Effects of Interactions Among Surfactants,Water and Oil on Equilibrium Configuration of Surfactant-W Effects of Interactions Among Surfactants,Water and Oil on Equilibrium Configuration of Surfactant-W

Effects of Interactions Among Surfactants,Water and Oil on Equilibrium Configuration of Surfactant-W

  • 期刊名字:武汉大学学报
  • 文件大小:360kb
  • 论文作者:Yuan Yin-quan,SUN Zhi-bo,XIE Y
  • 作者单位:School of Physics and Technology,Department of Physics
  • 更新时间:2020-07-08
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论文简介

Vol.9 No. 62004 895-898WUJNSWuhan University Journal of Natural SciencesArticle ID: 1007-1202(2004)06- 0895-04Effects of Interactions Among Surfactants,Water and Oil on Equilibrium Configurationof SurfactantWater-Oil Systems0 IntroductionYUAN Yin-quan1,2,SUN Zhi-bo' ,XIE Yun2,Z0U Xian-wulturfactants are amphiphilic molecules that are generally1. School of Physics and Technology,Wuhan University,Sabsorbed at interface between water and oil, and lowerWuhan 430072, Hubei,China;2. Department of Physics, Xiaogan University, Xiaoganthe interfacial tensionl. A typical surfactant contains a polar432100,Hubei, Chinahead group and a nonpolar tail. In a surfactant water\|oil (S-W-O) system, the head and tail of the surfactant are attractedAbstract: The distribution and configuration of surfactantsto the water and oil, respectively. Recently ,considerable in-at interface in surfactant- water oil systems have been investi-terest has been focused into the influence of surfactants on thegated using discontinuous molecular dynamic simulations.There exists a certain equilibrium concentration of surfactantsphase separation dynamics and equilibrium configuration ofat interface for the systems with certain interactions amongwater- oil systems52.3]. The discontinuous molecular dynamicsurfactant, water and oil. The interface length and equilibri-(DMD) simulation offers powerful means to study the phaseum morphology of the systems are dependent on the equilibri-separation and equllibrium configuration of systems-41 .um concentration of surfactants at interface and the total a-mount of surfactants. The interaction strengths among sur-Kawakatsu and Kawasaki have proposed a hybrid model infactant, water and oil determine the equilibrium concentrationwhich surfactants are microscopically treated[5]. But how theof surfactants at interface. Three typical configurations ofsurfactants at interface have been observed: ①surfactantinteraction between surfactant molecules and other phasemolecules are perpendicular to the interface and arrangedeffects the configuration of surfactants at interface is lack un-closely;②perpendicular to the interface and arranged at in-derstanding. In this report, we investigated the distributionterval of two particles; ③lie down in the interface partly.and configuration of surfactant molecules at interface in S W-OKey words: surfactant; phase separation; equilibrium con-figurationsystems using DMD simulation and bead-type model. It isCLC number: O 469, O 64found that there exists an equilibrium concentration of surfac-tants at interface,which is related to the interaction strengthamong surfactant, water and oil, and determines the interfacelength and equilbrium morphology of S W-O systems.Received date: 2004-05-10Foundation item; Supported by the National Natural Science Foun-中国煤化工dation of China (10274056) and the Foundation of the Education Min-1 Methistry of Hubei ProvinceYHCNMHGBiography: YUAN Yin quan (1964-) ,male, Ph. D.,research direc-tion: soft matter.Discontinuous molecular dynamic simulation and standardt To whom correspondence should be adressed. E mail: xwzou@bead- type model have been used to study the phase separationwhu. edu. cn895Wuhan Unprpfr数rnal of Natural Sciences Vol.9 No.6 2004of S W-O systems-6-12]. In the systems, the particles in-more complicate. For convenience, we use the total num-teract with each other via the collisions in the discontinu-ber of particles on one side of the interface to express the)us potential. Supposing there exist two types of beadsinterfacial length Leq. Figure 2 plots the interface length( particles):“illike” and“waterlike”. An oil moleculeas a function of time for the systems with various concen-corresponds to a single ollike bead, and a water moleculetrations of surfactants. It can be found from this figure,corresponds to a single waterlike bead. A surfactant mol-after 1. 0X 107 MDs, those S W-O systems come to theirecule is made up of p waterlike beads ( head beads) and qequilibrium states, and Leq approach their stable values .oillike beads (tail beads), and denoted by H, T。. For70,95,125and165forgs=0.12,0.16,0.24andconvenience, we used H and T, W and 0 to denote head0. 