Thermophysical Properties of NH3/IL+ Carbon Nanomaterial Solutions

This study proposes the use of new working fluids, refrigerant/IL+ carbon nanomaterials (CNMs), in absorption systems as an alternative to conventional working fluids. In this regard, the thermophysical properties of ammonia and carbon nanomaterials (graphene and single-wall carbon nanotubes) dispersed into [BMIM]BF4 ionic liquid are theoretically investigated. The thermophysical properties of NH3/IL+ CNMs solutions are computed for weight fractions of NH3 in the range of 0.018–0.404 and temperatures between 293 and 388 K. In addition, two weight fractions of CNMs are considered: 0.005 and 0.01, respectively. Our results indicate that by adding a small amount of nanomaterial to the ionic liquid, the solution’s thermal conductivity is enhanced, while its viscosity and specific heat are reduced. Correlations of the thermal conductivity, viscosity, specific heat, and density of the NH3/IL+ CNMs solutions are proposed.


Introduction
Ionic liquids (ILs) are considered a novel type of green working fluid used in various fields, such as absorption refrigeration, solar applications, chemistry (gas capture, storage), and electrochemistry (batteries, sensors). In recent years, ionic liquids have been considered a promising alternative to the conventional working fluids (NH 3 /H 2 O and H 2 O/LiBr) used as absorbents in absorption refrigeration systems due to their good thermal stability, high absorption capacity, and very low vapor pressure [1][2][3].
In one paper, Yokozeki and Shiflett [7] carried out a study on the performance of an absorption refrigeration system using NH 3 [Ac]). The results indicated that the COPs of all the studied solutions were lower than those of the NH 3 /H 2 O solution.
The thermophysical properties (vapor pressures and heat capacities) of the H 2 O + ([Dmim]dmp) system were investigated by Dong et al. [8]. The results revealed that the coefficient of the performance of the H 2 O + [Dmim]dmp system is close to that of the conventional working pair H 2 O + LiBr system.
Kim et al. [9] theoretically investigated the thermodynamic performance of a miniature absorption system using various refrigerant mixtures (R125, R152a, R32, R134a, R143a) /ILs ( 6 ]) as the working fluids. They found that refrigerant/IL solutions were promising materials for absorption refrigeration systems that utilize low-grade waste heat, such as those of electronic systems.  6 ], except in the case where the solubility difference between the absorber and desorber converged to zero.
In another paper, Kim and Kohl [11] investigated the cooling capability of the R134/[Bmim] [PF 6 ] used in an absorption refrigeration system. They [21] carried out a study on the use of ILs with supercritical CO 2 using a group contribution equation of state and found that the coefficient of performance was lower compared to a conventional NH 3 /H 2 O system. Table 1 presents the values of the coefficients of performance for absorption refrigeration systems using ammonia/ionic liquids as working fluids. Investigations into the application of ammonia/ionic liquids as working fluids in absorption refrigeration systems are limited in the open literature. Moreover, studies on absorption refrigeration systems using ammonia/ionic liquid+nanomaterials as working fluids are not reported in the literature. In order to improve the performance of absorption systems, new working fluids are herein proposed. The thermophysical properties of working fluids are the main data in this evaluation of the performance of absorption refrigeration systems. In this regard, the thermophysical properties of ammonia with graphene (GE) and single-wall carbon nanotubes (SWCNTs), respectively, dispersed into [Bmim]BF 4 ionic liquid, are analyzed and discussed. Correlations for the studied properties, required for the modeling and simulation of the performance of various absorption refrigeration systems, are also proposed.

Thermophysical Properties of the Solutions
The thermophysical properties of the working fluids used in absorption refrigeration systems must be determined as an essential step in the evaluation of the thermodynamic performance of these fluids. In this study, ammonia and two types of carbon nanomaterials (CNMs-graphene (GE) and single-wall carbon nanotubes (SWCNTs)) with two weight fractions (0.005 and 0.01), dispersed into a [B mim ]BF 4 ionic liquid, will be analyzed and discussed. The thermophysical properties of ammonia and CMNs/[B mim ]BF 4 were taken from the NIST database [24] and Fang et al. [25], respectively.
Since there are no data on the thermo-properties of IL+CNMs mixed with NH 3 solutions, the properties (thermal conductivity, specific heat, and density) were calculated using a general equation, based on the weighted average of the properties of both components of the mixture [26,27]: in which the mass fraction of NH 3 is calculated as: The solution dynamic viscosity is calculated as:

