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Enhancing Understanding of Siloxane Surface Properties and Functional Group Effects on Water Deoxygenation

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Abstract
The de-oxygenation process in water used in well injection operations is an important matter to eliminate corrosion in petroleum industry. This study used molecular dynamics simulations to understand the behavior of siloxane surface through studding the surface properties with two functional groups attached to the end of siloxane and their effect on de-oxygenation process. The simulations were performed using LAMMPS to characterize of surface properties. Jmol software was used to generate siloxane chains with (8, 20, and 35) repeat units. Firstly, we evaluated properties such as total energy, surface tension and viscosity. Then, we used siloxane as a membrane to compare the efficiency of de-oxygenation for the both types of functional groups. The results indicated that longer chains length increased total energy, viscosity while decreased surface tension. Replacing methyl groups with trifluoromethyl (CF3) groups increased all the above mentioned properties in varying proportions. Trifluoromethyl (CF3) groups showed better removal efficiency than methyl (CH3) groups but allowed more water to pass. Furthermore, the simulations were run using the class II potential developed by Sun, Rigby, and others within an explicit-atom (EA) model.
Keywords: 
Subject: Engineering  -   Chemical Engineering

1. Introduction

Polyargansiloxanes are a type of silicone-based polymer known as siloxane. The properties of polyarganosiloxanes can be affected by attaching different organic groups to silicon atoms. Some common organic groups include methyl, phenyl, fluorine containing alkyl groups. These changes in chemical structure vary the properties of polyarganosiloxanes such as flexibility, heat stability, and chemical resistance [1].
Many industries use silicon in their fields due to its unique properties. It is used in coatings, oil and gas separation processes, adhesives, lubricants, and many other products because of its resistance to heat, cold, moisture, and chemicals [3].
The most common methods to prepare polyarganosiloxanes are condensation polymerization and addition polymerization. The properties of the synthesized siloxane can be affected by the choice of synthesis method [4].The main type polymer in the group of polyorganosiloxanes is polydimethylsiloxane (PDMS). PDMS consists of repeating units of dimethylsiloxane, which is represented as shown in Figure 1 [5].
In PDMS, each silicon atom is bonded to two oxygen atoms and two methyl groups (CH3), resulting in a flexible and relatively inert polymer chain. The methyl groups in PDMS have low polarity and contribute to a low surface tension, meaning that PDMS tends not to wet surfaces easily. This property makes PDMS suitable for applications where low adhesion to surfaces is desired, such as in mold release agents or coatings for water-repellent surfaces. Another property of PDMS is that it is permeable to gases due to its flexible polymeric chains and large space between methyl groups, this high permeability to gases makes PDMS a valuable membrane in applications such as oil/gas separation [5].
The methyl group in PDMS can be replaced by other functional groups to synthesize polymers for a wide range of other exciting applications, for example fluorine-containing alkyl groups Figure 2 lead to high resistance to hydrocarbon solubility while maintaining low surface tension. This property is important for its application in gas/oil separation and is of direct importance for applications in the oil industry [5,6].
Many fundamental questions related the relationship between the backbone structure and the functional groups attached to the backbones can be addressed using molecular simulations, with carefully chosen interaction potentials and target boundary conditions with correct sampling procedures.
These good advantages and qualities of PDMS, motivated us to study and improve understanding of the surface behavior of siloxane to enhance the process of removing oxygen from water used in many applications in the petroleum industry to reduce corrosion.
In this study MD simulation by LAMMPS were performed to evaluate the surface properties of siloxane with two different of functional group attached to the end of the chain which are methyl group and fluorinated alkyl represented by Trifluoromethyl. Three types of chains were chosen (8, 20, 35) monomers per chain. Firstly, surface tension, viscosity, and total energy were examined for both cases. Subsequently we replicated the 20 monomer per chain for both end functional groups and used to evaluate efficiency the de-oxygenation of water.

