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Surfactants have been widely employed to debundle, disperse and stabilize carbon nanotubes in aqueous solvents. Yet, a thorough understanding of the dispersing mechanisms at molecular level is still warranted. Herein, we investigated the influence of the molecular structure of gemini surfactants on the dispersibility of multiwalled carbon nanotubes (MWNTs). We used dicationic n-s-n gemini surfactants, varying n and s, the number of alkyl tail and alkyl spacer carbons, respectively; for comparisons, single-tailed surfactant homologues were also studied. Detailed curves of dispersed MWNT concentration vs. surfactant concentration were obtained through a stringently controlled experimental procedure, allowing for molecular insight. The gemini are found to be much more efficient dispersants than their single-tailed homologues, i.e. lower surfactant concentration is needed to attain the maximum dispersed MWNT concentration. In general, the spacer length has a comparatively higher influence on the dispersing efficiency than the tail length. Further, scanning electron microscopy imaging shows a sizeable degree of MWNT debundling by the gemini surfactants in the obtained dispersions. Our observations also point to an adsorption process that does not entail the formation of micelle-like aggregates on the nanotube surface, but rather coverage by individual molecules, among which the ones that seem to be able to adapt best to the nanotube surface provide the highest efficiency. These studies are relevant for the rational design and choice of optimal dispersants for carbon nanomaterials and other similarly water-insoluble materials.

Gemini surfactants and bile acid salts are interesting amphiphiles owing to their unconventional and versatile molecular structure, and consequent unusual aggregation properties. In this work, we investigated the phase behavior of binary surfactant/water systems for the cationic bis-quat gemini surfactants 12-2-12 (short spacer) and 12-6-12 (intermediate spacer) and subsequently catanionic mixtures of each gemini surfactant and the bile salt sodium taurodeoxycholate, STDC. The phase diagrams and phase structure were unraveled by polarized light microscopy, differential scanning calorimetry, deuterium NMR and small angle X-ray scattering. The 12-2-12 and 12-6-12 gemini form a micellar-lamellar-cubic and micellar-hexagonal-cubic phase sequence, respectively, denoting the strong influence of the spacer length on the self-assembly properties. Both the catanionic 12-2-12/STDC/water and 12-6-12/STDC/water systems display a rich mesomorphic behavior. One of the most striking features is that the STDC micelles are able to incorporate enormous amounts of gemini surfactant, forming an extremely wide solution region of mixed micellar structures. By contrast, the gemini micelles can only solubilize limited amounts of STDC. The liquid crystalline phases from the binary systems incorporate sizeable amounts of the counterpart surfactant, forming mixed catanionic aggregates at high concentration. Interestingly, the dilute region of both systems is dominated by liquid-liquid phase separation implying the formation of aqueous two-phase micellar systems. The observed features are qualitatively rationalized in terms of packing and electrostatic effects between the surfactants at play.

Carbon nanotubes (CNTs) loading in a polymer matrix strongly affect the rheological behavior and in turn hamper the overall performance of the composite. The majority of the research in this topic ...

Mechanical agitation is commonly used to fragment and disperse insoluble materials in liquids. However, here we show that when pristine single-walled carbon nanotubes pre-dispersed in water are subject to vortex-shaking for very short periods (typically 10–60 s, power density ∼0.002 W mL −1 ), re-aggregation counterintuitively occurs. The initial dispersions are produced using surfactants as dispersants and powerful tip sonication (∼1 W mL −1 ) followed by centrifugation. Detailed imaging by light and electron microscopies shows that the vortex-induced aggregates consist of loose networks (1–10 2 μm in size) of intertwined tubes and thin bundles. The average aggregate size increases with vortexing time in an apparently logarithmic manner and depends on the dispersant used, initial concentration of nanotubes and size distribution of bundles. The aggregation is, nonetheless, reversible: if the vortex-shaken dispersions are mildly bath-sonicated (∼0.03 W mL −1 ), the flocs break down and re-dispersal occurs. Molecular insight for the mechanism behind this surprising phenomenon is put forth.

