胆酸盐与Gemini表面活性剂复合物构建的纳米纤维和囊泡.pdf

A物理化学学报(Wuli Huaxue Xuebao)Acta Phys.-Chim.Sin.2013,29(11),2415-2421NovemberReceived:July 18,2013;Revised:September 23,2013;Published on Web:September 24,2013.Corresponding author.Email:;Tel:[phone].The project was supported by the National Natural Science Foundation of China(20973104,21033005,21373127).国家自然科学基金(20973104,21033005,21373127)资助项目 Editorial office ofActa Physico-Chimica Sinicadoi:10.3866/PKU.WHXB201309243胆酸盐与Gemini表面活性剂复合物构建的纳米纤维和囊泡朱丽杰黄丹丹李钦堂许冠辰陈晓*(山东大学胶体与界面化学教育部重点实验室,济南250100)摘要:Gemini表面活性剂(CsH2s-,-(Me2N+CmH2m+1Br-)2,m-s-m)与胆汁酸盐(BS)利用静电作用构建超分子复合物,通过改变m-s-m和BS的结构,可以分别得到纳米纤维和囊泡状超分子聚集体.聚集体的形貌、结构和性质分别通过偏光显微镜(POM)、透射电子显微镜(TEM)、场发射扫描电子显微镜(FE-SEM)、X射线粉末衍射(XRD)及其它测试手段进行了表征.研究发现,通过改变m-s-m的烷基链长和间隔基团的长度,以及BS胆甾骨架上羟基的数目和位置,聚集体的形貌会发生明显的变化,聚集体主要依靠静电相互作用,并在疏水作用及氢键的协同作用下形成,本文的结果将有助于更好地理解离子自组装机理,并对超分子聚集体的设计提供新的思路.关键词:自组装;Gemini表面活性剂;胆酸盐;聚集体;超分子作用中图分类号:O648Nanofibers and Vesicles Self-Assembled from GeminiSurfactant Complexes with Bile SaltsZHU Li-JieHUANG Dan-DanLI Qin-TangXU Guan-ChenCHEN Xiao*(Key Laboratory of Colloid and Interface Chemistry of the Ministry of Education,Shandong University,Jinan 250100,P.R.China)Abstract:Nanofibers and vesicles are fabricated in aqueous solution by self-assembly of complexes ofcationic Gemini surfactants(CsH2s-,-(Me2N+CmH2m+1Br-)2,m-s-m)and anionic bile salts(BS).Their morphology,structure,and properties are characterized by polarized optical microscopy(POM),transmission electronmicroscopy(TEM),field-emission scanning electron microscopy(FE-SEM),and X-ray powder diffraction(XRD).The morphology of these aggregates was significantly influenced by minor structural changes of the buildingblocks,including the spacer and alkyl chain lengths of m-s-m,and the hydroxyl group number and position onthe steroid skeleton of the BS.The formation of these aggregates is considered to arise mainly from electrostaticinteractions,with contributions from hydrogen bonding and hydrophobic interactions.The results obtained maybe helpful for understanding the mechanisms of ionic self-assembly and aid the design of novel supramolecularaggregates.Key Words:Self-assembly;Gemini surfactant;Bile salt;Aggregate;Supramolecular interaction1IntroductionOrdered supramolecular aggregates formed through self-assembly are among the promising candidates for functionalmolecular devices and nanomaterials.1Extensive efforts havebeen directed to such materials to explore their novel proper-ties and functions,which are readily available from specific as-sembly of molecular components.2-6Various driving forces,like electrostatic interaction,hydrogen bonding,-interac-tion,van der Waals force,host-guest interaction,metal-ligandcoordination,amphiphilic association,and hydrophobic interac-tion,have been adopted for fabrication.As a continuation of our previous studies,7-9the preparation2415Acta Phys.-Chim.Sin.2013Vol.29of self-assembled nanofibers or vesicles with building blocksof a series of cationic Gemini surfactants(m-s-m)and anionicbile salts(BS)with chemical structures shown in Scheme 1 hasbeen reported here.Both of these two components exhibit un-usual properties and aggregation behaviors in aqueous solu-tions for their unique structures.10,11The former,Gemini mole-cules,12consist of two monomer surfactants covalently attachedby a spacer group at or close to the head groups,which pro-vides an opportunity to expand their structural diversities topresent their enhanced performances.13-15Among them,the di-cationic quaternary ammonium compounds(with a molecularformula of(CsH2s-,-(Me2N+CmH2m+1Br-)2,abbreviated as m-s-m here)are a type of Gemini molecules,which have been wide-ly studied on their aggregation properties in aqueous solution.16The specialties of the