Effect of SDS Surfactant on Methane Hydrate Formation: A Molecular

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Effect of SDS Surfactant on Methane Hydrate Formation: A Molecular Dynamics Study Nilesh Choudhary, Vrushali R Hande, Sudip Roy, Suman Chakrabarty, and Rajnish Kumar J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02285 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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The Journal of Physical Chemistry

Effect of SDS Surfactant on Methane Hydrate Formation: A Molecular Dynamics Study Nilesh Choudhary,a, c, Ϯ Vrushali R. Hande,b, c, Ϯ Sudip Roy,b Suman Chakrabarty,d, * and Rajnish Kumar e, ∗: a

Chemical Engineering and Process Development Division, CSIR-National Chemical

Laboratory, Pune 411008, Maharashtra, India. b

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune

411008, Maharashtra, India. c

Academy of Scientific and Innovative Research, Delhi – Mathura Road, New Delhi

110025, India. d

School of Chemical Sciences, National Institute of Science Education and Research

(NISER) Bhubaneswar, HBNI, P.O. Bhimpur-Padanpur, 752050, Odisha, India. e

Department of Chemical Engineering, Indian Institute of Technology – Madras, Chennai

600036, Tamilnadu, India. Ϯ

These two authors contributed equally.

∗ Corresponding authors: R. K. Tel.: +91 442 2574180; email: [email protected] and S. C. Tel.: +91 674 2494064; email: [email protected]

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ABSTRACT In experimental studies, it has been observed that presence of sodium dodecyl sulfate (SDS) significantly increases the kinetics of hydrate formation and the final water-to-hydrate conversion ratio. In this study we intend to understand the molecular mechanism behind the effect of SDS on the formation of methane hydrate through molecular dynamics simulations. Hydrate formation conditions similar to that of laboratory experiments were chosen to study hydrate growth kinetics in 1 wt% SDS solution. We also investigate the effect of interactions with isolated SDS molecules on methane hydrate growth. It was observed that the hydrophobic tail part of SDS molecule favourably interacts with the growing hydrate surface and may occupy the partial hydrate cages while the head groups remain exposed to water.


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INTRODUCTION Gas hydrate is potential energy source attributable to its abundance in deep oceans. Also, it can assist several industrial unit operations like water desalination, gas storage, and transportation as well as refrigeration and selective gas separation.1–4 Gas hydrates are clathrate, and ice-like crystalline solids made up of 85% water and 15% gas. The gasses like methane, propane, CO2 can form gas hydrates through entrapping themselves into water cages typically at moderate pressure (of 1-10 MPa) and lower temperature (close to 0oC). During the gas hydrate formation, gas and water molecules simultaneously coordinate with each other such that the water cages develop around guest (gas) molecules with the help of intermolecular hydrogen bonding of water. Subsequently, grown cages act as nucleating sites for further gas hydrate growth.

Size and interactions of guest molecules regulate different types of cages, and longrange ordering of cages results in final crystalline structure.1 Different gases thus have their own thermodynamic conditions for hydrate formation, which is appropriate for applications like selective separation of gas mixture and gas storage.2,4 In such applications, the focus is to enhance the hydrate formation kinetics and identify suitable operating conditions such that a favourable separation efficiency is obtained. On the other hand, the formation of gas hydrate plug may cause blockage in oil and gas transmission pipelines, which is one of the major challenges for flow assurance.5 Therefore, the utilization of gas hydrates in industrial applications like gas separation, flow assurance, requires enhanced/altered gas hydrate formation and decomposition process.2,4–7 Usage of additives (and change in temperature and pressure condition for hydrate growth) can alter the hydrate growth rate and is considered a promising method to scale up the formation and decomposition processes. Several known



