Solvent-induced crystal morphology transformation in a ternary soap system: Sodium stearate crystalline fibers and platelets

A ternary model system, sodium stearate (NaSt) crystallites dispersed in mixtures of water and propylene glycol (PG) at 1-5 wt % NaSt concentrations, has been studied. Two different morphologies, fibers and platelets, were observed with light and electron microscopy. At the molecular level, however, these two morphologies share the same layered crystalline structure as revealed by X-ray diffraction. NaSt fibers occur in water-rich mixtures, and by cryogenic electron microscopy they are shown as lamellar ribbons. With increasing PG/H2O ratios in the NaSt/H2O/PG system, platelike crystallites become the preferred morphology. The rheological properties of these suspensions were studied and correlated with the observed morphologies. Decreasing solvent polarity induced morphological transformation from fibers to plates, as further demonstrated by changing solvents from water, to glycerin, to ethylene glycol, and to propylene glycol. The delineation of these trends can be useful in the design of cleaning products. Additionally, in a brief study of sodium myristate (NaMy) in water, we conclude that the NaMy fibers are lamellar crystals.