Kinetics of precipitation of surfactants. I. Anionic surfactants with calcium and with cationic surfactants

Isoperibol calorimetry was used to measure the rates of precipitation for a queous solutions of several anionic surfactants with calcium and of anionic and cationic surfactants. A monomer concentration-dependent supersaturatio n ratio was used to describe the relative rates of precipitation for the su rfactant systems studied. This supersaturation ratio allows for the relativ e rates of precipitation of any surfactant solution to be compared whether micelles are present in solution or not. In general, as the supersaturation ratio increases, the rate of precipitation decreases and the induction tim e decreases, both above and below the critical micelle concentration (CMC). The rate of precipitation of sodium dodecyl sulfate (SDS) with dodecyl pyr idinium chloride is much slower than the rate of precipitation of the anion ic surfactants with calcium for similar supersaturation ratios. The rate of precipitation of SDS with calcium is slightly faster than the rate of prec ipitation of sodium octyl benzene sulfonate for similar supersaturation rat ios. Studies of precipitate crystals, conducted using image analysis, showe d that size and shape depended on the initial supersaturation, the precipit ating surfactant molecule, and the extent of aging (until an equilibrium si ze and shape was reached). Also, differences in the appearance of crystals formed from solutions above and below the CMC were observed. These were mos t likely due to the difference in supersaturation of these solutions. The c rystals formed due to precipitation of SDS with calcium at a concentration above the CMC formed flat trapezoidal, rhombic, and hexagonal shapes. These aged into clusters by 1 wk. For a solution that was precipitated at concen trations beginning below the CMC, the crystals began as elongated and rhomb ic flat plates and aged into trapezoidal, rhombic, and needle-like structur es.