Oil Wetting vs. Water Wetting

Tony O’Lenick explores the difference between oil wetting and water wetting.

Surfactants are molecules that consist of two groups that are normally insoluble in each other--commonly, an oil-soluble and a water-soluble group. This makes them amphiphilic since oil and water are two immiscible phases. The dual nature of the molecule makes the molecule surface active.

In the case of materials that possess water-soluble and oil-soluble groups, surfactant properties are observed in the solvent in which they are soluble, most often water. The molecules are in the lowest free energy state at the interface between water and air or in micelles, depending upon concentration. The orientation of the molecules at the surface and in micelles allows the surfactant to reduce surface tension, wet, foam and provide detergency depending the exact structure. While surfactants possessing water- and oil-soluble groups are the most common type of surfactant, they are by no means the only type.

Another equally important but often overlooked surfactant system is one in which the molecule possesses a silicone-soluble group and an oil-soluble group. Such a molecule is stearyl dimethicone. The molecule is amphiphilic just like the molecule possessing the water-soluble and oil-soluble group. Stearyl dimethicone, when added to oils like mineral oil, or more polar oils like fatty esters, orientates itself at the air/oil interface.

The presence of the silicone molecule alters surface tension, generally lowering it from around 32 dynes/cm² to 22 dynes/cm². As the concentration is increased, micelles form. The lowering of surface tension results in oil wetting, improving the ability of the oil to wet pigments. The lowering of surface tension also alters the aesthetics, making the oil spread better on hair and skin.

The ability to alter the surface tension of oils is every bit as powerful a formulating tool as the ability to alter surface tension in aqueous systems. The ability to make micelles from alkyl silicones results in the ability to gel oils and alter melting point. These effects are important not only in oily systems but also in both invert and standard emulsions.