Results and Discussions
In this study, the thermo-properties of the NH 3 /[B mim ]BF 4 and NH 3 /[B mim ]BF 4 + CNMs solutions were evaluated for the mass fractions of NH 3 in a range of 0.018-0.404 and at temperatures from 293 K to 388 K. Two types of carbon nanomaterials with two weight fractions (0.005 and 0.01), dispersed into an ionic liquid, were considered: graphene (GE) and single-wall carbon nanotubes (SWCNTs). No data for the thermal properties of the [B mim ]BF 4 +CNMs mixed with NH 3 solutions have been reported in the literature. With increasing temperatures can be seen that the thermal conductivities of all solutions have an upward trend up to w_NH 3 = 0.048, then with increasing NH 3 fractions (≥0.102), the thermal conductivities decrease with increasing temperatures, but increasing with increasing NH 3 fractions. The addition of carbon nanomaterials to the ionic liquid leads to an enhancement in the solution's thermal conductivity compared to the base solution. The enhancements in the thermal conductivity of the studied solutionscalculated as (k NH 3 /Il+CN Ms − k NH 3 /Il )/k NH 3 /Il × 100-at minimum and maximum NH 3 fractions-w NH3 = 0.018 and w NH3 = 0.404, respectively-are shown in Table 2: In addition, a descending trend in the thermal conductivity of solutions with higher NH 3 fractions may be seen. These results may be explained by the thermal conductivities of carbon nanomaterials dispersed into the ionic liquid. Graphene (GE) exhibits a thermal conductivity of ∼ 4000 W/mK [28][29][30], while the thermal conductivity of SWCNTs is usually reported to be in the range of 2000-6000 W/mK at a standard temperature (25 • C) [31]. Yu et al. [32] measured the thermal conductivity of SWCNTs using a chemical vapor deposition method and found a value higher than 2000 W/mK. The experimental results related to the thermal conductivity of ionic liquids revealed the increase in thermal conductivity achieved by adding nanoparticles into the ionic liquid and the minor influence of temperature on several ionic liquids containing nanoparticles. The main arguments for these trends are thermal boundary resistance, layering phenomena, and clustering [33]. The thermal conductivity values are correlated by means of a linear equation as a function of temperature:

Thermal Conductivity
The coefficient values , , and are given in Table 3.  The thermal conductivity values are correlated by means of a linear equation as a function of temperature:

Dynamic Viscosity
The coefficient values a, b, and R 2 are given in Table 3. Table 3. Coefficient values a, b, and R 2 obtained by fitting Equation (4).

Dynamic Viscosity
Figure 2a-e depict the variation in the viscosity of the solutions, with various NH 3 fractions, at rising temperatures. As shown in Figure 2, the viscosities of the solutions decrease exponentially with higher temperatures. By adding the carbon nanomaterials into the ionic liquid, a reduction in viscosity may be seen compared to the base solution. Higher fractions of carbon nanomaterials lead to an increase in the viscosity of the studied solutions, but these viscosity values do not exceed those of the base solution. With higher NH 3 fractions, a decrease in viscosity may also be seen. The diminution in viscosity is more obvious at the lower mass fractions of NH 3 in the solutions. The viscosities of the NH 3 /IL+CNMs solutions are lower than that of the base solution, indicating that these solutions are suitable for NH 3 absorption. The reduction in the viscosity of the studied solutions-calculated as (µ NH 3 /Il − µ NH 3 /Il+CN Ms )/(µ NH 3 /Il × 100-at minimum and maximum NH 3 fractions-w NH3 = 0.018 and w NH3 = 0.404, respectively-is shown in Table 4: The data related to the dynamic viscosity of ionic liquids are still contradictory. Most experimental studies indicate an increase in viscosity with the addition of nanoparticles to the ionic liquid, while on the other hand there are studies that have found a decrease in viscosity. The reduction in viscosity can be explained by the interaction between the molecules of the ionic liquid and the nanoparticles, as well as by the lubricating properties of the nanoparticles.
The dynamic viscosity values are correlated by means of an exponential equation as a function of temperature: The coefficient values a, b, and R 2 are given in Table 5.

Specific Heat
Figure 3a-e illustrate the variation in the specific heat of the solutions, with various NH 3 fractions, at rising temperatures. As shown in Figure 3, the specific heat of the solutions increases with both higher temperatures and higher fractions of NH 3 . In addition, by adding nanoparticles to the ionic liquid, a reduction in specific heat may be seen compared to the base solution. Higher CNMs fractions led to a decrease in the specific heat of all the solutions. The presented results are according to an equation proposed by Raud et al. [34], which indicates the increase in a solution's specific heat with rising temperatures, and also the reduction in specific heat by the addition of nanomaterials into the base solution. The dynamic viscosity values are correlated by means of an exponential equation as a function of temperature: The coefficient values , , and are given in Table 5.   The reduction in the specific heat of the studied solutions-calculated as (c p,NH 3 /Il − c p,NH 3 /Il+CN Ms )/(c p,NH 3 /Il × 100-at minimum and maximum NH 3 fractions-w NH3 = 0.018 and w NH3 = 0.404, respectively-is shown in Table 6: The data available in the open literature related to the specific heat of ionic liquids are, as in the case of viscosity, contradictory. The main reasons for this are the interaction between the molecules of the nanomaterials and the ionic liquid and the chemical structure of the ionic liquid.

