Comparatively Speaking: Trisiloxane vs. Dimethicone Copolyol

By: Anthony J. O'Lenick Jr., Siltech LLC

Posted: January 19, 2011

Dimethicone copolyol compounds, which are referred to in the INCI nomenclature as PEG/PPG dimethicone, have “D” units. Figure 1 shows the structure of PEG-8 dimethicone, a dimethicone copolyol, with “a” units substituted for "D" units. There are two classes of dimethicone copolyol compounds, one being trisiloxanes, which do not have "D" units (see Figure 2).

There are several reasons a formulator would be interested in the difference between dimethicone copolyols and trisiloxane; one is hydrolytic stability and another is eye irritation. Trisiloxanes are not as stable as dimethicone copolyols in formulations with high or low pH levels, and if the compounds degrade over time, their use is not appropriate in personal care products.

Hydrolytic Stability—Surface Tension Study

Silicone trisiloxane surfactants are generally added to a formulation to provide wetting, and the pre-requisite for wetting is the lowering of surface tension. Thus, surface tension measurements were conducted with 0.1% solution at RT, which was made by mixing 0.6 g silicone polyether with 600 mL of deionized water.

The solution was then divided into three parts in 4-oz bottles: silicone polyether “as is” without any pH adjustment; silicone polyether adjusted to pH 10 with ammonium hydroxide; and silicone polyether adjusted to pH 4 with acetic acid. The pH of the solution could be re-adjusted to the desired value after the first week, as needed.

The surface tension of the trisiloxane PEG-8 dimethicone over time at different pH levels is shown in Table 1, whereas the surface tension of a silicone polyether over time at different pH levels is shown in Table 2.