MPharm Programme: Surface Tension & Surfactants

The force that holds the surface molecules of a liquid together, resulting in a tension-like effect.

A substance that lowers surface tension by reducing the cohesive forces between liquid molecules. They have both hydrophilic and hydrophobic parts.

Water molecules have strong cohesive forces due to hydrogen bonding, resulting in a high surface tension.

Substances that tend to repel water and have a low affinity for water.

Substances that attract and readily mix with water.

The work needed to increase the surface area of a liquid by 1 square meter, measured in Joules per square meter (J/m²).

Surface tension and surface free energy are numerically equivalent. They represent the same property, but described in different units.

The symbol used for both surface tension and surface free energy, with the subscript referring to the system involved, e.g., γw/a for water/air interface.

Surface tension increases with decreasing temperature. This means hot water has lower surface tension than cold water.

The tendency for liquid molecules on the surface to be pulled inwards due to stronger attractive forces between them compared to molecules in the bulk.

The boundary region between two immiscible phases, like liquid and gas (surface tension) or liquid and liquid (interfacial tension).

The force that acts along the boundary between two immiscible liquids.

The tendency of systems to minimize their total free energy, leading to a decrease in surface area and achieving greater stability.

Increasing the surface area of a solid can make it more reactive, as there are more exposed molecules. This can be achieved by processes like milling.

Processes that require energy to increase the surface area of materials, like emulsion formation.

The tendency of a material to adhere to another surface, resulting in a stable bond.

A device that measures surface tension by measuring the force required to detach a platinum ring from the surface of a liquid.

The force required to detach a platinum ring from the surface of a liquid is directly proportional to the surface tension and the perimeter of the ring.

The property of a surfactant molecule having both a hydrophilic and hydrophobic region.

The part of a surfactant molecule that is repelled by water.

The part of a surfactant molecule that is attracted to water.

Surfactants tend to orient themselves at the surface or interface between two phases, with the hydrophobic portion pointing away from the aqueous phase.

The structure of a surfactant molecule with a hydrophobic 'tail' and a hydrophilic 'head'.

The angle between the tangent to the liquid surface at the point of contact with a solid surface and the solid surface. It describes the wettability of a surface by a liquid.

Describes a liquid spreading completely on a solid surface. The contact angle is 0 degrees, meaning the liquid wets the surface completely.

Describes a liquid that does not spread on a solid surface. The contact angle is 180 degrees, meaning the liquid does not wet the surface at all.

A method used to measure surface tension by measuring the force required to detach a thin, rectangular plate from the surface of a liquid. The plate is completely wetted by the liquid, ensuring that the surface tension acts vertically

The force acting around the perimeter of the Wilhelmy plate that is used to calculate the surface tension.

The principle behind the Wilhelmy Plate method, that the plate must be completely wetted by the liquid, i.e., the contact angle must be zero, to ensure that the surface tension acts vertically on the plate

The Wilhelmy Plate method can be used to measure changes in surface tension over time or with different variables.

The tendency of a molecule to adsorb at an interface, increasing with the size of its hydrophobic portion. The increase in adsorption from one –CH2 group to the next is approximately threefold.

The reduction of surface tension by a surfactant, caused by the replacement of water molecules at the surface with surfactant molecules. This disruption of water-water bonding reduces the cohesive forces and thus surface tension.

A mixture of two or more immiscible liquids, commonly water and oil, stabilized by a surfactant. The surfactant forms a layer at the interface preventing the liquids from separating.

A substance like sodium dodecyl sulfate (water-soluble) or cetostearyl alcohol (oil-soluble) that interacts with both water and oil phases, promoting the formation of a stable emulsion.

The formation of a stable emulsion with very small droplets, minimizing the interfacial area and reducing the tendency for the droplets to coalesce. This stability is enhanced by the formation of a charged film around the droplets.

The process of spontaneously separating a mixture into layers, driven by the reduction of interfacial free energy and the tendency to minimize the area of contact between the phases.

The reduction of the interfacial free energy between two phases, leading to a decrease in the surface tension and a tendency to minimize the interface area.

The state of equilibrium in which the rate of droplet formation equals the rate of droplet coalescence, resulting in a stable emulsion.