Specific Heat
Figure 3a-e illustrate the variation in the specific heat of the solutions, with various NH3 fractions, at rising temperatures. As shown in Figure 3, the specific heat of the solutions increases with both higher temperatures and higher fractions of NH3. In addition, by adding nanoparticles to the ionic liquid, a reduction in specific heat may be seen compared to the base solution. Higher CNMs fractions led to a decrease in the specific heat of all the solutions. The presented results are according to an equation proposed by Raud et al. [34], which indicates the increase in a solution's specific heat with rising temperatures, and also the reduction in specific heat by the addition of nanomaterials into the base solution.
The reduction in the specific heat of the studied solutions-calculated as , / , / / , / • 100 -at minimum and maximum NH3 fractions-0.018 and 0.404, respectively-is shown in Table 6: Table 6. Reduction in specific heat.
/    The specific heat values are correlated by means of a linear equation as a function of temperature: (6) In Table 7, the coefficient values , , and are given: Table 7. Coefficient values , , and obtained by fitting Equation (6). The specific heat values are correlated by means of a linear equation as a function of temperature: In Table 7, the coefficient values a, b, and R 2 are given:

Density
The densities of the solutions, with various NH 3 fractions and at rising temperatures, are illustrated in Figure 4a-e. As can be seen, the density decreases with both higher temperature and higher NH 3 fractions. The addition of carbon nanomaterials into the ionic liquid increases the solution's density compared to the base solution. In addition, higher CNMs fractions lead to increased density for all solutions. Most experimental studies regarding the density of ionic liquids report an increase in density with the addition of nanoparticles and a decrease with higher temperatures. The presented results show the same trend as the experimental results obtained by other studies [35].
The enhancement in the density of the studied solutions-calculated as (ρ NH 3 /Il+CN Ms − ρ NH 3 /Il )/(ρ NH 3 /Il × 100-at minimum and maximum NH 3 fractionsw NH3 = 0.018 and w NH3 = 0.404, respectively-is shown in Table 8:    The density values are correlated by means of a linear equation as a function of temperature: The coefficient values , , and are given in Table 9. Table 9. Coefficient values , , and obtained by fitting Equation (7).  The density values are correlated by means of a linear equation as a function of temperature:

Conclusions
The coefficient values a, b, and R 2 are given in Table 9. Table 9. Coefficient values a, b, and R 2 obtained by fitting Equation (7).

Conclusions
In this study, the thermophysical properties of ammonia and carbon nanomaterials (CNMs), dispersed into [B mim ]BF 4 ionic liquid, were analyzed and discussed. The results showed that the thermal conductivity of the solutions decreases with higher NH 3 fractions. By adding carbon nanomaterials into the ionic liquid, an enhancement in the solution's thermal conductivity may be seen compared to the base solution, with the maximum enhancement in thermal conductivity having been achieved by NH 3 /[B mim ]BF 4 + 0.01 GE. The viscosities of the NH 3 /IL+CMNs solutions were lower than that of the base solution, indicating that these solutions are suitable for NH 3 absorption. In this case, the maximum reduction in viscosity was recorded for NH 3 /[B mim ]BF 4 + 0.01 GE. In addition, by adding CMNs to the ionic liquid, a reduction in the specific heat of the solutions may be seen compared to the base solution. At a temperature of 293 K, the maximum reduction in specific heat was achieved by the solutions with a 0.01 fraction of nanomaterials (NH 3 /[B mim ]BF 4 + 0.01 GE and NH 3 /[B mim ]BF 4 + 0.01 SWCNTs). Moreover, the addition of CMNs to the ionic liquid led to an increase in the solution's density. At a temperature of 293 K, the maximum enhancement in density was achieved by NH 3 /[B mim ]BF 4 + 0.01 GE. Finally, correlations for all studied properties were proposed.
The results of this study may contribute to the consolidation of the property database of NH 3 /IL+NMs for applications in absorption refrigeration. Further investigations concerning the thermophysical characteristics of ammonia with other types of ionic liquids are needed. In addition, for the practical implementation of NH 3 /ILs+CNMs in absorption refrigeration systems, experimental studies to support the reported theoretical results are needed.