I am working on a cream and I have noticed that the pH changes over time in the stability test, more noticeable in accelerated conditions.
Ingredients: Methyl salicylate, menthol, sodium cetostearyl
sulfate and Cetostearyl alcool, xanthan gum, glycerine, water
I have noticed in the preparation process, if I melt the oil phase at 60-70 C the pH is around 6.5 and it drops to 6 after a month is the stability in accelerated conditions
If I melt the oil phase at 50-60 C the pH is about 7.5 but it drops to 7 in normal conditions and 6.1 in accelerated conditions (1 month).
What might be the cause?