Wednesday, February 2, 2011

A little chemistry on thickening...

Here's a little chemistry to start your day! Let's take a look at colloids, hydrocolloids, and shear thinning.

What is a colloid? A colloid is "a substance microscopically dispersed evenly through another substance." It can be a gas, solid or liquid. It consists of two phases - the dispersed or internal phase and the continuous or dispersed medium. Does this sound familiar? It should because lotions are considered colloids (liquid-liquid colloids)! The dispersed medium or continuous phase is the water phase and the dispersed or internal phase is the oil phase. So you've been making colloids and didn't know it!

What is a hydrocolloid? A hydrocolloid it's a colloid system where the colloid particles are dispersed in water, and it can be a gel or a liquid. If it's a liquid, it's called a sol - a colloidal suspension of solid particles in a continuous liquid medium, like blood or toner (I probably shouldn't have put those two things together - what an association!). The thickeners we'll be discussing like guar gum, xanthan gum, cationic guar gum, and so on create hydrocolloids.

What is shear thinning? We see this phrase thrown around quite a lot in lotion making, but it's applicable to gels and just about everything else we make. Shear thinning refers to a situation where the viscosity of something decreases with the rate of shear stress. It can refer to when we're mixing our lotions - when we're stick blending or mixing it, it will seem thinner than when the lotion is at rest - or to when we're using our products - squeezing a bottle will reduce the viscosity of the product. When we stop applying stress - stop squeezing, mixing, or pumping - the product returns to its original viscosity. (Materials that exhibit shear thinning are called pseudoplastic materials.)

What is thixotropy? It is a property of certain gels or viscous fluids: They become more fluid when we apply stress like shaking or squeezing a bottle. It takes a finite time to attain its original viscosity when introduced to a change in shear stress.

Thixotropic materials see a reduction in viscosity over time at a constant shear rate.
Pseudoplastic materials see a reduction in viscosity at an increased shear rate. Meaning, the viscosity decreases as we apply more stress. If we keep the stress the same, the viscosity will remain the same. With a thixotropic material, the viscosity will reduce over time if we keep the stress the same. Guar gum is an example of a thixotropic material when used at 1% (but not at 0.3%).

So let's get back to our regularly scheduled programming and take a look at guar gum!

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