Accepted Manuscript Thermodynamic effect of ammonium based ionic liquids on CO2 hydrates phase boundary Muhammad Saad Khan, Cornelius B. Bavoh, Behzad Partoon, Bhajan Lal, Mohamad Azmi Bustam, Azmi Mohamad Shariff PII: DOI: Reference: S0167-7322(17)30866-8 doi: 10.1016/j.molliq.2017.05.045 MOLLIQ 7337 To appear in: Journal of Molecular Liquids Received date: Revised date: Accepted date: 27 February 2017 ###REVISEDDATE### 9 May 2017 Please cite this article as: Muhammad Saad Khan, Cornelius B. Bavoh, Behzad Partoon, Bhajan Lal, Mohamad Azmi Bustam, Azmi Mohamad Shariff , Thermodynamic effect of ammonium based ionic liquids on CO2 hydrates phase boundary, Journal of Molecular Liquids (2017), doi: 10.1016/j.molliq.2017.05.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Thermodynamic Effect of Ammonium Based Ionic Liquids on CO2 Hydrates Phase Boundary Muhammad Saad Khan, Cornelius B Bavoh, Behzad Partoon, Bhajan Lal*, Mohamad Azmi Bustam, Azmi Mohamad Shariff AC CE PT ED M AN US CR IP T Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia. *Corresponding author: bhajan.lal@utp.edu.my +60103858473 /+6053686176 Telephone/Fax: +6053687684; ACCEPTED MANUSCRIPT Abstract In this experimental study, the phase boundary behaviour of CO2 hydrate is reported in the presence of 1, 5, and 10 wt% of three aqueous ammonium based ionic liquids (AILs) solutions. The T-cycle technique is used to measure the hydrate equilibrium conditions of AILs + CO2 + CR IP T H2O hydrate systems within the ranges of 274 – 283 K and 1.80 – 4.20 MPa. All studied AILs inhibited CO2 hydrate with the inhibition effect increasing with AILs concentration. The 10 wt%, TEAOH showed the highest inhibition effect with an average suppression temperature (∆Ŧ) of 1.7 K, followed by TMACl (∆Ŧ = 1.6 K) and then TPrAOH (∆Ŧ = 1.2 K). Furthermore, US COSMO-RS analysis is performed to understand the molecular level inhibition mechanism of AN AILs. In addition, the enthalpies of hydrate dissociation for all studied systems are also determined. The calculated hydrate dissociation enthalpies revealed that all the studied AILs ED not form semi-clathrate hydrates. M show insignificant participation in CO2 hydrate cage formation at all concentrations, hence, do AC CE PT Keywords: CO2 hydrate; ammonium based ionic liquids; phase equilibrium; THI. ACCEPTED MANUSCRIPT 1. Introduction In natural gas processing and transmission, gas hydrates formation is considered as a major flow CR IP T assurance problem. Clathrate hydrates or gas hydrates are non-stoichiometric solid inclusion compounds formed under high pressures and low temperatures conditions whereby water molecules (host) encapsulate gas molecules (guest) through H- bonding [1–3]. Gas hydrates do not only cause pipeline blockages but can also lead to catastrophic damages due to excess US pressure buildup in solid hydrate plugs [4]. Conferring to Xiao-Sen et al. [5], the oil and gas AN industry spends almost $ 200 M annually to avoid hydrate formation. The issue is more critical for high CO2 content gas reservoirs, as carbon dioxide is more prone to form hydrates than CH4 M [6]. The composition of CO2 in the existing natural gas wells varies at different geographical ED locations. High carbon dioxide (CO2) natural gas mostly are encountered in areas such as; South China Sea, Gulf of Thailand, Central European Pannonian Basin, Australian Cooper-Eromanga PT Basin, Colombian Putumayo basin, Ibleo platform, Sicily, Taranaki Basin, New Zealand and the CE North Sea South Viking Graben [1]. Malaysia is one of the major natural gas producers and exporters in the world with estimated reserve of 88.0 tcf [1]. In Malaysia, the K5 and J5 fields AC located in offshore Sarawak produce about 70 and 87 mol% CO2 natural gas [7]. This high amount of CO2 coupled with the deepwater conditions of these fields pose new problems for