Aliphatic branched chain hydrocarbons – always in style

Petroleum, a complex mixture of hydrocarbons, has long been used in cosmetics either as a starting material to synthesize other compounds. or as a material processed to give specific fractions to be used as is. Those materials include such long time favorites as mineral oil, petrolatum and the like.

Aliphatic-branched chain hydrocarbons or isoparaffins (known by the trade name Permethyl®) belong to a class of petroleum-based compounds which offer some very interesting formulation opportunities. These materials offer the benefits of being highly branched chained structures of high purity, non-reactive, inert molecules with excellent solvent and emollient properties.

Their basic structure is:

Purity

A major advantage of the aliphatic-branched chain hydrocarbons is their purity as determined by the high percentage of the material being comprised of one major chain length molecule. Table 1 reflects the purity of the three main isoparaffins in the Permethyl product line.

While other materials using the isoparaffin nomenclature are available for cosmetic use their composition is governed by a chain length distribution rather than by single fractions. Table 2 reviews the purity of other isoparaffinic hydrocarbons. This distribution of chain lengths can contribute to potentially increased skin irritation, discoloration and odor problems that can affect a finished product in a negative way.

A comparison of the data in Tables 1 & 2 confirms that not all materials classified as isoparaffins are the same and as such cannot be expected to possess similar properties and / or behave similarly in the same formulation.

Properties of the aliphatic branched chain hydrocarbons

Compatibility with silicones and other cosmetic fluids

The aliphatic-branched chain hydrocarbons exhibit excellent compatibility with many of the most widely used cosmetic fluids but especially with the polydimethylsiloxanes. This property means that the aliphatic-branched chain hydrocarbons can be used to modify high viscosity silicones to make them easier to handle for inclusion in a variety of cosmetic formulations. A major reason for the high compatibility with silicones can be explained by reviewing the structure of both materials. Figures 1 and 2 show the typical structure of polydimethylsiloxanes and of aliphatic-branched chain hydrocarbons.

A comparison of both indicates the strong structural similarities between the molecules, which explains the excellent compatibility and miscibility of these materials.

Effect of branching on product properties

Several physical properties were studied to determine what effect branching may have versus similar straight chain molecules. In Table 3 the data indicates differences are seen in specific gravity, pour point and to a lesser extent refractive index. The most interesting difference is seen in the pour points. The isoeicosane becomes an easily handled liquid due to branching.

Spreading

The performance of a wide range of products that are applied to the skin is determined by the ability of the product to spread evenly on the skin and maintain a continuous film. This is especially important in sun care products where good protection and high SPFs rely of uniform, continuous films being applied to the skin.

The spreading coefficient of a liquid determines how well that material will wet the surface to which it is applied and is determined by the equation:

So/w = sw/a - Yo/w - so/a

Where:

S = Spreading coefficient

s = Surface tension

Y = Interfacial tension

Very low and on occasion negative spreading coefficients are obtained for non-polar emollients, whereas vegetable oils tend to have high spreading coefficients due to the inclusion of surface-active materials, which effect the interfacial tensions. Another equation used to determine the spreading coefficient makes use of the contact angle of a droplet of liquid on a solid surface (in cosmetics the solid surface is usually skin or hair). The equation then becomes:

Sl/s = sl/a (Cosq - 1)

and is represented diagrammatically in Figure 3.

The contact angles of several common cosmetic fluids were measured (see Table 4) using a goniometer with the oil droplet applied through a 27 gauge needle onto dermal tape skin strippings (1). Isohexadecane was found to show spontaneous spreading, which means that the contact angle = 0º and could not be measured.

Another study was done on human skin using volunteers where 4 mg of the test product was applied and the area covered was measured. The data in Table 5 are comparable with the contact angle data reflecting the poor spreading properties of vegetable oils and an improvement as we get to the more non-polar materials. In this test the isohexadecane was shown to be the most efficacious of the oils tested.

Applications

The following table gives an overview of the various properties and applications of the aliphatic-branched chain hydrocarbons for use in cosmetics.

Sample formulations

The utility of the aliphatic-branched chain hydrocarbons can be demonstrated in Formulations 1-3 overleaf.

Double action make-up remover (Formulation 1)

The make-up remover formulation makes use of the fact that Isohexadecane is an excellent solvent for makeup and sebaceous deposits. In addition the formulation demonstrates the good compatibility of the aliphatic-branched chain hydrocarbons with silicone.

Moisture lotion spray (Formulation 2)

This formulation demonstrates the ability of the aliphatic-branched chain hydrocarbons to be blended to produce a desirable effect. The isododecane is a good solubilizer and more volatile and this helps to improve fragrance lift, whereas the isohexadecane provides a light silky feel to the skin.

O/W mascara (Formulation 3)

This formulation demonstrates the application of our newest product a blend of isododecane and polyisobutene (Permethyl 98B). This product has been found to be an excellent addition to mascara products to give added curl, lash thickening and lash lengthening effects.

Properties

An overview of the physico-chemical properties and information regarding chemical nomenclature is given in Table 6 on page 132. These properties can be used when determining which materials is best suited for a specific product application.

Safety

The assessment of a new class of compounds would not be complete without reviewing their safety profile. We are at a time in the development of cosmetic science where safety has become a primary concern of marketers and consumers alike. A summary of the safety testing we used on these products is given in Table 7. The aliphatic-branched chain hydrocarbons have been tested and found to be

non-comedogenic.

Summary

The aliphatic-branched chain hydrocarbons have been shown to have some very interesting properties and compatibilities making them ideal for use in a wide variety of cosmetic and personal care applications.

In addition to their compatibility with silicones and many other cosmetic fluids, their excellent spreading properties, purity and safety profile, make them obvious choices for use in a wide variety of new formulations.

The use of these materials in the cosmetic industry currently extends to their inclusion in well over 200 finished products marketed in the USA alone and over 450 finished products globally.