EMULSIFIERS Caroline Recardo – Alfa Chemicals, UK
Challenges of formulating with natural emulsifiers
As all formulators know, the challenge of creating an emulsion in cosmetic science is to achieve maximum stability combined with superior sensoriality. It is not enough to make a totally stable cream if the rheology or the feel of the product is wrong. Formulating with natural emulsifier systems brings its own challenges; in a later section we will look at the advantages and disadvantages of natural versus synthetic emulsifiers and the specific issues that have to be addressed when using natural emulsifiers.
Types of emulsions Standard emulsion This is the sort of emulsion we learn about at school or college. Figure 1a shows a typical oil-in-water system, where tiny droplets of oil are dispersed in the continuous water phase. The water phase may or may not be thickened, but the aim of the emulsion is for the oil droplets to remain dispersed over a long period of time. This is generally achieved using an emulsifier, which sits on the interface between the oil and water, and prevents coalescence of the particles. Figure 1b shows a typical water-in-oil
emulsion. It is the exact opposite of the water-in-oil in that here water droplets are suspended in the continuous oil phase. This type of emulsion tends to be less common in cosmetics, as most facial skin care, body lotions etc. are the oil-in-water type. Again the emulsifier is specifically chosen to stabilise this type of system.
Lamellar emulsion Oil and water can also form layers with the help of certain combinations of emulsifiers (Fig. 2). The emulsifiers sit on the interface between the layers and help stability by forming stable semi-solid structures known as liquid crystals. These exist in nature and in manmade objects, and have many useful properties. When they are present in emulsions they give excellent thermal stability, and also a shear thinning quality which make them spread easily over the skin and absorb well. The formation of liquid crystals in a cream can be
56 PERSONAL CARE February 2016 A: Oil-in-water system Oil droplets (dispersed medium) B: Water-in-oil system Water droplets (dispersed medium) Water (dispersion medium) Figure 1: Standard emulsions.
encouraged using certain ingredients, for example fatty alcohols, glyceryl stearate and lecithin.1
Microemulsion Microemulsions (Fig. 3) form when certain classes of surfactant are used which enable close packing on the water/oil interface, thereby reducing the size of the oil droplets to less than 100 nm. At this size they are unable to be seen with the naked eye, making the suspension appear transparent or translucent. The key to this system is a specific combination of
several emulsifiers and co-emulsifiers that enable full stability at low viscosity. Microemulsions have experienced something of a revival lately through ‘micellar waters’, popular for removing makeup.
The hydrophilic-lipophilic balance system (HLB) Before we go into a more detailed discussion about emulsifiers, it is worth mentioning the HLB system, which was introduced in the 1950s2
to classify
Water layer Oil layer
Figure 2: Lamellar emulsion.
different emulsifiers as having a tendency to form oil-in-water emulsions or water-in- oil emulsions. The scale was originally devised for synthetic emulsifiers, and for natural emulsifiers it may only be an estimate, but it is still a very useful tool. The scale goes from 1 to 20, 1 being a virtually entirely oil soluble substance with low emulsification characteristics, and 20 being a highly water soluble molecule used as a solubiliser. In between you find emulsifiers for oil-in-water emulsions (between 10 and 16), emulsifiers for water-in-oil systems (between 3 and 7) and other functional molecules such as wetting agents (7-9) and detergents (16-20). Knowledge of the effective HLB of an emulsifier, whether as a pure substance or a mixture, is a fairly reliable indicator of how it will behave in an emulsion.
Oil (dispersion medium)
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