Molecular dynamics study of room temperature ionic liquids with water at mica surface

1.Introduction

Room temperature ionic liquids(RTILs)have recently been a focal point of green chemistry due to their physicochemical properties,such as negligible vapor pressures,high temperature stability,high ionic conductivity and wide electrochemical window,in particular,tunable properties through systematic changes in the molecular structure of the cation-anion pairs [1-3].Nevertheless,the potential toxicity of some ionic liquids should also be taken into account[4-6].These properties offer RTILs distinct advantages in many applications such as microlubrication and nanotribology[7,8],batteries,supercapacitors,solar cells[9],micro fluids[10]and many other areas[11,12],all of which are dominated by interfacial interactions.Therefore,understanding the interfacial behavior of RTILs is of significant importance for those applications.Some efforts have been focused on revealing the interfacial phenomena between RTILs and solid surface in recent work[11].For instance,Fitchett et al.[13,14]showed the orientations of the imidazolium cations at a silica interface by sum-frequency vibrational spectroscopy (SFVS) experiments.Mezger et al.[15]revealed the existence of pronounced molecular layering of fluoroalkyl phosphate([FAP])-based RTILs at a charged sapphire(0001)surface,starting with a cation layer and decaying exponentially into the bulk liquid.Atkin and Warr[16]found that the surface charge,surface roughness and orientation of the cations in the interfacial layer are of great importance for the formation of solvation layers in RTILs.These interfacial phenomena were found to be very different from the bulk properties of RTILs,which attracted increasing research interests recently.

Mica is a kind of smooth,hard and easily prepared material in nature[17].Therefore,it has become a representative hydrophilic surface to understand the fundamental interactions between liquid molecules by means of the surface force apparatus(SFA)[18,19]and atomic force microscopy(AFM)[20,21].Many striking phenomena have been reported concerning the properties of RTILs-mica interfaces[22].Perkin et al.[23]employed the surface force balance(SFB)technique to investigate layering and shear properties of nano films of 1-ethyl-3-methyl imidazolium ethylsulfate([Emim][EtSO4])con fined between mica surfaces.They observed that the Friction coefficients of IL films were about one to two orders of magnitude lower than their non-polar counterparts.Liu et al.[22]found multilayer structures including liquid and solid phases as well as the ordered phenomena of 1-butyl-3-methylimidazolium hexafluorophosphate([Bmim][PF6])on mica surface at room temperature by AFM technique,which was attributed to the electrostatic interactions between the IL cations and the ordered anionic silicate layer on the mica surface.Payal and Balasubramanian[24,25]studied the self-assembly of ions and the effect of the cation symmetry on the microscopic organization of ILs near a charged mica surface using molecular dynamics(MD)simulations.In their w+ork,ILs consisting of 1,3-alkylimidazolium([CnC-mim])cation and bis(trifluoromethylsul fonyl)imide([TFSI])anion were investigated to reveal that symmetric cations with alkyl groups of intermediate length were more highly structured at the interface than their asymmetric counterparts.Apart from symmetry of the cation,the length of the alkyl group attached to the cation was found to crucially determine the ion structure near the solid surface.All of these investigations were concentrated on pure ILs confined between mica surfaces.However,most RTILs are hygroscopic-even the hydrophobic ionic liquids[26,27]can absorb a considerable amount of water[28,29],so that complete removal of water is nearly impossible.Thus,the impacts of water content on RTILs behaviors must be taken into account in all of the experimental works and applications of RTILs[30].Recently,the effect of water on RTIL-mica interfaces was studied by experiments.For instance,the SFB experiments showed that water strongly influences lubrication and friction mechanism on RTILs-mica interface[31,32].Smith et al.[31]observed that the incorporation of water from the environment dramatically alters the shear at ionic interfaces but leaves alkyl plane shearing unaffected.Espinosa-Marzal et al.[32]found that adsorbed water in RTILs appears to change both the ion-pair orientation and the slip condition for film-thickness transitions,that is,the resistance of the IL layers squeezed out from the contact.Sakai et al.[33]have presented the effects of water on solvation layers of imidazolium-type RTILs on mica and silica surfaces through AFM measurements.They observed that the hydrogen bond is a key factor in determining whether water molecules can be adsorbed on the solid surfaces,but it is also necessary to take into account the hydrophilic/hydrophobic nature of the RTILs.Cheng et al.[34]have studied the influence of water on charging and layering at hydrophobic ionic liquid[C2mim][TFSI]-mica interfaces by AFM and SFB techniques.They found that on hydrophilic and ionophobic mica surfaces water-saturated [C2mim][TFSI]dissolves surface-bound potassium ions,which leads to high surface charging and strong layering.In contrast,layering of dry RTILs at weakly charged mica surfaces is weakly structured.Gong et al.[35]have reported that water adsorption on mica surface is the key to the extended layering of hydrophobic IL[Bmim][FAP]by the aid of AFM,attenuated total reflectance Fourier transformed infrared(ATR-FTIR)and contact angle measurements.These studies proposed that water serves as an effective e+lectrolyte and thus facilitates the ion exchange between Kions at the mica surface and the cations of ILs,which initiate the ordered packing of cations and anions of ILs.However,until now,the detailed interface structures of ionic liquids in general and water near a mica surface is still unclear as well as how water content influences such microstructure.Moreover,although some researches have been done to explore the pure RTILs at mica surface using MD simulation[24,25],molecular modeling about interfaces between water-RTILs mixture and mica has not been reported yet,for instance,where would the water be?In this paper,we focus on the microstructure of the water-RTIL mixture to investigate the effects of water content,RTIL nature and surface charge on the interfacial distribution of water in hydrophilic RTIL[Emim][BF4]and hydrophobic RTIL[Emim][TFSI]at mica surface by MD simulation.