The most common synthesis methods for copper hydride (CuH) employ hard ligands that lead to the formation of considerable amounts of metallic Cu as side-product. Here we explore a synthesis method for CuH(s) through the reaction of CuCO3·Cu(OH)2(s) with hypophosphorous acid (H3PO2) in solution, via the formation of the intermediate Cu(H2PO2)2(aq) complex. The reaction products were characterized with XRD, FTIR and SEM at different reaction times, and the kinetics of the transformation of Cu(H2PO2)2(aq) to CuH(s) were followed with NMR and are discussed. We show that our synthesis method provides a simple way for obtaining large amounts of CuH(s) even when the synthesis is performed in air. Compared to the classic Wurtz method, where CuSO4 is used as an initial source of Cu2+, our synthesis produces CuH particles with less metallic Cu side-product. We attribute this to the fact that our reaction medium is free from the hard SO42− ligand that can disproportionate Cu(I). We discuss a mechanism for the reaction based on the known reactivity of the reagents and intermediates involved. We explored the possibility of using CuH(s) for making electrically conductive films. Tests that employed water-dispersed CuH particles show that this compound can be reduced with H3PO2 leading to electrically conductive thin films of Cu. These films were made on regular office paper and were found to be Ohmic conductors even after several weeks of exposure to ambient conditions. The fact that the synthesis reported here produces large amounts of CuH particles in aqueous media, with very little impurities, and the fact that these can then be converted to a stable electrically conductive film can open up new applications for CuH such as for printing electrically conductive films or manufacturing surface coatings.

When using amphiphilic polymers to exfoliate and disperse carbon nanotubes in water, the balance between the hydrophobic and hydrophilic moieties is critical and nontrivial. Here, we investigate the mode of surface attachment of a triblock copolymer, Pluronics F127, composed of a central hydrophobic polypropylene oxide block flanked by hydrophilic polyethylene oxide blocks, onto single-walled carbon nanotubes (SWNTs). Crucially, we analyze the composition in dispersant of both the as-obtained dispersion (the supernatant) and the precipitate-containing undispersed materials. For this, we combine the carefully obtained data from

The binding strength of dispersants to the surface of carbon nanotubes is of crucial importance for the efficiency of the dispersion process and for potential applications, yet data are scarce on this subject. Here we present the results of diffusion NMR experiments in dispersions of single-walled carbon nanotubes (SWNTs) prepared by either the polymer Pluronics F127 or the protein bovine serum albumin (BSA). The experiments detect the amount of F127 molecules adsorbed onto the SWNT surface. This quantity is recorded (i) in F127-SWNT dispersions to which BSA molecules are added and (ii) in BSA-SWNT dispersions to which F127 molecules are added. The data clearly show that F127 replaces BSA adsorbed at the SWNT surface, while BSA leaves the adsorbed F127 coverage intact. Consequently, F127 binds to the nanotube surface more strongly than BSA. Hence, we provide a way to categorize dispersants by adsorption strength. We also provide evidence showing that the nanotubes dispersed by BSA form loose aggregates wh...

Numerous reports have shown that the self-assembling properties of 12-s-12 bis(quaternary ammonium) gemini surfactants in aqueous solution are significantly influenced by s, the number of methylene groups in the covalent spacer. However, the role played by s on the phase behavior of the single compounds has not been investigated in a similarly systematic way. Here, we report on the thermotropic phase behavior of the anhydrous compounds with s = 2–6, 8, 10, and 12, resorting to differential scanning calorimetry (DSC), polarized light microscopy (PLM), and X-ray diffraction (XRD). All of the compounds show a stepwise melting behavior, decomposing at 200 °C. As the spacer length increases, nonmonotonic trends are observed for the thermodynamic parameters of the thermotropic phase transitions, mesophase formation, and solid-state d00l spacings. In particular, the number and type of mesophases (ordered smectic phases and/or fluid smectic liquid crystals) depend critically on s. Further, upon heating molecules ...

The synthesis of novel cationic gemini surfactants based on serine, with long lipophilic alkyl chains and a spacer linked to the nitrogen atom of the amino acid residue by amine linkages, is described. The most efficient synthetic pathway involves introduction of the spacer into the monomeric precursors, N-alkyl derivatives, by reductive amination of dialdehydes followed by methylation and deprotection. Characterization of the basic micellization properties of the new compounds was carried out by tensiometry and conductimetry. These surfactants present enhanced interfacial properties compared to the monomeric analogues, and show improved performance, namely lower critical micelle concentration (cmc), lower surface tension at cmc and increased micellar ionization, with respect to conventional bis-quaternary ammonium salts (bis-quats).

