Understanding solvents in skin care
What is a solvent?
A solvent is a liquid used to dissolve a powder; the solute dissolves because its molecules interact with the molecules of the solvent. For example, sugar will dissolve in water but not in oil.
In skincare, solvents are ingredients used to dissolve other solid ingredients. The usual solvents include water, vegetable or animal fats, silicones, alcohols, etc. When I plan a formulation, my objective is to use actives at the optimal concentration. If the active ingredient is not soluble in water or oil, I may have to find an alternative solvent because the un-dissolved active will just “seat there”. In most Skin Actives products, there are many different actives of many different chemical structures, so I may need a mix of solvents (rather than just water) to give all those actives a chance to dissolve and stay dissolved. For example, a powder may dissolve in hot water but may precipitate when the liquid cools down.
The physical properties of an ingredient, including its capacity to dissolve in a particular solvent, are fixed, they depend on the chemical structure. The ability to dissolve depends on the relationships that the molecules of solute and solvent can establish at a certain temperature.
Finding the right solvent, or a mix of solvents is a craft. There are indeed chemical indexes that provide solubility information of thousands of chemicals, but the solvents used in books (like the Merck Index) are water, ethanol, and acetone. Of these, only water is suitable for skincare use. The formulator has no choice but to experiment until she finds a suitable mix of solvents. The ingredient industry facilitates this job by providing sample formulas that employ their own products and by selling pre-mixes in which the desired ingredient may already be dissolved and preserved.
Floral waters are sometimes used instead of water; they are a byproduct of the production of essential oils by distillation. Floral waters are mostly water, but some chemicals in the petals of the flowers used as sources are also present in the floral water, and they can give a pleasant fragrance to the final product. “Floral water” also looks better on a label than plain old water/aqua/eau.
Your body is mostly water. Water will always be a suitable solvent. Others, not so much. At Skin Actives, we don’t use denatured alcohol, a mix of alcohol with additives that make it unsuitable for drinking. Because ethanol is sold with a huge tax assuming it will be consumed as a drink, companies save money by buying these denatured mixes. Some are unsuitable for the skin, and even pure ethanol should be avoided as it will act as a desiccant.
Other solvents you will see in our formulations include oils, alcohols, esters, and other liquids that the skin “likes”. In ELS serum, all the ingredients are lipids, a mix of liquids and solids that will dissolve in the liquid lipids.
Questions? Just ask.
Hannah