Assembly of Acid-Functionalized Single-Walled Carbon
Nanotubes at Oil/Water Interfaces
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Abstract
The efficient segregation of water-soluble,
acid-functionalized,
single-walled carbon nanotubes (SWCNTs) at the oil/water interface
was induced by dissolving low-molecular-weight amine-terminated polystyrene
(PS-NH<sub>2</sub>) in the oil phase. Salt-bridge interactions between
carboxylic acid groups of SWCNTs and amine groups of PS drove the
assembly of SWCNTs at the interface, monitored by pendant drop tensiometry
and laser scanning confocal microscopy. The impact of PS end-group
functionality, PS and SWCNT concentrations, and the degree of SWCNT
acid modification on the interfacial activity was assessed, and a
sharp drop in interfacial tension was observed above a critical SWCNT
concentration. Interfacial tensions were low enough to support stable
oil/water emulsions. Further experiments, including potentiometric
titrations and the replacement of SWCNTs by other carboxyl-containing
species, demonstrated that the interfacial tension drop reflects the
loss of SWCNT charge as the pH falls near/below the intrinsic carboxyl
dissociation constant; species lacking multivalent carboxylic acid
groups are inactive. The trapped SWCNTs appear to be neither ordered
nor oriented