Surfactant-Free Emulsions & Polymer-Stabilization of Emulsions Used in Skincare Products

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Most of the skincare products are intrinsically unstable, as per the second law of thermodynamics, as these products signify a combination of two or more substances that are immiscible in each other. For assured shelf life, appropriate stabilizers need to be added to these products. Conventionally, ionic or non-ionic surfactants are added as emulsifiers.

Amphiphiles with such low molecular weight are identified to cause incompatibility of cosmetics with the skin. Hence, the cosmetic industry has been looking for surfactant-free emulsions to substitute the traditional formulations. To produce adequately stable products with a pleasing appearance, polymeric emulsifiers or solid particles are used as stabilizers in the most potential substitutes.

Adding Polymers to Increase the Stability of Emulsions

Rather than using common formulation methods, emulsions can also be stabilized using suitable macromolecules instead of low molecular weight surfactant. The stability of an emulsion is often increased by adding polymers for thickening and adding yield value to the continuous phase.

However, to be more productive, surface-active polymers like hydroxypropyl methylcellulose or carbomer 1342 can be used as primary emulsifiers. Structured interfacial films that successfully prevent the amalgamation of oil drops are produced by these polymers. In such cases, the increase of viscosity of the external phase plays an insignificant role in the stabilization.

Defining the Properties of an Emulsifier

A formulation concept such as this is usually called as hydrolipid dispersion or hydro dispersion gel, which is rather used for sun care products that are thus called “emulsifier-free” formulations. From a physicochemical perspective, this is incorrect. (According to the International Union of Pure and Applied Chemistry, the properties of an emulsifier are defined as follows: An emulsifier is a surface-active substance. It lowers the interfacial tension of the medium in which it is dissolved and, accordingly, is positively adsorbed at interfaces. Small amounts of emulsifiers facilitate the formation of an emulsion or enhance its colloidal stability by decreasing either or both of the rates of aggregation and coalescence.)

Polymeric Emulsifiers and Traditional Emulsifiers

These formulations can be differentiated from emulsions stabilized with “traditional” emulsifiers pertaining to their irritative potential: Polymeric emulsifiers have a high molecular weight and hence are unable to penetrate into the stratum corneum. Therefore, undesired interactions like Mallorca acne have not been anticipated. For this reason, they are termed “emulsifier-free.” Table 1 lists a few classic examples.

Table 1. Examples of polymer-stabilized emulsions.

How Polymer-Stabilized Emulsions are Processed

Acrylate/C10-30 alkyl acrylate cross-polymer is used as a polymeric emulsifier in Formulation A. Hydroxypropyl methylcellulose and polyacrylic acid are used as co-stabilizers. The copolymers of acrylic acid are Carbomer 1342 polymeric emulsifiers that are modified by C10-30-alkyl acrylates and cross-linked with allylpentaerithrol.

The lipophilic alkyl acrylate portion is dominated by the hydrophilic acrylic acids portion. The resulting giant molecule’s molecular weight is 4 x 10⁹. The material does not dissolve, but after neutralization with a suitable base, it expands 1000-fold.

Carbomer Polymeric Emulsifiers

Carbomer polymeric emulsifiers in aqueous phase with low electrolyte concentration, form thick protective gel layers surrounding every oil droplet, where the hydrophobic alkyl chains are anchored in the oil phase. Emulsification up to 20% oil is enabled with standard usage levels of just 0.1% to 0.3% of the polymeric emulsifier.

What Happens When Carbomer Polymeric Emulsifiers Touch the Electrolyte-Containing Surface of the Skin?

The emulsions become unstable as soon as they come into contact with the electrolyte-containing surface of the skin, as the protective gel layer de-swells immediately. Once the oil phase is discharged, a thin oil film gets deposited on the skin. This process enables an easy formulation of sun care products that are waterproof even with their hydrophilic properties, during application.

Preparing Stabilized Emulsions Using the Homogeneous Indirect or Direct Method

Emulsions that are stabilized with acrylate/C10-30 alkyl acrylate cross-polymer can be produced using the direct method or the indirect method (see Table 2).

