| 1. |
- Abreu, Barbara, et al.
(författare)
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Gemini surfactants as efficient dispersants of multiwalled carbon nanotubes : Interplay of molecular parameters on nanotube dispersibility and debundling
- 2019
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Ingår i: Journal of Colloid and Interface Science. - : ACADEMIC PRESS INC ELSEVIER SCIENCE. - 0021-9797 .- 1095-7103. ; 547, s. 69-77
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Cullari, Lucas Luciano, et al.
(författare)
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Decoupling rheology from particle concentration by charge modulation : Aqueous graphene-clay dispersions
- 2024
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 0021-9797 .- 1095-7103. ; 655, s. 863-875
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Tidskriftsartikel (refereegranskat)abstract
- Hypothesis: Aqueous graphene dispersions are usually obtainable by treating the surface of graphene chemically or physically. In these dispersions, the rheological properties (e.g., viscosity) are governed by a direct coupling to the graphene concentration, which limits their applicability. An alternative approach for dispersing graphene is trapping them in a viscoelastic-network formed by a co-dispersed charged fibrous-clay, Sepiolite. Contrary to surface treatment, the rheological properties of these dispersions are set by the clay particles. The rheology of charged-colloidal dispersions is governed by various parameters, including interparticle interactions. Hence, the rheology of the dispersion could be modulated by changing the clay surface charge without compromising the dispersed graphene concentration. Experimental: The surface charge of Sepiolite was modulated either by charge-screening (by NaCl added to the solution) or by surface-charging (by attachment of highly charged ions, e.g., HexaMetaPhosphate, HMP−) and the effect on rheology and graphene concentration was assessed. In particular, loading the dispersion with HMP− yielded low viscosity, storage, and loss moduli (two orders of magnitude lower than the corresponding HMP−-free dispersion) while the graphene concentration was maintained. We demonstrate that by this charge-modulation approach, reaching the rheological requirements of different applications without compromising on graphene concentration is plausible.
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| 3. |
- Cullari, Lucas Luciano, et al.
(författare)
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Trapped and Alone : Clay-Assisted Aqueous Graphene Dispersions
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:5, s. 6879-6888
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Tidskriftsartikel (refereegranskat)abstract
- Dispersing graphene sheets in liquids, in particular water, could enhance the transport properties (like thermal conductivity) of the dispersion. Yet, such dispersions are difficult to achieve since graphene sheets are prone to aggregate and subsequently precipitate due to their strong van der Waals interactions. Conventional dispersion approaches, such as surface treatment of the sheets either by surfactant adsorption or by chemical modification, may prevent aggregation. Unfortunately, surfactant-assisted graphene dispersions are typically of low concentration (<0.2 wt %) with relatively small sheets (<1 mu m lateral size) while chemical modification is punished by increased defect density within the sheets. We investigate here a new approach in which the concentration of dispersed graphene in water is enhanced by the addition of a fibrous clay mineral, sepiolite. As we demonstrate, the clay particles in water form a kinetically arrested particle network within which the graphene sheets are effectively trapped. This mechanism keeps graphene sheets of high lateral size similar to 4 mu m) dispersed at high concentrations (similar to 1 wt %). We demonstrate the application of such dispersions as cooling liquids for thermal management solutions, where a 26% enhancement in the thermal conductivity is achieved as compared to that in a filler-free fluid.
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| 4. |
- Dai, Jing, et al.
