  Polyelectrolyte-surfactant complexes (PESCs) have long been employed as
oil-in-water (o/w) emulsions stabilizers, but never in the structure of
colloidal complex coacervates providing a Pickering effect. The complexed state
of PESCs could make them unsuitable o/w Pickering emulsifiers, which instead
require a balance between colloidal structure and stability, amphiphilicity and
wettability. Here we hypothesize that PESCs coacervates are efficient Pickering
stabilizers. Instead of classical surfactants, we employ sophorolipid (SL)
biosurfactants, atypical anionic/neutral stimuli-responsive biosurfactants.
Despite their tunable charge and mild amphiphilic character, they can be used
in combination with cationic/neutral polyelectrolytes (chitosan, CHL, or
poly-L-lysine, PLL) to form PESC coacervates for the development of biobased,
but also pH-switchable, Pickering emulsions.Aqueous solutions of SL-CHL (or
SL-PLL) complex coacervates are emulsified with dodecane. Confocal laser
scanning microscopy (CLSM) and scanning electron microscopy under cryogenic
conditions (cryo-SEM) demonstrate the Pickering effect, while optical
microscopy and oscillatory rheology respectively assess the emulsion formation
and relative viscoelastic properties.Both SL-CHL and SL-PLL PESCs stabilize o/w
emulsions up to $\Phi$oil of 0.7 only in the pH region of complex coacervation
(6 < pH < 9): outside this range, phase separation occurs. Rheology shows a
typical solid-like response and mechanical recovery upon applying large
deformations. CLSM and cryo-SEM highlight a colloidal structure, associated to
the complex coacervates, of the oil/water interface and suggest a Pickering
effect. These findings demonstrate the Pickering effect from PESC coacervates
and the possibility to use biobased and biocompatible components, with
application potential in cosmetics, food science, or oil recovery.
