MesoDyn Simulation Study on Phase Diagram of Aerosol OT/isooctane/water System

Chinese Chemical Letters Vol. 13, No.10, pp 1025- 1028, 20021025http://www.imm.ac.cn/jourmalccl.htmlMesoDyn Simulation Study on Phase Diagram of AerosolOT/isooctane/water SystemShi Ling YUAN1.2*, Gui Ying XU, Zheng Ting CAI'Institute of Theoretical Chemistry, Shandong University, Jinan 250100'Institute of Colloid & Interface Chemistry, Shandong University, Jinan 250100Abstract: A simple model, i.e. sodium di(2- -ethylhexyl) sulfosuccinate (AOT) represented by one-head and two-tail beads tied together by a harmonic spring and water or isooctane by one bead,was put forward via Dissipative Particles Dynamics (DPD) simulation method. Using thechanges of interfacial tension between water and oil phase, a ternary phase diagram ofAOT/water/isooctane system was drawn. From the simulation, one conclusion is shown thatDPD simulation can be considered as an adjunct to experiments.Keywords: Dissipative particles dynamics, interfacial tension, phase diagram, mesodyn simulation.There is a growing interest in the study of surfactant self-assemble in oil/water/surfactantsystem because of their applications not only in traditional colloid chemistry but inanalytical, synthetic, and medicinal chemistry as well2. In these systems, one of themost commonly studied sufactants which can form reverse micelles is sodium bis(2-ethylhexyl) sulfosuccinate, i.e. Aerosol OT (AOT)3. The properties of the AOT reverse :micelles have been discussed by some experimental methods+ 7 including IR, NMR, lightscattering, and so on. These experimental results enhance the understanding for thetwo-tail surfactant. At the same time, some molcular simulation methods have alsogained a growing interest about reverse micelle in recent years8-10.Dissipative Particle Dynamics (DPD) simulation method is an effective MesoDyn(Mesoscopic Dynamic) based on solving Newton' s motion equation with Verletalgorithm'". In this technique surfactant molecules are described by beads that act ascenters of mass, and a series of beads are connected through harmonic springs. Usingdifferent parameters representing the liquid compressibility and mutual solubility, realsurfactants molecules can be introduced into DPD model. In the model, the AOTmolecule is represented by three beads, which divided into two tails and one head beadtied together by a harmonic spring. Using the molecular simulation, the interfacialtension between water and oil can be calculated. The phase diagram of water/oil/AOTsystem can be obtained by the changes of the interfacial tension.* E-mail: shilingyuan @ sdu.edu.cn中国煤化工MHCNMH G.1026Shi Ling YUAN et al.Results and DiscussionsCalculation of the interaction parameterFor the AOT surfactant structure, it is difficult to distinguish hydrophobic tails from ahydrophilic domain, because the COO ester groups (referred to as the “elbow" regionsbetween the polar head and apolar tails) play an important role in the interaction withwater molecules at the origins of the tails. In this letter, in term of our calculations, the-SO.Na group and the nearby - _COO- ester groups are considered as the hydrophilicgroup. In Figure 1, the structures on the left of the dashed line are selected as thehydrophilic group, in which we use the hydrogen atoms instead of the tails.Accordingly, the leavings, ie. the structures on the right of the dashed line, are selectedas the hydrophobic tails using the hydrogen atom instead of the head.Figure 1 The structure of hydrophilic head and hydrophobic tails of AOTNa* ~OgS_When the head, the tail, isooctane and water are considered as the simulated objects,the interaction parameter, i.e. the Flory-Huggins parameter， between two simulatedobjects can be calculated using the computer simulation. And the parameters can betranslated into the DPD interaction parameters depending on the calculationl2. Theseparameters are listed in the following Table 1.Table 1 The interaction parameter among the molecules25.0027.708 4430 42 5350.0001 0.828 16.85925. 362427.70824. 