2.Simulation methods and models

The molecular structures of hydrophilic[Emim][BF4]and hydrophobic[Emim][TFSI]are shown in Fig.1a.The simulation system is modeled by a channel(Fig.1b),which consists of a slab of water-RTIL mixture enclosed between two mica walls,and each wall is modeled by two layers of mica sheets with an area of 4.24 nm×4.59 nm,which is consistent with the lateral box dimensions in all systems.The distance between two mica surfaces is about 8 nm,which was long enough to ensure a bulk-like behavior of ILs in the channel center and the box size in z direction(i.e.,the channel width direction)was set to be 4 times the channel width.Z=0 in the density pro files corresponds to the location of the oxygen atoms in the atom layer of bottom mica nearest to ILs(Fig.1b).All the MD simulations were performed in the NPT ensemble for bulk system and NVT ensemble for channel system using the MD package GROMACS 3.3[36].MD trajectories were visualized and snapshots of simulations were generated using VMD[37].Temperature and pressure controls were achieved through Nosé-Hoover thermostat and Rahman-Parrinello barostat.The equations of motion were integrated using the velocity Verlet algorithm with a time step of 2 fs.A spherical cutoff of 1.1 nm was employed for the non-bonded interactions.Long-range electrostatic interactions were treated using the PME method.Prior to simulating the ILs between mica,pure ILs were simulated first to obtain the bulk properties.Then,for each channel simulation,the MD system was simulated at 1000 K for 2 ns,and subsequently annealed gradually to the target temperature of 333 K.After that,a 20-ns simulation was run to ensure that the system was equilibrated and the number of ions was tuned so that ILs in channel center had the same properties as in bulk condition.Finally,a 40-ns production run was performed for analysis.

Fig.1.MD modeling of water-RTIL mixture between two mica surfaces.(a)All-atom model of ions[Emim]+,[BF4]-and[TFSI]-.(b)Snapshot of the MD system.Color planar walls means mica,color lines represent ionic liquids,dark green spheres represent K+ions and red/white spheres represent water molecules.The water molecules were enlarged for clarity.The cyan dash lines represent the position of oxygen atoms on mica surface.

We had performed 14 cases(Table 1)in this work to study how K+ions,water content and ion type affect the interfacialmicrostructure near mica surfaces.The K+ions on the mica surface were disposed to 0%,50%and 100%in order to simulate different surface charges.Correspondingly,the same number of anions was removed to maintain the electrical neutrality of the overall system.Moreover,we chose two kinds of RTILs:hydrophilic[Emim][BF4]and hydrophobic[Emim][TFSI]to model the humid RTILs-mica interfaces.The concentration of added water in hydrophilic[Emim][BF4]varied from 0.33 wt%to 15.06 wt%,since[Emim][BF4]can be miscible with water in the whole range of water concentrations[33].However,[Emim][TFSI]is a kind of more hydrophobic RTIL,the solubility of water in which is limited[38].The concentration of water saturated in[Emim][TFSI]is estimated to be about 2.0 wt%[33].Thus,we set the water concentrations from 0.26 wt%to 2.02 wt%.In contrast,pure ILs con fined between mica surfaces were also simulated in our work.The precise numbers of particles in each case are included in Table 1.In all simulations,mica was modeled using the CVFF force field developed by Heinz and coworkers[39],and ILs were modeled using the all-atom force field developed by Lopes group[40].The SPC/E model was used for the water molecules.These force fields used in this work have successfully captured key features of the water-RTIL mixture in prior simulations[30].

