To better understand mechanisms of enhanced oil production by smart waterflooding, the coalescence time of crude oil droplets in different brines was measured. Sulfate ions were found to hinder the coalescence of crude oil droplets, whereas magnesium ions significantly enhanced coalescence of the crude oil droplets. To shed light on the observed effect of an individual types of water-soluble inorganic ions in brine on coalescence of crude oil droplets, interfacial shear rheology of crude oil brine interfaces was measured at both ambient and elevated temperatures. zeta potential of crude oil in brine emulsions was measured at ambient temperature. The major objective of these measurements is to investigate interactions of individual water-soluble inorganic ions with the crude oil-water interface and study their impact on the coalescence of crude oil droplets. Interfacial shear rheology results showed significantly higher viscoelastic moduli for brines comprising of sulfate ions, while the brines with sodium, calcium or magnesium ions showed comparable interfacial shear rheology. The transition time, for the interface to become elastic-dominant from a viscous-dominant regime, was found to be the shortest for sulfate brine, followed by sodium brine and calcium brine, with the highest being magnesium brine. The most negative zeta potentials, indicating the strongest electrostatic repulsion between two oil droplets, were observed in the sodium brine and sulfate brine. The magnesium brine and calcium brine showed the lowest negative potentials. The results on coalescence time showed a good agreement with the combined effect of film interfacial rheology and zeta potential. This study establishes the impact of individual water-soluble inorganic ions, for the first time, on coalescence between oil droplets in specific relation to waterflooding and enhanced oil recovery.