the exploration and production of these natural gas including the estimation of the type of the gas in the reservoir, the deepwater transportation of the gas, separation of the CO2 from the natural gas and storage of CO2 produced [8]. ACCEPTED MANUSCRIPT Conventional hydrate mitigation methods such as; depressurization, water removal, heating and chemical insertion are the common approaches to preventing hydrate formation, however, in most cases, chemical inhibition is the viable solution for gas hydrate mitigation [1]. Basically, there are three types of gas hydrate inhibitors; thermodynamic hydrate inhibitors (THI) which CR IP T shifts the hydrate equilibrium curve to lower temperatures and higher pressures regions, kinetic hydrate inhibitors (KHI) which delays hydrate formation and lastly, anti-agglomerates (AA) which does not resist hydrate formation. They prevent hydrate crystals from pluging [1,9]. US Traditional THIs such as; methanol and ethylene glycol are widely used, but as exploration and production moves to deeper seas, the addition of such inhibitors offers many problems such as AN large storage volumes and pumping requirements. Also, the volatile nature of the chemicals M reduces their amount available for inhibition. The overuse of methanol may contaminate the hydrocarbons, causing it to be evaluated. The environmental unfriendly nature of these ED traditional inhibitors has drawn ecological and regulatory concerns (HSE) [1,10,11]. Thus, some PT environmentally friendly and efficient chemical based gas hydrate inhibitors are needed to replace the conventional ones. These, ionic liquids have gained much attention as novel gas CE hydrate inhibitors and they are molten salt at room temperature. In addition to their unique AC properties such as tunable cations and anions, extremely low vapor pressures, and easy synthesis from relatively inexpensive materials, they possess electrostatic charges and can form H-bonding with water. This makes them potential candidates for effective and efficient THIs [12,13]. Studies have shown that ILs can be regenerated after been used, thus encouraging their potential application in the oil and gas industry [14,15]. Xiao and Adidhirama (2009) performed a gas hydrate mitigation study using different ionic liquids (ILs) by employing a High-Pressure MicroDSC [16] and found an inhibition influence for CH4 hydrate up to 0.9 K. Since then, there ACCEPTED MANUSCRIPT have been several studies on gas hydrate inhibition via ILs. However, most of the studied ILs are imidazolium bases [17–21]; very limited researchers have studied the effects of other families of ILs (mainly AILs) on gas hydrate mitigation [6,22–25]. Based on this, ammonium based ILs have received some attention in the hydrate community as they exhibit more hydrate inhibition CR IP T potentials due to the presence of nitrogen donor atom in their structures [26]. Also, they provide better ‘greener’ potentials than imidazolium based ILs’ [27,28]. Currently, ammonium based ILs (AILs) are being studied as potential additives for mixed gas separation and gas storage [29]. Li et al.[5], examined the THI influence of tetramethyl ammonium chloride (TMACl) and dialkyl US imidazolium on methane hydrate formation and found that TMACl has shown better CH4 AN hydrate inhibition impact than other studied ILs. In addition, their inhibition performance was in the range of ethylene glycol (EG) [5]. Tariq et al. [23], evaluated AILs as dual functional M inhibitors (DFI) for CH4 gas hydrate. Their findings suggested that all studied AILs exhibited ED CH4 hydrate thermodynamic inhibition behavior at moderate pressure conditions. However, at higher pressures above 7 MPa, the inhibition impact seems to be reduced, and some of the ILs PT worked as thermodynamic promoters. They further suggested that tetramethyl ammonium CE acetate (TMAA) worked as the best THI, while choline octanoate (Ch-Oct) effectively worked as a KHI. There are limited studies in literature on the thermodynamic inhibition influence of CO2 AC hydrate in the presence of AILs [13]. In our recent work on the inhibition effect of TMAOH on CO2 and CH4 hydrates, we reported that TMAOH significantly inhibits CO2 hydrate equilibrium curve with an average suppression temperature (∆Ŧ) of 2.24 K at 10 wt%, owing to its strong Hbonding affinity for water molecules [22]. In this work, phase equilibrium measurements have been performed on CO2 + H2O, TMACl + CO2 + H2O, TEAOH + CO2 + H2O and TPrAOH + CO2 + H2O systems. The hydrate equilibrium ACCEPTED MANUSCRIPT measurements are performed at pressure and temperature ranges of (1.80 to 4.20) MPa and (274 to 285) K, respectively. COSMO-RS study is further performed to understand the molecular level inhibition mechanism of AILs. In addition, the hydrate dissociation enthalpy of each CR IP T system is also calculated by employing the Clausius–Clapeyron equation. 2. Methodology US 2.1 Materials The details of the materials used in this study are shown in Table 1. The aqueous AILs solutions AN are purchased from Merck milli-pore company Germany. CO2 is obtained from Air Products Singapore Private. Deionized water produced by RO membrane plant TKA-LabTowe is used M for the preparation of desired concentration (1, 5 and 10 wt%) of aqueous AIL solutions. ED Table 1: Material used for gas hydrate mitigation study Symbol Purity 1 Carbon Dioxide CO2 99.95 mol% 2 Tetramethylammonium Chloride TMACl 95.95 wt.% 3 Tetraethylammonium Hydroxide TEAOH 95.95 wt.% Tetrapropylammonium Hydroxide TPrAOH 95.95 wt.% Water H2O Deionized 5 CE AC 4 PT Chemical Name 2.2 Experimental Method 2.2.1 Details of experimental setup ACCEPTED MANUSCRIPT The particulars of the experimental setup and procedures adopted are provided in [22],[30]. The apparatus consists of high-pressure equilibrium cell with a volumetric capacity of 500 cm3, with temperature range from 253–523 K and maximum operating pressure of 20 MPa. Conditions such as temperature, and pressure are recorded for every subsequent second with an accuracy of CR IP T ±0.1K, and ±0.01 MPa respectively. Furthermore, the apparatus is fitted with magnetic stirrer consisting of 2-bladed pitch impeller and a 400 rpm motor to provide adequate mixing of the sample in the cell. The cell is submerged in a thermostatic bath, equipped with PID controller to control the bath temperature within ±0.3 ºC accuracy. Moreover, the bath temperature set points US are programmable through a data acquisition system. AN 2.2.2 Experimental procedure for THI measurements M In measuring the phase equilibriums behavior of CO2 + H2O and AILs + CO2 + H2O hydrate ED systems, the isochoric T-cycle method [22] with step heating is applied. Before each experimental run, the cell is washed with distilled water and loaded with the desired liquid PT sample of 100 ml (with or without AILs). Then the cell is inserted into the reactor, and the CE water bath temperature is set to the chosen initial operating condition. A small amount (0.5 MPa) of CO2 is inserted into the reactor and then vacuumed to guarantee there are no hints of AC the air in the reactor. The reactor is then pressurized with CO2 to the desired experimental pressure. In this experiments, the pressure and temperature ranges for CO2 + H2O, AILs and CO2 + H2O hydrate systems are 2.0 – 4.0 MPa and 273 – 283 K, respectively. Once the stabilized conditions are attained, the mechanical stirrer is set at 300 rpm to break the interface boundary of the liquid water and provide adequate mixing during hydrate formation. The fast cooling method is employed to reduce the temperature of the system and to facilitate the hydrate formation. After the desired cooling temperature is accomplished, the system ACCEPTED MANUSCRIPT temperature is allowed to stay constant for an extended time (from 4 to 8 hours). A sudden pressure drops attained in the data logging provide evidence of hydrate formation. When no further pressure drop is observed in the data logging, and hydrate is formed fully, then the reactor is slowly heated step-wise at a rate of 0.01 K/min until complete hydrate dissociation. CR IP T To determine hydrate equilibrium point accurately, the length of every step usually needs 2 to 6 hours. 