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.     

Surface modification of carbon black by thiol-ene click reaction for improving dispersibility in aqueous phase

Carbon black (CB) particles were firstly encapsulated by γ-Methacryloxypropyltrimethoxysilane (MEMO) using a sol-gel method and then grafted with sodium 3-Mercapto-1-propanesulfonate (MPS) via thiol-ene click reaction. Morphology characterization reveals that modified CB particles have a core-shell structure. Element composition and chemical status derived from X-ray photoelectron spectroscopy (XPS) results prove the grafting of MPS molecules. Moreover, the crystal structure and thermal behavior of modified CB particles were characterized by Raman spectra and Thermogravimetric analysis (TGA) curves, respectively. The modified CB particles exhibit excellent self-dispersing ability in aqueous media and the dispersion has high thermal and centrifugal stability. This research provides a new insight into the preparation of inkjet printing ink with excellent stability.     

Effect of surfactant structure on the stability of carbon nanotubes in aqueous solution

The suspending behaviors of multiple-wall carbon nanotubes (MWNTs), including pristine MWNTs (p-MWNTs) and acid-mixture-treated MWNTs (MWNTCOOH), stabilized by cationic single-chain surfactant, dodecyltrimethylammonium bromide (DTAB), and cationic gemini surfactant hexyl-alpha,beta-bis(dodecyldimethylammonium bromide) (C 12C 6C 12Br 2) were studied systematically. The surfactant structure influences the suspendability of MWNTs dramatically as well as the surfactant adsorption behavior on the nanotubes. Although both the surfactants can disperse the MWNTs effectively, they actually show different stabilizing ability. DTAB is not capable of stabilizing these two MWNTs below critical micelle concentration (CMC). However, C 12C 6C 12Br 2 can suspend both the nanotubes effectively even well below its CMC. Moreover, the adsorption of these two surfactants reaches equilibrium at twice the CMC with the original MWNT concentration of 2 mg/mL, 2 mM for C 12C 6C 12Br 2, and 30 mM for DTAB. After the adsorption equilibrium, the maximum amounts of the two suspended MWNTs in C 12C 6C 12Br 2 solution are about twice as much as those in DTAB solution. The strong hydrophobic interaction among the C 12C 6C 12Br 2 molecules and between the C 12C 6C 12Br 2 molecules and the nanotubes as well as the high charge capacity of C 12C 6C 12Br 2 lead to its much stronger adsorption ability on the MWNTs and result in its superior stabilizing ability for the MWNTs in aqueous phase. The gemini surfactant provides a possibility to effectively stabilize the MWNTs in aqueous solutions even at very low surfactant concentration well below its CMC.     

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.     

Aggregation behavior of imidazolium-based chiral surfactant in aqueous solution

A novel imidazolium-based chiral surfactant with a Y-type hydrophobic chain, (S)-(+)-1-(2,3-bis(octanoyloxy)propyl)-3-methylimidazolium chloride ([Bopmim]Cl), was synthesized. The aggregation behavior of [Bopmim]Cl in aqueous solution was then investigated by surface tension, electrical conductivity, ¹H NMR, and fluorescence measurements. Compared with [C₁₂mim]Cl, the critical micelle concentration for [Bopmim]Cl is lower, indicating that the novel chiral surfactant has superior capacity to form micelles. A larger value of pC ₂₀, a greater minimum area per surfactant molecule (A _min), a smaller degree of counterion binding (β), and a looser aggregate are caused by the relatively larger Y-type hydrophobic chain of [Bopmim]Cl. Furthermore, analysis of the ¹H NMR spectra revealed that the introduced Y-type hydrophobic chain may prevent the hydrophobic group from forming an extended chain configuration and cause a changeover from trans to gauche conformations upon micellization. The micelles of the novel chiral surfactant may provide some potential applications in the stereochemical recognition of surfaces or of biological structures.     