2. Results and Discussion

2.1. Surface Tension

The study of surface tension is a great importance in understanding the behavior of fluids for petroleum gas/oil separation processes [7]. We mentioned in the introduction that the presence of trifluoromethyl groups (CF3) has the unique property of providing high resistance to the solubility of hydrocarbons while maintaining low surface tension. This property makes it valuable in improving the efficiency of separation processes. Table 1 and Figure 3 showed that the surface tension increased slightly by changing end functional group from methyl group to Trifluoromethyl as well as it is decreased when the chain became longer for both type. These results can be used to check the accuracy for the results as the surface tension maintaining in low value.
The strong electronegativity and space barriers in trifluoromethyl groups on siloxane molecules leads to increase surface tension compared to siloxane molecules with methyl (CH3) groups [8]. On the other hand, the surface tension decrease with increasing chain length, conforms with the hypothesis that the interactions of longer chains increases with van der Waals and surface area, which lead to decrease the surface tension [9,10,11]. This result is observed for both types of end groups, indicating that chain length is a critical factor in determining surface tension. The results show that although the surface tension of siloxane is slightly increased in the presence of a trifluoromethyl group compared to a methyl group, it still maintains a relatively low value, making it suitable for applications in gas/oil separation processes. The observed results confirm the accuracy of the simulation results and provide insights into the molecular interactions.

2.2. Viscosity

This section showing simulation results of viscosity at different chains length with two functional end groups. The results in Table 2 and Figure 4 indicate that as the chain length increase, viscosity increased. As well as, the results showed the viscosity for Trifluoromethyl functional group is higher than methyl group. Longer chain lengths and higher viscous decrease the water permeability through membrane [12]. Increasing viscosity with increasing the chains length is due to increased molecular interactions and crosslinking, which impedes the flow of molecules [12]. The observation in results of viscosity has significant effects for membrane technology. Higher viscosity leads to decreased water permeability through the membrane [13]. This understanding can be essential in designing of membranes for various applications.

2.3. Total Energy

Evaluation the total energy of molecular interaction of siloxane is essential to understanding its behavior in various applications [13]. This section shows the simulated results of total energy of siloxane chains with (CF3) and (CH3) functional end groups. We observe from Table 3 and Figure 5, the total energy is increased as the chain became longer. The other important noticed is that the total energy for both functional groups approximately is equal. Increasing chain length meaning increasing number of bonds between atoms and then increase interaction between the siloxane chains and this led to increase the total energy.

2.4. Evaluation Dissolved Oxygen Removal

This section highlights understanding the effect of functional end groups on the removal of dissolved oxygen by preparing different concentrations of dissolved oxygen in water. Table 4 represented summary of all results that are gain from second part of investigation. We studied 4 oxygen concentrations, represented by (20, 50, 80, 100) % of dissolved oxygen in water. The system for these concentrations consists of for the three types of chain length of PDMS; (8, 20, 35) monomers per chain and each thus contain (87, 207, 357) atoms respectively. To this we then add 100 molecules of water and (20, 50, 80, 100) molecules of oxygen to represent the four concentrations mentioned. To provide a driving force for both water and oxygen molecules we also applied a force of (0.5, 1, 2, 4) (Kcal/mol.°A) to enable the proposed separation within the timescale limitations.
We calculated the percentage of water molecules permeating through membrane then the ratio of dissolved oxygen to water flux or water passage through membrane. Figure 6 represented simulation snapshot of water and dissolved oxygen on siloxane surface during period time of simulation.
We showed in Figure 7 that siloxane with CF3 functional group is more active than in methyl group in the removal of dissolved Oxygen at all concentrations. This is because CF3 groups are more polar and electronegative than CH3 groups and this could make stronger interactions with oxygen molecules and increasing the removal efficiency [15].
On the other hand, although that CF3 functional group have more efficiency than CH3 functional group however the water percentage pass through membrane are more than in CH3 functional group and this noticed can be seen in Figure 8.
In both types of membrane, we evaluate the ratio of percentage of oxygen removal to percentage water passage as shown in Table 4 and Figure 9. We can conclude in over all that ratio of removing oxygen to water passage in CH3 is better than CF3.

