Download Lecture - Surfactants

Surfactants – Surface Active Agents
(Chapter 4, pp. 76-84 in Shaw)
Short chain fatty acids and alcohols are
soluble in both water and organic media:
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-OH
hydrophobic tail and hydrophilic head
These molecules preferentially
position themselves at the waterorganic interface due to energetic
effects – they are surface active!
AIR
WATER
e.g. Stearic Acid: CH3-(CH2)16COOH

Strong adsorption gives rise to
monolayers and is termed surface
activity.

Surface active materials are also called
surfactants and are amphiphlic in
nature.

Surface activity is a dynamic
phenomenon since there is a balance
between complete adsorption and
complete mixing (entropy vs. enthalpy
effects).

Surfactant molecules will expand the
surface and lower the surface tension
(contracting forces).
If p is the expanding pressure (or surface
pressure) of an adsorbed layer of
surfactant, then the reduction in surface
tension will be:
g = go – p
The surface tension also depends on the
concentration of surfactant , as follows:
g - go = Bc
which is emperically known as Traub’s
rule.
80
60
g,
dyne/cm
40
Ethanol
n-Propanol
n-Hexanol
20
0
0.4
0.8
n-Butanol
1.2 Conc., mol/L
The longer the hydrocarbon chain the
greater the tendency to adsorb at the
interface.
Traube’s rule:
In homologous series each additional
CH2 group increases the surface tension
reduction effect three fold.
If the interfacial tension between two
liquids is reduced to a sufficiently low
value, emulsification takes place
because only a relatively small increase
in surface free energy is involved.
SURFACTANT CLASSIFICATION
The hydrophilic part is often an ionic
group
--- Ionic means better solubility.
For instance palmitic acid is unionized
and insoluble in water. However, the
sodium or potassium salt is readily
soluble and shows high surface activity
(Palmolive soap).
It is also possible to have non-ionic
groups as hydrophilic parts as for
instance in poly(ethylene oxide).
Surfactants are thus classified as:
--- Anionic
--- Cationic
--- Non-ionic and
--- Ampholytic
Anionics: Most widely used because
they are cheap and perform well.
Cationics: Are expensive but have
germicidal properties.
Non-ionics: Can be tailored to specific
applications (e.g. detergency, wetting
agent, emulsifier, stabilizer).
ANIONICS:
Sodium Oleate:
CH3(CH2)7CH=CH(CH2)7COO - Na+
Sodium Dodecylsulphate:
CH3(CH2)11SO4- Na+
Sodium Dodecylbenzenesulphonate:
CH3(CH2)11C6H4SO3- Na+
Sodium Stearate:
CH3(CH2)16COO- Na+
CATIONIC:
Dodecylamine hydrochloride:
CH3(CH2)11NH3+ Cl-
NON-IONICS:
Polyethylene Oxides:
e.g. CH3(CH2)11(O-CH2-CH2)nOH
Spans (sorbitan esters)
Tweens (polyoxyethylene sorbitan esters)
AMPHOLYTICS:
Dodecyl betaine:
C12H25N+(CH3)2(CH2COO-)
OTHER ADDITIVES:
Why do a lot of cleaning products
have a name that associates them
with lemon or lime?
Citrus Miracle
Lemon Tide
Swiss Chalet gives a cup with water
and a slice of lemon in it.
Lemon Fresh dishwashing detergent
What is the function of cirtic acid?
Sequestering agent which forms
soluble complexes with Ca2+ and Mg2+
to prevent the formation of soap scum.
GIBBS’S ADSORPTION EQUATION
(pp. 80-83 in Shaw)
In ideal systems
a
a
S
A
S
b
In real systems
S
A
s
B
S
B
b
In real systems there exists a surface
phase, s. The surface excess
concentration of component i is given
as:
G = ni/A
where A is the interfacial area and ni is
the amount of component i in the
surface phase s in excess of what
would have been in the bulk.
Gibbs’s surface excess relationships
(see pp. 81-83 in Shaw):
1 dg
GB  
RT dCB
1 dg
GB  
2RT dCB
(for non-ionic)
(for ionic)
In the presence of excess electrolyte the
first equation is valid since electronic
shielding takes place.
These relationships have been verified
in two ways:
1) McBain and Swain shaved off 0.1
mm layers from solutions of
surfactant.
2) Other researchers used b-ray
detection to measure concentrations.
Example:
The surface layer was scooped from 300
square cm of the surface of a soap
solution of bulk concentration of 0.02 M.
The volume of liquid collected was 2 mL
and was found to contain 4.013 x 10-5
mol of soap. If the surface tension of
pure water at the temperature of the
experiment (298 K) is 72.8 mN/m
calculate the surface tension of the soap
solution.
Do this question at home:
Excess concentration:
Gsoap = 4.33 x 10-6 mole/m2
Use Traube’s rule to find:
g = 61.3 mN/m
Next lectures: Micelles