emulsion help

How would these ingredients form an emulsion if glyceryl stearate (HLB ~ 3.8) is being used to emulsify lanolin (rHLB ~ 10) and stearic acid (rHLB ~15)? Or would this be w/o emulsion because of the low HLB of the emulsifier?

Active ingredients: menthol 10%; methyl salicylate 15%. Inactive ingredients: citric acid, glyceryl monostearate, lanolin, methylparaben, propylene glycol, propylparaben, purified water, stearic acid, trolamine.

Comments

  • letsalcidoletsalcido Member
    edited June 26
    There is less water than glyceryl stearate and lanolin, if in the right order. This is definitely w/o in that case.

    Maybe someone experienced will be able to tell for sure.

    If you have a sample of this product, does it “dissolve” in water or just floats?
  • mhart123mhart123 Member
    @letsalcido for otc labeling, the inactive ingredients can be listed in alphabetical order so that makes it a little trickier when trying to determine use level. I do not have a sample of the product yet but when I get it, I will try that out.  
  • PharmaPharma Member, Pharmacist
    Mixing stearic acid with trolamine results in TRIS stearate, a soap or in other words a high HLB emulsifier.
    Just based on composition, it can be either. Lanolin does bind water nicely and 25% oil soluble actives... a water saturated 'oil' would be my guess.

  • mhart123mhart123 Member
    Thanks @Pharma, I did some searching on here through older posts and found that the trolamine would I guess kind of activate the stearic acid and help with the emulsification.

    I mixed trolamine into the water (pH was 8.5) before adding the oil to the water phase and the emulsion turned out better then my previous tries, however it was still pretty thin.  Then with the addition of citric acid the oil separated out (pH 6.9). I will check to see what the pH of the competitor sample is when I receive it, but are there pH limitations for the stearic acid/trolamine combo?
  • PharmaPharma Member, Pharmacist
    If you add too much citric acid (or any acid), pH drops and reverts TRIS stearate back to non-emulsifying stearic acid.
  • Coming back to this discussion to see if anyone has some insight..

    Formula is listed below, it emulsifies but is water thin and pH is 8.6. The competitor product is 6.5 and viscosity of 20,000cps. The emulsion gets super thick after phase B is added to A and it cools but as soon as C is added it thins out and has no viscosity.  Any thoughts on how to increase the viscosity?
    A
    Water50.40
    Methylparaben0.200
    TEA0.30
    B


    Stearic Acid

    7.00
    Lanolin2.00
    Glyceryl Stearate5.00
    Propylparaben0.10
    C

    Propylene Glycol
    Methyl Salicylate

    10.00
    15.00
    Menthol10.00

    D

    Citric Acid

    0.00
  • mhart123 maybe you can try adding the Propylene glycol into the water phase, and the Methy salicylate and Menthol in the oil phase? Not sure how heating's gonna affect Methyl salycilate and Menthol's performance, but that much Propylene glycol added during cooling phase can indeed reduce emulsion's viscosity.
  • @ketchito the menthol will solubilize in the methyl salicylate but I thought maybe the propylene glycol would help it mix into the emulsion better.   I did make a batch with the propylene glycol in the water phase but ran into the same issue. 

    For whatever reason, I thought menthol would have to be added below 50C but I think I got the melt point temp confused with the flash point temp.  Just checked that the flash point is 90C so I guess I could add both in the oil phase?
  • @mhart123 I've seen emulsions where Menthol is added in the oil phase, so it's always nice to try. Maybe you could add it as the last ingredient of the oil phase, so it doesn't stay "hot" for too long. 
  • PharmaPharma Member, Pharmacist
    Hot process of menthol and methyl salicylate will simply tell everyone in the vicinity what you're doing :smiley: . Loss isn't much, though, and I would try incorporating into the oil phase and avoid PG.
    Essential oils and constituents thereof tend to show such a viscosity drop if added at high %. You could try a gelling agent and 'cheat your way around it'.
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