Stability Of Multi-walled Carbon Nanotubes Research Articles

Self-Assembly of Multiwalled Carbon Nanotubes on a Silicone Rubber Foam Skeleton for Durable Piezoresistive Sensors.

Conductive nanomaterial/flexible polymer composite foams are of great interest in the field of flexible and wearable piezoresistive pressure sensors. However, the existing composite foam sensors are faced with stability issues from conductive nanomaterials, which tends to decrease their long-term durability. Herein, we developed a solvent evaporation-induced self-assembly strategy, which could significantly improve the stability of multiwalled carbon nanotubes (MWCNTs) on a silicone rubber foam skeleton. The process for self-assembly of MWCNTs was straightforward. Aqueous MWCNT dispersion droplets were first hierarchically enclosed in silicone rubber via water-in-oil (W/O) Pickering high internal phase emulsions (HIPEs). Then, the high pressure generated by fast evaporation of the solvent from the droplets could break the thinnest pore walls to form interconnected pores. As a result, very dense and firm MWCNT layers were self-assembled on the pore wall surface. Due to the excellent stability of MWCNTs and tetramodal interconnected porosity, our MWCNTs/silicone rubber composite foam showed the following "super" properties: low density of 0.26 g/mL, high porosity of 76%, and excellent mechanical strength (the maximum stress loss of 8.3% at 80% strain after 100 compression cycles). In addition, excellent piezoresistive performance, including superior discernibility for different amplitudes of compressive strain (up to 80%), rapid response time (150 ms), and high sensitivity (gauge factor of 1.44), was demonstrated for such foams, together with prominent durability (39,000 compression cycles at 60% strain in air) and excellent stability of resistance response in water and organic solvents (5000 compression cycles at 30% strain in water and ethanol). Regarding its application, a wearable piezoresistive sensor, which was assembled from the as-prepared conductive silicone rubber composite foam, could capture various movements from tiptoeing and finger bending to small deformations resulting from human pulse.

Effect of surfactant types on the foam stability of multiwalled carbon nanotube stabilized foam

Multiwalled Carbon nanotube (MWCNT) serves as one of emerging and attractive materials for stabilizing foams due to its small size, special surface property and rod-like structure. The combination of MWCNT and surfactant has been regarded as an effective strategy to improve foam stability in porous media. However, the effects of surfactant types on the dispersity of MWCNT and on the stability of MWCNT stabilized foams have yet to be clearly understood. More importantly, the lack of comprehensive study of the synergistic interactions between surfactants and MWCNTs seriously limits the understanding of MWCNT’s foam-stabilizing mechanism, restricting its application as the foam stabilizer. Herein, this paper aims to systematically study the foam-stabilizing performance and mechanism of different surfactant/MWCNT foam systems. Combining with the experimental results obtained from ultraviolet and visible spectrophotometry and zeta potential tests, environmental scanning electron microscope analyses, contact angle measurements, optical microscope observations, and surface tension and dilational viscoelastic modulus determinations, the foam-stabilizing mechanism of different surfactant/MWCNT foam systems can be conjectured as follows. Compared with anionic and nonionic surfactants (SDS and OP-10) modified MWCNT, the MWCNT modified by the cationic surfactant (CTAB) can yield better dispersity due to the larger electrostatic repulsion and higher steric resistance between MWCNTs after the adsorption of CTAB. The highly dispersed MWCNTs have small sizes, which make them enter the foam film easily and tend to aggregate at Plateau border. These aggregations can significantly slow down the film drainage. In addition, MWCNTs can improve the viscosity of foam film, which helps to increase the resistance of gas diffusion and mitigate bubble coalescence. After being adsorbed by surfactants, the modified MWCNT can adsorb on the air-water interface. Due to the higher detachment energy, the CTAB modified MWCNT has a stronger adsorption capacity at the air-water interface compared with SDS and OP-10 modified ones. This tight adsorption can improve the dilational viscoelastic modulus and thus enhance foam stability. Finally, the dual-sand pack experiments proved that the CTAB/MWCNT stabilized foams have good potential for conformance control in heterogenous reservoirs.

