Investigation Of Assembling Behavior Of Surfactant At Oil/Wter Interface Using Mesoscopic Simulations

Behaviors of surfactants at the interface play an important role in many field, suchas oil extraction, textile chemistry and dyeing and finishing engineering, domesticchemistry engineering and pharmaceutical industries. And this is closely related withthe assembling behavior of surfactant at oil/water interface. The behavior isinfluenced by the surfactant concentration and structure, oil/water ratio, polymerconcentration and the number of monomers. A deeper investigation of interfacialproperties of surfactant at the oil/water interface is crucial for understanding theworkings of surfactant systems from fundamental interests of colloid and interfacescience, as well as practical applications.In the past decades, a vast number of experimental techniques have been used toinvestigate the properties of surfactant systems. However, only a few techniques areavailable for the investigation of oil/water interface, such as nonlinear vibrationalsum-frequency spectroscopy and second harmonic generation. So it is difficult toobtain detailed information on the behavior of the surfactant molecules at the interfaceexperimentally. Therefore, computer simulations are an attractive alternative toprovide additional information, enhancing the understanding of interfacial propertiesof surfactants. Compared with usual dynamics simulations, for example all-atomsimulation, Dissipative particle dynamics (DPD) allows for a much larger time stepand length scales than which is commonly used in usual dynamic simulations. So weuse DPD simulation to study the assembling behavior of surfactant at oil/waterinterface, the interfacial and structural properties. These DPD studies can throw morelight on the interfacial behavior of surfactant at the oil/water interface and therebygive some guidance on the study and application of surfactants.In the present work, we used DPDsimulation to set up proper molecule modeland interaction parameters for surfactants. The interfacial properties, structural properties and their dependence on the surfactant concentration and structure,oil/water ratio, polymer concentration and the number of monomers were investigatedin detail by contrast with other experimental and simulation results. Based on thesimulation results presented, the following conclusions can be drawn:1. The effect of cetyltrimethylammonium bromide (CTAB) concentration andoil/water ratio were investigated. A well defined interfacial phase can be clearlyobserved in the range of0.5-3.0. CTAB surfactants absorb on the interface orderly.The interface thickens with CTAB concentration and oil/water ratio. The CTABmolecules are more highly packed at the interface and more upright with bothconcentration and oil/water ratio. And the interfacial thickness increases. Theend-to-end distance and order parameter have a very weak dependence on theoil/water ratio. But both of them show an increase with CTAB concentration,indicating that the surfactant molecules at the interface become more stretched andmore ordered at high concentration. As CTAB concentration increases further, theorder parameter decreases instead because the bending of the interface. At the sametime, it is shown that CTAB has a high interfacial efficiency at the oil/water interface.2. DPD simulations of surfactant with different structures at the oil/waterinterface are performed, the interfacial density, interfacial thickness, interfacialtension, the order parameters show how their dependence on the surfactant structures.It is found that the surfactants with two tails or two head groups are significantly moreefficient at the oil/water interface than single head and single tail ones if their headgroups are strongly hydrophilic. Branched surfactants attain higher interfacialthickness due to cooperation among attractive tails. Simulations of the conformationof surfactant chain at the interface indicate that surfactant chains are straighter andmore ordered with interfacial coverage increase. The increase in the number ofhydrophobic tails or head groups lowers the stretching and orienting of the surfactantchains. Finally, studies on the supersaturated surfactant monolayer show that theprocess of forming a swollen micelle is not viable in very rigid monolayers.3. DPD is used to simulate the oil/water/surfactant system in the absence andpresence of polymer. Polymer chain absorbed at the interface as well as surfactants and surfactants bind to the polymer chain with hydrophobic interaction. The numberof polymer chains increases, its interfacial density grows higher and wider and thesurfactants at the interface are more ordered. The formation of polymer/surfactantcomplexes is favorable for the decrease of oil/water interfacial tension. In thepresence of polymer, the interface is obtained at a lager range of oil/water ratio and issupersaturated at a lower surfactant concentration. The surfactants end to end distance,interfacial thickness increases and the peak of density grows higher with surfactantconcentration. With the increase of surfactant concentration, some polymer chains arepressed into the oil or water phase, the density grows lower and wider. The polymerchains stretched into oil phase with oil/water ratio, density grows lower and wider.4. DPD is used to simulate the oil/water/surfactant system with polymer whichhas different number of monomers. The end to end distance increases with numberof monomers, and the increasement is lagerer with big number of monomers. Theinterfacial density of the head and tail beads grows higher and more narrow with thenumber of monomers which surfatcnats can bind with. The interface thicknessincreases but the interfacial tension decreases with the number of monomers.