Surfactants – Surface Active AgentsSurfactants – Surface Active Agents(Chapter 4, pp. 76-84 in Shaw)(Chapter 4, pp. 76-84 in Shaw)

Short chain fatty acids and alcohols are soluble in both water and organic media:

These molecules preferentially position themselves at the water-organic interface due to energetic effects – they are surface active!

CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-OH

hydrophobic tail and hydrophilic head

AIR

WATER

e.g. Stearic Acid: CH3-(CH2)16COOH

Strong adsorption gives rise to monolayers and is termed surface activity.

Surface active materials are also called surfactants and are amphiphlic in nature.

Surface activity is a dynamic phenomenon since there is a balance between complete adsorption and complete mixing (entropy vs. enthalpy effects).

Surfactant molecules will expand the surface and lower the surface tension (contracting forces).

If is the expanding pressure (or surface pressure) of an adsorbed layer of surfactant, then the reduction in surface tension will be:

= o –

The surface tension also depends on the concentration of surfactant , as follows:

- o = Bc

which is emperically known as Traub’s rule.

80

60

40

200 0.4 0.8 1.2 Conc., mol/L

, dyne/cm

Ethanol

n-Propanol

n-Butanoln-Hexanol

The longer the hydrocarbon chain the greater the tendency to adsorb at the interface.

Traube’s rule:

In homologous series each additional CH2 group increases the surface tension reduction effect three fold.

If the interfacial tension between two liquids is reduced to a sufficiently low value, emulsification takes place because only a relatively small increase in surface free energy is involved.

SURFACTANT CLASSIFICATION

The hydrophilic part is often an ionic group

--- Ionic means better solubility.

For instance palmitic acid is unionized and insoluble in water. However, the sodium or potassium salt is readily soluble and shows high surface activity (Palmolive soap).

It is also possible to have non-ionic groups as hydrophilic parts as for instance in poly(ethylene oxide).

Surfactants are thus classified as:--- Anionic--- Cationic--- Non-ionic and--- Ampholytic

Anionics: Most widely used because they are cheap and perform well.

Cationics: Are expensive but have germicidal properties.

Non-ionics: Can be tailored to specific applications (e.g. detergency, wetting agent, emulsifier, stabilizer).

ANIONICS:

Sodium Oleate:CH3(CH2)7CH=CH(CH2)7COO - Na+

Sodium Dodecylsulphate:CH3(CH2)11SO4

- Na+

Sodium Dodecylbenzenesulphonate:CH3(CH2)11C6H4SO3

- Na+

Sodium Stearate:CH3(CH2)16COO- Na+

CATIONIC:

Dodecylamine hydrochloride:CH3(CH2)11NH3

+ Cl-

NON-IONICS:

Polyethylene Oxides:e.g. CH3(CH2)11(O-CH2-CH2)nOH

Spans (sorbitan esters)

Tweens (polyoxyethylene sorbitan esters)

AMPHOLYTICS:

Dodecyl betaine:C12H25N+(CH3)2(CH2COO-)

OTHER ADDITIVES:

Why do a lot of cleaning products have a name that associates them with lemon or lime?

Citrus MiracleLemon TideSwiss Chalet gives a cup with water and a slice of lemon in it.Lemon Fresh dishwashing detergent

What is the function of cirtic acid?

Sequestering agent which forms soluble complexes with Ca2+ and Mg2+ to prevent the formation of soap scum.

GIBBS’S ADSORPTION EQUATION(pp. 80-83 in Shaw)

In ideal systems In real systems

S S S S

A A

B B

In real systems there exists a surface phase, . The surface excess concentration of component i is given as:

= ni/A

where A is the interfacial area and ni is the amount of component i in the surface phase in excess of what would have been in the bulk.

Gibbs’s surface excess relationships (see pp. 81-83 in Shaw):

BB dC

dRT1 (for non-ionic)

BB dC

dRT21 (for ionic)

In the presence of excess electrolyte the first equation is valid since electronic shielding takes place.

These relationships have been verified in two ways:

1) McBain and Swain shaved off 0.1 mm layers from solutions of surfactant.

2) Other researchers used -ray detection to measure concentrations.

Example:

The surface layer was scooped from 300 square cm of the surface of a soap solution of bulk concentration of 0.02 M. The volume of liquid collected was 2 mL and was found to contain 4.013 x 10-5 mol of soap. If the surface tension of pure water at the temperature of the experiment (298 K) is 72.8 mN/m calculate the surface tension of the soap solution.

Do this question at home:

Excess concentration:soap = 4.33 x 10-6 mole/m2

Use Traube’s rule to find: = 61.3 mN/m

Next lectures: Micelles