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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.

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.

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

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 ...

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.

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.