Molecular Dynamics Simulation And Entropy Calculation On Self-assembly Behaviors Of Surfactant

Self-assembly is a common phenomenon in nature,which is an important factor in the formation of living organism.Through the non-bonded interactions between molecules,building units can be assembled layer by layer to form different complex and functional structures.The basic attribute of these building units is that they have hydrophilic and lipophilic groups at the same time.Surfactants are such amphiphilic substances,which can self-assemble to form different kinds of aggregates,including micelle,vesicle,liquid crystal and other aggregates in solution,and can adsorb on the interface to form semi micelle,monolayer,multilayer and other aggregates.The formation of one micro area,which is distinguished from surrounding solvent,is the key for the excellent performance and application of self-assembly.Studying the accumulation state and structure of surfactants in the aggregating micro area is helpful to have deeper understanding of the self-assembly system of surfactants,especially the understanding and prediction of these self-assembly structures at the molecular level.These scientific studies can bring new scientific knowledge and laws into traditional basic principles of surfactant on the one hand,and can promote the design and development of new functional self-assembly structure of surfactant on another hand.Molecular dynamics simulation(MD)is an effective method to provide molecular-level structures and dynamic properties of self-assembly.The configurational entropy of molecule can directly describe the packing states of surfactant molecules in different self-assemblies.Therefore,this paper combines MD simulation with Quasi-harmonic(QH)approximation method to calculate the configurational entropy of surfactant molecules.Two typical aggregates(micelle and interfacial monolayer)have been studied in the following series:(1)Using MD simulation,the phase transition behavior of the sodium dodecyl sulfate(SDS)monolayer at the air-water interface,which is induced by the change of surface coverage,is systemically studied from the view of molecular configuration entropy.When surface coverage is large,the two-dimensional(2D)monolayer is unstable,and it will collapse into the solution to form a three-dimensional(3D)aggregate.This work gives detailed description of the collapse mode from interfacial film to solution micelle.Based on the change of configurational entropy-adsorption surface area isotherm and the deviation angle of hydrophobic chain relative to interfacial normal direction,three 2D phase states of SDS monolayer are defined,including gaseous(L),liquid expanded(LE)and liquid condensed(LC)phases.Using the QH approximation method,entropy changes of SDS for 2D phase transitions are first introduced.Based on the change of dynamic and structural properties of SDS interfacial monolayers under different surface coverages,a schematic diagram of 2D monolayer in different phases is proposed.(2)The N-dodecyl betaine(NDB)and n-tridecane(C13),which possess the same number of methylene group,are selected to study the packing state of alkyl chain in micelle interior.The configurational entropy and entropic contribution(translational,rotational,and vibrational entropies)of NDB alkyl chain in the micelle and liquid C13 alkyl chain are calculated and compared.Moreover,local entropies of NDB alkyl chain and those between micelle and C13 alkyl chain are also considered.Through these comparisons,it is clearly proposed that the accumulation of hydrophobic chain segments with no more than 2 carbon atoms inside the micelle is the same as that of pure liquid alkanes,while the outer core of micelle is in liquid-like state.Detailed information of micelle interior is the foundation for understanding micelle applications such as solubilization,drug delivery,and catalysis.(3)Combining molecular dynamics simulation and entropy calculation,the state of surfactant in the solution monomer,interfacial monolayer,and micelle is studied and compared.Through calculating configurational entropy and entropic decomposition(translational,rotational,and vibrational entropies)of NDB molecule,a new Born-Haber cycle is proposed to describe the state of NDB in different environments.This cycle also presents entropy changes of NDB molecules in the mass transport processes between different environments.Results show that in three processes(interfacial adsorption,bulk micellization,and surfactant transport from micelle to interfacial monolayer),NDB molecules all exhibit entropic decrease.(4)Using molecular dynamics simulation,the effects of different molar ratios on their respectively stacking state of SDS and NDB in mixed micelle are studied.Different stacking states are characterized by calculating and comparing configurational entropy and entropic contributions of SDS and NDB molecules with the change of molar ratio.Results show that packing states of different kinds of surfactants(SDS and NDB)have different changing tendency with their mole fractions in mixed micelles,and different surfactants can interact with each other.In addition,structural properties of mixed micelles with different molar ratios are analyzed,and two typical mixed micelle models are summarized.(5)Using MD simulation,the self-assembly structures of surfactants with special structure and special anti-ion combination are studied.First,aggregation behaviors of different sodium dodecyl benzenesulphonate positional isomers(x?12),where x=1,2,3,4,5 and 6,are studied.The solvent accessible surface area of micelle,probability distribution and angle distribution of hydrophobic chain in micelle,chain conformation,hydration number,distribution of polar head on micelle surface and interaction between benzene rings are mainly analyzed.Simulated results show that these six isomer micelles are more elliptical than spherical,and micelle radius increases with the movement of benzenesulphonate group from one side to the middle of alkyl chain.In micelle assembly,short alkyl chains tend to the polar layer of micelle while long alkyl chains aggregate in the central region of micelle.Due to the strong interaction between benzene rings,1?12 micelle exhibits some special characteristics.(6)The self-assembly structures of two green surfactants,choline dodecyl sulfate(ChDS)and choline laurate(ChC 12),are studied through MD simulation.Ion-pair effect between polar head and choline ion is directly analyzed at the molecular level,including binding pattern,strength and stability of counter ions.Results show that six binding patterns exist in both two micelles.The difference is that the hydroxyl groups of choline have more possibilities to orient to the interior of ChDS micelle while they are more likely to be exposed to aqueous solution in ChC12 micelle.Through analyzing micelle surface properties,an intuitive description of charge distribution on the micelle surface is presented.Furthermore,micelle systems with solubilized pyrene molecule are studied,and information about pyrene molecule passing in and out the micelle is obtained.