An Improved Approach to Manufacture Carbon Nanotube Reinforced Magnesium AZ91 Composites with Increased Strength and Ductility

Multiwalled carbon nanotubes (MWCNTs) are decorated with Pt nanoparticles by a “layer-by-layer” approach using poly (sodium 4-styrene sulfonate) (PSS) and poly (diallyl dimethylammonium chloride) (PDDA). Transmission electron microscopy (TEM) images and Energy Dispersive X-ray (EDX) analysis of the samples confirm Pt deposition on surfaces of CNTs. Dispersibility and dispersion stability of MWCNTs in the solvents are enhanced when MWCNTs are coated with Pt nanoparticles. Mg AZ91 composites reinforced with MWCNTs are then produced by a melt stirring process. Compression tests of the composites show that adding 0.05% wt Pt-coated MWCNTs in AZ91 improves the composite’s mechanical properties compared to the pure AZ91 and pristine MWCNT/AZ91. Fracture surface analysis of the composite using a scanning electron microscope (SEM) shows individual pulled out MWCNTs in the case of the Pt-coated MWCNT/AZ91 composites. This finding can be attributed to the uniform dispersion of Pt-coated MWCNTs in Mg due to the improved wettability of Pt-coated MWCNTs in Mg melts. The study of the pull-out behaviour of pristine and Pt-coated CNTs from an Mg matrix using molecular dynamics simulation supports this interpretation.

Application of nanotechnology in the enhancement of elasticity of vital sign blood walls

To analyze the application of nanotechnology in the enhancement of the elastic strength of vital sign blood vessel walls, to study the effect of vascular endothelial growth factor (VEGF)-loaded multi-walled carbon nanotube (MWCNT) composite stents on blood vessels and to verify that nanotechnology can enhance the elastic strength of blood vessel walls. Through the purification of MWCNTs, the dispersion and stability of MWCNTs were improved, and functionalized MWCNTs were obtained. VEGF-loaded MWCNT composite scaffolds were constructed by using the infection method, and the mechanical properties of the composite scaffolds were tested. The higher the concentration of MWCNTs in the composite scaffold, the greater the maximum load and the stronger the ability of anti-elastic deformation. The 5% MWCNT composite scaffold had the best effect on promoting the growth of human umbilical vein endothelial cells. Compared with polycaprolactone scaffolds, MWCNT composite scaffolds had a more significant effect on promoting the proliferation of vascular cells and the endothelialization of blood vessels. The mechanical properties of MWCNT composite scaffolds are good, and they can increase endothelial cells and elastin. The application of MWCNT composite scaffolds in blood vessels can improve the elastic strength of blood vessels and provide reference for manufacturing better bone repair materials.

Effect of particle size, surface area and conductivity of nano-carbon additives on deep discharge lead-acid battery

The effect of carbon nano- and micro-particle additives on performance of lead-acid battery (LAB) was studied by considering two different carbon blacks, both having low electrical conductivity. Full-scale 150 Ah flooded-electrolyte stationary batteries were prepared in a battery manufacturing unit and subjected to deep discharge cyclic conditions at depth-of-discharge (DOD) ~65%. We report that carbon particle size played a significant role in LAB performance. In comparison to nano-size, micron-scale particles demonstrated better results and this was completely in accordance with Pavlov’s findings. Moreover, an improvement in LAB charging regime and its charge return percentage was observed by the inclusion of multi-walled carbon nanotubes (MWCNTs) in negative active material (NAM). Intriguingly, upon testing with 100 Ah deep discharge traction battery by replacing 0.075 wt% of micron additive with MWCNTs, the battery back-up time was increased by half an hour roughly (during C5 discharge with 100% DOD), while its charging time was reduced by 15–25 min, which seems to be significant. Such enhancement in LAB’s charge–discharge efficacy can be attributed to high specific surface area and electrical conductivity of MWCNTs, which with its tubular morphology was helpful in establishing an effective conductive network within NAM. The microscopic analysis confirms the useful dispersion and structural stability of MWCNTs in NAM.

