State Key Laboratory of Fine Chemicals,School of Chemical Machinery and Safety,Dalian University of Technology,Dalian 116024,China

Keywords:Molecular simulation Supercritical carbon dioxide microemulsion Ionic liquids Microstructure Structure parameters

ABSTRACT Supercritical carbon dioxide microemulsions are great medium to combine two immiscible substances through forming nanoscale polar cores in nonpolar continuous phase with the help of proper surfactants.The properties of microemulsions could be signifciantly affected by their constituents and structures.In this work,molecular dynamics simulation was implemented to study supercritical carbon dioxide microemulsions containing ionic liquid[bmim][PF6]and water by adding surfactant Ls-36.Results showed that the above components could form spherical aggregates in CO2bulk phase with [bmim][PF6]and some water as the inner core,surfactant headgroups and water as the intermediate shell,and surfactant tails as the outer shell.The microstructure information about the outer shell was further investigated by defining an angle between the surfactant tail and the normal direction of the aggregate outer surface,which ranged from 78°to 125°.The infulence of the ionic liquid content on the size and structure of microemulsions was explored and the best molar ratio between the ionic liquid and surfactant was around 1.25 for getting maximum water solubility.

1.Introduction

Supercritical carbon dioxide(scCO2)microemulsion is a green solvent,which developed on the base of supercritical fluid(SCF)and combined both advantages of SCF and microemulsions [1-3].Because of their thermodynamically stable and tunable properties,supercritical carbon dioxide microemulsions have promising applications in chemical reaction [4],material preparation [5],separation [6]and extraction[1].Polar compounds,with the help of appropriate surfactants,could form thermodynamically stable microemulsions in scCO2.Recently,the investigation about scCO2microemulsion still obtained large interests.Zhang et al.[7]synthesized scCO2microemulsions with ionic liquid(IL)domains and applied them to material preparation and chemical reaction.Yin et al.[8,9]studied the phase behavior of microemulsions with scCO2as the continuous phase and Ls-mn(codicil(poly(ethylene-methyl ethylene))ether)as surfactants.These microemulsions could be used to selectively extract the 1,3-propanediol from its dilute aqueous solution effectively.

Except experimental studies,molecular dynamics(MD)simulation has become a powerful method in better understanding microstructures of scCO2microemulsions[10-12].Cummings et al.[10]reported the first MD simulation of dichain surfactant molecule((C7F15)(C7H15)Na+,F7H7)assembling into stable spherical aggregates in scCO2as proved by earlier experiments[13],which demonstrated the potential of MD simulation for modeling such complex scCO2microemulsion systems.The four regions of reverse micelles'structures were firstly proposed through simulating an aerosol-octyl-ketone(AOK)/H2O/scCO2system[11].To discuss the impact of different surfactants,especially the differences between fluorous and hydrocarbon surfactants,Lu et al.[12]simulated two systems composed of scCO2/H2O/fluorinated polyether(CF3-(O-CF2-CF(CF3))3-O-CF2-COO-,PFPE)and scCO2/H2O/PE.It was found that,compared with PE,PFPE had a larger solubility in CO2because its tails' difficulty in bending would allow more CO2to get into the intermolecular spaces.Furthermore,according to the research of Sagisaka[14]and Gelbart[15],the surfactants applied to form a microemulsion should possess three conditions,including:(1)They should be interfacial active and be able to lower the interfacial tension to nearly zero.(2)The surfactants have to be compatible with the curvature of the microemulsion droplets.(3)Surfactants could well minimize contacting between the H2O and CO2,which can also explain the performing differences between hydrocarbon surfactants and fluorocarbon surfactants.Meanwhile a new parameter has been developed to quantify the effect of“contact”,which specified“stubby”surfactants are promising for future surfactant design[16].

Fig.1.Schematic of molecule structures of systems.Color scheme:white=hydrogen atom;gray=carbon atom in surfactant;red=oxygen atom;dark blue=nitrogen atom;green=carbon atom in ionic liquids and CO2;light blue=fluorine atom.