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additives can control these processes either thermodynamically or kinetically. For example, additives like methanol and glycol act as thermodynamic hydrate inhibitor (THI) and polyvinylpyrrolidone (PVP) acts as kinetic hydrate inhibitor (KHI) making them suitable for flow assurance application.8,9 On the other hand, tetrahydrofuran (THF) acts as thermodynamic hydrate promoter, and sodium dodecyl sulfate (SDS) acts as kinetic hydrate promoter.8–12 These kinetic hydrate promoters are utilized for reducing induction time of hydrate nucleation, enhancing hydrate growth rate, and maximizing the water to hydrate conversion efficiency. Therefore, usage of gas hydrate promoters is beneficial in reducing the operational cost of the gas hydrate formation process. The hydrate-based gas separation (HBGS) process has been comprehensively studied for separating CO2, H2S and other gasses from fuel and flue gasses using promoters like SDS, sodium tetradecyl sulfate (STS), and sodium hexadecyl sulfate (SHS).11,13–15 Such surfactant molecules containing hydrophilic head and hydrophobic tail groups are liable for several surface active properties, e.g., the surfactant can change the surface tension of liquid phase, make self-assembled structures and alter the fluid properties.14,16 A self-assembled structure such as micelle may interact with the growing hydrate surface and induce hydrate nucleation.17–19 Yagasaki, Matsumoto, and Tanaka have also investigated the adsorption of additives on hydrate surface and its effect on hydrate growth.20,21 In experimental studies, use of less than 1 wt % of SDS or other surfactants has shown to increase the rate of hydrate formation significantly.11,15 Literature suggests several reasons for growth promotion in the presence of a surfactant. Kalogerakis, Jamaluddin, Dholabhai, Bishnoi and Kalogerakis proposed that surfactants reduce the nucleation barrier without altering the phase equilibria.22 Zhong and Rogers observed that presence of micelle not only


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enhances the gas solubility, it also acts as a nucleating site for faster hydrate growth.23,24 Some studies also suggest that micelle can adsorb on solid gas hydrate surface and may affect the growth kinetics.25 However, the theory of the formation of micelle at gas hydrate formation condition is debatable due to the fact that micelles should not form below the Krafft temperature.26 The surfactant like SDS has Krafft temperature ≈ 282K and in general gas hydrate formation experiments are carried out at ≈ 274 K.11 However, the measurements of Krafft temperature reported in the literature are usually carried out in the absence of methane at atmospheric pressure. Hence the possibility of micelle formation at gas hydrate formation conditions cannot be ruled out.

Several molecular simulation studies for SDS have been performed for surface tension calculation, micellization, and reverse micellization of surfactant in the water/oil system, SDS bilayer, and thin film, self-assembly in the presence of other compounds etc.27–32 Volkov, Tuzov, and Shchekin have performed micelle formation studies at 298 K and atmospheric pressure while varying the concentration of salt.33 Poghosyan, Arsenyan, Gharabekyan, Falkenhagen, Koetz, and Shahinyan have studied the bilayer and reverse micelle structure of SDS in the presence of poly diallyl dimethyl ammonium chloride and polyethyleneimine at 298 K and atmospheric pressure.34,35 They have observed that SDS tail interacts with a polymer which accelerates the reverse micellization. Yoshii, Iwahashi, and Okazaki have reported the free energy of SDS micelle formation at 300 K and atmospheric pressure and had shown that the average size of micelle falls in between 55–75 SDS molecules.16 Fujimoto, Yoshii, and Okazaki have studied the free energy profiles of methane and water penetration inside SDS micelle.36 They observed that methane was more stable inside micelle



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rather than in water due to hydrophobic interactions with water and the favorable interaction with the hydrophobic tail of SDS. In another work, Fujimoto, Yoshii, and Okazaki had shown that higher molecular weight polar molecule such as octylamine and octanol goes inside the SDS micelle.37 While lower molecular weight polar molecules such as methanol stay in the water phase and methylamine stays at the interface of SDS micelle and water. Several studies have shown varying conditions and surfactants may result in different types of self-assembled structures.38,39

