Dispersing carbon nanotubes using surfactants

We review the recent advances in dispersing single-wall carbon nanotubes (SWNTs) using amphiphilic surfactants in aqueous solutions. Three aspects are discussed. (1) On the organization of surfactant molecules with SWNTs, new insights at the microscopic level arise from electron microscopy and detailed computer simulation studies. (2) Quantitative measurements, such as molecular interactions between functional groups and SWNTs, the coverage of surfactant on SWNTs in solution, the charge state of the SWNT/surfactant complex, and the degree of dispersion are critical for better understanding dispersion mechanisms and for the further development of dispersion strategies. (3) The thermodynamic driving forces and the role of metastability in the structure of surfactant dispersed SWNT suspensions are analyzed. An outlook on practical and fundamental issues is also presented.     

The science of dispersing carbon nanotubes with surfactants

Self-assembled structures adsorbed on carbon nanotubes and other nanofibres offer a plethora of opportunities to endow them with new functions and to integrate them into devices and materials. At the same time they are key to solve the greatest problem in carbon nanotube utilisation--debundling and individualisation. Success will inevitably require an understanding of the underlying structure-function relationship of the adsorbed surfactant layer. Computer simulations are ideally suited to develop this understanding as they enable us to study the structure-function relationship in great detail. Combining the results from mesoscale and atomistic simulations we begin to develop this understanding and derive a number of recommendations for optimal dispersion design.     

Dispersing carbon nanotubes in aqueous solutions by a silicon surfactant: Experimental and molecular dynamics simulation study

The dispersion effect of carbon nanotubes (CNTs) in aqueous solutions by a silicon surfactant (ethoxy modified trisiloxane, named Ag-64) was investigated in detail using experimental method and molecular dynamics simulation. The Si–O–Si chain of silicon surfactant was flexible due to long Si–C bond and it could easily wrap onto the surface of CNTs through hydrophobic and other intermolecular interactions. The hydrophilic part of PEO provided the CNTs dispersed in the aqueous solution and prevented CNTs from aggregating in water through steric stabilization. It was found that Ag-64 could disperse CNTs with different diameters and it was an effective dispersing agent. The results of molecular dynamics simulation indicated that Ag-64 molecules could wrap onto the surface of CNTs leading to steric stabilization so that it could well disperse CNTs, and Van der Waals attraction was the dominating force of Ag-64 adsorbing onto CNTs. Our study may provide experimental and theoretical basis for using silicon surfactants to disperse CNTs, which can open the avenue of new applications for silicon surfactants.     

Competitive adsorption of proteins and low-molecular-weight surfactants: computer simulation and microscopic imaging

Proteins and low-molecular-weight (LMW) surfactants are used in the food industry as emulsifying (and foaming) ingredients and as stabilizers. These attributes are related to their ability to adsorb at fluid-fluid (and gas-fluid) interfaces lowering the interfacial (and surface) tension of liquids. Hence, the study of the properties of adsorbed layers of these molecules can be expected to lead to a better understanding of their effect on food products. Direct proof of the validity of mesoscopic models of systems of proteins and LMW surfactants can only be achieved by quantitative theoretical predictions being tested against both macroscopic and mesoscopic experiments. Computer simulation constitutes one of the few available tools to predict mathematically the behaviour of models of realistic complexity. Furthermore, experimental techniques such as atomic force microscopy (AFM) now allow high resolution imaging of these systems, providing the mesoscopic scale measurements to compare with the simulations. In this review, we bring together a number of related findings that have been generated at this mesoscopic level over the past few years. A useful simple model consisting of spherical particles interacting via bonded and unbonded forces is described, and the derived computer simulation results are compared against those from the imaging experiments. Special attention is paid to the adsorption of binary mixtures of proteins, mixtures of LMW surfactants, and also protein+surfactant mixed systems. We believe that further development of these mathematically well-defined physical models is necessary in order to achieve a proper understanding of the key physico-chemical processes involved.     

