A triptycene-based approach to solubilising carbon nanotubes and C60

We describe herein the synthesis of a triptycene-based surfactant designed with the ability to solubilise single-walled carbon nanotubes (SWNTs) and C(60) in water through non-covalent interactions. Furthermore, an amphiphilic naphthalene-based surfactant with the same ability to solubilise SWNTs and C(60) has also been prepared. The compounds synthesised were designed with either two ionic or non-ionic tails to ensure a large number of supramolecular interactions with the solvent, thereby promoting strong solubilisation. The surfactants produced stable suspensions in which the SWNTs are dispersed and the surfactant/SWNT complexes formed are stable for more than one year. UV/Vis/NIR absorption spectroscopy, TEM and AFM were employed to probe the solubilisation properties of the dispersion of surfactants and SWNTs in water.     

Influence of surfactant and lipid chain length on the solubilisation of phosphatidylcholine vesicles by micelles comprised of polyoxyethylene sorbitan monoesters

Photon correlation spectroscopy and freeze-fracture electron microscopy have been used to determine the ability of a range of micelle-forming, polyoxyethylene (20) sorbitan monoesters (Tweens) to solubilise vesicles prepared from phosphatidylcholines of different acyl chain lengths and degrees of saturation with a view to rationalising (in terms of their membrane toxicity) which of the micelle-forming surfactants to use as drug delivery vehicles. The phosphatidylcholines used were dimyristoyl-, dipalmitoyl-, distearoyl- and dioleoylphosphatidylcholine (DMPC, DPPC, DSPC and DOPC, respectively) while the nonionic polyoxyethylene sorbitan monoesters studied were polyoxyethylene (20) sorbitan monolaurate (Tween 20), a 9:1 weight ratio mixture of polyoxyethylene (20) sorbitan monopalmitate and monostearate (Tween 40), a 1:1 weight ratio mixture of polyoxyethylene (20) sorbitan monopalmitate and monostearate (Tween 60), and polyoxyethylene (20) sorbitan monooleate (Tween 80). The ability of the Tween micelles to solubilise phospholipid vesicles was found to depend both upon the length of the surfactant acyl chain and the length of the acyl chains of the phospholipid comprising the vesicle. Vesicles composed of long saturated diacyl chain phospholipids, namely DSPC and DPPC, were the most resistant to solubilisation, while those prepared from the shorter acyl chained DMPC were more readily solubilised. In terms of their solubilisation behaviour, vesicles made from phospholipids containing long, unsaturated acyl chains, namely DOPC behaved more akin to those vesicles prepared from DMPC. None of the Tween surfactants were effective at solubilising vesicles prepared from DPPC or DSPC. In contrast, there were clear differences in the ability of the various surfactants to solubilise vesicles prepared from DMPC and DOPC, in that micelles formed from Tween 20 were the most effective solubilising agent while those formed by Tween 60 were the least effective. As a consequence of these observations it was considered that Tween 60 was the surfactant least likely to cause membrane damage in vivo and, therefore, is the most suitable surfactant for use as a micellar drug delivery vehicle.     

Effect of Amino‐Acid‐Based Polar Oils on the Krafft Temperature and Solubilization in Ionic and Nonionic Surfactant Solutions

Abstract

The Krafft temperature and solubilization power of ionic and nonionic surfactants in aqueous solutions are strongly affected by added polar oils such as amino‐acid‐based oils (e.g., N‐acylamino acid esters, AAE), because they tend to be solubilized in the surfactant palisade layer. The Krafft temperatures of 5 wt.% sodium dodecyl sulfate (SDS)‐water and octaoxyethylene octadecyl ether (C₁₈EO₈)‐water systems largely decreases upon addition of AAE and 1‐hexanol, whereas it decreases very slightly in isopropyl myristate (IPM) and n‐dodecane. The lowering of the Krafft temperature can be explained by the same mechanism as the melting‐temperature reduction of mixing two ordinary substances. Namely, the polar oils are solubilized in the surfactant palisade layer of micelles and reduce the melting temperature of hydrated solid‐surfactant (Krafft temperature). On the other hand, non‐polar oil such as dodecane is solubilized deep inside micelles and makes an oil pool. The solubilization of non‐polar oil is enhanced by mixing surfactant with AAE due to an increase in micellar size.     

