Why emulsions separate, and how thickeners keep them stable

You made a lotion, it looked perfect, and a week later there is a watery layer at the bottom or a film of oil on top. This is the most common thing that goes wrong with emulsions, and it is rarely bad luck. This guide explains why emulsions fall apart and how thickeners (the professionals call them rheology modifiers) slow that down. No chemistry background is needed.

Why every emulsion wants to separate

Oil and water would rather be apart. An emulsion is a truce: the emulsifier wraps each tiny oil droplet so it cannot rejoin the others. But the truce is never permanent. In scientific terms an emulsion is thermodynamically unfavourable: left alone for long enough, it will always head back to separate oil and water.

So the formulator's goal is not an emulsion that lasts for ever. It is one that stays stable for the whole shelf life: months or years, in a warm bathroom, on a cold windowsill, being opened and closed every day.

Four ways an emulsion fails

Before you reach for a thickener, work out which kind of failure you have. They look different and they have different fixes.

1. Creaming and sedimentation

Oil is lighter than water, so in an oil-in-water (O/W) lotion the droplets slowly drift upwards and collect at the top, exactly like cream rising in fresh milk. In a water-in-oil (W/O) cream the heavier water droplets sink instead (that is sedimentation). The droplets are still intact, which is why a good shake often brings the product back.

How fast this happens follows a rule called Stokes' law. You do not need the formula, just the three things that matter: the bigger the droplets, the bigger the difference in density between oil and water, and the thinner the liquid they sit in, the faster they move. You cannot change density, but you can shrink the droplets (blend well, with enough mixing energy) and you can thicken the liquid. That is exactly where thickeners help.

2. Flocculation

Droplets stick together in clusters without actually merging, like grapes in a bunch. It is reversible, so it is not a disaster by itself, but clusters move and cream faster than single droplets, and they bring droplets close enough to merge later. Typical signs are a grainy or lumpy look, or a lotion that seems thinner than it should be.

3. Coalescence

This is the one you cannot undo. Droplets merge into bigger and bigger ones until oil visibly pools on the surface (an oily slick) or the whole thing splits into two layers. It usually means the emulsifier is not doing its job: too little, the wrong type for the oils you chose, a pH outside its working range, or not enough mixing. A thickener can slow coalescence by keeping droplets apart, but it cannot repair a weak emulsifier system.

4. Ostwald ripening

A slow, invisible process. Small droplets have higher internal pressure than large ones, so material gradually moves from the small droplets to the big ones. The small ones shrink and vanish and the big ones grow, so the lotion slowly turns coarser over weeks and months. A thickener does little here. The remedies are a narrower, finer droplet size from good mixing at the start, and in professional formulas special ripening inhibitors.

In real products these failures happen together and feed each other: creaming packs droplets closely, which causes flocculation, which leads to coalescence. That is why catching the first stage matters.

Words you will see

Viscosity is measured in mPa·s (millipascal seconds). For scale, water is about 1, and a spoonable cream is in the tens of thousands.

How thickeners keep an emulsion stable

Thickeners work in three ways, and most do more than one.

1. They make the water phase thicker. Thick liquid slows droplets down (Stokes' law again). Fewer collisions also means less flocculation and coalescence.
2. They build a network. Many thickeners form a loose three-dimensional web through the water. The web gives a yield stress: droplets are physically locked in place and cannot drift upwards however long the jar sits on the shelf. This is the strongest protection against creaming.
3. They create barriers. Long chains floating between droplets get in each other's way, a bit like cushions between people in a crowd (steric hindrance). If the chains carry a charge, droplets with the same charge push each other away, like two magnets with the same pole (electrostatic repulsion). Chains that sit on the droplet surface add a protective layer.

