Monday, April 5, 2010

Surfactant chemistry: Micelles

What are micelles? They are defined as "an aggregate of surfactant molecules dispersed in a liquid colloid". So what's a colloid? It is a type of mixture in which one substance is evenly dispersed throughout another. There's generally two phases - the dispersed or internal phase and the continuous phase. In the case of oil-in-water lotions, the oil phase is the dispersed phase because it is dispersed into the water phase. In the case of detergent mixtures, the dispersed phase would be the detergents mixed dispersed into the water phase. (You've been making colloids for years and didn't even know it!)

All surfactants have a hydrophobic (water hating) tail and a hydrophilic (water loving) head. (You might see hydrophobic tails listed as "lipophilic" meaning fat loving...) As you can see from the picture above, the hydrophobic tails migrate to the centre to get away from the water and the hydrophilic heads orient to the outside to get nearer to the water. To get the micelles to form, we need two things - we need to add enough surfactant to get to the right critical micelle concentration for the specific product, and we need to reach the critical micelle temperature (or Krafft temperature).

The critical micelle concentration and temperature is different for each product and really complicated, so we'll just go with the idea that if we use an appropriate amount of emulsifier for a lotion - say 25% of the oil phase for a lotion - and heat and hold our lotion ingredients at 70˚C for 20 minutes, we'll reach that point!

Why do we care about micelles? In oil-in-water lotions and creams, this is what gives us emulsification. The oil attaches to the hydrophobic tails inside the micelle and the water attaches to the hydrophilic heads on the outside. Without micelles, there would be no emulsification! In detergent mixes - like body washes and shampoos - the micelles help to create the viscosity of the product and emulsify small amounts of oils (like fragrance, essential, or speciality oils).

Note: For water-in-oil emulsions, the micelle is switched around with the heads in the middle and the tails on the outside. But that's a whole other post!

So we want to reach the critical micelle concentration for our detergent products so we can get good viscosity. We can do this by making sure we add a nice amount of surfactants or by thickening the products with salt, thickeners, or other surfactants.

Join me tomorrow for the first of a series on types of surfactants!

1 comment:

kontakt said...

Wow. You just solved me a riddle: if there are any micelles when a small amount of water and fairly much tensid is mixed with a larger amount of oil. I make and use a mixture like this in the shower, great for dry skin. I guess it is these "reverse micelles" that makes the mixture thicker, since no extra thickener is needed.