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.
Ternary crystalline dispersions of 10 wt % sodium stearate (NaSt) in mixtures of water and propylene glycol (PG) have been investigated at the atomic, molecular, and microscopic levels. Two lamellar crystalline structures, R-NaSt and β-NaSt, have been determined by X-ray diffraction. Compared with β-NaSt, R-NaSt exhibits a larger bilayer thickness, a more ordered packing of hydrocarbon chains, and dissolves at a lower temperature in the PG/H 2 O mixture. A transformation from β-NaSt to R-NaSt is observed upon aging in a PG concentration range of 60-95 wt % in mixtures of H 2 O and PG. The aging transformation progresses faster and to a greater extent as the PG content increases. Additionally, the fibrous crystallites of R-NaSt are more bundled and oriented compared to those of β-NaSt, as revealed by two-dimensional SAXS (small-angle X-ray scattering) and cryo-SEM (cryogenic scanning electron microscopy). The conformation and mobility of the alkane chains in R-NaSt and β-NaSt have been further studied by solid-state 13 C CP MAS (crosspolarization magic-angle spinning) NMR. A greater percentage of trans chain conformation is found in R-NaSt, and the rigidity of the chains is higher. A stronger association of PG with β-NaSt crystals than with R-NaSt crystals is also indicated by the appearance of the PG carbon resonances in the solid-state NMR spectrum.
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