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Surfactant-Free Emulsions and Polymer-Stabilization of Emulsions Used in Skin Care Products

Topics Covered

Background

Adding Polymers to Increase the Stability of Emulsions

Defining the Properties of an Emulsifier

Polymeric Emulsifiers and Traditional Emulsifiers

How Polymer-Stabilized Emulsions Are Processed

Carbomer Polymeric Emulsifiers

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

Preparing Stablized Emulsions Using the Homogeneous Indirect or Direct Method

The Processing Method for High Performance Homogenizers

Using Hydroxypropyl Methylcellulose (HPMC) as Polymeric Emulsifier

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

How to Prepare Hydroxypropyl Methylcellulose (HPMC) Stabilized Emulsions

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

Sterilizing Hydroxypropyl Methylcellulose (HPMC) Stabilized Emulsions in an Autoclave

What are “Quasi”-Emulsions?

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

Background

Since most skin care products represent a mixture of two or more materials that are not miscible in each other, they are, according to the second law of thermodynamics, inherently unstable. They require the addition of suitable stabilizers to guarantee an appropriate shelf life. Traditionally, ionic or non-ionic surfactants are used as emulsifiers. However, such low molecular weight, amphiphiles are known to cause incompatibilities of cosmetics with the skin. Consequently, the cosmetic industry has been seeking for surfactant-free emulsions as alternatives to the conventional formulations. The most promising alternatives use polymeric emulsifiers or solid particles as stabilizers in order to yield sufficiently stable products with a pleasant appearance.

Adding Polymers to Increase the Stability of Emulsions

In contrast to the traditional formulation concept, emulsions can be stabilized also by appropriate macromolecules without using any low molecular weight surfactant (see figure 1). Polymers are frequently added to increase the stability of an emulsion by thickening and adding yield value to the continuous phase. But it is, however, much more effective to use surface-active polymers, e.g. carbomer 1342 or hydroxypropyl methylcellulose, as primary emulsifiers. Such polymers form structured interfacial films, which effectively prevent the coalescence of oil drops. In this case, the increase of the viscosity of the external phase plays only a minor role for the stabilizing action.

Figure 1. Schematic representation of the structure of different interfacial films.

Defining the Properties of an Emulsifier

Such a formulation concept - often named hydrolipid dispersion or hydro dispersion gel - is preferably used for sun care products which are consequently called "emulsifier-free" formulations. This is not correct from a physicochemical point of view. (The International Union of Pure and Applied Chemistry defines the properties of an emulsifier 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 enhances its colloidal stability by decreasing either or both of the rates of aggregation and coalescence.)

Polymeric Emulsifiers and Traditional Emulsifiers

However, such formulations can be distinguished from emulsions stabilized with "traditional" emulsifiers concerning their irritative potential: Due to their high molecular weight, polymeric emulsifiers are not able to penetrate the stratum corneum. Thus, unwanted interactions, such as Mallorca acne, have not to be expected. Therefore, the term "emulsifier-free" is used. Table 1 shows some typical examples.

Table 1.  Examples of polymer-stabilized emulsions.

Ingredient (INCI Name)

Weight (% m/m)

Function

Formulation A

Water phase

Aqua

73,37

Diluent

 

Glycerine

2,50

Humectant

 

Disodium EDTA

0,03

Chelating agent, Stabliizer

Lipid phase

Octyl Methoxycinnamate

7,50

UV B-filter

 

Octyl Salicylate

5,00

Broad spectrum UV-filter

 

Benzophenone-3

5,00

Broad spectrum UV-filter

 

C12-C15 Alkyl Benzoate

4,00

Emollient

 

Sorbitan Oleate

0,30

Water in Oil Emulsifier

 

 

Acrylates/C10-30 Alkyl Acrylate
Cross-polymer

0,30

Polymeric emulsifier

 

 

Carbomer

0,30

Thickener

Additives, hydrophilic

Propylenglycol

0,80

Humectant

 

 

Diazolidinyl Urea
Parabene

0,80

Preservative

 

Triethanolamine

0,70

Neutralizer

Formulation B

Water phase

Aqua

78,0

Diluent

 

Hydroxypropyl Methylcellulose

2,0

Polymeric emulsifier

 

Lipid phase

Caprylic/Capric Triglyceride

15,0

Emollient

 

Triticum Vulgare

5,0

Emollient

How Polymer-Stabilized Emulsions Are Processed

Formulation A uses acrylate/C10-30 alkylacrylate cross-polymer as a polmeric emulsifier. Polyacrylic acid and Hydroxypropyl methylcellulose are used as co-stabilizers. Carbomer 1342 polymeric emulsifiers are copolymers of acrylic acid, modified by C10-30-alkyl acrylates, and cross-linked with allylpentaerithrol. The hydrophilic acrylic acids portion dominates the lipophilic alkyl acrylate portion. The molecular weight of the resulting giant molecule is 4 x 10 exp. 9. The substance swells 1000-fold after neutralization with an appropriate base, but it does not dissolve.

Carbomer Polymeric Emulsifiers

In aqueous media with low electrolyte concentration, carbomer polymeric emulsifiers form thick protective gel layers around each oil droplet with the hydrophobic alkyl chains anchored in the oil phase. This allows us to emulsify up to 20 per cent oil with typical usage levels of only 0.1 to 0.3 per cent of the polymeric emulsifier.

