The insolubility of single-walled carbon nanotubes (SWNT) in either water or organic solvents has been a limitation for the practical application of this unique material. Recent studies have demonstrated that the suspendability of SWNT can be greatly enhanced by employing appropriate surfactants. Although the efficiency of anionic, cationic, and nonionic surfactants has been demonstrated to different extents, the exact mechanism by which carbon nanotubes and the different surfactants interact is still uncertain. To deepen the understanding of this interfacial phenomenon, we have investigated the effects of chemical modifications of the surface on the extent of nanotube−surfactant interaction. Such changes in the surface chemistry of the SWNT can be achieved by simply varying the pretreatment method, which can be acidic or basic. We have found that intrinsic surface properties such as the PZC (point of zero charge) are greatly affected by the purification method. That is, the electrical charge of the SWNT surface varies with the pH of the surrounding media. However, it has been found that during the adsorption of the anionic surfactant sodium dodecylbenzenesulfonate (NaDDBS) on SWNT Coulombic forces do not play a central role, but are overcome by the hydrophobic interactions between the surfactant tail and the nanotube walls. Only at pH values far from the PZC do the Coulombic forces become important. The hydrophobic forces between the surfactant tail and the nanotube determine the structure of the surfactant-stabilized nanotubes. In such a structure, each nanotube is covered by a monolayer of surfactant molecules in which the heads form a compact outer surface while the tails remain in contact with the nanotube walls. It is important to note that although the final configuration can be described as a cylindrical micelle with a nanotube in the center, the mechanism of formation of this structure does not proceed by incorporation of a nanotube into a micelle, but rather by a two-step adsorption that ends up in the formation of a surfactant monolayer.

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In this work, we propose a mathematical model to reproduce the solubilization, equivalent droplet radius, interfacial tension, and phase transitions of anionic surfactant microemulsions by scaling the curvature of the surfactant membranes to the electrolyte concentration required to obtain an optimum microemulsion formulation. At optimum formulation, equal amounts of oil and water are cosolubilized in a bicontinuous media that has a zero net curvature. Our first modeling approach is to use a single curvature term (inverse of an equivalent spherical droplet ratio) which proves to be inadequate as the system transitions to a bicontinuous microemulsion (supersolubilization), where the micelles become swollen and are no longer spherical. Later we introduce two curvature terms (net and average curvature) to interpret bicontinuous microemulsion behavior. The scaling constant (L), which has a length scale, was obtained for sodium dihexyl sulfosuccinate microemulsions with styrene, trichloroethylene, and limonene. This scaling constant (L) is shown to be independent of the oil type, temperature, surfactant, or additive concentration. We use this net-average curvature model to reproduce selected published data. We also compare the scaling constants (L values) for the different microemulsion systems studied, finding that this parameter is proportional to the length of the extended tail of the surfactant and reflects the surfactant solubilization potential. Additionally, the model was modified to account for palisade micellar solubilization. Finally, we introduce the interfacial rigidity concept to reproduce the interfacial tension of these systems.

Along a formulation scan, solubilization is maximal when a bicontinuous microemulsion is in equilibrium with both oil and water excess phases in a so-called Winsor III system. The logical way to enhance solubilization is to increase the interaction of the surfactant for both the oil and water phases, which can be easily attained by increasing the size of both the head and tail groups. However, this approach is limited by solubility constraints. Additional solubilization enhancement can be attained by introducing a molecule(s) that bridge the bulk phase and the adsorbed surfactant layer; this can be accomplished by using the so-called lipophilic and hydrophilic "linker effect" or by using block copolymer additives. In either case, the goal is to modify an extended zone in the oil and water domains close to their boundary. The intramolecular grafting of a linker group between the hydrophilic and lipophilic moieties in a surfactant results in a so-called "extended" surfactant structure, which produces enhanced solubilization, as does the surfactant/linker combination, but with the added benefit that the self-contained extended surfactant structure does not undergo selective partitioning. We conclude that an improvement in solubilization is directly related to the presence of a smooth, blurred, and expanded transition across the interfacial region from polar to apolar bulk phases.KEY WORDS: Extended surfactants, future trends, interfacial tension, lipophilic and hydrophilic linkers, microemulsions, solubilization, state of the art.