# Review on Application of Quaternary Ammonium Salts for Gas Hydrate Inhibition
## 3 Gas Hydrate Inhibition
### Quaternary Ammonium Salts
The average suppression temperature when inhibitor is tested with CO\({}_{2}\) gas is better compared to CH\({}_{4}\) gas since it gives a slightly higher value, as shown in Figure 4.  
It has been suggested that anions of ILs play a leading role in inhibiting gas hydrates thermodynamically, while cations can also contribute to gas hydrate inhibition when specific functional groups, like a hydroxyl group (-OH), are added to them, or their chain length is modified, thus making them able to interact with water molecules [29; 31; 38; 71]. Furthermore, the thermodynamic gas hydrate inhibition efficiency for ILs with the same anion attached to shorter alkyl chain (C\({}_{2}\)) substituent is better than that of ILs with longer alkyl chain (C\({}_{4}\)) substituent [10; 72]. This indicates that the length of alkyl chain [73] is one of the factors that contributes to the performance of hydrate inhibitor. Larger alkyl chain carries hydrophobicity and prohibits cations in ILs to have a bond with the hydroxyl  
Figure 4: Average Suppression Temperature at 5 wt% and 10 wt% for Different Molecular Weight of AmmoniumBased ILs. ion [58]. In studying the effect of IL cations, the efficiency of ILs as THIs reduced with increasing chain length of the cation [43; 74]. This is further evidenced by a study conducted by [20] using several ammonium based ILs at 10 wt% by maintaining the same anion, where it shows a similar trend of increasing average suppression temperature as the molecular weight decreases. It is also observed that the concentration of ILs is directly proportional to the average suppression temperature, as shown in research done by [37]. Inhibition performance is also enhanced when higher concentrations of ILs are used. \(\Delta H_{diss}\) is referred to the gas hydrate dissociation enthalpy. Dissociation enthalpies is calculated by employing Clausius-Clapeyron equation. For this purpose, derivative of the experimental HLVE data is obtained. Dissociation enthalpy is calculated by finding slope of hydrate liquid vapor equilibrium data using the Clausius-Clapeyron equation as the following formula (2) [58]. \[\frac{\mathrm{d}\ln P}{d\frac{1}{T}}=\frac{\Delta H_{diss}}{zR} \tag{2}\]  
where in \(T\) is the equilibrium temperature and \(P\) is the pressure at which equilibrium temperature is inspected. Universal gas constant is presented as \(R\) and \(z\) is the compressibility factor of the gas used [58]. Hydrate dissociation enthalpy, \(\Delta H_{diss}\) is dependent upon two main factors which are, the ability of clathrate structure to form hydrogen bond and cage occupancy of gas molecules [75]. Since the dissociation enthalpy values do not significantly change, it can be concluded that the chemical compounds used as inhibitor did not contribute significantly in the hydrate crystallization phase [58]. Other than average suppression temperature and hydrate dissociation enthalpy, the performance of hydrate inhibitor is also evaluated on its electrical conductivity. Recently, Ref. [56] have studied on the effects of tetrabutyl ammonium bromide (TBAB), tetraethyl ammonium bromide (TEAB), and tetramethyl ammonium bromide (TMAB), [56] on inhibiting CH\({}_{4}\) gas hydrate formation by isochoric pressure search method. The experiment was conducted at temperature range of 282.4-276.8 K and pressure range of 4.2-7.6 MPa by varying concentrations, i.e., 0.05 and 0.10 mass fraction. It is observed that the addition of TMAB and TEAB shifts the phase equilibrium curve of CH\({}_{4}\) gas hydrate to lower temperature and higher pressure regions, whereas the addition of TBAB demonstrates characteristic of hydrate formation effect. Electrical conductivity experiments results indicate that the shorter alkyl chain length of the ammonium ILs yield to higher electrical conductivity, and the sequence of electrical conductivity is TMAB \(>\) TEAB \(>\) TBAB [56]. The electrical conductivity is also found to be directly proportional to the concentration.