The effects of heterocyclic atoms (O, N, S) in non-hydrocarbons on the properties at non-hydrocarbon/surfactant/water interface were investigated by molecular dynamics simulation. The model surfactant was sodium cetyl metaxylene sulfonate (2 ,4-Dimethyl-5-(1'-butyl) Sodium dodecyl benzene sulfonate). The interface properties were analyzed , which include density profile, interfacial formation energy and interfacial tension, radial distribution function and hydrophobic chain order parameters. The simulation results indicated that interface thickness is relevant to the electronegativity of heterocyclic atoms, and the arrangement of hydrophobic tails is relevance with the energy of heterocyclic atoms bond with carbon atoms. It is also bound up with the self-loop strain. Then, the radial distribution function and hydrophobic tail sequence parameters were calculated to further verify and explain this phenomenon in the equilibrated model systems. The stability of the interface formed with sulfonate surfactants is O> N> S according to the heterocyclic atom. When the number of rings in non-hydrocarbons changes, the interface stability also follows this rule. At the same time, it is found that the stability of the interface increases as the number of rings adding.
Research Article
Effect of Heterocyclic Atoms (O, N, S) on surfactant solutions investigated by molecular dynamics simulation
https://doi.org/10.21203/rs.3.rs-1249348/v1
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Molecular dynamics
Heterocyclic atoms
Non-Hydrocarbon/Water interface
Surfactant
Crude oil compounds are mainly with highly complex fluids such as alkanes, olefins, aromatics and non-hydrocarbon[1, 2]. Among them, the non-hydrocarbon, O, N, and S atoms have always been the research target of scholars[3]. On the one hand, thiophene sulfur is very difficult to separate from the oil[4], and a large amount of nitrogen oxide compounds were produced by petroleum products during the combustion process, which cause pollution to the atmosphere[5]. On the other hand, the physical properties of non-hydrocarbon compounds vary with the heterocyclic atoms[6]. When forming an oil/surfactant/water interface, due to the interaction between non-hydrocarbon and surfactants at the interface during the film formation process, the interface will show different status.
Molecular dynamics simulation is an effective method to explore the influence of oil phase composition changes at the oil/water interface in the molecular scale[7, 8]. According to the reports, a large number of modelings and experiments have been carried out on the study of the interface characteristics between the oil phase and the water phase in a wide range of temperature and pressure[9]. Jian et al.[10] found that increasing the temperature and reducing the amount of salt can reduce the interfacial tension by a combination of simulation and experiment. Shi et al.[11] used the dissipative molecular dynamics method to quantitatively predict the interfacial tension between caprolactam and benzene/water. Gao et al.[12] found that carboxyl asphaltenes with different electrical had different properties either in oil or at the oil/water interface by the molecular dynamics methods.
Scholars have made preliminary research on non-hydrocarbons. Chitranjan Sah et al.[13] calculated the decomposition stability of five-membered six-ring free radicals, and the results showed that there was a competition between the delocalization of free radical electrons or spins and the ring strain., which determined the overall stability of the five-membered heterocyclic/carbocyclic radical. Diego et al.[14] used density functional theory to study the toxicity of a six-membered nitrogen ring to a five-membered heterocyclic ring for hydrotreating catalysts. The results showed that the protons of the five-membered heterocyclic ring destroy the flatness due to the interaction. KE et al.[15] also used density functional theory to study the isotropic shielding behavior in the spatial region of five-membered heterocyclic molecules and found that it was related to the aromaticity of heterocyclic molecules. Belen et al.[16] used quantum chemistry to study the substitution of N, O, S atoms in five-membered heterocyclic molecules by Se, and explained the best positions for electrophilic substitution. Although these works are valuable, they only provide the limited information about individual molecules, and do not aggregate individual molecules into a system for systematic comparative studies.
There are not only single-ring non-hydrocarbon in crude oil, but also non-hydrocarbon with fused benzene rings[17]. It is very important to study the selectivity of surfactants to non-hydrocarbon at the non-hydrocarbon/water interface. However, in the current research, there are few reports on the influence of heterocyclic atoms in non-hydrocarbon on the properties at the non-hydrocarbon/water interface. Therefore, the all-atom molecular dynamics method was adopted to solve the problem, and selected non-hydrocarbon compounds containing N, O and S atoms in the oil phase, which were thiophene, furan and pyrrole, respectively. For further exploration, the corresponding aromatic derivatived: benzothiophene, benzofuran, benzopyrrole and dibenzothiophene, dibenzofuran, and dibenzopyrrole were selected as the oil phase for a comprehensive study. The sodium cetyl xylene sulfonate was choosed as surfactant[18]. In order to explore the effect of different heterocyclic atoms in non-hydrocarbon at the non-hydrocarbon/water interface on the molecular level. The density profile and interfacial thickness, interface formation energy (IFE) and interfacial tension (IFT), radial distribution function (RDF) and alkyl chain end order parameter (Order parameter) were calculated and analysed in this paper.