The present study explores the effect of SDS on methane hydrate formation. Generally, 1 wt % of SDS has been used in experiments of gas hydrate formation where SDS molecules can be in a homogeneously dispersed phase.11 In the dispersed SDS solution, local regions will have isolated SDS molecules as well as aggregation of SDS as schematically portrayed in Figure 1. In atomistic molecular dynamics simulation, the system size is computationally limited up to a few nanometers length. Hence, the thermodynamic equilibrium between isolated SDS molecules and SDS aggregates is challenging to achieve in a single simulation. Therefore, we use only 1 wt % concentrations of SDS to mimic the isolated SDS molecules in the solution. We analyzed the gas hydrate formation in the presence of 1 wt % SDS and compared with pure water system (a system with no additive). Here we attempt to understand the nature of interactions between SDS and growing hydrate surface.

METHODOLOGY GROMACS software suite has been used to perform all the molecular dynamics (MD) simulations.40 Rigid TIP4P/Ice model was used for water, united atom (OPLS-UA) model was used for methane, and SDS was modelled using CHARMM forcefield.41–43


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The leap-frog algorithm was used to integrate Newton’s equations of motion with a time step of 2 fs.44 LINCS algorithm was applied to constrain the bond lengths.45 Lorentz-Berthelot combination rule was applied to calculate Lennard-Jones potentials between different atom types.40 Short range non-bonded interactions between the molecules were truncated at 12.0 Å. Periodic boundary condition was employed in all three spatial directions, and Particle Mesh Ewald (PME) summation method was used to handle the long-range electrostatic interactions.46

Simulation procedure for methane hydrate formation The gas hydrate forming condition (270 K and 100 bar) was chosen to ensure sufficient driving force for attaining practical growth kinetics in MD simulations, while at the same time not being far away from the operating conditions which were utilized in the general gas hydrate formation experiments.47 A temperature of 270 K provides optimum sub-cooling and mass transfer in the system so that the methane and water molecules can come together to form hydrate crystals.47 The initial structure contained uniformly distributed methane, water, SDS and one seed as shown in Figure 2a. Presence of seed, which is composed of two unit cells of methane hydrate (having 16 methane and 92 water molecules) mimics a scenario of heterogeneous hydrate nucleation.7,10,48–50 The seed was kept position restrained during entire simulation and rest of the components of the system were free to move. Literature reports that hydrate formation with 0.01 wt% to 1 wt % SDS-water solution results in faster hydrate growth.11 Thus 1 wt % concentration of SDS was chosen for simulation. Total 7 molecules of SDS with 11000 water molecules were used to obtain 1 wt % SDS solution. The total number of methane molecules in the solution is 650; this number mimics the methane concentration in water just before hydrate nucleation.



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This number was obtained from experimental data which reports a range of 350 to 650 gas molecules.4,7,47,51,52 Hydrate growth kinetics of 1 wt % SDS system was compared with the pure water system, which does not contain SDS molecules and has 650 methane molecules dissolved in 11000 water molecules. Summary of each system is reported in Table 1. The simulation of formation study was initiated via energy minimization of initial configurations

using

the

steepest-descent

algorithm. The

energy

minimized

configurations were subsequently subjected to NVT equilibration of 2 ns at 270 K. Finally, the equilibrated configuration was put into NPT ensemble at 270 K and 100 bar for 1 microsecond. Both systems (with SDS and without SDS) was subjected to three independent MD runs. The pressure was isotropically maintained using Parrinello–Rahman barostat with a relaxation time of 4 ps.53 The temperature was controlled using Nose-Hoover thermostat with the coupling time constant of 4 ps.54

RESULTS AND DISCUSSIONS Gas hydrate formation In the gas hydrate formation study, gas hydrate growth was observed for both the systems (with and without additive). The water molecules that contribute to hydrate formation become more structurally ordered during the formation process. The extent of clathrate-like order in water molecules was measured using the four body order parameter (F4) defined as follows:55