Meet the main families

Carbomers (acrylic polymers)

Carbomer (also sold as Carbopol) is a cross-linked polyacrylic acid and the classic gel former. The powder does very little in water until you neutralise it with a base such as sodium hydroxide, triethanolamine or arginine. Neutralising gives the chains a negative charge, so they repel each other and swell into a dense, entangled network. It is very efficient: 0.1 to 1% is enough, and it makes clear gels, which no other family does as well. Its network gives a strong yield stress, so it locks oil droplets in place.

• Needs neutralising before it thickens, and works best at about pH 6 to 10.
• Sensitive to salts. Electrolytes (mineral salts, aloe powder, sodium PCA and many actives) shorten the chains' reach and the gel collapses and thins. Salt-tolerant grades exist.
• Do not make it a surprise. Add salts and actives after it has thickened, and re-check the thickness.

Xanthan gum

Xanthan is a natural polysaccharide made by fermenting sugar with a bacterium (Xanthomonas campestris). It is the workhorse of home formulating, and for good reason. It is strongly shear-thinning: at rest its chains form a rigid network that holds droplets in place, under shear (pumping, spreading) the network breaks and the product flows, and when you stop it recovers. Usage is typically 0.1 to 1%, and the ingredient library lists its range.

• Stable over a wide pH range (about 4 to 10), through heating and freeze-thaw cycles.
• Tolerates salts far better than carbomer, which is why it is the usual choice for formulas with electrolytes.
• Natural, so it is usually the first choice for natural and certified (for example Ecocert) formulations.
• Do not overdo it. Above about 1% it turns stringy and slimy. Most lotions need only 0.1 to 0.5%.
• It carries a negative charge. It can clash with cationic ingredients such as conditioning emulsifiers (BTMS), giving a gummy mess. Check the charge in the ingredient library when mixing thickeners with conditioners.

Xanthan also teams up with other natural gums. With guar gum, locust bean gum or konjac it often gives more viscosity, or even a gel, than either would alone. The exact result depends on the amounts and the salts present.

Other natural gums

• Gellan gum: effective at very low levels. It makes fluid gels that hold droplets suspended without making the product heavy.
• Carrageenans (especially kappa and iota): form structured networks and interact with proteins, which helps prevent separation in emulsions.
• Alginates (from brown seaweed): form gels when they meet calcium or other divalent ions, which immobilises droplets.
• Konjac glucomannan: holds a lot of water, and combines with xanthan for better texture and stability.
• Pullulan: not usually the main stabiliser, but adds film-forming and some thickening.
• Guar gum: a common partner for xanthan, typically around 0.3 to 0.8%.

Cellulose derivatives: HEC and HPMC

Hydroxyethyl cellulose (HEC) and hydroxypropyl methylcellulose (HPMC) dissolve in water and thicken it as their long chains tangle. HEC in particular acts as a protective colloid and a thickener at once, forming an elastic three-dimensional network that helps hold emulsions together. Typical use is about 0.5 to 1.5%.

• Easier to use than carbomer: no neutralising step.
• Less clear and a softer gel: they do not match carbomer for crystal-clear gels or gel strength.
• Modified versions exist, such as hydrophobically modified HEC (HMHEC), used as an "associative" thickener in water-based paints.

Clays and fumed silica (mineral thickeners)

Clays such as bentonite, hectorite (including the synthetic clay Laponite) and attapulgite are tiny flat plates. In water they attract each other and stack in a loose "house of cards" that gives a yield stress. Fumed silica does a similar job by hydrogen bonding between particles. They come from mineral sources and give very precise control. Use rates are usually a few percent, so follow the supplier's data sheet, and let clays swell in water before adding anything else.

Thickener families at a glance

How to choose the right one

A thickener that works brilliantly in one product can fail in another. Ask these questions about your formula:

1. What is the pH? Carbomer needs about pH 6-10. Xanthan copes with about 4-10. Check your target with the pH guide.
2. Are there salts or lots of actives? Electrolytes thin carbomer gels. Choose xanthan, cellulose or a salt-tolerant carbomer grade.
3. Is there a surfactant or a charged ingredient? Free surfactants can disrupt a network, and a thickener's charge can clash with cationic ingredients (conditioners).
4. Do you need it clear? Carbomer is the best choice for clear gels. Others will look hazier.
5. Is it natural or certified? Xanthan, other gums and clays suit natural formulas; carbomer does not.
6. How easy should it be to make? Cellulose and xanthan need no neutralising step.
7. Where will it be sold? See the EU note above.

