§8.5 Surfactants and their properties and Applications

The substances that drastically lower the surface tension of water even at low concentrations 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 <  mol dm -3. 1) Structure of surfactants

hydrophilic group -COOH, carboxyl -OSO 3 H, sulphate -SO 3 H, sulphonic acid -NR 4 +, ammonium -CH 2 - CH 2 -O-, poly-ethylene oxide etc. hydrophobic group (CH 2 ) n (CF 2 ) n (SiR 2 -O-) n (-CH 2 -CH 2 -CH 2 -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

8.5.2 critical micelle concentration and micelle When concentration (c) below ca mol  dm -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  mol  dm -3, a sharp breaks occur in the conductivity and surface tension. NaCl C 12 C 14 c  detergence Osmotic pressure Surface tension Molar conductivity interface tension

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  S n Rod-like micelleLayered micelle An equilibrium exsits between micelle and individual surfactant Micelles are usually charged particles

vesicle

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 Na 2 SO 4 at 40 o C.

Phase diagram of surfactant-water system TKTK T / o C Two-phase region micelle solution

MediumcmcAggregation number Average charge per micelle Water M NaCl M NaCl M NaCl Effect of electrolyte on cmc and micelle

8.5.3 Important application of surfactants (1) wetting: Pesticide, oil extraction, paint, etc.

(2) beneficiation

Cr OH - = 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% C 17 H 35 COOH. 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 % 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 C 16 H % C 17 H 35 COOH + KOH  emulsion (thermodynamically unstable, opaque) Emulsion + n-C 4 H 9 OH  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