STABILISATION OF CARBON NANOTUBE SUSPENSIONS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 5

sidewalls or defect sites can be modiied by various grafting reactions or
covalent binding of hydrophilic moieties to the CNT surface, which enhances
their solubility and biocompatibility proiles. Non-covalent adsorption or
wrapping of various functional molecules is used to form supramolecular
complexes. Typical examples of molecules that can be adsorbed onto the
hydrophobic surface of CNTs to form stable suspensions are surface-active
agents, which include surfactants, synthetic molecules and biopolymers. 2
The stability of non-covalently functionalised CNT dispersions depends
on the eficiency of the physical wrapping of molecular units around CNTs.
This “physical wrapping” involves forces that are relatively weaker than
those involved in covalent functionalisation, and hence the latter is expected
to produce the most stable dispersion. However, covalent functionalisation
alters the electronic structure of CNTs and hence potentially also affects
their physical properties. 2 Non-covalent chemical modiication of CNTs
is particularly attractive as it offers the facility of associating functional
groups to the CNT surface without modifying the π system (conjugation) of
the graphene lattice and thereby not modifying their electrical or physical
properties. 14 This indicates non-covalent modiication of CNTs to be the
preferred approach, and in the following section we will focus on surfactant
adsorption onto the CNT surface in order to obtain stable and homogeneous
aqueous dispersions.
1.3 SURFACEACTIVE AGENTS IN STABILISING CNT SUSPENSIONS
Surface-active agents have a tendency to accumulate at the boundary
between two phases because of their amphiphilic nature, whereby they
exhibit both hydrophilic and lipophilic properties. Surfactant molecules
possess a hydrophobic “tail” and a hydrophilic “head”, which have been
shown to lower the interfacial tension between insoluble particles and the
suspending medium through adsorption onto the insoluble particles. This
process enables particles to be dispersed in the form of a suspension. The
hydrophobic regions usually consist of saturated or unsaturated hydrocarbon
chains, rarely heterocyclic or aromatic systems. The hydrophilic regions can
be anionic (negatively charged), cationic (positively charged) or non-ionic
(no charge). Surfactants are usually classiied by the charge and nature of the
hydrophilic portion. 15
The surface tension of a surfactant solution reduces as the concentration
of the surfactant increases where an increasing number of molecules
enter the interfacial layer. At a particular concentration termed the
critical micelle concentration (CMC), this layer becomes saturated and the
surfactant molecules adopt a supramolecular micellar structure in which
the hydrophobic regions of the surfactant molecules orient themselves in