Table 1 Number of IL ions,water molecules,and K+ions on mica surface(anion is[BF4]-in case 1-8 and[TFSI]-in case 9-14).The second column denotes the weight percentage of water in the mixture of ionic liquids and water.D(nm)represents the distance between two mica surfaces(see Fig.1b).

when the number of K+ions is 80,the mica surface is neutral.

Case Percentage(wt%) Cation Anion Water K+ion D(nm)1 0.00 561 521 0 40 8.00 2 0.33 581 501 20 0 8.00 3 0.33 564 524 20 40 8.00 4 0.33 551 551 20 80 8.00 5 0.64 486 446 33 40 7.00 6 1.00 560 520 60 40 8.00 7 7.00 520 480 414 40 8.00 8 15.06 466 426 874 40 8.00 9 0.00 378 338 0 40 8.00 10 0.26 376 336 20 40 8.00 11 0.33 486 446 33 40 10.40 12 1.00 372 332 75 40 8.00 13 1.40 368 328 105 40 8.00 14 2.02 363 323 150 40 8.00

3.Results and discussion

3.1.The effect of surface charge on the interfacial structure

Fig.2 shows the number density pro files of water,K+ions,cations and anions in hydrophilic[Emim][BF4]-water mixture at mica surfaces with 0%,50%,100%surface K+ions,when water content is 0.33 wt%.Throughout this work,all the number densities were computed based on the mass center of molecules/ions.It could be found that the water layer is closer to the mica surface than the anion layer which is furthest regardless of surface charges.As shown in Fig.2a,while the mica surface becomes more neutral(i.e.,more K+ions),the K+ions would be further to surface and there would be less water in contact with the surface.In particular,at mica surface with 50%K+ions,two water layers were present,one of which was located within 0.2 nm(inner layer)and another approximately at 0.2 nm-0.3 nm(outer layer)from mica surface.However,the twin-peak structure is not obvious in 0%and 100%cases.Fig.2b and c show different trends between cations and anions.In Fig.2b,with the number of K+ions increasing from 0%to 100%,the position of the first cation layer slightly shifts from 0.37 nm to 0.41 nm.Fig.2c reveals that when there is no K+on the surface,the first anion layer appears at 0.73 nm and ranges from 0.6 nm to 0.9 nm,however,with 50%K+ions on the mica surface,the first anion layer appears at about 0.46 nm,and the second anion layer starts at～0.6 nm,which is same to the position of first anion layer in the case without K+ions on the surface,and extends to～1.15 nm.Meanwhile,in the case with 100%K+ions,the second anion layer ranges from 0.8 nm to 1.15 nm.Thus,it could be concluded that the mica surface charge,in other words,the number of K+ions on mica surface,significantly affects the interfacial structure of water and anion.

Fig.2.The number density pro files of(a)water(solid lines)and K+ions(dotted lines),(b)cations and(c)anions in[Emim][BF4]-water mixture with 0.33 wt%water content,con fined between mica surfaces with 0%,50%,100%K+ions.Here,the number density of K+ions has been scaled down by a factor of 7.The positions of all the ions are represented by their center of mass.The x-axis(z(nm))means the location of ions'or water's mass center with respect to mica surface.