3. Materials and Methods

Molecular dynamics simulation through LAMMPS was performed for this study [16]. Jmol software was used to generate chains of siloxane structures with lengths (8, 20, and 35) repeat units, as illustrated in the Table 5 [17]. Methyl (CH3) and trifluoromethyl (CF3) functional groups were independently incorporated into each sample. The simulations were run using the class II potential developed by Sun, Rigby, and others, Table 6 [18,19]. This class II potential is applied within an explicit-atom (EA) model, treating Si, O, C, and H atoms separately. A time step of 0.01 fs and rc = 11 Å was used for all of the simulations. The temperature T was controlled with a Nose-Hoover thermostat at 300 K
The methodology consist of two parts, firstly using both the NVT ensemble and NPT ensemble with constant density 0.95 g/cm3 to evaluate the surface properties of siloxane with both of functional group such as total energy, surface tension and viscosity
In the second part of investigation we chose a chain length (20) repeat unit and replicated four times for both types of functional groups. The main aim of this part is to study efficiency of removing dissolved oxygen in water. In this part, pdms was equilibrated using NVT ensemble with density 0.98 g/cm3 for methyl functional group and 1.2 g/cm3 for trifluoromethyl functional groups, then Packmol [20] was used to configure simulation box containing equilibrated pdms in both functional group as a membrane then water with TIP3P force fields and dissolved oxygen randomly placed above the membrane as shown in Figure 10. We choose four different concentrations (20, 50, 80, 100) % of dissolved oxygen with four driving force (0.25, 1, 2, 4). The configured simulation box was run with NVE ensemble and Langevin to control the temperature.
We used the common form of class I intermolecular potentials for bonds, angles, and torsional bonds. Bond lengths, bond angles, and torsional bond angles are preferred in Potential Harmony as mentioned in the equations below respectively [18,19];
E = kb (r - r0)2
E = kθ (θ - θ0)2
E = kt [1 + cos(nØ)]
Where r0 is the equilibrium bond distance, kb is (energy/distance^2), θ0 is the equilibrium value of the angle, kθ and kt is energy, n integer > = 3 and Ø = 180 degrees.
All potential parameters for bond interaction and non-bond interaction used in the simulations are summarized in Table 6 and Table 7 respectively.
The van der Waals and Coulomb potential interactions occurs through electrical interactions between two or more atoms or molecules that are very close together, these interaction parameters are listed in Table 8. Non- bonded potential is defined as follows [18,19]:
  U α γ nonbond r = U α γ vW r + k q q α q γ r r < r c k q q α q γ r r > r c
Where U α γ vW ( r ) are the van der Waals interactions, r is the distance between two atoms of type α and ɣ, q is the electric charge, and rc is the cutoff distance for the van der Waals interactions.
To determine interaction parameters for non-bonded interaction we used sixthpower mixing rule as below [17];
∈ α γ = 2 σ α   3    σ γ 3 ∈ α ∈ γ σ α 6 + σ γ 6
σ α γ = σ α   6 + σ γ   6 2 1 6
Table 8. Van der Waals Interaction Parameters for the class II force field.
Table 8. Van der Waals Interaction Parameters for the class II force field.
Bonds ∈ [kcal/mol] ơ [Å]
C-C 4.0100 0.0540
C-F 3.7119 0.0459
C-H 3.6691 0.0233
C-O 3.8091 0.1062
C-Si 4.1583 0.0603
F-F 3.2000 0.0598
F-H 3.1059 0.0339
F-O 3.3881 0.1146
F-Si 3.9199 0.0459
H-H 2.9950 0.0200
H-O 3.3189 0.0615
H-Si 3.8875 0.0229
O-O 3.5350 0.2400
O-Si 3.9952 0.1107
Si-Si 4.2840 0.0700

4. Conclusions

In this research, the effect of functional groups on the behaviors of siloxane surface and deoxygenation efficiency was studied. It has been observed that trifluoromethyl functional groups (CF3) are more resistant to dissolution in hydrocarbons compared to methyl groups (CH3) and slightly increased the surface tension. Furthermore, longer chains length, mainly in both types of functional group led to increased viscosity requiring while the total energy approximately was equal. We concluded in deoxygenation processes, membranes with CF3 functional groups gives more efficiency than methyl group due to stronger interactions with oxygen molecules. Despite this, the percentage of water passing through it is also greater compared to siloxane membrane with CH3 functional group. So the evaluation of deoxygenation ratio to water passage showed that membrane with CH3 functional group performed better in this part. These results enhanced the understanding the nuanced balance between functional group properties and membrane performance, offering valuable insights for optimizing siloxane membranes in various applications such as gas/oil separation.