(författare)
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Dispersing Carbon Nanotubes in Water with Amphiphiles : Dispersant Adsorption, Kinetics, and Bundle Size Distribution as Defining Factors
- 2018
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 122:42, s. 24386-24393
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Tidskriftsartikel (refereegranskat)abstract
- Debundling and dispersing single-walled carbon nanotubes (SWNTs) is essential for applications, but the process is not well understood. In this work, aqueous SWNT dispersions were produced by sonicating pristine SWNT powder in the presence of an amphiphilic triblock copolymer (Pluronic F127) as dispersant. Upon centrifugation, one obtains a supernatant with suspended individual tubes and thin bundles and a precipitate with large bundles (and impurities). In the supernatant, that constitutes the final dispersion, we determined the dispersed SWNT concentration by thermogravi-metric analysis (TGA) and UV-vis spectroscopy, and the dispersant concentration by NMR The fraction of dispersant adsorbed at the SWNT surface was obtained by H-1 diffusion NMR Sigmoidal dispersion curves recording the concentration of dispersed SWNTs as a function of supernatant dispersant concentration were obtained at different SWNT loadings and sonication times. As SWNT bundles are debundled into smaller and smaller ones, the essential role of the dispersant is to sufficiently quickly cover the freshly exposed surfaces created by shear forces induced during sonication. Primarily kinetic reasons are behind the need for dispersant concentrations required to reach a substantial SWNT concentration. Centrifugation sets the size threshold below which SWNT particles are retained in the dispersion and consequently determines the SWNT concentration as a function of sonication time.
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| 5. |
- Dai, Jing, et al.
(författare)
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The dispersion process of carbon nanotubes sonicated in aqueous solutions of a dispersant
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Single-walled carbon nanotube (SWNT) dispersions are created by sonicating pristine SWNT powders added to aqueous solutions of the dispersant block copolymer Pluronic F127. In those dispersions, the amount of the dispersed SWNT is determined by the combination of TGA and UV-Vis methods, while the dispersant concentration is estimated by 1H NMR spectroscopy. In addition, the amount of dispersant adsorbed at the SWNT surface is obtained by 1H NMR diffusion experiments. A part of the dispersant is taken up by non-dispersed and precipitated particles. Dispersion curves recording the amount of the dispersed SWNT as a function of either the initial dispersant concentration or the final dispersant concentration are obtained at different initial SWNT loadings and sonication times. The results show in detail the way the original SWNT particles are divided into smaller and smaller sizes thereby increasing the available SWNT surface to be covered by dispersant. Centrifugation sets the size-threshold above which SWNT particles are retained in the dispersion which determined the SWNT content as a function of sonication time.
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| 6. |
- di Gregorio, Maria Chiara, et al.
(författare)
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pH Sensitive Tubules of a Bile Acid Derivative: a Tubule Opening by Release of Wall Leaves
- 2013
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Ingår i: Physical Chemistry Chemical Physics. - 1463-9084. ; 15:20, s. 7560-7566
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Tidskriftsartikel (refereegranskat)abstract
- Tubules formed by self-assembly of organic molecules have a vast potential in nanotechnology applications and the introduction of sensitivity to stimuli into self-assembly tubules represents a particularly attractive feature. Here we report on the preparation and characterization of a molecule obtained by chemical modification of a natural bile acid, a biological surfactant, that self-assembles in pH sensitive tubules in aqueous solutions. The tubules, that are rigid, single-walled and with a diameter of 60 nm, form at pH 8-9 and open up when the pH is increased. The transition is reversible, it occurs in the pH range of 9-10 with an opening mechanism that is remarkably different from those so far proposed in the literature. It involves a release of wall layers similar to leaves, and is determined by a drastic pH-triggered change in the molecular arrangement, which in turn induces a radical modification of the wall curvature. The description of the morphological transformation is performed by means of cryogenic transmission electron microscopy and represent, to our knowledge, the first detailed visualization of pH stimulated tubule opening. UV and circular dichroism spectroscopies are used to investigate the evolution at the molecular level.
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| 7. |
- Fernandes, Ricardo M.F. 1983-, et al.
(författare)
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Assessing Surfactant Binding to Carbon Nanotubes via Competitive Adsorption : Binding strength and critical coverage
- 2015
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The displacement of a nonionic polymeric dispersant, Pluronic F127, adsorbed at the surface of single-walled carbon nanotubes, by low molecular-weight ionic dispersants (surfactants) is studied in aqueous dispersion. The method applied is diffusion NMR spectroscopy that can accurately measure the fraction of F127 molecules adsorbed at the tube surface because of the slow exchange (over the experimental time scale) of F127 between bulk and surface. In a series of surfactants with varying chain length and headgroups, we find that anionic surfactants replace in general more nonionic F127 than do cationic surfactants. The data collected show a strong correlation with the critical dispersibility concentration of the different surfactants, a parameter that signifies the concentration at which one obtains significant dispersed nanotube concentration by ultrasonication. We posit that this finding indicates the existence of a threshold surface coverage for dispersants that constitutes a necessary condition for de-bundling by ultrasonication. The results are discussed in relation to previous findings in the literature.
