Determination of the surface coverage of exfoliated carbon nanotubes by surfactant molecules in aqueous solution

To determine the surface coverage of exfoliated carbon nanotubes by surfactant molecules, we propose four experimental methods based on thermogravimetric analysis, UV-vis spectroscopy, surface tension measurements, and a variant of Maron's titration. We apply all four methods to aqueous mixtures of carbon nanotubes and the surfactant sodium dodecyl sulfate and consistently find a surface coverage of between 1.5 and 2 g of surfactant per gram of carbon nanotubes. This corresponds to slightly more than two SDS molecules per square nanometer, which is comparable to the known maximum packing density of SDS at the air-water interface.     

Understanding surfactant aided aqueous dispersion of multi-walled carbon nanotubes

Dispersions of multi-walled carbon nanotubes (MWNTs) assisted by surfactant adsorption were prepared for a number of ionic and non-ionic surfactants including sodium 4-dodecylbenzenesulfonate (NaDDBS), hexadecyl(trimethyl)azanium bromide (CTAB), sodium dodecane-1-sulfonate (SDS), Pluronic? F68, Pluronic? F127, and Triton? X-100 to examine the effects of nanotube diameter, surfactant concentration, and pH on nanotube dispersability. Nanotube diameter was found to be an important role in surfactant adsorption rendering single-walled carbon nanotube studies as unreliable in predicting MWNT dispersive behavior. Similar to other reports, increasing surfactant concentrations resulted in a solubility plateau. Quantification of nanotube solubility at these plateaus demonstrated that CTAB is the best surfactant for MWNTs at neutral pH conditions. Deviations from neutral pH demonstrated negligible influence on non-ionic surfactant adsorption. In contrast, both cationic and anionic surfactants were found to be poor dispersing aids for highly acidic solutions while, CTAB remained a good surfactant under strongly basic conditions. These pH dependent results were explained in the context of nanotube surface ionization and Debye length variation.     

Dispergation and modification of multi-walled carbon nanotubes in aqueous solution

Multi-walled carbon nanotubes (MWCNTs) are widely applied in development of composite materials. However, their properties are directly influenced by the degree of uniformity of dispersion of MWCNTs in the material’s matrix. In this paper, the dispersing of raw MWCNTs (R-MWCNTs) and decorated MWCNTs (D-MWCNTs) was studied in aqueous solution. The D-MWCNTs were obtained by chemical modification method by treatment of initial MWCNTs with the mixture of concentrated nitric and sulfuric acids (3: 1 vol/vol). To achieve a good dispersion of the MWCNTs, a method utilizing ultrasonic processing and surfactant (polyvinylpyrrolidone, PVP) was employed. MWCNTs were characterized by Fourier transform infrared spectroscopy (FT–IR) and X-ray diffraction (XRD). The prepared MWCNTs suspensions were investigated by UV spectroscopy, zeta potential measurements, surface tension and transmission electron microscopy (TEM). The D-MWCNTs have better dispersibility in aqueous solution; this attributed to the functional groups formed on their surface during chemical modification. The PVP surfactant in a certain concentration of 0.6 g/L has the maximum dispersing effect on MWCNTs in aqueous solution, the optimum concentration ratio of PVP and MWCNTs was 3: 1.     

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.     

Fluorophore and dye-assisted dispersion of carbon nanotubes in aqueous solution

DNA short oligo, surfactant, peptides, and polymer-assisted dispersion of single-walled carbon nanotube (SWCNTs) in aqueous solution have been intensively studied. It has been suggested that van der Waals interaction, π-π stacking, and hydrophobic interaction are major factors that account for the SWCNTs dispersion. Fluorophore and dye molecules such as Rhodamine B and fluorescein have both hydrophilic and hydrophobic moieties. These molecules also contain π-conjugated systems that can potentially interact with SWCNTs to induce its dispersion. Through a systematic study, here we show that SWCNTs can be dispersed in aqueous solution in the presence of various fluorophore or dye molecules. However, the ability of a fluorophore or dye molecule to disperse SWCNTs is not correlated with the stability of the fluorophore/dye-SWCNT complex, suggesting that the on-rate of fluorophore/dye binding to SWCNTs may dominate the efficiency of this process. We also examined the uptake of fluorophore molecules by mammalian cells when these molecules formed complexes with SWCNTs. The results can have potential applications in the delivery of poor cell-penetrating fluorophore molecules.     