Table 1 The molecule numbers of components for MD simulation systems

Nowadays,the combination of ILs and scCO2is one of the most promising directions of green chemistry[17,18].Normally,ILs solubilized little in scCO2and microemulsions can effectively improve the ILs solubility in scCO2.Han and co-workers[7]showed the existence of IL domains in continuous CO2phase from their spectroscopic experiments on the system N-EtFOSA(C2H5NHSO2C8F17)/scCO2/IL.The UV-vis and TEM (transmission electron microscope)results proved that NEtFOSA can load TMGA,TMGL and TMGT in scCO2bulk phase through assembling into aggregates.In addition,[bmim][PF6]-based microemulsions were much less stable than [TMG][Ac]-based microemulsions.To study the microstructure of this complex system thoroughly,Senapati and Chaitanya[19]did MD simulation to provide the direct evidence of the formation of ellipsoid microemulsion based on experimental data obtained by Han et al.[7].The atomic level exploration of a variety of IL-scCO2-surfactant systems revealed that the stability of microemulsions mainly pertained to the interaction between surfactant headgroups and IL anions,while the interaction between surfactant headgroups and IL cations played a secondary role.As to form stable ILs/CO2microemulsion,three conclusions should be considered that:(1)ILs should have relatively strong interaction with the headgroup of surfactants.(2)The surfactants should have enough solubility in CO2.(3)The interaction between ILs and CO2should also be strong.

Compared with fluorous surfactants,hydrocarbon surfactants are cheaper and more environmentally benign[20-22].In general,hydrocarbon surfactants have low solubility in scCO2,while they could self-assemble successfully with the help of co-solvents.Yin et al.[8,22]investigated Ls-mn/H2O/scCO2system through both experiments and MD simulations.It was put forward that water was the main driving force of the self-assembled process of Ls-mn in scCO2.Finding out the forming conditions and assembling properties of hydrocarbon surfactants was the key to expanding ILs/CO2microemulsions'applications.In this paper,MD simulations were applied to investigate the microstructures and interactions of Ls-36/H2O/[bmim][PF6]/scCO2systems.

2.Simulation Details

The selection of force field is important to MD simulations.The molecular models used in this work were quoted from papers and were well examined.This paper applied EPM2[23]for CO2and SPC/E[24]developed by Berendsen for H2O.The models for Ls-36 were sited from earlier work[25].The[bmim]cation was described in CHARMM force field with united methyl groups while the model for[PF6]remained the same as Bhargava[26].Schematic molecular structures of the Ls-36,[bmim][PF6],H2O and CO2were shown in Fig.1.The size of the cubic box was set as 20 nm at the beginning.MD simulations were performed with NPT ensemble at 308.15 K and 23 MPa coupled by the Berendsen model with a constant time of 1.7 ps.The time step was set as 0.001 ps.The cutoff length for intermolecular potential calculations was 1.2 nm.GROMACS package(v4.6.2)was used to compute the equilibrium progress.Ewald summation was adopted to compute the longrange electrostatic interactions.The intermolecular potential between the partial charges on atomic sites consisted of Coulomb and Buckingham-type.Periodic boundary conditions(PBC)were applied.All the systems were kept running for more than 100 ns after energy minimization to ensure the equilibrium of these complex systems.

Fig.3.Site-site radial distribution function of(a)EO and[bmim][PF6]in 5th system.(b)[PF6]and EO,PO in 5th system.

Fig.4.Gyration radius of Ls-36/CO2/H2O/[bmim][PF6]system,include the Rgof three dimensions and the average Rgof space.

Fig.5.The evolution curve of the total energy.

To ensure the self-assembly of surfactants,the concentrations of Ls-36 were above the critical microemulsion concentration(0.008 mol·L?1)[25].ILs were added to systems and the molar content of ILs changed from WIL=nILs/nLs-36=0 to WIL=nILs/nLs-36=2.5.Thus a total of eight simulation cases were performed in this work.The details of systems were listed in Table 1.