Effect of NaCl concentration on the dispersion, stability and rheological properties of MWNTs by CMC

A detailed study of the dispersion, rheological and adsorption behaviors between multi-walled carbon nanotubes (MWNTs) and sodium carboxymethylcellulose (CMC) at different NaCl concentration were presented. The experimental results suggested that NaCl concentration governed the dispersion and stability of MWNTs. A small amount of NaCl was contributed to the dispersion and stability of MWNTs, while as the NaCl concentration increased, the MWNTs formed aggregates and the MWNTs suspension became unstable. Adsorption amount, Raman spectroscopy, zeta potential, and adsorption conformation were investigated systematically to elucidate the mechanism of this interesting phenomenon. Surprisingly, results from thermogravimetric analysis (TGA) shown that adsorption amount of CMC on MWNTs didn’t change under any circumstances. The value of zeta potential increased slightly at first and then dropped sharply as the increasing of NaCl concentration. The microstructures were measured by TEM. It was found that uniform CMC adsorption layers were formed on the surface of MWNTs at low NaCl concentrations. However, for the high-salt solution, the MWNTs were wrapped by CMC agglomerates, resulting in poor dispersion stability.

Effect of Ball Milling and Oxidation on Dispersibility and Dispersion Stability of Multiwalled Carbon Nanotubes in High Viscous Heat Exchange Fluids

AbstractMultiwalled carbon nanotubes (MWCNTs) tend to agglomerate resulting in poor dispersion in most of the common solvents due to possessing long tubular structure, high Vander Waal force of attractions and poor interfacial interactions with the surrounding matrix. To improve the dispersion stability, MWCNT, were subjected to ball milling with 300 rpm for 1 h to reduce the length of the MWCNT to improve the functional groups to the MWCNT wall during chemical functionalization. MWCNTs were subjected to thermal oxidation in air at different temperatures before acid treatment to remove the amorphous carbon from MWCNT. Morphology studies of MWCNTs nanofluids were carried out by using a SEM and TEM methods to characterize the surface deformity of MWCNT structure and extent of defect formation, respectively. FT IR and XRD methods were used for the identification of different functional groups and crystalline nature of pristine, ball milled and oxidized MWCNT, respectively. Raman spectroscopy was applied to measure the relative intensities (ID/IG) to estimate the defect ratios. Preparation of nanofluid by dispersing high defect ratio MWCNTs with a concentration of 100 ppm in the base fluid by the ultra‐sonication method. Nanofluids dispersion stability and cycle stability were studied by using UV‐Visible spectroscopy and MWCNTs treated with combined ball milling and oxidation method have exhibited high stability and high dispersibility.

Influence of solvents on the enhancement of thermophysical properties and stability of multi-walled carbon nanotubes nanofluid

Multi-walled carbon nanotubes (MWCNTs) are a contemporary class of nanoparticles that have a prominent thermal, electrical and mechanical properties. There have been numerous studies on the enhancement of thermophysical properties of nanofluids. However, there is only limited research on thermal and stability analysis of MWCNT nanofluids with various kinds of solvents or base fluids, namely propylene glycol, ethanol, ethylene glycol, polyethylene glycol, methanol and water. This paper reports the enhancement of thermophysical properties and stability of MWCNTs with six different base fluids in the presence of sodium dodecyl benzene sulfonate surfactant with a mass concentration of 0.5 wt%. Thermal and dispersion stabilities were determined using a thermogravimetric analyzer (TGA) and Zeta potential, along with a visual inspection method to evaluate the agglomeration or sedimentation of MWCNT nanoparticles over a period of one month. Ultraviolet–visible spectroscopy and Fourier transform infrared spectroscopy were utilized to identify the molecular components and light absorption of the formulated nanofluids at their maximum wavenumber (4500 cm−1) and wavelength (800 nm). In addition, thermophysical properties such as thermal conductivity, specific heat capacity, viscosity and density with a peak temperature of 200 °C were also experimentally evaluated. The TGA results illustrated that MWCNT/ethylene glycol nanofluid achieved maximum thermal stability at 140 °C and it revealed a maximum zeta potential value of −61.8 mV. Thus, ethylene glycol solution was found to be the best base liquid to homogenize with MWCNTs for acquiring an enhanced thermophysical property and a long-term stability.