Model construction
All simulation processes were calculated by using the Materials Studio software package[19]. Firstly, the Visualizer module was used to build the oil phase, water phase and surfactant molecular models. Then, the Amorphous Cell tool was used to construct the water layer, oil layer and monolayer surfactants. The water layer had 800 water molecules, the single oil layer had 80 oil molecules(the system had 160 oil molecules), and the surfactant layer was a monomolecular layer composed of 6 sodium cetyl xylene sulfonate molecules (the system had 16 surfactant molecules). Finally, Layer tool was used to combine the individual layers into an oil/surfactant/water system. The model was that the oil layers on both sides, the water layer in the middle, the surfactants were along the Z-axis, the hydrophilic group faced the water layer, and the hydrophobic group faced the oil layer. To facilitate subsequent discussions, thiophene, furan, pyrrole, benzothiophene, benzofuran, benzopyrrole and dibenzothiophene, dibenzofuran, and dibenzopyrrole were named as 1-S, 1-O, 1-N, 1.5-S, 1.5-O, 1.5-N, 2-S, 2-O and 2-N. The structure and naming of each atom are shown in Figure 1.
COMPUTATIONAL DETAILS
The calculation process was completed in the Forcite module in this paper[20], the force field was COMPASS Ⅱ[21], and the water molecule was the SPC model[22]. Before establishing the entire system, firstly, the oil molecules, water molecules, and surfactant molecules were initialized with 30,000 steps using the Smart geometry optimization method to minimize the molecular energy. The energy of the system was minimized to eliminate possible molecular overlap during the construction process after the system was built. First of all, the optimized system performed 1 ns dynamic simulation under the canonical ensemble (NVT), and then following 1 ns isothermal-isobaric ensemble (NPT) and 1 ns NVT MD simulations were sequentially carried out to equilibrate the system. Finally the last 500 ps equilibrium system for oil/water interface analysis were extracted. The Dmol 3 module was used to calculate the non-hydrocarbon charge distribution. This calculation used the Generalized Gradient Approximation (GGA) Perdew-Burke-Ernzerhof (PBE) functional[23] and the Double Numerical Pathway (DNP)[24, 25].
Through all simulation processes, the time step was 1 fs, the trajectory information was recorded every 1 ps, the initial rate was random. Van der Waals interaction (vdW) and electrostatic interaction (Electrostatic) were calculated by using Atom based method. The temperature was 318K and controlled by the Andersen method[26, 27], the pressure was 101kPa and controlled by the Berendsen method[28].
Each system aggregation state was similar after reaching equilibrium and the 1-O system was showedas an example for structure display. Figure 2 showed the system initial establishment and the aggregation state. It can be seen from Figure 2 that the hydrophilic end extended into the water phase, and the hydrophobic end extended into the oil phase in a disordered manner to cause the disorder of surfactant aggregation morphology.
Density profile and interface thickness
The density profile of each system can be obtained by calculating the slices of the simulation data generated along the Z-axis. In order to analyze the density profile and the interface thickness, Figure 3 shows the density distribution on the same side of the oil/surfactant/water interface. It can be seen from Figure 3 that when no surfactant is added, the thickness of the oil-water interface is narrow, that is, the oil phase and the water phase form a relatively "clear" interface. After adding surfactants, the thickness of the interface becomes wider, indicating that the surfactant forms a "fuzzy" interfacial film that blends with the oil phase and the water phase. At this time, the surfactants in the interface cross between the oil phase and the water phase. This cross behavior of the surfactants increases the thickness of the oil-water phase interface film and thus the interface strength. Also, it can be found that the density of water after the equilibrium of each system model is 0.98 ± 0.02 g/cm3, which is consistent with the density of pure water at 318 K of 0.99 g/cm3[29], indicating that the simulation system is correctly constructed and large enough. The force field parameters are correct, which can be used to study the interface properties of each system.
Figure 3 The density of oil water and surfactant moleculars along the Z-axis
The interface thickness refers to the distance from 90% of the water phase density to 90% of the oil phase density[30, 31], the interface thickness without surfactants is δREF, the interface thickness after adding surfactant is δSUR, the increased interface thickness value is δINC=δSUR-δREF, and the calculation results of the interface thickness are shown in Table 1. It can be seen from Table 1 that when no surfactants are added, the interface thickness is 0.43~0.66 nm. After adding surfactants, the interface increases to 1.38~1.77 nm, and the increased value is 0.87~1.34 nm. The interface thickness can directly reflect the strength of the surfactant adsorption at the interface. The greater the interface thickness of the system, the stronger the adsorption of the surfactants[32].
