§8.5 Surfactants and their properties

and Applications

Definition: 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

§8.5 Surfactants and their properties and Applications

8.5.1 Surfactants

§8.5 Surfactants and their properties and Applications

(2) Classification of surfactant

(1) Classification by structure/active moiety

Group work:Find at least one example for

each type.

8.5.1 Surfactants

§8.5 Surfactants and their properties and Applications

(2) Classification on functions

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

8.5.1 Surfactants

§8.5 Surfactants and their properties and Applications

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 as other

strong electrolytes.

Concentration-dependence of

some physical properties of

sodium dodecyl sulfate:

between 1~2 10-3 moldm-3,

sharp breaks occur.

NaCl

C12

C14

c

0 0.2 0.4 0.6

detergence

Osmotic

pressure

Surface tension

Molar conductivity

interface tension

§8.5 Surfactants and their properties and Applications

6.0x10-4

7.0x10-4

8.0x10-4

9.0x10-4

1.0x10-3

1.1x10-3

0.034

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

8.5.2 Critical micelle concentration and micelle

§8.5 Surfactants and their properties and Applications

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.

8.5.2 Critical micelle concentration and micelle

§8.5 Surfactants and their properties and Applications

vesicle

8.5.2 Critical micelle concentration and micelle

§8.5 Surfactants and their properties and Applications

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

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.Done in our research group.

8.5.2 Critical micelle concentration and micelle

§8.5 Surfactants and their properties and Applications

8.5.2 Critical micelle concentration and micelle

§8.5 Surfactants and their properties and Applications

Phase diagram of surfactant-water system

0.04 0.06 0.08

20

30

40

TK

T / oC

Two-phase region

micellesolution

8.5.2 Critical micelle concentration and micelle

§8.5 Surfactants and their properties and Applications

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

8.5.2 Critical micelle concentration and micelle

§8.5 Surfactants and their properties and Applications

What is the effect of Na2SO4 in detergent powder?

8.5.3 Important application of surfactants

(1) wetting:

Pesticide, oil extraction, paint, etc.

§8.5 Surfactants and their properties and Applications

(2) beneficiation

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

Cr3+ + 3OH- = Cr(OH)3

(3) Phase transfer (extraction)

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

(4) solubilization

Lesol: an old disinfectant used in hospital.

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.

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

(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 stirseparation

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

Separation of oil from water.

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

Continuous phase

domain

O/W emulsion: oil in water W/O emulsion: water in Oil

Bicontinuous phase

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

emulsification

Applications: 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.

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

demulsification

Water into oil?

gasoline + 1.5 % emulsifying agent + 25 % water.

the oil consumption can be reduced by 18.7 %, and thesmoke 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

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

(6) vesicant

Lower surface tension

Stiffen the film

Increase the viscosity.

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

The way for soap bubbles

intercepts with each other.

(7) Dispersion

Liquefaction of coal:

70~ 75 % coal powder

25 ~ 30 % water

surfactant

Paint, pigment

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

(8) detergence

Wetting, emulsification,

solubilization, frothing.

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications

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

8.5.3 Important application of surfactants

§8.5 Surfactants and their properties and Applications