Tuesday, September 29, 2009

Silicones and the HLB system

Mich brings up a very good point that I think needs addressing!

Here you have the HLB requirement for dimethicone as either 9 or 10, depending on c.s. It is listed as 5 on both LabRat's PDF and the Herbarie's list. (Neither of those specify the c.s.)

In the texts I've been reading, I keep seeing 9 to 10 as the required HLB value for dimethicone. But it makes more sense for silicones to have a required HLB value of 5. A low required HLB value of 5 indicates the molecule is more hydrophobic (and more lipophilic) and is less water soluble than one with a required HLB of 9 or 10. We know how hydrophobic dimethicone is - just try mixing it in water! - so it should be a 5!

The HLB value of a molecule is determined by the molecular weight of the hydrophilic portion and divided by 5. Silicones - like the dimethicone and cyclomethicone - are inorganic molecules, meaning they are based on silicon and oxygen. (Organic molecules have hydrocarbon groups that contain hydrogen and carbon). As a silicone is almost completely oil soluble (and therefore, hydrophobic) it should have a low number!

And yes, I know there are water soluble silicones, but this issue is already complicated enough!

But silicones don't act like our normal lotion ingredients. Their behaviour in lotions are hard to predict. To quote from Surfactants in Personal Care Products...

Direct application of the HLB concept to these materials has resulted in approximate and often misleading values due to the fact that silicones are organic/inorganic hybrids and the thermodynamics of their interactions with the oil and water phases in emulsions scale differently than do the interactions of purely organic amphiphiles.

In other words, it's hard to predict what a silicone will do and put a required HLB value on it.

A new system needed to be created for molecules like silicones which are difficult to categorize. It's called the 3D HLB system and there is no way I can explain it all here (click on the link for 3D HLB fun!) Instead of thinking about oil-in-water emulsions and water-in-oil emulsions, we must now consider silicone-in-oil emulsions, silicone-in-water emulsions, water-in-silicone emulsions, and oil-in-silicone emulsions. (Please consult the PDF linked above for detailed information this system...it is a very good PDF and will make more sense than I will!)

So what the heck does all of this mean? Well, it means we can't predict what a silicone will do in an emulsion without using the 3D HLB system. Since most of us don't have access to fancy labs and hundreds of different ingredients (sigh...) then we have to go by what the suppliers tell us.

From yesterday's post...

Macadamia nut - 0.147 x 7 = 1.029
Safflower oil - 0.147 x 8 = 1.176
Mango butter - 0.441 x 8 = 3.528
Cetyl alcohol - 0.088 x 15.2 = 1.338
IPM - 0.0588 x 11.5 = 0.6762
Cyclomethicone - 0.0588 x 8 = 0.4704
Dimethicone - 0.0588 x 9 = 0.5292
Total HLB for the oil phase: 8.7468 - we'll round this up to 8.75 or 8.8.

So our HLB target number for the emulsifiers is 8.8.

What if we change the dimethicone number to be 5? Dimethicone - 0.0588 x 5 = 0.294

Add the numbers together, and our HLB target number for the emulsifiers is 8.4816 or 8.5.

Is that a big difference? I would say yes. We have to re-work the emulsifiers for this lotion and do more math. More math = significant.

My head is starting to hurt, so I'll summarize my thoughts...On the one hand I have various textbooks and data sheets telling me dimethicone has a required HLB value of 9 to 10, and on the other hand I have the Herbarie and Lab Rat telling me it's 5. I've gone to a number of manufacturers' sites and they won't give me the number; they tell me it's hydrophobic, as if I didn't already know that.

It makes sense to me for dimethicone to be a 5 - it's hydrophobic, lipophilic, and certainly doesn't have any hydrophilic tendencies (why does that sound rude to me?), all of which point to a low required HLB value. I trust the Herbarie and Lab Rat, so I'm going to go with 5 as the required HLB for dimethicone.

Thanks, Mich! Great topic!


Mich said...

I am always happy to share my confusion with others!

Thanks for taking the time to discuss the topic. I love them 'cones!

Marlene Taylor said...

Thank you so much for sharing this article! It has given us a new step toward researching and understanding the silicone issue with microemulsions!