Selecting Silicone Surfactants for Personal Care Formulations

Silicone surfactants such as a dimethicone copolyol contain hydrophobic and hydrophilic portions enabling them to lower the surface tension of water.¹ The reduction of surface tension is a necessary first step in providing foam, emulsification, wetting and other surfactant properties. Each of these surfactant properties requires a molecule that lowers surface tension. Put another way, all molecules capable of foaming, emulsifying or wetting must be able to lower the surface tension, but not all molecules that lower surface tension provide these properties. The lowering of surface tension depends on the presence of hydrophilic and hydrophobic portions in the molecule. Additional surfactant properties depend on the structure of the molecule and its activity at the surface.

The function of dimethicone copolyol or any other silicone compound alone in aqueous solution may be of academic interest. However, it is of limited interest to a formulator because formulations are never simply water and dimethicone copolyol. The key to formulation is the interaction between the surfactants and other ingredients that alter the performance of the surfactants at the surface. There are interactions between different formulation components and understanding them and optimizing them for a given effect is key to formulation success.

This article is intended to educate cosmetic chemists in the chemistry of dimethicone copolyols and their potential effects in surfactant systems, such as shampoos or body washes. It will investigate some of the interactions between selected dimethicone copolyol compounds and a fatty surfactant and how they alter the properties of a solution or formulation. The structures of materials chosen for evaluation are shown in Figure 1. Sodium lauryl sulfate (SLS) and sodium laureth-2 sulfate (SLES-2) were chosen because they are commonly used in personal care products. Table 1 outlines the molecular weight information, the INCI name and the shorthand used to designate the compounds in this article. 

Surface Tension in Aqueous Solutions 

Aqueous solutions were prepared with the various materials at 1% by weight. The surface tension of each material was determined using a tensiometer^a.

Table 2 lists the results and clearly shows that the sulfated fatty alcohol surfactants have a surface tension in the range of 30–32 dynes/cm². The silicone surfactants have lower surface tension, in the range of 21–28 dynes/cm². The variation of surface tension within the class of silicone compounds is noteworthy. There has been a tendency to make generalizations that all silicone surfactants have essentially identical surface tension values. Clearly, this is not the case. As the silicone molecule contains less and less silicone, the surface tension becomes more like that of a fatty surfactant. 

The surface tension is determined by the orientation of the surfactant molecule at the air/water interface. More specifically, surface tension is determined by the orientation of the organic functional groups on the surfactant molecule. These groups include silicon-containing portions, methyl groups, methylene groups and polyoxyalkylene groups. Action at the interface depends on the group that predominates at the surface when the molecule is in the lowest free energy conformation. The silicone portion of the molecule has an abundance of methyl groups, which makes the surface tension lower. The fatty surfactant groups have an abundance of methylene groups (-CH₂-), which makes the surface tension higher. 

It is important to note that all silicone surfactants do not have the same low surface tension. Molecules that have long chains of ethylene oxide or propylene oxide have surface tensions like fatty surfactants, not silicone surfactants. As will be shown, the performance in formulations is complex; it depends upon the other components present.

Surface Tension in Binary Mixed Systems 

Water is a unique material in that it orientates itself by hydrogen bonding. A hydrogen bond is a special type of dipole-dipole force that exists between an electronegative atom and a hydrogen atom bonded to another electronegative atom. Hydrogen bonding results in an orientation of molecules that have the lowest energy in the solution. This lowest energy state is favored. It results in the high surface tension of water. The reason oil and water separate from each other is that the two separate phases are at lower energy than when they are together. Simply stated, the number of hydrogen bonds between water molecules that need to be disrupted to keep oil in a water phase results in the separation of the phases being the lowest energy.

Surfactants (fatty or silicone) experience hydrogen bonding in water. If there are several different surfactant types in water the interaction becomes more complicated albeit still driven by achieving the lowest energy.

The combination of SLS or SLES-2 with the various dimethicone copolyols suggests numerous possible interactions:

•Interactions from incompatibilities of the silicone, fatty and water-soluble domains in the surfactant. As with the oil and water interaction just described, these domains are incompatible with each other.

