8.5 Surfactants and their properties and...

§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


(2) Classification of surfactant

(1) Classification by structure/active moiety

Group work:Find at least one example for

each type.

8.5.1 Surfactants


(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


8.5.2 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


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



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.



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

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.



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



What is the effect of Na2SO4 in detergent powder?

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: 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.



(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



Separation of oil from water.



Continuous phase

domain

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

Bicontinuous phase



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.



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



(6) vesicant

Lower surface tension

Stiffen the film

Increase the viscosity.





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) 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



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