F = ∑ cos 3ɸ

(1)

where n is the number of oxygen-oxygen pairs of water molecules present within 0.3 nm distance of each other. ɸ is the torsional angle between farthest O-H vectors of


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both water molecules as represented in the inset of Figure 3. The F4 parameter value stays around 0.7 for pure gas hydrate and -0.04 for liquid water.55

It should be noted that the F4 parameter in Eq.(1) is averaged over all water molecules present in the system. F4 value of 0.7 requires that all the water molecules of the system have transformed into crystalline hydrate with a methane-water ratio of 1:5.75.1 In our study, the methane-water ratio was fixed near the experimentally measured methane concentration in water just before nucleation, i.e. 1:17 – 1:32. Thus complete conversion of water to hydrate was not expected, and the F4 parameter would not reach 0.7. As shown in Figure 3, the trend of F4 values (when averaged over entire simulation box) illustrates hydrate nucleation and growth with time. We expedite the nucleation step by inserting a position restrained seed crystal and thus the induction time is quite short. In the case of 1 wt % SDS and without additive system, it was observed that as hydrate grows, the F4 parameter starts increasing from ≈ -0.04 and saturates up to ≈ 0.15 as shown in Figure 3. Since most of the methane molecules were utilized in hydrate formation before 400 ns, lesser number of methane molecules were available to continue the hydrate growth, resulting in a significantly lower value of the F4 parameter ≈ 0.15. This is evident from the Figure 2b where the hydrate cages formed can be seen clearly. For the first 50 ns, the kinetics of growth was relatively slow. Further, it was observed that the “without additive” system shows marginally lower hydrate growth in the first 50 ns compared to 1 wt % SDS solution. However, one out of the three runs carried out with 1 wt % SDS system shows relatively slower growth kinetics compared to the other two and that of without additive system. Thus the presence of 1 wt% SDS does not affect the kinetics of hydrate growth significantly. Based on the above observation



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it can be concluded that the intrinsic kinetics of hydrate growth is not very different with 1 wt% SDS concentration. However, experimental observation consistently shows better hydrate growth in the presence of 1wt% SDS when compared to hydrate growth from pure water. Clearly, SDS might play a role through other mechanism resulting in faster hydrate growth. Moreover, hydrate formation with 10 wt% SDS was also tried and it resulted in no hydrate growth, which matches with experimental observation (discussed in supplementary section). In experimental studies it has been regularly reported that the morphology of methane hydrate in the presence of SDS is entirely different; it has been argued that in the presence of SDS a porous hydrate layer grows above the aqueous phase and thus enhances hydrate growth by improving the mass transfer.14,56

In our simulation

studies, we explore this view by analyzing the gas hydrate surface during growth. Due to the hydrophilic head - hydrophobic tail nature of surfactants, they can play a specific role of adsorption on growing hydrate surfaces and change the morphology of hydrate.11,56 Therefore, to explore SDS adsorption on hydrate surface, the F4 parameter was calculated for water molecules within 1 nm of individual SDS molecules (a representative snapshot is shown in Figure 4a). The time evolution of F4 values for each of the seven SDS molecules is shown in Figure 4b. Higher F4 parameter value (≈ 0.15) around the single SDS molecule signifies that SDS was close to the hydrate surface and a lower F4 value (≈ -0.04) suggests that SDS stays in the liquid phase. It was observed that the F4 parameter for fourth SDS molecule starts increasing after ≈ 50 ns and remains at a higher value during entire simulation time, suggesting its proximity to solid hydrate surface. The F4 parameter for fifth SDS molecule starts increasing after 100 ns, and it continued to show higher F4 value up to 450 ns. However, the F4 value dropped to a lower number towards the end of the simulation.