SAXS study on complexes formed by anionic poly(sodium methacrylate‐co‐N‐isopropylacrylamide) gels with cationic surfactants

Small‐angle X‐ray scattering (SAXS) has been used to study the nanostructures of complexes formed by slightly crosslinked anionic copolymer gels of poly(sodium methacrylate‐co‐N‐isopropylacrylamide) [P(MAA/NIPAM)] interacting with cetylpyridinium bromide (CPB), and alkyltrimethylammonium bromide (C_nTAB, 10 ≤n ≤ 18), respectively. Both the charge density of polyelectrolyte gels and the surfactant alkyl tail length could induce the phase structure transition from Pm3n space group cubic to hexagonal close packing of spheres (HCP), while the different polar groups of pyridinium and trimethylammonium with the same hydrophobic cetyl chain in surfactants had no significant effects on the structures of complexes formed with the same gels. The highly ordered structures were shown to be formed by the self‐assembly of ionic surfactants inside the anionic gel network, driven by both electrostatic and hydrophobic interactions. Freeze drying the water‐equilibrated complexes could collapse the formed ordered structures. However, the highly ordered structures could be restored after the dried complexes were reswollen by water under the same conditions, indicating that the highly ordered water‐equilibrated complexes were thermodynamically stable. Copyright © 2000 John Wiley & Sons, Ltd.     

Single‐Walled Carbon Nanotubes in Highly Viscous Media: A Comparison between the Dispersive Agents [BMIM][BF4], L121, and Triton X‐100

Dispersions of single‐walled carbon nanotubes (SWNTs) have been prepared by using the room‐temperature ionic liquid [BMIM][BF₄] (1‐butyl‐3‐methylimidazolium tetrafluoroborate), the triblock copolymer Pluronic L121 [poly(ethylene oxide)₅‐poly(propylene oxide)₆₈‐poly(ethylene oxide)₅] and the non‐ionic surfactant Triton X‐100 (TX100) in the pure state. The size of the SWNTs aggregates and the dispersion degree in the three viscous systems depend on the sonication time, as highlighted by UV/Vis/NIR spectroscopy and optical microscopy analysis. A nonlinear increase in conductivity can be observed as a function of the SWNTs loading, as suggested by electrochemical impedance spectroscopy. The generation of a three‐dimensional network of SWNTs showing a viscoelastic gel‐like behavior above a critical percolation concentration has been found at 25 °C in all the investigated systems by oscillatory rheology measurements.     

Hybrids of Copolymers of Fluorene and C60‐Carrying‐Carbazole with Semiconducting Single‐Walled Carbon Nanotubes

Three different copolymers of C₆₀‐carrying‐carbazole and fluorene units with different copolymer composition ratios were designed and synthesized. On the basis of photoluminescence, atomic force microscopy, and Vis‐NIR and Raman spectroscopic analysis, we found that these copolymers solubilize only semiconducting single‐walled carbon nanotubes (sem‐SWNTs) to form copolymer/sem‐SWNT hybrids, in which energy transfer from the copolymer/C₆₀ moieties to the SWNTs was revealed. By comparing two possible hybrid structures with molecular‐mechanics simulations, the greatest stabilization was found when the C₆₀ moieties lay on the sem‐SWNT surfaces.     

Effects of additional salts on the interfacial tension of crude oil/zwitterionic gemini surfactant solutions

Zwitterionic gemini surfactants, which have the advantages of both zwitterionic and gemini surfactants, have been widely used in various disciplines. Sulfobetaine-type zwitterionic gemini surfactants consisting of 1,2-bis[N-methyl-N-(3-sulfopropyl)-alkylammonium]ethane (2C_nSb with 6, 8 and 10 carbon atoms) were evaluated for their interfacial activities at the water/crude oil interface. The 2C₁₀Sb molecules showed a remarkable ability to decrease the interface tension (IFT) of water/crude oil, and the degree of decrease was much greater than those in either zwitterionic or gemini surfactant systems by at least two orders of magnitude. Furthermore, the effects of salts (NaCl, CaCl₂, and MgCl₂) on the IFT of the 2C₁₀Sb system were thoroughly investigated. Interestingly, the delicate balance between the effects of additional cations and the intramolecular interactions of 2C₁₀Sb molecules played crucial roles in the interfacial arrangements of 2C₁₀Sb molecules, which were mainly dependent on the bonding abilities of the cations. Moreover, a zwitterionic surfactant and a cationic gemini surfactant were employed in control experiments to verify the proposed mechanisms.     

