Hydrocarbons

The hydrocarbon propellants are propane (freezing point —42.1), n-butane (freezing point — 0.5) and isobutane (freezing point — 11.7). Commercial butane is always a mixture of n-butane and isobutane. Mixtures with propane in different ratios are usually used to adjust the pressure. Hydrocarbons are cheap, stable and of low toxicity, but are highly flammable.

Aerosol propellants

A few years ago only one group of aerosol propellants, the fluorocarbons, was used. Due to the well-founded suspicion that these chemicals damage the ozone layer, they are now virtually obsolete. The gap has been filled by the increased use of pump systems, and by hydrocarbons and dimethyl ether.

W/O stabilizers

A relatively simple way of obtaining highly stable W/O emulsions is to use block copolymers such as PEG-45 dodecyl glycol copolymer. These products, with molecular weights of 2000 to 4000, can improve the stability of W/O emulsions by protection of the droplets from coalescence. The hydrophilic middle part is responsible for a good anchorage in the water phase, while the highly branched ends give a steric hindering effect. The high molecular weight imparts excellent skin compatibility. Contrary to other stabilizers, for example metal soaps, which increase the viscosity of the outer emulsion phase, these copolymers typically reduce the viscosity. It is possible to obtain light, highly stable W/O lotions that are good bases for sun preparations and which give a skin-feel very close to emulsions.

Single W/O emulsifiers

A limited number of W/0 emulsifiers are available. This is because ionic emulsifiers will not work in the case of W/0 emulsions and, since very low HLB is required. variations using ethylene oxide are not possible. W/0 emulsifiers have HLB values between 3 and 6. However, the type of the alkyl chain is also important. In practire, oleyl derivatives have shown good effects. The most widely used W/O emulsifiers are sorbitan monooleate, sorbitan sequioleate and glycerol monooleate, and with increasing importance, polyglycerol esters.
In addition to oleate, ricinoleates and isostearates have aIso found use.

The methoxy PEG-22 dodecyl glycol copolymer is a high molecular weight W/O emulsifier, which is especially effective in formulations containing mineral oils. It is a saturated ether and is therefore stable against oxidation and hydrolysis.

Water-in—oil (W/O) emulsifiers

From the dermatological point of view, W/O emulsions are preferable to O/W emulsions. The natural lipid fiIm on the skin is also a W/O emulsion. W/0 emulsions can improve the hydration of the skin and are an ideal base for lipid-soluble active ingredients. A problematic point is that they usually leave a ‘fatty-feeling’ on the skin, and this is not particularly appreciated by customers.
Another problem can be ‘oiling-off’ or separation of the oil phase on storage. Viscosity a ad stability are extremely process sensitive.

O/W stabilizers

Very efficient and widely used stabilizers for O/W emulsions are polymers, especially the carbomers (CTFA-name). These are polyacrylate resins which have to be neutralized in order to form gels, usually by the use of triethanolamine (TEA). Amounts up to 0.5% are recommended for cosmetic emulsions. Too much can leave an unpleasant feel on the skin. The principle of stabilization is the thickening of the outer phase. The use of preneutralized copolymers, e.g. acrylamide/sodiurn acrylate copolymer can make incorporation and handling easier.

Non-ionic O/W emuIsifiers

A wealth of non-ionic 0/W emulsifiers is available. These are mainly PEG derivatives such as êthoxylates or PEG esters. Typical O/W emulsifiers are :
• ethoxylated fatty a!coho
• PEG esters of fatty acids
• ethoxylated sorbitan esters
• ethoxylated monoglycerides
• ethoxylated castor-oil derivatives.
The degree of ethoxylation determines the primary properties of that part of the molecule that results in the HLB classification. Good 0/W emulsifiers can be found in the HLB range between 8 and 18. However, the type of hydrophobic moiety is also very important for the stability of the emulsions. Contrary to anionic emulsifiers, the non-ionics are unaffected by changes in pH and, in the case of pure ethers, they can also be used at extreme pH values.
Following the trend towards more ‘natural’ cosmetics, trials have been made in order to replace the E0 chain by polyglycerol or sugars but up till now it has not been possible to reach high HLB values.

Cationic O/W emulsifiers

Cationic emulsifiers have generally been avoided in skin-care products because they are often more irritant compared with anionic emulsifiers. They are important in hair-care formulations, where they act as conditioners and anti- static agents. Their advantage, particularly distearylammonium chloride, is that they produce emulsified products with excellent cushion feel on skin and mitigate the heavy feel imparted by glycerin.