3.2.The influence from the hydrophilic and hydrophobic nature of RTILs

Fixing mica surface with 50%K+ions,we examined the distribution of interfacial water molecules and K+ions with hydrophilic and hydrophobic RTILs.Firstly,the adsorbed water layer was analyzed in hydrophilic IL[Emim][BF4]system with different water contents from 0.33 wt%to 15.06 wt%(Fig.3a):when the water concentration is 0.33 wt%and 0.64 wt%,a twin-peak structure was observed with one peak located at 0.13 nm(inner layer)and another at 0.23 nm(outer layer);as the concentration increases to 7.00 wt%and 15.06 wt%,the water layer becomes denser,especially in the outer layer,thus the twin-peak structure is blurred.The feature of adsorbed water layers agrees with previous experimental observations of water accumulating near mica surface in imidazolium-based RTILs[33,41].Secondly,Fig.3b shows that the distribution of K+ions is farther from mica surface with more water content.The peak of K+ions layer with 15.06 wt%water content is located at 0.11 nm while that with 0.33 wt%water content is located at 0.09 nm from mica surface.The increasing number of water molecules and shift of K+peak position observed here probably result from the interaction between water and mica surface like hydrogen bond and van der Waals as well as the hydration effect between water and K+ions on mica surface[41,42].Thirdly,Fig.3c and d exhibit the distribution of water and K+ions in hydrophobic IL[Emim][TFSI]system with different water contents.The similar results were observed as in the[Emim][BF4]system irrespective of the hydrophobicity of RTILs,except that the amount of water molecules accumulating at mica surface.specifically,it is found that regardless of the same water concentration(0.33 wt%and 1.00 wt%(Fig.S1)water cases)or the same number of ions(33 water molecules,486 cations and 446 anions,0.64 wt%case of[Emim][BF4]and 0.33 wt%case of[Emim][TFSI],see Table 1),a denser and thicker water layer was found at the interface of the hydrophobic[Emim][TFSI]than the hydrophilic[Emim][BF4],probably due to the hydrophobicity of[Emim][TFSI]which pushes water away from ionic liquids to accumulate at the hydrophilic mica surface.

除去转化这一数学的方式外我们在数学问题解答上也需要采用更多的学习方式，学生的思维是各具特色，应对方法也是应该多样。特别是在具有一定操作的实际问题上。例如“平行四边形面积”教学内容中，探索平行四边形面积公式时可以采用多样的数学方式：从不规则图形转化为规则图形上去探究公式、也可以利用教具“四杆机构”来帮助学生研究公式、从特殊的平行四边形（长方形）中推导公式。根据不同的学生采用不同的数学方式。

在合适的情况下，与媒体记者和网络知名人士建立合作，主动拉拢虚拟知识社群进行统一管理，提高思想认识，在舆情发生时能主动发声正确引导舆情正确走向。通过这样的方式减少错误言论，对公众存疑的问题上以知识社群为主体进行正面解答。配合政府监管平台，有效杜绝谣言的产生。对思想行为不端的主动进行处理，对发表危害国家言论的，一经发现立即关闭其社交账号，使其无法继续使用网络平台煽动群众。

Fig.3.The number density pro files of(a)water,(b)K+ions in[Emim][BF4]-water mixture and(c)water,(d)K+ions in[Emim][TFSI]-water mixture con fined between mica surface with 50%K+ions on it.Concentration of water given by weight percentage(wt%)are 0.33,0.64,7.00,15.06 wt%in hydrophilic[Emim][BF4]system and 0.33,1.00,1.40,2.02 wt%in hydrophobic[Emim][TFSI]system.The black dot lines in(b)and(d)represent the K+ions distribution in the system of pure ILs at mica surface(i.e.,no water content).The positions of water molecules are represented by their center of mass.The x-axis(z(nm))means the location of ions'or water's mass center with respect to mica surface.

Fig.4.The(a)cation and(c)anion number density pro files in[Emim][BF4]-water mixture and the(b)cation and(d)anion number density pro files in[Emim][TFSI]-water mixture near mica surface with 50%K+ions.Concentrations of water given by weight percentage(wt%)are 0.33,7.00,15.06 wt%in hydrophilic[Emim][BF4]system and 0.33,1.00,2.02 wt%in hydrophobic[Emim][TFSI]system,respectively.The black dot lines represent the cations and anions distribution in the system of pure ILs at mica surface(i.e.,no water content).The x-axis(z(nm))means the location of ions'mass center with respect to mica surface.

