Title:  Emulsifier-free finely disperse systems of the oil-in-water and water-in-oil type

United States Patent:  6,391,321

Assignee:  Beiersdorf AG (Hamburg, DE)

Filed:  September 15, 1999

Foreign Application Priority Data:  Sep 18, 1998[DE] (198 42 730)

Pickering emulsions, which are finely disperse systems of the water-in-oil or oil-in-water type, comprising (1) an oil phase which contains at least one wax and/or one oil thickener, (2) a water phase, (3) at least one type of microfine particles which a) have an average particle size of less than 200 nm, which b) display both hydrophilic and lipophilic properties, i.e. which have amphiphilic character, and are dispersible both in water and in oil and which c) have optionally been coated on the surface, and (4) at most 0.5% by weight of one or more emulsifiers.

Description of the Invention

The present invention relates to emulsifier-free finely disperse systems of the oil-in-water and water-in-oil type, preferably as cosmetic or dermatological preparations.

Emulsions are generally taken to mean heterogeneous systems which consist of two liquids which are immiscible or have only limited miscibility with one another, which are usually referred to as phases. In an emulsion, one of the two liquids is dispersed in the form of very fine droplets in the other liquid.

If the two liquids are water and oil and if oil droplets are finely dispersed in water, then this is an oil-in-water emulsion (O/W emulsion, e.g. milk). The basic character of an O/W emulsion is defined by the water. In a water-in-oil emulsion (W/O emulsion, e.g. butter), the principle is reversed, the basic character here being determined by the oil.

In order to achieve permanent dispersion of one liquid in another, emulsions in the traditional sense require the addition of an interface-active substance (emulsifier). Emulsifiers have an amphiphilic molecular structure, consisting of a polar (hydrophilic) and a nonpolar (lipophilic) molecular moiety, which are spatially separate from one another. In simple emulsions, finely disperse droplets of one phase, surrounded by an emulsifier shell, (water droplets in W/O emulsions or lipid vesicles in O/W emulsions) are present in the second phase. Emulsifiers lower the interfacial tension between the phases by positioning themselves at the interface between two liquids. At the phase boundary, they form oil/water interfacial films, which prevent irreversible coalescence of the droplets. Emulsions are frequently stabilized using emulsifier mixtures. Traditional emulsifiers can, depending on their hydrophilic molecular moiety, be divided into ionic (anionic, cationic and amphoteric) and nonionic:

The most well-known example of an anionic emulsifier is soap, which is usually the term used for the water-soluble sodium or potassium salts of saturated or unsaturated higher fatty acids.

Important examples of cationic emulsifiers are quaternary ammonium compounds.

The hydrophilic molecular moiety of nonionic emulsifiers frequently consists of glycerol, polyglycerol, sorbitans, carbohydrates and polyoxyethylene glycols, and, in most cases, is linked to the lipophilic molecular moiety via ester and ether bonds. The lipophilic molecular moiety usually consists of fatty alcohols, fatty acids or isofatty acids.

By varying the structure and the size of the polar and nonpolar molecular moiety, the lipophilicity and hydrophilicity of emulsifiers can be varied within wide limits.

A decisive factor for the stability of an emulsion is the correct choice of emulsifiers. The characteristics of all substances present in the system are to be taken into consideration here. In the case of, for example, skincare emulsions, polar oil components and, for example, UV filters lead to instability. As well as the emulsifiers, therefore, other stabilizers are also used which, for example, increase the viscosity of the emulsion and/or act as protective colloid.

Emulsions are an important type of product in the field of cosmetic and/or dermatological preparations.

Cosmetic preparations are essentially used for skin care. The main aim of skin care in the cosmetics sense is to strengthen or rebuild the skin's natural function as a barrier against environmental influences (e.g. dirt, chemicals, microorganisms) and against the loss of endogenous substances (e.g. water, natural fats, electrolytes). If this function becomes impaired, increased resorption of toxic or allergenic substances or attack by microorganisms may result, leading to toxic or allergic skin reactions.