Table 2. Scheme for the preparation of hydro dispersion gels with a polymeric emulsifier by the indirect or direct method

Polymeric emulsifier 1 = INCI-Name: Acrylates/C10-30 Alkyl Acrylate Cross-polymer.

The Processing Method for High-Performance Homogenizers

To prevent mechanical degradation of the polymeric emulsifiers with high molecular weight, high-performance homogenizers should be cautiously used, since the stability of emulsion would be reduced. In general, such formulations exhibit a mean droplet diameter of 20–50 µm. But this has no negative effect on physical stability.

It is recommended to add an amphiphilic co-emulsifier like sorbitan monooleate if a finely dispersed system (1–5 µm) is chosen for aesthetic purposes. Yet, formulations such as these can never be stated as “emulsifier-free.”

Using Hydroxypropyl Methylcellulose (HPMC) as Polymeric Emulsifier

Although Formulation B (see the bottom portion of Table 1) is also of the hydrolipid dispersion type, it uses only hydroxypropyl methylcellulose (HPMC) as a polymeric emulsifier.

What are the Results When Hydroxypropyl Methylcellulose (HPMC) is Used as Polymeric Emulsifier?

Preparations with HPMC as polymeric emulsifier are less reactive to electrolytes, in contrast to the hydrolipid dispersions with carbomer 1342 polymeric emulsifiers. Consequently, o/w emulsions in which a normal saline solution is used as the external phase remain stable on storage.

A partial breakdown of the emulsions may occur due to the mechanical stress upon application to the skin and a thin oil film develops on the skin, thus minimizing the wettability of the skin. The emulsion remains partially on the skin after the water gets evaporated, thus forming a flexible film in which oil droplets are fixed into a polymer matrix.

How to Prepare Hydroxypropyl Methylcellulose (HPMC) Stabilized Emulsions

HPMC stabilized emulsion is prepared by using a rotor stator homogenizer, such as Ultra Turrax^®. The homogenizer produces a small droplet size of 2–5 µm. High-energy input from ultrasound or high-pressure homogenization can be used to achieve nanoemulsions with a mean diameter of 100–500 nm.

The Technique for Cold Processing of Hydroxypropyl Methylcellulose (HPMC) Stabilized Emulsions from Liquid Lipid Phases

It is possible to cold-process HPMC-stabilized nanoemulsions from liquid lipid phases. To obtain a crude pre-emulsion, the liquid oil phase and the aqueous polymer solution are combined at room temperature. The pre-emulsion is passed several times via a high-pressure homogenizer at pressures of 20–90 MPa to obtain the final nanoemulsion.

Although an additional increase in pressure above the optimal range is technically possible without any issues, it usually results in an increase in droplet size and not in the desired higher degree of dispersion as anticipated. This phenomenon, known as over-processing, is a common characteristic of polymer-stabilized emulsions.

Sterilizing Hydroxypropyl Methylcellulose (HPMC) Stabilized Emulsions in an Autoclave

One more notable characteristic of HPMC-stabilized emulsions is that they can be sterilized in an autoclave without significantly affecting their quality. This is because they exhibit a thermally reversible sol-gel transition. The external phase gels and arrests the movement of dispersed oil drops, above a temperature of 60 °C.

The droplets are unable to collide and the rate of coalescence is almost insignificant. Therefore, a preservative-free o/w emulsion can be formulated by the formulators, if a recontamination-proof packaging is used.

What are “Quasi”-Emulsions?

As previously stated, emulsions can also be stabilized exclusively by the viscosity-optimizing effect of the added polymer, such as carbomer (polyacrylic acid). These preparations are termed as “quasi”-emulsions as there is no involvement of interfacial activity in the stabilizing action of the polymer. Suitable commercial products usually named “balm” often contain much smaller amounts of lipids spread in a hydrogel.

Achieving Physical Stability by Dispersing Lipids and How to Avoid Creaming and Coalescence of Oil Droplets

Fine dispersion of the lipids ensures physical stability and sufficient shelf life. Droplet mobility is minimized by this measure and the yield value of the external phase, thereby effectively inhibiting creaming and coalescence of the oil droplets.