Derivatization of fatty alcohol ethoxylate non‐ionic surfactants using 2‐sulfobenzoic anhydride for characterization by liquid chromatography/mass spectrometry

A derivatization procedure has been developed for the improved characterization of fatty alcohol ethoxylate non‐ionic surfactants by liquid chromatography/mass spectrometry. The end hydroxyl group of each surfactant species was converted into an oxycarbonylbenzene‐2‐sulfonic acid group with 2‐sulfobenzoic anhydride under mild conditions. The produced sulfonic acid group allows all species, including fatty alcohols and those with less than three ethoxylates, to be uniformly ionized by electrospray ionization (ESI) mass spectrometry. Both acid and base can be used as a mobile phase additive for liquid chromatography without affecting M_n and average ethoxylate values, although ion intensities are suppressed during the ESI process. The method was used to analyze seven commercial fatty alcohol ethoxylate non‐ionic surfactants, and the determined M_n and EO values were comparable with the results obtained by NMR. The relative ratio of different fatty alcohol based ethoxylates in a sample can also be determined using the summed mass spectral data. Copyright © 2009 The Dow Chemical Company     

Self‐Assembly of Imidazolium‐Based Surfactants in Magnetic Room‐Temperature Ionic Liquids: Binary Mixtures

The phase behaviour of binary mixtures of ionic surfactants (1‐alkyl‐3‐imidazolium chloride, C_nmimCl with n=14, 16 and 18) and imidazolium‐based ionic liquids (1‐alkyl‐3‐methylimidazolium tetrachloroferrate, C_nmimFeCl₄, with n=2 and 4) over a broad temperature range and the complete range of compositions is described. By using many complementary methods including differential scanning calorimetry (DSC), polarised microscopy, small‐angle neutron and X‐ray scattering (SANS/SAXS), and surface tension, the ability of this model system to support self‐assembly is described quantitatively and this behaviour is compared with common water systems. The existence of micelles swollen by the solvent can be deduced from SANS experiments and represent a possible model for aggregates, which has barely been considered for ionic‐liquid systems until now, and can be ascribed to the rather low solvophobicity of the surfactants. Our investigation shows that, in general, C_nmimCl is a rather weak amphiphile in these ionic liquids. The amphiphilic strength increases systematically with the length of the alkyl chain, as seen from the phase behaviour, the critical micelle concentration, and also the level of definition of the aggregates formed.     

Explaining Fullerene Dispersion by using Micellar Solutions

An effective computational strategy to describe the dispersion of C₆₀ by surfactants is presented. The influence of parameters such as surfactant concentration and molecular length on the final morphology of the system is explored to explain the experimental results and to understand the incorporation of C₆₀ inside micelles. Both neutral and charged amphiphilic molecules are simulated. The long‐discussed problem of the location of fullerenes in micelles is addressed and C₆₀ is found in the hydrocarbon‐chain region of the micelles. If the available hydrophobic space increases, C₆₀ is localized in the inner part of the micellar core. Short, charged amphiphilic stabilizers are more efficient at dispersing fullerenes monomolecularly. Two different phases of C₆₀ are observed as the C₆₀/surfactant ratio varies. In the first, aggregates of C₆₀ are entrapped inside the micelles, whereas, in the second, colloidal nanoC₆₀ is formed with surfactants adsorbed on the surface.     

Effects of surfactant and boron doping on the BWF feature in the Raman spectrum of single-wall carbon nanotube aqueous dispersions

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.     

Interactions of two homologues of cationic surface active ionic liquids with sodium carboxymethylcellulose in aqueous solution