To quantitatively understand the role of ion-water interactions in hydrophilic and hydrophobic nature of RTILs and the distribution of water molecules and ions near mica surface,the average number of hydrogen bonds formed by each water molecule in the water layer(both inner layer and outer layer)adsorbed on mica surface was evaluated.Fig.5a shows the average number of hydrogen bonds in[Emim][BF4]system.With the increase of water content,the average number of hydrogen bonds between water molecules gradually raises,suggesting the water distribution at the interface becomes more and more compact.The similar trend was also observed in[Emim][TFSI]system.However,even water was saturated in[Emim][BF4]or[Emim][TFSI]system,the water-water interaction at the interface is less intense than in bulk water:the average number of hydrogen bonds between water molecules in bulk water is about 1.72,while it is 0.97 in 15.06 w%case of[Emim][BF4]system and 0.51 in 2.02 wt%case of[Emim][TFSI]system.On the contrary,as the water content increases,the average number of hydrogen bonds between water and mica surface gradually decreases,possibly because the increase of water clustering weakens hydrogen bonds between water and mica surface.However,Fig.S2 shows that the total number of hydrogen bonds between water and mica surface raises with the increasing water concentration,possibly due to the enough interaction sites in mica surface to form hydrogen bonds with water molecules.In addition,when we compared the same water concentration(0.33 wt%water cases)or the same number of ions(0.64 wt%case of[Emim][BF4]and 0.33 wt%case of[Emim][TFSI])in Fig.5a and b,we found the average number of hydrogen bonds formed by water-water in hydrophobic[Emim][TFSI]system is more than in hydrophilic[Emim][BF4]system,while the average number of hydrogen bonds formed by water-mica has an opposite trend,suggesting that a denser and thicker water layer was found at the interface of the hydrophobic[Emim][TFSI]than of the hydrophilic[Emim][BF4](the same result as Fig.3).

How could the water content affect the interfacial RTILs?With different water contents,the distribution of ions in RTI Ls-water mixture near mica surface is shown in Fig.4,in which Fig.4a and c present the cation and anion number density pro files of[Emim][BF4]-water mixture.With the increase of water content,the number of cations and anions at the interface gradually decreases,probably because the denser water layer(especially the outer one)at mica surface squeezes out the ions as well as screening the surface charge to attenuate the interactions between mica and ions taking into account that the water layer is closer to mica surface than the ion layer.What's more,the number of water in their first layer changed more than that of cation and anion(Table S1),probably due to the different ions'size.Fig.4b and d show the cations and anions number density pro files of[Emim][TFSI]-water mixture,which indicate the twin-peak adsorbed cation layer becomes a single layer with water content enhancement.Meanwhile,the first layer of anions near mica surface still gradually decreases as water content increases.The peak of the first cation layer in[Emim][BF4]-water mixture and[Emim][TFSI]-water mixture is located at about 0.40 nm,however,the peak of the first anion layer in[Emim][TFSI]-water mixture is located at about 0.62 nm,which is farther from mica surface than that in[Emim][BF4]-water mixture(～0.48 nm),this phenomenon may be attributed to the hydrophobicity and the anion size[43](i.e.,the adsorbed water layer prefer anion than TFSI-anion,which drivesanion closer to mica surface,and the smaller size of anion facilitates itself closer to mica as well).

3.3.Adsorbed water layer with hydrophilic RTIL[Emim][BF4]

房产税是以不动产业态存在的房产为征收对象，按房产的原值或房产出租的租金收入为计税依据，向不动产权利所有人征收的一种财产税。分为对个人产权所有人和企业产权所有人征收两大类，目前对个人产权所有人的房产税只在上海和重庆进行试点征收，其他地方暂未征收；企业不动产权所有人的房产税除了少数特殊减免外，均在正常征收中。本文只讨论综合性商业体作为不动产权所有人涉及的房产税的筹划思路。

