Modification and dispersion of multi-walled carbon nanotubes in water

In the present work, the decorated purified raw multi-walled carbon nanotubes (R-MWCNTs) were obtained by chemical modification (CM) by treatment with concentrated sulfuric acid and concentrated nitric acid mixture with a certain volume ratio of 1: 3. The R-MWCNTs and CM-MWCNTs samples were investigated by X-ray Diffraction (XRD) analysis and Fourier transform infrared spectroscopy (FT-IR). The prepared MWCNTs were homogeneously dispersed in water using a commercial surfactant (Polyvinyl pyrrolidone, (PVP)) and ultra-sonication. The dispersion of MWCNTs was obtained by UV-Vis analysis. The results show that chemical modification purified MWCNTs and more effective functional groups were attached on the surface of MWCNTs. Meanwhile, R-MWCNTs and CM-MWCNTs were uniformly distributed in aqueous PVP solution and the dispersion of CM-MWCNTs in water was better.     

Debundling of multiwalled carbon nanotubes in N,N-dimethylacetamide by polymers

Structure and properties of the dispersions of multiwalled carbon nanotubes (MWCNTs) in N,N-dimethylacetamide (DMAc) with different dispersing polymers: polyvinylpyrrolidone (PVP), poly(ethyleneoxide), triblock copolymers poly(ethyleneoxide)-b-poly(propyleneoxide)-b-poly(ethyleneoxide) (Pluronic F127 and Pluronic F108), ethylenediamine tetrakis(ethoxylate-b-propoxylate) tetrol, and ethylenediamine tetrakis(propoxylate-b-ethoxylate) tetrol (Tetronic) of different molecular weights were studied. All studied polymers were shown to be able to disperse MWCNT in DMAc, and MWCNT dispersions appear free of aggregates by visual inspection even after 3 months of keeping at room temperature. Dispersions were characterized by UV–VIS absorption spectroscopy and dynamic light scattering measurements. PVP was found to be the best dispersing polymer for MWCNT in DMAc. It was shown that the yield of the dispersed MWCNT and the average particle size of the MWCNT in DMAc depend on the chemical nature, molecular weight of the dispersing polymer, and solvent quality. The difference in dispersive capacity of the studied polymers is attributed to different dispersion mechanisms for PVP (“polymer wrapping” model) and for other studied dispersing polymers (“loose adsorption” model), which have different efficiencies in DMAc. It was revealed that an increase of dispersing polymer (PVP) concentration at the range of 4.7–37.6 g l^?1 results in an average particle size enlargement and MWCNT final concentration reduction.     

Optimizing functionalization of multiwalled carbon nanotubes using sodium lignosulfonate

Multiwalled carbon nanotubes (MWCNTs) were functionalized with sodium lignosulfonate (SLS) at various SLS/MWCNT ratios, and the solubility of the functionalized MWCNTs was examined using ultraviolet-visible (UV-Vis) spectroscopy. Then, the effects of SLS on the dispersion and conductivity of MWCNTs were investigated. A calibration curve was constructed to measure the concentration of MWCNTs in water using the absorbance measured from UV-Vis spectroscopy. Using the curve, the change in the functionalized MWCNT concentration was investigated as a function of time. The results showed that the solubility of the MWCNTs did not increase significantly with further increases in SLS after the appropriate amount of SLS was employed. Excessive use of SLS rather decreased the conductivity of functionalized MWNTs. Also, the solubility of MWCNTs was influenced by dispersing method even when the same amount of SLS was used. Our method could functionalize the MWCNTs with a small amount of SLS, and the solution could remain stable for lengthy periods of time.     

Surfactant--polymer aggregates formed by sodium dodecyl sulfate, poly(N-vinyl-2-pyrrolidone), and poly(ethylene glycol)

The interaction of sodium dodecyl sulfate (SDS) in aqueous solution with poly(N-vinyl-2-pyrrolidone) (M(w) = 55,000 g/mol) in the presence of poly(ethylene glycol) (M(w) = 8000 g/mol) is investigated by electrical conductivity, zeta potential measurements, viscosity measurements, fluorescence spectroscopy, and small-angle X-ray scattering (SAXS). The results indicate that SDS-polymer interaction occurs at low surfactant concentration, and its critical aggregation concentration is fairly dependent on polymer composition. The polymer-supported micelles have average aggregation numbers dependent on surfactant concentration, are highly dissociated when compared with aqueous SDS micelles, and have zeta potentials that increase linearly with the fraction of PVP at constant SDS concentration. The analysis of the SAXS measurements indicated that the PVP/PEG/SDS system forms surface-charged aggregates of a cylindrical shape with an anisometry (length to cross-section dimension ratio) of about 3.0.     

