Thursday, October 1, 2009

Phase inversion

I know I recently wrote a post on this topic, but I thought I'd add a little more to it here...

What is phase inversion? We know we have two phases in our lotions - the oil phase and the water phase (if you want more technical terms or to learn more about emulsification, please click here). In an oil-in-water lotion you have the external, continuous phase (the water) and the internal dispersed lipid phase (the oil).

When we have heated and held our two phases, we add them together. In the past, I'd add the oil to the water phase because the container was usually bigger because I had more water based ingredients. But I was wrong!

If you add your oil to the water phase, you are creating an oil-in-water emulsion. This is what we want, right? Well, yes and no. The oil-in-water emulsion is our end goal, but if we add the water to the oil, we can get a more stable emulsion and, therefore, a more stable lotion.

When we add the water phase to the oil phase it produces a temporary water-in-oil emulsion (w/o). It will eventually "flip" to become an oil-in-water lotion at the point of phase inversion. This makes the emulsion more stable, which is, again, something we like.

How exactly does phase inversion work? Most of the emulsifiers we use are ethoxylated fatty materials. The hydrogen bonding between the ethoxylated molecules and the water is what helps with the emulsion. When they are heated two things happen: their HLB changes from about 12 to about 5, which means they are more suitable for water-in-oil emulsions, and their ability to form these hydrogen bonds with water decreases. So they're really acting as water-in-oil emulsifiers when heated.

As the lotion cools, the HLB of these emulsifiers starts to increase back to the proper HLB value and their ability to hydrogen bond with water increases. The lotion switches from being a water-in-oil emulsion to being an oil-in-water emulsion.

This is more stable because the oil droplets in a phase inverted lotion will be smaller than those found in a non-phase inverted lotion. We know that oil droplet clumping can be one of the biggest reasons for separation - the smaller the droplets, the easier it is to keep them apart, thus less likelihood of clumping.

Isn't chemistry fun?

For a fantastic PDF on phase inversion, click here for Dr. Z's explanation!

Join me tomorrow for something...

5 comments:

Lalla said...

Does that mean that if I use a non ehtoxylated emulsifier (cetearyl glucoside for example) there won't be a phase inversion?

Shine555 said...

i do think ethoxylated is one of example of functional group that can be bonded to Water by Hydrogen bond and the cetearyl glucoside is also had a lot of OH (Hydroxyl Group from glucoside) so cetearyl glucoside could act as emulsifier too but i'm not your how potence it is

Anonymous said...

Hi Susan, what I understand from reading the Dr. Z explanation you linked is that the oil phase can be added to the water phase and phase inversion will still occur. Dr. Z described adding the oil phase to the water phase and having an initial w/o emulsion form because of the lack of H-bonding and low HLB of the heated emulsifier. It then said that at the Phase Inversion Temperature the emulsion changes to o/w. It did not mention anything about adding the oil to the water or the water to the oil and any effect of either on phase inversion occurring so I don't understand why you say the water phase must be added to the oil phase.

Anonymous said...

Adding to my comment above Susan, I don't see why it would matter which way you combine the phases because isn't the emulsifier heated either way? So wouldn't it still display the same change of properties regardless of if you add the oil phase to the water phase or the other way around? Either way, wouldn't the emulsifier still have that low HLB and lack of H-bonding when heated thus initially forming a w/o emulsion?

Susan Barclay-Nichols said...

I thought I'd update this post by linking it to a post in which I note that we can add our ingredients any way we wish! Thanks for encouraging me to look at this again!