  Monolayers of colloidal particles at oil-water interfaces readily crystalize
owing to electrostatic repulsion, which is often mediated through the oil.
However, little attempts exist to control it using oil-soluble electrolytes. We
probe the interactions amongst charged hydrophobic micospheres confined at a
water/hexadecane interface and show that repulsion can be continuously tuned
over orders of magnitude upon introducing minor amounts of an organic salt into
the oil. Our results show that charged groups at the particle/oil interface are
subject to an associative discharging mechanism, analogous to the charge
regulation kinetics observed for charged colloids in non-polar solvents.
