What it is
Isostearic acid is a branched-chain isomer of stearic acid, sharing the formula C18H36O2 but carrying a methyl branch on the chain instead of a straight backbone. That branching lowers its melting point so it stays liquid and oil-soluble at room temperature, and it gives the material good odor, thermal, and oxidative stability compared with straight-chain fatty acids. In cosmetics it acts as an emollient that imparts a silky feel and as a co-emulsifier and dispersant, where it is especially good at wetting and stabilizing pigments and mineral particles in oil-based systems such as foundations and mineral sunscreens.
Origin & sourcing
It is typically manufactured from oleic acid, an unsaturated plant-derived fatty acid, by isomerization that rearranges the molecule into a saturated, branched structure. It is often described as a vegetable-derived fatty acid, and the branched form is valued for its liquid state and stability.
Questions everyone asks
How is isostearic acid different from stearic acid?
They have the same chemical formula, but isostearic acid is branched, which keeps it liquid and oil-soluble, while stearic acid is straight-chained and waxy.
Why is it common in mineral makeup and sunscreens?
It helps wet and disperse pigments and mineral particles evenly in the oil phase, improving spread and a smooth finish.
Is it plant-derived?
It is typically made from oleic acid sourced from vegetable oils, then isomerized into its branched form.