Interfacial Phenomena Flashcards
Define interface
The boundary between two phases e.g. Between Oil and water
Define surface
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
What are the different symbols representing different interfaces
L/L liquid liquid
L/V liquid vapour
S/L solid liquid
S/V solid vapour
Which forces cause surface tension
Vertical forces
I’m the bulk liquid what do molecules experience
They experience equal attraction from all directions
What will molecules experience at the surface
They will experience a net inward attraction force.
The molecules at the surface tend to move onward and create surface tension/energy
Define surface tension and give units
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
What happens to the energy level once work is done
Energy level becomes high. It increases from E1 (initial energy) to E2 (final energy)
Work done to a system equals the net energy increase
When is a system more stable?
At Lower energy
What can work be converted to
Energy
Equation to calculate work
W= E2 - E1
What force is produced by the object
F1 =weight = mg
What happens if F1 is too big
The sliding bar is pulled to the right
What happens if f1 is too small
The film shrinks pulling the bar to the left
Force and counter force are..
Always equal
f1 = f2 = mg
Equation: force per unit length of film
Gamma = f/2l = mg/2l
What happens if the film expands from X1 to X2
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)
Relevant CGS units
Force: dyne = g cm s-2
Energy: erg = g cm2 s-2
Relevant SI units
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
Surface energy equation
Surface energy = surface tension x surface area, E= gamma x A
Force equation
Force = surface tension x length
Relationship between surface area and surface energy
The larger the surface area the higher the surface energy
Relationship between surface tension and surface energy
The higher the surface tension the higher the surface energy
Surface tension of the curved film
Generate a inward pressure so the pressure inside the bubble is higher than atomospheric pressure outside
Pint > Pext
Curved surfaces - what happens if surface decreases
Pressure increases
When the film shrinks it generates an inward force
Energy(triangle) p =
Pint - Pext = 2gamma/r
Gamma is the ST
R is radius of the bubble
The smaller the radius the greater the trianglep
Spreading and wetting (L/L)
Define work of adhesion
Work required to separate 2 phases (unlike) at their interface
Wa = (gamma A + gamma B) - gamma A/B
Spreading and wetting (L/L)
Define work of cohesion
Work required to produce 2 new surfaces between the like molecules
Wc = 2 gamma B Wc = 2 gamma A
S and W (L/L)
The larger the work of adhesion..
The stronger the attraction between the unlike surface
Wa
Indication of the attractive strength between unlike surface
Wc
Indication of the attractive strength between like surface
S and W (L/L)
How is spreading coefficient calculated and when does it occur
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
S and W (L/S)
What is a contact angle
When liquid contacts solid, the contact angle determines the behaviour of the drug
S and W (L/S)
Youngs equation
Gamma A = gamma A/B + gamma B cos0
S = gamma B (Cos0 - 1)
Wa = WA/B = gamma B (Cos0 + 1)
What is contact angle affected by
Pressure
Dynamic contact angle
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
When adhesion is higher
Attraction between solid and liquid is stronger so the liquid will come into solid faster
What are surface active agents
Amphiphiles - SAA go to the surface and surface/interfacial tension is reduced
Adsorption at solid surfaces - S/V systems
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
Adsorption at solid surfaces - S/L systems
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
Factors affecting adsorption
Conc of solute
High temp decreases adsorption
SA increased leads to increased adsorption
pH
Pharmaceutical applications
Removal of orally taken toxics by activated charcoal
