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Interfacial tension

• Interfacial tension is somewhat similar to surface tension in that cohesive forces are also involved.
  
  However the main forces involved in interfacial tension are adhesive forces (tension) between the liquid phase of one substance and either a solid, liquid or gas phase of another substance.
  
  The interaction occurs at the surfaces of the substances involved, that is at their interfaces.
  
  
• Glass, due to its composition, has molecules that are able to hydrogen bond to water and will have a pull on the water molecules that it contains. This results in the water being pulled up by the attractive force of the glass where they make contact at the interface.
  
  
  • This effect can be witnessed as capillary action in a glass tube with a small internal diameter, where the surface area of the glass is large in relation to the volume of the water (see diagram). Water in such a tube will be drawn up higher inside the tube than would be normally expected.
    
    
  • Similarly, in larger diameter tubes, such as a measuring cylinder, a meniscus results from the interfacial tension between the glass and water.
    
    
  • The formation of tears above the wine's surface of a wine contained in a wine glass, also involves interfacial tensions
    
    
• This lifting action is limited by the surface tension of the water and forces of gravity.
  
  
• If a glass tube is replaced with a plastic tube, the water will bead into convex shape, as opposed to the convex shape involving glass.
  
  This is because the internal cohesive forces in water are stronger then any adhesive forces between the water and plastic.
  
  Similarly mercury in a glass thermometer will also bead to minimize the surface energy or tension (see diagram).
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  

• Evaporation occurs when a liquid absorbs heat energy from its surroundings and energises some of the liquid molecules up to a point where the internal bonding energies between molecules are overcome and is able to escape the liquid to form a vapour.
  
  The independent vapour molecules can also loose energies to the surroundings and on collision with similar molecules, bond and reform into the liquid phase.
  
  In a closed, isolated liquid/vapor situation, at a steady temperature, the vapour phase will become supersaturated and a stable equilibrium situation is established, with no increase in the vapour concentration or a decrease in the liquid volume.
  
  That is no more (increase in) evaporation .
  
  Conversely if the vapor phase is constantly removed by air movement a greater capacity for evaporation is established.
  
  
• Evaporation from a mixture of liquids sees the mixture become more concentrated in the less volatile substance, with time, due to the higher evaporation rate of the more volatile liquid.
  
  This will occur up to a point where the altered concentration ratios of the liquids involved changes the interaction dynamics between the molecules involved.