Molecular Dynamics Simulation on Behavior of Common Surfactants at the Oil/Water Interface in Complex Systems†

doi:10.7503/cjcu20170242

Abstract

Abstract:

In order to explore the behavior of common surfactants on oil/water interface in complex systems, five models were designed to study using the molecular simulation method. Except a blank model, four different surfactants were placed in the same complex systems as follows: sodium dodecyl benzene sulfonate(SDBS), dodecyltrimethylammonium bromide(DTAB), nonylphenol ethoxylates(NPE), and betaine. These four surfactants represented the anionic surfactant, the cationic surfactant, the nonionic surfactant, and the zwitterionic surfactant, respectively. For these models, radial distribution function(RDF) of surfactants and the oil molecules, mean square displacement(MSD) along z direction of oil molecules, oil/water interfacial tension(IFT), interaction energy between the oil layer and the mineral stone layer, the distribution of relative concentration of oil molecules along z direction, etc. were analyzed. For the SDBS, NPE, Betaine system, at the beginning, the surfactants arranged regularly at the interface of oil/water, the lipophilic parts of the surfactants were partially inserted into the oil phase, the hydrophilic parts were stretched into the water phase. Then the hydrophilic parts of the surfactants got together and formed to a micelle gradually. The micelle, which of its interior is hydrophilic and its exterior is lipophilic, moved to the oil layer gradually. For DTAB, at first, the surfactants were distributed regularly at the interface of oil/water, they gradually became disorderly as the simulation time increased. But the lipophilic head stretched into oil phase and the hydrophilic head distributed at the oil/water from the beginning to end. The order of the interaction between surfactants and oil molecular were: Betaine≈DTAB<SDBS<NPE; The order of oil displacement efficiency of different surfactants were: Betaine>SDBS>NPE>DTAB>None. Above all, the model systems constructed were rationalized. The results obtained from the simulation were agreement with the experimental data. The research gave partly the illustration of actual oil displacement efficiency of different surfactants at the molecular level.

Key words: Molecular dynamics, Oil/water interface, Interfacial tension, Interaction, Oil displacement efficiency

CLC Number:

O641
O631

TrendMD:

JIANG Rongjun, LUO Jianhui, BAI Ruibing, JIANG Bo, ZHOU Ge. Molecular Dynamics Simulation on Behavior of Common Surfactants at the Oil/Water Interface in Complex Systems^†[J]. Chem. J. Chinese Universities, 2017, 38(10): 1804.

Figures/Tables 9

Fig.1 Construction of common surfactants model systems(such as SDBS)

Fig.2 Energy of the DTAB model as the function of simulation time

Fig.3 Model systems for calculating the interfacial tension(such as SDBS)

Fig.4 Behavior variation of different surfactants systems(highlight for surfactants)(A1—A5) SDBS; (B1—B5) NPE; (C1—C5) DTAB; (D1—D5) Betaine. (A1—D1) 0 ps;(A2—D2) 250 ps; (A3—D3) 500 ps; (A4—D4) 1000 ps; (A5—D5) 2000 ps.

Fig.5 RDF for different surfactants and oil layers

Fig.6 Centroid variation(Zoil center) of oil molecules along the simulation time

Fig.7 Snaps of different surfactants systems at 0 ps(A1—E1), 500 ps(A2—E2), 2000 ps(A3—E3), respectively(A1—A3) None; (B1—B3) DTAB; (C1—C3) SDBS; (D1—D3) Betaine; (E1—E3) NPE.

Fig.8 Relative concentrations of oil molecules of different surfactants systems at 500 ps(a) and 2000 ps(b)(A) None; (B) DTAB; (C) SDBS, (D) Betaine; (E) NPE.

Fig.9 MSD of oil molecular in different surfactants systems along z-direction

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