Improving the stability of multi-walled carbon nano-tubes in extremely environments: Applications as nano-plugging additives in drilling fluids

Owing to the harsh downhole environments, poor dispersion of multi-walled carbon nanotubes (MWCNTs) at high salinity and high temperature can severely limit their application in oil & gas exploitation industry. Herein, the objective of this study is to improve salt tolerance and thermal stability of MWCNTs by modification. In this study, MWCNTs was successfully functionalized with an anionic polymer by a SI-ATRP (Surface-initiated atom transfer radical polymerization) method. By varying the ratio of monomer and initiator, two composites were obtained, named MWCNTs-g-SPMA-1 and MWCNTs-g-SPMA-2, respectively. Both were well dispersed in saturated brine and standard API brine (8 wt% NaCl + 2 wt% CaCl2) for 60 days at room temperature. Moreover, the modified MWCNTs were stable at 170 °C for 24 h, as well as stable in weakly alkali environment. Afterwards, MWCNTs-g-SPMA-2 as the nano-plugging additive in water-based drilling fluids exhibited excellent plugging performance. The plugging efficiency reached 50.96% with adding a small amount of 0.3 wt% MWCNTs-g-SPMA-2.

Strongly coupled sulfur nanoparticles on graphene-carbon nanotube hybrid electrode for multifunctional sodium and aluminium ion storage

The practical utility of multiwalled carbon nanotubes in sodium and aluminium ion batteries are limited due to its low interlayer spacing of 0.34 nm. Herein, we have developed a simple approach of enhancing the interlayer spacing of multiwalled carbon nanotubes by incorporating sulfur nanoparticles into graphene-multiwalled carbon nanotube (GCNT) structure. This proof of concept is realised from its excellent specific capacity and cyclic stability for the first time in sodium and aluminium ion batteries. GCNT/S as the anode in sodium ion battery exhibits a specific capacity of about 510 mA h g−1 at a current density of 50 mA g−1 with a cyclic stability of 2500 cycles which results in about 42% enhancement in specific capacity when compared to GCNT. GCNT/S as cathode in aluminium ion battery shows a specific capacity of 507 mA h g−1 at a current density of 50 mA g−1 and a stable cyclic stability for 600 cycles. This is about 7.2 times enhancement in specific capacity when compared to GCNT. The highly durable nature is due to the small sulfur particles which offer low volume expansion and the GCNT structure acts as a buffer matrix for controlling the volume expansion. Our study provides a new and a simple strategy to enhance the specific capacity and cyclic stability of multiwalled carbon nanotubes for sodium ion batteries and aluminium ion batteries which can be a promising approach for other cost-effective battery technologies as well.

An effective and novel approach for enhancement of the oxidative thermal stability of multiwalled carbon nanotubes loaded polymer blend

Multiwalled carbon nanotubes (MWCNTs) have wide range of applications. Commercial MWCNTs have impurities, disorder and entangled structure, which limit its applications. In this respect, novel, low-energy consumption and friendly technique was applied to enhance its structure. The technique is operated during isometric process inside autoclave and based on thermal treatment of MWCNTs in the presence of solvent and pressure. It was used to enhance the oxidative thermal stability of MWCNTs. Thermal gravimetric analysis results showed an increase in onset temperature, activation energy of decomposition, and the temperature required to loss 75, 50, and 33 wt% as the treating temperature increased. Raman spectroscopy tests showed a decrease in the degree of disorder from 1.29 for untreated samples to 0.45 for samples treated at 250°C. The electrical resistivity of MWCNTs was increased from 0.33 Ω.cm for untreated samples to 0.54 Ω cm for 250°C treated samples. Advantages of thermal treatment of MWCNTs using this technique appeared in the presence of a percolation behavior for the AC electrical conductivity when a polymer matrix was loaded with it with a threshold percolation concentration of 0.7 wt%. Furthermore, Cole–Cole Z semicircles as well as Argand plots of the electric modulus appeared for polymer matrix loaded with thermally treated MWCNTs beyond the threshold percolation concentration.