latter,bile salts,come from their slightlycurved backbone shapes and the availability of a variable num-ber(up to 3)of hydroxyl groups.As one kind important physio-logical constituent,their aggregation behaviors and differentialchemical reactivities have been also extensively exploited by anumber of research groups in diverse areas of chemistry.17-21Based on these structural characteristics and their oppositecharges,it is then interesting to explore the aggregate forma-tion using both m-s-m and BS as building blocks.Such a com-bination could provide a useful model to explore the relation-ship between the aggregate morphology and the componentstructure due to their rich structural tuning options.By directlymixing the stock solutions of m-s-m and BS,the self-organizedhierarchical nanofibers or vesicles could be obtained respec-tively,depending on the used structures of m-s-m and BS.Vari-ous effects,including the spacer and alkyl chain lengths of m-s-m,the number or position and direction of hydroxyl groups onthe steroid skeleton of BS,could be investigated to modulatethe self-organized structures.The possible aggregation mecha-nisms respectively for nanofibers or vesicles have been ana-lyzed by considering various molecular interactions.The ob-tained results may provide valuable information on self-assem-bly in such catanionic systems and contribute to design novelsupramolecular aggregates.2Experimental2.1MaterialsThe m-s-m type Gemini surfactants were synthesized accord-ing to procedures reported previously.22,23Except for sodiumcholate(SC,98%)from TCI and sodium deoxycholate(SDC,98%)from Sinopharm Chemicals Reagent Co.,chenodeoxy-cholic acid(CDC,99%),ursodeoxycholic acid(UDC,99%)and hyodeoxycholic acid(HDC,99%)were all from AladdinChemistry Co.Ltd.The other chemical reagents used were ofanalytical grade.They were used without further purification.The solvent was triply distilled water.2.2Preparation of aggregatesThe aqueous solutions of m-s-m,SC,and SDC at certain con-centrations were prepared by directly dissolving quantitativeamounts of corresponding compounds in water.The solutionsof sodium chenodeoxycholate(SCDC),sodium ursodeoxycho-late(SUDC),and sodium hyodeoxycholate(SHDC),however,were prepared by mixing NaOH with CDC,UDC,and HDC at1:1 molar ratio,respectively.The samples with aggregate formation were prepared bymixing the aqueous solutions of m-s-m and BS directly.Takethe system with nanofiber formation as an example,the aque-ous solution of SC at 4.0 mmolL-1was mixed with anotherequal volume aqueous solution of m-s-m at 2.0 mmolL-1.Themixed system was then kept at 25 C for one day to lead theelectrostatic attraction induced aggregates to precipitate suffi-ciently.Then the precipitates were collected by vacuum filtra-tion and washed three times with water to remove the pro-duced salts and any other residual precursors.The productswere dried under vacuum at room temperature for 24 h.Thesamples with dispersed vesicle,however,could be prepared bydirectly mixing desired solutions of BS(all kinds of BS usedhere except for SC)and m-s-m at certain concentrations andmolar ratios.2.3Aggregate characterizationsOptical microscopy(OM)and polarizing optical microscopy(POM)observations were taken on an OLYMPUS BX51,witha MicroPublisher 5.0 RTV CCD Camera(Olympus Optical Co.Ltd.,Japan).The sample solution for TEM was placed on aFormvar-covered TEM grid(200 mesh),then they were exam-ined using a Hitachi 100CX-II TEM(JEOL Ltd.,Japan)operat-ing at 100 kV.The field-emission scanning electron microsco-py(FE-SEM)images were taken on a SUPRATM55 system(Carl Zeiss AG,Germany).The sample was prepared by cast-ing the aqueous solution of complexes onto a clean siliconslide,followed by drying under vacuum at