In summary,the effect of surface charges,water content and the hydrophilicity/hy drophobicity of ionic liquids on the micro structure at RTILs-mica interface were studied in this work by molecular dynamics simulation.The modeling reveals that with more K+ions on mica surface,the distribution of water,K+ions and cations is farther away from the wall while anions become closer.Moreover,from modeling the mica surface with 50%K+ions,we found that(1)as water content increases,the water layer adsorbed on the mica surface becomes denser and thicker,and correspondingly the peak of K+ions would move further from mica surface;(2)regardless of the same water concentration(0.33 wt%and 1.00 wt%water cases in Fig.S1)or the same number of ions(33 water molecules,486 cations and 446 anions,0.64 wt%case of[Emim][BF4]and 0.33 wt%case of[Emim][TFSI]),more water and a thicker water layer was found at the interface of the hydrophobic[Emim][TFSI]than of the hydrophilic[Emim][BF4],due to the RTIL hydrophobicity and anion size(3)the peak of anion's first layer in the nanoconfined hydrophobic[Emim][TFSI]-water mixture is farther from mica surface than in hydrophilic[Emim][BF4]system;(4)the average number of hydrogen bonds formed by water-water increases with the water content,which accounts for more water accumulation;(5)the 2D number density map of the inner layer and outer layer in[Emim][BF4]revealed that the water distribution area in the outer layer is wider than in inner layer,and the distribution areas of water layer,especially th+e high-density areas,seem to be related to hexagonal cavity,K

Fig.5.The average number of hydrogen bonds formed by each water molecule in the adsorbed water layer in(a)[Emim][BF4]-water mixture and(b)[Emim][TFSI]-water mixture with different water concentrations.The orange diagonal column means the average number of hydrogen bonds between single water and ionic liquid.The olive cross column means the average number of hydrogen bonds between water molecules.The dark cyan column means the average number of hydrogen bonds between single water and mica surface.

Fig.6.2D number density map of water in the(a)inner layer(0-0.2 nm)and(b)outer layer(0.2-0.3 nm)according to the water number density pro files in[Emim][BF4]system with 15.06 wt%water content.The hexagon composed of yellow and red lines is the structure of mica surface.The dark green spheres represent K+ions and the violet spheres are aluminum atoms.The green area in(a)means the number density of water is higher than 50#/nm3in the inner layer and 100#/nm3in the outer layer;on the contrary,the density of blue area is less than those values.Red lines show a triangle formed by high-density areas.

Fig.7.The radial density pro files of(a)hydrogen and(b)oxygen atoms of the water molecules with respect to the K+ions on mica surface in the inner layer and outer layer according to the water number density pro files in[Emim][BF4]system with 15.06 wt%water.

Fig.8 shows orientation distributions of the water dipole and H-H vector in the adsorbed water layer near mica surface.The orientation of water dipole in inner layer shows a single peak at an orientation of θ =150°,suggesting that the water dipole is titled to mica surface with one hydrogen atom closer to surface.Meanwhile,the orientation of water dipole in outer layer is similar to that in inner layer.The H-H vector orientation in Fig.8b provided more information on the configuration of adsorbed water molecules,which reveals that the H-H vector is parallel to mica surface in the inner layer and is tilted(45° or 135°)in outer layer.The joint probability distribution of the two vectors can accurately characterize the orientation of water molecules at the interface.

To scrutinize the adsorbed water at RTILs-mica interfaces,the 2D number density maps,the radial density pro files and the orientation distributions of water were analyzed in[Emim][BF4]system with 15.06 wt%water content.Fig.6 shows the 2D number density map of water in the inner layer(0-0.2 nm)and outer layer(0.2-0.3 nm)in[Emim][BF4]system.It shows that the water prefers to distribute surrounding K+ions and the high-density areas of water seem to be related to K+ions and aluminum atoms on mica surface[44].Combining the 2D number density map and the number density pro files in Fig.3,we can find that the water molecules in the inner layer prefer to stay above the hexagon cavity,with the very similar distance to mica surface as the silicon atoms(i.e.,the peak location of the inner water layer and silicon atom is nearly the same),while the water molecules in the outer layer tend to stay around the aluminum atoms.Moreover,the structure of the outer layer in high-density areas is similar to be a triangle centered on aluminum atoms.A more intuitive structure can be seen in Figs.S3 and S4.

4.Conclusions

Fig.8.The(a)water dipole and(b)HH-vector of water orientation distributions in the inner layer and outer layer in[Emim][BF4]system with 15.06 wt%water.The orientation in(a)is de fined by the angle between the surface normal(positive direction of z axis)and the vector from the oxygen atom to the center of hydrogen atoms in water molecule.The orientation in(b)is defined by the angle between the surface normal and the H-H vector.