Interaction between functionalized multiwalled carbon nanotubes and MS2 bacteriophages in water

Fate and transport of carbon nanomaterials can be strongly dependent on the interaction with secondary particulates in the aquatic systems. Bio-particulates in water, e.g., viruses with charged and hydrophobic surface moieties, may profoundly influence the interfacial behavior and hence the environmental fate of nanomaterials (and vice versa). This study systematically evaluates the interfacial interaction of acid-functionalized multiwalled carbon nanotubes (MWNTs) with MS2 bacteriophages, or heteroaggregation behavior of these particulates, under mono- and di-valent cations and with Suwannee River humic acid (SRHA). Results indicate that the highest concentration of MS2 (i.e., MWNT:MS2 of 100:1) renders exceptional stability of MWNTs, even in high salinity conditions. However, at lower MS2 concentrations (i.e., MWNT:MS2 of 1000:1 and 10,000:1), the suppression of MWNT heteroaggregation rate is not as significant. The observed enhanced stability is likely caused by the preferential attachment of the MS2 capsids onto MWNT surfaces, which is mediated by electrostatic attraction (between negatively charged oxygen-containing moieties on MWNTs and positively charged amino acid residues on MS2 surfaces) and/or by MS2 capsids with positive hydropathy index (indicating strong hydrophobicity). Presence of SRHA also shows stability enhancement; however, at lower MS2 concentrations, SRHA dominated the heteroaggregation behavior. These results implicate that preferential interaction between virus capsids (i.e., MS2 and may be other waterborne viruses) and carbonaceous nanomaterials may influence environmental transport of both in aquatic environments.

Dispersion and stability of multi-walled carbon nanotubes in water as affected by humic acids

The effects of two different humic acids (HAs), commercial Aldrich HA (AHA) and HA extracted from forest soils (FHA), on the dispersion and stability of three multi-walled carbon nanotubes (MWCNTs) in water were investigated. The amount of MWCNTs suspended in pure water increased with increases in the MWCNT surface oxygen content and solution pH, and the amount decreased as the ionic strength increased. These results indicated that electrostatic repulsion drove the dispersion of MWCNTs. Adsorbed HA greatly enhanced the dispersion and stability of MWCNTs due to the increased steric hindrance and electrostatic repulsion between individual MWCNTs. Suspended MWCNT concentrations and HA adsorption to MWCNTs were positively correlated with low ionic strengths (0–40 mM) and/or pH values between 3 and 7, which were independent of the change in electrostatic repulsion, suggesting that the dispersion of MWCNTs was primarily controlled by steric hindrance. At higher ionic strengths, MWCNTs rapidly precipitated due to the reduction of the electrostatic repulsion between individual MWCNTs. AHA could suspend more MWCNTs than FHA, which was ascribed to the higher molecular weight and aromatic carbon content of AHA that led to greater steric hindrance between individual MWCNTs. This result verified that steric hindrance was the dominant mechanism by which HAs disperse MWCNTs.

Improved rheological properties and stability of multiwalled carbon nanotubes/polymer in harsh environment

ABSTRACTThis work aims to improve the rheological properties and stability of multiwalled carbon nanotubes (MWCNTs)/acrylamide (AA) base skeleton polymer blends at harsh environment of high salinity‐high temperature (HS‐HT) or various pH. Different co/terpolymers have been accomplished to modify the structure of AA polymer by free‐radical copolymerization of AA‐based monomers. Anionic, cationic, and hydrophobic functional groups were used for the synthesis of polyelectrolyte, polyampholytic, and partially hydrophobic AA polymer types. The conversion, molecular weight, and poly dispersity of co/terpolymers have been evaluated by nuclear magnetic resonance (1H‐NMR), gel permeation chromatography, and differential scanning calorimetry analysis. The effects of sonication power, concentration of polymer, and concentration of MWCNTs were also investigated on rheological behavior of co/terpolymers. The results show that negative polyelectrolyte and polyampholytic polymers are the best candidates for the improvement of MWCNTs/polymer stability and viscosity at HS‐HT and alkali environment, respectively. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47205.

