Environmental Water Treatment: Emulsified Oil — Definition and Function
Emulsified oil is typically produced by blending fuel oil (gasoline, diesel, or heavy oil) at 70%–90% with water at roughly 10%–30% (by mass), plus 0.5%–1% additives, and then emulsifying the mixture with dedicated equipment. The resulting liquid forms water-in-oil (W/O) molecular clusters; the smaller and more uniform the droplets, the longer the emulsion stays stable—usually 1 to 6 months. Once the emulsion "breaks" (demulsifies), it loses its properties.
The surface of emulsified-oil droplets is often covered by amphoteric molecules such as surfactants, giving the droplet a double electric layer. Removal of emulsified oil is usually achieved by dissolved air flotation (DAF), preceded by adding a coagulant to destabilize the emulsion and improve oil-removal efficiency.
Composition
Emulsified oil consists of base oil and additives (emulsifier, rust inhibitor). It is stable only when the proportions are properly balanced. The base oil acts as the carrier for the corrosion inhibitor; its viscosity affects emulsion stability. Low viscosity is easy to emulsify, but too low a viscosity yields a thin protective film that is poor for rust prevention, so the base-oil viscosity must be moderate. Different oil phases require emulsifiers with different HLB (hydrophile–lipophile balance) values, and combinations of oil phases shift the required HLB.
(1) Emulsifier: A surfactant that lowers interfacial tension between immiscible liquids to form an emulsion. A surfactant molecule has a non-polar, oil-loving (hydrophobic) hydrocarbon chain and a polar, water-loving (hydrophilic) group at opposite ends—a dual-affinity ("amphiphilic") structure.
(2) Rust inhibitor: Mainly water-soluble and oil-soluble types. Rust prevention is driven by free alkali reacting with water-soluble corrosion inhibitors; e.g., triethanolamine reacts with the metal to form an insoluble, dense oxide film that blocks electrochemical corrosion. It must be dosed properly—excess can prevent dissolution, disturb emulsification, or even break the emulsion.
(3) Stabilizer: Stabilizes the oil phase (no layering, transparent and homogeneous) and keeps the emulsion stable at room and high temperature, preventing separation and oil bleed. Common stabilizers: isopropanol, polyols (glycols), ethanol, higher alcohols, di-/triethanolamine, phenylethanolamine.
Micro-explosion theory
In a W/O cluster, oil is the continuous phase and water the dispersed phase. Because oil's boiling point is higher than water's, water evaporates/boils first when heated. When the pressure inside the droplet exceeds the sum of oil surface tension and ambient pressure, the steam breaks the oil film and the droplet "explodes" into finer droplets—the so-called micro-explosion or secondary atomization—making the oil burn more completely and improving energy efficiency in engines or oil burners. Chemically, the water-gas reaction lets carbon particles from high-temperature cracking react with steam to form CO₂ and H₂, raising combustion efficiency and lowering soot.
HLB and emulsion type
The type of emulsion depends mainly on the emulsifier. "Hydrophile–lipophile balance" (HLB) expresses the relative strength of the hydrophilic vs. lipophilic groups; the larger the HLB, the more hydrophilic. An HLB of 0 means oil-soluble only; above 20 means almost water-soluble, insoluble in oil. A W/O emulsion needs HLB 2–6; an O/W emulsion forms at HLB 12–18. Normally a W/O emulsion has water droplets 2–5 μm in diameter and stays stable for days or weeks. Notably, emulsions made from industrial heavy oil or residual oil can yield satisfactory W/O emulsified oil even without any surfactant.
Emulsified oil is classified by use into grades 1–4 (per industry standard). Grade 1 has better rust resistance, grade 2 better cleaning, grade 3 better extreme-pressure performance, and grade 4 is transparent (for special metalworking cooling or as engine coolant). Mixed with water at a set ratio, the resulting emulsion provides rust-proofing, cleaning, and extreme-pressure properties and is used as a coolant in metal cutting and machining.
