br Introduction Gas hydrates are crystalline solid compounds

Introduction Gas hydrates are crystalline solid compounds in which gas molecules (called guest molecules) are surrounded by water molecules, forming a regular cavity crystalline structure. Gas molecules with a desirable shape and size such as methane, ethane, propane, carbon dioxide and etc. can be placed in the solid crystalline as guest molecules [1,2]. Gas hydrates have been studied in the literature due to either their industrial applications (in the gas separation, natural gas storage and transportation, etc.) or their industrial problem in blockage of natural gas transportation pipelines [3,4]. In the field of gas pipelines blockages, some chemicals called thermodynamic hydrate inhibitors have been introduced to prevent or delay the natural gas hydrate formation. Traditionally, particular types of these materials have been used as industrial inhibitors such as methanol, glycerol, Celastrol mg glycol, and triethylene glycol. However, new gas hydrate inhibitors such as ionic liquids have been recommended in the recent years [2,5]. Ionic liquids, known as salts with a melting point below the boiling point of water, though these materials are usually in the liquid phase at the room temperature [6]. Along with experimental studies performed to identify the promising candidates of hydrate inhibitors, the thermodynamic modeling should also be conducted. There are two main approaches for thermodynamic modeling of gas hydrate formation and dissociation; 1) van der Waals-Platteeuw, 2) two-step hydrate formation theory of Chen and Guo [7,8]. The CPA EoS was presented by Kontogeorgis et al., in 1996 [23]. The proposed equation is composed of two parts (SRK EOS + association term) such that it can well predict the thermodynamic behavior of hydrogen bonding materials as a result of considering the association term. Yakoumis et al. (1997) examined the ability of CPA EoS for VLE calculation of alcohol/hydrocarbon systems in comparison with the SRK equation of state. Their results indicated that the CPA EoS yields far better predictions for these systems [24]. Kontogeorgis et al. (2000) used the sCPA EoS for polymer solutions and they achieved very satisfactory results [25]. In addition to hydrogen bonding effects, the association term of CPA might be used to consider the effect of ions in calculating the activity coefficient of ionic liquids in liquid phase. This speculation has previously been applied to model the ionic liquids phase behavior through PC-SAFT EoS. Kamil et al. (2012) performed thermodynamic modeling of ionic liquids with PC-SAFT. They considered the effects of ions in the association term and achieved to the good results [26]. Therefore, given the similarity of association terms in SAFT and CPA equation of states, ionic liquids might be modeled with CPA EoS, such that the effect of ions might be considered in the association term of CPA EoS. With regards to this speculation, as already mentioned, H. Soltani Panah (2017) used the CPA EoS to model prediction of the solubility of CO2 and H2S across various ionic liquids [22].