油酸以及双面包覆的磁流体的合成

Preparation and magnetic performance of the magnetic fluid stabilized by bi-surfactantBuhe Bateer,Yang Qu,Xiangying Meng,Chungui Tian,Shichao Du,Ruihong Wang,Kai Pan,Honggang Fu nKey Laboratory of Functional Inorganic Material Chemistry,Ministry of Education of the People’s Republic of China,Heilongjiang University,150080Harbin,PR Chinaa r t i c l e i n f oArticle history:Received 9August 2012Received in revised form 22November 2012Available online 22December 2012Keywords:Bi-surfactant coated Fe 3O 4Phase-transferStable oil-dispersion Magnetic fluidMagnetic performancea b s t r a c tA stable poly-alpha olefin oil based magnetic fluid of bilayer surfactant coated (oleic acid and succinimide)Fe 3O 4nanoparticles was prepared by a using phase transfer method.The bilayer surfactant-coated magnetic particles are characterized by Fourier-transform infrared spectroscopy,the result shows that the Fe 3O 4nanoparticles are coated by oleic acid,and the bilayer surfactant succinimide wraps the oleic acid coated Fe 3O 4nanoparticles.Transmission electron microscopy images indicate that the dispersibility of the bilayer coated Fe 3O 4nanoparticles is better than that of the uncoated ones.Vibrating sample magnetometer measurement confirms that both the coated and uncoated Fe 3O 4nanoparticles are super-paramagnetic.Stability measurements show the magnetic fluids prepared by bilayer coated Fe 3O 4nanoparticles is nearly constant for 360days at room temperature.&2012Elsevier B.V.All rights reserved.1.IntroductionMagnetic fluid (ferrofluid)is a kind of suspension composed of magnetic nanoparticles (NPs),such as magnetite (Fe 3O 4),[1,2]iron nitride (e -Fe 3N),[3]iron (Fe),[4]cobalt (Co)[5]with the size about 10nm that were dispersed into the carrier liquid (aqueous or organic solvent)since their first appearance in the middle of the 1960s [6].The magnetic particles and the liquid are not separated even in the presence of centrifugal force or magnetic field [7].Magnetic fluids have the characteristics of both magnetic and fluid properties and have a diverse range of applications in various fields.It was estimated that 500million hard drives,350million loud speakers,15million DVD-ROM drives,and 200,000rotary vacuum seals had been manufactured using magnetic fluids in 2006[8].And there are other uses of magnetic fluids in magnetic resonance imaging,[9]magnetic ink,[10]micropumps,[11]medicines [12]and art [13].Stability of magnetic fluids is of great importance in applica-tions,especially in industry,and thus it has been a topic of great interest in recent years.The suitable organic surfactants and carrier liquids are important factors that can effect the stability of magnetic fluids [14–16].Surface coating can restrict the agglomeration and improve the dispersibility of the Fe 3O 4NPs simultaneously,[17–19]and consequently,can improve thestability of the magnetic fluids largely.There are many reports about magnetic fluids prepared by dispersing of magnetic NPs coated with monolayer surfactant (oleic acid)in different organic solvents (kerosene,transformer oil,dodecane,etc.)[20,21].Although it is simple and easy,the organic solvents have low boiling point and can volatilize fast,so the magnetic fluid cannot last for a long time.There are also reports regarding to the synthesis of magnetic fluids in oil based carriers with high boiling point.In this case,a single surfactant,such as the lauroyl sarcosine surfactant,was used in silicone oil based magnetic fluid [22].The evaporation rate of these oil-based carriers is slower than that of the organic solutions with low boiling point,but the dispersing stability of magnetic NPs in them becomes poor.To improve the stability of magnetic fluids,the NPs coated with bilayer surfactant are used [23].Shimoiizaka et al.[24]found bilayer stabilization as early as 1980s by first precipitating oleic acid-coated particles and then re-dispersing them in aqueous solutions of sodium dodecylbenzene sulfonate,poly(oxyethylene)nonylphenyl ethers,and di(2-ethylhexyl)-adipate those acted as a second layer on the top of oleic acid.Wooding et al.