The ability of cationic-rich and anionic-rich mixtures of CTAB (cetyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) for dispersing of carbon nanotubes (CNTs) in aqueous media has been studied through both the experimental and molecular dynamics simulation methods. Compared to the pure CTAB and SDS, these mixtures are more effective with the lower concentrations and more individual CNTs, reflecting a synergistic effect in these mixtures. The synergistic effects observed in mixed surfactant systems are mainly due to the electrostatic attractions between surfactant heads. In addition, the surface charge related to the colloidal stability of mixed surfactant-covered nanotubes has been characterized by means of ζ-potential measurements. The results indicate that the hydrophobic interactions between surfactant tails also give rise to the higher adsorption of surfactant molecules. Furthermore, molecular dynamics (MD) simulations have been performed to provide insight about the structure of surfactant aggregates onto nanotubes and to attempt an explanation of the experimental results. The MD simulation results indicate that the random and disordered adsorption of mixed surfactants onto carbon nanotubes may be preferred for a low surfactant concentration. Our research may provide experimental and theoretical bases for using mixed surfactants to disperse CNTs, which can open an avenue for new applications of mixed surfactants.

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The insolubility of carbon nanotubes (CNTs) in aqueous media has been a limitation for the practical application of this unique material. Recent studies have demonstrated that the suspend ability of CNT can be substantially improved by employing appropriate surfactants. Although various surfactants have been tested, the exact mechanism by which carbon nanotubes and the different surfactants interact is not fully understood. To deepen the understanding of molecular interaction between CNT and surfactants, as well as to investigate the influence of the surfactant tail length on the adsorption process, we report here the first detailed large-scale all-atomistic molecular dynamics simulation study of the adsorption and morphology of aggregates of the cationic surfactants containing trimethylammonium headgroups (C12TAB and C16TAB) on single-walled carbon nanotube (SWNT) surfaces. We find that the aggregation morphology of both C12TAB and C16TAB on the SWNT is dependent upon the number of the surfactants in the simulation box. As the number of the surfactants increases the random monolayer structure gradually changes to the cylinder-like monolayer structure. Moreover, we make a comparison between the C12TAB and C16TAB adsorption onto SWNTs to clarify the role of the surfactant tail length on the adsorption process. This comparison indicates that by increasing the number of surfactant molecules, the larger number of the C16TAB molecules tend to adsorb onto SWNTs. Further, our results show that a longer chain yields the higher packed aggregates in which the surfactant heads are extended far into the aqueous phase, which in turn may increase the SWNTs stabilization in aqueous suspensions.

We have investigated micellization in systems containing two surfactant molecules with the same structure using a lattice Monte Carlo simulation method. For the binary systems containing two surfactants, we have varied the head-head interactions or tail-tail repulsions in order to mimic the nonideal behavior of mixed surfactant systems and to manipulate the net interactions between surfactant molecules. The simulation results indicate that interactions between headgroups or tailgroups have an effect on thermodynamic properties such as the mixed critical micelle concentration (cmc), distribution of aggregates, shape of the aggregates, and composition of the micelles formed. Moreover, we have compared the simulation results with estimates based on regular solution theory, a mean-field theory, to determine the applicability of this theory to the nonideal mixed surfactant systems. We have found that the simulation results agree reasonable well with regular solution theory for the systems with attractions between headgroups and repulsions between tailgroups. However, the large discrepancies observed for the systems with head-head repulsions could be attributed to the disregarding of the correlation effect on the interaction among surfactant molecules and the nonrandom mixing effect in the theory.

We have studied the behavior of binary surfactant mixtures using the Monte Carlo (MC) simulation technique with a three-dimensional lattice model of a binary surfactant mixture, in which the constituent surfactant species are represented by a series of connected grid sites. Head-head interactions, alone and along with tail-tail interactions, among identical surfactant species were varied to imitate non-ideal mixing and to manipulate the net attractions and repulsions between surfactant species. We found that the head-head and tail-tail interactions affect both the mixed critical micelle concentration and distribution of aggregates. The simulation results are analyzed in the light of the phase separation model, which considers micelles as separate bulk pseudo-phase. Our studies reveal that regular solution theories do not present a satisfactory description for such systems. The discrepancies observed between the theoretical and simulation results for the studied systems could be attributed to the nonrandom mixing effect in simulation, which is neglected in regular solution theory.