Facile synthesis of the necklace-like graphene oxide-multi-walled carbon nanotube nanohybrid and its application in electrochemical sensing of Azithromycin

A novel electrochemical platform was designed for the determination of Azithromycin (Azi), a widely used macrolide antibiotic, by combining the hydrophilic properties of graphene oxide (GO) and the excellent electronic and antifouling properties of multi-walled carbon nanotubes (MWCNTs). Stable MWCNTs aqueous dispersion has been prepared using GO nano-sheets as surfactant and the obtained GO-MWCNTs nanohybrid was characterized by UV–vis spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, transmission electron microscopy and electrochemical impedance spectroscopy, which confirmed that GO nano-sheets were attached onto the wall of MWCNTs to form a necklace-like structure. Electrochemical results obviously reveal that the oxidation peak currents of Azi obtained at the GC electrode modified with GO-MWCNTs hybrid are much higher than those at the MWCNTs/GC, GO/GC and bare GC electrodes. Under optimized conditions, the anodic peak current was linear to the concentration of Azi in the range from 0.1 to 10 μM with the detection limit of 0.07 μM. To further validate its possible application, the proposed method was successfully used for the determination of Azi in pharmaceutical formulations with satisfactory results.     

Studies on the interactions between anionic surfactants and polyvinylpyrrolidone: Surface tension measurement, 13C NMR and ESR

 The interaction between anionic surfactants and polyvinylpyrrolidone (PVP) are investigated using ¹³C NMR, ESR spectroscopy and surface tension measurements at the air/water interface. The behavior of single-chained surfactant, sodium dodecyl sulphonate (AS), is compared with that of the double-chained surfactant, sodium bis(2-ethylhexyl) phosphate (NaDEHP). The results showed that a surfactant–polymer complex of “necklace and head structure” is formed in AS aqueous solutions in the presence of PVP due to the hydrophobic interaction between PVP and AS. The AS micelles nucleate on the polymer hydrophobic sites, and the mobility of the AS head groups is not affected. But, for NaDEHP surfactant, it was found that PVP is little effective in influencing the monomer–micelle equilibrium and no surfactant– polymer complex formed in the NaDEHP aqueous solution. Received: 8 May 1996 Accepted: 14 August 1997     

Photoisomerization of merocyanine 540 in polymer-surfactant aggregate

Photoisomerization of merocyanine 540 (MC540) in a polymer-surfactant aggregate is studied using picosecond time resolved emission spectroscopy. The aggregate consists of the polymer, poly(vinylpyrrolidone) (PVP) and the surfactant, sodium dodecyl sulphate (SDS). With increase in the concentration of SDS in an aqueous solution of MC540 containing PVP, the emission quantum yield and lifetime of MC540 increase markedly. This indicates marked retardation in the nonradiative photoisomerization process of MC540, when it binds to the polymer-surfactant aggregate. The critical association concentration of SDS for binding to PVP has been found to be 0.5 mM. This is about 16 times lower than the CMC of SDS in pure water (8 mM).     

Self-assembled Gemini surfactant film-mediated dispersion stability

The force-distance curves of 12-2-12 and 12-4-12 Gemini quaternary ammonium bromide surfactants on mica and silica surfaces obtained by atomic force microscopy (AFM) were correlated with the structure of the adsorption layer. The critical micelle concentration was measured in the presence or absence of electrolyte. The electrolyte effect (the decrease of CMC) is significantly more pronounced for Gemini than for single-chain surfactants. The maximum compressive force, F(max), of the adsorbed surfactant aggregates was determined. On the mica surface in the presence of 0.1 M NaCl, the Gemini micelles and strong repulsive barrier appear at surfactant concentrations 0.02-0.05 mM, which is significantly lower than that for the single C(12)TAB (5-10 mM). This difference between single and Gemini surfactants can be explained by a stronger adsorption energy of Gemini surfactants. The low concentration of Gemini at which this surfactant forms the strong micellar layer on the solid/solution interface proves that Gemini aggregates (micelles) potentially act as dispersing agent in processes such as chemical mechanical polishing or collector in flotation. The AFM force-distance results obtained for the Gemini surfactants were used along with turbidity measurements to determine how adsorption of Gemini surfactants affects dispersion stability. It has been shown that Gemini (or two-chain) surfactants are more effective dispersing agents, and that in the presence of electrolyte, the silica dispersion stability at pH 4.0 can also be achieved at very low surfactant concentrations ( approximately 0.02 mM).     

