Magnesium sulfate makes my emulsions fail

Hello everybody. My name is Craig, and I make body emulsions in Georgia. My current method is to make a base o/w emulsion and use this to create a variety of different finished products. This base emulsion is smooth and stable, and it will accept just about any active ingredient (e.g., taurine, benfotiamine, leucine, resveratrol, vitamin D3, melatonin, etc). This is the recipe for the base emulsion:

WATER PHASE  

  • Distilled water - 63.00%
  • Glycerin - 4.50%
  • Hydroxyethyl cellulose - 0.75%
  • Phenonip - 1.00%            

OIL PHASE          

  • Flaxseed oil - 8.88%
  • Avocado oil - 8.88%
  • Isopropyl myristate - 4.00%
  • Emulsifying wax NF - 7.00%
  • Vitamin E Tocopherols - 1.00%

EMULSION PHASE           

  • Fragrance - 1.00%

Recently I created a batch of body cream with 10% magnesium sulfate. Powders like this are folded into the base emulsion, then homogenized at very low sheer. Within a day, the mag emulsion started to separate, with a portion of the water phase settling on the bottom of the jar. Does anybody know why the magnesium sulfate is causing the emulsion to fail? I made a 2nd batch with a more incremental addition of magnesium, but it also separated. Any advice you can provide is greatly appreciated.


Comments

  • It's typical for high amounts of electrolytes to destabilize W/O emulsion. Although HEC is fairly electrolyte tolerant, is it possible that the system is too overloaded? what was the incremental amount of MgSO4 added in the second batch?
     
  • chemicalmattchemicalmatt Member, Professional Chemist
    I doubt that system would hold up with even 1.0% MgSO4.  Divalent electrolytes are particularly adept at destabilizing.
  • PharmaPharma Member, Pharmacist
    Emulsifying wax NF per se shouldn't be too much of a problem since it's non-ionic but the fairly small quantity of true surfactant in the mixture (Polysorbate 60) is low and it's just one single surfactant; hence it's prone to 'surface charge neutralisation' by MgSO4. I suppose you should be able to find a work-around probably by adding a co-emulsifier. Maybe try a sorbitan ester and some fatty alcohol?
  • Bill_TogeBill_Toge Member, Professional Chemist
    instead of emulsifying wax NF, try an emulsifier with an electrolyte-tolerant co-emulsifier with a long chain, e.g. PEG-100 stearate or polyglyceryl-6 behenate
    alternatively, instead of magnesium sulphate try using magnesium oxide (less water-soluble)
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • PharmaPharma Member, Pharmacist
    Emulsifying wax NF should be electrolyte tolerant and has a relatively long chain ;) . But using either of the two proposed ones in addition might actually work (try and see).
    Magnesium oxide will form magnesium hydroxide in water and this is alkaline. Neutralising it will result in a soluble magnesium salt. Since magnesium ions are usually the culprit, using magnesium lactate or the like is pointless unless one were to use a cationic emulsifier.
  • Thank you for all the excellent advice. I think a co-emulsifier is a logical next step. 

    But here's another mystery. In the last couple of days I made a batch of cream that included 7.5% magnesium sulfate and 7.5% taurine. The taurine seems to "protect" the emulsion and it is still very robust. I've noticed a similar effect with taurine emulsions that are subjected to high heat for multiple days -- they don't separate. Why would taurine have that kind of preservative effect? 

    Taurine has some negative side effects in this particular case, but there's no doubting its ability to strengthen an emulsion. 
  • PharmaPharma Member, Pharmacist
    pccochran said:
    ...Why would taurine have that kind of preservative effect? ... 
    It is likely the coincidental result of ionic strength, ion type, buffer capacity, and probably pH which has an impact on Debye length, Zeta potential, and electrical double layer which in turn affect emulsion stability.
    Taurine being a zwitterionic aka inner salt makes it likely (but not obligatory) that it's addition can reduce adverse effects of other salts on emulsion stability.
    Given the high electrolyte concentration, I'm not sure how well predictions are possible and how far it's just a lucky find.
  • Thank you, @Pharma. Your response is very helpful. 
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