Creatine is a popular sports/fitness supplement, generally sold as a powder to be mixed with water (usually like 5-10g creatine per serving). However, it is not very "soluble" in water -- barely 13.3g/L at 18°C. To enhance solubility, some manufacturers sell it as a creatine-HCl formulation -- bringing it up to 88.8g/L!
This formulation, of course, is also more expensive. It makes sense why lowering the pH (making more of the creatine molecules positively charged) would make creatine more soluble, but I'm not sure if there is anything "special" about the HCl component here.
Namely, at home I have regular creatine monohydrate powder (not HCl) and a bag of anhydrous citric acid. I haven't measured the exact increase in creatine solubility after the addition of citric acid, but solubility does increase a little bit and the solution is quite sour as expected.
What I am wondering, is there anything "special" about the creatine-HCl formulation that would greatly bump up creatine solubility that could not just be done with regular creatine (non-HCl) powder and some citric acid powder to lower the pH? I know citric acid is not as "strong" an acid as HCl, but regardless whether it's HCl or citric acid that's used, the final solution pH still has to be safe for oral consumption. And adding citric acid already makes things really sour!