[Gabriella]

Hi Doreen, I had the same issue for a long time and found a solution. I mixed 30% 1,3-propanediol with 0.5% ferulic acid and then added all the water content of the formula which in my case was 53.5%. I heated this solution at about 80°C for 15-20 minutes. After this I added my other ingredients in the cool down phase. The pH was around 3.5. I have kept the solution at refrigerated temperature without any visibly noticable crystallization.

[Gabriella]

There is a recent permeability study about diproylene glycol, propylene glycol and butylene glycol and their ability to penetrate the stratum corneum of porcine ears by Kis et al. (2022) (https://www.sciencedirect.com/science/article/pii/S0168365922000815).
The above mentioned glycols are relatively inexpensive, safe and easy to formulate with. I find that they have great skin feels and add some nice slip.

Here is another recent in-vitro study about the use of Propylene Glycol as a co-solvent and penetration enhancer for topical applications on pigskin (Carrer et al., 2020). https://pubmed.ncbi.nlm.nih.gov/31786711/#:~:text=Propylene%20glycol%20(PG)%20has%20been,the%20skin%20from%20topical%20preparations.

Ethoxydiglycol and Dimethyl Isosorbate are great ones as well and I have seen those being used by La Roche Posay and Skinceuticals. Unfortunately not very easy to get my hands on as an at-home formulator without networks in the cosmetic industry.