Surface

Question 1

Interfacial Tension

Answer 1

Force per unit length that exists at an interface between 2 immiscible liquids or 2 solids or a solid and a liquid

Question 2

Interface

Answer 2

The boundary between 2 phases

Question 3

Why does interfacial tension exist?

Answer 3

Due to molecules at the interface experiencing unequal attractive forces compared to those in the bulk

Question 4

Emulsion

Answer 4

Oil and water are immiscible due to interfacial tension so a surfactant is used to allow them to mix by reducing tension

Question 5

Surface tension

Answer 5

Molecules on the surface of a liquid experience a force inwards towards the bulk as there is no force pulling them up

Question 6

Tension

Answer 6

Force per unit length

Question 7

Units of tension

Answer 7

N/mm or dynes per cm

Question 8

Tension of water

Answer 8

72.8 mN/m

Question 9

Why does water have a high surface tension?

Answer 9

Due to H bonding

Question 10

What happens to surface tension as temperature increases?

Answer 10

As temperature increases, surface tension decreases due to IM forces decreasing

Question 11

Ring Method

Answer 11

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

Question 12

Wilhelmy Plate Method

Answer 12

Use plate instead of ring with same principles

Question 13

Capillary Rise Method

Answer 13

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

Question 14

Spreading co-efficient equation

Answer 14

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

Question 15

Work of cohesion

Answer 15

Work required to break apart the forces within a single liquid

Question 16

Work of cohesion equation

Answer 16

Wc = 2γB
2 times the surface tension of the liquid being broken

Question 17

Work of adhesion equation

Answer 17

Wa = γΑ + γB - γΑ/Β

Question 18

Work of adhesion

Answer 18

Forces between 2 immiscible liquids

Question 19

Mutual saturation

Answer 19

Interfacial tension is reduced and Sfinal is negative

Question 20

What happens to spreading co-efficient as C chain length of an oil increases

Answer 20

Spreading co-efficient decreases

Question 21

Wetting

Answer 21

Spreading of a liquid on a solid

Question 22

What does higher contact angle mean for spreading?

Answer 22

Poorer spreading

Question 23

Young’s equation for contact angle and surface tension

Answer 23

Wsl = γLV (1 + cosΘ)

Question 24

Critical surface tension (γc)

Answer 24

The minimum surface tension of a liquid required to give you complete wetting.

Question 25

Critical surface tension of solid hydrocarbons

Answer 25

Low γc

Question 26

What are surfactants used for?

Answer 26

Used to improve wetting or to allow 2 immiscible liquids to mix by reducing interfacial tension

Question 27

What is the HLB?

Answer 27

Hydrophilic-Lipophilic balance is the size of the head group to the tail group

Question 28

What are surfactants used for in pharmacy?

Answer 28

Control the size of the droplets in eye dropper Wetting of particles in suspension Improve drug absoprtion

Question 29

What do surfactants do in water?

Answer 29

Reduce surface tension by adsorbing at the interface between air and water

Question 30

How do surfactants work in water?

Answer 30

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

Question 31

What happens when the surface of a liquid is saturated with surfactant?

Answer 31

The critical micelle concentration is reached

Question 32

What is the average lifespan of a micelle?

Answer 32

1/10-2 to 1/10-9 seconds

Question 33

What happens as the surface tension plateaus against surfactant concentration?

Answer 33

The CMC is reached

Question 34

What happens to the CMC as the hydrophobic tail length increases?

Answer 34

CMC value decreases but after 16-18C CMC does not change due to steric hindrance of the tail

Question 35

What is the aggregation number?

Answer 35

The number of surfactants that come together to form a single micelle

Question 36

What happens to CMC as aggregation increases?

Answer 36

CMC decreases