2.2.3 Phase equilibrium data analysis: US In this experimental work, the average suppression temperature (∆Ŧ) is calculated to determine AN the inhibition performance of three ammonium based ionic liquids CO2 hydrate formation at different concentrations (1, 5, and 10 wt%) in the temperature and pressure ranges of 273 – n  T  T  (T i 1 0 , pi n (Equation 1) PT n  T1, pi ) ED M 283 K and 2.0 – 4.0 MPa, respectively [31]. CE where, T0, pi and T1, pi denotes the equilibrium temperature of CO2 in water and aqueous solution of AILs, respectively. The values of both dissociation temperatures should be attained at the AC same pi and n refers to the number of pressure point considered. The dissociation enthalpies (∆Hdiss), of gas hydrates, are determined through the Clausius– Clapeyron equation by differentiation of the phase equilibrium data (see Equation 2). The values of ∆Hdiss above 273 K can be obtained from the slope of the phase equilibrium data acquired from the Clausius–Clapeyron equation as used by numerous researchers [18,32,33]. ACCEPTED MANUSCRIPT 𝜕 ln 𝑃 1 𝜕 𝑇 =− ∆𝐻𝑑𝑖𝑠𝑠 (Equation 2) 𝑧𝑅 where T and P are the equilibrium temperature and pressure, R denotes the universal gas constant, z represents the compressibility factor of CO2 and ∆Hdiss, the dissociation enthalpy of CR IP T CO2 hydrates. US 2.2.4 COSMO-RS analysis COSMO-RS software is employed to understand the CO2 hydrate inhibition mechanism of AN aqueous AILs at intermolecular level by observing their sigma profiles and H-bonding affinity M for water molecules. All the studied molecules are taken from the parametric file of BP_TZVP_C30_1301 (COSMOlogic GmbH & Co KG, Leverkusen, Germany) and estimated ∑𝑖 𝑥𝑖 PT ∑𝑖 𝑥𝑖𝑝𝑥𝑖(𝜎) CE 𝑝𝑠(𝜎) = ED the sigma profile via COSMOthermX software interface [34–36]. (Equation 3) The distribution of the division given on the sigma (σ) is called σ-profile (ps(σ)). The σ-profile AC of the solvent ps(σ), is defined as the mole fraction (xi) weighted sum of the σ-profiles of its compounds xi, pxi respectively in Equation 3 [37,38]. 3. Results and Discussion ACCEPTED MANUSCRIPT 3.1. Phase Equilibrium Measurement of AILs + CO2 + H2O Gas Hydrate System As stated afore, very limited number of studies are available in the open literature on the thermodynamic inhibition effect of AILs on CO2 hydrate systems [6,39,40]. The phase behavior AC CE PT ED M AN US tabulated in Table 2 and graphically shown in Figure 1. CR IP T of the studied AILs + CO2 + H2O systems at different AILs concentrations (1, 5 and 10 wt%) is ACCEPTED MANUSCRIPT Table 2: Hydrate phase equilibrium data of aqueous AILs for CO2 hydrates at different concentrations (1, 5 and 10) wt%. CE US AN PT TPrAOH + H2O + CO2 M TEAOH + H2O + CO2 ED TMACl + CO2 + H2O 1 wt% T (K) P(MPa) 282.3 4.010 281.5 3.500 279.9 3.011 279.0 2.541 276.7 1.921 282.3 4.10 281.4 3.61 280.2 3.04 278.9 2.56 276.9 2.05 282.3 4.020 281.6 3.511 280.1 3.011 278.9 2.511 277.0 1.991 CR IP T CO2 + H2O T (K) P(MPa) 282.9 3.99 282.2 3.51 280.4 2.95 279.6 2.58 277.4 1.85 5 wt% T (K) P(MPa) 282 3.990 280.9 3.560 279.4 2.960 278.4 2.550 276.4 1.940 282.1 4.110 281.1 3.612 279.8 3.110 278.6 2.560 276.7 2.068 282.4 4.110 281.3 3.501 279.7 3.001 278.7 2.491 276.7 1.955 ` 10 wt% T (K) P(MPa) 281.1 3.920 280.5 3.551 279.1 2.981 278.0 2.551 276.1 2.031 281.6 4.100 280.5 3.580 278.9 3.020 278.2 2.531 276.1 2.047 