[25,26]produced stable aqueous magnetic fluids by selecting various saturated and unsaturated fatty acids as primary and secondary surfactants.The n-alkanoic acids with 9–13carbons can be used as bilayer surfactant to stabilize magnetic fluids as demonstrated by Shen et al.[27,28].Hong et al.[29]prepared the aqueous magnetic fluids in which the oleate sodium and PEG-4000were used as the primary and the secondary layers respectively.These bilayer coated aqueous-based magnetic fluids are quite stable,Contents lists available at SciVerse ScienceDirectjournal homepage:/locate/jmmmJournal of Magnetism and Magnetic Materials0304-8853/$-see front matter &2012Elsevier B.V.All rights reserved./10.1016/j.jmmm.2012.12.009nCorresponding author.Tel.:þ8645186608458;fax:þ8645186673647.E-mail address:fuhg@ (H.Fu).Journal of Magnetism and Magnetic Materials 332(2013)151–156and can be used in biomedical,but they are hard to be used in industrial due to their low boiling point and poor thermal stability.A bilayer surfactant-coated oil-based magnetic fluid would be desirable due to their high stability that is favorable for the industrial application,but there is no report about this at present.Above analyses indicate that the traditional method of pre-paring magnetic fluids has some disadvantages,such as hard to disperse in oil and low stability.As known,the high stability and well dispersibility in oil is very significant for the industrial application of the magnetic NPs.In this paper,we designed a stable oil-based magnetic fluid of bilayer surfactants coated Fe 3O 4NPs by using simple phase transfer method.The Fe 3O 4NPs are coated with a primary surfactant that can bind strongly to the surface atoms of the Fe 3O 4NPs,and then the coated Fe 3O 4NPs are recoated with a second surfactant that has a good compatibility with the carrier of the magnetic fluid.We choose oleic acid and succinimide (PIBA)as primary and secondary surfactant respec-tively.By using phase transfer method,the bilayer surfactant coated Fe 3O 4NPs were prepared.After it was dispersed in PAO-2carrier,the stable magnetic fluid with high stability and good magnetic property were obtained.2.Experimental section 2.1.Chemicals and materialsFerric chloride (FeCl 3Á6H 2O,99%),ferrous sulfate (FeSO 4Á7H 2O,99%),oleic acid (OA),ammonium hydroxide (28%NH 3in water,w/w),deionized water,heptane,were purchased from Sinopharm Chemical Reagent Co.Ltd.Succinimide dispersant (PIBA),poly-alpha olefin synthetic oil (PAO-2)were purchased from China Petrochemical Corporation.2.2.Synthesis of oleic acid-coated Fe 3O 4magnetic NPsOleic acid coated (primary surfactant)Fe 3O 4magnetic NPs were firstly prepared by a modified chemical coprecipitation method.Aqueous solutions of 200mL of 2M FeCl 3Á6H 2O and 200mL of 1M FeSO 4Á7H 2O were mixed in a 2L beaker.A total of 200mL of NH 4OH (28%(w/w))was quickly added,and the mixture was vigorously stirred at 601C for 30min.After adding of 12mL oleic acid,the temperature was raised to 801C and remained for 45min under constant stirring.After the beaker wascooled to room temperature,the Fe 3O 4magnetic NPs were obtained.2.3.Synthesis of bilayer-coated Fe 3O 4magnetic NPs200mL of heptane contained succinimide dispersant (second-ary surfactant)was added into the beaker contained OA-coated Fe 3O 4magnetic NPs and stirred for 10min.After standing for 30min,two liquid layers formed in the beaker,and the color of the aqueous phase became clear,which meant the Fe 3O 4NPs were transferred to heptane.The two phases were separated by a separating funnel.2.4.Synthesis of magnetic fluidsA certain number of PAO-2is added into the heptane solution including bilayer coated Fe 3O 4magnetic NPs and then stirred at 1001C until the heptane is evaporated to get a stable bilayer coated magnetic fluid.The content of magnetic NPs in the magnetic fluids is controlled by the adding amount of PAO-2.2.5.CharacterizationX-ray powder diffraction (XRD)patterns were obtained by a Rigaku D/max-IIIB diffractometer using Cu Ka radiation(l ¼1.5406˚A).The transmission electron microscopy (TEM)experiment was performed on a JEM-2100electron microscope (JEOL,Japan)with an acceleration voltage of 200kV.Carbon coated copper grids were used as the sample holders.Thermo-gravimetric