Supplementary Materials

The following supporting information can be downloaded at the website of this paper posted on Preprints.org.

Author Contributions

F.S.: investigation, data curation, methodology, and writing—original draft; S.M.: Supervision, conceptualization, investigation, writing—review and editing; S.M: Supervision, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We express our gratitude to Prof. Dr. Saadoun Taha Ahmed for his valuable guidance during the preparation of this research.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Noll W. Chemistry and technology of silicones,2nd ed.; Elsevier: New York, USA, 2012.
  2. Maciejewski H, Karasiewicz J, Dutkiewicz A, Dutkiewicz M, Dopierała K, Prochaska K. Synthesis and properties of polysiloxanes containing mixed functional groups. Reactive and Functional Polymers. 2014;83:144-54. [CrossRef]
  3. Andriot M, Chao S, Colas A, Cray S, de Buyl F, DeGroot J, et al. Silicones in industrial applications. Inorganic polymers. 2007:61-161.
  4. Yur’evich KA, Valer’evich DF, Ivanov VS, Yegorov AS, Men’shikov VV. Methods for the Preparation of Modified Polyorganosiloxanes (A Review). Oriental Journal of Chemistry. 2018;34(2):612. [CrossRef]
  5. Floess JK, Murad S. Molecular simulations of the competitive adsorption of siloxanes and water on amorphous silica surfaces as a function of temperature. Chemical Physics Letters. 2011;516(4-6):216-9. [CrossRef]
  6. Owen MJ. Surface tension of polytrifluoropropylmethylsiloxane. Journal of applied polymer science. 1988;35(4):895-901. [CrossRef]
  7. Abdul-Majeed GH, Al-Soof NB. Estimation of gas–oil surface tension. Journal of Petroleum Science and Engineering. 2000 Sep 1;27(3-4):197-200. [CrossRef]
  8. Clayden J, Greeves N, Warren S. Organic chemistry: Oxford University Press, USA; 2012.
  9. Schwarcz A, Farinato R. Surface tension of polymers with long unbranched side chains. Journal of Polymer Science: Polymer Physics Edition. 1972;10(10):2025-31.
  10. Dickinson E. The influence of chain length on the surface tensions of oligomeric mixtures. Journal of Colloid and Interface Science. 1975;53(3):467-75. [CrossRef]
  11. Razafindralambo H, Thonart P, Paquox M. Dynamic and equilibrium surface tensions of surfactin aqueous solutions. Journal of Surfactants and Detergents. 2004;7:41-6. [CrossRef]
  12. Ji C, Wang C, Cai X, Liu S, editors. Shear viscosity of Polydimethylsiloxane melt by molecular dynamics simulation. 2020 21st International Conference on Electronic Packaging Technology (ICEPT); 2020: IEEE.
  13. Zhang W, Hou J, Liu Y, Du Q. Effect of shear thickening property of polymer on relative permeability. In Journal of Physics: Conference Series; IOP Publishing: Harbin, China ,2022 Feb 1; Vol. 2194, No. 1, p. 012036. [CrossRef]
  14. Zhang J, Zeng H. Intermolecular and surface interactions in engineering processes. Engineering. 2021 Jan 1;7(1):63-83. [CrossRef]
  15. Oberhammer H. On the structural effects of CF3 groups. Journal of fluorine chemistry. 1983;23(2):147-62. [CrossRef]
  16. Thompson AP, Aktulga HM, Berger R, Bolintineanu DS, Brown WM, Crozier PS, In’t Veld PJ, Kohlmeyer A, Moore SG, Nguyen TD, Shan R. LAMMPS-a flexible simulation tool for particle-based materials modeling at the atomic, meso, and continuum scales. Computer Physics Communications. 2022 Feb 1;271:108171.
