One of the main environmental problems of our time is the atmospheric accumulation of gases that causes the so-called greenhouse effect, due to the use of fossil fuels. Due to its excellent fuel properties, renewability and biodegradability, biodiesel present a possible alternative to fossil fuels. In the last decade, considerable quantity of research is being done about the use of vegetable or animal oils as feedstock for biodiesel production [1, 2]. The most common way to produce biodiesel is through the transesterification of oils with an alcohol [3]. This yields fatty acid alkyl esters and glycerol. Methanol is the most commonly used alcohol, partly due to its low cost. The transesterification reaction can be catalyzed by alkaline and acidic catalysts, by enzymes [4] or can be carried out without added catalyst [5, 6]. The most common sources of oil for biodiesel production are soybean oil due to the best quality end product. A soy oil based diesel fuel tends to have more efficient conversion rates and better flowability in cold temperatures compared to other oils and fats.

Beside the fact that room temperature ionic liquids (ILs) were initially introduced as alternative green reaction media because of their chemical and physical properties of nonvolatility, nonflammability, thermal stability, and controlled miscibility [7], today show their significant role in controlling the reaction as solvents and catalysts. By combining different cations and anions a variety of ionic liquids can be obtained with desired solvent properties [8]. Moreover, the incorporation of specific functional groups dramatically enhances solubility with specific solutes, such as polar substrates or catalysts [9]. Although ionic liquids have been successfully employed for many organic reactions, their using for the transesterification is scarcely addressed. Wu et al. [10] used functional Brønsted acidic ionic liquids as solvents and catalysts for transesterification of cottonseed oil opening potential of application in the biodiesel production. In addition, mixture of quaternary ammonium salts with glycerol has been used to extract glycerol from biodiesel formed from the reaction of triglycerides with ethanol in the presence of KOH [11]. In general, traditional transesterification is biphasic (methanol + oil phase) and reversible reaction and therefore knowledge on mutual solubility of reactants and products is a paramount for the design of related technological process.Moreover, it was shown that the transesterification reaction takes place in the methanol phase, thus the reaction rate greatly depends on the solubility of oil in methanol phase which is quite low [12]. Therefore, utilization of co-solvent is desirable in order to enhance the solubility of oil in the phase where reaction takes place (methanol-rich phase). To do so, most studies reported utilization of flammable and hazardous solvents such as tetrahydrofuran [13], petroleum ether [14] and hexane [15].

The aim of this study was to examine various ionic liquids as a cosolvent for components involved in biodiesel production. Thus, in this contribution mutual solubility of binary and ternary mixtures containing ionic liquid, soybean oil and methanol in the range of 298 to 343 K is reported, whereas ionic liquid stands for 1-butyl-3-methylimidazolium thiocyanate [C₄MIM][SCN], 1-butyl-3-methylimidazolium bis(trifluoromet hylsulfonyl)imide [C₄MIM][NTf₂], 1-butyl- 3-methylimidazolium dicyanamide [C₄MIM][DCA], 1-Butyl-3-methylimidazolium hexafluorophosphate [C₄MIM] [PF₆], 1-butyl-3-methylzolium hydrogensulfate [C₄MIM][HSO₄], 1-decyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide [C₁₀MIM][NTf₂], methyltriocty lammonium bis(trifluoromethylsulfonyl) imide [ALIQUAT][NTf₂] and methyltrioctylammonium chloride [ALIQUAT][Cl].

The average molecular weight of soybean oil was adapted from our previous work [17]: 866.5 g(mol^-1. Even though the oil is a mixture of various triglycerides, we considered it as a single component. Acid number of the oil before and after deacidification is 0.95 and 0.04 (mg KOH)/(1 g of oil), respectively. The applied method sufficiently decreased the content of free fatty acids, making it suitable for conventional transesterification.

The cloud point data of ternary (methanol + soybean oil + ionic liquid) systems are presented in Table 2.

All ionic liquids used in this study showed complete miscibility with methanol in the whole range of concentrations and studied temperatures. On the contrary, with the exception of ALIQUAT Cl ionic liquid (completely soluble), all the other (ionic liquid + soybean oil) systems studied illustrated limited mutual solubility, which slightly increased as temperature raised. The mutual solubility of the binary (ionic liquid + soybean oil) systems at 298 K is presented in Fig. (1) as solubility of soybean oil in IL-rich phase (Fig. 1a) and solubility of ionic liquid in oil rich phase (Fig. 1b).