To further understand the special pattern of water distribution,the radial density pro files of the hydrogen and oxygen atoms of water adsorbed on mica surface were analyzed both in the inner layer and outer layer according to the water number density pro files in[Emim][BF4]with 15.06 wt%water.Fig.7a shows that the radial density pro files of the hydrogen atoms around in the inner layer starts from 0.29 nm,which is further than 0.23 nm in the outer layer;the peak location in the inner layer is 0.455 nm,while the peak in the outer layer is located at 0.285 nm.These suggest that the water molecules in the outer layer are closer to K+ions than in the inner layer.In addition,the water molecules in the outer layer may have stronger interactions with K+ions than those in the inner layer.The starting point and the peak location of K+-O pro files in the outer layer are 0.26 nm and 0.30 nm(Fig.7b),and both of which are further than that of K+-H in the outer layer(0.23 nm and 0.29 nm).It can be inferred that hydrogen atoms of water molecules are closer to K+ions than oxygen atoms,probably because of the hydrogen bonds formed by hydrogen atoms of water molecules and oxygen atoms of mica.

ions and aluminum atoms of mica surface;(6)the radial number density and orientation distributions of water molecules provided more information on the configuration of adsorbed water molecules.These results can help to understand the microstructure of RTIL-based electrical double layers and the behavior of the associated water content in RTILs used for microlubrication,electrical energy storage,and other applications.

（2）钻机开孔。设置钻机，校正机座和机架。为了保证岩基岩和坝体分开，下入套管。应用金刚钻头进行钻进，直到完成终孔。在钻进过程中，一旦发现卡钻的现象，可以将钻具上拉或适当扭转钻具，保证冲洗液可继续灌进。应用钻机进行钻进的过程中，应格外留意钻孔倾斜度，一旦发现倾斜度超标，立即采取有效的措施进行纠斜；当钻探深度达到设计深度后，等待灌浆结束，应用覆盖物对孔口进行保护，防止杂物落入孔内。

Conflicts of interest

Supplementary data related to this article can be found at https://doi.org/10.1016/j.gee.2017.11.002

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51406060)and Shenzhen Basic Research Project(JCYJ20170307171511292).This work was carried out at the National Supercomputing Centers in Tianjin(Tianhe-1A)and Guangzhou(Tianhe II).

Appendix A.Supplementary data

The authors declare that they have no competing interests.

References

[1]E.Frackowiak,G.Lota,J.Pernak,Appl.Phys.Lett.86(2005)164104.

[2]K.K.Seddon,Nat.Mater.2(2003)363-365.

[3]J.Wang,J.Luo,S.Feng,H.Li,Y.Wan,X.Zhang,Green Energy Environ.1(2016)43-61.

[4]P.Losada-Pérez,M.Khorshid,F.U.Renner,PLoS One 11(2016)e0163518.

[5]S.-K.Ruokonen,C.Sanwald,M.Sundvik,S.Polnick,K.Vyavaharkar,F.Duša,A.J.Holding,A.W.T.King,I.Kilpeläinen,M.Lämmerhofer,P.Panula,S.K.Wiedmer,Environ.Sci.Technol.50(2016)7116-7125.

[6]A.H.Rantamäki,S.-K.Ruokonen,E.Sklavounos,L.Kyllönen,A.W.T.King,S.K.Wiedmer,Sci.Rep.7(2017)46673.

[7]A.Somers,P.Howlett,D.MacFarlane,M.Forsyth,Lubricants 1(2013)3-21.

[8]C.Ye,W.Liu,Y.Chen,L.Yu,Chem.Commun.(2001)2244-2245.

[9]A.Abate,D.J.Hollman,J.l.Teuscher,S.Pathak,R.Avolio,G.D'Errico,G.Vitiello,S.Fantacci,H.J.Snaith,J.Am.Chem.Soc.135(2013)13538-13548.

[10]Y.S.Nanayakkara,H.Moon,T.Payagala,A.B.Wijeratne,J.A.Crank,P.S.Sharma,D.W.Armstrong,Anal.Chem.80(2008)7690-7698.

[11]R.Hayes,G.G.Warr,R.Atkin,Phys.Chem.Chem.Phys.12(2010)1709-1723.

[12]J.Lu,F.Yan,J.Texter,Prog.Polym.Sci.34(2009)431-448.

[13]B.D.Fitchett,J.C.Conboy,J.Phys.Chem.B 108(2004)20255-20262.

[14]J.B.Rollins,B.D.Fitchett,J.C.Conboy,J.Phys.Chem.B 111(2007)4990-4999.

