[Victor]

Thank you for your reply.

The pH of NaOH edition is similar to KOH one.

At the beginning, I think the precipitation of the solution with NaOH is caused by plenty of sodium ions, and that might causes the  solubility of mandelic acid decreasing. 

(Even if more solvent is added, it will still precipitate)

May strong electrolyte interfer hydrogen bond forming ?

Based on this doubt, I did a test to add a large amount of NaCl to the mandelic acid aqueous solution (Solubility is considered), instead of NaOH. Therefore, the test rules out the effect of pH on the solubility of mandelic acid. However, the precipitation of crystals still formed.

Thus, I assume that electrolytes is possible until I finding that KOH edition did not produce crystal precipitation…….haha……..

So, I come up to ask and hope someone could give me some advice.

Anyway , thank you again.

[Victor]

In my opinion, increasing surfactants (polysorbate 20) or solvents (Isopropyl Alcohol/Alcohol) seems rather feasible.

Add additional carbomer might not be avoided precipitation. Unless you want to develop an emulsion, but still need enough solvent/surfactant/oil to avoid menthol crystallization.