•Interactions from hydrogen bonding occurring between polyoxyalkylene domains of one molecule interacting with polyoxyalkylene domains or polar domains on another molecule. The nature of all of these interactions collectively determines the surface tensions of the various blends. 

DMC SLES-2 systems: Blends of SLES-2 at 95%, 90% and 50% with each DMC were prepared in solution with 1% of the blend and evaluated for surface tension. Table 3 shows the results. Only DMC-1 had an impact on the surface tension of the solution. Of the four DMCs tested, DMC-1 had the lowest molecular weight. The interaction between the various functional groups in a formulation and the stability of the resulting complexes is critical to functionality of the formulation. If lowering surface tension is the goal of the addition, DMC-1 is the only DMC that will effectively accomplish the goal.

DMC SLS systems: Blends of SLS at 95%, 90% and 50% with each DMC were prepared in solution with 1% of the blend and evaluated for surface tension. Table 3 shows the results. As in the case of SLES-2, only DMC-1 had an impact, albeit slight, on the surface tension of the solution.

Foam and Wetting in Aqueous Systems

Table 4 shows the Draves wetting times for the neat surfactants at 1% in water. SLS and SLES-2 are both good wetting materials and good foaming compounds. DMC-1 is a good wetter and a fair foaming agent. DMC-2 and DMC-3 are neither good wetting agents nor good foaming compounds.

Foam and Wetting in Binary Mixed Systems

DMC and SLES-2 systems: Because SLES-2 is a high foaming surfactant, it was expected that the addition of DMC to SLES-2 would not improve foam. In fact, that is what happened. At concentrations of up to 10% added DMC, there was no negative effect upon foam or wetting with all blends of SLES-2. The foam was adversely effected with 50% added DMC. Table 3 shows the results.

DMC and SLS systems: At all concentrations of added DMC, there was no negative effect upon foam or wetting with all blends of SLS. However, DMC-4 improved wetting in SLS systems. 

Table 3 shows the result.

Simple Shampoo System

Materials and methods: The effect of DMC compounds on simple shampoos was studied using Formula 1. The results are shown in Table 5. Conditioning on hair swatches was evaluated on a scale from 1 (worst) to 5 (best).

Results and discussion: The selection of a silicone to add to a shampoo formulation—even a very simple one—depends upon the effect desired. The appropriate silicone can be determined only in the formulation and can have no relationship to the properties of the silicone in solution alone. 

Table 5 makes the following points for this simple formulation:

•For wetting effects, DMC-1 provides the best results;

•For foaming effects, DMC-4 provides the best results; and

•For conditioning effects DMC-2 provides the best results. 

These results would not have been predicted from the data generated by evaluating either surfactant in water. The finished formula’s raw materials, taken as a whole, are critical to determining the effectiveness of adding the silicone. There are significant interactions between surfactants in a formulation that alter the properties obtained when formulated together. The cosmetic formulation is more than merely the sum of its ingredients.

Conclusion 

The selection of dimethicone copolyol for inclusion in hair care products is a complex process. The use of INCI names alone will be fruitless for picking the proper dimethicone copolyol for use in formulations. Likewise, the use of dimethicone copolyol’s properties themselves in water to predict the functionality in formulation can be misleading. This is because there are various interactions between the dimethicone copolyol and the other surface active agents in the formulation. The formulation itself needs to be tested to determine if the formulation performs as desired. 

The best test will be in the salon because in the final analysis consumer perception is the key to formulation performance. Dimethicone copolyols can be engineered to be formulator-friendly and provide the desired effect(s) in formulations. 

It also needs to be noted that the compounds studied in this project are nonionic silicone compounds, an important but limited class of materials. Improved conditioning can be obtained by working complexes of anionic and cationic silicones, designed specifically for that application.

Acknowledgements: The authors gratefully acknowledge the financial support of SurfaTech Corp. and Colonial Chemical for funding the research. The authors are also grateful to Siltech LLC and Colonial Chemical for providing the silicone and fatty surfactants used in this study.

References

1.AJ O’Lenick Jr, TG O’Lenick and L Anderson, Mixed fatty/silicone surfactant systems, Cosmet Toil 122(8) 49–54 (2007)