Enhanced mechanical properties of PDMS/PMMA composite membrane using MWCNTs and its application in phenol separation from saline wastewater

ABSTRACTThis work describes the enhanced mechanical properties of composite polydimethylsiloxane/poly(methyl methacrylate) (PDMS/PMMA)/multi‐walled carbon nanotubes (MWCNTs) nanofiber membrane and its application in the phenol separation from saline wastewater. MWNCTs with varied content were immobilized in the membrane matrix using electric field of electrospinning. The neat PDMS/PMMA and composite PDMS/PMMA/MWCNTs membranes were characterized by scanning electron microscopy, universal testing machine, contact angle measurement, 3D automatic optical profiler, and pore size analyzer. Results indicated that tensile strength of composite PDMS/PMMA/MWNCTs membrane was drastically increased six times with a water contact angle (WCA) of 163.3° due to increased roughness parameters compared to neat membrane. However, porosity and fiber diameter of PDMS/PMMA/MWCTs membrane decreased with the increase of MWCNTs content. Moreover, phenol extraction efficiency of PDMS/PMMA/MWNCTs membrane was found to be 34.5% higher than neat one with similar salt rejection efficiency of 99.97%. The stability of MWCNTs in the membrane matrix was confirmed by the cross‐sectional morphology, which indicated the robust and novel design of membrane. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47123.

Debundling, Dispersion, and Stability of Multiwalled Carbon Nanotubes Driven by Molecularly Designed Electron Acceptors.

Carbon nanotubes (CNTs) have attracted significant attention because of their outstanding physical and chemical properties, and yet, their high natural tendency to form bundles, ropes, or aggregates, as a consequence of their strong π-π interactions, limits their solvent processing and further applications. Efficient processing solvents, mostly amide-based, that partially compensate for these strong inter-CNT π-π interactions have been widely reported. However, the yield of debundled/dispersed CNTs and the stability of subsequent dispersions in these solvents remain key challenges. Moreover, there are major concerns related to the large-scale use of conventional solvents, as they are fossil fuel based and intrinsically highly toxic, hence the need to identify environmentally friendly and safer alternatives. Herein, we address these challenges by using a ternary system composed of multiwalled CNTs (MWCNTs), tailored electron-deficient acceptors, and an organic solvent. Not only do the electron-deficient acceptors interrupt the inter-CNTs π-π interactions, thereby enabling the subsequent debundling and dispersion of MWCNTs aggregates in the solvent, they also act as stabilizers, after dispersion, by inhibiting inter-CNT π-π interactions and re-agglomeration. The use of electron acceptors increases the yield by a factor of 165 in N-methyl 2-pyrrolidone, improves the long-term stability of the debundled and dispersed MWCNTs, and reduces the energy input to only 30 min of mild bath sonication, compared with prolonged high-energy sonication reported in the literature. We also report for the first time, the use in MWCNT processing of a "green" biosolvent, dihydrolevoglucosenone, as an environmentally friendly and nontoxic alternative to the more conventional amide-based solvents.

Electrostatic stabilization of multi-walled carbon nanotubes dispersed in nonaqueous media

Dispersing carbon nanotubes is an easy and low-cost way to manipulate these solids and allows the preparation of more complex materials or devices, so it is fundamental for further uses that these dispersions have controlled properties and high colloidal stability. In this work we report the spontaneous electrical charge build-up in pristine multi-walled carbon nanotubes dispersed in common organic solvents such as chloroform and tetrahydrofuran and the achievement of dispersions stable for long periods without adding passivant agents or functional groups on nanotubes surface. Results from electrokinetics, homo- and heterocoagulation provided macroscopic evidences that carbon nanotubes acquire electric charges after dispersion in some organic liquids and we confirmed this process by measuring in situ Raman spectra of the nanotubes dispersions with higher surface electric potentials. We also show that the signal of electric potential of the dispersions can be predicted by the acid-base behaviour of the dispersing medium, corroborating previously reports for other dispersions of carbon nanomaterials.

