pH adjustment and emulsion stability

I've been working on a moisturiser and have achieved the viscosity I was looking for but the when I add citric acid solution to correct the pH the emulsion thins out rapidly and begins falling apart. I'm using sucrose stearate as the main emulsifier with glyceryl stearate as the co-emulsifier. I've read sucrose stearate emulsions can be a little unstable sometimes so the recommendation was adding a stabiliser like Xanthan Gum. Has anyone else had issues with acidic pH adjustment and found a solution. Is it likely that adding Xanthan Gum will give the stability I'm looking for. Everything else was great, so it's a bit disappointing for it to fall down at the last step (pH adjustment). 

Comments

  • Just realised I have been measuring the pH incorrectly so I may have been making it to acidic which resulted in destablisation. Will make another batch and test correctly to see if this makes any difference!
  • chemicalmattchemicalmatt Member, Professional Chemist
    You are encountering one of the steadfast rules of emulsion chemistry: avoid adding strong electrolytes to o/w emulsions at any time, but if you must, then add while at elevated temperature. IF it were w/o or w/Si, that's a different story.
  • @chemicalmatt thanks for the reply. Learning more all the time! It seems the glyceryl stearate I was using was contributing to the pH sensitivity. Measuring the pH correctly has also helped!
  • @chemicalmatt further to your point. How would you suggest lowering the pH of an O/W emulsion other than doing it at higher temperature?
  • chemicalmattchemicalmatt Member, Professional Chemist
    Reheat to a tolerable level, usually above the PIT (phase inversion temperature), then adjust. Another easier method is to heavily dilute your acid or alkali and add very slowly to the batch while mixing is underway. This works in most cases, but tragically not all. 
  • @chemicalmatt thanks for the feedback. I'm guessing I'll need to go with the second method as I will have added heat sensitive ingredients in the cool down phase. 
  • @chemicalmatt when you say to add the diluted acid solution while mixing is underway do you mean the initial high shear mixing or can it be just stirring during cool down? 
  • So in doing some research into this I tested the pH of cheap supermarket moisturiser, one which I consider to be reasonably high end and one pre-made by a cosmetics ingredients supply. All were in the range of 7.1-7.2 which I found really surprising as this would be working against your skins acid mantle but I can also see that at this pH they would also avoid a lot of formulation issues as my emulsion is creamy and stable at this pH but becomes thin and light/fluffy at pH 5.5. More research needed.
  • So turns out I just need to mix the emulsion properly. I got an homogeniser and made another batch, adjusted to pH 5.3 and it's great. Was previously mixing via overhead stirrer only. Thanks for all the tips so far. 
  • You are encountering one of the steadfast rules of emulsion chemistry: avoid adding strong electrolytes to o/w emulsions at any time, but if you must, then add while at elevated temperature. IF it were w/o or w/Si, that's a different story.
    Sorry to jump in...kinda out of context.... but at what point would you add a 1% Hyaluronic acid solution to a lotion emulsion?
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