 I have be working on my chemistry series to build on to two topics that I get asked about the most; solubility and polarity. I was talking with Allison about solubility and thought I could give a quick thing. Without getting into the complexities of the topic. In a typical general chemistry textbook, you spend 6 chapters talking about soluble. It is a big topic. For everyone, make sure you look up the solubility of every ingredient. Either form the supplier or a quick google search. Unfortunately, suppliers for small brands (re-packers) are not always great about stating the solubility or just plain posting the wrong information. You can also check the SDS sheet, it may be under the chemical properties section. Solubility is usually a selling point. I know some may give you a weird answer. The wording is very important. You want to see what they say about the solubility, there will use some terms that you need to know what they mean. Some terms to look out for, soluble, "soluble in", insoluble, and dispersible. We are going to dive into what all of them mean. When something is soluble, it means that a lot of the material can be dissolved in it. Usually in large quantities. Sugar for example is soluble in water. There is a point you can use to much, but in most cases it will be soluble. The same is true for formulating. We are not using 50% salt in our formulations. "Soluble in", means that it has preferences of the solvent and has a solubility quirk. The statement will usually follow with a g/mL statement. Some are even in mg/mL or g/L. You may have to do some conversions. Check out this post here to learn how. Let's say an ingredient has a solubility of 5g/100mL in water. That means, at room temperature, 5 grams of the material will dissolve in 100 mL of water. At room temperature, water is 1g is equal to 1mL. So in a 100 g batch you can use up to 5%. I do not recommend that because if you measure incorrectly you will have some that is not soluble. So I would use less than 5%. The real world example of this is allantoin. It has a solubility of 0.50g/100mL. You have to be careful. Our formulations are not 100% water. We have preservatives and other ingredients as well. So, it is important to know how much water is actually in your formulation. If your distilled water is 60%, you are not going to dissolve 0.50g at room temperature. Aloe vera and hydrosols also contribute to the water. When it is not water you also have to be careful. For example some ingredients are soluble in alcohol or glycols. The 1g per 1mL does not apply, you have to know the density. For example, propylene glycol has a density of 1.036g/mL at 25C. Let's say a ingredients has a solubility of 0.75g per 100mL in propylene, what percentage can you use it in a formulation? 0.75/100=0.0075 For every, 0.0075g per 1 mL. 1mL of propylene glycol is 1.036g 0.0075/1.036=0.0072 So, 0.0072g of the ingredient for every 1 g of propylene glycol. We can use this to write into our formulation. Let's say we want to use 10% propylene glycol in our formulation. We would could use 0.072% or less in the formulation. This means it has very little solubility. Insoluble is self-explanatory. No matter the portion added, it is not going to dissolve. This is when you need to consider, emulsifier, solublizer, different product, etc. For example vegetable oils are insoluble in water. It does not matter 0.00001g or 10000g, it is not going to mix with water. Dispersible is where you can get in trouble as well. It does not mean it is soluble. The classic example is pigments; for example, iron oxides are not soluble in water or oil. However, it is dispersible in oils. It means that you can get the material fine enough thought a dispersion, that it will stay in-suspension. Solubility, in most cases, increase as you increase the temperature. You usually have to heat the solvent and the ingredient to get it to dissolve. You may want to heat the water phase as well. Adding the solvent mixture to room temperature water may affect it. I did this with a salicylic acid serum. There are nuances to solubility. It is more of a spectrum than absolute. This should get you by the basics of it. I cannot wait until I can explain all the chemistry of it. I will share some related posts on solubility where we can use it to our advantage and where we have to work around it. Sources: Related PostsWant to Learn More? Consider getting access to the Diamond Level Blog 1 Month Access to Diamond Level
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