Charge Regulation at the Nanoscale as Evidenced from Light-Responsive Nanoemulsions

Emulsions are indispensable in everyday life, and the
demand for
emulsions’ diversity and control of properties is therefore
substantial. As emulsions possess a high internal surface area, an
understanding of the oil/water (o/w) interfaces at the molecular level
is fundamental but often impaired by experimental limitations to probe
emulsion interfaces in situ. Here, we have used light-responsive surfactants
(butyl-AAP) that can photoisomerize between E and Z isomers by visible and UV light irradiation to tune the
emulsion interfaces. This causes massive changes in the interface
tension at the extended o/w interfaces in macroemulsions and a drastic
shift in the surfactants’ critical micelle concentration, which
we show can be used to control both the stability and phase separation.
Strikingly different from macroemulsions are nanoemulsions (RH ∼90 nm) as these are not susceptible
to E/Z photoisomerization of the
surfactants in terms of changes in their droplet size or ζ-potential.
However, in situ second-harmonic scattering and pulsed-field gradient
nuclear magnetic resonance (NMR) experiments show dramatic and reversible
changes in the surface excess of surfactants at the nanoscopic interfaces.
The apparent differences in ζ-potentials and surface excess
provide evidence for a fixed charge to particle size ratio and the
need for counterion condensation to renormalize the particle charge
to a critical charge, which is markedly different compared to the
behavior of very large particles in macroemulsions. Thus, our findings
may have broader implications as the electrostatic stabilization of
nanoparticles requires much lower surfactant concentrations, allowing
for a more sustainable use of surfactants.