What makes a silicone-based polymer water soluble?
Simple silicone polymers, such as dimethicone (polydimethylsiloxane) are comprised of a linear inorganic backbone of silicone and oxygen, with organic (carbon-based) pendant groups. These materials are extremely hydrophobic oils. However, several different types of chemical reactions can be utilized to add hydrophilic character to the polymers. These new polymers are amphiphilic, containing both hydrophobic and hydrophilic portions, and are classified as silicone surfactants.
Perhaps the most straightforward and popular method for rendering a silicone molecule water soluble is by adding multiple units of ethylene glycol (-OCH2CH2O-) to sites along the polymer chain. The oxygen atoms in these segments add polarity to the silicone and are readily available for association with water molecules. This process is called ethoxylation or polyethylene glycol (PEG)-modification.
PEG-modification can be done on sites that dangle from the silicone backbone, which results in a polymer shaped like a comb with hydrophilic tendrils. PEG can also be added to the terminal ends of the silicone polymer, making a straight chain surfactant type copolymer, with a hydrophilic block-hydrophobic block-hydrophilic block structure. Star-like molecules can be created by PEG-substitution occurring both at the ends of the polymer and on the pendant groups. Each type of polymer has slightly different properties.
Happily, polymer chemists have for various reasons spent time developing and optimizing water soluble silicone-based polymers...These silicones provide more options to curly ladies and gentlemen.
On product labels, these polymers were formerly denoted by the name dimethicone copolyol. The preferred nomenclature for the comb-shaped polymers now is PEG-X dimethicone, with X being the number of repeat units of ethylene glycol. The block copolymers are designated Bis-PEG-X dimethicone, and the star-shaped polymers are designated Bis-PEG-X/PEG-X dimethicone. The higher the number “x” is, the greater the water solubility. Below a threshold of approximately PEG-6, the polymer is only sparingly soluble, and when the degree of ethoxylation equals or exceeds 8, the material can be considered to be highly water soluble.
Similar modification of a different silicone results in the novel polymer Bis-PEG-18 methyl ether dimethyl silane, which is completely water soluble and highly moisturizing to skin and hair. Another interesting water soluble silicone polymer is one modified with side chain copolymers of poly glucosides (sugars), PEG-8-PG-coco glucoside dimethicone. This material is completely water soluble, has high substantivity to hair and skin, is very moisturizing, and also has sufficient surfactant qualities that it can be used as a foaming agent and mild cleanser in gentle shampoos. Silicone phosphate esters (INCI name: Dimethicone PEG-X phosphate) are another category of water soluble silicone surfactants that provide excellent moisturizing properties and act as foam boosters.
Read More: What’s the Scoop on Silicones?