Kinetics of precipitation of surfactants. III. Atomic force microscopy of precipitate crystals

The overall time required for precipitation to occur can increase dramatically for surfactant mixtures compared to single components. Atomic force microscopy (AFM) has been used in this study to examine the precipitated surfactant crystals, which gives insight into both the thermodynamics and kinetics of surfactant precipitation. Calcium dodecyl sulfate, Ca(DS)(2), shows regular, layered growth that is consistent with a screw-dislocation crystallization mechanism. Calcium octylbenzene sulfonate, Ca(OBS)(2), forms crystals with uneven, jagged edges. At high magnification, the head groups of both Ca(DS)(2) and Ca(OBS)(2) show hexagonal packing. For mixtures that have surprisingly slow precipitation kinetics, the jagged Ca(OBS)2 crystals were observed to grow on the layered Ca(DS)2 crystals. Other surfactant mixtures that exhibited a stepwise rate curve formed crystals with incomplete layers and holes that are not observed with pure surfactants. In general, this type of crystal formation can occur when an impurity is present, which in this case is most likely the second surfactant. This work indicates that interruption of crystal growth by crystallization of a dissimilar supersaturated surfactant onto the first crystal is one explanation for the slower precipitation of surfactant mixtures compared to single surfactants. Entrapment of the nonprecipitating surfactant (inclusion) in the forming crystal of the precipitating surfactant in mixed surfactant systems is another mechanism responsible for the reduction in precipitation rate.