Thin-layer chromatography in the analysis of surfactants: At a glance

This review presents the research work appeared in the literature during 1983–2016 on identification, separation, and determination of surfactants using different modes of thin-layer chromatography (TLC), e.g., normal phase TLC (NP-TLC), high-performance TLC (HP-TLC), and reversed-phase TLC (RP-TLC). The relative work done on these techniques follows the trend: NP-TLC?>?HP-TLC?>?RP-TLC. Most of the work performed has been concentrated on the analysis of nonionic surfactants compared to the other types of surfactants. The analysis trend of surfactants by TLC follows the order: nonionic?>?cationic?>?anionic?>?amphoteric. Green solvents as an eluant in TLC analysis of surfactants have been recently introduced. In addition, the entry of ionic liquids (ILs) as a mobile phase or as an impregnant of stationary phase tremendously increased the scope of TLC as green chromatographic technique in the analysis of surfactants. The advantageous features of ILs including negligible vapor pressure, good thermal stability, tunable viscosity, nonflammable, and miscibility with water and organic solvents as well as good extractability for various organic compounds and metal ions have expanded the scope of TLC in chemical analysis.     

Van der Waals interactions: evaluations by use of a statistical mechanical method

In this work the induced van der Waals interaction between a pair of neutral atoms or molecules is considered by use of a statistical mechanical method. With use of the Schro?dinger equation this interaction can be obtained by standard quantum mechanical perturbation theory to second order. However, the latter is restricted to electrostatic interactions between dipole moments. So with radiating dipole-dipole interaction where retardation effects are important for large separations of the particles, other methods are needed, and the resulting induced interaction is the Casimir-Polder interaction usually obtained by field theory. It can also be evaluated, however, by a statistical mechanical method that utilizes the path integral representation. We here show explicitly by use of this method the equivalence of the Casimir-Polder interaction and the van der Waals interaction based upon the Schro?dinger equation. The equivalence is to leading order for short separations where retardation effects can be neglected. In recent works [J. S. H?ye, Physica A 389, 1380 (2010); Phys. Rev. E 81, 061114 (2010)], the Casimir-Polder or Casimir energy was added as a correction to calculations of systems like the electron clouds of molecules. The equivalence to van der Waals interactions indicates that the added Casimir energy will improve the accuracy of calculated molecular energies. Thus, we give numerical estimates of this energy including analysis and estimates for the uniform electron gas.     

Electrochemical analysis of surfactants: An overview

This work presents an overview of electrochemical techniques, namely potentiometry, amperometry, tensammetry, electrocapillary measurements and biosensors, recently applied for the determination of surfactants.     

Study of ionic surfactants interactions with carboxylated single-walled carbon nanotubes by using ion-selective electrodes

Potentiometric measurements based on the use of ion-selective electrodes have been used to study the interaction (adsorption) of anionic and cationic surfactants with functionalized single-walled carbon nanotubes (SWCNT–COOH). According to results, the interaction is driven by hydrophobic forces between the hydrocarbon tails of the surfactants and the nanotube walls. Electrostatic interactions practically exert no influence on the interaction. Driving forces in the dispersion processes are different.     

Toward a statistical mechanical theory for water: analytical theory for a short-ranged reference system

Starting from a realistic Hamiltonian and making use of recent findings that the properties of associating fluids are determined primarily by short-ranged interactions, this methodology has been implemented using statistical mechanical approaches and thermodynamic perturbation theory for the TIP4P model of water. We focus on the short-range reference system for which an analytic expression for the Helmholtz free energy is derived. It is found that the model (reference system) exhibits, in addition to a faithful representation of the structure of water, the same features that are characteristic for real water, namely, (i) the temperature of the density maximum and its pressure dependence, including the inflection point at high pressures and (ii) the temperature minima of the constant pressure heat capacity and the coefficient of isothermal compressibility.     

Stringent test of the statistical quasiclassical trajectory model for the H+3 exchange reaction: a comparison with rigorous statistical quantum mechanical results

A complete formulation of a statistical quasiclassical trajectory (SQCT) model is presented in this work along with a detailed comparison with results obtained with the statistical quantum mechanical (SQM) model for the H+ +D2 and H+ +H2 reactions. The basic difference between the SQCT and the SQM models lies in the fact that trajectories instead of wave functions are propagated in the entrance and exit channels. Other than this the two formulations are entirely similar and both comply with the principle of detailed balance and conservation of parity. Reaction probabilities, and integral and differential cross sections (DCS's) for these reactions at different levels of product's state resolution and from various initial states are shown and discussed. The agreement is in most cases excellent and indicates that the effect of tunneling through the centrifugal barrier is negligible. Some differences are found, however, between state resolved observables calculated by the SQCT and the SQM methods which makes use of the centrifugal sudden (coupled states) approximation (SQM-CS). When this approximation is removed and the full close coupling treatment is used in the SQM model (SQM-CC), an almost perfect agreement is achieved. This shows that the SQCT is sensitive enough to show the relatively small inaccuracies resulting from the decoupling inherent to the CS approximation. In addition, the effect of ignoring the parity conservation is thoroughly examined. This effect is in general minor except in particular cases such as the DCS from initial rotational state j=0. It is shown, however, that in order to reproduce the sharp forward and backward peaks the conservation of parity has to be taken into account.     

