Colloid chemistry

Lecture 10: Surfactants

Applications of surfactants: cleaning/detergents (40%);textiles; cosmetics; pharmacy; paint; food; etc.

Etymology

Surfactant micellessurfactant molecule

hydrophobicalkyl chain

hydrophilichead group

self-assembling

spherical micelle

hydrophilic shellhydrophobic core

cationic surfactantanionic surfactantnonionic surfactant

orientation → energy minimumHardy-Harkins principle

30-100 moleculesd-3-5 nm(association)

OSOOSO33--NaNa++

surfactant = surface active agent

If enough soap is added to water the molecules arrange themselves into a structure called a micelle

Hydrophilic ( lyophobic, water-loving) head containing a charged functional group

Hydrophobic ( lyophilic,water-fearing ) tail containing a hydrocarbon chain

Soap molecules/micelles

Soaps are produced from complex esters:

animal fat

How are soap produced?

The saponification of a triglyceride

Classification of Surfactants

Anionic

Cationic

Zwitterionic

Nonionic

Sodium dodecylsulfate (SDS)

Cetylpyridinium bromide

Dipalmitoylphosphatidylcholine (lecithin)

O O O O OHTetraoxyethylene lauryl ether

SO-Na+

O

OO

OO

P

O

OO

OCH2CH2N(CH3)3+

O-

Classification of Surfactants

N+

Br-

Micellar shapes

Packing parameter affects micellar shape

Surfactant phases

Surfactants as biocolloids

Surfactants as biocolloids

plasma membranes are primarily lipid bilayers with associated proteins and glycolipids(cholesterol is also a major component of plasma membranes)

Surfactants as biocolloids

Small, viral sized (10-50 nm) particles

Similar to lipid micelles

Composed of peptide core (hydrophobicpart) and PEG shell (hydrophilic part)

Peptide core composition allows peptide/protein solubilization

Also good for small molecules

Peptide micelles

Diblock copolymer micelles

molecule micelle

PB202-PEO360

02468

101214

0 1Surfactant concentration

CMC

Con

cent

ratio

n unimers

micelles

02468

101214

0 1Surfactant concentration

CMC

Osmotic pressureπ

02468

101214

0 1(Surfactant concentration)1/2

CMC

Molar conductivityΛ

02468

101214

0 1Surfactant concentration

CMC

Light scattering

Solution Properties

02468

10

0 1log (surfactant concentration)

CMC

Surface tensionγ

τ

cmc

I. Above the Krafft point, the solubility is sharply increases due to the formation of micelles.

II. Some surfactants (e.g. poly(ethylene)-poly(ethylene-oxide) = PE−PEO = CmEn) - based surfactants) dehydrate and phase separate upon temperature elevation (“cloud point”).

III. For pluronics; poly(propylene-oxide)-poly(ethylene-oxide) = (PPO-PEO) diblock copolymer surfactants, the micelles form as a result of the dehydration of the PEO segments (“critical micelle temperature” or “CMT”).

Krafft point; cloud point; cmT

Factors affecting the cmc: molecular structure of the surface active agent

Factors affecting the cmc: effect of additives

HLB of surfactants

• HLB ca. 1 to 3: Antifoams / inverse micelles• HLB ca. 3 to 8: Water-in-Oil Emulsifiers• HLB ca. 7 to 9: Wetting and spreading agents• HLB ca. 8 to 16: Oil-in-Water Emulsifiers• HLB ca. 13 to 16: Detergents• HLB ca. >15: Solubilizers

Amphiphilic surfactants are characterized by the hydrophilic-lipophilic balance (HLB): a relative ratio of polar and non-polar groups in the surfactant

HLB and use of surfactants

Griffin’s scale: it is an arbitrary (group additivity) scale of values serving as a measure of the HLB of surfactants

(polysorbates)

(sorbitan monooleates)

• What is the HLB of the mixture of 40 % Span 60 (HLB = 4.7) and 60 % Tween 60 (HLB = 14.9)?– HLB of mixture:

4.7 x 0.4 + 14.9 x 0.6 = 10.8• In what proportion should Span 80 (HLB = 4.3) and

Tween 80 (HLB = 15.0) be mixed to obtain “required” HLB of 12.0?– 4.3.(1-x) + 15.x = 12 x = 0.72 – 72 % Tween 80 and 28 % Span 80

Surfactant blends are commonly used to obtain desired emulsifying properties.

HLB of surfactant blends

polar compound

reverse micelles

non-polar compound

normal micelles

amphiphilic compound

Solubilization

Spontaneous transfer of a compound insoluble in the bulk solventinto solution due to incorporation into the surfactant micelles:

in polarsolvents

in non-polarsolvents

Detergency

(contact angle increases)(contact angle increases)

elementary steps of washing solubilization efficiency of K-soaps(solubilization capacity: tg α)

amou

nt o

f sub

stan

ce so

lubi

lized

K-soap concentration, mol/L