  17. Cass ME, Rzepa HS, Rzepa DR, Williams CK. The use of the free, open-source program Jmol to generate an interactive web site to teach molecular symmetry. Journal of chemical education. 2005;82(11):1736. [CrossRef]
  18. Frischknecht AL, Curro JG. Improved united atom force field for poly (dimethylsiloxane). Macromolecules. 2003;36(6):2122-9. [CrossRef]
  19. Sides SW, Curro J, Grest GS, Stevens MJ, Soddemann T, Habenschuss A, et al. Structure of poly (dimethylsiloxane) melts: Theory, simulation, and experiment. Macromolecules. 2002;35(16):6455-65.
  20. Martínez L, Andrade R, Birgin E, PACKMOL JM. A package for building initial configurations for molecular dynamics simulations., 2009, 30. [CrossRef]
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Figure 1. Structure of the PDMS (methyl end group).
Figure 1. Structure of the PDMS (methyl end group).
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Figure 2. Structure of the PDMS (fluorine alkyl end group).
Figure 2. Structure of the PDMS (fluorine alkyl end group).
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Figure 3. Relation of Surface tension with chain Length and Functional group.
Figure 3. Relation of Surface tension with chain Length and Functional group.
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Figure 4. Relation of Viscosity with chain Length and Functional group.
Figure 4. Relation of Viscosity with chain Length and Functional group.
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Figure 5. Relation of Total Energy with chain Length and Functional group.
Figure 5. Relation of Total Energy with chain Length and Functional group.
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Figure 6. snapshot of water and dissolved oxygen on siloxane surface during Simulation. Siloxane with CF3 (b) Siloxane with CH3. Color code: dissolved O, pink; Water O, blue; Siloxane O, red; C, grey; H, white; Si, yellow; F, green.
Figure 6. snapshot of water and dissolved oxygen on siloxane surface during Simulation. Siloxane with CF3 (b) Siloxane with CH3. Color code: dissolved O, pink; Water O, blue; Siloxane O, red; C, grey; H, white; Si, yellow; F, green.
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Figure 7. Percentage Dissolved Oxygen Removal with respect to driving Force.
Figure 7. Percentage Dissolved Oxygen Removal with respect to driving Force.
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Figure 8. Percentage Water Pass through membrane with respect to driving Force.
Figure 8. Percentage Water Pass through membrane with respect to driving Force.
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Figure 9. Ratio of percentage removal dissolved oxygen to water pass through membrane with respect to driving Force.
Figure 9. Ratio of percentage removal dissolved oxygen to water pass through membrane with respect to driving Force.
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Figure 10. Configuration of water and dissolved oxygen slab in contact with siloxane. (a) Siloxane with CF3 (b) Siloxane with CH3. Color code: dissolved O, pink;. Water O, blue; Siloxane O, red; C, grey; H, white; Si, yellow; F, green.
Figure 10. Configuration of water and dissolved oxygen slab in contact with siloxane. (a) Siloxane with CF3 (b) Siloxane with CH3. Color code: dissolved O, pink;. Water O, blue; Siloxane O, red; C, grey; H, white; Si, yellow; F, green.