Adsorption dynamics of binary mixture of Gemini surfactant and opposite-charged conventional surfactant in aqueous solution

The maximum bubble pressure technique has been used to study the adsorption kinetics of binary mixtures of an anionic Gemini surfactant C9pPHCNa with a cationic conventional surfactant C10TABr in aqueous solutions. The dynamic surface tension data were analyzed using the revised Ward and Tordai equations as well as the micelle dissociation kinetic model suggested by Joos et al. The apparent diffusion coefficient Da below the cmc, the adsorption barrier epsilona and the micelle dissociation constant kmic were obtained. The Da s at short times and at long times were respectively 0.2-16 x 10(10) and 0.08-0.9 x 10(10) m2s(-1), the latter corresponded to the adsorption barrier epsilona of 10-20 kJ mol(-1). The minimum epsilona appeared at the mole fraction of C9pPHCNa (alpha1, on a surfactant-only basis) in the bulk solution being 0.33. The kmic s of the mixed micelles were about 16-2300 s(-1). The most stable mixed micelles were formed at alpha1=0.2 rather than at alpha1=0.33 owing to great discrepancy of hydrophobicity between the two components. These results indicated that the composition of mixed solution was an important factor affecting the adsorption kinetics and the micelle stability.     

A facile method for gold decoration of Te@CdTe nanorods in aqueous solution

Colloidal synthesis of metal-semiconductor hybrid nanostructures is mainly achieved in organic solution. In some applications of hybrid nanoparticles relevant in aqueous media, phase transfer of hydrophobic metal-semiconductor hybrid nanostructures is essential. In this work, we present a simple method for direct synthesis of water-soluble gold (Au) decorated Te@CdTe hybrid nanorods (NRs) at room temperature by using aqueous Te@CdTe NRs as templates, which were preformed by using CdTe nanocrystals (NCs) as precursor in the presence of hydrazine hydrate (N(2)H(4)). Our results showed that NRs were decorated with Au islands both on tips and along the surface of the NRs. The size and density of Au islands can be controlled by varying the amount of Au precursor (mixture of HAuCl(4) and thioglycolic acid (TGA)) and TGA/HAuCl(4) ratio. A possible growth mechanism for the Au decoration of Te@CdTe NRs is concluded as three steps: (1) the formation of AuTe(1.7) via the substitution reaction of Cd(2+) by Au(3+), (2) adsorption of Au-TGA complex onto the preformed AuTe(1.7) anchors and following reduction by CdTe and N(2)H(4), leading to the formation of small Au NCs, (3) Au NCs grow to bigger ones, followed by reduction of more Au precursor by N(2)H(4).     

Self-assembly of hydrophobin and hydrophobin/surfactant mixtures in aqueous solution

The self-assembly of the protein hydrophobin, HFBII, and its self-assembly with cationic, anionic, and nonionic surfactants hexadecylterimethyl ammonium bromide, CTAB, sodium dodecyl sulfate, SDS, and hexaethylene monododecyl ether, C(12)E(6), in aqueous solution have been studied by small-angle neutron scattering, SANS. HFBII self-assembles in solution as small globular aggregates, consistent with the formation of trimers or tetramers. Its self-assembly is not substantially affected by the pH or electrolytes. In the presence of CTAB, SDS, or C(12)E(6), HFBII/surfactant complexes are formed. The structure of the HFBII/surfactant complexes has been identified using contrast variation and is in the form of HFBII molecules bound to the outer surface of globular surfactant micelles. The binding of HFBII decreases the surfactant micelle aggregation number for increasing HFBII concentration in solution, and the number of hydrophobin molecules bound/micelle increases.     

Complexation of DNA with cationic gemini surfactant in aqueous solution

Interactions between DNA and the cationic gemini surfactant trimethylene-1,3-bis(dodecyldimethylammonium bromide) (12-3-12) in aqueous solution have been investigated by UV-vis transmittance, zeta potential, and fluorescence emission spectrum. Complexes of DNA and gemini surfactant are observed in which the negative charges of DNA are neutralized by cationic surfactants effectively. The DNA-induced micelle-like structure of the surfactant due to the electrostatic and hydrophobic interactions is determined by the fluorescence spectrum of pyrene. It is found that the critical aggregation concentration (CAC) for DNA/12-3-12 complexes depends little on the addition of sodium bromide (NaBr) because of the counterbalance salt effect. However, at high surfactant concentration, NaBr facilitates the formation of larger DNA/surfactant aggregates. Displacement of ethidium bromide (EB) by surfactant evidently illustrates the strong cooperative binding between surfactant and DNA. In contrast to that in the absence of surfactant, the added NaBr at high surfactant concentration influences not only the binding of surfactant with DNA, but also the stability of DNA/EB complex.     