Surface decoration and dispersibility of carbon nanofibers in aqueous surfactant solution

As a novel functional nanomaterial, the dispersion effect of carbon nanofibers (CNFs) has a significant influence on the application of CNFs in the composites. Two effective surfactants, methylcellulose (MC) and polycarboxylate superplasticizer, were used to analyze the dispersion of CNFs in aqueous solution. A method utilizing ultrasonic processing was employed to achieve a homogenous CNF suspension, and the dispersion effect was further characterized by the method of measuring ultraviolet absorbency (UV absorbency), zeta potential, surface tension and transmission electron microscopy (TEM) micrographs. The results show that the zeta potential and surface tension reach the saturation plateau at MC concentration and polycarboxylate superplasticizer concentration of about 0.4 and 0.8 g/L, respectively, which reflects that the optimum concentration ratio of MC to CNFs is 2: 1, and the optimum dispersing polycarboxylate superplasticizer to CNFs ratio of 4: 1 is required to achieve dispersions with maximum achievable dispersion of CNFs.     

Influence of polyvinyl pyrrolidone on the dispersion of multi-walled carbon nanotubes in aqueous solution

The sonication-driven suspensions of multi-walled carbon nanotubes (MWNTs) were prepared using the polyvinyl pyrrolidone as a dispersant. UV-visible absorbency, surface tension, zeta potential, adsorption isotherm and transmission electron microscopy, were used to characterize the homogeneity and stability of MWNTs suspensions with different concentrations of polyvinyl pyrrolidone. All results show that the optimum concentration of polyvinyl pyrrolidone in suspension is 0.5 g/L.     

Adsorption of roxarsone from aqueous solution by multi-walled carbon nanotubes

Molecularly imprinted particle for bisphenol A (BPA-MIP) was prepared using the surface molecular imprinting technique with a sol-gel process on the surface of silica nanoparticles. The dosages of diethylenetriaminepentaacetic acid (DTPA) as a functional monomer and teraethyl orthosilicate (TEOS) as a cross-linker were optimized, respectively. The prepared BPA-MIP was characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Fourier transform infrared spectrometer (FTIR), thermogravimetric analysis (TGA), and a standard Brunauer-Emett-Teller (BET) analysis. Moreover, the proper binding and selective recognition ability were also investigated by a single batch binding experiment. The equilibrium data fitted well to the pseudo-second-order kinetic and the Langmuir model for BPA binding onto BPA-MIP, respectively. The saturate binding capacity of BPA-MIP was found to be 30.26 μmol g(-1), which was three times higher than that of BPA non-molecular imprinted particle (BPA-NIP). The satisfactory results demonstrated that the obtained BPA-MIP showed an appreciable binding specificity toward BPA than similar structural compounds in water phase. The BPA-MIP could serve as an efficient selective material for determining or removing BPA from water environment.     

苯胺在碳纳米管上的吸附特征

苯胺是一种重要的有机化工原料,能通过皮肤和呼吸道进入人体而引起中毒,还会严重污染环境。目前,国内外处理含苯胺废水的方法主要有氧化法、萃取法、生化法、吸附法等。本文使用碳纳米管进行液相吸附除去苯胺。     

A study of the stability of aqueous suspensions of functionalized carbon nanotubes

The properties of aqueous suspensions of carbon nanotubes have been studied as depending on the conditions of their functionalization in a mixture of sulfuric and nitric acids. The elemental composition and contents of carboxyl, lactone, and hydroxyl groups in carbon nanotubes have been determined at different durations and temperatures of functionalization. The influence of functionalization conditions on the value of the electrokinetic potential of carbon nanotubes in aqueous suspensions and the nanotube solubility in water has been investigated. It has been found that the absolute value of the electrokinetic potential of nanotubes and their solubility in water increase with both the duration and temperature of functionalization due to a rise in the number of functional groups on their surface. The optimal regimes of functionalization of carbon nanotubes have been determined from the point of view of preserving their structure and stability in aqueous dispersions.     

