§8.5 Surfactants and their properties and Applications

8.5.1 Surfactants 1) Structure of surfactants The substances that drastically lower the surface tension of water even at low concentrations. The surface tension declines steeply even at c < 10-3 mol dm-3. 1) Structure of surfactants The Amphiphilicity is the basic characters of surfactants They have both polar and nonpolar moieties. Hydrophilic group, hydrophobic / lipophilic group.

hydrophilic group hydrophobic 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 anionic Group work: Find at least one example for each type. ionic cationic amphoteric ester ether nonionic amine amide nonionic-cationic mixed nonionic-anionic

(2) Classification on applications Emulsifier; deemulsifier, wetting agent, anti-wetting agent, dispersant, vesicant, (3) Classification on molecular weight M > 10000, polymeric surfactant. Find one example (4) Classification on composition Find one example Organosilicon surfactant Organometallic surfactant: Zn soap, Mg soap (5) Classification on solubility water-soluble surfactant oil-soluble surfactant Find one example

8.5.2 critical micelle concentration and micelle When concentration (c) below ca. 10-3 moldm-3, most ionic surfactants in aqueous solution display similar conductance to other strong electrolytes. NaCl C12 C14 c  0.2 0.4 0.6 detergence Osmotic pressure Surface tension Molar conductivity interface tension Concentration-dependence of properties for sodium dodecyl sulfate: between 1~2  10-3 moldm-3, a sharp breaks occur in the conductivity and surface tension.

A result presented by a student research group

Owing to the hydrophobic groups, surfactant is unstable in water 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?

Micelles are usually charged particles In 1925, Mcbain postulated that at high concentration, surfactant molecules aggregate to form micelle. nS  Sn An equilibrium exsits between micelle and individual surfactant Small micelle Ball-like micelle Layered micelle Rod-like micelle Micelles are usually charged particles

vesicle

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 micelle solution Phase diagram of surfactant-water system

Average charge per micelle Effect of electrolyte on cmc and micelle 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

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

(2) beneficiation

(3) Phase transfer (extraction) Cr3+ + 3OH- = Cr(OH)3

(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 Continuous phase domain

emulsification 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. emulsion polymerization, floor wax, oil extraction, oil transport, dry washing, micro-reactor for enzyme-catalyzed reaction.

gasoline + 1.5 % emulsifying agent + 25 % water. 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%. demulsification 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