A new insight into pure and water-saturated quaternary phosphonium-based carboxylate ionic liquids: Density, heat capacity, ionic conductivity, thermogravimetric analysis, thermal conductivity and viscosity

Highlights

• ILs appeared as prospective heat transfer fluids based on their specific properties.

• ILs mixtures with water might be a good alternative for common heat transfer fluids.

• Thermal conductivity of ILs was found to be independent on the anion chain length.

• An experimental approach (Walden rule) proved specific cation-anion interactions.

Abstract

Ionic Liquids (ILs) are task specific materials with tunable properties which may be applied as heat transfer fluids (HTFs) due to their characteristic properties including low vapour pressure, wide liquid range and high thermal stability. This study provides an evaluation of their potential as HTFs from both their physicochemical properties as well as an economic comparison with commonly used materials. The paper presents the thermophysical properties (density, isobaric heat capacity, ionic conductivity, viscosity, thermal stability by TGA, thermal conductivity) and their thermodynamic derivatives (isobaric thermal expansion coefficient, excess molar volume, excess molar heat capacity), as well as the Walden rule of ionicity for a range of hydrophobic ILs based on the trihexyl(tetradecyl)phosphonium, ([P_14,6,6,6]⁺), cation. The ionic liquids studied are trihexyl(tetradecyl)phosphonium acetate ([AcO]⁻), butanoate ([ButO]⁻), hexanoate ([HexO]⁻), octanoate ([OctO]⁻), and decanoate ([DecO]⁻), as well as their mixtures with water. It was shown that investigated systems are highly promising materials as HTFs, especially ILs + water mixtures, which may be the solution for many engineering issues.

Introduction

Thermophysical properties occupy a very important place in the physical and chemical sciences and their values are needed to design a wide range of unit operations including heat exchangers, distillation, extractions, etc. These thermophysical properties include the thermal conductivity coefficient, both isobaric and isochoric heat capacity, density, viscosity, ionic conductivity, electrical conductivity, speed of sound, liquid-gas surface tension, thermodynamic derivatives (isobaric thermal expansion coefficient, isentropic and isothermal compressibility), etc.

One of the most important unit operations are heat exchangers as most processes being carried out in factories, laboratories, power stations or daily life, require cooling or heating and, therefore, the development of heat transfer fluids (HTFs) used for heating or cooling has been the result of a large number of studies [1], [2]. There are a wide variety of HTF materials and these can be divided into three groups: low temperature (up to 563.15 K, i.e. water, silicone polymers, propylene and ethylene glycol), medium temperature (563.15–863.15 K, i.e. mixture of phenylcyclohexane and bicyclohexyl, terphenyl, synthetic hydrocarbons, alkyl substituted aromatic compounds, white mineral oils) and high temperature (above 863.15 K, i.e. eutectic sodium-potassium alloy and lead) [3]. The most important property when considering materials as HTF is the temperature range of operation. In addition, the mode of operation (liquid or vapor phase) needs to be considered; although predominantly a liquid phase operation is used industrially [4].

The main properties which need to be studied for HTF applications include the thermal conductivity, heat capacity, density, viscosity, corrosivity, flash point, pumpability, durability, maintenance time, energy conservation, toxicity and environmental impact (namely ecologic aspects) [4], [5]. The most common materials used as HTFs are petroleum, engine oils, water, ethylene glycol, propylene glycol, synthetic fluids derived from mineral oils, white/paraffinic oils, synthetic aromatics – polyphenols or alkylated benzenes, silicones, fluorocarbons.

Recently, a number of studies have considered ILs as an alternative approach for HTFs as replacements for conventional HTFs, i.e. 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1,2-dimethyl-3-propylimidazolium bis[(trifluoromethyl)sulfonyl]imide [5]. The disadvantages of using ILs as HTFs have been reported by Chernikova et al. [6], and include the high cost of the ILs, lower thermal stability in comparison to commonly used HTFs (i.e. the maximum temperature of operation of Dowtherm Q, Dowtherm G and Paratherm HT are 573.15 K, 644.15 K and 631.15 K, respectively), higher electrical conductivity and corrosivity. Whilst there are advantages of using ILs as HTFs, they have several other disadvantages as well as those reported by Chernikova et al. [6], compared with conventional HTFs. The ILs have, in general, higher viscosities and sometimes lower densities. However, these properties can be tailored by the addition of molecular solvents, for example water [7], [8]. By adding water to ILs, the viscosity is significantly reduced and, in case of ILs with low density, the density can be modified to more practical levels [9]. Furthermore, water has very high thermal conductivity and heat capacity so it is expected to greatly enhance ILs properties when mixed with water.

Nevertheless, ILs exhibit unique properties in comparison to conventional HTFs, namely their relatively low freezing point [10], high specific heat capacity [11], very low vapor pressure [12], high energy storage density [13], relatively high thermal conductivity [5], non-flammability (under standard conditions) [14], wide liquid range [10], and their ability to be tailored to match desired properties. Many publications have been devoted to the thermophysical characterization of ILs for the application as HTFs [15], [16], [17].

In this work, the thermal conductivity, specific heat capacity, density and dynamic viscosity of trihexyl(tetradecyl)phosphonium ([P_14,6,6,6]⁺) acetate ([AcO]⁻), butanoate ([ButO]⁻), hexanoate ([HexO]⁻), octanoate ([OctO]⁻), and decanoate ([DecO]⁻), are reported. The properties of the ILs were investigated as a function of the temperature at atmospheric pressure. It has been shown in many previous reports that the [P_14,6,6,6]⁺ cation has a significant contribution to the thermal conductivity and heat capacity of such ILs and, therefore, these properties are relatively high in comparison to other ILs or commonly used HTFs [18], [19], [20]. However, in general, the availability of thermophysical properties for tetraalkylphosphonium-based ILs is limited. This results in reduced performance ability to use predictive models for thermophysical properties [21], [22]. Furthermore, ILs based on carboxylate anions, especially when composed with quaternary phosphonium-based ILs have not been studied extensively. The carboxylate anion contribution to thermal conductivity and heat capacity is also not negligible [23], [24].

This work presents the investigation of density, isobaric thermal expansion coefficient derived from density, isobaric heat capacity, ionic conductivity, thermogravimetric analysis, thermal conductivity coefficient and viscosity of pure and mixtures with water composed of the [P_14,6,6,6]⁺ cation with the [AcO]⁻, [ButO]⁻, [HexO]⁻, [OctO]⁻ or [DecO]⁻ anion. In addition, the excess molar properties and Walden rule were also determined for these systems to analyze the interactions between cation, anion and water. In addition, an evaluation of the heat exchanger cost for these systems compared with commonly used HTFs has been undertaken.