744 31 28827.6220.8281 -0.0782 1 .90470.801 947.43031.288 25 .67 528.9976.85921.9047 0 20641 222242.53527.e2228.997237255.3624 0.8019 1 .22 22 -0 3899Note: h represents the head group, t the tail group, W water molecule, and 0 isooctane.Dynamic interfacial tension in the process of phase separationWhen these DPD parameters are used in the simulation, the interfacial tension betweenoil and water interface in isooctane/water/AOT system can be calculated using theGroot s result"In Figure 2, the change of interfacial tension with the increase of simulated steps .shows the change process of aggregates in the ternary system. For the fixed-concentration water/oil/surfactant system, the three components are dispersed in thesolution at the beginning of simulation, so the value of interfacial tension is high. Whensmall aggregates like water or oil drops begin to occur, surfactant molecules may arrangeat the interface between small oil and water drops. This means that the interfacialtension decreases. Some bigger water or oil drops begin to .中国煤化工THCNMH G.MesoDyn Simulation Study on Phase Diagram of Aerosol1027OT/isooctane/water Systembetween small drops with the increase of simulated times. At last, interfacial tensionreaches the lowest as soon as two incompatible discontinuous phases separate in thesystem. Figure 2 also shows that the simulated system has already gotten equilibrationstate after 5000 steps, and additional simulated time does not affect the equilibrationresult. In order to average out all the thermal fluctuations in the interfacial region, thevalues of interfacial tension are calculated at 10000 simulated steps.Figure 2 Simulated times dependence of interfacial tension for ternary systemThe simulated steps (X1000)Making phase diagram using the interfacial tensionIn Figure 3a, three peaks are found in the curve of interfacial tension via surfactantconcentration. It indicates that three phase transitions occur with the increase of theAOT concentration and three points are pointed out in the phase diagram ofwater/AOT/isooctane system at the 5% concentration of isooctane. Similar to this, itcan be seen that two-phase transitions in the systems occur (see Figure 3b, at the 50%concentration of isooctane). Using this method, if many systems of fixed isooctaneconcentrations are provided, a whole ternary phase diagram can be drawn through thecurves of the interfacial tension, as shown in Figure 4.Figure 3 Interfacial tension in isooctane/water/AOT system. a, 5% isooctane; b, 50% isooctane.5-.3-0.4-0.2-0.0。2Surfactant Concentration (%)abA typical phase diagram for the water/AOT/isooctane system (Figure 4b) wasprovided by Tamamushil3 using the experimental technique.中国煤化工TYHCNMH G.1028Shi Ling YUAN et al.between our simulated phase diagram (Figure 4a) and his experimental result is that theformer has bigger O/W and W/O micelle regions and a smaller liquid crystals region.The different results could be explained as follows. Firstly, the elbow regions, i.e. theCOO ester groups, are assigned to the hydrophilic domain in our simulation. Althoughthe spring constant is used between the head and tail in DPD simulation, maybe the Cooester groups have more important roles in the molecular structure. Secondly, in thepartition of surfactant molecule (including the head and tails), H atom is used tosubstitute the leavings. Maybe this substitute is a lttle unfit to surfactant molecule inthe theoretical calculations. Although there are some differences in the liquid crystalsregion, the main phase region is uniform, like W/O reverse micelle and the solutionregion. From the simulated phase diagram, one conclusion is shown that the ternaryphase diagram can be drawn via the DPD simulation method.Figure 4 The simulated phase diagram of ternary systemIsooctaneWO0%/.,Two-phase0/--/wo-phas5%/-- O/W--Tiquid crystals-/orw }Liquid crystalsWaterAOT WaterAOTReferences1. J. Faeder, B. M. Ladanyi, J. Phys. Chem. B, 2000, 104, 1033.S. P. Moulik, B. K. Paul, Adv. Colloid Interface Sc, 1998, 78, 99.3. T. K. De, A. Maitra, Adv. Colloid Interface Sci, 1995, 59, 95.M. B. Temsamani, M. Maeck, I. El Hassani, J. Phys. Chem. B, 1998, 102, 3335.. O. A. El Seoud, J. Mol. Liq., 1997, 72, 85.E. Junquera, L. 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