Wet-grinding assisted ultrasonic dispersion of pristine multi-walled carbon nanotubes (MWCNTs) in chitosan solution

Ultrasonication is often used to disperse nano-particles in aqueous solution. However, a good dispersion of nano-particles in aqueous solution is not always achieved, due to the fact that incoming ultrasonicwaves in liquid are usually reflected and damped at the gas/liquid interface. In this work, we report a so-called wet-grinding assisted ultrasonication (GU) method, in which wet-grinding of multi-walled carbon nanotubes (MWCNTs) in chitosan solution is carried out before ultrasonication. The dispersions of MWCNTs were characterized by visual comparison, UV/vis spectroscopy, and scanning electron microscopy (SEM). The results demonstrate that the dispersion quality of chitosan/MWCNT suspension prepared by wet-grinding assisted ultrasonication is much better than that by ultrasonication or wet-grinding alone. It was found that wet-grinding could improve the water wettability of MWCNTs and eliminate the barrier of air layer around MWCNTs to ultrasonicwaves. Meanwhile, the composite from the chitosan/MWCNTs suspension prepared by GU method has an obvious improvement in mechanical property compared to pure chitosan. This simple method for integrating MWCNTs and biocompatible chitosan into a homogeneous dispersion may have great potential application in biotechnology, such as preparing composite materials for medicine, bio-fiber, biosensor, antibacterial coating, and cell cultivation.     

Purification and dispersibility of multi-walled carbon nanotubes in aqueous solution

In this paper, the pristine multi-walled carbon nanotubes (P-MWCNTs) were purified either by the high temperature treatment (HT-MWCNTs) or by concentrated acid treatment (CA-MWCNTs). The HT-MWCNTs were prepared by heating at 500°C, while the CA-MWCNTs were treated by the mixture of concentrated nitric and sulfuric acids taken in a volume ratio of 3: 1. Ultrasonic processing and surfactants were utilized to achieve homogenous MWCNTs suspensions. The HT-MWCNTs and CA-MWCNTs were characterized by thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). Among these three MWCNTs, the prepared homogeneously dispersed MWCNTs suspensions were characterized by UV–Vis absorbency and transmission electron microscopy (TEM). Finally, the dispersion mechanism was discussed. The results showed that both high temperature treatment and concentrated acid treatment can be used for purification of the P-MWCNTs, removing the amorphous carbon and other impurities. In these suspensions, the purified MWCNTs showed a better dispersibility in aqueous solution. The high temperature treatment was a kind of physical purification treatment method and it just burned the amorphous carbon away and strengthened the structure of MWCNTs, while the concentrated acid treatment was a chemical purification treatment method and this chemical treatment method grafted more effective groups to improve the dispersibility of MWCNTs.     

Electrically conducting electrospun silk membranes fabricated by adsorption of carbon nanotubes

We present a simple method of obtaining electrically conducting electrospun silk non-woven membranes consisting of nanofibers with multi-walled carbon nanotubes (MWCNTs) adsorbed on their surface. Nanofibrous membranes with fibroin diameters of 460 ± 40 nm were formed from aqueous Bombyx mori fibroin solution by electrospinning. The MWCNTs adhered well to the surface of the highly porous silk nanofibrous membranes when Triton X-100 was used as the surfactant for the dispersion of the MWCNTs in aqueous media. The electrical conductivity of the membranes was 2.4 × 10⁻⁴ S/cm due to the presence of the MWCNTs on their surface. In addition, the strong interaction between the MWCNTs and nanofibers keeps them from separating each other, even after ultrasonication. The combination of the high conductivity of the membranes and the simple process used to fabricate them could lead to significant advances in the development of new materials, such as electromagnetic interference shielding or electrostatic dissipation membranes.     

Surfactants for CNTs dispersion in zirconia-based ceramic matrix by sol–gel method

Zirconium oxide is a ceramic material widely studied due to its mechanical and electrical properties that can be improved with the use of carbon nanotubes (CNTs) as reinforcement. The synthesis of CNT/zirconia composites by sol–gel method is still very scarce, due to the hydrophobic nature of the CNTs, being their dispersion in aqueous medium an intrinsic difficulty to the synthesis. In this work, we present a sol–gel synthesis for MWCNTs/zirconia composites, where two kinds of surfactants, sodium and ammonium stearates dissolved in water (1 g/100 mL), were used as dispersant agents for multiwall carbon nanotubes (MWCNTs). They are cheap and easy to prepare, and were very effective in dispersing the MWCNTs. Different quantities of MWCNTs (up to 5 wt%) were added in the solution of stearate/water and this solution with the highly dispersed MWCNTs was added to the zirconia sol–gel, producing composites of MWCNTs/zirconia with different concentrations of MWCNTs. All the powders were heat treated at 300 and 500 °C and the powder characterization was performed by transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and infrared spectroscopy (FTIR). The composite MWCNTs/zirconia remained amorphous at 300 °C and presented a tetragonal phase at 500 °C with an average grain size of about 20 ± 3 nm, determined by the Scherrer equation from the XRD patterns. For these crystalline samples, TEM images suggest a more effective interaction between MWCNTs with ZrO₂ matrix, where it can be observed that the carbon nanotubes are fully coated by the matrix.     