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| 8. |
- Fernandes, Ricardo M.F. 1983-, et al.
(författare)
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Dispersing Carbon Nanotubes with Ionic Surfactants under Controlled Conditions : Comparisons and Insight
- 2015
-
Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 31:40, s. 10955-10965
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Tidskriftsartikel (refereegranskat)abstract
- 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, andcmc) 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,cs,max, and the number of surfactant molecules per unit CNT area at cs,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.
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| 9. |
- Fernandes, Ricardo M.F. 1983-, et al.
(författare)
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Mechanical Agitation Induces Aggregation of Pre-Dispersed Carbon Nanotubes
- 2015
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Mechanical agitation is typically used to fragment and disperse insoluble materials in a solvent. We report here an aggregation process that, contrary to expectation, is induced by mechanical agitation: when aqueous dispersions of single-walled carbon nanotubes (SWNTs) are subject to vortex-shaking, weakly bound micron-sized aggregates are formed. The SWNT dispersions are prepared by adding various dispersants employing a sonication followed by centrifugation approach. While surfactant adsorption to the SWNTs during sonication results in stabilized exfoliated tubes and thin bundles, we find that vortex-shaking the fresh dispersions for short periods (10-60 s) results in re-aggregation into flocs in the 1-102 µm range. The aggregation is reversible: if the vortexed dispersions are mildly sonicated, the flocs break down and re-dispersal occurs. Imaging at different resolutions shows that the aggregates consist of loose networks of intertwined tubes and bundles. The data further indicate that the average aggregate size increases logarithmically with vortex time and is critically influenced by dispersant type (ionic or nonionic), centrifugation time (prior to vortexing) and initial concentration of dispersed SWNTs. These results are relevant if stabilization or destabilization of dispersions is sought for, i.e., in drug delivery or sensing applications, and could also be of interest for chiral sorting of SWNTs and percolation conductivity.
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| 10. |
- Fernandes, Ricardo M.F. 1983-, et al.
(författare)
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Surface Coverage and Competitive Adsorption on Carbon Nanotubes
- 2015
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:38, s. 22190-22197
-
Tidskriftsartikel (refereegranskat)abstract
- 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 where a large part of the surface is not in direct contact with the surrounding liquid. The results are discussed in relation to previous findings in the literature.
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| 11. |
- Ferreira Fernandes, Ricardo Manuel, 1983-, et al.
(författare)
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Block copolymers adsorbed on single-walled carbon nanotubes. Block polydispersity and the modes of surface attachment
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 1H NMR peak intensities, 1H NMR diffusion measurements and TGA experiments were used to clarify the fate of the dispersing molecules, block copolymer Pluronic F127, during preparation of single-walled carbon nanotube (CNT) dispersions and their state in the created dispersions. In the dispersions, a fraction of the F127 molecules is adsorbed to the CNT surface. The mode of adsorption is the attachment and significant immobilization of the hydrophobic polypropylene oxide (PPO) block to the CNT surface and, as a result, the 1H NMR signal from the attached PPO blocks is lost. On the other hand, the hydrophilic polyethylene oxide (PEO) blocks remain highly mobile and thereby detectable by NMR. The F127 is revealed to exhibit significant block polydispersity. Molecules with large PPO blocks become enriched upon the surface of that fraction of the initial CNT powder that does not become dispersed. The molecular motions involved in creating the observed NMR features are clarified.
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| 12. |
- Ferreira Fernandes, Ricardo Manuel, 1983-, et al.
