Is there any reason to have two versions?
I know that in the industry, the word serum is used to convey the idea that it has a higher concentration of the magic ingredient. This is not necessarily true.
In my case, I use a serum when the ingredients are either water soluble OR oil soluble; and I use a cream when I want to include two types of ingredients, some soluble in water and others in oil.
A cream is an emulsion of oil and water, so you have more choice in terms of what ingredients you can use. When you add ingredients to a cream, they will partition according to which side they prefer.
In the case of antioxidants, it can be important to have the choice. Why? In our body we have antioxidants that work in the water phase of the cell, like the cytosol, for example. But we also need antioxidants capable of working in the lipid phase, like in the cell membrane. Having a cream gives us a chance to produce a product with a wider range of antioxidants, and it will provide better overall protection.
If you add an insoluble ingredient to a serum, then you don’t have a “working” active.It may be on the label but it is not in your skin.
And always remember that solubility is determined by the chemical structures of the solvent and the solute, and you can’t change that.