While cationic/anionic surfactant mixtures have been reported to form strongly non-ideal mixed aggregates in aqueous solution, the micellization of true catanionic surfactants, i.e. the salt-free ion-paired compounds obtained from the mixtures, has been much less investigated. However, if there is a significant solubility mismatch between the paired chains, these surfactants may show less conventional aggregation features in water, such as a temperature-driven vesicle–micelle transition and the coexistence of two lamellar phases in equilibrium. In this work, we carry out micellization studies for a catanionic surfactant of the type C 16 + C 8 − , cetyltrimethylammonium octylsulfonate (TASo), by surface tension and conductivity measurements. For comparison purposes, the catanionic mixture cetyltrimethylammonium bromide (CTAB)/sodium octylsulfonate (SOSo) at different mixing ratios and the neat SOSo have also been studied. For the non-ideal CTAB/SOSo mixture, Rubingh's model has been applied to determine the β interaction parameter and the mixed micelle composition. The cmc of TASo has been investigated as a function of temperature, salt and excess ionic surfactant, displaying some differences with respect to conventional ionic and nonionic surfactants. The trends in the thermodynamic parameters of micellization of the three types of systems are comparatively rationalized.

A fundamental understanding of the mechanisms involved in the surfactant-assisted exfoliation and dispersion of carbon nanotubes (CNTs) in water calls for well-controlled experimental methodologies and reliable comparative metrics. We have assessed the ability of several ionic surfactants to disperse single and multiwalled carbon nanotubes, resorting to a stringently controlled sonication-centrifugation method for the preparation of the dispersions. The CNT concentration was accurately measured for a wide range of surfactant concentration, using combined thermogravimetric analysis and UV-vis spectroscopy. The obtained dispersibility curves yield several quantitative parameters, which in turn allow for the effects of nanotube morphology and surfactant properties (aromatic rings, chain length, headgroup charge, and cmc) to be assessed and rationalized, both in terms of dispersed nanotube mass and surface area. The data also indicate that the CNT-surfactant association follows patterns that are markedly different from other equilibrium processes governed by hydrophobicity (such as micellization); in particular, the surfactant concentration needed for maximum dispersibility, c(s,max), and the number of surfactant molecules per unit CNT area at c(s,max) are shown to depend linearly on chain length. The results further suggest that the presence of micelles in the exfoliation process is not a key factor either for starting CNT dispersibility or attaining its saturation value.

Os compostos que apresentam a mesma formula molecular sao designados por isomeros. O fenomeno caraterizado pela existencia de compostos que apresentam a mesma formula molecular mas que diferem na formula de estrutura ou na formula estereoquimica e designado por Isomerismo. Este fenomeno foi descoberto em 1827, quando o quimico alemao Friedrich Wohler (1800-1882) preparou acido ciânico e verificou que este composto, apesar de apresentar propriedades diferentes, tinha uma composicao igual ao acido fulminico (figura 1).

Amino acid-derived surfactants have increasingly become a viable biofriendly alternative to petrochemically based amphiphiles as speciality surfactants. Herein, the Krafft temperatures and critical micelle concentrations (cmc) of three series of novel amino acid-derived surfactants have been determined by differential scanning microcalorimetry and surface tension measurements, respectively. The compounds comprise cationic molecules based on serine and tyrosine headgroups and anionic ones based on 4-hydroxyproline headgroups, with varying chain lengths. A linear dependence of the logarithm of cmc on chain length is found for all series, and in comparison to conventional ionic surfactants of equal chain length, the new amphiphiles present lower cmc and lower surface tension at the cmc. These observations highlight their enhanced interfacial performance. For the 18-carbon serine-derived surfactant the effects of counterion change and of the presence of a cis-double bond in the alkyl chain have also been investigated. The overall results are discussed in terms of headgroup and alkyl chain effects on micellization, in the light of available data for conventional surfactants and other types of amino acid-based amphiphiles reported in the literature.