Molecular perspective on diazonium adsorption for controllable functionalization of single-walled carbon nanotubes in aqueous surfactant solutions

Functionalization of single-walled carbon nanotubes (SWCNTs) using diazonium salts allows modification of their optical and electronic properties for a variety of applications, ranging from drug-delivery vehicles to molecular sensors. However, control of the functionalization process remains a challenge, requiring molecular-level understanding of the adsorption of diazonium ions onto heterogeneous, charge-mobile SWCNT surfaces, which are typically decorated with surfactants. In this paper, we combine molecular dynamics (MD) simulations, experiments, and equilibrium reaction modeling to understand and model the extent of diazonium functionalization of SWCNTs coated with various surfactants (sodium cholate, sodium dodecyl sulfate, and cetyl trimethylammonium bromide). We show that the free energy of diazonium adsorption, determined using simulations, can be used to rank surfactants in terms of the extent of functionalization attained following their adsorption on the nanotube surface. The difference in binding affinities between linear and rigid surfactants is attributed to the synergistic binding of the diazonium ion to the local "hot/cold spots" formed by the charged surfactant heads. A combined simulation-modeling framework is developed to provide guidance for controlling the various sensitive experimental conditions needed to achieve the desired extent of SWCNT functionalization.     

Removal of radiocobalt from aqueous solution by different sized carbon nanotubes

Batch adsorption technique was applied to study the adsorption of radiocobalt on multiwalled carbon nanotubes (CNTs) with deferent sizes. The aim of this work was to examine the effect of contact time, pH, solid content, foreign ions and CNT particle sizes on the removal of Co(II) ions from aqueous solutions by CNTs. The results indicated that the adsorption of Co(II) was strongly dependent on pH and the adsorption capacity was in inverse proportion to the particle sizes of CNTs. The adsorption of Co(II) was weakly affected by ionic strength and foreign ions. Ion exchange and surface complexation were the main adsorption mechanisms. The kinetics of Co(II) adsorption on CNTs was described well by pseudo-second-order model. The Langmuir and Freundlich models were applied to interpret the adsorption data. The results are important to understand the physicochemical behavior of Co(II) with CNTs, and for the application of CNTs in the preconcentration of radiocobalt from large volumes of aqueous solutions.     

Amino-functionalized multi-walled carbon nanotubes for removal of cesium from aqueous solution

Journal of Radioanalytical and Nuclear Chemistry - Amino-functionalized multi-walled carbon nanotubes (MWCNTs) were synthesized by a simple, cost-effective method using 3-aminopropyltriethoxysilane...     

Effect of oxidation treatment of multi-walled carbon nanotubes on the adsorption of pentachlorophenol from aqueous solution: Kinetics study

Multi-walled carbon nanotubes (MWCNTs) were oxidized using different oxidizing agents and the produced oxidized MWCNTs were characterized using different techniques. IR measurements showed the presence of carboxylic acid function groups especially for the MWCNTs oxidized with nitric acid and hydrogen peroxide. The adsorption of pentachlorophenol (PCP) to pristine and oxidized multi-walled carbon nanotubes (MWCNTs) has been studied. The results showed that the oxidation of the MWCNTs decreased their abilities to adsorb PCP compared with the pristine MWCNTs. The adsorption was studied kinetically and the results showed that the adsorption process occurs in two different steps. The first step involves the transfer of PCP to the surface of the oxidized MWCNTs, which was very fast due to the diffusion of PCP from the liquid phase to the solid phase. This step followed by a second slower step of adsorption could be due to intra-particle diffusion.     