32,respectively. These data show that the equilibriumand tail beads, water and oil beads respectively. Theinterface length increases almost linearly with the surfac-whole system consists of Nw water beads, No oil beadstant concentration (see the inset of Fig. 2). The equilib-and Ns surfactant molecule, so the total number of beadsrium concentration of surfactants at interface Teq, whichN= Nw+ No+ (p+q) Ns, the concentration of surfac-is defined as the ratio of the number of surfactant mole-tants gs= (p+q)Ns/N. For simplicity, we assume allcules at interface to the total number of particles at onebeads have the same diameter σ and mass m. The systemside of the interface in the equilibrium states, Pa=0. 45consists of 1 000 particles ( beads), they spread on a two-士0.05. It is independent of the concentration of surfac-dimensional square box, and the average density is takentants and fixed as a constant in the range of gs betweenthe value of 0. 6. The periodic boundary condition is0.12 and 0.32 for present S W-O systems. It is under-used. The initial configuration was prepared by spreadingstandable. Due to the amphiphilic characteristic, the sur-a certain number of particles in the right proportion on afactant molecules prefer to be situated at the W-O inter-two- dimensional square lattice, and by labelling theseface. The distribution of surfactant molecules at interfaceparticles head, tail, oil and water randomly. In this oper-is determined by the strength of hydrophilic and hydro-ation it is important to ensure the required number of thephobic interactions, so the equilibrium concentration ofHp Tq molecules, water and oil particles. After beingsurfactants at interface is definite for the S W-O systemsheated to a high temperature (temp= 10) and relaxing forwith the same interaction parameters in the equilibrium. a long time [2. 0X 106 molecule dynamical time stepsstate.(MDs)],the systems were quenched into the tempera-Table 1 Interaction parameters used for the HTr-W-O systemsture temp=1. To keep the systems at such constant tem-HHHTHWTTTWwwHOTOWOO0perature velocity rescaling was used every 10* MDs in the-0.2 0.2 -3.0 -0.2 0.2 -1.0 0.2 -3.0 1.0 -1.0whole simulating process, and each of the systems was e-volved to a total run of about 2. 0X 107 MDs. Through-out this paper, the reduced units are used. The absolutevalue of water- water attraction |eww|( is selected as theenergy unit, so we have eww= - 1.0. |leww |/kB is takenas the temperature unit (kB is the Boltzmann constant), σas the length unit, and σ(m/ |eww)1/2 as the time unit.2 Simulation and DiscussionTo understand the equilibrium morphology of S W-O systems, we simulated four S W-O systems with vari-ous concentrations of surfactants. The interaction param-eters are fixed and shown in Table 1. The typical equilib-Fig.中国煤化ITations of HT-wateroilrium configurations are displayed in Fig. 1. It can besysten:YHCN M H GHT,(a)qs=0.12.(b)seen that the surfactant molecules are concentrated on the0.16,(c) 0.24,and (d) 0, 32. Square: head of H2 Tg; black circle:interface, and as the surfactant concentration gs increa-tail of H2T2; dark gray circle: oil; light gray cirle: waterses, the interface becomes longer and the pattern becomes896YUAN Yin-quan et al: Effects of Interactions Among Surfactants, ..EHw =ETo,and Eww=Eoo=-1. In addition, the repul-400180sion parameters EHO and εTw, EHT and Ewo are the same as140the values listed in Table 1. In this case, Teq is deter-mined by ETT and ETO.300100Figure 3(a) shows the dependence of Fa on εTo for0.1 0.2 03the systems with EHu=εττ=→3. 0. Figure 3(b) shows200央the dependence of Peq on ETT for the systems with EHw =ETo= -1.0. At pointIof Fig. 3(a),∈Hw=εro= -1.100 |0,andeHu=ετr=一3. 0, the strong head-head and tail-tail attractions enforce the surfactant molecules to arrange5(150Time 1 10' MDsclosely and to be perpendicular to the interface. The cor-responding equilibrium morphology is the patternFig. 2 Dependence of interfacial length on the time for H2T2-water-oil systems with various concentrations of surfactantsshowed in Fig.4(a). At point IIof Fig.3(a), EHw=εToThe length unit is the diameter of water particle and the time unit=一3.0,andεmu=επr= 一3. 