3.Results and Discussion

3.1.The assembling process

Fig.2 illustrates the spontaneous evolution of the 5th system in Table 1 from the random mixture (Fig.2(a))to ordered polar cores surrounded by surfactants in scCO2phase.Some small aggregates clustered fast in the first 8 ns(Fig.2(b)).Then small aggregates slowly fused to form two to three reverse micelles(RM)at around 27 ns(Fig.2(c)).Next,these small aggregates gathered together to form a single RM at around 74 ns(Fig.2(d))with unique size and one free Ls-36 dissociating and re-associating in the continuous CO2phase.It was not until 123 ns did all the surfactants assembled together(Fig.2(e))and sustained stable during the last 27 ns(Fig.2(f)).The polar cores referred to water cores in the water in oil microemulsion.Water molecules aggregate and nucleate through hydrogen bond interaction and hydrogen bond between water and polar headgroups of surfactants (EO groups)attracted surfactants to gather around.In order to obtain some insights about the microstructure of aggregates,the radius distribution function(RDF),the gyration radius(Rg),density and angle distributions,as well as distance information were studied in detail.

3.2.Microstructures of aggregates

The detailed structure of aggregates could be described by density distribution.The local population density distribution around one central point could be represented by radial distribution function(RDF),which was:

Fig.6.Distances and density distributions of the system.(a)Distances between P and EO,H2O,bmim,PO respectively.(b)Average densities of H2O and CO2around C10,OE1 and OP1 atoms of Ls-36.

Fig.7.Distribution of angles between the C00-C10 vector and the normal to the outside surface of the aggregate.(a)line chart;(b)schematic diagram.

where“N”is the number of any kind of species including atoms,residues,molecules and so on.“r”is the distance away from the origin at“t”ns.“ρN”is the average population density of species which was selected.

RDFs were plotted for the[bmim][PF6]and Ls-36 in Fig.3.In Fig.3(a),the RDF of ethylene oxide(EO)group-[PF6]showed a higher peak than that of EO-[bmim],and the RDF of propylene oxide(PO)group-[PF6]showed a much lower peak than the RDF of EO-[bmim]in Fig.3(b).The results indicate that the anions (PF6)prefer the EO groups rather than the PO groups.

The Prince was immediately seized with the most ardent79 desire to go on board the ship, and shouted loudly to attract the notice of her crew, but no one answered

The morphology of the aggregates could be elucidated by gyration radius (Rg)in three dimensions:Rg-X,Rg-Y and Rg-Z.In Fig.4,Rgobtained from X/Y/Z three dimensions was similar to each other,indicating that the aggregate was in good agreement with spherical structure.The calculation formula of Rgwas listed as below:

where riwas the distance between every species“i”and the mass center of their molecules.miwas the mass of species“i”.

The average Rgof space from 80000 ps to 160000 ps was calculated and this value was about 2.08 nm.According to the conversion formula between Rgand the radius of aggregates,which was shown as follows in Eq.(3),one can calculate that the R was about 2.68 nm.This R was in similar size with the radius 2.7 nm reading from RDFs.

Fig.8.Three different conformations of assembled surfactants.

Fig.9.The effective assembled percentage of surfactant in different systems.On the base of stable microemulsion formed in ternary systems,1-5 ILs were added gradually to the system.

where R was the radius of aggregates.The Rginformation together with RDFs helped people understand the structure of microemulsions.

To verify that the microemulsion system has reached its equilibrium state.The evolution curve of the total energy was plotted in Fig.5.From the data,we can conclude that the equilibrium state was reached at about 360 ps.

In order to get more information about aggregates'microstructures,the surfactant molecules were divided into three parts,i.e.the tail chain,EO groups,and PO groups,in typically,took C10,OE1 and OP1 atoms as research objects respectively.The interaction between ILs-CO2and ILssurfactants were the main influence factor for creating microemulsions.Attention should be paid on the interaction especially between anions and other components in systems.In Fig.6(a),P(phosphorus)atom in [PF6]was selected as the central atom and the distances between the mass center of P and EO,H2O,bmim,and PO were calculated respectively.In Fig.6(a),the distance between P and H2O was the smallest,even smaller than the distance between[bmim]and P,while the EO group laid closer to P compared to the PO group.Meanwhile,average density distributions of H2O and CO2were calculated around C10,OE1 and OP1 to reveal the interactions'differences of surfactants.Fig.6(a)and(b)both showed that the H2O was in the core region,Ls-36 in the middle layer,and CO2bulk phase in the outermost layer.As was shown in Fig.6(b),the boundary of H2O and CO2bulk phase was at about 2.7 nm inside the core region.