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.     

Spontaneous vesicle formation in aqueous solution of zwitterionic and anionic surfactant mixture

Vesicles form spontaneously in the aqueous mixtures of dodecyl sulfonate betaine (DSB) and sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT (AOT)) at certain mixing ratios, which has been demonstrated by microcalorimeter, negative-staining transmission electronic microscopy (TEM) and quasi-elastic light scattering (QELS) methods. The addition of NaCl will expand the range of vesicle formation, and monodispersed vesicles are obtained in the solution from the salinity of 0.03 to 0.09 M at the mixing molar ratio of 7/3 (DSB/AOT, mol/mol), with the polydispersity of the system lower than 0.1. To learn more about the structural change in the mixture, ultrasonic was employed finally. Meanwhile analysis was made from the viewpoint of molecular geometry structure.     

Conductometric study of antidepressant drug-cationic surfactant mixed micelles in aqueous solution

Conductivity measurements have been carried out on aqueous solutions of two antidepressant drugs (nortriptyline hydrochloride and clomipramine hydrochloride) with four cationic surfactants (monomeric: cetyltrimethylammonium bromide, tetradecylammonium bromide; dimeric: 1,5-pentanediyl-alpha-omega-bis(hexadecyldimethylammonium bromide), 1,4-butanediyl-alpha,omega-bis(hexadecyldimethylammonium bromide) as well as with sodium chloride. Counterions from NaCl adsorb to the charged headgroup of the drug molecules and reduce the repulsion, hence cmc decreases. cmc values decreased with the addition of surfactants indicating mixed micelle formation. Experimental mole fraction of surfactants in micelle (X1) and their ideal values (X1 id) also support this explanation. Interaction parameter, beta, and excess free energy of micellization are negative suggesting synergism in mixed state. Activity coefficients are less than unity which means non-ideal mixing.     

Aggregation behavior in mixed system of double-chained anionic surfactant with single-chained nonionic surfactant in aqueous solution

The aggregation behavior of mixed systems of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) or sodium bis(4-phenylbutyl) sulfosuccinate (SBPBS) with nonionic surfactant pentaethylene glycol mono-n-dodecyl ether (C12E5) have been studied by means of steady-state fluorescence, electrical conductivity, dynamic light scattering, transmission electron microscopy, electrophoretic light scattering and pyrene solubilization measurements. The critical concentrations for aggregation, micropolarity, mobility, solubilization capacity and morphology of aggregates are characterized. Two critical concentrations for aggregation are observed in the mixed surfactants, which may correspond to the formation of different kinds of aggregates. Moreover, it is more favorable for AOT-C12E5 to form mixed vesicles compared to SBPBS-C12E5 at higher mole fraction of C12E5. In addition, it is revealed that SBPBS-C12E5 mixture has larger solubilization capacity for pyrene than AOT-C12E5 system.     

Cloud point of nonionic surfactant Triton X-45 in aqueous solution

Triton X-45, a nonionic surfactant with a low hydrophile-lypophile balance value and dispersible in aqueous solution at room temperature, has a Krafft point above the room temperature. The cloud point of Triton X-45 in an aqueous solution is different from the conventional aqueous nonionic surfactant micelle solution. It was further confirmed by a determination of the effect of additives on the phase behavior of aqueous solutions containing Triton X-45. The experimental fact eliminates the prevalent concept that the cloud point of Triton X-45 is below room temperature, which is helpful to exploit a biocompatible medium for a microbial growth and then for whole cell microbial transformation in a nonaqueous medium.     