Oxidation and stability of multi-walled carbon nanotubes in hydrogen peroxide solution

The oxidation and stability of multi-walled carbon nanotubes (CNTs) have been investigated by exposing CNTs in 30% w/v H2O2 solution at room temperature (RT) for up to 8 weeks and at 80 °C for up to 8 h. H2O2 oxidation not only generated surface oxygen-containing groups, but also created surface defects, as disclosed by results of temperature-programmed desorption and X-ray Photoelectron Spectroscopy. The total surface oxygen content was found to be correlated to the final H2O2 concentration. The higher the total surface oxygen content on CNTs, the lower the final H2O2 concentration. Meanwhile, the carbon oxidation and simultaneous H2O2 decomposition were observed and confirmed by an online analysis of evolved gases during the oxidation stepwise heated from room temperature to 80 °C. Raman study showed that the D/G and D'/G ratios of the CNTs oxidized at RT first decreased with an oxidation time of 4 weeks and then increased when prolonging the oxidation time up to 8 weeks. Similar trend was also observed on the CNTs oxidized at 80 °C. The size of CNTs was gradually reduced with increasing oxidation time as shown by SEM studies. Our work reveals the critical changes in the surface oxygen groups as well as the changes in morphology at two distinct stages of hydrogen peroxide treatment, purification and then functionalization. CNTs can withstand 30% w/v H2O2 oxidation for only a certain time, while they may be damaged or consumed eventually in long-term applications. Our study contributes to filling in the knowledge gap about CNT surface oxidation and structural changes with H2O2 treatment under industrial conditions.

Automatic dispersion, long-term stability of multi-walled carbon nanotubes in high concentration electrolytes

Nanoparticles have been known as the useful materials in working fluids for petroleum industry. But the stabilization of nano-scaled materials in water-based working fluids at high salinities is still a big challenge. In this study, we successfully prepared the anionic polymer/multi-walled carbon nanotubes (MWNTs) composites by covalently wrapping of MWNTs with poly (sodium 4-styrenesulfonate) (PSS) to improve the stability of MWNTs in high concentration electrolytes. The PSS/MWNTs composites can automatically disperse in salinity up to 15 wt% NaCl and API brines (8 wt% NaCl + 2 wt% CaCl2). Hydrodynamic diameters of composites were measured as a function of ionic strength and API brines by dynamic light scattering (DLS). By varying the concentration of brines, hydrodynamic diameter of PSS/MWNTs composites in brines fluctuated between 545 ± 110 nm for 14 days and 673 ± 171 nm for 30 days. Above results showed that PSS/MWNTs could be well stable in high salts solutions for a long period of time. After wrapped with PSS, the diameters of nanotubes changed from 30 ~ 40 to ~ 430 nm, the thickness of wrapped polymer is about ~ 400 nm by analysis of morphologies. The zeta potentials of PSS/MWNTs composites in various salinity of brines kept at approximately − 41 ~ − 52 mV. Therefore, the well dispersion of PSS/MWNTs in high salinity is due to large negative charges of poly (sodium 4-styrenesulfonate), which provide enough electrostatic repulsion and steric repulsion to hinder compression of electric double layer caused by high concentration electrolytes.

Dispersion stability of multi-walled carbon nanotubes in catanionic surfactant mixtures

The power of binary mixtures of dodecyl trimethyl ammonium bromide and sodium dodecyl sulfate for dispersion of multi-wall carbon nanotubes (MWCNTs) in aqueous media was investigated. Stable dispersions were obtained at lower total surfactant concentration as compared to their concentration when used alone, indicative a synergistic effect in these catanionic mixtures. The dispersion of carbon nanotubes was characterized using UV–vis spectroscopy and zeta potential measurement. Optimal surfactant concentrations for dispersions of MWCNTs were determined by UV–vis spectroscopy. Surface tension observations demonstrated that the surfactant molecules have more tendency to the surface of CNTs than air/solution interface. This tendency is more in binary suspensions in comparison with individual surfactant suspensions. Based on the concerns of the cost and environmental effect, the use of mixed surfactants should be more encouraging for the stable dispersion of MWCNTs. Raman analysis shows that the non-covalent functionalization used in this study does not disturb sp2 structure of the CNTs significantly. Also, for the first time, we have introduced a new simple approach to estimate surface charge of surfactant-covered CNTs. To do this, we have utilized the well-known Poisson-Boltzmann equation and thermo gravimetric analysis (TGA) results. According to the Poisson equation, calculation of electric potential needs charge density, a parameter which we have estimated using thermo gravimetric results in this new approach. The TGA-based estimated potentials are in good agreement with experimental results. Ultimately, using the surface charge results, we have calculated interaction energy between two parallel tubes. Calculated results clearly show that the dispersions could be stable, which has been observed experimentally too.