From Table 1, it finds that when the number of rings is constant, δSUR decreases in the order of oxygen, nitrogen, and sulfur atoms, and δINC also increases in this order. The reason for this phenomenon may be that the electronegativity decreases in the order of oxygen atoms (3.5), nitrogen atoms (3.0), and sulfur atoms (2.5). The electrostatic interaction between heterocyclic atoms and the hydrophobic groups of surfactants is different. In addition, the heterocyclic atoms and the hydrogen atoms on the hydrophobic group may form hydrogen bonds, and the strength order of the hydrogen bonds is also consistent with the interface thickness and the increase in interface thickness[33]. With two benzene rings non-hydrocarbon systems (2-O, N, S) after adding surfactant molecules, the interface thickness is greater than that of non-benzene ring non-hydrocarbon systems (1-O, N, S) after adding surfactants. Similarly, the corresponding increase in interface thickness also has the same law, but for non-hydrocarbon systems with only one benzene ring (1.5-O, N, S) have no such phenomenon. The reason for this phenomenon may be that the non-hydrocarbon system with two benzene rings and the non-hydrocarbon system with non-benzene ring have symmetry, while the non-hydrocarbon system with one benzene ring has no symmetry. For the non-hydrocarbon system corresponding to the same heterocyclic atom (except 1.5-S), as the number of rings increases, the interface thickness and the increase value of the interface thickness increase after adding surfactants.
Table 1 Thickness of each system
|
|
δSUR / nm |
δREF/ nm |
δINC / nm |
|
1-O |
1.54 |
0.52 |
1.02 |
|
1-S |
1.43 |
0.49 |
0.94 |
|
1-NH |
1.52 |
0.56 |
0.96 |
|
1.5-O |
1.64 |
0.61 |
1.03 |
|
1.5-S |
1.38 |
0.52 |
0.87 |
|
1.5-NH |
1.54 |
0.58 |
0.96 |
|
2-O |
1.77 |
0.43 |
1.34 |
|
2-S |
1.69 |
0.66 |
1.03 |
|
2-NH |
1.73 |
0.59 |
1.14 |
Interface formation energy and interfacial tension
The influence of different non-hydrocarbon/water interface is quantitatively analyzed on the stability by calculating the interface formation energy and interfacial tension. The stability of the interface can be compared by the interface formation energy[34], which represents the decrease in energy of the system after adding surfactant molecules. This value is negative, the greater the absolute value, the more stable the interface. It is related to the oil phase molecules, water molecules, surfactant molecules and the interaction between molecules[35]. Adding the surfactants reduce the system energy, and it also reduces the interfacial tension, which is also an important indicator of the system properties. In the molecular simulation process, the interface formation energy is calculated according to formula (1)[36], when the interface is perpendicular along the Z-axis, the interfacial tension is calculated by formula (2)[37]:
In formula (1), Etotal is the total energy, calculated when molecular dynamics simulation reaches equilibrium; Esurfactant, single is the energy of a single surfactant molecule, calculated under the same conditions as the non-hydrocarbon/water system; Eoil/Water is the non-hydrocarbon/water interface energy, calculated with no surfactants adding; n is the number of surfactant molecules (n=12). In formula (2), LZZ is the length of the box along the Z-axis, and Pαα (α=x, y, z) is the pressure along the α axis. The calculation results are shown in Figure 4, where IFE is the absolute value, and the interfacial thickness is also listed in Figure 4 for comparison with IFE and IFT.
It can be seen from the Figure 4 that when the number of rings is the same, the interfacial tension corresponding to the non-hydrocarbon system containing oxygen atoms is the smallest, and the interfacial tension corresponding to the non-hydrocarbon system containing sulfur atoms is the largest. Interestingly, the non-hydrocarbon system containing oxygen atoms corresponds to the largest interface formation energy, and the non-hydrocarbon system containing sulfur atoms corresponds to the smallest interface formation energy. The change trend of the interface thickness is basically consistent with the interfacial tension, indicating that the "fuzziness" of the interface film can make the formed interface more stable and the hydrophobic chain more stretched. The bond strength of X (O, N, S)-C in non-hydrocarbons is consistent with the trend of bond energy, that is, O-C> N-C> S-C, and is consistent with the ring strain energy of the formed monocyclic molecule[38].
