Properties of Molecules at Surfaces

Interfacial Tension or Surface Tension

Keeps Interfaces by counteracting net inward pull towards bulk of the liquid

cohesive attraction

When the _____ between molecules of like phases is greater than the adhesive attraction of unlike molecules found in the bulk phase, interfacial tension, and surface tension increases.

adhesive attraction

When the ____ is greater than the cohesive attraction, interfacial tension, and surface tension decreases.

Gas-Gas

INTERFACIAL TENSION: -
EXAMPLES OF INTERFACE: No interface possible

Gas-Liquid

INTERFACIAL TENSION: YLV
EXAMPLES OF INTERFACE: Body of water exposed to atmosphere

Gas-Solid

INTERFACIAL TENSION: YSV
EXAMPLES OF INTERFACE: Table top

Liquid-Liquid

INTERFACIAL TENSION: YLL
EXAMPLES OF INTERFACE: Emulsion

Liquid-Solid

INTERFACIAL TENSION: YLS
EXAMPLES OF INTERFACE: Suspension

Solid-Solid

INTERFACIAL TENSION: YSS
EXAMPLES OF INTERFACE: Powder particles in contact

1. Capillary Rise Method

2. Dun Nouy Ring Method

Methods of Measuring Interfacial Tension

Capillary Rise Method

• The force of adhesion between the liquid molecule and the capillary wall is greater than the cohesion between the liquid molecules, so the liquid spread over it and rise in the tube 

• In this ____, the force of adhesion between the liquid molecule and the capillary wall is GREATER than the cohesion between the liquid molecules, so the liquid spread over it and rise in the tube

Dun Nouy Ring Method

• Used to measure surface and interfacial tensions 

• The principle is based on the force necessary to detach a platinum-iridium ring immersed at the surface/interface is proportional to the surface/interfacial tension

Work of Adhesion

energy required to break attraction between unlike molecules

Work of Cohesion

energy required to separate two identical molecules (or surfaces of the same liquid) from each other and create two new surfaces

Surface free energy

the energy required to create new surface area by overcoming cohesive molecular forces. As surface area increases, the total surface free energy increases, making the system more energetically unstable — especially for fine particles or nanoparticles.

Amphiphiles

• Are surface active agents or surfactants (emulsifying agents) 

• The dual character of their molecule (amphiphilic = hydrophilic & lipophilic)

Head or hydrophilic head

hydrophilic part or the polar part

Tail or lipophilic tail

lipophilic or the non-polar part

Surface Active Agents

• Substances with part of their molecule lipophilic and part hydrophilic move on their own to the surfaces or interfaces of the liquid.

• Surfactants are materials that increase surface area of dispersed phase with the dispersion medium

Lowering the surface tension or interfacial

between two liquids or between a liquid and a solid