§8.5 Surfactants and their properties and Applications.
-
Upload
amberly-wade -
Category
Documents
-
view
222 -
download
1
Transcript of §8.5 Surfactants and their properties and Applications.
§8.5 Surfactants and their properties and Applications
The substances that drastically lower the surface tension of water
even at low concentrations.
8.5.1 Surfactants
The Amphiphilicity is the basic characters of surfactants
They have both polar and nonpolar moieties.
Hydrophilic group, hydrophobic / lipophilic group.
The surface tension declines steeply even at c < 10-3 mol dm-3.
1) Structure of surfactants
hydrophilic group
-COOH, carboxyl
-OSO3H, sulphate
-SO3H, sulphonic acid
-NR4+, ammonium
-CH2- CH2-O-, poly-ethylene oxide etc.
hydrophobic group
(CH2)n
(CF2)n
(SiR2-O-)n
(-CH2-CH2 -CH2-O-)n
2) Classification of surfactant
(1) Classification by structure
ionic
nonionic
anionic
cationic
amphoteric
mixed
ester
ether
amine
amide
nonionic-cationic nonionic-anionic
Group work:Find at least one example for each type.
(2) Classification on applications
Emulsifier; deemulsifier, wetting agent, anti-wetting agent, dispersant, vesicant,
(3) Classification on molecular weight
M > 10000, polymeric surfactant.
(4) Classification on composition
Organosilicon surfactant
Organometallic surfactant: Zn soap, Mg soap
water-soluble surfactant
oil-soluble surfactant
(5) Classification on solubility
Find one example
Find one example
Find one example
8.5.2 critical micelle concentration and micelle
When concentration (c) below ca. 10-3 moldm-3, most ionic surfactants in aqueous solution display similar conductance to other strong electrolytes.
Concentration-dependence of
properties for sodium dodecyl
sulfate: between 1~2 10-3
moldm-3, a sharp breaks occur
in the conductivity and surface
tension.
NaCl
C12
C14
c
0 0.2 0.4 0.6
detergence
Osmotic pressure
Surface tension
Molar conductivity
interface tension
6.0x10-4 7.0x10-4 8.0x10-4 9.0x10-4 1.0x10-3 1.1x10-30.034
0.036
0.038
0.040
0.042
0.044
0.046
/ N
m
c / mol dm-3
A result presented by a student research group
Owing to the hydrophobic groups, surfactant is unstable in
water. It tends to approach to the solution surface with the
hydrophobic groups pointing outwards.
When all the surface is occupied, how can surfactants
stabilize themselves in the solution?
In 1925, Mcbain postulated that at high concentration, surfactant molecules aggregate to form micelle.
Small micelle
Ball-like micelle
nS Sn
Rod-like micelleLayered micelle
An equilibrium exsits between micelle and individual surfactant
Micelles are usually charged particles
vesicle
0 0.2 0.4 0.6
detergence
Osmotic pressure
Surface tension
Molar conductivity
interface tension
CMC:critical micelle concentration
The lowest concentration at which individual surfactant molecules aggregate to form micelles.
Variation of the surface tension of the solution with the sodium decanoate concentration in 0.2 mol dm-3 Na2SO4 at 40 oC.
Phase diagram of surfactant-water system
0.04 0.06 0.08
20
30
40
TK
T / oC
Two-phase region
micellesolution
Medium cmc Aggregation number
Average charge per micelle
Water 0.0081 80 14.4
0.02 M NaCl 0.00382 94 13.2
0.10 M NaCl 0.00139 112 13.4
0.40 M NaCl 0.00052 126 16.4
Effect of electrolyte on cmc and micelle
8.5.3 Important application of surfactants
(1) wetting:
Pesticide, oil extraction, paint, etc.
(2) beneficiation
Cr3+ + 3OH- = Cr(OH)3
(3) Phase transfer (extraction)
(4) solubilization
lesol
The solubility of benzene is 0.07 g in water, 9 g in solution containing 10% C17H35COOH.
Solubilization occur only when the concentration of surfactant is over cmc.
(5) emulsification
An emulsion is a liquid-liquid (oil-water) suspension that is small drops of one liquid dispersed in another liquid.
Any liquid immiscible with water is named as oil.
Oil droplet
Strong mechanical stir separation
Separation of oil from water.
Continuous phase
domain
O/W emulsion: oil in water W/O emulsion: water in Oil
Bicontinuous phase
emulsification
emulsion polymerization, floor wax, oil extraction, oil transport, dry washing, micro-reactor for enzyme-catalyzed reaction.
Due to the large specific interfacial area, emulsion is not stable thermodynamically. In order to stabilize an emulsion a third substance known as an emulsifying agent should be added.
demulsification
Water into oil?
gasoline + 1.5 % emulsifying agent + 25 % water.
the oil consumption can be reduced by 18.7 %, and the smoke and dust in the exhaust gas can be reduced by 50%.
microemulsion
C16H34 + 10% C17H35COOH + KOH
emulsion (thermodynamically unstable, opaque)
Emulsion + n-C4H9OH microemulsion (thermodynamically stable, transparent) < 0.1 m
(6) vesicant
Lower surface tension
Stiffen the film
Increase the viscosity.
(7) Dispersion
Liquefaction of coal: 70~ 75 % coal powder 25 ~ 30 % water surfactant
Paint, pigment
(8) detergence
Wetting, emulsification, solubilization, frothing.
Common composition of a commercial detergent
1) Sodium dodecylbenzenesulfonate (surfactant)
2) Water softeners
3) Tween-80: Polysorbate 80
4) cmc: Cellulose acetate
5) fluorescent whitening agent
6) essence