Methylparaben and propylparaben precipitating out

MaydayMayday Member
edited January 28 in Formulating
I'm working with 100g test batches. I had some trouble dissolving methylparaben (200mg, 0.2%) and propylparaben (100mg, 0.1%) in phenoxyethanol (500mg, 0.5%). The parabens created a slurry in the phenoxyethanol rather than dissolving. Room temp about 20C, so theoretically the parabens should be soluble in water at this level, at least individually.
Adding this to my water phase resulted in the dense slurry hitting the bottom of my beaker and getting smeared around by the magnetic stirrer. Heating up the batch did dissolve the residue, but it later precipitated out after cooling. (The other ingredients in solution were 0.02% Calcium Disodium EDTA and 1.7% Sodium PCA.)
I see that Clariant Phenonip and Euxyl K300 both use 72% phenoxyethanol in their blends. Mine was 62.5%. Do I need more solvent (phenoxyethanol) for the parabens? I'm thinking about making a stock of phenoxyethanol and the parabens. Would being more patient and/or heating it up help dissolve the parabens at these levels, or would heating likely just lead to precipitating out of the phenoxyethanol?
Trying to avoid propylene glycol or ethanol, though I'm aware those can both help solubilize the parabens.
I only have 2oz phenoxyethanol at the moment. I would appreciate some advice on what to try next, so I don't waste my sample. (Aside from just buying Phenonip, which I want to try once it's in stock from Lotion Crafter.)
⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️

Comments

  • PhilGeisPhilGeis Member, Professional formulator
    What is the product?
  • MaydayMayday Member
    edited January 29
    Water-based personal lubricant, so it is only the water phase. Still testing gelling agents, but want to get a working preservative system defined before doing more work in that area. Not for commercial purposes at the moment.
    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
  • I have seen some premade blends that all had ~4% propyl, ~17% methyl Paraben in phenoxyethanol and i was always thinking why this ratio. 

    Maybe because more than this can not be dissolved.

    Do you know how much methyl Paraben maximum alone can we dissolve in phenoxyethanol that dissolve in final water based product well too? 
  • MaydayMayday Member
    edited January 29
    I think if the paraben is fully dissolved (not a slurry) in the solvent, then as long as that solvent is in turn miscible in water (or soluble at that level), it should not precipitate out.

    I have not been able to find information about, e.g. solubility of methylparaben in phenoxyethanol. I think these sorts of mundane but difficult to answer questions are why people hire experienced cosmetic chemists. Sometimes I can find hints in research papers (...thanks to Miss Elbakyan).
    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
  • Mayday said:
    I think if the paraben is fully dissolved (not a slurry) in the solvent, then as long as that solvent is in turn miscible in water (or soluble at that level), it should not precipitate out.

    I now think this conclusion is incorrect given others have reported problems with precipitation while diluting a paraben stock with distilled water. My new thinking is that unless you have surfactants to help keep the parabens in solution, the solubility will be more-or-less the solubility of the main solvent (water) rather than the stock solvent (phenoxyethanol/ethanol/propylene glycol, etc.).

    Given the solubility limit of propylparaben in water at 25C is 500ppm, it's likely the propylparaben that is precipitating out of solution as it cools. Phenonip's solubility limit of 0.5% in water (at 25C? Brochure doesn't say.) gives individual paraben concentrations far below the solubility limit in 25C water, and a total paraben concentration of only 1400ppm.

    Getting 1000ppm propylparaben into an aqueous solution without surfactants would then require either a correspondingly large proportion of cosolvent (propylene glycol, etc.) or using sodium propylparaben with pH adjusted below 6.

    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
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