Vesicle Solubilization Of Fullerenes And Stable Dispersion Of Carbon Nanotubes In Salt-Free Cationic/Anionic Surfactant System

As novel nanostructures,fullerenes and carbon nanotubes are known to have many unique properties and great potential applications.However,their poor solubility in water together with their propensity to form large aggregates has long been an obstacle for further study and practical applications.To solve these problems,the means originating from colloid and interface science especially those from surfactant science are frequently adopted.In this doctoral dissertation,the detailed phase behavior of tetradecyltrimethylammonium hydroxide(TTAOH)/lauric acid(LA)surfactant mixture has been investigated both in aqueous and non-aqueous media.After that.this mixed surfactant system is selected to solubilize fullerene C₆₀and C₆₀/C₇₀mixture as well as to disperse carbon nanotubes.In addition to improve the solubility of pristine fullerenes and carbon nanotubes in aqueous media,we have also investigated the properties and aggregation behavior in water of some C₆₀amphiphiles given by Andreas Hirsch from Germany.The interactions between these C₆₀amphiphiles and traditional surfactants as well as carbon nanotubes are also discussed.The outline and contents of this doctoral dissertation are as follows:Chapterâ… is a brief introduction of the research background of this work,in which the history and recent progress in surfactant science as well as fullerene chemistry are reviewed from a worldwide angle of view.The objective and the scientific significance of this doctoral dissertation are also pointed out at the end of this part.In Chapterâ…¡,the phase behavior and rheological properties of TTAOH/LA/H₂O system are investigated in detail.It is found the system exhibits a rich phase behavior with the variation of molar ratio of LA to TTAOH(n_LA/n_TTAOH)and total surfactant concentration (C_TTAOH+LA)at 25℃.In TTAOH rich side,an isotropic L_l phase(micellar phase)forms at n_LA/n_TTAOH＜0.87.whose viscosity changes significantly with solution parameter.As n_LA/n_TTAOHincreases from the border of L_l phase,birefringent L_αphase begins to form.In certain region,L_l phase and L_αphase coexist with birefringent L_αphase at the top.An obvious interface can be found between bottom L_l phase and top L_αphase and the volume ratio of L_αphase to L_l phase varies with solution parameter.Around equimolar mixing ratio of LA to TTAOH,bluish vesicular phase is observed which extends to the region where LA is in slight excess.At low concentration of total surfactants,the system tends to macroscopically phase separate and this phenomenon will be more obvious at higher LA content.The rich phase behavior of TTAOH/LA/H₂O system with variation of solution parameter is a macroscopic reflection of aggregate growth and evolution among different kinds which occurs at microscopic length scale.The growth and evolution of aggregates also induce significant rheological property changes of the system.Take the series of samples with c_TTAOH+LA=25 mg/mL for example,a Newton fluid region,a shear-thickening region,a worm-like micellar region which exhibits Maxwell fluid character and a gel-like vesicular region are observed in turn with increasing n_LA/n_TTAOH. We have investigated the detailed rheological properties by means of both steady state and oscillatory measurements within each region as a function of total surfactant concentration and temperature.It is found micelle growth is greatly suppressed at higher temperatures while temperature rise has a much less influence on vesicular phases.The rheological property of the L_l and L_αphase within the two-phase region has been carried out as a whole as well as separately.The textures of the birefringent L_αphase are also investigated by a polarized microscope as a function of shear and time.In Chapterâ…¢,the phase behavior of TTAOH/LA mixed system is investigated in various organic solvents.After monitoring the experimental conditions,reverse vesicles with dimensions usually at micrometer scale are successfully constructed in several non-polar organic solvents including toluene,tert-butyl benzene,cyclohexane and n-heptane.The formation of reverse vesicles occurs within a rather narrow range of surfactant composition and a small amount of water is always necessary.The reverse vesicular phases show a blue color against room light and exhibit strong birefringence under polarized microscope.It is found the property and stability of reverse vesicles depends greatly on the selected organic solvent and temperature.In toluene,tert-butyl benzene and cyclohexane,reverse vesicles can form spontaneously and show high stability at room temperature.An increase in temperature can make the composition of surfactants within which reverse vesicles are constructed move to LA-rich side and vice versa.In n-heptane,however,the formation of reverse vesicles usually needs external energy and these reverse vesicles are not stable at room temperature.Instead.they will disappear from the top upon time and centrifugation.Ultimately macroscopic phase separation is observed,which will be accelerated at higher temperatures,and a gel-like phase forms at the bottom.Reverse vesicles may not be destroyed if suitable amount of aqueous solutions containing certain inorganic ions such as Zn²⁺and S^2-is added,but adding too much will of course destroy the structure of reverse vesicles.By designing experiments with great care,reverse vesicles can be used as nano-reaction center for inorganic particle synthesis.Investigation toward this direction has also been carried out and some interesting results have been obtained.Based on the knowledge of TTAOH/LA mixed system both in aqueous and non-aqueous media obtained from the former two chapters,in Chapterâ…£we investigate the solubilization of