analysis was performed on a TG (TA,Q600)thermal analyzer under air with a heating rate of 101C min À1.Fourier-transform infrared spectra (FT-IR)of the samples were acquired with a PE Spectrum One B IR spectrometer.Magnetization was measured at 300K with a Lake Shore Vibrating Sample Magnet-ometer Model 7404,applying field strengths up to 20kOe.3.Results and discussion3.1.Characterization of magnetic NPsThe synthetic process of magnetic fluid of bilayer coated Fe 3O 4NPs is shown in Scheme 1.The Fe 3O 4NPs are prepared by chemical co-precipitation method,and they are coated with oleic acid as the primary layer surfactant in aqueous phase.The oleic acid molecules bind strongly to the surface atoms of the Fe 3O4Scheme 1.Synthetic process of magnetic fluid of bilayer coated Fe 3O 4magnetic NPs.B.Bateer et al./Journal of Magnetism and Magnetic Materials 332(2013)151–156152NPs[30].The oleic acid coated Fe3O4NPs are transferred to heptane to be coated with succinimide.Succinimide is amphi-philic molecular composed of two long non-polar tails and a polar head.The polar heads can attach to the oleic acid coated Fe3O4 NPs,a restricted mobility of the OA is due to theirfirm anchoring on the surface of Fe3O4NPs,while PIBA is formed with polar and nonpolar groups,and the polar groups are bound with the OA layer by strong van der Waals interactions,[27]the long(non-polar)hydrocarbon tails are favorable for the sustentation of succinimide in PAO-2,therefore,even the application of an external strong magneticfield,the bilayer-coated Fe3O4NPs cannot easily reorient the spins.The bilayer coated magnetic NPs can be used to the preparation of magneticfluids.Phase-transfer method is commonly used in the preparation of nano-material[31–33].In water–oil systems,the particles tend to be assembled at the water–oil interface[34–36]and transferred to the oil phase in the presence of surfactant[37–39].The hydrophilic group of the surfactant is adsorbed on the particle surface by electrostatic and/or hydrogen-bonding interactions [40].The hydrophobic chain of the surfactant is directed to the outside,so the hydrophobic particles modified with surfactant could be extracted and dispersed in the oil phase.The re-coated step of oleic acid coated Fe3O4NPs through phase-transfer to heptane can simplify the preparation process,prevent the agglomeration of the NPs and improve the dispersing efficiency simultaneously.The mixture of PAO-2and heptane solution including bilayer coated Fe3O4magnetic NPs is stirred at1001C until the heptane is evaporated(the evaporated heptane can be recycled).And a stable bilayer coated magneticfluid isfinally prepared.Magneticfluids made of uncoated and monolayer coated Fe3O4 NPs dispersing in PAO-2are easily precipitated(Supporting Information,Fig.S1).The uncoated Fe3O4NPs are easy to precipitate from the carrier due to their bald surface,and the upper layer of the magneticfluid becomes clear after2h.And the magneticfluid of monolayer coated Fe3O4NPs is unstable because the nonpolar groups can bind weakly to PAO-2molecular.While the magneticfluid of bilayer coated Fe3O4NPs is stable due to the virtue of the bilayer surfactants.Fig.1A shows the TEM image of uncoated Fe3O4NPs prepared by chemical coprecipitation method.The uncoated Fe3O4NPs are not modified by any surfactants,so they are easily reunited and badly re-dispersed in heptane or PAO-2,even under vigorous stirring or long-time ultrasonic vibration.Fig.1B shows the TEM image of oleic acid coated Fe3O4NPs prepared by a modified chemical coprecipita-tion method,which are well-dispersed in heptane but bad-dispersed in PAO-2.Fig.1C shows bilayer coated Fe3O4NPs prepared by phase-transfer,they are well-dispersed in both heptane and PAO-2.We can see that the coating of bilayer surfactants improves the dispersibility of the Fe3O4NPs without any effect on their particle size.And we can see from Fig.1B and C,both the single coated and bilayer coated Fe3O4NPs have a good dispersibility.From Fig.1A,B and C,we can see that the particle shape of the single coated and bilayer coated Fe3O4NPs is reasonably spherical with better dispersion than the uncoated ones.Also,the bilayer coated Fe3O4NPs