Molecular-visualizable software PyMOL was used to“observe”surfactants carefully and three conformations of Ls-36 were displayed in Fig.8(a),(b),(c).The EO groups always had contact with H2O bulk phase while PO groups had three states:one is to fold themselves to interact with water core(Fig.8(a)),shaped like the letter“d”;one is to stretch in CO2bulk phase (Fig.8(b)),shaped like the letter“l(fā)”;and some shaped like“c”with PO groups stretched out in larger scale(Fig.8(c)).

From the above analysis,the detailed structure information of microemulsions was obtained.Ls-36/H2O/[bmim][PF6]/scCO2systems could form sphere like aggregates with ILs and water solubilized in the core region,while some of the trapped water together with CO2-phobic groups made up the middle layer.The carbon tails stretching in CO2were arranged on the outside of the microemulsion with the angle relative to the normal direction of aggregates' surface ranging from 78°to 125°.

3.3.The influence of ILs

For the purpose of studying the influence of ILs on the forming process of systems,[bmim][PF6]was added into the Ls-36/H2O/scCO2ternary system one by one and the number of assembled surfactants was observed carefully.In this work,surfactants that can successfully assemble around the water core were defined as effective surfactants to reflect the assembling efficiency of surfactants,and which was indicated as effective ratio of aggregates(ERA)signified in percentage.

Fig.10.The effective assembled percentage of surfactants in Ls-36/H2O/CO2and Ls-36/H2O/CO2/[bmim][PF6]systems.(a)8 Ls-36/160 H2O/12000 CO2and 8 Ls-36/160 H2O/12000 CO2/3[bmim][PF6].(b)6 Ls-36/160 H2O/12000 CO2and 6 Ls-36/160 H2O/12000 CO2/3[bmim][PF6].

Fig.11.The fraction of trapped and free water in systems with different concentrations of ILs.

In Fig.9,the black line illustrated the change of ERA of the ternary system during 0 ns to 80 ns.The maximum of ERA was 83%at 20 ns.The first[bmim][PF6]was added to the well simulated ternary system and quickly the IL attracted several Ls-36 and H2O molecules around it.Thus the system exists two aggregates (the gray region):pure water core (the red line)and ILs+water core (the blue line).At 15 ns,these two aggregates combined together with 75% ERA.After the system reached a stable state,the second IL was added randomly to the system and quickly solubilized in ILs+water core(the green line).As a consequence,the ERA rose from 50%up to 83%.After totally adding five ILs into the system,people can observed that the ERA of Ls-36 could get 100%at first and remained stable at 90%.The similar phenomenon was observed in systems with lower concentration of Ls-36 as well.Comparison results were shown in Fig.10.From Fig.10(a)and(b),one can notice that systems with ILs had higher ERA than systems without ILs.Even in the 6 Ls-36/160 H2O/12000 CO2/3[bmim][PF6]system,the added ILs could improve the ERA up to 100%.

Thus,adding ILs into the scCO2microemulsion system could help Ls-36 assemble faster and get a larger ERA.

Adding ILs could also affect the solubility of water in microemulsion.In order to investigate this influence,six systems with different contents of ILs were compared in detail.In this paper,the water solubilized in polar cores was defined as the trapped water and the water dissolved in scCO2was defined as the free water.In Fig.11,changes of molar fractions of trapped water and free water versus WILwere represented as polyline.The molar fraction of trapped water in the ternary system was around 78%while that of free water was 22%.When the added IL molar ratio ranged from 0.25 to 2.5,all systems have higher trapped water fraction than the ternary system.The maximum of trapped water in polar cores reached up to 87%when WIL=1.25.It should be noted that the simulation results only suggested that the appropriate WILshould be close to 1.25 and the specific content should be tested by experiments thoroughly.

4.Conclusions

Ls-36/H2O/[bmim][PF6]/scCO2systems were studied through MD simulation.The results demonstrated that stable scCO2microemulsion with IL domains could be formed under appropriate conditions.The microemulsion presented like sphere with radius around 2.7 nm.The analysis of RDFs and density distribution function revealed the three layer structure of aggregates:ILs and water solubilized in the core region,and some of the combination water together with CO2-phobic groups made up the middle shell.The carbon tail separated with each other in CO2should lay on the outside of the microemulsion,which stretched in an angle range from 78°to 125°.The content of ILs could affect the solubility of water in polar cores.The maximum of trapped water in polar cores could reach up to 87%when WIL=1.25.