posted on 2023-12-14, 16:04authored byPeng Wang, Rahul Prasanna Misra, Chiyu Zhang, Daniel Blankschtein, YuHuang Wang
Surfactants
are widely used to disperse single-walled carbon nanotubes
(SWCNTs) and other nanomaterials for liquid-phase processing and characterization.
Traditional techniques, however, demand high surfactant concentrations,
often in the range of 1–2 wt/v% of the solution. Here, we show
that optimal dispersion efficiency can be attained at substantially
lower surfactant concentrations of approximately 0.08 wt/v%, near
the critical micelle concentration. This unexpected observation is
achieved by introducing “bare” nanotubes into water
containing the anionic surfactant sodium deoxycholate (DOC) through
a superacid–surfactant exchange process that eliminates the
need for ultrasonication. Among the diverse ionic surfactants and
charged biopolymers explored, DOC exhibits the highest dispersion
efficiency, outperforming sodium cholate, a structurally similar bile
salt surfactant containing just one additional oxygen atom compared
to DOC. Employing all-atomistic molecular dynamics simulations, we
unravel that the greater stabilization by DOC arises from its higher
binding affinity to nanotubes and a substantially larger free energy
barrier that resists nanotube rebundling. Further, we find that this
barrier is nonelectrostatic in nature and does not obey the classical
Derjaguin–Landau–Verwey–Overbeek (DLVO) theory
of colloidal stability, underscoring the important role of nonelectrostatic
dispersion and hydration interactions at the nanoscale, even in the
case of ionic surfactants like DOC. These molecular insights advance
our understanding of surfactant chemistry at the bare nanotube limit
and suggest low-energy, surfactant-efficient solution processing of
SWCNTs and potentially other nanomaterials.