Thursday, March 14, 2013

Chemistry Thursday: The Bancroft rule

While looking at the comments section looking for questions I could answer, I found a link to the Bancroft rule, and I thought I should post it here. (I can't remember who suggested this, sorry, but you are awesome!)

The Bancroft rule states: "The phase in which an emulsifier is more soluble constitutes the continuous phase."

What does this mean? In an oil in water emulsion, we have the internal or discontinuous or discrete phase of the product - the oil part, the part that is in particles - and the external or continuous phase of the product - the water part. The internal phase is suspended in the continuous phase. (Check out this post for more in depth information.)

Oil phase = internal, discontinuous, or discrete phase
Water phase = external or continuous phase

Looking at this picture, the oil is the orange part. 

More quoting from Wikipedia: "What the Bancroft rule states is that contrary to common sense, what makes an emulsion oil-in-water or water-in-oil is not the relative percentages of oil or water, but which phase the emulsifier is more soluble in. So even though there may be a formula that's 60% oil and 40% water, if the emulsifier chosen is more soluble in water, it will create an oil-in-water system.
There are some exceptions to Bancroft's rule, but it's a very useful rule of thumb for most systems."

Wow! So what this says is that the emulsifier determines if it's an oil-in-water lotion (what we generally make) or a water-in-oil lotion, not the ratio of oils to water! Wow!

When we take a look at the hydrophilic-lipophilic balance system (HLB system), we can see that some emulsifiers are more soluble in oil and others in water. The lower the number, the more oil soluble. So glycol distearate (HLB 1) will more soluble in oil than polysorbate 20 (HLB 15). Which makes sense as most of those low HLB emulsifiers are waxy or oily things like glycol distearate or lecithin.

Yet more quoting: "To have a stable emulsion, the surfactant must be soluble in the continuous phase." So anything we use must be water soluble to create an oil-in-water product. Interesting...

This isn't a hard and fast rule, and it seems to be have been surplanted by the HLB system, but it gives us something to think about when creating lotions. As with so much of what we learn about making bath & body products, common sense isn't always the correct answer!

If we're using all-in-one emulsifiers like Polawax, emulsifying wax, Ritamulse SCG, Incroquat BTMS-50 we don't have to worry about solubility. And, to be honest, we really don't need to worry so much about it if we're using HLB system emulsifiers. I just thought this was really interesting! 

Related posts:
Phase inversion
More on phase inversion- Creating products: Mixing the two phases

Related websites:
Colloid chemistry power point (do a search for Bancroft, although the whole thing is fascinating!)
Colloid chemistry power point (page 16, or do a search)


melian1 said...

omg! THANK YOU! i have been told (on the dish) that the emulsifier determined if it was a w/o or a o/w emulsion and it never ever made any sense. you have just explained something to me that i have failed to understand for literally years.

holy cow. you are good.

Lise M Andersen said...

On a completely unrelated topic: Susan have you ever come across this ingredient: Magacamianussöl (from an ingredients list with purely INCI names.
There is a make-up company advertising their stuff as 100% natural with this ingredient listed in almost every product. It's not often I hit a brick wall researching an ingredient but this is looking a lot like crocodile oil to me and I'd be ghrilled to hear any input you may have on this.

Lise M Andersen said...

thrilled... jeez.. excuse the iphone typo

Susan Barclay-Nichols said...

Great question, Lise! I've turned it into today's Weekend Wonderings. I have no idea what the ingredient might be, but I have contacted the company and I hope that posting it here will garner some responses.

Hi melian! I know, right! It's so interesting. Damn, I love chemistry!

7slaper said...

My guess would be Macadamia.

Explanation: I found another result on Google (German version). It was a scanned article saved as a PDF-document, which I opened. Searching for Magacamia it jumped to the Magnesium in de text ....

So I think it's either a typo or an OCR-error. ;)