Oppositely charged nanoparticles precipitate rapidly only at the point of electroneutrality,
wherein their charges are macroscopically compensated. We investigated the aggregation
and precipitation of oppositely charged nanoparticles at concentrations ranging from
10 to 10−3 mm (based on gold atoms) by using
UV/Vis measurements. We employed solutions of equally sized (4.6 nm) gold nanoparticles,
which were functionalized and stabilized with either positively or with negatively
charged alkanethiols. Results showed that oppositely charged nanoparticles do not
precipitate if their concentration is below a certain threshold even if the electroneutrality
condition is fulfilled. This finding suggests a universal behavior of chemical systems
comprising oppositely charged building blocks such as ions and charged nanoparticles.
