Quick Guide To Solubility

Knowing solubility is important when formulating your own products. It is handy to know what is oil soluble, what is water soluble, and what is partially soluble or insoluble, or even soluble in other substances such as alcohol. The vast majority of natural ingredients are either water soluble or oil soluble.

The solubility of ingredients will help guide your formulation in terms of the water phase, oil phase and if you require additional ingredients such as emulsifiers to bring your ingredients together.

Terminology

• Solute: The substance that would be dissolved
• Solvent: A substance capable of dissolving certain substances
• Solubility: The degree to which the solute is able to dissolve in the solvent
• Solution: A mixture where one substance is completely dissolved into another

In formulation, mostly we will refer to the solvents and solutes as substances that are 'water-based', 'oil-based', 'alcohol-based', etc. (eg. water-based substances are all highly soluble to each other, can be easily combined and hence is called water based).

Water Soluble Ingredients

Water is probably the most common solvent in natural cosmetics.

Water-based ingredients will be soluble in water as well as in other water-based ingredients. For instance, you can happily mix plain water, hydrosols, water-based botanical infusions, aloe vera liquid gel, vegetable glycerine, hyaluronic acid and a whole host of other water-soluble ingredients together.

What are Other Water Soluble Ingredients?

• Certain vitamins, such as Vitamin C (ascorbic acid) and Vitamin B.
• Gum or cellulose thickeners such as xanthan gum, guar gum and hydroxyethyl cellulose are dissolvable in water to form gels.
• Titanium dioxide
• Botanical extracts.
• Alpha hydroxy acids (AHAs) such as citric acid, lactic acid and malic acid.
• Specialised ingredients such as marine collagen, hydrolyzed wheat protein, keratin and D-panthenol.
• Baking soda, sugar and salt are soluble in water (see more under partial solubility below).

Water-soluble ingredients will always go in the water phase of your formulation.

Oil Soluble Ingredients

Oil is another very common solvent in the cosmetics world.

What are Oil Soluble Ingredients?

• All types of oils and butters are soluble in other oils, including essential oils and carrier oils.
• Certain vitamins are also oil soluble, such as Vitamin A and Vitamin E.
• Lecithin, lanolin, squalane, beeswax and plant waxes.
• Emulsifying waxes and ingredients such as stearic acid, vegetable cetyl alcohol and cetostearyl alcohol.

Oil-soluble ingredients will always go into the oil phase of your formulation.

Alcohol Soluble Ingredients

Alcohol is a powerful solvent and it can also be used as a Solubilizer.

Solubilizer: dissolves small amounts of oils such as essential oils so that they can be added to water with no separation.

What are Alcohol Soluble Ingredients?

• Small amounts of certain carrier oils and essential oils are soluble in alcohol (eg. in an alcohol-based air freshener).

Alcohol is commonly used to make alcohol-based botanical extracts and tinctures.

Partially Soluble Ingredients

Partially soluble ingredients are pretty self-explanatory: they can only partially dissolve in a solvent/base.

The most common example of a partially soluble substance is Allantoin - 0.5% soluble in water till it reaches it's saturation point.

Certain substances do dissolve in water but have an upper limit after which no more will dissolve (sometimes heat can increase this limit). Examples are sugar and salt. Sugar has a higher solubility than salt but eventually it will saturate the water and you just won't be able to dissolve more.

Insoluble Ingredients

Insoluble ingredients cannot dissolve at all. They can still be mixed into formulas though, and serve a myriad of purposes. Here are some common insoluble ingredients:

What are some Insoluble Ingredients?

• Micas and iron oxides.
• Zinc oxide is insoluble but can be dispersed in heavy creams.
• Clays will form a suspension in water and will form a paste if only a little water is used, however, they are not soluble.
• Botanical powders - these don't dissolve in anything, but you can make infusions or extracts out of them.
• Arrowroot powder is insoluble but is often added to cut greasiness and give a silky skin feeling.
• Activated charcoal is insoluble.
• Exfoliating elements such as apricot grinds, clays, oats and seeds are all insoluble.
• Sugar is not soluble in oil.
• Salt is not soluble in oil.

What is Soluble in Surfactants?

You may notice that surfactants can have emulsifying or solubilising properties. You can add limited amounts of oils straight into surfactants and the water phase and when you mix it all up, everything will be blended (it will most likely turn whitish - you've essentially made an emulsion).