We can do this by using salt, by adding surfactants like the betaines, or by adding fatty materials, like Crothix, PEG-150 distearate, EZ Pearl (glycol distearate), or other thickening esters. Because surfactants have those hydrophilic heads and lipophilic tails, the fatty materials will become trapped in the middle of the micelles, causing them to swell. This creates viscosity in our products.
If you are using these fatty materials, be warned that some will pearlize or cloud your product. Crothix will create a clear system, but EZ Pearl is definitely something that will make it very opaque, which can be very pretty! (This was thickened with EZ Pearl - you can't see the pearly look, but it is lovely.)
These products thicken very well, but may not play well with your fragrances. Keep a list of those fragrances that thin or thicken your mixture so you can compensate for that change. (For instance, I find the black amber lavender and cream cheese icing fragrance oils from Brambleberry thicken my products like silly - I love it - whereas the jewelled citrus thins it like crazy!) I like to use liquid Crothix to thicken so I have more control over the product - I make something like a bubble bath, add everything but the Crothix, and wait until the next morning when it becomes clear. I can add the Crothix then - 0.5% at a time - to see how it thickens up.
Join me tomorrow for fun increasing our viscosity by making gels!