Particle Zeta Potentials Remain Finite in Saturated Salt Solutions

The zeta potential of a particle
characterizes its motion in an
electric field and is often thought to be negligible at high ionic
strength (several moles per liter) due to thinning of the electrical
double layer (EDL). Here, we describe zeta potential measurements
on polystyrene latex (PSL) particles at monovalent salt concentrations
up to saturation (∼5 M NaCl) using electrophoresis in sinusoidal
electric fields and high-speed video microscopy. Our measurements
reveal that the zeta potential remains finite at even the highest
concentrations. Moreover, we find that the zeta potentials of sulfated
PSL particles continue to obey the classical Gouy–Chapman model
up to saturation despite significant violations in the model’s
underlying assumptions. By contrast, amidine-functionalized PSL particles
exhibit qualitatively different behaviors such as zero zeta potentials
at high concentrations of NaCl and KCl and even charge inversion in
KBr solutions. The experimental results are reproduced and explained
by Monte Carlo simulations of a simple lattice model of the EDL that
accounts for effects due to ion size and ion–ion correlations.
At high salt conditions, the model suggests that quantitative changes
in the magnitude of surface charge can result in qualitative changes
in the zeta potentialmost notably, charge inversion of highly
charged surfaces. These findings have important implications for electrokinetic
phenomena such as diffusiophoresis within salty environments such
as oceans, geological reservoirs, and living organisms.