Stearic acid is a fatty acid that increases hydration and helps keep other skincare ingredients intact and stable. It’s also used as an Emulsifier; Cleaning agent; Emulsion Stabilizer; Surfactant; Refatting agent and Lipophilic Thickener.

Stearic acid is naturally found in skin’s stratum corneum (SC) and is used in cleansers to reduce cleansing-induced damage to the SC, aid in moisturization, and replenish the SC with skin-natural lipids. In this case, stearic acid has two functions: to act buffer against solubilization of the SC lipids into surfactant micelles and to deposit stearic acid into the SC to replace the fatty acids lost during normal cleansing.

The amount of deposited stearic acid is equivalent to that extracted during washing and is incorporated into the SC 10 layers deep. Because fatty acids are readily extracted during surfactant use, their replenishment during cleansing is important for barrier protection.
This approach of mild surfactant combined with skin-identical lipids is enhanced by the inclusion of triglyceride oils (e.g., sunflower, soybean), which can bind to SC proteins, impeding surfactant access and providing protection from cleanser-induced damage. The visual and clinical benefits of this DEFI-based mild and moisturizing cleansing technology have been seen in patients with eczema, winter xerosis, and patients with skin of color.

Sources
Journal of Clinical Aesthetic Dermatology. 2013 Jun; 6(6): 23–30.