Melt mixing of polycarbonate with multiwalled carbon nanotubes: microscopic studies on the state of dispersion

The focus of the paper is to investigate several issues related to the state of dispersion of multiwalled carbon nanotubes (MWNTs) in a polycarbonate (PC) matrix. A masterbatch of PC-MWNT (15 wt.%) was diluted with different amounts of PC in a small scale conical twin screw extruder (DACA Micro Compounder) to obtain different compositions of MWNT. In this system, electrical measurements indicated percolation of MWNT between 1.0 and 1.5 wt.%. We report TEM and AFM investigations of the state of dispersion of MWNT, in the entire volume of the matrix, in selected composites with compositions below (1 wt.% MWNT) and above the percolation threshold (2 and 5 wt.% MWNT). In addition, it was investigated if surface segregation of MWNT and flow induced orientation of nanotubes within the extruded strands had been occurred. It is found that the nanotubes dispersed uniformly through the matrix showing no significant agglomeration in the compositions studied. TEM micrographs seem to be able to detect the percolated structure of the carbon nanotubes. Furthermore, by comparing AFM micrographs from the core region and near to surface region no evidence of segregation or depletion of MWNT at the surface of the extruded strand was found. Comparison of TEM and AFM micrographs on surfaces cut along and perpendicular to the strand direction led to the conclusion that no preferred alignment had occurred as a result of extrusion. Aside from TEM technique, AFM is shown to be suitable to characterize the state of nanotube dispersion along with the issue of surface segregation and orientation of the nanotubes.     

Downhill versus two-state protein folding in a statistical mechanical model

The authors address the problem of downhill protein folding in the framework of a simple statistical mechanical model, which allows an exact solution for the equilibrium and a semianalytical treatment of the kinetics. Focusing on protein 1BBL, a candidate for downhill folding behavior, and comparing it to the WW domain of protein PIN1, a two-state folder of comparable size, the authors show that there are qualitative differences in both the equilibrium and kinetic properties of the two molecules. However, the barrierless scenario which would be expected if 1BBL were a true downhill folder is observed only at low enough temperature.     

Polymeric surfactants with tricyclic rigidity from natural rosin: Synthesis and dispersion of single-walled carbon nanotubes in aqueous solution

Polymeric surfactants from natural rosin containing a unique tricyclic rigid structure were prepared and their structures were characterized by IR, ¹H NMR and GPC. Their surface activities including hydrophile—lipophile balance values (HLB), emulsification properties, foaming properties, critical micelle concentration (CMC) and the minimum surface tension (γCMC) were evaluated. With the increase in ratio of hydrophobic to hydrophilic in polymeric surfactant, the CMC values of the surfactants decreased, and the emulsification and foaming properties of these surfactants increased, and the HLB values of them decreased. This type of polymeric surfactants was used to disperse single-walled carbon nanotubes (SWCNTs) in water, and their dispersal capacities were comparatively determined by UV-visible absorption spectroscopy. Thermogravimetric analysis (TGA) was used to quantify the amount of surfactant attached onto the nanotubes. More intuitive image of their dispersion states were obtained by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The results showed that this type of polymeric surfactants had good dispersion capacity and dispersion stability to SWCNTs in water through strong hydrophobic attraction and weak van der Waals interactions.     

Numerical analysis of a dynamic problem involving bulk-surface surfactants

In this paper, a dynamic problem which models the evolution of the concentration of surfactants is analyzed from the numerical point of view. Both bulk and surface diffusions are taken into account into the model, and the relationship between both concentrations, in the bulk and at the surface, is considered by using the well-known Langmuir–Hinshelwood equation. Two convective terms are also included. The variational formulation is then written as a coupled system of parabolic partial differential equations, for which an existence and uniqueness result is stated in an earlier paper (Fernández et al. in SIAM J Math Anal 48(5):3065–3089, 2016). Then, fully discrete approximations are introduced by using the classical finite element method to approximate the spatial variable and the implicit Euler scheme to discretize the time derivatives. An a priori error estimates result is proved, from which the linear convergence of the approximation is derived under suitable additional regularity conditions. Finally, some numerical simulations are presented in order to show the accuracy of the algorithm and the behaviour of the solution in real situations.     