Another aim of skin care is to compensate for the loss by the skin of lipids and water caused by daily washing. This is particularly important if the natural regeneration ability is inadequate. Furthermore, skincare products should protect against environmental influences, in particular against sun and wind, and delay skin ageing.

Cosmetic preparations are also used as deodorants. Such formulations are used to control body odour, which is produced when fresh perspiration, which is in itself odourless, is decomposed by microorganisms.

Medicinal topical compositions usually comprise one or more medicaments in an effective concentration. For the sake of simplicity, in order to distinguish clearly between cosmetic and medicinal use and corresponding products, reference is made to the legal provisions in the Federal Republic of Germany (e.g. Cosmetics Directive, Foods and Drugs Act).

The use of customary emulsifiers in cosmetic or dermatological preparations is in itself acceptable. Nevertheless, emulsifiers, like ultimately any chemical substance, may in certain circumstances cause allergic reactions or reactions based on oversensitivity of the user.

For example, it is known that certain light dermatoses are triggered by certain emulsifiers, but also by various fats and simultaneous exposure to sunlight. Such light dermatoses are also called "Mallorca acne". There has thus been no lack of attempts to reduce the amount of customary emulsifiers to a minimum, in the ideal case even to zero.

A reduction in the required amount of emulsifier can, for example, be achieved by taking advantage of the fact that very finely divided solid particles have an additional stabilizing action. The solid substance accumulates at the oil/water phase boundary in the form of a layer, as a result of which coalescence of the disperse phases is prevented. It is not the chemical properties of the solid particles which are of fundamental importance here, but the surface properties.

Around 1910, Pickering prepared paraffin/water emulsions which were stabilized merely by the addition of various solids, such as basic copper sulphate, basic iron sulphate or other metal sulphates. This type of emulsion is thus also referred to as a Pickering emulsion.

The original forms of Pickering emulsions initially surfaced, as it were, as undesired secondary effects in a variety of industrial processes, such as, for example, in secondary oil recovery, the extraction of bitumen from tar sand and other separation processes involving two immiscible liquids and fine, dispersed solid particles. These are generally W/O emulsions which are stabilized by mineral solids. Accordingly, investigation of corresponding systems, such as, for example, the oil/water/soot or oil/water/slate dust systems was initially the focus of research activity.

Basic experiments have shown that one characteristic of a Pickering emulsion is that the solid particles are arranged at the interface between the two liquid phases, where they form, as it were, a mechanical barrier against the combining of the liquid droplets.

It is a relatively new technical development to use Pickering emulsions as a base for cosmetic or dermatological preparations.

One way of achieving solids stabilization in the sense of a pickering emulsion in a cosmetic or dermatological preparation is, according to May-Alert (Pharmazie in unserer Zeit [Pharmacy in our time], Vol. 15, 1986, No. 1, 1-7) for example, to use emulsifier mixtures which comprise both anionic and cationic surfactants. Since mixing anionic and cationic surfactants always results in the precipitation of insoluble, electroneutral compounds, deliberate precipitation of these neutral surfactants in the oil/water interface makes it possible to achieve additional solids stabilization.

European Laid-open Specification 0 686 391, moreover, describes emulsions of the water-in-oil type which are free from surface-active substances and are stabilized only by solids. Stabilization is achieved here using spherical polyalkylsilsesquioxane particles which have a diameter of from 100 nm up to 20 .mu.m. These emulsions can be referred to as Pickering emulsions according to that mentioned above.

Pickering emulsions are stabilized by the use of suitable solids or pigments. However, the preparations of the prior art generally have the disadvantage that they are limited to a narrow field of application or a restricted choice of starting materials since they can only be stably formulated in this manner. For many areas of cosmetics (e.g. for the field of face care), Pickering emulsions of the prior art have unsatisfactory cosmetic properties.

The object was therefore to remedy the disadvantages of the prior art. In particular, the intention was to provide cosmetic and dermatological bases for cosmetic and dermatological preparations which are characterized by good skin tolerability. In addition, an object of the present invention was to provide products with the widest possible variety of applications. For example, the intention was to provide bases for preparation forms such as cleansing emulsions, facecare and bodycare preparations or deodorants, but also distinctly medicinal-pharmaceutical presentations, for example preparations against acne and other skin conditions.