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A similar trend was observed for second SDS molecule where the higher F4 value was observed from 550 to 900 ns. The similar trend of F4 was observed for third and seventh SDS molecule while the first and sixth SDS molecule stays in the liquid phase during the entire simulation time. It can be concluded from the above observations that SDS molecules exhibit affinity to get adsorbed on the hydrate surface. However, the binding affinity is not very high, and they remain in a dynamic equilibrium or exchange with the solution phase within the simulation time scale (a few hundred ns).

The above observation suggests that there is a weak attractive interaction between SDS molecules and the hydrate surface. Therefore, we attempted to dissect the role of individual SDS atoms that might be responsible for adsorption. Hence, water structure around each heavy atom of hydrophobic and hydrophilic parts of SDS was studied separately. The distribution of F4 parameter was plotted for the water molecules which are within 1 nm of each selected atom as shown in Figure 5b and the atom labels of SDS are shown in Figure 5a. If a particular atom stays inside or close to the hydrate, then the value of an F4 parameter falls towards 0.7, and if it is in the liquid phase, then the value will be close to zero. In Figure 5c, the distribution is plotted for first 100 ns of the trajectory and in Figure 5d, it is shown for 300 to 400 ns of trajectory just before the saturation of hydrate growth. As can be seen from the Figure 5c, among all the atoms the terminal four carbon atoms show a slightly higher propensity towards the higher F4 parameter, while for other atoms it is close to zero. Whereas, at a longer time duration when further growth is not expected, the population of the F4 parameter for the tail part shifts considerably towards the higher values as shown in Figure 5d. This highlights that the tail part is responsible for the adsorption and which was not



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observed for middle and head part of SDS since they were not showing a significant change in distribution within the simulation time. In order to analyze the time evolution of local structural features of different components of the system (SDS, methane, and water), radial distribution functions (RDF) between various components of the system were calculated during the different segments of trajectories as shown in Figures. 6 and 7. Initially, during 0 to 50 ns, the system consists of mostly liquid water and randomly distributed methane molecules. As hydrate grows (200-250 ns), both methane and water molecules get arranged in a hydrate like structure (Figure. 7). Figure. 7a shows that the second peak in methanemethane RDF becomes more pronounced with time signifying formation of hydrate like structure, whereas the first peak signifies the direct methane-methane contacts predominant in the liquid phase initially. The methane-water RDF also captures the increased structuring with time (Figure 7b). Subsequently, we turn our attention to the structuring around different parts of SDS molecule (Figure. 6). Figures. 6a and 6c show the RDF between C12 (the tail atom of SDS) and S (a head group of SDS) with methane. Clearly, there is significant structure formation (second peak) around C12, whereas there is a significant decrease in methane population around the head group (S). This noticeably demonstrates that SDS anchors on the hydrate surface through the hydrophobic tail region, and C12 can be incorporated into the hydrate cage with enhanced structuring, while the polar head group remains exposed to the water phase. This is similar to the usual surfactant action of SDS, and may lead to the lower surface free energy of the hydrate nucleus thus improving the nucleation and growth kinetics.

Interestingly, despite the observed preferential adsorption of SDS molecule on hydrate surface, we do not observe a significant difference in the hydrate growth kinetics in


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the presence of SDS within our simulation timescale. This suggests that even though the initial intrinsic kinetics might be same in both the systems, at a longer timescale such anchoring of SDS molecules on the hydrate surface may contribute to porous hydrate morphology at macro level thus enhancing mass transfer of water and gas. Enhanced mass transfer leads to faster hydrate growth as observed in experimental studies. Computational modeling of such phenomena is quite challenging and would be explored in future through long simulations (beyond microseconds) using coarsegrained models of larger systems.