As the number of rings increases, the interfacial tension of the non-hydrocarbon-containing oxygen atom and nitrogen atom system decreases. But for the non-hydrocarbon system containing sulfur atoms, when the number of rings is one and two, the corresponding interfacial tension of the system is roughly the same, and the interfacial tension decreases with the three rings. Through comparison, it can be found that the change law of the interface formation energy corresponding to the non-hydrocarbon system of a heterocyclic atom at a certain time is the same as the change law of the interfacial tension. When the heterocyclic atom is the oxygen atom or nitrogen atom, as the number of rings increases, the interface formation energy of the non-hydrocarbon system increases. When the heterocyclic atom is the sulfur atom, the interface formation energy of the non-hydrocarbon system first basically unchanged and then increases with the increase of the ring number.
The interfacial tension of the three-ring non-hydrocarbon system is generally less than one and two rings. Correspondingly, the interfacial thickness is the largest and the interfacel formation energy is the largest. It shows that the three rings non-hydrocarbon system with two benzene rings has the best interface performance, and the surfactant is the most active in this series of systems.
Radial distribution function
In order to calculate the accumulation of non-hydrocarbon near the surfactants alkyl chain at the non-hydrocarbon/water interface, the radial distribution function of the hydrogen atoms on the dodecyl end and butyl end carbon of the surfactant molecules and the heteroatoms in non-hydrocarbons were calculated. The dodecyl and butyl groups in 2,4-dimethyl-5-(1'-butyl)dodecylbenzene sulfonate extend into the oil phase respectively in this series models. The radial distribution function represents the probability that the alkyl chains of surfactant molecules appear within the specified radius of the oil phase molecules[39], which can reflect the aggregation state and strength of the alkyl chains. The calculated radial distribution function results are shown in Figure 5.
It can be seen from Figure 5 that the positions corresponding to the main peaks are all at 0.5 ± 0.1 nm, where non-hydrocarbons are most likely to appear in surfactant molecules, indicating that the surfactants are most closely bound to non-hydrocarbons at 0.5 ± 0.1 nm and the first non-hydrocarbon layer appears. A secondary peak with a smaller peak appears behind the main peak, indicating that the combination of surfactant molecules and non-hydrocarbons is similar to the combination of surfactant molecules and water molecules[40], and a second non-hydrocarbon layer appears. By comparing Figure 5, when the alkyl chain is dodecyl, the primary and secondary peaks are larger than when the alkyl chain is butyl. When the alkyl chain is dodecyl, the distance of the secondary peak is shoter. when the alkyl chain is butylt, the distance of the secondary peak is bigger. This indicates that the adsorption tightness of the dodecyl end of the surfactant to the non-hydrocarbon phase is greater than the adsorption tightness of the butyl end to the non-hydrocarbon phase.
At the same time, it can be found that the trend in the graph is basically the same. When the number of rings is the same, the combination of surfactant molecules and heterocyclic molecules is in the order of O> N> S. For the dodecyl end, when the ring number is one, the g(r) is significantly greater than the g(r) of the other two rings, while for the butyl end, the g(r) is arranged more uniformly. When the heterocyclic atoms are the same, as the number of heterocyclic molecules increases, the tightness of the binding between the surfactant molecules and the heterocyclic molecules decreases. By comparing the difference between primary and secondary peaks, the difference becomes smaller as the number of rings increases.
In order to further investigate the reasons for the above results, the charge distribution calculations were performed on the non-hydrocarbon models and surfactant molecular models, and the calculation results are shown in Figure 6. It can be seen from Figure 6 that the overall charge distribution of the alkyl chain ends is approximately neutral. But comparing the charge distribution of butyl and dodecyl, it can be seen that the charge distribution of dodecyl is neutral, while the charge distribution of butyl is weakly negative. As a result, the tightness and occurrence probability are that the combination of dodecyl with oil phase molecules is higher than butyl with oil phase molecules.
When the atom is oxygen, the charges corresponding to the three molecular models heterocyclic atoms are negative; when the atom is nitrogen, the charges are positive; when the atom is sulfur, the charges are neutral. The absolute charge value is in the order of O> N> S as the number of rings in the non-hydrocarbon system is the same, so the tightness and probability of binding to the surfactant are in the order of O> N> S.The absolute charge value decreases according to the increase in the number of rings as the heterocyclic atoms are the same, so the tightness and probability of binding with the surfactant decrease according to the increase in the number of rings.