In the preceding paper of this series, we studied the interactions of copolymers with the ionic liquids, 1-alkyl-3-methylimidazolium bromide (C _n mimBr, n?=?8, 10, 12, 14, 16) and N-alkyl-N-methylpyrrolidinium bromide (C _n MPB, n?=?12, 14, 16). An obvious difference was detected between the interaction mechanism and the alkyl chain length of the surfactant. In the present study, we performed a systematic study on the interaction of sodium carboxymethylcellulose (NaCMC) with ionic liquids in aqueous solution by isothermal titration microcalorimetry (ITC), conductivity, turbidity, and dynamic light scattering (DLS) measurements. The existence of electrostatic attraction between NaCMC and ILs could increase the complexity of these systems. The results show that the monomers of C₈mimBr can bind to the NaCMC chains and form free surfactant micelles in the solution, while no micelle-like C₈mimBr/NaCMC cluster is detected. For other surfactants, the formation of surfactant/NaCMC clusters in the solution is driven by electrostatic and hydrophobic interactions, which could be divided into two types. One type is the polymer-induced surfactant/NaCMC complexes that form in the solution for the surfactant of C _n mimBr (n?=?10, 12, 14) or C _n MPB (n?=?12, 14). The other type is that the surfactant-induced surfactant/NaCMC complexes come into being for the surfactant of C₁₆mimBr or C₁₆MPB. Finally, the different modes of complex formation proposed have a good interpretation of the experiment results, unraveling the details of the effect of surfactant alkyl chain length and headgroup on the surfactant–NaCMC interactions.     

Spectrophotometric study of interaction and solubilization of procaine hydrochloride in micellar systems

The interaction of Procaine hydrochloride (PC) with cationic, anionic and non-ionic surfactants; cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and triton X-100, were investigated. The effect of ionic and non-ionic micelles on solubilization of Procaine in aqueous micellar solution of SDS, CTAB and triton X-100 were studied at pH 6.8 and 29°C using absorption spectrophotometry. By using pseudo-phase model, the partition coefficient between the bulk water and micelles, K_x, was calculated. The results showed that the micelles of CTAB enhanced the solubility of Procaine higher than SDS micelles (K_x = 96 and 166 for SDS and CTAB micelles, respectively) but triton X-100 did not enhanced the solubility of drug because of weak interaction with Procaine. From the resulting binding constant for Procaine-ionic surfactants interactions (K_b = 175 and 128 for SDS and CTAB surfactants, respectively), it was concluded that both electrostatic and hydrophobic interactions affect the interaction of surfactants with cationic procaine. Electrostatic interactions have a great role in the binding and consequently distribution of Procaine in micelle/water phases. These interactions for anionic surfactant (SDS) are higher than for cationic surfactant (CTAB). Gibbs free energy of binding and distribution of procaine between the bulk water and studied surfactant micelles were calculated.      

A lyotropic nematic phase of lamellar micelles (N L) obtained by a non-ionic surfactant in aqueous solution

A non ionic surfactant with a rigid rod-like hydrophobic group has been synthesized. Owing to the molecular geometry of the surfactant only lamellar micelles are formed in aqueous solution.This system exhibits a lyotropic nematic phase (N _L), which for the first time has been found for a binary non ionic surfactant/water system.Herrn Professor Dr. H.-G. Kilian mit herzlichen Glückwünschen zum 60. Geburtstag gewidmet.     

The influence of surfactants on the structure of transparent gelatin films

The interference image of cold-dried transparent gelatin films containing different surfactants was determined in the conoscopic ray of monochromatic light. From the results obtained the order of the structure of gelatin in the film was characterized. Unlike the ionic surfactants (sodium dodecyl sulfate and hexadecyltrimethyl ammonium bromide), which lead to a reduction of the ordered structure, the addition of nonionic and amphoteric surfactants [ethoxylized octylphenole and a technical C₁₂/C₁₄-dipoly(oxyethylene)ammoniopropane sulfonate] causes an increase of the ordered structure of gelatin. The results quantitatively agree with those found for the influence of surfactants on the secondary structure of gelatin in diluted gelatin solutions. The influence is independent of the gelatin/surfactant ratio and has been explained by gelatin/gelatin interaction competing with the gelatin/surfactant interaction if the gelatin concentration becomes sufficiently high.     

Lithium storage in single-walled carbon nanotubes

Here, we investigated the lithium insertion/extraction mechanism in single-walled carbon nanotubes (SWNTs) based both on the empty SWNTs and filled SWNTs, including ferrocene-filled SWNTs (Fc@SWNTs) and C₆₀-filled SWNTs (C₆₀@SWNTs). SWNTs, C₆₀@SWNTs and Fc@SWNTs were systematically investigated as anode materials for Li-ion batteries. The electrochemical performance of the C₆₀@SWNT electrode was slightly better than that of the SWNTs, and the reversible capacity of Fc@SWNTs per unit weight was ～1.7 times greater than that of the empty SWNTs due to its special tube internal structure. It was proved that the dominant reversible sites for lithium storage in empty SWNTs are the trigonal interstitial channels. Meanwhile, lithium can reversibly insert or extract the inner channels of the tubes after doping with ferrocene; the reversible capacity presented in the inner channels of Fc@SWNTs is about Li_1.13C₆.     

