Emulsified Oil Explained: Composition, Stability, HLB and Why DAF Needs a Coagulant First

Emulsified oil is one of those terms that appears in every wastewater specification and is defined in almost none of them. The material itself is usually a deliberate product rather than an accident: fuel oil — gasoline, diesel or heavy oil — blended at 70%-90% with water at roughly 10%-30% by mass, plus 0.5%-1% additives, then passed through dedicated emulsifying equipment.

What comes out is a water-in-oil structure, a dispersion of water droplets inside a continuous oil phase. The smaller and more uniform those droplets are, the longer the emulsion holds together; typical stability runs from 1 to 6 months. Once the emulsion breaks, it loses the properties it was made for, which is precisely why formulators work so hard to prevent that happening — and why treatment plants struggle to make it happen on demand.

Why the Droplets Resist Coalescence

The surface of an emulsified oil droplet is covered by amphoteric molecules, surfactants among them, and that covering gives the droplet a double electric layer. Two droplets approaching each other therefore meet as two charged surfaces rather than as two oil volumes, and the electrical repulsion keeps them apart long enough for Brownian motion to carry them away again.

Removal of emulsified oil is usually achieved by dissolved air flotation (DAF), preceded by dosing a coagulant to destabilise the emulsion. The order matters. Flotation provides the lift; the coagulant provides the reason for droplets to merge into something liftable. A DAF unit run without that chemical step will remove free oil efficiently and emulsified oil barely at all, which is the most common explanation for a plant that performs well on commissioning and poorly six months later.

What Goes Into the Blend

Emulsified oil consists of base oil and additives — emulsifier and rust inhibitor — and it is stable only when the proportions are properly balanced. The base oil acts as the carrier for the corrosion inhibitor, and its viscosity affects emulsion stability. Low viscosity is easy to emulsify, but too low a viscosity yields a thin protective film that is poor at rust prevention, so the base-oil viscosity has to sit in a moderate band.

Different oil phases require emulsifiers with different HLB values, and combining oil phases shifts the required HLB again. That is why an emulsified oil formulated for one base stock cannot simply be re-formulated by swapping a component.

The Three Additive Families

The emulsifier is a surfactant that lowers interfacial tension between immiscible liquids so an emulsion can form at all. A surfactant molecule carries a non-polar, oil-loving hydrocarbon chain at one end and a polar, water-loving group at the other — a dual-affinity structure, which is exactly why it sits at the interface and stays there.

The rust inhibitor comes mainly in water-soluble and oil-soluble types. Rust prevention is driven by free alkali reacting with water-soluble corrosion inhibitors; triethanolamine, for instance, reacts with the metal to form an insoluble, dense oxide film that blocks electrochemical corrosion. Dosing has to be right. Excess inhibitor can prevent dissolution, disturb emulsification, or break the emulsion outright — an interesting failure mode, since over-treatment produces the very instability the operator was trying to avoid.

The stabiliser keeps the oil phase from layering, holds the blend transparent and homogeneous, and maintains stability at both room and elevated temperature so the product does not separate or bleed oil in storage. Common stabilisers include isopropanol, polyols such as glycols, ethanol, higher alcohols, di- and triethanolamine, and phenylethanolamine.

Micro-Explosion: the Behaviour Behind the Combustion Claim

In a water-in-oil cluster, oil is the continuous phase and water the dispersed phase. Because oil boils at a higher temperature than water, the trapped water evaporates and boils first when the fuel is heated. When the pressure inside the droplet exceeds the sum of the oil surface tension and the ambient pressure, the steam ruptures the oil film and the droplet breaks into finer droplets — micro-explosion, or secondary atomisation.

The result is more complete combustion, which is the whole purpose of the formulation: finer droplets mean more surface area, more surface area means faster evaporation and mixing with air, and better efficiency in an engine or an oil burner. Chemically, the water-gas reaction lets carbon particles from high-temperature cracking react with steam to form CO2 and H2, which raises combustion efficiency and lowers soot. From a treatment perspective, the same behaviour explains why emulsified oil can be surprisingly stubborn in a tank that is heated: the emulsion does not simply separate when warm, it changes physically.

HLB and Which Emulsion You Get

The type of emulsion depends mainly on the emulsifier. Hydrophile-lipophile balance expresses the relative strength of the hydrophilic and lipophilic groups in the molecule, and the larger the HLB, the more hydrophilic the surfactant. An HLB of 0 means oil-soluble only; above 20 means almost water-soluble and insoluble in oil.

The practical bands are narrow and worth memorising. A water-in-oil emulsion needs HLB 2-6. An oil-in-water emulsion forms at HLB 12-18. Normally a water-in-oil emulsion has water droplets of 2-5 μm in diameter and stays stable for days or weeks — small enough that gravity settling is not a realistic separation mechanism, which is the whole reason flotation and coalescence exist.

One detail catches people out: emulsions made from industrial heavy oil or residual oil can yield a satisfactory water-in-oil emulsified oil even with no surfactant at all. Natural surface-active components in the heavy fraction do the work, and a plant that assumes "no added emulsifier means no emulsion" will be surprised by its own separator performance.

Grades and End Uses

Emulsified oil is classified by use into grades 1-4 under the relevant industry standard. Grade 1 has better rust resistance, grade 2 better cleaning, grade 3 better extreme-pressure performance, and grade 4 is transparent, used for special metalworking cooling or as an engine coolant.

Mixed with water at a set ratio, the resulting emulsion provides rust-proofing, cleaning and extreme-pressure properties, and serves as the coolant in metal cutting and machining. Every one of those properties is a surfactant effect, and every one of them is a reason the spent fluid arriving at a treatment plant will not separate on its own.