room temperature.Scheme 1Structures of the studied bile salts and m-s-m typeGemini surfactantsSC,SDC,SCDC,SUDC,and SHDC are sodium salts of cholate,deoxycholate,chenodeoxycholate,ursodeoxycholate,and hyodeoxycholate,respectively.2416ZHU Li-Jie et al.:Nanofibers and Vesicles Self-Assembled from Gemini Surfactant Complexes with Bile SaltsNo.11The dried sample was then coated by gold.1H NMR spectrawere recorded on a Bruker AV-400 NMR spectrometer(BrukerBioSpin,Switzerland)at room temperature with the chemicalshifts()referenced to=2.5 for dimethyl sulfoxide(DMSO).X-ray powder diffraction(XRD)experiments were performedon a Rigaku D/max-rA diffractometer(Rigaku Inc.,Japan)withCu Kradiation(=0.154 nm,40 kV,60 mA).Thermal analy-ses through differential scanning calorimetry(DSC)and thermo-gravimetric analysis(TGA)were performed using the MettlerToledo DSC-822e and TA SDTQ-600 controllers(Mettler To-ledo,Switzerland)at a scan rate of 10 Cmin-1under nitrogenat a?ow rate of50 mLmin-1.3Results and discussion3.1Aggregates with different morphologiesBy changing the molecular structures of m-s-m and BS usedin aggregation systems,either nanofibers or vesicles could beformed at room temperature.3.1.1NanofibersThe nanofibers are observed,resulting mainly from the sys-tems containing SC or SDC molecules.To illustrate the macro-scopic morphologies and properties of such formed aggregates,the 12-2-12/SC complex system is taken as an example for dis-cussion.The1H NMR spectra measured respectively for CD3OD solu-tion of complex aggregates and pure 12-2-12 or SC compo-nents are shown in Fig.1.It can be noted from Fig.1(c,d)thatthe aggregate composition could hardly be changed in systemswith different mixing ratios.The stoichiometric ratio between12-2-12 and SC in the complex was determined as 1:2 molarratio by comparing the peak integral intensities,even whenthey were mixed at 1:1 molar ratio.The macroscopic morphologies of aggregates have beencharacterized by SEM and TEM,with the typical imagesshown in Fig.2.Lots of fiber-like aggregates can be clearlyseen with the length ranging from several hundred microme-ters to several millimeters,and the width of 100-200 nm.It isobvious that the fibrous aggregates are not crystalline but softthreads as reflected by the entanglement of them and also thenot clear edges under TEM.The thermal stability of such 12-2-12/SC complex aggregateshas been evaluated by DSC and TGA measurements.As shownin Fig.3a,there are two main endothermic peaks at about 110and 210 C in the DSC curve,corresponding possibly to thecollapse of alkyl chains packing and the solid to liquid phasetransition.24,25The TGA profile shown in Fig.3b indicates a two-step degradation of aggregates,about 39%of the complex be-ing decomposed at 230 C and the other at 390 C.It is indicat-ed that the degradation at 230 C is from 12-2-12,which is con-Fig.11H NMR spectra of 12-2-12/SC complexes andstarting materials in CD3OD solution(a)12-2-12;(b)SC;(c,d)12-2-12/SC complexes obtained at 12-2-12:SC mixingmolar ratios of 1:1 and 1:2,respectively.The asterisks denote the solvent peaks.Fig.2SEM(a-c)and TEM(d)images of 12-2-12/SC aggregates2417Acta Phys.-Chim.Sin.2013Vol.29sistent with the Geminisurfactantsmass percentage(36%).This also supports that our assembled complex is comprised by12-2-12 and SC in 1:2 molar ratio.26As for the decompositionat 390 C,it is perhaps caused by the carbonization pyrolysisof these organic components.To explore the structure of produced nanofibers,the XRDanalyses have been done respectively for the ionic interactedcomplex and the individual building blocks with the resultsshown in Fig.4.Not the same XRD patterns partially supportthe formation of ionic complexes.The main Bragg peaks ofcomplexes are