(författare)
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Block Copolymers as Dispersants for Single-Walled Carbon Nanotubes : Modes of Surface Attachment and Role of Block Polydispersity
- 2018
-
Ingår i: Langmuir. - : AMER CHEMICAL SOC. - 0743-7463 .- 1520-5827. ; 34:45, s. 13672-13679
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Tidskriftsartikel (refereegranskat)abstract
- 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 H-1 NMR peak intensities and self-diffusion and thermogravimetric analysis. The molecular motions behind the observed NMR features are clarified. We find that the hydrophobic blocks attach to the dispersed SWNT surface and remain significantly immobilized leading to H-1 NMR signal loss. On the other hand, the hydrophilic blocks remain highly mobile and thus readily detectable by NMR. The dispersant is shown to possess significant block polydispersity that has a large effect on dispersibility. Polymers with large hydrophobic blocks adsorb on the surface of the carbonaceous particles that precipitate, indicating that although a larger hydrophobic block is good for enhancing adsorption, it may be less effective in dispersing the tubes. A model is also proposed that consistently explains our observations in SWNT dispersions and some contradicting findings obtained previously in carbon nanohorn dispersions. Overall, our findings help elucidating the molecular picture of the dispersion process for SWNTs and are of interest when looking for more effective (i.e., well-balanced) polymeric dispersants.
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| 13. |
- Ferreira Fernandes, Ricardo Manuel, et al.
(författare)
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Lateral Diffusion of Dispersing Molecules On Nanotubes As Probed by NMR
- 2014
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:1, s. 582-589
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Tidskriftsartikel (refereegranskat)abstract
- Noncovalent dispersion of carbon nanotubes is essential to most applications but still poorly understood at the molecular level. The interaction of the dispersing molecule with the nanotube, wrapping or nonwrapping, still awaits consensus. Herein, we have studied by H-1 NMR diffusometry some features of molecular dynamics in the system of carbon nanotubes dispersed by triblock copolymer Pluronics F127 in water. The diffusional decays obtained at different diffusion times, Delta, are not single-exponential and have a complex Delta-dependent profile, ultimately implying that the polymer is observed in two states: free (in unimeric form) and nanotube-bound. Fitting a two-site exchange model to the data indicates that at any instant, only a small fraction of polymers are adsorbed on the nanotubes, with polydisperse residence times in the range of 100-400 ms. Most significantly, we further provide an estimate of D = (3-8) x 10(-12) m(2) s(-1) the coefficient of lateral diffusion of the polymer along the nanotube surface, which is an order of magnitude slower than the corresponding self-diffusion coefficient in water. The emerging picture is that of a nonwrapping mode for the polymer-nanotube interaction.
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| 14. |
- Frise, Anton E., et al.
(författare)
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Protein Dispersant Binding on Nanotubes Studied by NMR Self-Diffusion and Cryo-TEM Techniques
- 2010
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 1:9, s. 1414-1419
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Tidskriftsartikel (refereegranskat)abstract
- Carbon nanotubes can be dispersed by a variety of molecules. We investigate the dynamics of protein-assisted carbon nanotube dispersion in water. We find that in equilibrium only a small fraction of the dispersants is indeed adsorbed to the nanotube Surface, while there is a fast exchange process between the adsorbed and free protein molecules. Self-diffusion NMR spectroscopy in combination with cryo-transmission electron microscopy imaging are employed.
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| 15. |
- Frise, Anton, et al.
(författare)
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Polymer Binding to Carbon Nanotubes in Aqueous DispersionsStudied by NMR Diffusometry
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The diffusion behavior of block copolymer Pluronic F-127 in aqueous dispersions of single-walled (SW) and multi-walled (MW) carbon nanotubes (CNT) has been studied by pulsed-field-gradient (PFG) 1H NMR spectroscopy. We show that the polymers are either free monomers or bound to the CNT and exchange between these two states. The residence times and relative populations in those states could also be obtained. For both classes of CNT, the residence time in the bound state was found to be similar and in the order of 10-100 milliseconds. Thermal gravimetric analysis (TGA) showed that the SWCNT concentration dispersed by F-127 was higher than that for MWCNT.
