(738c) Segregation of Single-Walled Carbon Nanotubes to Liquid-Liquid Interfaces

The interfacial assembly of carbon nanotubes (CNTs) is needed for many of their applications, and we have developed a new process for driving this assembly at water-organic liquid-liquid interfaces.  Our interest in such interfaces was motivated by a desire to find a practical, continuous process for the preparation of CNT membranes, one that would exploit interfacial polymerization to prepare membrane films containing the interface-segregated CNTs.  Acid treatment of single-walled CNTs (SWCNTs), which can reduce their lengths to hundreds of nanometers, decorates the tubes with carboxyl functionalities that make them water-dispersible.  When an aqueous SWCNT suspension is placed in contact with an organic liquid containing hydrophobic solutes functionalized with tertiary amine(s), carboxyl-amine bonding drives the CNTs to the liquid-liquid interface, as demonstrated by a dramatic lowering of interfacial tension.  With amine-terminated polystyrene (PS-NH2) in toluene, for example, the interfacial segregation of SWCNTs can drop the interfacial tension of the water-toluene interface from ~35  mN/m to ~10 mN/m, a change sufficient for the preparation of stable emulsions; the segregation to the interface was further verified by fluorescence microscopy of labeled SWCNTs.  To understand the assembly better, interfacial tension for these systems was examined as a function of SWCNT and PS-NH2 concentration as well as PS-NH2 molecular weight and SWCNT degree of carboxylation.  Unexpectedly, the interfacial tension fell sharply over an extremely narrow window of SWCNT concentration.  Further study showed the same behavior when the SWCNT suspension was replaced by a poly(acrylic acid) solution.   These trends suggest that pH of the aqueous phase is a controlling variable, the SWCNT segregation sensitive to the degree of protonation of the carboxyl groups.   The sharp SWCNT concentration dependence is thereby a consequence of SWCNT impact on pH.  Unfortunately, microscopic observation of dried emulsion droplets indicated that the segregated SWCNTs, while trapped in a film, do not have a preferential orientation relative to the interface.