Thermogravimetric analysis of organoclays intercalated with the gemini surfactants

Sodium montmorillonite has been modified via cation exchange reaction using gemini surfactants. Montmorillonite modified by cetyltrimethyl ammonium bromide (CTAB) is used for comparation. Basal spacings and thermal stability of these organo-montmorillonite clays have been characterized using X-ray diffraction analysis and thermogravimetric analysis. The d(001) spacings of montmorillonite-Gemini14, montmorillonite-Gemini16, montmorillonite-Gemini18 can reach above 35 Å compared with the 23.66 Å of the montmorillonite-CTAB at 2.2CEC. The thermogravimetric analysis show four-step degradation which corresponds to residual water desorption, dehydration, followed by decomposition of the organic modifier and the dehydroxylation of the organo-montmorillonite. In addition, DTG enables two different structural arrangements of gemini surfactant molecules intercalating the montmorillonite to be proposed that is different from montmorillonite-CTAB.     

Adriblastina-single stranded DNA interaction with statistical analysis

The electrochemical behavior of adriblastina has been studied at in situ mercury film electrode (in situ MFE) and platinum electrode (PtE) in the presence of phosphoric acid as supporting electrolyte using Osteryoung square-wave stripping voltammetry (OSWSV) and cyclic voltammetry (CV). Optimal experimental and operational parameters have been selected for the drug accumulation and determination in aqueous medium. The interaction of the drug with single stranded DNA (ssDNA) has been studied and validated by using classical least square and partial least square with propagation of error. The formal potentials E° and E°′ and the equilibrium constants K₁ and K₂ have been calculated. It was found that K₂ for the oxidized form of adriblastina is 63 times than K₁ for the reduced form. Among several possible interfering metal ions, a complex formation reaction was observed between adriblastina and Cu(II) ions at in situ MFE. Cu(II) ions formed 1:2 metal:drug complex which is more stable than ssDNA–drug interaction and consequently it inhibits drug biochemical damage effects. The copper complex offers sub-nanograms determination of adriblastina in that 5.80 and 180 pg/ml could be easily detected in aqueous and urine media, respectively, with R.S.D. less than 4%. F-test and t-test have been applied in urine media giving good results that indicated the high accuracy of the proposed method.     

Gemini surfactants with a disaccharide spacer.

A gemini surfactant is an amphiphile possessing (in sequence) the following: hydrocarbon tail/polar group/spacer/polar group/hydrocarbon tail. Widespread interest in geminis has emerged recently from both industrial and academic laboratories. In the present contribution, two related families of geminis have been synthesized, both with trehalose, a disaccharide, as a polar spacer. One family, Series-A, is nonionic and has amide groups separating the long chains from the trehalose spacer. The other family, Series-B, has quaternary ammonium ions connecting the long chains to the trehalose spacer. It was found that Series-A geminis are water insoluble despite the two amides and multiple hydroxyls. When hydrated or extruded, these geminis form microscopically visible vesicular and tubular structures above their transition temperatures (which were determined calorimetrically). Insoluble monomolecular films, constructed from these geminis, have interfacial areas that are dominated by the sugar spacer although intermolecular chain/chain interactions seem to stabilize the films. Thus, the behavior of Series-A geminis in many ways parallels that of phospholipids and simple double-chain surfactants. It is as if the trehalose is less of a spacer than a large but conventional headgroup. In contrast, cationic Series-B geminis are water soluble and form micelles with critical micelle concentrations an order of magnitude lower than that of corresponding conventional surfactants. Molecular modeling using the Amber force field explains the difference in properties between the two families of geminis. Series-A are tubular in shape and thus prefer bilayer packing as do other amphiphiles in which the headgroups are similar in width to the sum of the tail diameters. Series-B geminis are conical-shaped and pack more readily into spherical micelles. This work entails synthesis, tensiometry, conductance, microscopy, surface balance studies, calorimetry, light scattering, and molecular modeling. In colloid chemistry, a balanced perspective cannot be achieved by one methodology alone but only through the pursuit of consilience among multiple approaches.