0, since the head-head,is 105 MDs. Inset: the equilibrium interfacial length with the surfac-tail\| tail, head- water and tail-oil attractions are all stron-tant concentrationger than the water- water and oil-oil attractions, the sur-To understand the relationship between the equilib-factant molecules arrange themselves perpendicular to therium concentration of surfactants at interface Fa and theinterface and at interval of two particles. The corre-interaction strength among surfactant, water and oil, wesponding equilibrium morphology is the pattern II showedsimulated the S W-O systems with gs=0.16. It can bein Fig.4(b). At point II of Fig. 3(b), εHw=ετo =seen from Fig. 1, the head beads only adjoin head beads-1.0, and eHH=εrτ= -0. 2, all interactions are smal-or water particles and the tail beads only border on tailler than or equivalent to the thermal fluctuation (kB T=beads or oil particles in the equilibrium state. Therefore,1. 0). On one hand, due to the amphiphilic property,thethe effect of the interactions on the configuration and e-surfactant molecules concentrate on the interface. On thequilibrium concentration of surfactants at interface can beother hand, owing to the influence of relatively strongreduced to the influence of interaction parameters EHH , .thermal fluctuation, one end of the surfactant molecule :EHW and Eww,along with ETτ, ETO and Eoo. For simpli-lies down on the interface, and another end distributesty, we use the symmetric systems, in which, EHH=ετ,randomly. It is just the pattern II showed in Fig. 4(c).(a)(b).8-0.8F↑I80.6 F0.6).4 F0.4-2ετoπFig, 3 Dependence of the equilibrium density of the surfactant HT2 at interface on interaction parameters. φs=0. 16It can be seen from Fig. 4(a) that, by lack of the .ical equilibrium concentrations of surfactants at interfacesurfactant molecules, only a part of the interface is em-Teq are中国煤化工4. The obtained r.bedded by surfactants and the rest does not join up with .are 0.terns I, II and II, re-CNMHGsurfactants. In this case, only the part of interface em-spectively. uccaust ui icllici riuctuation, these valuesbedded by surfactants is taken into account in the calcula-approach, but are not equal to their idea values 1 (for thetion of the surfactant concentration at interface. The typ-pattern I) and 0.5 (for patterns II and II). .897Wuhan Unipit数ernal of Natural Sciences Vol.9 No.6 2004a):)?e必题Fig. 4 Typical equilibrium configurations of surfactants in the interface region for the H2 I;-water- oil systems with φs=0. 16(a) Pattern I;(b) Pattern I;(c) Pattern IDynamics Simulations of Phase Separation in the Presence of3 ConclusionSurfactants. Phys Rev E,1994,50: 1243-1252.[4] Franzese G, Malesolo G, Skibinsky A, et al. Generic Mech-anism for Generating a Liquid Liquid Phase Transition. NaThis work shows that the combination of DMD sim-ture, 2001,409: 692-695.ulation and bead- type surfactant model is suitable for in-[5] Kawakatsu T, Kawasaki K, Furusaka M, et al. Theoriesand Computer Simulations of Self- Assembling Surfactant So-vestigating the distribution and configuration of surfac-lutions. J Phys: Condens Matter, 1994. 6: 6385- 6408.tants at interface in surfactant water oil systems. The in-[6] Elltt J R,Hu L. Vapor Liquid Equilibria of Square Wellteraction strengths among surfactants, water and oil de-Spheres. J Chem Phys, 1999, 110: 3043- 3048.termine the equilibrium concentration of surfactants at in-[7] Alder B J, Wainwright T E. Studies in Molecular Dynamics.I. General Method. J Chem Phys, 1959,31: 459-466.terface, which further determines the interface length and8] Allen M P, Tildesley D J. Computer Simulation of Liquid.equilibrium morphology of S W-O systems.New York: Oxford University Press, 1987.[9] Smit B. Computer Simulations of a Water/ oil Interface in theReferencesPresence of Mieclles. Nature, 1990, 348: 624-625.[10] Larson R G. Monte Carlo Simulation of MicrostrueturalTransitions in Surfactant Systems. J Chem Phys, 1992, 96:[1] Safran S A. Statistical Thermodynamics of Surfaces, In-7904-7918.terfaces and Membranes. MA: Addison Wesley Publishing[11] Liu H Y, ZouX W,Yuan Y Q, etal. Elfects of PropertiesCompany, 1994.of the Surfactant on Its Aggregate Behavior. Eur Phys J E,2] RoanJ R,HuC K. Crossover from the Hydrodynamic Re2002, 8: 373-376.o the Thermal Fluctuation Regime in a Two-Dimen-[12] YuanYQ, ZouX w, LiuH Y. Efects of Concentration andsional Phase Separation Binary Fluid Containing Surfactants.Conformation of Surfactants on the Phase Separation of Sur-Phys Rev E, 2000, 62: 766-774.factant- Water-Oil System. Chin Phys Lett, 2004, 21: 709-[3] Laradji M, Mouritsen OG,Toxvaerd s, et al. Molecular712.中国煤化工MHCNMHG898YUAN Yin-quan et al: Effects of Interactions Among Surfactants, ..

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