Order parameter
The arrangement of the surfactant alkyl chain ends at the non-hydrocarbon/water interface has an extraordinary important influence on the interfacial activity. In different non-hydrocarbon systems, the alkyl chain ends extend into the oil phase in different configurations. Calculating the order parameter (SCD) of the alkyl chain ends along the Z-axis can explore the adsorption arrangement of the butyl and dodecyl groups at the alkyl chain ends in the non-hydrocarbon phase. It is of great significance to reveal the adsorption law of different heterocyclic atoms by surfactants. SCD can be calculated according to the formula (3)[41], and the calculation result is shown in Figure 7.
In the formula (3): θ is the angle between the vector of Cn-1 and Cn+1 atoms with the Z-axis. From formula (3), it can be seen that the SCD is -0.5 to 1. When SCD approaches 1, the smaller the angle between the alkyl chain with the Z-axis, the greater the degree of the surfactant perpendicular to the interface. When the SCD approaches -0.5, the greater the angle between the alkyl chain with the Z-axis, the greater the degree of the parallel to the interface. When SCD approaches 0, it means that the surfactants are arranged disorderly at the interface[42].
It can be seen from Figure 7 that the alkyl end order parameters are not much different. Among them, the alkyl chain greater than 0 corresponds to the dodecyl. The maximum value of the alkyl end order parameter in each system is C8, and the maximum value ranges from 0.27 (2-S) to 0.32 (1-O). The minimum value is C1, and ranges from 0.11 (2-S) to 0.19 (1-N). The alkyl chain less than 0 part is the butyl. The maximum value is C13, and the maximum value ranges from -0.13 (2-S) to 0 (1-O). The minimum value is C11, and the value ranges from -0.26 (2-N) to -0.06 (1-O ). The swing space of the distal carbon atom is larger than the swing space of the proximal carbon atom, so that the vertical degree is less than the vertical degree of the proximal carbon atom. However, the degree of the vertical interface at the butyl end is less than the degree of the vertical interface at the dodecyl end, probably because the number of non-hydrocarbons around it is less than the number of non-hydrocarbons near the dodecyl end.
When the number of non-hydrocarbon rings is constant, the end order parameters of the alkyl chain are sorted in the order of O> N> S, and the degree of the vertical interface is also sorted in this order, which is consistent with the calculation results in the previous. To the same heterocyclic atoms in the non-hydrocarbon system, the order parameters are sorted in the order of increasing and decreasing non-hydrocarbon rings number, and the vertical interface degree is consistent with the parameters order. This is because the thicker the interfacial film thickness, the higher the interfacial formation energy, and the lower the interfacial tension, resulting in the hydrophobic end of the heterocyclic molecule being more perpendicular to the interface.
Using MD simulation, the effect of different non-hydrocarbon heterocyclic atoms (O, N, S) on the properties at the non-hydrocarbon/water interface were studied. At the same time, interfacial properties of the systems including the different rings number were also reported.
When the non-hydrocarbon system contains different heterocyclic atoms with the same ring number, the stability of the non-hydrocarbon/water interface decreases in the order of the heterocyclic atoms O> N> S. At the same time, when the non-hydrocarbon system contains the same heteroatoms but the number of rings is different, the non-hydrocarbon/water interface stability increases with the rings number increase. The binding strength of the surfactant hydrophobic end and the non-hydrocarbon is arranged in the order of the heterocyclic atom O> N> S, which is consistent with the degree of the heterocyclic atom charge ability. The vertical interface degree of the surfactant alkyl chain end is arranged in the order of O> N> S, the rings number in the non-hydrocarbon system increases, and the vertical degree decreases. The dodecyl chain end vertical degree is greater than that of the butyl chain, and the carbon chain vertical degree becomes smaller as the carbon atom distance away from the benzene ring increases.
In this paper, the reach show many pieces of useful information at the non-hydrocarbon/surfactant/water interface with different heterocyclic atoms and different ring numbers in non-hydrocarbons. The results will help in further study on how the compatibility of non-hydrocarbons with surfactants was used in the improved crude oil recovery.
Funding (This work is supported by National Key Research & Development Program of China (2018YFC1801902).)
Conflicts of interest/Competing interests (We confirm that the manuscript has been read and
approved by all named authors and that there are no other persons who satisfied the criteria for
authorship but are not listed. We further confirm that the order of authors listed in the manuscript
has been approved by all of us.)
Ethics approval(N/A)
Consent to participate(N/A)
Consent for publication(N/A)
Availability of data and material(The datasets used or analysed during the current study are
available from the corresponding author on reasonable request)
Code availability(Some or all code generated or used during the study are proprietary or
confidential in nature and may only be provided with restrictions)
Authors' contributions(Zhinan Liu performed the experiment and wrote the manuscript;Bailin Li, Li Wang contributed significantly to analysis and manuscript preparation;Shuhai Guo helped perform the analysis with constructive discussions.)
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