observed mainly at 24.0,7.0,8.6,18.0,21.3 for the complex aggregates,corresponding to lattice spac-ings of 2.2,1.25,1.03,1.49,0.42 nm,respectively.The inter-digitation of cholate ring skeletons and their various face-to-face or back-to-back interactions are responsible for latticespacings lower than 1.25 nm.The value of 2.2 nm,however,equals approximately to the length of 12-2-12 alkyl chain plusthat of cholate steroids skeleton.Therefore,the nanofibersshould be resulted from the organized packing of 12-2-12/SCionic complexes.3.1.2VesiclesIt is interesting to find that all other BS molecules except forSC,could form vesicles with suitable m-s-m counterparts.Take12-3-12/SDC ionic complex system as the example,the aggre-gate appearance and morphology images by visual and TEMobservations at different molar ratios are shown in Fig.5,where the concentration of SDC was fixed at 2 mmolL-1in allsamples.With the 12-3-12/SDC molar ratio above 1:1,the mix-ture solution is clear and transparent(Fig.5a1).Increasing theSDC amount,however,leads the mixture solution to exhibit aslightly transparent appearance with opalescence(Fig.5a2).Acloudy suspension is observed at a 1:2 molar ratio of 12-3-12/SDC(Fig.5a3).Further increase of SDC will make the mixturesolution become transparent again(Fig.5(a4-a6).As shown inFig.5(b2-b5)for TEM images using uranyl acetate as a nega-tive staining agent,closed spherical vesicles with hundreds ofnanometers in diameter could be observed.It is also noted thatthe vesicle diameter becomes smaller with increasing SDCamount.These vesicles are basically with a single core,but thewall seems multi-lamellar from the rough observations.3.2Aggregate formation mechanismsWhat factors dominate the final aggregate morphology?Based on the above mentioned experimental results,two possi-ble self-assembly mechanisms are proposed respectively fornanofiber and vesicle formation.Fig.3DSC(a)and TGA(b)curves for 12-2-12/SC complex aggregatesFig.4XRD patterns of produced nanofibers(12-2-12/SC)and individual starting materialsFig.5Visual images(a1-a6)and TEM images(b1-b6)of12-3-12/SDC systems at different molar ratiosn12-3-12/nSDC:(a1,b1)1:1,(a2,b2)1:1.5,(a3,b3)1:2,(a4,b4)1:4,(a5,b5)1:10,(a6,b6)1:20;cSDC=2 mmolL-12418ZHU Li-Jie et al.:Nanofibers and Vesicles Self-Assembled from Gemini Surfactant Complexes with Bile SaltsNo.11The 12-2-12/SC complex system is still taken as an exam-ple.As concluded from1H NMR results,the basic buildingblock for nanofibers is constructed by one 12-2-12 and two SCmolecules,which is mainly combined by the electrostatic inter-action.27,28Such supra-amphiphilic building blocks may self-assemble themselves along one-dimensional direction underthe cooperation of hydrogen bonding and hydrophobic interac-tions,27as schematically shown in Fig.6.The hydroxyl groupson steroid skeleton of SC provide connection points betweenadjacent SC molecules.The m-s-m molecule,however,pro-vides not only the hydrophobic part of building block,but alsoa linkage between two SC molecules.The electrostatic attrac-tion between SC and m-s-m results in poor amphiphilicity forbuilding block and makes it aggregate easily in bulk solutioneven at very low concentration(below 1 mmolL-1)and vari-ous BS/m-s-m molar ratios.The aggregation behavior of suchkind ionic self-assembled complex system is quite differentfrom those observed for the complex between SC and tradition-al surfactants,natural lipids,or double chain surfactants.As for the formation of vesicles,the construction of basicbuilding block has a similar mechanism due to the strong elec-trostatic attraction between m-3-m and SDC,as shown inFig.7.However,the increased CH2 group in spacer and re-duced hydroxyl groups on steroid skeleton of SDC would