Surface/Interfacial Tension

Question 1

What is Surface Tension?

Answer 1

• Forces acting on a molecule
• Molecule at surface will feel an unbalanced force
  
  • Tendency to be pulled into the bulk (to minimise
• As chain length increases, surface tension increases

Question 2

What is the effect of temperature on Surface Tension?

Answer 2

• Forces of attraction between molecules decrease with temperature
• y (work required/unit area to maintain that position) found to decrease linearly with temperature

Question 3

Discuss Water as a major component of formulations

Answer 3

• Water has a higher y than other liquids
  
  • H-bonds control the interaction with other molecules and water structure

Question 4

Define Hydrophilic

Answer 4

Ions, polar molecules or charged surfaces can fit into water structure

Question 5

Define Hydrophobic

Answer 5

• Non-polar, uncharged, incapable of forming H-bonds
• Water minimised unused H-bonds, resulting in greater ordering and low solubility

Question 6

How is Surface Tension Measured?

Answer 6

• Wilhelmy Plate
• Capillary Rise

Question 7

Discuss Measuring Surface Tension with the Wilhelmy Plate

Answer 7

• Correct for weight
• Make contact with the liquid
• Raise the plate and measure the force

Question 8

Discuss Measuring Surface Tension with the Capillary Rise

Answer 8

• Not suitable for interfacial tension measurement
• Liquid rises up a capillary placed in liquid
• Liquid column supported by surface tension

Question 9

Define Cohesion

Answer 9

• Force of attraction between molecules of the same substance e.g. water molecules

Question 10

Define Adhesion

Answer 10

• Force of attraction between different substances e.g. water and glass

Question 11

Define Work of Adhesion

Answer 11

• If two phases in contact are pulled apart
  
  • Original interface destroyed
  • Two new interface created

Question 12

Define Work of Cohesion

Answer 12

If a single liquid is pulled apart

Question 13

Define Spreading Coefficient

Answer 13

• When a drop of oil is placed on another immiscible liquid (e.g. water) it may:
  
  • Spread as a thin film on the surface
  • Form a liquid lens if it cannot spread
  • Spread as a monolayer with areas that are identified as lenses

Question 14

Describe Solid Systems - Contact Angles θ

Answer 14

• Angle formed between solid surface and a drop of liquid, θ
• Balance of interfacial tensions
• Hydrophilic, water θ < ~40°C
• Hydrophobic, water θ > 90°C