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Ingredient Profile: PEG-150 Distearate
By: Michael J. Fevola, PhD, Johnson & Johnson
Posted: December 29, 2010, from the January 2011 issue of Cosmetics & Toiletries.
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PEG-150 distearate typically is supplied as solid, white to off-white waxy flakes that melt at approximately 52–57°C. Potential impurities in the raw material can include: unreacted stearic acid or methyl stearate; monofunctional PEG-150 stearate; unreacted PEG-150; (trans)esterification catalyst residues; trace organic peroxides that result from oxidation of PEG; and 1,4-dioxane, a by-product of ethylene oxide poly-merization to produce PEG-150 diol.5 PEG-150 distearate is water-soluble; however, it must be heated above its melting point to achieve effective dissolution and its solubility is tremendously enhanced in the presence of other surfactants.
Technology and Applications
PEG-150 distearate is mainly used as a micellar thickening agent for surfactant-based cleansers, especially shampoos, shower gels and face washes.5 Although the exact mechanism of rheology modification by PEG-150 distearate in surfactant solutions is not fully elucidated, it generally is accepted that PEG-150 distearate would function similarly to other low MW telechelic hydrophobically modified polymers.6–9
Such molecules incorporate into surfactant micelles, where they can:
- increase the hydrodynamic size and viscous drag of micelles in solution by extension of hydrated PEG chains into the aqueous phase;
- increase the micellar radius of curvature to promote wormlike micelle formation—due to incorporation of the bulky C18 stearate hydrophobes into the micellar core and shielding of electrostatic head group repulsions between ionic surfactants by the nonionic PEG chains; and
- physically crosslink micelles via intermicellar bridging.
Individually or in combination, each of these mechanisms typically leads to increases in the bulk viscosity of surfactant solutions. Besides an increase in bulk viscosity, PEG-150 distearate may impart other rheological effects; for example, intermicellar physical crosslinking can dramatically increase the storage modulus, i.e. elasticity, of a micellar surfactant solution.
An important specification for PEG-150 distearate is the saponification value, a measure of the degree of esterification of the PEG ester, which is reported in mg KOH/g PEG ester. For PEG-150 distearate, a higher saponification value generally indicates a higher level of disubstituted PEG chains in the material, which corresponds to higher thickening efficiency.