Surface Flashcards
Interfacial Tension
Force per unit length that exists at an interface between 2 immiscible liquids or 2 solids or a solid and a liquid
Interface
The boundary between 2 phases
Why does interfacial tension exist?
Due to molecules at the interface experiencing unequal attractive forces compared to those in the bulk
Emulsion
Oil and water are immiscible due to interfacial tension so a surfactant is used to allow them to mix by reducing tension
Surface tension
Molecules on the surface of a liquid experience a force inwards towards the bulk as there is no force pulling them up
Tension
Force per unit length
Units of tension
N/mm or dynes per cm
Tension of water
72.8 mN/m
Why does water have a high surface tension?
Due to H bonding
What happens to surface tension as temperature increases?
As temperature increases, surface tension decreases due to IM forces decreasing
Ring Method
Ring held on liquid surface and ring tensiometer measures the force per distance required to remove the ring from the liquid surface. Can be used in surface and interfacial tension with 0 contact angle and no wave motion
Wilhelmy Plate Method
Use plate instead of ring with same principles
Capillary Rise Method
Measure distance liquid travels up capillary tube - can only be used for surface tension and is more subjective than ring method but cheaper. The diameter of the rube has significant bearing
Spreading co-efficient equation
S = γΑ - (γB - γΑ/Β)
γΑ = surface tension of surface being spread on
γB = surface tension of material being spread
γΑ/Β = interfacial tension between the 2 materials
Work of cohesion
Work required to break apart the forces within a single liquid
Work of cohesion equation
Wc = 2γB
2 times the surface tension of the liquid being broken
Work of adhesion equation
Wa = γΑ + γB - γΑ/Β
Work of adhesion
Forces between 2 immiscible liquids
Mutual saturation
Interfacial tension is reduced and Sfinal is negative
What happens to spreading co-efficient as C chain length of an oil increases
Spreading co-efficient decreases
Wetting
Spreading of a liquid on a solid
What does higher contact angle mean for spreading?
Poorer spreading
Young’s equation for contact angle and surface tension
Wsl = γLV (1 + cosΘ)
Critical surface tension (γc)
The minimum surface tension of a liquid required to give you complete wetting.
Critical surface tension of solid hydrocarbons
Low γc
What are surfactants used for?
Used to improve wetting or to allow 2 immiscible liquids to mix by reducing interfacial tension
What is the HLB?
Hydrophilic-Lipophilic balance is the size of the head group to the tail group
What are surfactants used for in pharmacy?
Control the size of the droplets in eye dropper
Wetting of particles in suspension
Improve drug absoprtion
What do surfactants do in water?
Reduce surface tension by adsorbing at the interface between air and water
How do surfactants work in water?
They exchange molecules from the surface to the bulk over and over again. They interfere with H bonding between the H2O molecules at the surface by the head groups being present
What happens when the surface of a liquid is saturated with surfactant?
The critical micelle concentration is reached
What is the average lifespan of a micelle?
1/10-2 to 1/10-9 seconds
What happens as the surface tension plateaus against surfactant concentration?
The CMC is reached
What happens to the CMC as the hydrophobic tail length increases?
CMC value decreases but after 16-18C CMC does not change due to steric hindrance of the tail
What is the aggregation number?
The number of surfactants that come together to form a single micelle
What happens to CMC as aggregation increases?
CMC decreases
