We study the influence of different
surfactants on NaCl crystallization
during evaporation of aqueous salt solutions. We found that at concentrations
of sodium chloride close to saturation, only the cationic surfactant
CTAB and the nonionic surfactant Tween 80 remain stable. For the nonionic
surfactant, the high concentration of salt does not significantly
change either the critical micellar concentration (CMC) or the surface
tension at the CMC; for the cationic surfactant, the CMC is reduced
by roughly 2 orders of magnitude upon adding the salt. The presence
of both types of surfactants in the salt solution delays the crystallization
of sodium chloride with evaporation. This, in turn, leads to high
supersaturation which induces the rapid precipitation of a hopper
crystal in the bulk. The crystallization inhibitor role of these surfactants
is shown to be mainly due to the passivation of nucleation sites at
both liquid/air and solid/liquid interfaces rather than a change in
the evaporation rate which is found not to be affected by the presence
of the surfactants. The adsorption of surfactants at the liquid/air
interface prevents the crystallization at this location which is generally
the place where the precipitation of sodium chloride is observed.
Moreover, sum frequency generation spectroscopy measurements show
that the surfactants are also present at the solid/liquid interface.
The incorporation of the surfactants into the salt crystals is investigated
using a novel, but simple, method based on surface tension measurements.
Our results show that the nonionic surfactant Tween 80 is incorporated
in the NaCl crystals but the cationic surfactant CTAB is not. Taken
together, these results therefore allow us to establish the effect
of the presence of surfactants on sodium chloride crystallization.