Oil-in-water (0/W) emulsifiers

Oil-in-water (0/W) emulsifiers

- Anionic O/W emulsifiers

Anionic 0/W emulsifiers can be used to obtain very stable emulsions because they can build an electrical double layer around the droplets, which prevents the droplets from coalescing. On the other hand, anionic emulsifiers are sensitive to low pH and electrolytes.
The most important anionic 0/W emulsifier is soap, which forms the stearate creams. Neutralized with potassium hydroxide, the stearate is alkaline. The use of triethanolamine as a neutralizing agent is more common. A disadvantage of stearic acid s the typical ‘soap-up’ effect, a type of foaming that occurs when the cream is rubbed into the skin. To prevent this, silicon oil is frequently added to the formulations.
Other very effective anionic emulsifiers are alkyl sulfates, particularly sodium cetearyl sulfate. In principle, all anionic surfactants can be used. However, due to the fact that the soaps are relatively alkaline and fatty alcohol sulfates are irritant to the skin, the mild surfactants are of more interest. These include sodium cocoyl isethionate as an extremely mild emulsifier, and phosphoric acid esters, like potassium cetyl phosphate which are very effective at low concentrations.

Fatty acids

Fatty acids, like the fatty alcohols, are important raw materials for cosmetic ingredients such us surfactants, emulsifiers and emollients. Fatty acids are obtained by saponification of naturally-derived triglycerides. As such, they are seldom used in emulsions. They act as consistency regulators but tend to crystallize in the formulation.
Fatty acids especially stearic acids, are mainly used in the form of soaps. As the pH increases, they become truly anionic and act as oil-in-water (O/W) emulsifiers. forming a group of so-called stearate creams.
Commercially available stearic acids are often mixtures of C16 and C18 fatty acids, mainly I : 1. The preferred cosmetic quality with low odor is a triple-pressed stearine with an iodine value below 2. A special application of the fatty acids is their use in alcohol stick formulations, where stearic acid is dissolved in ethanol and then neutralized with sodium hydroxide. These hard sticks are used as deodorant bases.

Polyol esters

The most important polyol ester is the so-called glycerol monostearate (GMS). Generally, it is not a pure product but a mixture of mono- and diesters of stearic and palmitic acids. The main distinguishing feature is the content of monoester. Products with approx. 4O% monoester are obtained by direct esterification of stearic acid and glycerol. Products with approx. 6O% monoester are produced by glycerolysis of triglycerides with glycerol. The 9O% material can be obtained by molecular distillation but is seldom used in cosmetic formulations because the mono/diesters provide the best applicational properties.
GMS is poorly soluble in waler. However, as with the fatty alcohols, when combined with more hydrophilic emulsifiers, it is an excellent co-emulsifier
and becomes self-emulsifying. Typical combinations are :
• GMS plus potassium stearate (GMS self-emulsifying’)
• GMS plus sodium lauryl sulfate
• GMS plus ethoxylated fatty alcohols (‘GMS sell-emulsifying, acid stable’)
Other polyol esters used a co-emulsifiers are the sorbitan esters, especially the monostearate. These are obtained by dehydration of sorbitol to sorbitan followed by esterification with fatty acid. Such products are typically offered as pure products, and function as water-in-oil (W/O) emülsifiers. In oil-in- water formulations they are most often combined with ethoxylated products of the same family.

Cream bases

Cream bases

- Fatty alcohols

Fatty alcohols are important raw materials for surfactants, emollients and emulsifiers. Pure fatty alcohols, mainly cetyl alcohol and stearyl alcohol, are also used per se as consistency regulators and co-emulsifiers in, for example, creams, lotions and hair rinses. The so-culled natural fatty alcohols are obtained by hydration of fatty acid methyl esters. Similar, linear primary alcohols can be obtained from the Ziegler process. The branched types obtained from oxo-synthesis are important as raw material feedstock, but are not used as alcohols perse. Mixtures of cetyl and stearyl alcánois combined with hydrophilic emulsifiers are known as the emulsifying waxes’ of the British Pharmacopoeia. In such combinations the fatty alcohols are self-emulsifying. Common combinations are:

(i) cetearyl alcohol plus sodium cetearyl sulfate
(ii) cetearyl alcohol plus PEG-1000 monocetyl ether
(iii) cetearyl alcohol plus alkyl trimethyl ammonium bromide

As examples for(i) anionic;(ii) non-ionic: and (iii) cationic cream bases. Type (iii) combinations are mainly used in hair-rinse formulations. The fatty alcohol gives texture and body to the formulation and acts as consistency regulator.

Silicone oils

The high molecular weight organo polysiloxanes (dimethicone) are hydrophobic oils with good skin protection and non-sticky skin-feel. They show very high spreading and are used in small amounts in stearate creams to avoid the soap-up effect. properties, silicone oils are important for waterproof sun products. They are usually classified by viscosity.
Cyclic methyl polysiloxanes (cyclomethicone) are particularly volatile. They are used in hair-care products to improve the gloss of the hair and the compatibility of the product.

Waxes

Waxes

The word wax has two different meanings. From a chemical point of view it means an ester of a fatty acid and a fatty alcohol. Jojoba oil is therefore a liquid wax. In this Section, however, the word wax is considered from a physical point of view, and means compounds having a high melting point (approx 50—100°C).

- Natural waxes

The most important wax, and a classical component for creams, is beeswax, which is the construction material of honeycombs.

