Dimethicone refers to a silicone compound that has “D” units, meaning only silicon atoms with two methyl groups attached. Dimethicone compounds are common and this term encompasses silicone fluids, PEG-8 dimethicone, cetyl dimethicone and many others (see Figure 1).
The presence of the “D” unit, written in parenthesis with an “a” subscript, makes a compound a dimethicone. The “D” unit is shown in Figure 2.
A methicone, on the other hand, completely lacks the “D” unit. In place of a "D" unit are organofunctional groups. An example is cetyl methicone, which conforms to the structure shown in Figure 3.
If any “D” units are present, the compound is a dimethicone. Consequently, the molecule shown in Figure 4 is cetyl dimethicone. Strangely enough, cyclomethicone has four "D" units in cyclic form, yet it is called cyclomethicone not cyclodimethicone. No one ever said the names would be logical!