is poly-dispersed with the size in the range of8to17nm.The mean particle size is about12nm(Fig.S2).Fig.1D gives the high resolution TEM(HRTEM) image of the Fe3O4NPs,from which an interplanar spacing about 0.240nm can be observed,corresponding to the(311)plane of the cubic Fe3O4.The uncoated Fe3O4(Fig.2a),monolayer coated Fe3O4(Fig.2b) and bilayer coated Fe3O4(Fig.2c)samples are examined by XRD. All the diffraction peaks in Fig.2can be indexed as the face-centered magnetite(Fe3O4)crystallite(JCPDS cardfile No. 74-0748).The average particle size of the uncoated NPs is calculated to be about11nm by using Scherer’s equation from the half-maximum width of the(311)X-ray diffraction line.After the coating of surfactants,all of the peaks(Fig.2b,c)related to magnetic Fe3O4can also be observed.Also,the peaks of the coated Fe3O4NPs have lower intensity than that of the uncoated ones, which should be contributed to the low content of magnetic Fe3O4in coated Fe3O4NPs than that of the uncoatedones.Fig.1.TEM images of(A)uncoated Fe3O4(B)monolayer coated Fe3O4NPs(C)bilayer coated Fe3O4NPs(D)HR-TEM images of uncoated Fe3O4.B.Bateer et al./Journal of Magnetism and Magnetic Materials332(2013)151–156153An FT-IR spectrometer is used to study the surface organic groups of the coated and uncoated Fe 3O 4NPs (Fig.3).We can see that the uncoated Fe 3O 4(Fig.3a)has only a strong characteristic peak of Fe–O stretch at 590cm À1(Fig.3a).The spectrum of free oleic acid shows an IR band at 1713cm À1corresponding to the vibration of free –COOH (Supporting Information,Fig.S3).For the oleic acid coated Fe 3O 4NPs,the IR band at 1713cm À1was absent,which indicates no free oleic acid in the NPs.In addition,the peaks of –CH 2stretch for oleic acid coated Fe 3O 4NPs (Fig.4b)can be found at 2921and 2851cm À1,which have a shift toward the high wave number in comparison with the pure oleic acid,indicating the absorption of oleic acid on the surface of Fe 3O 4NPs.Two new intense and broad peaks at 1530and 1404cm À1are characteristic of COO –asymmetric and COO –symmetric stretch [41].The wave number separation (D )between COO –asymmetric and COO –symmetric of 116cm À1is ascribed to bridging bidentate,where the COO –group is covalently bonded to surface atoms of Fe 3O 4.The FT-IR spectra of bilayer coated Fe 3O 4NPs is shown in Fig.3c,the strong characteristic peak at590cm À1is assigned to Fe–O stretch,two sharp bands at 2923and 2854cm À1are attributed to the asymmetric CH 2stretch and the symmetric CH 2stretch of the second surfactant layer of PIBA.The peak at 1707cm À1corresponds to the imide absorption peak.The peak at 1404cm À1is assigned to the CH 2bending vibration of the PIBA.IR test indicates the successful modification of both OA and PIBA on the Fe 3O 4particles.To calculate the Fe 3O 4content of the uncoated and coated Fe 3O 4NPs,TGA analysis is performed (Fig.4).The weight loss of uncoated Fe 3O 4NPs is about 7.4wt%in whole temperature range (Fig.4a),which is largely due to the removal of absorbed physical and chemical water.For the monolayer coated Fe 3O 4NPs,the weight loss of 24.5%is mainly caused by thermal degradation of OA over the temperature range from 200to 4001C and the absorbed water (Fig.4b).By comparasion,the content of OA is about 17.1%in the coated Fe 3O 4NPs.The TGA curve of bilayer coated Fe 3O 4NPs (Fig.4c)shows a total weight loss of about 41.5%,which includes the degradation of OA and PIBA disper-sants.In addition,its largest weight loss can be observed from 200to 4501C.By comparison with Fig.4b,the content of PIBA dispersant is about 17%.Bulk Fe 3O 4is ferromagnetic at room temperature,but below a critical particle size it becomes super-paramagnetic.Fig.5shows the magnetization curve of the Fe 3O 4NPs at 300K and magnetic field of 20kOe.We can see from Fig.5that the uncoated,monolayer coated and bilayer coated Fe 3O 4NPs have super-paramagnetic properties because their remanence and coercivity are zero.Their saturation magnetization is 65emu/g,53emu/g and 38emu/g,respectively.The decrease of saturation magneti-zation of the coated Fe 3O 4NPs may be caused by the coating of surfactants.3.2.Characterization of magnetic fluidsThe viscosity and density