[15]M.Mezger,H.Schröder,H.Reichert,S.Schramm,J.S.Okasinski,S.Schöder,V.Honkimäki,M.Deutsch,B.M.Ocko,J.Ralston,Science 322(2008)424-428.

[16]R.Atkin,G.G.Warr,J.Phys.Chem.C 111(2007)5162-5168.

[17]J.L.Parker,D.L.Cho,P.M.Claesson,J.Phys.Chem.93(1989)6121-6125.

[18]U.Raviv,P.Laurat,J.Klein,Nature 413(2001)51-54.

[19]U.Raviv,J.Klein,Science 297(2002)1540-1543.

[20]A.Ulcinas,G.Valdre,V.Snitka,M.J.Miles,P.M.Claesson,M.Antognozzi,Langmuir 27(2011)10351-10355.

[21]G.B.Kaggwa,P.C.Nalam,J.I.Kilpatrick,N.D.Spencer,S.P.Jarvis,Langmuir 28(2012)6589-6594.

[22]Y.Liu,Y.Zhang,G.Wu,J.Hu,J.Am.Chem.Soc.128(2006)7456-7457.

[23]S.Perkin,T.Albrecht,J.Klein,Phys.Chem.Chem.Phys.12(2010)1243-1247.

[24]R.Singh Payal,S.Balasubramanian,ChemPhysChem 13(2012)1764-1771.

[25]R.S.Payal,S.Balasubramanian,J.Phys.Condens.Matter 26(2014)284101.

[26]N.Nishi,H.Murakami,Y.Yasui,T.Kakiuchi,Anal.Sci.24(2008)1315-1320.

[27]J.A.Widegren,A.Laesecke,J.W.Magee,Chem.Commun.12(2005)1610-1612.

[28]Y.Wang,H.Li,S.Han,J.Phys.Chem.B 110(2006)24646-24651.

[29]T.Welton,Chem.Rev.99(1999)2071-2084.

[30]G.Feng,X.Jiang,R.Qiao,A.A.Kornyshev,ACS Nano 8(2014)11685-11694.

[31]A.M.Smith,M.A.Parkes,S.Perkin,J.Phys.Chem.Lett.5(2014)4032-4037.

[32]R.Espinosa-Marzal,A.Arcifa,A.Rossi,N.Spencer,J.Phys.Chem.C 118(2014)6491-6503.

[33]K.Sakai,K.Okada,A.Uka,T.Misono,T.Endo,S.Sasaki,M.Abe,H.Sakai,Langmuir 31(2015)6085-6091.

[34]H.-W.Cheng,P.Stock,B.Moeremans,T.Baimpos,X.Banquy,F.U.Renner,M.Valtiner,Adv.Mater.Interfaces 2(2015)1500159.

[35]X.Gong,A.Kozbial,L.Li,Chem.Sci.6(2015)3478-3482.

[36]E.Lindahl,B.Hess,D.Van Der Spoel,Mol.Model.Annu.7(2001)306-317.

[37]W.Humphrey,A.Dalke,K.Schulten,J.Mol.Graph.14(1996)33-38.

[38]A.Heintz,J.K.Lehmann,C.Wertz,J.Jacquemin,J.Chem.Eng.Data 50(2005)956-960.

[39]H.Heinz,H.Koerner,K.L.Anderson,R.A.Vaia,B.Farmer,Chem.Mater.17(2005)5658-5669.

[40]J.N.C.Lopes,J.Phys.Chem.B 108(2004)2038-2047.

[41]H.-W.Cheng,J.-N.Dienemann,P.Stock,C.Merola,Y.-J.Chen,M.Valtiner,Sci.Rep.6(2016)30058.

[42]B.Docampo-Alvarez,V.Gomez-Gonzalez,H.Montes-Campos,J.Otero-Mato,T.Mendez-Morales,O.Cabeza,L.Gallego,R.Lynden-Bell,V.Ivanistsev,M.Fedorov,J.Phys.Condens.Matter 28(2016)0953-8984.

[43]J.M.Black,M.Zhu,P.Zhang,R.R.Unocic,D.Guo,M.B.Okatan,S.Dai,P.T.Cummings,S.V.Kalinin,G.Feng,Sci.Rep.6(2016)32389.

[44]M.Ricci,P.Spijker,K.Voïtchovsky,Nat.Commun.5(2014)4400.