It was surprising and in no way predictable by the person skilled in the art that Pickering emulsions, which are finely disperse systems of the water-in-oil or oil-in-water type, comprising

(1) an oil phase which contains at least one wax and/or one oil thickener,

(2) a water phase,

(3) at least one type of microfine particles which

a) have an average particle size of less than 200 nm, which

b) display both hydrophilic and lipophilic properties, i.e. which have amphiphilic character, and are dispersible both in water and in oil and which

c) have optionally been coated on the surface and

(4) at most 0.5% by weight of one or more emulsifiers, overcome the disadvantages of the prior art.

According to the invention, it is particularly advantageous if the preparations comprise significantly less than 0.5% by weight of one or more emulsifiers or are even entirely free from emulsifiers.

The preparations according to the invention are extremely satisfactory preparations in every respect which have considerably higher stability compared with traditional Pickering emulsions and are therefore suitable in particular to serve as bases for preparation forms having diverse application purposes. In particular, W/O Pickering emulsions within the meaning of the present invention are, surprisingly, markedly stable.

In addition, the preparations according to the invention are characterized by excellent skin tolerability. In addition, it was surprising that preparations according to the invention, which are in the form of a sunscreen, exhibit higher effectiveness than customary sunscreen formulations.

While preparations of the prior art having a pigment content of as little as 1% by weight produce a dull feel following their application to the skin, which increases further with higher pigment concentrations, the preparations according to the invention, surprisingly, do not leave a dry or dull impression on the skin, but on the contrary exhibit excellent cosmetic properties.

Although the prior art recognizes, in addition to Pickering emulsions, emulsifier-free, finely disperse cosmetic or dermatological preparations, which are generally referred to as hydrodispersions and which are dispersions of a liquid, semisolid or solid inner (discontinuous) lipid phase in an outer aqueous (continuous) phase, the prior art was unable to point the way to the present invention.

In hydrodispersions of a liquid lipid phase in an external aqueous phase, the stability can be guaranteed, for example, by building up a gel structure in the aqueous phase in which the lipid droplets are stably suspended.

German Laid-Open Specification 44 25 268 describes stable finely dispersed, emulsifier-free cosmetic or dermatological preparations of the oil-in-water type which, in addition to an oil and a water phase, contain one or more thickeners from the group consisting of the acrylic acid polymers, polysaccharides and their alkyl ethers, where for these thickeners a lowering of interfacial tension must not be measurable.

Based on similar hydrodispersions, German Laid-Open Specification 43 03 983 discloses cosmetic or dermatological sunscreen formulations which are essentially free of emulsifiers, inorganic micropigments which serve as UV filter substances being incorporated into the lipid phase of the hydrodispersion.

O/W Pickering emulsions within the meaning of the present invention, however, are obtainable by first dispersing amphilic particles according to the invention, which are suitable for the preparation of O/W Pickering emulsions, in the aqueous chase and then combining the aqueous phase with the fatty phase. W/O Pickering emulsions according to the invention, however, are obtainable by dispersing amphilic particles which are suitable for the preparation of W/O Pickering emulsions in the fatty phase.

Claim 1 of 11 Claims

What is claimed is:

1. A Pickering emulsion, said Pickering emulsion being a finely dispersed water-in-oil or oil-in water system, said Pickering emulsion comprising:

a) an oil phase comprising at least one wax and/or at least one oil thickener;

b) an aqueous phase;

c) microfine particles, said microfine particles:

i) having an average particle size of less than 200 nm;

ii) being dispersible both in water and in oil;

iii) having both hydrophilic and lipophilic properties resulting in amphiphilic character; and

iv) being selected from the group consisting of metal oxides, which are coated on the surface thereof with:

(A) a dimethylpolysiloxane and/or silica gel; and

(B) aluminium hydroxide and/or alumina and/or silicon dioxide;

d) at most 0.5% by weight of one or more emulsifiers.

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