Quick answer: A multiple (or double) emulsion is an emulsion inside an emulsion. In water-in-oil-in-water (W/O/W), small water droplets are trapped inside larger oil droplets, which are themselves dispersed in a continuous water phase. In oil-in-water-in-oil (O/W/O) the arrangement is reversed. Because they have two interfaces to break instead of one, multiple emulsions are far more stable and harder to treat than simple emulsions. They show up in desalters, slop and rag layers, and produced water, and breaking them usually needs the right combination of demulsifier and reverse demulsifier plus adequate residence time.
Key figures at a glance
- Structure: an emulsion inside an emulsion (two nested interfaces)
- W/O/W: water droplets inside oil droplets, dispersed in continuous water
- O/W/O: oil droplets inside water droplets, dispersed in continuous oil
- Common locations: desalters, slop/rag layers, produced-water systems
- Treatment: demulsifier + reverse demulsifier together, plus residence time
Most emulsion problems are simple W/O or O/W. But some of the most stubborn, treatment-resistant fluids on a facility are multiple emulsions — a structure many operators do not realise they are dealing with. This guide explains what they are, where they form, and why they defeat single-chemistry treatment. There is currently no page on the site covering this, so it fills a genuine gap.
What a multiple emulsion is
A multiple emulsion is a hierarchical structure: droplets of one phase are contained within droplets of a second phase, which are dispersed in a continuous third phase (chemically the same as the innermost). Instead of one oil-water interface, there are two nested interfaces. That nested structure is what makes multiple emulsions distinctive — and difficult, because resolving them means breaking both interfaces.
W/O/W vs O/W/O
The two arrangements are mirror images:
- Water-in-oil-in-water (W/O/W): tiny water droplets sit inside larger oil droplets, which float in continuous water. The bulk behaves like water. This is common where a W/O emulsion becomes re-dispersed into a water phase.
- Oil-in-water-in-oil (O/W/O): small oil droplets sit inside larger water droplets, which are dispersed in continuous oil. The bulk behaves like oil.
Which one you have follows from the same continuous-phase logic as simple emulsions — the outermost phase sets the bulk behaviour.
Where they occur (desalters, slop tanks, produced water)
Multiple emulsions tend to form where fluids are repeatedly sheared and re-mixed across both oil-continuous and water-continuous conditions:
- Desalters — intense mixing of wash water and crude, plus recycling, can generate nested structures.
- Slop and rag layers — the persistent interface layer in tanks and separators is often a multiple emulsion, which is why rag layers are so hard to clear (see the slop-oil breaker page).
- Produced water systems — re-processing and repeated shear can create W/O/W structures that carry oil into the water.
Why they are hard to break
Two factors make multiple emulsions stubborn. First, there are two interfaces, each stabilised by natural emulsifiers, so a single demulsifier that targets one interface leaves the other intact. Second, the inner droplets are physically shielded by the outer phase, so treatment chemistry and coalescence have to work through an extra layer. The result is a fluid that resists the dosing that would easily break a simple emulsion.
Treatment approaches
Breaking a multiple emulsion generally means addressing both interfaces and giving the system time:
- Combined chemistry — a demulsifier to break the oil-continuous interface and a reverse demulsifier to break the water-continuous interface, selected together so they are compatible and complementary.
- Adequate residence time and gentle heat — nested structures need time (and often mild heat) to resolve in stages as each interface breaks.
- Bottle testing — because these fluids are unpredictable, laboratory screening to find the working chemistry and dose is essential rather than optional.
Recognising a multiple emulsion in the field
Multiple emulsions are easy to miss because, from the outside, they can look like an ordinary stubborn emulsion — the tell is that they refuse to resolve under a dose that should work. A persistent rag layer at a separator interface that will not clear, a produced-water stream that carries oil despite adequate deoiler, or a slop that resists both demulsifier and reverse demulsifier are all classic signs of a nested W/O/W or O/W/O structure with two interfaces to break instead of one.
Confirming it usually means looking at a sample under magnification, where the nested droplets-within-droplets structure is visible, and testing response to combined chemistry. Because the inner droplets are physically shielded by the outer phase, a single product almost never resolves the system; the fix is a matched demulsifier-plus-reverse-demulsifier programme with enough residence time and gentle heat for each interface to break in turn.
Treating a multiple emulsion as if it were a simple one is the common error — it leads to escalating doses that never quite work. Diagnosing the structure first is what turns an intractable slop or rag problem into a solvable one.
Why residence time and heat matter for nested emulsions
Breaking a multiple emulsion is as much about the conditions as the chemistry, because two nested interfaces have to resolve in sequence rather than at once. Even with a correctly matched demulsifier and reverse demulsifier, the outer interface must break before the inner droplets are exposed and can coalesce, and that takes time. A process that gives a simple emulsion enough residence time can still fail on a multiple emulsion simply because the fluid moves on before the second interface has resolved.
Gentle heat helps by lowering viscosity and weakening the interfacial films, so both interfaces break more readily and the freed droplets settle faster. The combination — matched chemistry, adequate residence time, and mild heat — is what turns an intractable rag or slop into a resolvable one, and it is why simply increasing the demulsifier dose on a multiple emulsion so often fails.
The design implication is that persistent rag layers may need a dedicated slop-treatment step with its own residence time and heating, rather than an expectation that the main separator train will clear them. Recognising the nested structure is what justifies giving it the conditions it needs.
Frequently asked questions
What is a water-in-oil-in-water (multiple) emulsion?
It is an emulsion inside an emulsion — for example water-in-oil-in-water (W/O/W), where small water droplets are trapped inside larger oil droplets that are themselves dispersed in continuous water. It has two nested interfaces instead of one.
Why are multiple emulsions so hard to treat?
Because they have two stabilised interfaces rather than one, and the inner droplets are shielded by the outer phase. A single demulsifier breaks only one interface, so resolving them usually needs both a demulsifier and a reverse demulsifier plus enough residence time.
Where do W/O/W emulsions occur?
In desalters, in slop and rag layers in tanks and separators, and in produced-water systems — anywhere fluids are repeatedly sheared and re-mixed across both oil-continuous and water-continuous conditions.
Related Rodanco resources: See Rodanco’s water-in-oil emulsions overview and slop oil demulsifier / emulsion breaker page for persistent rag-layer problems.
Where Rodanco fits: Rodanco resolves stubborn rag-layer and multiple-emulsion problems with matched demulsifier and reverse-demulsifier programmes proven by bottle testing. See our Water-in-Oil Emulsions and Slop Oil Demulsifier pages, or discuss a persistent emulsion through the Rodanco contact page.