# Review on Application of Quaternary Ammonium Salts for Gas Hydrate Inhibition
## 3 Gas Hydrate Inhibition
### Quaternary Ammonium Salts  
Quaternary ammonium salts (quats) (QAS) are one of the common economical IL compounds applied in industry [48]. Among their advantageous characteristics include having surface-active properties, bioactive, and possess anti-microbial activity [49; 50]. Over the last decade, researchers have been investigating on this class of IL and found their improved chemical and thermal stability compared to imidazolium and pyridinium based ILs, their solvating properties and unique miscibility which can be utilized in specific applications [45; 51; 52; 53; 54]. A typical QAS consists of a positively-charged nitrogen atom attached to four carbon atoms [55]. Figure 3 shows a typical structure of QAS, tetramethyl ammonium chloride. Figure 2: Schematic diagram of kinetic hydrate inhibition by adsorption and perturbation [36]. Table 1 shows a summary of several studies conducted on QAS for gas hydrate inhibition and a brief findings on their inhibition performance. In evaluating the performance of QAS for gas hydrate inhibition, several methods has been applied, such as the T-cycle method and using highpressure micro differential scanning calorimetry equipment. The Isochoric constant cooling method or T-cycle method is one of the most used method, carried out by [37; 56; 57; 58; 59; 60; 61; 62] with different type of gases to study on the kinetic inhibition of the selected compounds. In this method, 100 mL of liquid sample is poured into the cell which is then vacuumed thoroughly to remove excess air that may still exist in the cell [20; 63]. The initial gas temperature is set to a temperature above the hydrate equilibrium temperature to avoid gas hydrate formation during pressurization. Then, the cell was pressurized to the specified experimental pressure. The reactor was kept for stabilization until the pressure reaches at its equilibrium with the experimental condition. The pressure and temperature measurements start getting logged when the temperature begins to decrease by rapid cooling method to facilitate the hydrate formation as it quickly rises to the desired experimental temperatures [20]. The pressure and temperature profiles are recorded for every 10 s through data acquisition software. Figure 3: Molecular Structure of tetramethyl ammonium chloride. In order to establish the effects of additional chemicals and salts on water activity, there are different types of thermodynamic models available, such as the Dickens and Quinby Hunt [58; 66] which is useful in determining the hydrate liquid vapor equilibrium for CH\({}_{4}\) hydrates. This model has also been used by other researchers [67; 68] due to its accuracy in predicting the hydrate phases stability boundaries [69] in the presence of IL solutions and amino acids. In evaluating the performance of a thermodynamic hydrate inhibitor, the average suppression temperature, \(\Delta\)T is one of the key indicator [70]. The suppression temperature (\(\Delta\)T) is calculated by the following formula (1) [20]. \[T=\frac{\Delta\text{T}}{n}=\frac{\sum_{i=1}^{n}\left(T_{0,\;pi}-T_{1,\;pi} \right)}{n} \tag{1}\]  
where \(T_{0,\;pi}\) is the temperature at equilibrium conditions for gas in a blank sample which does not contain any compound. The equilibrium temperature of gas containing inhibitor is represented by \(T_{1,\;pi}\). In both cases, the dissociation temperatures must be determined at the same pressure values. Symbol 'n' denotes the number of points for pressure considered in the experimentation. [58]. In researches conducted by [58; 59] using different types of gases, they demonstrate that as the molecular weight of the IL reduces, the average suppression temperature, \(\Delta\)T increases.