Effect of salt concentration on the phase separation of bitumen emulsions

This work has studied pronouncedly different phase separation situations for bitumen emulsions at various salt concentrations. With increasing sodium chloride concentration (CNaCl), the added salt promotes the degree of phase separation of bitumen emulsions at CNaCl lower than 5.2 mM, where the separated water is the upper phase and the bitumen is the lower phase. Nevertheless, this kind of phase separation is gradually reduced as CNaCl increases from 5.2 to 250 mM. At CNaCl = 250–500 mM, the bitumen emulsions with very low degree of phase separation are observed to transit from the aqueous phase in the upper position and the bitumen phase in the lower level into the reverse situation. Further increase of salt amount in bitumen emulsions leads to the enhanced phase separation at CNaCl = 500–830 mM. These results of phase separation of bitumen emulsions are explained as the combined influences of the screening effect on the electrostatic repulsion of the bitumen droplets and the density changes of the aqueous phase induced by the added salt. While the salt screening effect is responsible for the increased phase separation at CNaCl smaller than 5.2 mM, other phase separation behaviors at higher salt amounts may be mainly attributed to the increasing density values of the aqueous phase due to the salt addition.