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Interfaces summary for Food physics (FPH20306)
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Complete summary of all the relevant information needed to understand interfaces in this course.
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Chapter 8: InterfacesIntroduction
Static description of internal pressure and surface tension of a liquid
- Surface tension: energy you need to insert in the system to increase the free surface [N.m-1]
o Effect/tendency of materials (more evident in liquids) to acquire the smallest surface
area possible
o Surface tension can vary depending on the temperature and what is dissolved in the
material
o Attraction of molecules due to cohesion
o Net effect: inward force at surface
- The amount of work depends on the strength of attractive forces between molecules
- Laplace reasoned that attractive forces could be determined by calculating the force per unit
area needed to divide an infinite body of liquid into two semi-infinite bodies with planar
surfaces
SURFACE TENSION OF WATER (25°C) = 72 mN/m
, Measurement of interfacial tension
Wilhelmy-plate method
- Plate is placed on top of a liquid surface and then because of the existence of a surface
tension, the force F, needed to keep the plate in position just on top of the liquid surface is
given by;
- Measuring the force needed to keep the plate in position and substituting the other
experimentally accessible variables = surface tension
Du Nouy-ring method
- A ring is placed on top of a liquid surface. The force needed to keep the ring on top of a
liquid surface is given by;
Surface active molecules
Surfactants, amphiphiles and emulsifiers
- Usually the main ingredients of an emulsion or foam do not mix spontaneously
- Shaking and stirring can help the missing process but will eventually lead to phase
separation after a while (oil and water)
- Addition of surface-active molecules leads to a situation where this phases separation does
not occur so readily anymore
o The surface-active molecules reside mostly in the interface of the water droplet
dispersed in the oil, thereby stabilizing the interface of the water droplets against
coalescence
- SURFace-ACTive molecules are also called surfactants
o Other terms; amphiphiles (have affinity for both liquids because they have a
hydrophobic and hydrophilic part)
o Emulsifiers, help to form emulsions
o Also called soaps
- The surfactants are not only in the interface but divides themselves over the water phase, oil
phase and interface so that the chemical potential is equal in all phases
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