Application of modified multiwall carbon nanotubes as a sorbent for zirconium (IV) adsorption from aqueous solution

Modified multiwall carbon nanotubes (MWCNTs) by nitric acid solution were used to investigate the adsorption behavior of zirconium from aqueous solution. Pristine and oxidized MWCNTs were characterized using nitrogen adsorption/desorption isotherm, Boehm’s titration method, thermogravimetry analysis, transmission electron microscopy and Fourier transform infrared spectroscopy. The results showed that the surface properties of MWCNTs such as specific surface area, total pore volume, functional groups and the total number of acidic and basic sites were improved after oxidation. These improvements are responsible for their hydrophobic properties and consequently an easy dispersion in water and suitable active sites for more adsorption of zirconium. The adsorption of Zr(IV) as a function of initial concentration of zirconium, contact time, MWCNTs dosage, HCl and HNO₃ concentration and also ionic strength was investigated using a batch technique under ambient conditions. The experimental results indicated that sorption of Zr(IV) was strongly influenced by zirconium concentrations, oxidized MWCNTs content and acid pH values. The calculated correlation coefficient of the linear regressions values showed that Langmuir model fits the adsorption equilibrium data better than the Freundlich model. Kinetic data of sorption indicated that equilibrium was achieved within 60 min and the adsorption process can be described by the pseudo second-order reaction rate model. Based on the experimental results, surface complexation is the major mechanism for adsorption of Zr(IV) onto MWCNTs. Also, Study on the desorption process of zirconium showed that the complete recovery can be obtained using nitric or hydrochloric acids of 4 M.     

Radiation-induced grafting of multi-walled carbon nanotubes in glycidyl methacrylate–maleic acid binary aqueous solution

With the aim to improve the compatibility between multi-walled carbon nanotubes (MWCNTs) and nylon-6, purified MWCNTs (p-MWCNTs) were grafted successfully with glycidyl methacrylate–maleic acid in aqueous solution using a single-step radiation method. The chemical structure and morphology of grafted p-MWCNTs (g-MWCNTs) was investigated by micro-FTIR, Raman spectroscopy and transmission electron microscopy. The prepared nylon-6/g-MWCNTs composite has higher mechanical strength and heat distortion temperature due to improved dispersion and compatibility than those of nylon-6/p-MWCNTs.     

Structure and properties of aqueous dispersions of sodium dodecyl sulfate with carbon nanotubes

The dispersing action of the surfactant (sodium dodecyl sulfate, SDS) on the carbon nanotubes (CNT) in aqueous medium has been studied. Electron microscopy, molecular docking, NMR and IR spectroscopies were applied to determine the physical-chemical properties of CNT dispersions in SDS—water solutions. It was established that micellar adsorption of the surfactant on the surface of carbon material and solubilization of SDS in aqueous medium contribute to improving CNT dispersing in water solutions. It was shown that the non-polar hydrocarbon radicals of a single surfactant molecule form the highest possible number of contacts with the graphene surface. Upon increase of the SDS in solution these radicals form micelles connected with the surface of the nanotubes. At the sufficiently high SDS concentration the nanotube surface becomes covered with an adsorbed layer of surfactant micelles. Water molecules and sodium cations are concentrated in spaces between micelles. The observed pattern of micellar adsorption is somewhat similar to a loose bilayer of surfactant molecules.     

聚乙烯基吡咯烷酮修饰多壁碳纳米管的研究

采用超声波辅助技术,研究了两亲性聚合物聚乙烯基吡咯烷酮(PVP)修饰多壁碳纳米管(MWNTs)的效果、作用机理及影响因素.研究结果表明在适合的条件下两亲性聚合物PVP可以被引入到多壁碳纳米管表面,修饰后的MWNTs在DMF、乙醇和水等溶剂中具有良好的分散性.通过红外光谱(FTIR)和拉曼光谱(Raman)分析表明,两亲性聚合物与MWNTs之间产生了化学接枝作用,高分辨透射电镜分析表明两亲性聚合物不均匀地存在于MWNTs的表面和端部.两亲性聚合物的浓度对接枝量的影响不大,但超声波作用时间对MWNTs表面两亲性聚合物PVP的接枝量有较大的影响,在超声时间为4h时接枝量最大.两亲性聚合物修饰效果不同于表面活性剂,采用表面活性剂十二烷基苯磺酸钠(SDBS)修饰的MWNTs经过洗涤过滤后,不能重新溶于水中,而两亲性聚合物PVP修饰的MWNTs可以重新溶解.     