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| 16. |
- Marques, Eduardo, et al.
(författare)
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Micelles, Dispersions and Liquid Crystals in the Catanionic Mixture Bile Salt-Double-Chained Surfactant. The Bile Salt-Rich Area
- 2000
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 16:22, s. 8255-8262
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Tidskriftsartikel (refereegranskat)abstract
- The phase behavior and phase structure for the catanionic pair sodium taurodeoxycholate - didodecyl-dimethylammonium bromide (DDAB) are investigated, at 25 degreesC. A combination of techniques is used including light and electron microscopy, small-angle X-ray scattering, and pulsed field gradient NMR self-diffusion. The bile salt micellar solution incorporates large amounts of the double-chained amphiphile, with the solution region extending to equimolarity. On the contrary, the hexagonal liquid-crystalline phase is destabilized by the addition of small amounts of DDAB. At equimolarity, coacervation instead of precipitation is observed, with formation of a viscous isotropic solution and a very dilute one. In the water-rich part of the phase diagram, a peculiar type of phase separation occurs, involving the formation of very fine bluish dispersions and a region of coexistence of two dispersions (double dispersion region). Microscopy and self-diffusion data for the solution region indicate limited growth of the mixed micelles. Large domains in which the micellar structure appears to be maintained are imaged in the bluish dispersions by electron microscopy. No other type of aggregate such as vesicles or precipitates is observed in the dilute bile salt-rich area of this mixture.
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| 17. |
- Nadiv, Roey, et al.
(författare)
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Polymer nanocomposites : insights on rheology, percolation, jamming and molecular mobility
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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 has focused on bulk rheological properties, while here we employ NMR diffusion experiments to explore the mobility (diffusivity) of epoxy molecules when loaded with CNTs. Rheology and light microscopy indicate percolation, or jamming events of CNT aggregates, caging a substantial amount of epoxy molecules while small angle X-ray scattering indicates rearrangements of epoxy molecules in the vicinity of the nanotubes. NMR diffusion experiments distinguish between the diffusion of the caged molecules and that of the free ones, and relate the fraction of the former to the system viscosity. These findings demonstrate the utility of NMR diffusion experiments as an additional method applied to the rheological behavior of polymer mixtures.
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| 18. |
- Rocha, Jessica, et al.
(författare)
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Gemini Surfactants as Dispersants of Multiwalled Carbon Nanotubes : a Systematic Study on the Role of Molecular Structure
- 2015
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Surfactants have been widely used as non-covalent dispersants of carbon nanotubes and yet a deeper and systematic understanding of the role of their molecular properties on dispersibility still awaits consensus. Herein, we report on the dispersibility of multiwalled carbon nanotubes (MWNTs) using a set of dicationic gemini surfactants of the n-s-n type, where both the length of the covalent spacer (s) that bridges the two cationic headgroups and the length of the tails (n) are systematically varied. Thus, 12-s-12 gemini with s = 2, 6, and 12 are studied together with 16-s-16 (s = 2 and 12). In addition, the single-tailed homologues dodecyltrimethylammonium bromide, DTAB (n = 12), and cetyltrimethylammonium bromide, CTAB (n = 16), are employed for comparisons. High precision dispersion curves (dispersed NT vs. surfactant concentration) are presented, obtained through a well-controlled sonication/centrifugation procedure combined with an accurate determination of MWNT concentration. The gemini amphiphiles, despite being double-tailed and double-charged, are found to be less effective dispersants than CTAB and roughly as effective as DTAB. Among the gemini, the following pattern emerges as concerning dispersion behavior. (i) The tail length, n, is less influential than spacer length, s, in dispersing ability, implying that the spacer hydrophobicity rather than that of the tail may govern the affinity for the nanotube surface. (ii) In the 12-s-12 series, the surfactant concentration needed for maximum MWNT dispersibility depends linearly on s, while it is known that the neat cmc depends non-monotonically on s. (iii) Similarly to single-tailed ionic surfactants, the presence of micelles has no direct effect on the dispersion behavior. In combination, these observations also point to an adsorption mechanism that does not involve the formation of micelle-like aggregates on the nanotube surface but rather coverage by individual dispersant molecules.
