Precipitation kinetics were measured for calcium-induced precipitation of m
ixtures of two anionic surfactants. The overall time required for precipita
tion to occur increased dramatically in specific ranges of compositions for
the surfactant mixtures when compared to single components. Adsorption of
the nonprecipitating surfactant onto the precipitate surface was shown to b
e responsible for this remarkable synergism. The higher the supersaturation
of surfactant monomers, the more rapidly precipitation occurred. Under con
ditions where both surfactants were supersaturated, precipitation sometimes
occurred in stepwise fashion, where crystals of different composition were
formed with different induction times. Image analysis of the crystalline p
recipitate showed that crystal habit was affected when the two surfactants
were mixed, indicating that processes such as adsorption and coprecipitatio
n (most likely by inclusion) were occurring. When the crystals were allowed
to age in solution for a period of 1 wk, the crystalline phase from the mi
xed surfactant solutions was found to separate into two types of crystals,
which resembled week-old crystals formed from single surfactant systems.