SURFACTANTS IN
SOLUTION
Amphiphilic Surfactants
O
O
O
+
Na-O S
O
O
O
Aerosol OT
Amphiphilic surfactants contain a non-polar
portion and a polar portion.
Classification of Surfactants

O
S - +
O Na
O
Anionic
Sodium dodecylsulfate (SDS)

+
N
Br-
Cationic
Cetylpyridinium bromide

O
Zwitterionic
O
O
OCH2CH2N(CH3)3+
P OO
O

Nonionic
Dipalmitoylphosphatidylcholine (lecithin)
O
O
O
Polyoxyethylene(4) lauryl ether (Brij 30)
O
OH
Surfactant Aggregates
Unimers
Normal micelles
cylindrical
spherical
Inverted hexagonal phase
Reverse micelles
Bilayer lamella
4 nm
Molecular Architecture
Aerosol OT
Sodium dodecylsulfate (SDS)
Critical Micelle Concentration
CMC
14
12
10
8
6
CMC
4
2
0
0
1
Surfactant concentration
Below CMC only unimers
are present
Above CMC there are
micelles in equilibrium
with unimers
Solution Properties
14
12
10
8
6
4
2
0
Concentration
unimers
CMC
micelles
0
14
10
8
6
4
2
0
CMC
0
1 1/2
(Surfactant concentration)
Osmotic pressure
10
8
6
4
2
0
Surfactant concentration1
Molar conductivity
1/R
12
14
 12
CMC
0
14
Isc12
10
8
6
4
2
0
Surfactant concentration1
Light scattering
CMC
0
Surfactant concentration1
Solubilization
Spontaneous transfer of a compound insoluble in the
bulk solvent into solution due to incorporation into
the surfactant micelles
Normal micelles
non-polar compound
amphiphilic compound
Reverse micelles
polar compound
Solubility Effects
Solubility of a poorly soluble compound increases
as a result of solubilization in the micelles
Solubility
14
12
10
8
6
CMC
4
2
0
0
1
Surfactant concentration
HLB and Use of Surfactants
Amphiphilic surfactants are characterized by the
hydrophilic-lipophilic balance (HLB): a relative
ratio of polar and non-polar groups in the surfactant
HLB ca. 1 to 3.5: Antifoams
HLB ca. 3.5 to 8: Water-in-Oil Emulsifiers
HLB ca. 7 to 9: Wetting and spreading agents
HLB ca. 8 to 16: Oil-in-Water Emulsifiers
HLB ca. 13 to 16: Detergents
HLB ca. 15 to 40: Solubilizers
Required HLB
HLB needed for emulsification of the oil phase. If there
are several oil ingredients the required HLB is
calculated as a sum of their respective required HLB
multiplied by the fraction of each.
Calculate the required HLB for the oil phase of the following o/w
emulsion: cetyl alcohol 15 g., white wax 1g. Lanolin 2 g, emulsifier
(q.s.), glycerin 5 g. water 100 g.
Required HLB
Cetyl alcohol
White wax
Lanolin
Total required HLB
(from reference)
15
x
15/18
12
x
1/18
10
x
2/18
Fraction
12.5
0.7
1.1
14.3
HLB of Surfactant Blend
Surfactant blends are commonly used to obtain
desired emulsifying properties.
What is the HLB of the mixture of 40 % Span 60 (HLB =
4.7) and 60 % Tween 60 (HLB = 14.9)?
HLB of mixture:
4.7 x 0.4 + 14.9 x 0.6 = 10.8
In what proportion should Span 80 (HLB = 4.3) and
Tween 80 (HLB = 15.0) be mixed to obtain “required”
HLB of 12.0?
4.3.(1-x) + 15.x = 12
x = 0.72
72 % Tween 80 and 28 % Span 80