13G NMR and ESR studies on the association between sodium 3-dodecy I ether-2-hydroxypropy1-1-sulfonate and polyvinylpyrrolidone

The surfactant, sodium 3-dodecy] ether-2-hydroxypropyl-l-sulfonate(SDEHS) was synthesized. The association and standard free energy of formation of the complex between sodium 3-dodecyl etheT-2-hydroxypropyl-l-sulfonate(SDEHS) and polyvinyl-pyrrolidone(PVP) in an aqueous solution have been investigated using C NMR, ESR spectra, and surface tension measurements at the air/ water interface. ¹³C NMR and ESR spectra all indicate that the basic structure of the complex is a micelle-like aggregate, the SDEHS molecules assembling on the methylidync(a) the methylene(α ) carbon in the backbone, and the methyleneβ carbon attached to the nitrogen of PVP molecules, and shield hydrocarbon groups on the surface of the micelle from contacting with water. The measurement results ofsurface tensions show that the amount of surfactant bound to the polymer are linear function of the polymer concentrations ( φ,WI% )i. e( c₂ -c₁ and the miceltization in the presence of PVP occurs at a lower concentration than the critical micelle concentration of SDEHS. The effectiveness of PVP in lowering the free energy of formation of the surfactant aggregates in aqueous solutions increases with the concentrations of PVP.     

Protecting nanoscaled non-oxidic particles from oxygen uptake by coating with nitrogen-containing surfactants

To suppress the reactivity of nanoscaled non-oxidic powders of titanium nitride (TiN) and silicon carbonitride (SiCN) against hydrolysis and oxidation, chemical surface modification with nitrogen-containing surfactants was investigated. Among these surfactants, long-chain primary amines, ethylenediamines, guanidines, nitriles, isocyanates, and succinimides were examined. Thermogravimetry, elemental analysis, and behavior against the water-vapor adsorption of the modified particles were used as methods to estimate the protective capacity of the organic coating material. The best results were obtained by using the long-chain amines and octadecylisocyanate, which were indicated by a significant shift of the powder oxidation toward the higher temperatures and an increase of the particle hydrophobicity. A long-chain succinimide was found to be the most effective in dispersing nanoscaled TiN in organic media. Preparation of a stable aqueous dispersion without significant changes in the elemental composition of the powder was achieved by the application of an ionic surfactant to the surface-modified particles.     

Electrochemical reduction of oxygen on double-walled carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions

The oxygen reduction reaction has been investigated on double-walled carbon nanotube (DWCNT) modified glassy carbon (GC) electrodes in acid and alkaline media using the rotating disk electrode (RDE) method. The surface morphology and composition of DWCNT samples was examined by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Aqueous suspensions of DWCNTs were prepared using Nafion and non-ionic surfactant Triton X-100 as dispersing agents. The RDE results indicated that the DWCNT modified GC electrodes are active catalysts for oxygen reduction in alkaline solution. In acid media DWCNT/GC electrodes possess poor electrocatalytic properties for O₂ reduction which indicates lack of metal catalyst impurities in the DWCNT material studied. The oxygen reduction behaviour of DWCNTs was similar to that of multi-walled carbon nanotubes (MWCNTs) observed in our previous studies.     

Dispersion of multi-wall carbon nanotubes by an ionic liquid-based polyether in aqueous solution

Multi-wall carbon nanotubes (MWCNTs) can be effectively dispersed by an ionic liquid-based polyether, poly(1-glycidyl-3-methylimidazolium chloride) (PGMIC) in aqueous solution. The amount of dispersed MWCNTs increases with the increasing of PGMIC concentration, and then decreases. Reaggregation of MWCNTs is observed when PGMIC exceeded the optimal concentration, which may be due to the conformational change of PGMIC molecules around MWCNT. The ultrasonic dispersion method is better than stirring method in the PGMIC solution. Furthermore, the acidic solution is convenient to prepare stable MWCNTs suspensions. Through the characterizations of ultraviolet–visible–near infrared, thermogravimetric analysis and Fourier transform infrared, it can be concluded that electrostatic repulsions, hydrophobic effect, n–π, and cation–π interactions played important roles in the dispersion of MWCNTs.