磁性碳纳米管对水中孔雀石绿的吸附性能研究

（1）以磁性碳纳米管吸附处理孔雀石绿溶液,考察了吸附剂用量、孔雀石绿浓度以及温度等对吸附平衡的影响。结果表明,该吸附不受溶液pH值的影响,符合准二级动力学方程（R2＞0.999）。不同温度下,该吸附过程满足Langmuir方程（R2>0.97）和Freundlich方程（R2>0.98）,qmax、KF随温度升高而变大。D-R等温方程拟合结果表明该吸附的机理是以化学吸附为主。热力学参数计算结果表明,该吸附是一个熵增的自发吸热过程,温度越高越利于吸附的进行。     

Effect of initial concentration on stability of panipenem in aqueous solution

The effect of initial panipenem (CAS No. 87726-17-8) concentration on its degradation in an aqueous solution was investigated. The degradation of panipenem followed pseudo-first-order kinetics at all the pH values tested. However, in an acidic solution, the degradation rate increased with the initial panipenem concentration. On the other hand, in an alkaline solution, the degradation rate was not affected by its initial concentration. In an acidic solution, the plots of the first-order rate constants versus initial panipenem concentrations showed a linear relationship. Theoretically, the first-order rate constant is independent of the initial concentration, and therefore, the results suggested unusualness. We investigated the results obtained under acidic conditions in detail to find a very complex reaction mechanism: panipenem and its degradation products are factors causing the unusual increase in the degradation rate. Moreover, it was shown that the dissociation of the carboxyl group played an important role in the degradation of panipenem.     

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.     

Dispersion of single-walled carbon nanotubes in aqueous solution with a thermo-responsive pentablock terpolymer

Single-walled carbon nanotubes (SWNTs) were dispersed in pure water with a thermo-responsive amphiphilic PNIPAM₁₅₀-F108-PNIPAM₁₅₀ pentablock terpolymer in comparison with its precursor PEO₁₃₆-PPO₄₅-PEO₁₃₆ (F108) triblock copolymer. The stability, dispersibility, and thermo-responsive behaviors of the polymer/SWNT hybrids were characterized by UV–vis–NIR spectroscopy, thermal gravimetric analysis, viscosity measurement, Raman spectroscopy, and high-resolution transmission electron microscopy. The pentablock/SWNT hybrids showed superior ability in stabilization over F108/SWNT hybrids, and no sign of sedimentation was found at room temperature for 6 weeks or even 2 months of storage. The pentablock terpolymer can efficiently disperse SWNTs into individual tubes or small bundles with average diameter of about 5 nm, and their chains were helically wrapped onto the nanotube surface, whereas the larger bundles of the nanotubes with sizes of 15–25 nm were observed in F108/SWNT hybrids. Moreover, the pentablock/SWNT hybrids switched reversibly between the well-exfoliated and the aggregated states when cyclically increasing and decreasing temperature.     

Effect of residual electrolyte on dispersion stability of graphene in aqueous solution

Journal of Solid State Electrochemistry - The stability of graphene dispersions in water is of both scientific and technological significance. We studied the dispersion stability of...     

Molecular theory of surfactant micelles in aqueous solution

This paper presents an overview of recent theoretical work on the molecular theory of micelle formation. A primary emphasis is given to the role of computer simulation of condensed materials in understanding micelle structure and thermodynamics. Much of the detailed discussion focuses on recent Monte Carlo studies of a simple molecular model of micellar aggregates. For clarity of presentation, a compact, physical organization of micelle thermodynamic equilibrium ratios is advocated. This procedure provides a simple basis for physical reasoning about the molecular roles of attractive and repulsive forces in micellization thermodynamics. The molecularly coarse-grained micellar structural information available from current small angle neutron scattering (SANS) measurements is surveyed. The structural predictions of the reviewed Monte Carlo calculations are shown to be in good qualitative agreement with the SANS data. The Monte Carlo results indicate that micelles should be viewed as fluid aggregates with a low surface free energy relative to water-hydrocarbon interfaces. The computer experimental results suggest that dynamic surface and shape fluctuations should be considered in understanding micelle structure at a molecular level. Several instantaneous structures are graphically displayed to illustrate that these transitory structures could be qualitatively described as “dry” but irregularly shaped. Configurations drawn from Monte Carlo calculations on cylindrical and bilayer structures of infinite extent are used to illustrate the role of surface flexibility in these systems.