**3. Progress and discussion**

### **3.1 Correlation development and validation**

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 related to IL inhibition ability.

## *3.1.1 Interpretation of sigma profile graphs*

A sigma profile graph in COSMO-RS allows us to understand certain aspects of an IL-water system. The main information we can obtain from the graph is to learn about the hydrophobicity of IL and the tendency of IL to act as a hydrogen bond donor or hydrogen bond acceptor. According to Klamt [5, 58], the sigma profile graph can be divided into three regions. The first region is the hydrogen bond donor region (at the left of −1.0 e/nm<sup>2</sup> ), the second region is nonpolar region (between −1.0 and 1.0 e/nm2 ), and the thirdly region is the acceptor region (at the right of 1.0 e/nm2 ). By judging at which region the peak of an IL locates, the tendency of IL to act as hydrogen bond donor or acceptor would be identified. Generally, a peak that locates at the right side of the sigma profile graph indicates the more electronegative area and acts as an H-bond acceptor.

Now, **Figure 4** shows the sigma profile graph of EMIM-Cl, BMIM-Br, and water molecules. Looking at the sigma profile of water molecules as shown in **Figure 4**, it is observed that water has two high peaks, one in the hydrogen bond donor region and another in the acceptor region [39]. This indicates that water has a high affinity toward both acceptor and donor. Furthermore, **Figure 4** shows the sigma profile of two ILs, which are EMIM-Cl and BMIM-Br. From the figure, it is observed that cations EMIM and BMIM both have their peak in the nonpolar region. However, water molecules which have peaks in the polar region tend to have higher affinity only with strong hydrogen bond donor or acceptor, but not cation that lays its peak in the nonpolar region [59]. As a result, cations do not interact much with water molecules. Meanwhile, anions that have their peaks in hydrogen bonding acceptor region are more attractive to water molecules. Hence, this inferred that anion is the main ion that interacts with water molecules to prevent hydrate formation, whereas cation merely contributes very slightly in the process [57].

Moreover, we can see that EMIM, which has a shorter alkyl chain length, has its peak nearer to the polar region than BMIM. As consequences, EMIM is also more polarized and hydrophilic than BMIM, which is a desired characteristic of a good hydrate inhibitor. This also proves that a cation with shorter alkyl chain length is preferable during the tuning of IL inhibitor, as a shorter cation is less bulky and hence can more effectively interact with water molecules [49, 60]. For anion, Cl<sup>−</sup> proves itself to be a better H-bond acceptor as it has a peak at the right side of the graph, which is the indication of its further electronegative. This at the same time means that Cl<sup>−</sup> will be more effective in accepting H-bond from water molecules than Br<sup>−</sup>. Therefore, this makes Cl<sup>−</sup> more hydrophilic and serves as a better anion for hydrate inhibitor.

**151**

**Figure 5.**

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

IL effectiveness as EMIM-Cl > EMIM-Br > BMIM-Cl > BMIM-Br.

In short, referring to **Figure 4**, we can see that EMIM-Cl is the best combination of ions among the two types of ILs. Due to its lower alkyl chain length cation and a more electronegative anion, it should perform the best among the four ILs. This deduction is supported by the work of Xiao et al. [30], which reported the order of

Although hydrogen bonding strength has been widely quoted to have a relationship with the effectiveness of IL as hydrate inhibitor [29, 30], so far, no work has been conducted to prove this relationship. In this work, validation is done and has successfully proven that a linear relationship exists between hydrogen bonding strength and the effectiveness of IL as hydrate. This linearity is validated through four different sets of data that comes from three papers [25, 30, 61]. All four sets of data show good linearity relationship, with the highest regression value as *R*<sup>2</sup>

effectiveness could be made through the comparison of hydrogen bonding strength. Besides proving this relationship, several interesting findings have also been observed throughout the process. Firstly, computation of COSMO-RS, in total, will calculate three kinds of energy value for an IL, namely, misfit energy (*E*MF), hydrogen bonding energy (*E*HB), and van der Waals energy (*E*vdW). The summation of these three energies leads to the value of total interaction energy (*E*int). Although hydrogen bonding strength is known to affect the effectiveness of IL, the significance of other energies could not be neglected yet. Hence, in **Figure 8**, all types of predicted energies including *E*MF, *E*vdW, *E*HB, and *E*INT are plotted against average depression temperature to determine if these energies could also affect the effec-

**Figure 5** demonstrates that for ILs with BMIM cation, it is evidently shown the anion contributes more to the total interaction energy than the cation. The reason behind this is virtually consistent; van der Waals energies are nearly constant for all of the tested ILs and have thus no effect on the temperature depression. The contribution of misfit energy, having only a regression value of 0.2247, is also negligible. This leaves the hydrogen bonding energy to be the only energy that plays an essential role in affecting the effectiveness of BMIM-ILs. Furthermore, the relationship between total interaction energy (*E*INT) and temperature depression is also not convincing. This graph hence supports the earlier statement that hydrogen

*Average temperature depression from Sabil et al. [25] work vs. types of predicted energy (binary components).*

= 0.8926. As a result, this implies that the prediction of IL

= 1

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

*3.1.2 Hydrogen bonding*

and the lowest as *R*<sup>2</sup>

tiveness of ILs as hydrate inhibitor.

**Figure 4.** *Sigma profile graph of EMIM-Cl and BMIM-Br.*

In short, referring to **Figure 4**, we can see that EMIM-Cl is the best combination of ions among the two types of ILs. Due to its lower alkyl chain length cation and a more electronegative anion, it should perform the best among the four ILs. This deduction is supported by the work of Xiao et al. [30], which reported the order of IL effectiveness as EMIM-Cl > EMIM-Br > BMIM-Cl > BMIM-Br.