282.1 4.071 281.1 3.521 279.4 3.011 278.3 2.491 276.3 1.955 AC The obtained results reveal that all studied AILs are able to reduce the CO2 hydrate equilibrium curves towards lower temperature and higher pressure ranges and therefore reflect the thermodynamic inhibition effect. The inhibition effect of all studied AILs are compositiondependent, as increasing AILs composition from 1 – 10 wt% in the solution resulted in more inhibition effect, as shown in Figure 1. The inhibition influence of AILs is due to their strong Hbonding and electrostatic interaction ability with water molecules. This is mainly due to the presence of OH- and Cl- anions in the studied AILs. ACCEPTED MANUSCRIPT To measure the thermodynamic inhibition performance of AILs + CO2 + H2O systems, the ∆Ŧ values of AILs at studied concentrations are obtained and presented in Figure 2. Based on Figure 2, TEAOH + CO2 + H2O system shows the best thermodynamic inhibition performance compared with the two other systems at all concentrations. 1 wt% TEAOH + CO2 + H2O system CR IP T showed a ∆Ŧ value of 0.8 K, while that of 5 and 10 wt% are found to be 1.2 K and 1.7 K, respectively. The ∆Ŧ of TMACl + CO2 + H2O systems are 0.7 K, 1.1 K, and 1.6 K at 1, 5 and 10 wt%, respectively, which is slightly lower than TEAOH + CO2 + H2O hydrates system. Similarly, the ∆Ŧ of TPrAOH + CO2 + H2O are 0.7 K, 0.9 K, and 1.2 K at 1, 5 and 10 wt%, US respectively. AN It should be noted that due to the quadruple moment of CO2; TMACl, TEAOH, and TPrAOH M efficiently interact with CO2. The strong inhibition effect of TEAOH than TMACl and TPrAOH is due to the existence of hydroxyl OH- group, as it is categorized as the most suitable anion for ED H-bonding. The presence of OH- in TEAOH increase its H-bonding affinity, thus, causes more PT disturbance in the activity of water in hydrate formation, which further leads to effective hydrate inhibition. In addition, tetraethylammonium (TEA+) cation is less hydrophobic in ILs AC CE perspective, as it has a shorter alkyl chain, thus, helps to improve its inhibition strength. ACCEPTED MANUSCRIPT 4.5 Pressure (MPa) 3.5 This Work CO TEAOH (10%)2 TEAOH (1%) TEAOH (5%) 4 TMACl (10%) TMACl (5%) TMACl (1%) 3 2.5 2 3.5 3 2.5 2 1 (a) 1.5 1.5 276 277 278 279 280 281 282 283 276 This Work CO2 TPrAOH (10%) TPrAOH (5%) TPrAOH (1%) 3.5 3 2.5 AN Pressure (MPa) 4 277 1.5 277 ED 276 M 2 278 278 279 280 281 Temperature (K) 1 (b) 282 283 US Temperature (K) 4.5 CR IP T 4 Pressure (MPa) 4.5 This Work CO2 279 280 281 Temperature (K) 1 (c) 282 283 Figure 1: Hydrate equilibrium data for 1 wt%, 5 wt% and 10 wt% concentrations of (a) TMACl AC CE PT + CO2 + H2O systems (b) TEAOH+ CO2 + H2O systems and (c) TPrAOH+ CO2 + H2O systems. ACCEPTED MANUSCRIPT 1.8 TMACl 1.6 TEAOH TPrAOH 1.4 1 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 CR IP T ∆Ŧ (K) 1.2 6 7 8 9 10 US Concentration of AILs Figure 2: Average suppression temperature (∆Ŧ) for different concentrations (1, 5 and 10) wt% AN of AILs + CO2 + H2O systems. M TMACl exhibits a slightly less inhibition strength (∆Ŧ values) compared with TEAOH. The ED potential reason is that TMACl is composed of tetramethylammonium (TMA+) cation and chloride (Cl-) anion. TMA+ cation is regarded as the most hydrophilic cation as it has the least PT possible alkyl chain compounds. Also, Cl- is one of the best anion among the halide family in CE forming hydrogen bonds with water. The combination of the lowest possible alkyl chain and the hydrogen bonding affinity of the Cl- in TMACl enables it to show a better inhibition strength AC close to TEAOH. TPrAOH showed the least inhibition activity among the studied AILs; this is due to the relatively long alkyl chain in the tetra propyl ammonium (TPrA+) cation. The presence of longer alkyl chain in TPrA+ increases the hydrophobicity, which hinders its miscibility with water