Interfacial Phenomena

Question 1

Define interface

Answer 1

The boundary between two phases e.g. Between Oil and water

Question 2

Define surface

Answer 2

When one of the phases is gas/vapour. E.g. The surface of a cup of water
A surface is a special type of interface

Question 3

What are the different symbols representing different interfaces

Answer 3

L/L liquid liquid
L/V liquid vapour
S/L solid liquid
S/V solid vapour

Question 4

Which forces cause surface tension

Answer 4

Vertical forces

Question 5

I’m the bulk liquid what do molecules experience

Answer 5

They experience equal attraction from all directions

Question 6

What will molecules experience at the surface

Answer 6

They will experience a net inward attraction force.

The molecules at the surface tend to move onward and create surface tension/energy

Question 7

Define surface tension and give units

Answer 7

The work required to create new surface per unit area. The free energy per unit area of the surface
ST units erg cm-2 or dyne cm-1

Question 8

What happens to the energy level once work is done

Answer 8

Energy level becomes high. It increases from E1 (initial energy) to E2 (final energy)
Work done to a system equals the net energy increase

Question 9

When is a system more stable?

Answer 9

At Lower energy

Question 10

What can work be converted to

Answer 10

Energy

Question 11

Equation to calculate work

Answer 11

W= E2 - E1

Question 12

What force is produced by the object

Answer 12

F1 =weight = mg

Question 13

What happens if F1 is too big

Answer 13

The sliding bar is pulled to the right

Question 14

What happens if f1 is too small

Answer 14

The film shrinks pulling the bar to the left

Question 15

Force and counter force are..

Answer 15

Always equal

f1 = f2 = mg

Question 16

Equation: force per unit length of film

Answer 16

Gamma = f/2l = mg/2l

Question 17

What happens if the film expands from X1 to X2

Answer 17

The energy increases equals the work done
W= f x d = mg x d
SA increased: A =2l x d

So surface tension can be expressed as surface energy per unit area of film (e.g. 1 dyne/cm = 1Erg/cm2)

Question 18

Relevant CGS units

Answer 18

Force: dyne = g cm s-2
Energy: erg = g cm2 s-2

Question 19

Relevant SI units

Answer 19

Force: N = kg m s-2
Energy: J = N m=kg m2 s-2

1N = 10 power 5 dyne
1J = 10 power of 7 erg

Question 20

Surface energy equation

Answer 20

Surface energy = surface tension x surface area, E= gamma x A

Question 21

Force equation

Answer 21

Force = surface tension x length

Question 22

Relationship between surface area and surface energy

Answer 22

The larger the surface area the higher the surface energy

Question 23

Relationship between surface tension and surface energy

Answer 23

The higher the surface tension the higher the surface energy

Question 24

Surface tension of the curved film

Answer 24

Generate a inward pressure so the pressure inside the bubble is higher than atomospheric pressure outside
Pint > Pext

Question 25

Curved surfaces - what happens if surface decreases

Answer 25

Pressure increases

When the film shrinks it generates an inward force

Question 26

Energy(triangle) p =

Answer 26

Pint - Pext = 2gamma/r

Gamma is the ST
R is radius of the bubble

The smaller the radius the greater the trianglep

Question 27

Spreading and wetting (L/L)

Define work of adhesion

Answer 27

Work required to separate 2 phases (unlike) at their interface

Wa = (gamma A + gamma B) - gamma A/B

Question 28

Spreading and wetting (L/L)

Define work of cohesion

Answer 28

Work required to produce 2 new surfaces between the like molecules

Wc = 2 gamma B
Wc = 2 gamma A

Question 29

S and W (L/L)

The larger the work of adhesion..

Answer 29

The stronger the attraction between the unlike surface

Question 30

Wa

Answer 30

Indication of the attractive strength between unlike surface

Question 31

Wc

Answer 31

Indication of the attractive strength between like surface

Question 32

S and W (L/L)

How is spreading coefficient calculated and when does it occur

Answer 32

Spreading coefficient calculated from interfacial energy
S = gamma A - (gamma B + gamma A/B)

S= Wa - Wc

Spreading occurs if a liquid adheres to the substrate more strongly than adhering to itself
S>=0 or Wa>=Wc

Question 33

S and W (L/S)

What is a contact angle

Answer 33

When liquid contacts solid, the contact angle determines the behaviour of the drug

Question 34

S and W (L/S)

Youngs equation

Answer 34

Gamma A = gamma A/B + gamma B cos0
S = gamma B (Cos0 - 1)

Wa = WA/B = gamma B (Cos0 + 1)

Question 35

What is contact angle affected by

Answer 35

Pressure

Question 36

Dynamic contact angle

Answer 36

Cos0 = F/ 2 gamma (L+T)

Compress powder into rectangle plate and insert into liquid
Use a balance to record net detachment force (F)
Detachment force F equals the downward force by ST

Question 37

When adhesion is higher

Answer 37

Attraction between solid and liquid is stronger so the liquid will come into solid faster

Question 38

What are surface active agents

Answer 38

Amphiphiles - SAA go to the surface and surface/interfacial tension is reduced

Question 39

Adsorption at solid surfaces - S/V systems

Answer 39

When gas is bought into contact with solid some of the gas is adsorbed onto the solid
Physical adsorption (VDW forces) reversible
Chemisorption irreversible unless bonds broken

Question 40

Adsorption at solid surfaces - S/L systems

Answer 40

x/m = abC / 1 + bC

x is the amount of solute adsorbed
m is the amount of adsorbent
C is conc
a and b are constants

Question 41

Factors affecting adsorption

Answer 41

Conc of solute
High temp decreases adsorption
SA increased leads to increased adsorption
pH

Question 42

Pharmaceutical applications

Answer 42

Removal of orally taken toxics by activated charcoal