**2.2 Simulation of fundamental properties value in COSMO-RS**

After obtaining the data of IL inhibition ability, now it is the time to collect another set of data, which is the fundamental property value of IL-hydrate system. Here, COSMO-RS software will be used to carry out the simulation. In COSMO-RS, all calculation works are performed based on density functional theory (DFT), utilizing the triple-zeta valence polarized (TZVP) basis set [50]. **Figure 2** shows the entire computational method of COSMO-RS.

As regards **Figure 2**, COSMO-RS first requires the input of molecular structure [51]. After this, the charge density of a segment on each molecule surface will be calculated in a virtual conductor. The distribution of this charge density on the entire surface of the molecule will then generate a sigma profile (*σ*-profile) through the use of COSMOtherm software [52]. Then, the *σ*-profile will now be used as the basis by COSMO-RS to predict the desired thermodynamic properties. Nevertheless, it is to be noted that among the computational process being shown in **Figure 2**, a user is only required to insert the input, while all the computational process will be carried out by the software itself. Therefore, it is utmost important to input the right information to extract the desired output.

The input or simulation method of COSMO-RS in this work has been conducted by referring to the work of Kurnia et al. [39, 53]. **Figure 3** shows the required input for calculating hydrogen bonding value before a proper simulation could be run.

As observed from **Figure 3**, the required inputs are temperature and the mole fraction of IL-hydrate system. For this work, the temperature is fixed at

**Figure 2.** *Flowchart of predicting thermodynamic properties through COSMO-RS.*


#### **Figure 3.**

*Inputs required to run an IL-hydrate system simulation in COSMO-RS.*


*\* Assuming 100 g of mixture and IL is inserted at a mass fraction of 10 wt%, then 90 g will be water and 10 g will be IL.*

#### **Table 2.**

*Example of calculation of mole fraction.*

10° C, which is the normal temperature where hydrate will start to form. The effect of temperature is also proven not to be significant in this work, which will be explained later in the section of result and discussion. Next, the right value of mole fraction has to be entered for all four components including cation, anion, water, and involved gas. These mole fraction values need to be calculated beforehand as shown in **Table 2**. Similar to an experimental method that has been carried out by the chosen papers [25, 30, 49, 54], this simulation also considers that IL is inserted into the water at a mass fraction 10 wt%. Besides, since COSMO-RS considers IL is made up of equimolar cation and anion, a mole of IL will be divided equally into half a mole of cation and half a mole of the anion in the calculation [36, 55, 56].

When all inputs are inserted, the simulation can now be run. Similar simulation method is applied for all other desired properties including sigma profile, activity coefficient, and solubility of IL in water. When all fundamental property value is collected, the next step is the identification of pattern and, later, the development of correlation using multiple regression analysis.

#### **2.3 Prediction of inhibition ability of ammonium-based ILs**

In total, 20 ammonium-based ILs have been selected for this study based on literature review. For cations, only shorter alkyl chains cations starting from tetramethylammonium up to tetrabutylammonium cations are chosen because longer cations are not effective [29, 30]. This might be because shorter alkyl chains are easier to be adsorbed by crystal surface. Longer alkyl chain, on the other hand, might even promote the formation of hydrates due to their increased hydrophobicity to react with water [57]. On the other hand, anions are made up of halide group

**149**

**Table 3.**

**3. Progress and discussion**

*List of ammonium-based ILs being predicted.*

related to IL inhibition ability.

**3.1 Correlation development and validation**

*Pre-Screening of Ionic Liquids as Gas Hydrate Inhibitor via Application…*

and tetrafluoroborate [BF4]<sup>−</sup> and hydroxide [OH]<sup>−</sup> ions due to their strong electrostatic charges and tendency to form hydrogen bonding with water [30] (**Table 3**). All of the above chemicals will be simulated and calculated in COSMO-RS, which the calculations were carried out using TURBOMOLE6.1. The quantum chemical calculation follows the DFT, using the BP functional B88-86 with a TZVP

Using the four fundamental properties that have been identified earlier, an effort to relate them with the effectiveness of IL as a hydrate inhibitor has been carried out. These four properties are sigma profile, hydrogen bonding energy, activity coefficient, and solubility of IL in water. The following sections now thoroughly report and discuss if these four fundamental properties have successfully been

basis set and the resolution of identity standard (RI) approximation.

*DOI: http://dx.doi.org/10.5772/intechopen.86847*

*Pre-Screening of Ionic Liquids as Gas Hydrate Inhibitor via Application… DOI: http://dx.doi.org/10.5772/intechopen.86847*


**Table 3.**

*Solvents, Ionic Liquids and Solvent Effects*

*Inputs required to run an IL-hydrate system simulation in COSMO-RS.*

**148**

10°

**Table 2.**

**Figure 3.**

*\**

the calculation [36, 55, 56].

*Example of calculation of mole fraction.*

of correlation using multiple regression analysis.

**2.3 Prediction of inhibition ability of ammonium-based ILs**

C, which is the normal temperature where hydrate will start to form. The effect of temperature is also proven not to be significant in this work, which will be explained later in the section of result and discussion. Next, the right value of mole fraction has to be entered for all four components including cation, anion, water, and involved gas. These mole fraction values need to be calculated beforehand as shown in **Table 2**. Similar to an experimental method that has been carried out by the chosen papers [25, 30, 49, 54], this simulation also considers that IL is inserted into the water at a mass fraction 10 wt%. Besides, since COSMO-RS considers IL is made up of equimolar cation and anion, a mole of IL will be divided equally into half a mole of cation and half a mole of the anion in

 *Assuming 100 g of mixture and IL is inserted at a mass fraction of 10 wt%, then 90 g will be water and 10 g will be IL.*

When all inputs are inserted, the simulation can now be run. Similar simulation method is applied for all other desired properties including sigma profile, activity coefficient, and solubility of IL in water. When all fundamental property value is collected, the next step is the identification of pattern and, later, the development

In total, 20 ammonium-based ILs have been selected for this study based on literature review. For cations, only shorter alkyl chains cations starting from tetramethylammonium up to tetrabutylammonium cations are chosen because longer cations are not effective [29, 30]. This might be because shorter alkyl chains are easier to be adsorbed by crystal surface. Longer alkyl chain, on the other hand, might even promote the formation of hydrates due to their increased hydrophobicity to react with water [57]. On the other hand, anions are made up of halide group *List of ammonium-based ILs being predicted.*

and tetrafluoroborate [BF4]<sup>−</sup> and hydroxide [OH]<sup>−</sup> ions due to their strong electrostatic charges and tendency to form hydrogen bonding with water [30] (**Table 3**).

All of the above chemicals will be simulated and calculated in COSMO-RS, which the calculations were carried out using TURBOMOLE6.1. The quantum chemical calculation follows the DFT, using the BP functional B88-86 with a TZVP basis set and the resolution of identity standard (RI) approximation.