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

Upon contact with the electrolyte-containing surface of the skin, such emulsions become unstable because the protective gel layer deswells instantly. The oil phase is released and a thin oil film deposits on the skin. This mechanism allows an easy formulation of sun care products which are waterproof, despite their hydrophilic properties during application.

Preparing Stablized Emulsions Using the Homogeneous Indirect or Direct Method

Emulsions which have been stabilized with acrylate/C10-30 alkylacrylate cross-polymer may be prepared 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.

Step

Indirect Method

Direct Method

1

Mix homogeneously all ingredients of the oil phase.

Mix homogeneously all ingredients of the oil phase.

2

Combine the ingredients of the water phase (including neutralizing alkali) to yield a clear solution.

Combine the ingredients of the water phase (without neutralizing alkali) to yield a clear solution.

3

Disperse polymeric emulsifier 1 in the homogeneous oil phase.

Disperse polymeric emulsifier 1 by sifting slowly into rapidly agitating water phase

4

Add the oil phase (containing polymeric emulsifier 1) to the water phase (containing neutralizing alkali) under vigorous agitation (15-30 min).

Disperse oil phase homogeneously in water phase (containing polymeric emulsifier 1) under vigorous agitation (15-30 min).

5

Add remaining ingredients (preservatives, fragrance) and disperse homogeneously.

Neutralize with a suitable base and mix until smooth and uniform. Add remaining ingredients (preservatives, fragrance) under moderate stirring.

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

The Processing Method for High Performance Homogenizers

High performance homogenizers should be used carefully to avoid mechanical degradation of the high molecular weight polymeric emulsifiers, which would then decrease emulsion stability. Such preparations often exhibit a mean droplet diameter between 20 and 50 µm. However, this has no negative impact on the physical stability. If, for aesthetic reasons, a finely dispersed system (1 - 5 µm) is desired, the addition of an amphiphilic co-emulsifier, e.g. sorbitan monooleate, is recommended. However, such a formulation could no longer be claimed as "emulsifier-free".

Using Hydroxypropyl Methylcellulose (HPMC) as Polymeric Emulsifier

Formulation B (see the bottom half of Table 1, above) is also of the hydrolipid dispersion type, but uses solely hydroxypropyl methylcellulose (HPMC) as a polymeric emulsifier.

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

Contrary to those hydrolipid dispersions with carbomer 1342 polymeric emulsifiers, preparations with HPMC as polymeric emulsifier are less sensitive to electrolytes. Therefore, o/w emulsions with a normal saline solution as the external phase are still stable on storage. When applied on the skin, the mechanical stress may cause a partial breakdown of these emulsions and a thin oil film spreads on the skin, thus reducing the wettability of the skin. After the water evaporates, the emulsion remains partially on the skin and forms a flexible film where oil droplets are embedded into a polymer matrix.

Figure 2. Influence of the contact angle on the stabilizing action of solid particles.

How to Prepare Hydroxypropyl Methylcellulose (HPMC) Stabilized Emulsions

The preparation of HPMC stabilized emulsion can be performed using a rotor stator homogenizer, e.g. Ultra Turrax®, yielding a mean droplet size between 2 and 5 µm. Nanoemulsions with a mean diameter between 100 and 500 nm can be achieved by using high energy input from ultra sound or high-pressure homogenization.

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

HPMC-stabilized nanoemulsions from liquid lipid phases can be processed cold: The aqueous polymer solution and the liquid oil phase are mixed at room-temperature to yield a crude pre-emulsion. The final nanoemulsion is obtained by passing the pre-emulsion several times through a high-pressure homogenizer at pressures between 20 and 90 MPa. Further increasing the pressure above this optimal range, although technically feasible without further problems, does often end up with an increase in droplet size and not as expected in the desired higher degree of dispersion (see figure 3). This phenomenon is called over-processing and is a common feature of polymer-stabilized emulsions.

Figure 3. Influence of homogenizing pressure on the particle size of HPMC stabilized emulsions.

Sterilizing Hydroxypropyl Methylcellulose (HPMC) Stabilized Emulsions in an Autoclave

Another special feature of HPMC-stabilized emulsions is that they may be sterilized in an autoclave without substantial impact on their quality. This is due to the fact that they show a thermally reversible sol-gel transition. Above 60° C, the external phase gels and immobilizes the dispersed oil drops. The droplets cannot collide and the rate of coalescence is almost negligible. Thus, formulators have the opportunity to formulate a preservative-free o/w emulsion, presumed that a recontamination proof packaging is used.

What are “Quasi”-Emulsions?

As mentioned earlier, emulsions might also be stabilized solely by the viscosity enhancing effect of the added polymer, e.g. carbomer (polyacrylic acid). Such preparations where no interfacial activity is involved in the stabilizing action of the polymer are termed “quasi”-emulsions. Appropriate products on the market, often named "balm", usually consist of considerably small amount of lipids dispersed in a hydrogel.

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

The physical stability and an adequate shelf life are obtained through finely dispersing the lipids. This measure and the yield value of the external phase reduce droplet mobility, and thus effectively prevent creaming and coalescence of the oil droplets.

Note: A complete list of references can be found by referring to the original text.

Primary author: Professor Rolf Daniels.

Source: ‘Galenic Principles of Modern Skin Care Products’, Issue 25, Skin Care Forum.

For more information on this source please visit Skin Care Forum.

 

Date Added: Jul 5, 2005 | Updated: Jun 11, 2013
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