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Table 1. Chain Length and Surface Tension for methyl and Fluorinated alkyl Function Group.
Table 1. Chain Length and Surface Tension for methyl and Fluorinated alkyl Function Group.
Type of chain Chain Length Surface Tension (m N/m)
CH3 8 21.44
CH3 20 19.97
CH3 35 17.8
Average 19.74
CF3 8 24.76
CF3 20 20.183
CF3 35 19.81
Average 21.58
Table 2. Chain Length and viscosity for methyl and Fluorinated alkyl Function Group.
Table 2. Chain Length and viscosity for methyl and Fluorinated alkyl Function Group.
Type of chain Chain Length Viscosity (cp)
CH3 8 8.83
CH3 20 18.4
CH3 35 20
Average 15.74
CF3 8 11.9
CF3 20 17.8
CF3 35 29.9
Average 19.86
Table 3. Total Energy for Siloxane as a Function of Chain Length and Types.
Table 3. Total Energy for Siloxane as a Function of Chain Length and Types.
Type of chain Chain Length Total Energy (Kcal)
CH3 8 171.83
CH3 20 422.95
CH3 35 723.16
Average 439.32
CF3 8 177.44
CF3 20 424.47
CF3 35 722.9
Average 441.6
Table 4. Dissolved Oxygen Removal at 300K.
Table 4. Dissolved Oxygen Removal at 300K.
Chains Type Driving Force (kcal/mol)/Angstrom Od Concentration with water Od Reomoval % Water pass % (Od removal %) / (water moleules pass %)
CH3 0.25 20% 0.00 0.00 0.00
50% 1.98 40.11 4.95
80% 3.93 62.87 6.25
100% 157.56 95.68 164.67
average 40.87 49.67 82.29
CF3 0.25 20% 1.99 43.69 4.56
50% 63.70 91.62 69.53
80% 139.73 85.90 162.66
100% 185.15 82.36 224.80
average 97.65 75.89 128.66
CH3 1 20% 2.29 37.60 6.09
50% 94.66 67.51 140.21
80% 141.68 65.67 215.73
100% 188.82 69.53 271.55
average 106.86 60.08 177.86
CF3 1 20% 15.96 58.32 27.37
50% 91.90 72.52 126.72
80% 139.73 85.90 162.66
100% 185.15 82.36 224.80
average 108.19 74.78 144.68
CH3 2 20% 11.95 38.92 30.70
50% 75.62 38.28 197.55
80% 119.92 58.79 203.98
100% 172.35 52.41 328.86
average 94.96 47.10 201.62
CF3 2 20% 26.90 64.72 41.57
50% 63.70 91.62 69.53
80% 78.84 65.55 120.28
100% 170.20 71.53 237.95
average 84.91 73.35 115.76
CH3 4 20% 13.93 50.56 27.55
50% 53.72 35.75 150.24
80% 117.24 53.66 218.49
100% 133.07 47.50 280.13
average 79.49 46.87 169.60
CF3 4 20% 25.92 73.48 35.27
50% 69.80 60.19 115.96
80% 137.03 65.34 209.71
100% 157.75 62.91 250.75
average 97.62 65.48 149.09
Table 5. Number and Molecular Weight of PDMS Structures.
Table 5. Number and Molecular Weight of PDMS Structures.
End Functional Group PDMS structure per Monomer Total atoms of pdms in Simulation box Molecular weight (g/mol)
CH3 8 87 607.310
20 207 1497.169
35 357 2609.493
CF3 8 87 607.310
20 207 1497.169
35 357 2609.493
Table 6. Potential Parameters for bond interaction.
Table 6. Potential Parameters for bond interaction.
bond Kt r0 Angle Kθ θ0 dehidral KØ n
C-F 496 1.363 C-Si-C 44.4 113.5 F-C-Si-C 0.9006 3
C-H 340.6175 1.105 C-Si-O 42.3 113.1 F-C-Si-O 0.9006 3
C-Si 238 1.809 F-C-F 60 109.5 H-C-Si-C 0.9006 3
O-Si 392.8 1.665 F-C-Si 95 107.8 H-C-Si-O 0.9006 3
H-C-H 60 109.5 Si-O-Si-C 0.9006 3
H-C-Si 34.6 112.3 Si-O-Si-O 0.9006 3
O-Si-O 44.4 113.5
Si-O-Si 60 109.5
Table 7. Potential Parameters for non-bond interaction.
Table 7. Potential Parameters for non-bond interaction.
Nonbonded ∈[kcal/mol] ơ [Å] q mass
C 0.054 4.01 -0.294000 12.0115
F 0.0598 3.2 -0.344300 18.9984
H 0.02 2.995 0.053000 1.0079
O 0.24 3.535 -0.445000 15.9994
Si 0.07 4.284 0.715000 28.085501
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