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| 19. |
- Schoeman, Brian J., et al.
(författare)
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A study of the initial stage in the crystallization of TPA-silicalite-1
- 1996
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Ingår i: Zeolites. - 0144-2449. ; 17:5-6, s. 447-456
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Tidskriftsartikel (refereegranskat)abstract
- The initial stage in the crystallization of discrete colloidal crystals of tetrapropylammonium (TPA)-silicalite-1 has been studied with high effect laser light scattering and cryo-transmission electron microscopy (cryo-TEM). The apparently clear TPA-silicate precursor solutions contain discrete subcolloidal particles prior to hydrothermal treatment with an average size of 3.8 nm measured by dynamic light scattering (DLS) and about 5 nm when observed with cryo-TEM. These subcolloidal particles are present in the hydrothermally treated sol throughout the course of crystallization. Following the onset of hydrothermal treatment, the average DLS particle size of the subcolloidal fraction increases from 3.8 to 5.0 nm because of redistribution of silica via an Ostwald ripening mechanism. The first sampling point was at 1.5 h at which time the particle size distribution is monomodal. The apparent absence of growing particles at this point is thought to be a result of the inability of the light scattering technique to resolve two particle populations of similar average size. The first indication of a second growing particle fraction, shown to be TPA-silicalite-1, is after 2.5 h, the second sampling time, at which stage the crystal size is 26 nm. The crystal size increase as a function of time, calculated as 18.5 nm/h, is linear from this time onward. The number of subcolloidal particles prior to hydrothermal treatment is estimated to be of the order of 1017/g of sol, whereas the number of growing crystals is only 4.6 · 1010/g of sol. The possible role of the subcolloidal particles is discussed.
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| 20. |
- Wang, Xia, 1990-
(författare)
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Assemblies of Colloidal Hydrochar Nanoparticles and their Derived Activated Carbons for CO2 Sorption
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon-rich colloids are of great fundamental and technological interest and in this thesis, I tested a range of hypotheses and studied aspects of small hydrochar-based colloids and their colloidal and material chemistry. Crude hydrochar dispersions were synthesized by hydrothermal carbonization of glucose and purified by dialysis. After the purification, stable and monodisperse dispersions of colloidal hydrochar particles in water were obtained. Evaporation of water from the colloidal hydrochar dispersion led to that the hydrochar particles deposited into repeated strip patterns on glass substrates, or underwent directed assembly into macroscopic rods or yarn-shaped objects at the glass-water-air interface.In one study, we studied the strip patterns that comprised dense assemblies of hydrochar particles formed through directed assembly on the substrates during evaporation of water as a function of the sodium dodecylsulfate (SDS) addition, pH of the dispersion, geometry of the substrates, and concentration of the colloidal particle. The mechanisms were presented. In the published paper included in the thesis, the formation of the macroscopically large and assembled rods was studied during evaporation of water from the colloidal hydrochar dispersions. This assembly was studied along with the electrostatic stability of the dispersions at various pH and ion strengths and the redispersability of the assembled rods into the constituting colloidal particles. For matters of applications of the rod assemblies, pyrolysis and templating silicon carbide -tricopper silicide ((SiC-Cu3Si) by reactive infiltration with a copper silicon alloy by reaction infiltration were introduced.In two manuscripts, aspects of the dispersions of hydrochar particles were studied with means of KHCO3 activation into activated carbons (ACs). In one study, hydrochar particles were activated, and then the ACs were dispersed in a solvent after physical grinding. The morphology, porosity, and CO2 sorption properties, etc. of the activated carbons prepared by chemical activation were studied for freeze-dried hydrochar particles and the long bent yarn assemblies pretreated under different conditions. ACs of electrospun nanofibers of polyvinylpyrrolidone (PVP) and colloidal hydrochar were oxidized and chemically activated with KHCO3 or K2CO3 and studied for the adsorption of CO2.
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