CONCLUSIONS Methane hydrate formation in the presence and absence of 1 wt% SDS was studied at hydrate forming temperature and pressure conditions. During the hydrate formation, it was observed that SDS shows a tendency to adsorb on the hydrate surface through its hydrophobic tail by anchoring into the open cages of growing hydrate, while the polar head groups remain exposed to the solution phase. Such adsorption would lead to stabilisation of the nascent nuclei. Reduction in surface free energy should lead to reduction in nucleation barrier and induction time. Moreover, at longer time and length scales the adsorbed SDS molecules may alter the morphology of the growing hydrate with increased porosity. A porous hydrate would improve the mass transfer of methane significantly leading to enhanced hydrate growth kinetics.

AUTHOR INFORMATION

Corresponding Author



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Corresponding authors: R. K. Tel.: +91 442 2574180; email: [email protected] and S. C. Tel.: +91 674 2494064; email: [email protected]

AUTHOR CONTRIBUTIONS

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ϮThese authors (N. C. and V.H.) contributed equally as the first author.

ACKNOWLEDGEMENTS We thank CSIR 4-PI supercomputing facility for the CPU time. N. C. and V. H. thanks CSIR for fellowship. S.C. thanks, DST, India for Ramanujan Fellowship.

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TABLES Table 1. The composition of each system used to build their initial configurations. Nwater, NSDS and Nmethane are numbers of water, SDS and methane respectively. SDS concentration ( wt % )

Nwater

NSDS

Nmethane

Systems used for methane hydrate formation 0 Without additive)

11000

0

650

1 wt %

11000

7

650



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FIGURES

Figure 1. Representation of dispersed solution of SDS. Water and SDS molecules are marked blue and red, respectively. Green circle represents the isolated SDS molecules while yellow circle highlights SDS aggregate/micelle.


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Figure 2. (a) Initial structure of 1 wt % SDS system. (b) Snapshot of 1 wt % SDS system at 1 microsecond showing the significant formation of methane hydrate. The colour representation is as follows: Liquid water is light red, seed water is dark red, methane is blue, methane of seed is magenta, carbon of hydrophobic tail of SDS is green, sulfur is yellow, hydrogen is white, and oxygen of SDS is red. In (b), dark red is a large cage (51262), black is a small cage (512), and water is hidden for better visualization. The system is simulated at 270 K and 100 bar.

Figure 3. Time evolution of four body structural order parameter (F4) for 1 wt % SDS, and without additive system. Representation of F4 parameter is given in inset figure where two water molecules (red and black font) are making hydrogen bond (dashed line). Green lines mark the farthest O-H vectors that define the torsional angle (ɸ) used in Eq.1 for the calculation of F4 parameter.



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Figure 4. (a) Representation of water solvation shell of size 1 nm around a SDS molecule. (b) The F4 parameter for water molecules which are present in solvation shell of each SDS molecule.


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Figure 5. (a) Nomenclature of the individual atom (excluding hydrogen) of SDS molecule. (b) Representation of hydrate cages around C12 (terminal carbon) of SDS. (c) Distribution of F4 Parameter for the water molecules which are within 1 nm of a particular atom of SDS for initial 0 to 100 ns where initial growth takes place and (d) for 300 to 400 ns, just before saturation of growth. In (c) and (d) only C8 to C12 (hydrophobic tail) and OS1 (hydrophilic head) atom were highlighted because these are giving reasonable differences. The rest of the atoms were shown using grey lines, and all are giving distribution reasonably similar to OS1.



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Figure 6. Radial distribution function (RDF) between atoms of different components of system. (a) RDF between C12 (tail atom) of SDS and methane. (b) RDF between C12 of SDS and OW (oxygen of water). (c) RDF between S (head atom) of SDS and methane. (d) RDF between S of SDS and OW.


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Figure 7. Radial distribution function (RDF) between atoms of different components of system. (a) RDF between methane and methane. (b) RDF between methane and OW (oxygen of water). (c) RDF between OW and OW.



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TABLE OF CONTENTS (TOC) IMAGE

TOC: Snapshot of SDS adsorbed on hydrate surface


Effect of SDS Surfactant on Methane Hydrate Formation: A Molecular

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