compared to shorter alkyl chain cations such as TMA+ and TEA+. However, the presence of OH- anion improved the inhibition influence of TPrAOH. Therefore, for better understanding and to grasp the intermolecular information among the studied AILs-H2O systems, COSMO-RS simulations is further applied and the results discussed in section 3.2 of this paper. ACCEPTED MANUSCRIPT Figure 3 presents the comparison of AILs + CO2 + H2O system with different types of studied ILs such as; EMMor-BF4 [6], EMMor-Br [6], EMPip-BF4 [6] and EMPip-Br [6], reported by J.H. Cha et al. [6] at 10 wt%. Additionally, the results are also compared with Chen et al. [41] reported results for BMIM-BF4 [41] (imidazolium-based IL). Furthermore, the obtained results CR IP T in this work are compared with TMAOH + CO2 hydrate system at 10 wt% [22]. Based on the comparison results, TMAOH + CO2 slightly performs better than studied AILs in this work and other IL families. However, the inhibition strength of TEAOH (1.7 K) and TMACl (1.6 K) is found to relatively perform better than all compared ILs families at 10 wt%. However, the US inhibition strength of TPrAOH + CO2 + H2O is found to be in the range of other reported ILs AN (see Figure 3). M 4.5 ED PT 3.5 CE 3 2.5 2 1.5 AC Pressure (MPa) 4 274 275 276 277 278 279 Temperature (K) 280 281 282 283 Figure 3: Phase behavior of AILs + CO2 + H2O (10 wt%) in comparison with 10 wt% literature data of Cha et al. [6] and Chen et al. [41]. ACCEPTED MANUSCRIPT 3.2. COSMO-RS analysis of AILs-H2O system The thermodynamic inhibition results (in terms of ∆Ŧ) obtained in an earlier section (see Figure 2) are due to the strong H-bonding affinity of AILs with water molecules. The sigma profiles of CR IP T AILs and water (see Figure 4) are generated in COSMO-RS software to understand the molecular interaction phenomenon. In Figure 4, the left-hand side of the sigma profile shows the polar H-bond donor region (-0.04 to -0.01), the next region in the range of -0.01 to 0.01 is the US non-polar region; while the right hand region of the graph (0.01 to 0.04) represents the the most 35 Non-Polar TEAOH Water TPrAOH TMACl + TEA PT 20 + ED 25 TPrA Polar H-Bond acceptor M Polar H-Bond doner 30 p(σ) AN electronegative area i.e. act as H-bond acceptor [42–45]. 15 Cl- TMA+ CE 10 AC 5 0 -0.04 -0.03 -0.02 OH- -0.01 0.0 σ[e/Å2] 0.01 0.02 0.03 0.04 Figure 4: Sigma profile of TMACl, TEAOH, TPrAOH and H2O. In Figure 4, water shows H-bonding donor and acceptor affinity peaks lengths of -0.016 and 0.018 due to its unique molecular structure which consists of two H atoms and lone pairs of the oxygen atom. In the non-polar area, TMACl exhibited a relatively broad peak in H-bond donor ACCEPTED MANUSCRIPT area due to its methyl functional group. In addition, it also has a wide peak in the H-bond acceptor area for Cl- anion which allows TMACl to exchange H atoms with water and thus, leads to strong CO2 hydrate inhibition (1.6 K) via decrease in water activity. However, solvation of Clfor water is relatively lower than OH- anion due to peak intensity difference between Cl- (25.557) CR IP T and water (1.849) as shown in Figure 4. Additionally, TMA+ cation peak is relatively closer to the peak; this makes it more hydrophilic in nature and thus, exhibits high hydrate inhibition performance compared with other cations. TEAOH is composed of TEA+ (ethyl functional group) and OH- anion. It shows a broader peak in the non-polar region, indicating less US hydrophilic nature as compared to TMA+ cation. Furthermore, the TEA+ cation induces less AN inhibition effect as compared to the OH- anion, which can be observed in an extreme polar Hbond acceptor region peak (0.036). The OH- peak intensity 4.78 is relatively nearer to water peak M 1.849 causing better solvation and results in strong H-bonding and also leads to substantial ED inhibition impact of TEAOH (1.7 K). TPrAOH contains TPA+ cation which exhibits the least hydrophilicity among the studied AILs and shows the least inhibition performance (1.2 K) even PT in the presence of OH- anion. An earlier study on ILs via COSMO-RS [45] also revealed that CE the H-bonding energies (EHB) and sigma profiles of compounds are related to their hydrate suppression temperature (∆Ŧ). The higher electronegativity of OH- anion brings about higher H- AC bonding energy and thus interacts better with the water molecules, resulting in more hydrate inhibition in comparison with Cl- anion. A similar conclusion is stated in the latest study [46] via COSMO-RS sigma profile analysis that, the cationic and anionic activity and H-bonding ability of TMAOH affect its thermodynamic inhibition performance. For further analysis, the total area of sigma profile along with the areas of the various sigma profile regions for all studied AILs are presented in Figure 5. The total area of AILs along with ACCEPTED MANUSCRIPT water for different areas of sigma profile is depicted in Figure 5(a). For further quantification; the sigma profile area of water is subtracted among AILs and plotted in Figure 5(b). The obtained results revealed that the sigma profile peaks in various regions contribute separately to the inhibition influence of AILs. Considering the total area, if the total area of the sigma profile is CR IP T close to the water, it is considered as a good inhibitor since it has better overall miscibility and H-bonding ability than other molecules (see Figure 5(a)). TMACl is found to have the least total area among studied AILs. However, its overall inhibition at 10 wt% TMACl (1.6 K) is found to 0.3 (a) AN 0.27 0.21 0.21 M 0.18 0.15 0.12 ED σ [e/A2] (b) 0.24 0.24 0.09 0.06 0 Water TMACl PT 0.03 TEAOH σ [e/A2] 0.3 0.27 US be less than TEAOH (1.7 K). 0.18 0.15 0.12 0.09 0.06 0.03 0 TPrAOH TMACl Area of AILs Non_Polar HB_Accp CE Total Area TEAOH TPrAOH Area of AILs HB_Donor Total Area Non_Polar HB_Accp AC Figure 5: (a) Sigma surface area of various regions of AILs and water. (b) Sigma surface area of AILs subtracted by water. Therefore, the non-polar and H-Bond acceptor areas are calculated to know the significance or contribution of cations and anions in gas hydrate inhibition respectively. TMACl possesses maximum 0.048 area for H-Bond acceptor region, compared to 0.032 for TEAOH or TPrAOH due to the Cl- and OH- anion, respectively (see Figure 5(b)). This phenomenon clearly justifies the high inhibition of TEAOH compared to TMACl, which originally comes from and H-Bond acceptor region. Similarly, the non-polar region also induces their cationic effect in hydrate ACCEPTED MANUSCRIPT inhibition. TMACl have the less non-polar area among all the AILs due to methyl group while TEAOH and TPrAOH have higher areas because of ethyl and propyl functional groups, respectively. Regarding inhibition activity, TMACl is very close to TEAOH; although Cl- anion is not efficient as OH- anion. The lower peak of 0.074 for TMACl as compared to higher peaks CR IP T 0.171 and 0.250 for TEAOH and TPrAOH, respectively justifies the strong inhibition behavior of the latter THIs as shown in Figure 5(b). 3.3. Enthalpy of dissociation for AILs + CO2 + H2O gas hydrate US The calculated ∆Hdiss for AILs + CO2 + H2O hydrate systems are presented in Table 3. It is assumed that the system mainly contains only liquid and gas phases at hydrate equilibrium AN condition, therefore, the amount of hydrate phase is negligible [30]. The ∆Hdiss of H2O + CO2 is M 64.73 kJ/mol, which lies within the range of CO2 hydrate enthalpy data [7]. In Table 3, it can be ED observed that the enthalpies of all studied AILs + CO2 + H2O systems at various concentrations appears to be very close to that of pure CO2 hydrate. This behavior, suggests that the presence of PT AILs have no significant