fullerene C₆₀and C₆₀/C₇₀mixture in TTAOH/LA/H₂O system via a medium step where an organic solvent toluene is involved.The changes of spectral features of fullerenes before and after solubilization as well as fullerene solubilization induced phase behavior changes of the mixed system have also been clarified.It is found the solubilization capability is considerable high for the vesicular phases especially those formed at n_LA/n_TTAOH=1.For example,the 50 mmol/L vesicular phase at n_LA/n_TTAOH=1 Can solubilize C₆₀up to 0.588 mg/mL without macroscopic phase separation.This is already higher than the solubilization maximum of C₆₀in other surfactant systems reported in literatures,which is 0.4 mg/mL.Further investigations indicate the L_αphases also have considerable capability to solubilize fullerenes while the L_l phase especially the worm-like micellar phases can not effectively solubilize fullerenes.In LA rich side.the system loses the ability to solubilize fullerenes gradually with increasing n_LA/n_TTAOH.We have investigated the phase behavior of TTAOH/LA/H₂O system with fullerenes solubilized as a function of total surfaetant concentration.It is found precipitate will form when the samples are diluted by water and the amount of precipitate depends on the kind of phase as well as the extent of dilution.Compared with vesicular or lamellar phases. micellar phases with fullerenes solubilized are more stable against dilution. Thermogravimetric analysis indicates the precipitate is a mixture of fullerenes and surfactants rather than fullerenes only.UV-vis-NIR measurements indicate the characteristic absorptions of fullerenes disappeared after solubilization and an Einstein shift occurs for the whole absorption curve together with an interesting color change of the samples.Further investigations have revealed the color change of the samples and the disappearance of the characteristic absorptions of fullerenes are influenced little by light. but depends greatly on the content of TTAOH in total surfactants.So it seems that electron transfer reactions occur between fullerenes and OH^- during solubilization together with an aggregation of fullerene molecules.In Chapterâ…¤,the dispersion of carbon nanotubes(CNTs)in TTAOH/LA/H₂O system is investigated at the presence of two kinds of nonionic polymers.Pluronic F127 and polyvinylpyrrolidone(PVP).The rheological property changes of TTAOH/LA/H₂O system induced by polymer and CNTs addition are investigated in detail,respectively.The UV-vis-NIR measurements have also been carried out on the TTAOH/LA/polymer/CNTs mixed samples.It is found that the addition of polymers greatly suppresses aggregate growth both in micellar region and vesicular region,which can be seen simply by shaking. and visual inspection.This conclusion gains further proof from rheological measurements which are carried out on the shear-thickening region,the worm-like micellar region and the vesicular region.At the same weight fraction of polymers,the influence of F127 is much more obvious than that of PVP,which is also consistent with the results of surface tension measurements.This is due to the strong hydrophobic interaction as well as hydrogen bond formation between F127 and TTAOH/LA/H₂O system.TTAOH/LA/H₂O system itself can not disperse CNTs effectively.After addition of F127 or PVP.however. CNTs can be well dispersed into the bulk aqueous solutions.UV-vis-NIR measurements indicate the quality of the final dispersion is influenced greatly by the amount of added polymer.After introduction of CNTs into the matrix of ordered molecular assemblies such as worm-like micelles,the rheological properties of mixed system change obviously with a decrease in viscoelastic property and a rise in relaxation time.In the last chapter,we have investigated the properties and aggregation behavior in aqueous solutions of seven C₆₀amphiphiles with different molecular structures which are kindly given by Andreas Hirsch from Germany.The interaction between these C₆₀ amphiphiles and traditional surfactants as well as CNTs is also discussed.1 and 2 have 18 and 9 terminal carboxy groups as hydrophilic part,respectively,and are highly soluble in water.They belong to monoadducts of C₆₀derivatives and have a dendritic molecular structure.The aggregation behavior of 1 and 2 in water is not obvious as proved by transition electron microscopy(TEM)observations.UV-vis and dynamic laser light scattering(DLLS)measurements show,however,they can still form aggregates in water. This conclusion gains further proof from the fact that they can disperse MWNTs.It is found the aggregation behavior of 1 and 2 is different from each other due to their different hydrophilic parts.The aggregate formation of 1 and 2 can be greatly enhanced when a cationic surfactant,TTAOH is added.The[3:3]hexakisadducts of C₆₀amphiphiles 3-5 are star-like.They are also highly soluble in water and each has six terminal quartery ammonium headgroups together with different number of ethyloxide groups as hydrophilic part.Similar with 1 and 2,the aggregation behavior of 3-5 in water is not obvious,either.But they still exhibit moderate amphiphilic character in aqueous solutions as proved by surface tension measurements and can disperse CNTs which are rich in SWNTs.The[3:3]hexakisadducts of C₆₀amphiphiles 6 and 7 are also star-like.They have 6 and 18 terminal carboxy groups,respectively,and are almost insoluble in water but soluble in basic aqueous solutions.After being transformed into their corresponding sodium salts 6-Na and 7-Na,the solubility is greatly improved and they can be subsequently used to disperse MWNTs.Investigations have been carried out on the aggregation behavior of 1.2,6-Na and 7-Na in aqueous solutions containing highly concentrated salts and interesting fraetals are observed besides well-defined spherical aggregates.