Molecular Dynamics Simulations of Oil Detachment from Hydrophobic Surfaces by Using Janus Nanoparticles

Abstract

Janus particles composed of two parts with different chemical properties can be used for enhanced oil recovery. We investigated the role of Janus dimers in the process of detachment of oil aggregates from hydrophobic solid surfaces using molecular dynamics. Large-scale simulations were performed for different sets of parameters characterizing the system. The effects of interactions between Janus particles and a solid surface and with an oil droplet were considered. We found two main mechanisms of enhanced oil removal: the "kidnaping" of oil aggregates by Janus nanoparticles from the substrate and the competitive adsorption of nanoparticles at the solid surface. In the case of weak affinity of Janus particles with the substrate, the first mechanism dominated, whereas when the affinity was strong, the second mechanism played an important role. We showed how the amphiphilicity of Janus particles and their concentration influence the shape and internal structure of oil aggregates adsorbed on a hydrophobic surface. The high amphiphilicity of Janus particles and their increased concentration promote the process of removing oil from the surface. We analyzed the time evolution of the system after the addition of Janus particles in detail. In the "kidnaping" process, the flat oil aggregate left the surface as a large, sandwich-like aggregate, while the adsorption of Janus particles on the surface caused it to break up into small pieces that left the surface as nearly spherical droplets.

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Examples of the equilibrium configurations of oil aggregates adsorbed on the surfaces of different strengths of aqueous solvent–substrate interactions(a) the inert (hydrophobic) surface, and hydrophilic substrates with ε _WS ^*: 0.5 (b), 1.0 (c), 1.5 (d), 2.0 (e). The yellow spheres correspond to the oil segments (O), the light blue spheres represent the solvent (W), and the gray spheres are for the solid. For greater clarity of the drawing, the solvent molecules have been reduced.

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Local density profiles of the oil segments (a) and solvent molecules (b) on the surfaces with different strengths of solvent–substrate interactioninert surface (black), and hydrophilic substrates with ε _WS ^*: 0.25 (red), 0.5 (green), 0.75 (blue), 1.0 (orange), 1.25 (orange), 1.5 (brown), and 1.75 (magenta). In (b), the solvent density profile is also plotted for ε _WS ^* = 2.0 (dashed turquoise line). In this case, the oil droplet does not attach to the surface. The abscissas are scaled logarithmically.

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Average z-coordinate of the center of mass of oil segments, z _com ^*, (a) and the fraction of the number of oil segments which “touch” the surface, f, (b) as functions of the energy parameter characterizing the solvent–substrate interactions, ε _WS ^*.

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Examples of the equilibrium configurations of systems containing Janus particles with different BO interactions, ε _BO ^*: 1.0 (a, b), 3.0 (c, d), and 5.0 (e, f). The green spheres represent segments A, the red spheres represent segments B, the yellow spheres correspond to the polymer segments (O), the light blue spheres represent the solvent (W), and the gray spheres represent the solid. For greater clarity of the drawing, the solvent molecules have been reduced. The density of Janus particles, ρ _AB ^* = 0.042.

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Average z-coordinate of the center of mass of oil segments, z _com ^*, (a), and the fraction of the number of oil segments which “touch” the surface, f, (b) as functions of the energy parameter characterizing the BO interactions, ε_BO ^*. The density of Janus particles, ρ _AB ^* = 0.042.

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Local density profiles of the oil segments (a) and the segments of Janus dimers (b) on the strongly hydrophobic substrate for different BO interactions, ε _BO ^*: 1.0 (black), 3,0 (red), and 5.0 (green). In (b), the densities of segments B (A) are plotted as solid lines (dashed lines). The abscissas are scaled logarithmically. The density of Janus particles, ρ _AB ^* = 0.042.

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Average z-coordinate of the center of mass of the oil segments (a) and the fraction of the number of oil segments on the surface (b) as functions of time for different BO interactions, ε _BO ^*: 1.0 (black), 3.0 (red), and 5.0 (green). The density of Janus particles, ρ _AB ^* = 0.042.

8

Examples of the equilibrium configurations of systems with different densities of Janus particles ρ _AB ^*: 0 (a), 0.007 (b), 0.014 (c), 0.021 (d), 0.028 (e), and 0.035 (f). The green spheres represent segments A, the red spheres represent segments B, the yellow spheres correspond to the polymer segments (O), the light blue spheres represent the solvent (W), and the gray spheres are for the solid. For greater clarity of the drawing, the solvent molecules have been reduced. The parameter ε _BO ^* = 5.0.

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Local density profiles of the oil segments (a) and the segments of Janus dimers (b) on the strongly hydrophobic substrate for different average densities of Janus dimers, ρ _AB ^*: 0.007 (black), 0.014 (red), 0.021 (green), 0.028 (blue), and 0.035 (orange). In (b), the densities of segments B (A) are plotted as solid lines (dashed lines). The abscissas are scaled logarithmically. The parameter ε _BO ^* = 5.0.

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Average z-coordinate of the center of mass of oil segments, z _com ^*, (a) and the fraction of the number of oil segments which “touch” the surface, f, (b) as functions of the density of Janus dimers ρ _AB ^*. The parameter ε _BO ^* = 5.0.

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Average z-coordinate of the center of mass of the oil segments, z _com ^*, (a) and the fraction of the number of oil segments on the surface, f, (b) as functions time for different densities of Janus dimers, ρ _AB ^*: 0.007 (black), 0.014 (red), 0.021 (green), and 0.028 (blue). The parameter ε _BO ^* = 5.0.

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Time evolution of the system involving Janus dimers that are attracted by the substrate, ε_AS ^* = 5.0. The simulation time: 0 (a), 2.0·10⁸ (b), 6.0·10⁸ (c), 8.0·10⁸ (d), 9.15·10⁸ (e), and 1.0·10⁹ (f). The green spheres represent segments A, the red spheres represent segments B, the yellow spheres correspond to the polymer segments (O), the light blue spheres represent the solvent (W), and the gray spheres are for the solid. For greater clarity of the drawing, the solvent molecules and dimer segments have been reduced. Janus dimers are inert with respect to the oil and solvent. The density of Janus particles, ρ _AB ^* = 0.056.

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Final stage of oil droplet detachment is shown in Figure . The simulation time: 8·10⁸ (a), 9.15·10⁸ (b), 9.48·10⁸ (c), 9.63·10⁸ (d), 9.90·10⁸ (e), and 1.00·10⁹ (f). The yellow spheres correspond to polymer segments (O). For greater clarity of the drawing, the solvent molecules and Janus dimers have been omitted.

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Average z-coordinate of the center of mass of the oil segments, z _com ^*, (a) and the number of oil segments on the surface, f, (b) as functions of time for the system from Figure . The red arrows have letters corresponding to the snapshots shown in Figure .