Effect of Confinement on Proton‐Transfer Reactions in Water Nanopools

Proton transfer from the photoacid 8‐hydroxy‐1,3,6‐pyrenetrisulfonic acid (HPTS) to water is studied in reverse micelles with ionic (AOT=sodium dioctyl sulfosuccinate) and non‐ionic (BRIJ‐30=polyoxyethylene(4)lauryl ether) surfactants. The dynamics are studied by probing the transient electronic absorption and transient vibrational absorption, both with sub‐picosecond resolution. The reverse micelle sizes range from approximately 1.6 to 5.5 nm in diameter. For both surfactants it is found that the rate of proton transfer decreases with decreasing reverse micelle size, regardless of surfactant. In addition, for AOT reverse micelles, a fraction of the photoacid molecules exhibit non‐radiative decay, preventing proton transfer.     

How Nano‐Ions Act Like Ionic Surfactants

Recently, nanometric ions were shown to adsorb to hydrated neutral surfaces and to bind to the cavities of macrocyclic molecules with an unexpectedly strong affinity arising from a solvent‐mediated effect named superchaotropicity. We show here that nano‐ions at low concentrations (μm range), similarly to anionic surfactants, induce the spontaneous transformation of a swollen lyotropic lamellar phase of non‐ionic surfactant into a vesicle phase. This transition occurs when the neutral lamellae acquire charges, either by adsorption of the nano‐ions onto, or by anchoring of the ionic surfactant into the lamellae. In contrast to ionic surfactants, nano‐ions strongly dehydrate the neutral surfactant assemblies. As a conclusion, these purely inorganic nanometric ions act as alternatives to the widely used organic ionic surfactants.     

Effect of Alkyl Chain Length on the Interfacial Dilational Properties of Hydroxy-Substituted Alkyl Benzenesulfonates

Dilational rheological behaviors of adsorption layers of three surfactants, sodium 2-hydroxy-3,5-dioctyl benzene sulfonate (C₈C₈), sodium 2-hydroxy-3-octyl-5-decyl benzene sulfonate (C₈C₁₀), and sodium 2-hydroxy-3-octyl-5-dodecylbenzenesulfonate (C₈C₁₂) formed at air–water and decane–water interfaces, have been investigated as a function of concentration and frequency (0.002–0.1 Hz) by the oscillating bubble/drop method. The experimental results show that the dilational moduli of hydroxy-substituted alkyl benzenesulfonates are obviously higher than those of the common surfactants, because the interfacial interactions between alkyl chains are improved drastically by the unique arrangement of C₈C₈ molecules at the interface. However, the moduli at the decane–water interface are much lower than those at the surfaces because decane molecules will insert into the surfactant molecules adsorbed at the interface and destroy the interactions between alkyl chains. With an increase in the number of carbon atom of 5-alkyl, the surface dilational modulus decreases because the orientation of the surfactant molecules at the surface varies from parallel to tilt. On the other hand, the diffusion-exchange process dominates the interfacial behavior and the interfacial modulus improves with the increase in the length of the alkyl chain.     

Stable sequestration of single-walled carbon nanotubes in self-assembled aqueous nanopores

We demonstrate the ability to stably sequester individual single-walled carbon nanotubes (SWNTs) within self-contained nanometer-scale aqueous volumes arrayed in an organic continuum. Large areal densities of 4 × 10(9) cm(-2) are readily achieved. SWNTs are incorporated into a surfactant mesophase which forms 2.3 nm diameter water channels by lyotropic self-assembly. Near-infrared fluorescence spectroscopy demonstrates that the SWNTs exist as well-dispersed tubes that are stable over several months and through multiple cycles of heating and cooling. Absence of physical distortion of the mesophase suggests that the SWNTs are stabilized by adsorbed surfactants that do not extend considerably from the surface. Our findings have important implications for templated assembly of carbon nanotubes using soft mesophases and the development of functional nanocomposites.