dependence of the magnetic fluids consisting of bilayer coated Fe 3O 4NPs in PAO-2on the content of the bilayer coated Fe 3O 4NPs is analyzed.As shown in Figs.6and 7,the viscosity and density of the magnetic fluids increases with the increasing of the mass percentage of bilayer coated Fe 3O 4NPs in PAO-2.Also,the increasing of density is nearly proportional to the mass percentage.The viscosity and density of the magnetic fluid with 5%mass percentage of bilayer coated Fe 3O 4NPs in PAO-2are 7cP (251C)and 0.85g/cm [3].Fig.2.X-ray diffraction of (a)uncoated Fe 3O 4NPs (b)monolayer coated Fe 3O 4NPs (c)bilayer coated Fe 3O 4NPs.Fig.3.FT-IR spectra of (a)uncoated Fe 3O 4NPs (b)monolayer coated Fe 3O 4NPs (c)bilayer coated Fe 3O 4NPs.Fig.4.TGA curves of (a)uncoated Fe 3O 4NPs (b)monolayer coated Fe 3O 4NPs (c)bilayer coated Fe 3O 4NPs.B.Bateer et al./Journal of Magnetism and Magnetic Materials 332(2013)151–156154The magnetic property of the stable magnetic fluids is also studied.Fig.8shows the initial magnetization curve of magnetic fluids with different mass percentage.We can see that all the magnetic fluids have super-paramagnetic properties for the magnetization increases with the increasing of the magnetic field (H )simultaneously until it tends to be saturated and there is no remanence.The saturation magnetization of the magnetic fluids with 5%and 45%mass percentage of Fe 3O 4NPs is respectively 2.94emu/g and 21.92emu/g.The dispersion stability of the magnetic fluids is usually estimated by periodical measurement of density changement.Fig.9shows the dispersion stability of the PAO-2based magnetic fluids consisting of bilayer coated Fe 3O 4NPs with duration time.The index of stability (I )in the figure is calculated as below [5]:I ¼r 1Àr ÀÁ=r Àr wÀÁwhere r 1is the density of the upper layer of the magnetic fluid after duration,r is the density of the freshly prepared magnetic fluid,and r w is the density of the liquid carrier.As shown in Fig.9,the bilayer coated Fe 3O 4magnetic fluid does not show any change in stability even after storage of 360days,which indicates that the bilayer coated Fe 3O 4magnetic fluid can keep good stability and dispersibility for a long time.The duration lasts for more than 360days.Result proves the high stability of magnetic fluid of bilayer coated Fe 3O 4in PAO-2,which is favorable for its use in loudspeaker (Supporting Information,Fig.S4).4.ConclusionsIn conclusion,a stable magnetic fluid has been prepared by bi-surfactant assisted phase-transfer method,and its magnetic performance researched.The OA (primary surfactant)coated Fe 3O 4nanoparticles are prepared by chemical coprecipitation and transferred to heptane in which contains the PIBA (secondary surfactant).Then the heptane solution including the bilayer-surfanctant coated Fe 3O 4magnetic nanoparticles is mixed with PAO-2,and heptane is removed through constant heatingandFig.6.Viscosity of the magnetic fluids consisting of Fe 3O 4NPs in PAO-2versus their masspercentage.Fig.7.Density of the magnetic fluids consisting of Fe 3O 4NPs in PAO-2versus their masspercentage.Fig.5.Magnetic hysteresis curves of the NPs at 300K (a)uncoated Fe 3O 4NPs (b)monolayer coated Fe 3O 4NPs (c)bilayer coated Fe 3O 4NPs.Fig.8.Initial magnetization curves of Fe 3O 4NPs in PAO-2with different mass percentage.B.Bateer et al./Journal of Magnetism and Magnetic Materials 332(2013)151–156155stirring.Magnetic fluids with the bilayer surfactant-coated Fe 3O 4nanoparticles content of 5%to 45%are prepared.And their magnetization and viscosity are analyzed.The result shows that the magnetic fluids have good magnetic stability for no phase separation happens after 360days,which may have potential application in machinery.AcknowledgmentWe gratefully acknowledge the support of the Key Program Projects of the National Natural Science Foundation of China (No 21031001),the National Natural Science Foundation of China (Nos 20971040,91122018,21001042,21101061),the Cultivation Fund of the Key Scientific and Technical Innovation Project,Ministry of Education of China (No 708029),Specialized Research Fund for the Doctoral Program of Higher 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