  The electrostatic charge density of particles is of paramount importance for
the control of the dispersion stability. Conventional methods use
potentiometric, conductometric or turbidity titration but require large amount
of samples. Here we report a simple and cost-effective method called
polyelectrolyte assisted charge titration spectrometry or PACTS. The technique
takes advantage of the propensity of oppositely charged polymers and particles
to assemble upon mixing, leading to aggregation or phase separation. The mixed
dispersions exhibit a maximum in light scattering as a function of the
volumetric ratio X, and the peak position XMax is linked to the particle charge
density according to {\sigma} ~ D0 XMax where D0 is the particle diameter. The
PACTS is successfully applied to organic latex, aluminum and silicon oxide
particles of positive or negative charge using poly(diallyldimethylammonium
chloride) and poly(sodium 4-styrenesulfonate). The protocol is also optimized
with respect to important parameters such as pH and concentration, and to the
polyelectrolyte molecular weight. The advantages of the PACTS technique are
that it requires minute amounts of sample and that it is suitable to a broad
variety of charged nano-objects.