Basicity of some P1 phosphazenes in water and in aqueous surfactant solution

The pKa values in water and in dilute surfactant solution for 15 ring-substituted phenyl P1 pyrrolidino phosphazenes PhN=P(NC4H8)3 and the phenyl P1 dimethylamino phosphazene PhN=P(NMe2)3 previously studied in acetonitrile (AN) and tetrahydrofuran (THF) are reported. The nonionic surfactant Tween 20 was used for the basicity measurements of some compounds to overcome the solubility problems. Measurements with a control group of phosphazenes in both media were used to validate the use of the obtained pKa values as estimates of aqueous values. The pK(a) values of the studied phosphazenes in aqueous medium vary from 6.82 (2,6-dinitro-) to 12.00 (4-dimethylamino-). The basicity span is 5.18 pKa units. The aqueous pKa values of the P1 phosphazenes were correlated with the respective basicity data in AN and THF and from these correlations the pK(a) values in water for the parent compounds HN=P(NC4H8)3 and HN=P(NMe2)3 were estimated as 13.9 and 13.3. Also a comparison of the basicity of phosphazenes and some guanidines, amines and pyridines was made. In water the parent phosphazenes and guanidines are the strongest of all the groups of bases studied. In AN and THF the parent phosphazenes are clearly the strongest bases followed by guanidines, amines and pyridines which are bracketed between the basicities of phenyl phosphazenes. In the gas phase the phosphazenes for which data are available are clearly more basic than the other compounds referred to here. Comparison of the basicity data of P1 phosphazenes and some guanidines confirms earlier conclusions about the partly ylidic character of the N=P double bond.     

Poly(aniline-2-sulfonic acid) modified multiwalled carbon nanotubes with good aqueous dispersibility

Poly(aniline-2-sulfonic acid) (PASA)-modified multiwalled carbon nanotubes (MWNTs) were prepared by in situ oxidative polymerization. HRTEM images show that the PASA-modified MWNTs (PASANTs) are core (MWNT)-shell (PASA) tubular structures with a shell thickness of several nanometers. The strong interaction between the surface of the MWNTs and the conjugated structure of the PASA shell layer was proven by FTIR spectra. It was found that the aqueous dispersibility and dispersion stability of PASANTs were significantly improved compared to those of the raw MWNTs. Furthermore, when the PASA content is greater than 30 wt%, no precipitation is found for the dispersions of modified MWNTs after sedimentation for 500 h, exhibiting an excellent stability. The electrical conductivity of these modified MWNTs is 2-5 times higher than that of raw MWNTs and is 10(3) times higher than that of PASA.     

Light-controlled single-walled carbon nanotube dispersions in aqueous solution

We have succeeded in dispersing single-walled carbon nanotubes (SWNTs) into an aqueous solution of poly(ethylene glycol)-terminated malachite green derivative (PEG-MG) through simple sonication. It was found that UV exposure caused reaggregation of these predispersed SWNTs in the same aqueous medium, as adsorbed PEG-MG photochromic chains could be effectively photocleavaged from the nanotube surface. The observed light-controlled dispersion and reaggragation of SWNTs in the aqueous solution should facilitate the development of SWNT dispersions with a controllable dispersity for potential applications.     

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.     

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.     

Mesoscopic simulation of the aggregation behavior of fluorinated surfactant in aqueous solution

The aggregation behavior of fluorinated surfactant in aqueous solution was investigated using dissipative particle dynamics (DPD) simulation method. Simulation results show that fluorinated surfactants behave mainly as their hydrocarbon analogues, having similar sequences of phases and aggregate structures, which are capable of building micelle, hexagonal phase and lamellar phase. But fluorinated surfactants also show interesting differences from hydrocarbon analogues, which can easily form hexagonal and lamellar structures with comparative little curvature. They can also form ellipsoid or rod-like micelles even in very low concentrations instead of spheroid ones. The dynamic aggregation behavior of fluorinated surfactants, as well as the comparison with hydrogenated ones, was also investigated.