The most important factor that determines if a PEG-8 dimethicone polymer is soluble, insoluble or dispersible in water is the ratio of D:D* units present in the molecule.
Note: D* refers to water soluble groups; D refers to silicone soluble groups.
This critical parameter is not reflected in the INCI name. The D:D* ratio is determined as follows: the number of D units (i.e., the number of dimethyl containing groups, or a) divided by the number of D* units (i.e., the number of organofunctional units, or b). See Figure 1.
Figure 1. PEG-8 dimethicone structure (generic)
If the result is less than or equal to 3, the silicone compound will be water soluble. If the number is 4 or higher, the polymer will not be soluble.
The ability to regulate the structure of a silicone polymeric PEG/PPG surfactant to form a microemulsion in a formulation presents a useful mechanism to maximize deposition. Simply put, if PEG/PPG dimethicone is too water soluble, it will wash down the drain during rinsing and provide little or no additional benefit to the formulation. Thus, in order for PEG/PPG dimethicone to remain clear and soluble and efficiently impart lasting deposition in a system, the PEG/PPG dimethicone used should form a microemulsion in the formula tested.
The simplest way to achieve this is to alter the D/D* ratio (the ratio of a:b). In doing so, the solubility is altered such that the least soluble product without insolubility can be added to a system, in turn depositing to the substrate with the lowest free energy. Put another way: if you want the best delivery on a substrate from a clear formulation, deliver the silicone polymer as a microemulsion.
Figure 2 shows the effect of altering the D/D* ratio in PEG dimethicone. In fact, it is relatively easy to use a calculation to effectively determine the number of PEG-8 units in a silicone polymer to make a microemulsion.
Figure 2. PEG-8 dimethicone solubility
The calculation to determine PEG-8 dimethicone solubility is as follows:
Minimum Needed b Units = (0.1499)(Actual Number of a Units) + 1
where a and b are the subscripts shown in Figure 1. This means if the number of b units is above the calculated value, an appreciable concentration of insoluble oligomer would not be present. If the value of b in the polymer is lower than that required in the calculation (based on the number of a subunits), a soluble product is impossible since there will be an appreciable amount of insoluble polymer present in the oligomer mixture.
Choosing a D:D* ratio to provide a silicone polymer that forms a microemulsion is a simple, cost-effective method to improve the efficiency of a PEG/PPG dimethicone in formulation. And remember, the INCI name for all ratios of D:D* is the same.