This invention relates to new dispersions, and more particularly it relates to stable dispersions of water-insoluble substances in aqueous media which are based on biliquid foams entrapped in aqueous gels.
Dispersions of oils and oil-soluble materials in aqueous media are widely used in many industries, in particular the cosmetics, food, paints, pharmaceuticals and printing industries. Such dispersions typically depend for their stability on the presence of surface-active chemicals, generally known as emulsifying agents or surfactants, which migrate to the oil-water interface which surrounds each individual suspended oil droplet, and prevent said droplet from coalescing with other droplets with which it may come into contact. Such dispersions are generally known as emulsions and typically contain, depending on the chemical nature and concentration of the components of the emulsion, from 3 to 10% by weight of surfactant.
A disadvantage of the presence of surfactants in emulsions, particularly in the cosmetics and pharmaceuticals industries, is their potential, to a greater or lesser extent, to cause skin irritation. This arises from the physico-chemical nature of all surfactant molecules, especially from their ability to combine with skin lipids, and is concentration-dependent. It is therefore essential, in well-formulated cosmetic and pharmaceutical products, to use the minimum surfactant concentration consistent with providing stability of the emulsion, but said minimum concentration in practical terms is necessarily fairly high.
A further disadvantage of the presence of surfactants in emulsions is their detrimental effect on the efficacy of many preservatives, which are essential in emulsion formulations to prevent the growth of unwanted organisms such as bacteria, yeasts and other fungi. As a result, in the presence of surfactants, levels of preservatives need to be higher than they might otherwise be. The disadvantage is compounded because the preservatives themselves are skin-sensitizing, and higher levels thereof exacerbate the skin problems.
Apart from the problem with surfactants, many emulsions known for use as cosmetics or pharmaceuticals contain, in order to produce stable formulations of desired viscosity, fatty chemicals such as cetyl or stearyl alcohol, glyceryl stearate, petroleum, or waxes, either natural (for example beeswax, candelilla wax or carnauba wax) or synthetic (such as microcrystalline or paraffin wax). These form a fatty, pore-clogging, occlusive film on skin which users find abhorrent.
By virtue of the oily materials which they contain, many emulsions, particularly those used as skin cleansers, have to be removed from the skin by wiping with tissue material, and cannot be rinsed from the skin by aqueous means and remain effective, and these emulsions are not liked by their users for this reason.
An alternative cosmetic or pharmaceutical product for topical use comprises a liquid or gel wherein the gelling agent is either entirely water-based, or alternatively entirely oil-based. The disadvantage of the former type is that, despite feeling light and refreshing in use, it cannot deliver oil-soluble or oil-dispersible materials to the skin, and the disadvantage of the latter type is the unpleasantness in use, as explained in the previous paragraph.
There are also known aqueous gel products designed for cleaning and conditioning hair and skin, which comprise high levels of surfactants, either with or without additional gelling agents. Such products are known as hair shampoos, body shampoos, bath or foam gels, bubble or foam baths and products of similar descriptions. The primary purpose of such products is as hair or skin cleansers, but a secondary objective is to provide a lasting pleasant feel to the hair or skin, and to provide a lustrous shine to the hair, these effects being generally known as "conditioning". Oils used in skin care products, particularly silicones and their derivatives, are amongst the most effective conditioning agents but it is difficult to incorporate these into shampoos and similar products in sufficient quantities to provide an adequate conditioning effect, without destroying gel viscosity or foaming effect.
Dispersions of oil droplets in aqueous media are known which are stabilized by thin films containing low levels of surfactants, and these films are generally known as "biliquid foams". These foams are not emulsions (Sebba, Chemistry and Industry, 1984, pp 367-372) and may contain dispersed oil content of up to 95% by volume. They are insufficiently stable to form useable cosmetic or pharmaceutical products because the foam breaks at the air-water interface and deposits a layer of oil on the surface which, for reasons set out above, is unacceptable to the user.
The present invention is addressed to formulations for use in the cosmetics, pharmaceuticals and other industries which are based on biliquid foams, but which do not possess the disadvantages set out above and in particular are less skin-irritant by virtue of their requirement for only low levels of surfactants, and as a consequence only low levels of preservatives.