Interfacial tension

What is interfacial tension?

Interfacial tension is a property of the boundary between two phases that do not mix. When both phases are liquids, such as crude oil and brine, it is called interfacial tension; when one of the phases is a gas, usually air, the same property is called surface tension. Thermodynamically, interfacial tension is the Gibbs free energy per unit area of the interface at fixed temperature and pressure. It can equally be read as a force acting along the interface per unit length, which is why it is reported in millinewtons per meter (mN/m), numerically the same as the older unit dynes per centimeter.

Molecules at an interface are in a less favorable position than molecules inside either liquid, because they have fewer like neighbors to interact with. The system therefore resists any increase in interfacial area. The higher the interfacial tension, the more energy it takes to create new interface, for example to break oil into small droplets or to push an oil droplet through a narrow opening.

Interfacial tension and surface tension

The two terms describe the same physical quantity; the difference is what lies on the other side of the boundary.

	Surface tension	Interfacial tension
Boundary between	a liquid and a gas	two immiscible liquids
Typical example	water and air	oil and water
Units	mN/m	mN/m

For reference, the surface tension of pure water against its vapor is 71.97 mN/m at 25 °C, according to the International Association for the Properties of Water and Steam.

How surfactants lower interfacial tension

A surfactant molecule has a water-attracting head and an oil-attracting tail, so it is drawn to the oil-water boundary, where each end can sit in the liquid it prefers. As surfactant accumulates at the interface it replaces unfavorable oil-water contact, and the interfacial tension falls. How far it falls depends on the surfactant’s structure and on the salinity, temperature and oil composition of the system.

Surfactant formulations designed for chemical enhanced oil recovery can reduce oil-water interfacial tension to ultralow values, which SLB defines as below about 10^-2 mN/m.

Why interfacial tension matters in oil recovery

After a reservoir has been produced and waterflooded, much of the remaining oil is held as isolated droplets in pore spaces, trapped by capillary forces at pore throats. Whether a displacing fluid can move those droplets depends on the balance between viscous forces, which push the oil, and capillary forces, which hold it. That balance is expressed as the capillary number, which is inversely proportional to interfacial tension.

Lowering interfacial tension therefore raises the capillary number at a given flow rate. In SLB’s description, ultralow interfacial tension increases the capillary number enough that viscous forces during a flood can mobilize additional oil. The same principle applies after hydraulic fracturing, where lower interfacial tension reduces the capillary pressure that holds treatment fluid in the rock and can help fluid flow back.

Interfacial tension beyond the oilfield

• Emulsions. Lower interfacial tension reduces the energy needed to break one liquid into droplets within another, which is why emulsifiers are surfactants.
• Cleaning. Removing oily soil from a surface depends on the interfacial tension between the oil and the wash solution, together with how well the solution wets the surface.
• Formulation. Products that combine oil-soluble and water-soluble ingredients, such as emulsifiable agrochemical concentrates, rely on controlled interfacial tension to disperse evenly when diluted.

How interfacial tension is measured

Interfacial tension is measured with a tensiometer. Common methods include the pendant drop, in which the shape of a drop hanging in the second liquid is analyzed; the du Noüy ring and Wilhelmy plate, which measure the force needed to pull a probe through the interface; and the spinning drop, in which a drop of oil in a rotating tube of denser liquid is stretched by centrifugal force. The spinning drop method is the standard choice for the very low and ultralow values relevant to enhanced oil recovery.

Interfacial tension and Locus biosurfactants

Locus’s oil and gas pages describe interfacial tension reduction as one of the mechanisms by which its biosurfactants mobilize trapped hydrocarbons and improve flowback, alongside reducing capillary pressure and shifting rock toward a more water-wet state.

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