Combining thickeners

One thickener is often not enough, and two complementary ones can work better than either alone.

• Carbomer plus xanthan: the gel strength and clarity of carbomer plus the salt tolerance of xanthan.
• Xanthan plus guar, locust bean gum or konjac: the natural gums reinforce each other.
• HEC plus other polysaccharides or cross-linked polymers: improves stability over HEC alone.
• A thickener plus a fatty alcohol: cetyl or cetearyl alcohol (1-5%) stiffens the oil phase and adds body, which is why lotions use both. Research on combined networks has found gel-like behaviour comparable to commercial creams.

How to add a thickener without lumps

Powdered thickeners clump into jelly-coated lumps (often called "fish eyes") if the outside swells before the inside gets wet. These habits prevent it:

• Pre-mix the powder with a small amount of glycerin, or a little oil, to make a slurry before it meets the water. The powder spreads out before it swells.
• Sprinkle slowly into water that is already moving (a stick blender or vortex), not all at once.
• Give it time to hydrate. Many gums and clays need 15-30 minutes, some overnight, to reach full thickness.
• Carbomer: hydrate first, neutralise last. Disperse and hydrate it fully, then add the base drop by drop while stirring slowly. Mixing too hard traps air.
• Add salts and actives after the thickener has built its structure.
• Wait before judging. Most emulsions finish thickening over 24-48 hours. See the troubleshooting guide before you add more.

Fixing common problems

How to know your emulsion is stable

Choosing a thickener is only the first step. Labs check stability with specialist instruments:

• Oscillatory rheology (G' and G''): shows whether a product acts more like a solid (stretchy, elastic) or a liquid at rest.
• Flow curves: show how it thins under shear and how much push it needs to start flowing (yield stress).
• Thixotropy loop tests: show how quickly the thickness returns after it has been stirred or pumped.
• Turbiscan / backscattering: watches droplets rise or settle over time without diluting the sample.
• Dynamic light scattering (DLS): tracks droplet size over time to spot flocculation or Ostwald ripening.

At home you can learn a lot with far simpler tools:

1. Keep samples under different conditions: room temperature, a fridge, and a warm place around 40°C for several weeks. Heat speeds ageing, so it shows problems early. Some people also try a freeze-thaw cycle.
2. Use a clear jar so you can see layers form, and photograph it against a white background at the same time of day.
3. Compare to a retained reference from the day you made it: thickness, smell, colour and pH.
4. Record check-ins. Formularium asks how the batch looks at 24 hours, 1 week and 4 weeks and keeps the answers on the batch. Over time the Metrics page in the Studio shows how your check-ins went and which problems you note most often.

Quick checklist

1. Name the failure first. Layers forming slowly is creaming (thicken, shrink droplets). An oil slick is coalescence (fix the emulsifier). Grainy is flocculation.
2. Match the thickener to your pH and salt level. Carbomer for low-salt, neutral-to-alkaline, clear gels. Xanthan for salts, a wide pH range and natural formulas.
3. Think about the whole life of the product. It must be thick on the shelf, flow while you make it, and spread easily. Shear-thinning thickeners do all three.
4. Try combinations. Two complementary thickeners usually beat one, with a fatty alcohol adding body.
5. Always test in your own formula. Surfactants, temperature, pH and salts change the result. Wait 24-48 hours, then run a longer stability test.

Ready to try it? Build a lotion in the Formulation Studio and look up each thickener's usage range in the ingredient library. To work out the right emulsifier mix first, use the HLB calculator. To brush up on the basics, read Emulsions explained and Your first formula.