1. Chapter 15. Interfacial Phenomena

2. Contents Liquid Interface Adsorption at Liquid Interfaces
Adsorption at Solid Interfaces
Applications of Surface Active Agents
Electric Properties of Interfaces

3. Interface & Surface Interface Surface
The boundary between two phases (faces)
Surface
Ether a gas-solid or a gas-liquid interface

4. LIQUID INTERFACES

5. Liquid Interfaces
Fig. 15-1

6. Surface & Interfacial Tension
Surface tension
A force pulls the molecules of the interface together
Unit : dyne/cm
Interfacial tension
The force per unit length existing at the interface between two immiscible liquid phases

7. Surface Tension ()
Fig. 15-3
 = f / 2L

8. Surface Free Energy W =  A  : surface tension (dyne/cm)
W : The work done or surface free energy (ergs)
 : surface tension (dyne/cm)
A : The increase in area (cm)

9. Measurement of Surface and Interfacial Tension
Capillary rise method
DuNoüy ring method
Drop weight ….
 Temperature   Surface tension 
 Critical Temperature  Surface Tension = 0

10. Capillary Rise Method
2r = r2hg
 = 1/2 rhg
Fig. 15-5

11. DuNoüy Ring Method
Fig. 15-6

12. Work of Adhesion
Fig. 15-7
Wa = L + S - LS

13. Work of Cohesion
Fig. 15-8
Wc = 2L

14. Spreading Coefficient
S = Wa - Wc = (L + S - LS) - 2L
S = S - (L + LS)
S > 0 : spreading
S < 0 : form globules or a floating lens

15. ADSORPTION AT LIQUID INTERFACES

16. Adsorption at Liquid Interfaces
Surface free energy
The work that must be done to increase the surface by unit area
Adsorption
A phenomenon, where the added molecules are partitioned in favor of the interface
Absorption
The liquid or gas being absorbed penetrates into the capillary spaces of the absorbing medium

17. Surface-Active Agent Surface-active agent (= surfactant, amphiphile)
Molecule and ion that are adsorbed at interfaces
The molecule or ion has a certain affinity for both polar and nonpolar solvent

18. Hydrophilic-Lipophilic Balance ( HLB )
Fig
An arbitrary scale of values to serve as a measure of the hydrophilic-lipophilic balance of surface-active agents

19. HLB S : The saponification number of the ester
Nonionic surfactant
HLB = E/5
E : Percent by weight of nonionic surfactant
Polyhydric alcohol fatty acid esters
S : The saponification number of the ester
A : The acid number of the fatty acid

20. Required Hydrophile-Lipophile Balance (RHLB)
The specific HLB that the oil phase of an oil-in-water(O/W) emulsion required
s : The density of the surfactant mixture
 : The density of the dispersed phase
Q : The percent of the dispersant of the emulsion

21. ADSORPTION AT SOLID INTERFACES

22. The Solid-Gas Interface
Adsorbent : The material used to adsorb the gas
Adsorbate : The substance being adsorbed

23. Adsorption Physical adsorption Chemisorption the adsorbent)
Van der Waals forces
Reversible
Temperature, pressure
 desorption (the removal of the adsorbate from
the adsorbent)
Chemisorption
Chemical bond
Irreversible

24. Adsorption Isotherm
The relationship between the amount of gas physically adsorbed on a solid and the equilibrium pressure or concentration at constant temperature
Fig

25. Freundlich Isotherm
y : the mass of gas x adsorbed per unit mass m of adsorbent
k, n : constants

26. Langmuir Isotherm y : the mass of gas adsorbed per gram of adsorbent
y : the mass of gas adsorbed per gram of adsorbent
at pressure p and at constant temperature
ym : the mass of gas that 1 gram of the adsorbent can
adsorb when the monolayer is complete

27. Adsorption Isotherm
Fig

28. BET Equation
P : the pressure of the adsorbate in mmHg at which the mass y of vapor per gram of adsorbent is adsorbed
p0 : the vapor pressure when the adsorbent is saturated with adsorbate vapor
ym : the quantity of vapor adsorbed per unit mass of adsorbent when the surface is covered with a monomolecular layer

29. The Solid-Liquid Interface
Langmuir equation
c : the equilibrium conc. In milligrams of
alkaloidal base per 100mL solution
y : the amount of alkaloidal base x in milligrams
adsorbed per gram m of clay

30. Wetting
Wetting agent
: A surfactant that, when dissolved in water, lowers
the advancing contact angle
Fig

31. Young’s Equation s = SL + L cos  S = L ( cos  -1 )
Wa = WSL = L ( 1- cos  )

32. Critical Surface Tension
Fig
Surface tension at cos  = 1
(c)

33. APPLICATIONS OF SURFACE ACTIVE AGENTS

34. Applications of Surface Active Agents
Emulsifying agents
Detergents
Wetting agents
Solubilizing agents
Antibacterial agents
Protective agents
Aids to absorption of drug

35. ELECTRIC PROPERTIES OF INTERFACES

36. The Electric Double Layer
Fig

37. Nernst and Zeta Potentials
Fig
Nernst potential (E)
: aa’
Zeta potential ()
: bb’

38. Effect of Electrolytes
Concentration of electrolyte 
 Screening effect of the counterion 
 Potential falls off more rapidly with
distance