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.
The surface tension measurements shown in Table 1 indicate that the product hydrolyzes after five days at pH 4. In addition, surface tension measurements indicate the product hydrolyzes after seven days at pH 10. The development of haze predicts the instability even earlier. The trisiloxane at pH 4 became hazy the next day, while the pH 10 sample became hazy after three days.
The products tested without pH adjustment were stable at RT for at least eight weeks and there were no observable changes in surface tension for the silicone polyethers at 0.1% in water. The solutions remained clear and the surfactants did not hydrolyze in the water during the 28 days of sitting at RT.
Trisiloxane solutions adjusted to pH 10 were not stable after three days of sitting at RT. The solution became hazy after three days and remained hazy at the end of the experiment.
Trisiloxane solution adjusted to pH 4 with acetic acid was not stable after one day at RT. The solution became hazy over night and the surface tension increased rapidly in the first week of sitting. The solution became clear again and some oily liquid was found on the inside walls of the bottle. It also lost the foaming capability gradually during the first week of sitting.
Dimethicone copolyol at pH 10 and pH 4 was found to be stable at RT after eight weeks, and there were no obvious changes in the surface tensions and foaming properties for these silicone surfactants.