§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