additional hydrocarbons in the neat phase. The spatially alternating charge of the expanded IL network is shown to interact favourably with the stacked aromatic subphase, something not possible for aliphatic hydrocarbons. 

In previous work with thermally robust salts [Cassity et al., Phys. Chem. Chem. Phys. , 2017, 19 , 31560] it was noted that an increase in the dipole moment of the cation generally led to a decrease in the melting point. Molecular dynamics simulations of the liquid state revealed that an increased dipole moment reduces cation–cation repulsions through dipole–dipole alignment. This was believed to reduce the liquid phase enthalpy, which would tend to lower the melting point of the IL. In this work we further test this principle by replacing hydrogen atoms with fluorine atoms at selected positions within the cation. This allows us to alter the electrostatics of the cation without substantially affecting the sterics. Furthermore, the strength of the dipole moment can be controlled by choosing different positions within the cation for replacement. We studied variants of four different parent cations paired with bistriflimide and determined their melting points, and enthalpies and entropies of fusion through DSC experiments. The decreases in the melting point were determined to be enthalpically driven. We found that the dipole moment of the cation, as determined by quantum chemical calculations, is inversely correlated with the melting point of the given compound. Molecular dynamics simulations of the crystalline and solid states of two isomers showed differences in their enthalpies of fusion that closely matched those seen experimentally. Moreover, this reduction in the enthalpy of fusion was determined to be caused by an increase in the enthalpy of the crystalline state. We provide evidence that dipole–dipole interactions between cations leads to the formation of cationic domains in the crystalline state. These cationic associations partially block favourable cation–anion interactions, which are recovered upon melting. If, however, the dipole–dipole interactions between cations is too strong they have a tendency to form glasses. This study provides a design rule for lowering the melting point of structurally similar ILs by altering their dipole moment. 

Recent work by Wasserscheid, et al. suggests that PPh 4 + is an organic molecular ion of truly exceptional thermal stability. Herein we provide data that cements that conclusion: specifically, we show that aliphatic moieties of modified PPh 4 + -based cations incorporating methyl, methylene, or methine C–H bonds burn away at high temperatures in the presence of oxygen, forming CO, CO 2 , and water, leaving behind the parent ion PPh 4 + . The latter then undergoes no further reaction, at least below 425 °C. 

Drugs containing amine groups react with CO 2 to form crystalline ammonium carbamates or carbamic acids. In this approach, both the cation and anion of the salt, or the neutral CO 2 adduct, are derived from the parent drug, generating new crystalline versions in a ‘masked’ or prodrug form. It is proposed that this approach may serve as a valuable new tool in engineering the physical properties of drugs for formulation purposes. 

Thermally robust materials have been of interest since the middle of the past century for use as high temperature structural materials, lubricants, heat transfer fluids and other uses where thermal stability is necessary or desirable. More recently, ionic liquids have been described as ‘thermally robust,’ with this moniker often originating from their low volatility rather than their innate stability. As many ionic liquids have vanishingly low vapor pressures, the upper limit of their liquid state is commonly considered to be their degradation temperature, frequently reported from TGA measurements. The short duration ramps often used in TGA experiments can significantly overestimate the temperature at which significant degradation begins to occur when the compounds are held isothermal for even a few hours. Here, we review our recent work, and that of colleagues, in developing thermally robust ionic compounds, primarily perarylphosphonium and perarylsulfonium bistriflimide salts, in some of which cation stability exceeds that of the anion. We have used a combination of molecular design, synthesis, and computational modeling to understand the complex tradeoffs involving thermal stability, low melting point and other desirable physicochemical properties.