The melting point is around 61 to 66°C. Untreated beeswax is dark-yellow and is usually treated to improve the color.
Beeswax is a very good consistency regulator in creams and ointments and is also used in stick formulations. The addition of approx. 6 percent of borax, calculated on the beeswax, results in partial saponification and provides the beeswax with some emulsifying properties, This principle was used in the classic cold-cream formulations. Nowadays, European law restricts the use of borax in cosmetic products.
Two other natural waxes that are harder and therefore mainly used in stick formulations are carnauba wax (melting point approx. 85°C) and candelilla wax (melting point approx. 70°C). Both are extracted from South American plants.

- Synthetic waxes
Due to the varying qualities of the natural products, especially yearly and source variations of beeswax, synthetic substitutes are of interest. These are often mixtures of very different chemicals, and result from empirical research. An example of a beeswax replacement the mixture of glyceryl hydroxystearate (and) cetyl palmitate (and) microcrystalline wax (and) trihydroxy stearine. A single defined product with a structure close to beeswax is hydroxyoctacosanyl hydroxystearate, an ester of a C26-β-hydroxyalcohol and hydroxystearic acid. This product, which shows very similar behavior compared with beeswax, is easy to emulsify and can be used as a general Consistency regulator for cosmetic emulsions.
Another synthetic wax is synthetic spermaceti. The natural type is no longer available because it is obtained from whales, but was formerly an excellent wax compound for emulsions. Replacements are cetyl palmitate, which is the main component of the natural spermaceti or special mixtures like the cetyl esters wax NF, which comes very close to the natural product.

Synthetic oils

Synthetic oils
Synthetic oils are esters, usually obtained by direct reaction of fatty acids with alcohols. Common fatty acids are caprylic. caprinic. lauric, myristic. palmitic, stearic, oleic, linoleic and behenic acid, and also adipic acid, a dicarboxylic acid. Alcohols can be, for example, isopropyl, butyl, ethylhexyl, myristyl or oleyl alcohol, as well as polyvalent alcohols such as ethylene glycol, propylene glycol and glycerol. A wide range of combinations to produce different synthetic esters is possible, and the list of synthetic oils available to the cosmetic industry is long.
In addition to products obtained via normal esterification using chemical catalysts, esters obtained by enzymatic catalysis are now also available]. Possibly advantages can be found in the case of oleic acid derivatives, because esterification can be obtained under milder conditions. Consequently, a light color and weak odor can be expected.
- Isopropyl esters
The most important synthetic ester for cosmetics is isopropy myristate. It has a very dry character and shows good spreading on the skin. It can open films of mineral oil or other fatty oils in order to let water evaporate, and it is an excellent solvent for perfumes, ultraviolet filters, etc.
A disadvantage of isopropyl myristate is its high comedogenicity.
Due to their good solvent properties, the isopropyl esters can cause difficulties when used in formulation that are filled into polystyrene or polyethylene containers.
- Ethylhexyl esters
Other alternatives to isopropyl myristate are the ethylhexyl esters of palmitic acid or stearic acid. These have become important products for the cosmetic industry. Compared with isopropyl esters, the molecular weight is higher, but spreading properties are similar.

- Oleic acid esters

The two most important oleates are decyl oleate and oleyl oleate. The character of these esters is relatively fatty and, as with unsaturated esters, they can become rancid. The advantage of the oleates is that, even at high molecular weight, the esters are liquid.
Interesting alternatives to the oleates are the isostearates. Esters of branched isostearic acid are also liquid and present no oxidation problem. The limiting factor for use is the price, which is much higher than that of the oleates.

- Caprylic/capric acid esters

The most important caprylic/capric esters are the triglyceride and the propylene glycol diester. The glycerol ester is mainly used as a replacement for natural triglycerides, with the advantage that it is stable against oxidation. Like the natural triglycerides, the products are relatively difficult to emulsify. Due to the typical odor of short chain fatty acids, caprylic/capric esters must be produced in very pure form. Following long-term storage, hydrolysis, which leads to changes in the odor of cosmetic products, can take place.

Continuation of natural oils

safflower oil where no comedogenic effects have been found. Other examples of natural triglycerides are peanut oil, soy oil, olive oil, wheat germ oil and avocado oil.
A special triglyceride is castor oil, which is obtained from the seed of Ricinus communis. This oil is based on ricinoleic acid, an unsaturated C18 hydroxy fatty acid. Contrary to other triglycerides, it is easily soluble in alcohol. Castor oil is used in lipsticks and also in alcoholic preparations.

Jojoba oil

Unlike the oils described in the previous section, jojoba oil is not a triglyceride. It is a liquid, unsaturated wax from the esters of long carbon-chain fatty acids (C20 to C22) and long carbon-chain unsaturated alcohols (C20, C22). Its source is the jojoba plant, which grows in California and Mexico. Jojoba oil has a very fatty character and shows excellent cosmetic properties.