influence on the enthalpy of the system and therefore does not take part CE or affect the CO2 hydrate structure and cages occupancy. Hence, only sI CO2 hydrate structure is AC formed for all the studied systems. ACCEPTED MANUSCRIPT Table 3: Calculated molar enthalpies of hydrate dissociation, ∆Hdiss (kJ/mol), of CO2 hydrate in the presence of AILs + CO2 + H2O solutions. AIL concentration in the solution CO2 + H2O 64.739 ∆Hdiss (KJ/mol) AIL + CO2 + H2O 1 wt% 5 wt% 10 wt% TMACl + CO2 + H2O 64.560 64.486 64.425 TEAOH + CO2 + H2O 63.545 63.293 63.100 TPrAOH + CO2 + H2O 64.732 64.551 64.829 CR IP T ∆Hdiss (KJ/mol) US 4. Conclusion AN In this work, the hydrate phase equilibrium measurements for AILs+ CO2 + H2O systems are reported. The obtained results revealed that the presence of AILs disrupts the water activity in M hydrate formation by decreasing the hydrate phase boundary temperature of TMACl + CO2 + ED H2O, TEAOH + CO2 + H2O, and TPrAOH + CO2 + H2O systems up to 1.6 K, 1.7 K and 1.2 K at 10 wt%, respectively, which is relatively significant in ILs perspectives. Additionally, COSMO- PT RS analysis shows that the presence of both H-bond and the smaller non-polar area in sigma CE profile of AILs leads to better hydrate inhibition impact. Furthermore, the molar enthalpies of dissociation for the AILs + CO2 + H2O hydrate systems suggested that the studied AILs are not AC involving in the hydrate crystalline structure and their impact on the inhibition is mainly due to their influence on the activity of water molecules. Acknowledgment This study is supported by PETRONAS Research Sdn Bhd (PRSB) under TD Project Grant No. 053C1-024. The authors would like to thanks, Chemical Engineering Department, Universiti ACCEPTED MANUSCRIPT Teknologi PETRONAS for providing facilities throughout the studies. The authors also like to acknowledge and appreciate the Centre of Research in Ionic Liquids and Research Centre for CO2 Capture for providing laboratory and technical services. CR IP T Nomenclature Ammonium based Ionic liquid AAs Anti-agglomerates BMIM-BF4 1-butyl-3-methyl imidazolium tetrafluoroborate BMIM-Br 1-butyl-3-methyl imidazolium chloride BMIM-Cl 1-butyl-3-methyl imidazolium chloride BMIM-HSO4 1-butyl-3-methyl imidazolium hydrogen sulfate Ch-But choline butyrate Ch-iB choline iso-butyrate ED M AN US AIL choline hexanoate PT Ch-Hex Ch-Oct CE CH4 EHB EMF AC CO2 choline octanoate Methane Carbon Dioxide Hydrogen bonding energy Misfit energy ET Total internal energy EVdW Van der wall energy EMMor-Br N-ethyl-N-methyl morpholinium bromide EMMor-BF4 N-ethyl-N-methylmorpholinium tetrafluoroborate ACCEPTED MANUSCRIPT N-ethyl-N-methyl piperidinium bromide EMPip-BF4 N-ethyl-N-methylpiperidinium tetrafluoroborate ∆H dissociation enthalpies KHIs kinetic hydrate inhibitors LDHIs Low dosage hydrate inhibitors PEO Polyethylene oxide THI Thermodynamic hydrate inhibitor TMAA tetra-alkyl ammonium acetate TMACl Tetramethyl ammonium Chloride TMAOH Tetramethylammonium hydroxide TEAOH Tetraethylammonium hydroxide TPrAOH Tetrapropylammonium hydroxide σ-profile Sigma profile US AN M ED ∆Ŧ Average suppression temperature Suppression temperature CE References PT ∆T E.D. 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Mellon, Influence of tetramethylammonium hydroxide on methane and carbon dioxide gas hydrate phase AC CE PT ED M AN US CR IP T equilibrium conditions, Fluid Phase Equilib. Volume 440 (2017) 1–8. ACCEPTED MANUSCRIPT AC CE PT ED M AN US CR IP T Graphical Abstract ACCEPTED MANUSCRIPT Research Highlights  The phase boundaries measurement and ∆Ŧ of AILs + CO2 + H2O are reported.  COSM-RS applied to study the mechanism of the thermodynamic inhibition for AILs.  The enthalpy of hydrate dissociation for the studied (AIL+ CO2 + H2O) systems revealed AC CE PT ED M AN US CR IP T that AILs are not involved in hydrate crystalline structure.