Experiments And Molecular Dynamics Simulation On Self-Assembly Of Surfactants And Inclusion Of Surfactants

Surfactants can form a variety of microstructures and exhibit complicated phase behaviors,so they are widely used in many fields.Molecular dynamics（MD）simulation,which can explain and predict the phase behavior of surfactant at the molecular level,is used to verify and supplement the experimental results.In this paper,the inclusion ofα-,β-,γ-cyclodextrin（α-,β-,γ-CD）with surfactant and the effect of additives on the phase behaviors of surfactant mixed systems were studied by combining experiments with MD simulation.In the first chapter,the properties and applications of surfactants and cyclodextrins are briefly introduced.Several methods of molecular simulation are emphatically introduced and some basic concepts of MD simulation are introduced in detail.In the second chapter,the effects of ethanol and n-propanol on the phase behaviors and phase inversion of aqueous two-phase systems in0.1 mol/kg trimethylene-1,3-bis（dodecyl dimethyl ammonium bromide）（12-3-12）/sodium dodecyl sulfonate（AS）mixed systems at 318.15 K were investigated by MD simulation.The simulation results show that at lower x_n-alcohol,a large number of n-alcohol molecules are available to participate on the shell of aggregates,forcing surfactant molecules to pack loosely,which means that the increase of the distance between the headgroups and an increase in the apparent area per molecule at the interface.Therefore,n-alcohol molecules mainly act as cosurfactants at lower x_n-alcohol.At higher x_n-alcoholthe surfactant molecules are surrounded and dispersed by a large number of n-ethanol and n-propanol molecules to prevent the formation of larger aggregates,so n-alcohol molecules mainly act as cosolvents.Thus,the addition of n-alcohol leads to the phase transition from the heterogeneous system to aqueous two-phase system and finally to single phase solution.MD simulation results suggest that the variation of ratio of surfactant(MR_12-3-12/AS)has weak influence on the interactions between surfactant and ethanol.However,the interaction between surfactant and ethanol enhances with the increase of the total concentration of surfactant（m _T）.The m _Tof the bottom phase of the aqueous two-phase system is larger than that of the top phase,the bottom phase with large m _Tis capable to combine with more ethanol molecules with the increase of x_ethanol.This means that the decrease of the density for the bottom phase is more significant with the increase of x_ethanol,leading to the phase inversion of the aqueous two-phase system.The increase of m _T(or decrease of MR_12-3-12/AS)leads to enhancement of the attraction between surfactant and n-propanol in12-3-12/AS/ethanol/H₂O mixed systems.Both of MR_12-3-12/ASand m _Tare smaller in the top phase for aqueous two-phase,these mixed vesicles with smaller MR_12-3-12/ASwill spontaneously enter into the top phase accompanied simultaneously by certain amount of mixed solvent transferred to the top phase.However,the stronger attraction between surfactant and the mixed solvent due to the higher m _Tin the bottom phase prevents the transferring of the mixed solvent from the bottom phase to the top phase.Therefore,the combination of the two factors leads to the increase of m _Tin the top phase,thus inducing the phase concentration inversion of ATPS-C.In the third chapter,the difference of inclusion betweenα-,β-,γ-cyclodextrin（α-,β-,γ-CD）and cetyltrimethylammonium bromide（CTAB）at 298.15 K was explored.Experimental results show thatα-CD and CTAB mainly form an inclusion complex ofα-CD₂@CTAB₁,β-CD and CTAB mainly form inclusion complexes ofβ-CD₂@CTAB₁andβ-CD₁@CTAB₁,whileγ-CD and CTAB mainly form inclusion complex ofγ-CD₁@CTAB₁.Umbrella sampling simulation and MD simulation forγ-CD₁@CTAB₁show thatα-,β-CD and CTAB form an inclusion complex of CD₁@CTAB₁firstly,then CD₁@CTAB₁is combined with another CD to form CD₂@CTAB₁.For theα-CD containing system,the formation potential energy ofα-CD₂@CTAB₁is much lower than that ofα-CD₁@CTAB₁,thus the formation of the inclusion complexα-CD₂@CTAB₁is superior.The formation potential energies ofβ-CD₁@CTAB₁andβ-CD₂@CTAB₁are close to each other,soβ-CD₁@CTAB₁andβ-CD₂@CTAB₁coexist in the system.For the inclusion complexγ-CD₁@CTAB₁,CTAB in theγ-CD cavity is bent and folded,preventing the combination of anotherγ-CD molecule withγ-CD₁@CTAB₁to formγ-CD₂@CTAB₁,or preventing combination of another CTAB molecule withγ-CD₁@CTAB₁to formγ-CD₁@CTAB₂.Therefore,γ-CD₁@CTAB₁is the main inclusion complex in theγ-CD containing system.In the fourth chapter,the effect ofβ-CD on the phase behaviors of0.1 mol/kg CTAB/sodium laurate（SL）mixed systems at 298.15 K was investigated.There are four phase behaviors which include single phase,aqueous two-phase,liquid crystalline phase and heterogeneous phase for the CTAB/SL mixed systems in the absence ofβ-CD,the system is heterogeneous near the equimolar ratio(x _CTAB=0.5).When m_β-CD=0.01mol/kg,the heterogeneous region of the mixed systems becomes wider,the isotropic single-phase region becomes narrower,and the aqueous two-phase region and the liquid crystalline phase region move away from the equimolar ratio.When m_β-CD=0.02 mol/kg,the liquid crystalline phase regine disappears,the heterogeneous phase region becomes significantly wider,and the aqueous two-phase region moves away from the equimolar ratio to a greater extent.MD simulation results show that with the molar ratio of CTAB and SL approaching to equimolar ratio,interactions between surfactants become stronger,the formation of large aggregates are easier,the combination of aggregates are more compact.With molar ratio of CTAB and SL approaching to equimolar ratio,the morphology of surfactant aggregates changes from spherical to lamellar.Therefore,the phase behavior of the system with x _CTAB>0.5 changes from single phase to aqueous two-phase to liquid crystalline phase to heterogeneous phase.Theβ-CD molecules added will combine with surfactant molecules having higher content to form inclusion complex,leading to the composition of the mixed system approaching to equimolar ratio,thus the formation of large aggregates is easier.Therefore,with the addition ofβ-CD,the heterogeneous region becomes wider,the isotropic single-phase region becomes narrower,and the aqueous two-phase region and the liquid crystalline phase region move away from the equimolar ratio.MD simulation results show that the floccule precipitates in the water is a lamellar aggregates formed by the self-assembly of CTAB and SL,the density of the floccule precipitates is smaller than that of the aqueous solution.In the 0.02 mol/kgβ-CD aqueous solution,the precipitates are large aggregates formed by the self-assembly of CTAB,SL,β-CD₁@CTAB₁andβ-CD₁@SL₁.the density of the precipitates is larger than that of the solution.