Tony O’Lenick examines the difference between bis-PEG dimethicone and PEG dimethicone.
Silicone compounds are available in three types of structures. The most common is the comb or rake structure. In this type of structure the organofunctionality is on an internal position. The second type of compound is one in which the organofunctionality is present at the terminal ends. Finally, there are silicone compounds in which there are both internal and terminal groups.
Prior to 1999, all silicone compounds that contained PEG or PPG groups were classified as dimethicone copolyols. This term encompassed any amount of either polyoxyethylene (PEG) or polyoxypropylene (PPG). It also encompassed any ratio of silicone to organic functionality.
In 1999 the INCI rules were changed. The term PEG/PPG dimethicone was created. With this change, manufacturers were required to disclose the number of moles of ethylene oxide or propylene oxide present in a dimethicone copolyol; they were not required to disclose whether the organofunctionality was internal or at the molecule's terminal ends.
More recently a new nomenclature was introduced that tells the user if the molecule is terminal or internal.
Even under the current system the “a” to “b” value for the internal product or the number of “a” units in the terminal structure are present. The reason that this is critical to a formula's performance is that as the ratio of “a“ to “b” groups increases--or the number of “a” units in terminal groups increases--the silicone becomes less and less water soluble.