Silicones and oils don't mix. Although we consider them both oil soluble in lotions - for instance, when we are thinking about emulsifying something in a hair conditioner or a facial moisturizer - they really aren't, so they'll separate when combined.
We generally think of our ingredients as being
a) hydrophobic, or water hating
b) hydrophilic, or water loving
c) lipophobic, or oil hating
d) lipophilic, or oil loving
But there's a third category of ingredients that are siliphilic - silicone loving - or siliphobic - silicone hating. Silicones are hydrophobic, lipophobic, and siliphilic. They prefer to hang out with other silicones away from water and oils.
As an aside, you've probably read that dimethicone is considered a barrier protectant ingredient. How does that work? "It is the lack of solubility in oils and water that makes dimethicone a barrier when applied to skin."
When we create an emulsion or get things to combine that don't want to combine, we need a surfactant with one end of the molecule that loves water and the other end of the molecule that loves oils. We call these emulsifiers.
When we make an emulsion that includes silicones, we are actually creating something with three phases - water phase, oil phase, silicone phase - that could be more unstable than one with just water and oil. It's not hard to create something that works well - take a look at any number of my hair conditioner formulas using Incroquat BTMS-50 to see how simple it can be - but we definitely need an emulsifier of some sort to mix these things together.
Silicones and the HLB system
The problem with something that's just oils and silicones is that I can't find something to bring just those two things together or something with an oil loving end and a silicone loving end. Generally we have something like Lotioncrafter's Serum SE to make something with water, a small amount of oil, and silicones.
So what can you do? It might be easier to choose a silicone that can be combined with oils or esters. Something like regular old dimethicone 350 cs doesn't stay combined with carrier oils, but an elastomer might work.
What's an elastomer? "An elastomer is a polymer with viscoelasticity (having both viscosity and elasticity)..." (reference) The silicone molecules are linked with other silicone molcules to create a more viscous silicone, which can range from the scoopable loveliness you see in the picture above of Optiblur, to something very thick that has to be cut. They're used to film form in things like lipsticks, create that soft silkiness we like in make up primers, or smooth down our cuticle and prevent frizzies in hair are products.
For instance, Lotioncrafter is carrying a silicone elastomer called OptiBlur™ that can be combined with all kinds of oils and esters.
- 20% Optiblur™ Elastomer is miscible with 80% caprylic/capric triglycerides, isododecane, C12-15 alkyl benzoate, cyclomethicone, or phenyl trimethicone
- 80% Optiblur™ Elastomer is miscible with 20% phenyl trimethicone or cyclomethicone, but will separate with 20% caprylic/capric triglycerides, isododecane, C12-15 alkyl benzoate.
What this means is that you can mix 20% of this ingredient with 80% caprylic/capric triglycerides (also potentially known as fractionated coconut oil at some vendors), and it will remain mixed.
Something like Lotioncrafter's EL 3045 (INCI: Cyclopentasiloxane (and) C30-45 Alkyl Cetearyl Dimethicone Crosspolymer) might also be a good choice as it "is an excellent anti-syneresis (anti-weeping) agent in water in oil emulsions and anhydrous systems. Picture this product as a fluid-containing sponge. The sponge is the silicone copolymer network and cyclopentasiloxane is the fluid. This silicone sponge allows its carrier or fluid to travel in and out, and can be receptive to other cosmetic ingredients. As a result, it holds oils or liquid in the external phase reducing syneresis."
You could use something that might make it easier to incorporate cyclomethicone or dimethicone into a product, something like phenyl trimethicone, but I'm not sure how much to use yet.
The other alternative is to make a solid product. I've been making a lovely lotion bar that contains dimethicone and cyclomethicone for years, and it has never wept or separated. A balm might work as well.
Or you could accept this will separate and needs to be shaken before every use.
So the short answer is - I don't know. And the longer answer is that I still don't know, but there are a few ingredients that will make it easier to keep them together.
If you want to learn more about silicones, check out this free e-book by Anthony O'Lenick, who is a master of silicones.
Making a water-in-silicone serum with Lotioncrafter Serum SE
Why use silicone in personal care applications, part one by Anthony O'Lenick
Silicones in personal care products
Selecting the perfect silicone for your formula