| Surfactants are amphiphilic molecules. They spontaneously form various microstructures in water to accommodate the hydrophilic-hydrophobic interactions. Soaps are the oldest kind of man-made surfactants that are commonly used as washing and cleaning agents. In spite of the long history of soap research, many aspects of soaps in nonaqueous solvents remain unclear. Unlike the aqueous soap systems, which have been studied extensively, investigations of nonaqueous, polar soap systems are rather limited. Motivated by the applications of nonaqueous, polar solvents in soap products, we investigated sodium stearate (NaSt)/water/propylene glycol (PG) systems. The effects of gradual substitution of PG for H 2O on the phase behavior, morphology and crystalline structure of NaSt systems were studied by a combination of characterization techniques. The techniques include direct visual observation, differential scanning calorimetry, wide-angle and small angle x-ray scattering, light and cryo-electron microscopy, and solid-state nuclear magnetic resonance.; Anhydrous NaSt forms layered crystalline structures at 25°C. With increasing temperature, a distorted hexagonal phase and a hexagonal liquid crystalline phase form. Compared with aqueous soap systems, the regions of liquid crystalline phases in the phase diagrams are reduced as PG replaces or gradually substitutes for H2O.; Fibrous and plate-like NaSt crystallites were investigated in the NaSt/PG/H 2O system containing 1–5 wt% NaSt. Despite of the morphological difference, NaSt fibers and platelets share the same layered crystalline structure at the molecular level. NaSt fibers consist of stacked thin ribbons of NaSt bilayers. NaSt platelets exhibit large basal planes {lcub}001{rcub} surrounded by other faster-growing lateral planes.; Two lamellar crystalline structures, α-NaSt and β-NaSt, which formed in the NaSt/PG/H2O system with 10 wt% NaSt, were characterized on the atomic, molecular and microscopic levels. In a PG concentration range of 60–95 wt% in mixtures of H2O and PG, β-NaSt transforms to α-NaSt upon aging. Compared with β-NaSt, the hydrocarbon chains in α-NaSt consist of a higher percentage of trans conformation, which is characteristically more orderly packed and more rigid. α-NaSt exhibits a larger bilayer thickness, and dissolves at a lower temperature in the PG/H 2O mixture. The fibrous crystallites of α-NaSt are more bundled and oriented compared to those of β-NaSt. |