Thus, at 298 K, the solubility of soybean oil in imidazolium ionic liquid rich phase goes from 0.16 to 1.36 mol %. A bit higher solubilities of these ionic liquids in soybean oil rich phase were witnessed, typically ranging from (2.2 to 4.1 mol) % at 298 K. In general, imidazolium based ionic liquids, independently of their counter anions, exhibit lower solubility in soybean oil than ionic liquids with the aliquat cation.

Comparing ionic liquids with the same cation, [C₄mim ⁺], we can observe from (Fig. 1a) that the relative affinity of soybean oil to the various anions follows the order [SCN] ^- < [NTf₂]^- < [DCA ^-] < [PF₆] ^- < [HSO₄] ^-. However, a different order of affinities was observed for the soybean oil-rich systems (Fig. 1b).

Fig. (1). Solubility of soybean oil in an ionic liquid (a) and solubility of ionic liquid in soybean oil (b) for various ionic liquids at 298 K. xoil and xIL stand for mole fraction of soybean oil in IL-rich phase and mole fraction of ionic liquid in soybean oil-rich phase, respectively.

The longer alkyl chain in the imidazolium cation caused some what higher solubility of soybean in ionic liquids, ranging from 0.23 mol % in the case of C₄mim NTf₂ to 1.6 mol % in the case of C₁₀mim NTf₂. This is probably due to the fact that the van der Waals interactions increase between cation chain and the alkyl portion of the tryglicerides, similarly to ionic liquid behavior with alcohols [18].

In the case of the aliquat based ionic liquids, huge effect of anion on solubility was noticed. Thus, while aliquat NTf₂ is slightly soluble with soybean oil, the system aliquat [Cl] + soybean oil is completely soluble.

The data reported in Table 2 were used to construct the ternary phase diagrams of the (methanol + soybean oil + IL) systems at atmospheric pressure (Fig. 2). The area inside envelopes corresponds to two phase region.

The huge area of mutual immiscibility (two-phase region) is observed for systems involving C₄mim[SCN], C₄mim NTf₂, C₄mim DCA, C₄mim PF₆, C₄mimHSO₄ and C₁₀mim NTf₂. Somewhat better mutual miscibility was noted for ALIQUAT NTf₂ and ALIQUAT[Cl] systems. As it can be concluded from Figs. (2f-h), increasing the temperature enhanced the mutual solubility.

An excess of alcohol is used in biodiesel production to ensure the maximal yield. The optimal molar ratio of alcohol to triglycerides in most reports is 6:1 [2]. This corresponds to molar ratio of methanol in soybean 0.86 which is a binary mixture exhibiting two-phase system. It is clear from Figs. (2g, h) that both aliquat based ionic liquids can be used as cosolvents to solubilize binary (methanol + soybean oil) mixture.

Fig. (2). Ternary diagrams for ternary methanol + soybean oil + ionic liquid at atmospheric pressure (compositions are given in mole fractions). Ionic liquid stands for [C4mim][SCN] (a), [C4mim][NTf2] (b), [C4mim][DCA] (c), [C4mim][PF6] (d), [C4mim][HSO4] (e), [C10mim][NTf2] (f), [ALIQUAT][NTf2] (g), and [ALIQUAT][Cl] (h). The full lines and filled circles correspond to data at 298 K, while dashed lines and empty circles correspond to data at 343 K.

Data on mutual solubility of binary (soybean oil + ionic liquid) and ternary (soybean oil + methanol + ionic liquid) systems in the temperature range of 298 K to 343 K were reported. The following conclusions were made: 1) imidazolium based ionic liquids lower solubility in soybean oil than ionic liquids with the aliquat cation; 2) the solubility is enhanced by the longer alkyl chain in the imidazolium cation and by temperature increase; 3) aliquat based ionic liquids are good candidate to be used as cosolvents for biphasic (methanol + soybean oil) mixture.

The authors confirm that this article content has no conflict of interest.