Spontaneous Thermoreversible
Formation of Cationic
Vesicles in a Protic Ionic Liquid
- Publication date
- Publisher
Abstract
The search for stable vesicular structures is a long-standing
topic
of research because of the usefulness of these structures and the
scarcity of surfactant systems that spontaneously form vesicles in
true thermodynamic equilibrium. We report the first experimental evidence
of spontaneous formation of vesicles for a pure cationic double tail
surfactant (didodecyldimethylammonium bromide, DDAB) in a protic ionic
liquid (ethylammonium nitrate, EAN). Using small and ultra-small angle
neutron scattering, rheology and bright field microscopy, we identify
the coexistence of two vesicle containing phases in compositions ranging
from 2 to 68 wt %. A low density highly viscous solution containing
giant vesicles (<i>D</i> ∼ 30 μm) and a sponge
(L<sub>3</sub>) phase coexists with a dilute high density phase containing
large vesicles (<i>D</i> ∼ 2.5 μm). Vesicles
form spontaneously via different thermodynamic routes, with the same
size distribution, which strongly supports that they exist in a true
thermodynamic equilibrium. The formation of equilibrium vesicles and
the L<sub>3</sub> phase is facilitated by ion exchange between the
cationic surfactant and the ionic liquid, as well as the strength
of the solvophobic effect in the protic ionic liquid