2. Surfactants and self-assembly

Surfactant

Molecules that interact with surfaces or interfaces . Surfactants are usually amphiphilic organic compounds, that have hydrophobic part (tail) and hydrophilic part (head). Surfactant= surface active agents

Example: sodium dodecyl sulfate (SDS) or fatty acid salts in common hand soap

Anionic polar head groups

Cationic polar head group

Non-ionic polar head group

Surfactants in aqueous solution

The polar head groups are hydrated and take part in the H-bonding structure of water

Hydrocarbon tails do not interact with water molecules.

– “Hydrophobic effect” (the segregation of water and nonpolar substances)

– Tail in solution → free energy increase

– Aggregation → minimize contact between hydrocarbon tails and water molecules

Formation of micelles

•  Micelles are self-assembled equilibrium structures

• Equilibrium - Thermodynamically stable

• Constant rapid exchange of individual molecules between solution and surfactants particles

•  Micellar solutions are not dispersions – they are one-phase solutions!

Critical micelle concentration (CMC)

•  Micelles form at the critical micelle concentration (CMC)

• Below CMC, only dissolved particles, no micelles 

What happens with free energy at formation of micelles?

The standard free energy (ΔG= ΔH-TΔS) is affected by:

– Decrease

• Removal of non-polar chains from the aqueous phase.

– Increase

• Interface creation

• Repulsion between hydrophilic head groups.

Which force tries to couteract micelle formation?

Repulsion between head groups counteracts micelle formation

– Steric (e.g large head groups → problem with packing)

– Electrostatic

How does micelle formation affect entropy?

It gives an increase in entropy.

What gives a high vs low CMC?

A large hydrophilic group gives a high CMC
A large hydrophobic group (how long carbon chain is) gives a low CMC

Mixed micelles

• Micelles can consist of different types of surfactants

• Non-surfactant molecules can be accommodated in micelles

– Example: fatty alcohol in fatty acid micelle (breaks up repulsion between acid groups)

• Can influence the CMC

– Example: Less repulsion between polar head groups

Which properties of surfactants are determined by the CMC?

• Washing – detergency

• Foaming

• Emulsification

• Solubilization of hydrophobic compounds

Describe different steps for detergency

1. Micellar solution- Above CMC

2. Surfactant adsorb to the dirt 

3. Roll-up of dirt

Can either occur:

• Spontaneously

• Mechanical force (rubing etc) - most common 

4. Dispersion (Dirt inside micelle)

Krafft point

The Krafft point is the temperature surfactant solubility increases drastically.

• Below Krafft temperature is the solubility of the surfactant is low and micelles can not be formed. 

• It is a typical property for ionic surfactants.

Wetting

The ability of a liquid to maintain contact with a solid surface. Adsorption of surfactants changes the properties of the surface

Clean glass: High energy surface – often slightly anionic

Water on a surfactant monolayer of hexadecyltrimethylammonium bromide (cationic):

Low energy surface

Surfactant self-assembly

Concentration above CMC additional self-assembled structures start to form (affects viscosity)

Different micelle structures

Liposome

Self-assembled surfactant (polar lipid) structures for drug delivery

Ternary phase diagram

Three component system. 

• Every corner represents a pure condition

• Each side of the triangle represents all possible binary combinations of the three components. On any of those sides, the fraction of the third component is zero (0%).

• By drawing parallel lines at regular intervals between the zero line and the corner, fine divisions can be established for easy estimation.

Cloud point

Cloud point temperature at which a surfactant solution leads to a two phase transition, typically from a clear solution to a cloudy state due to phase separation (the point where precipitation starts).

Functionality lost above cloud point (solubility is lost)

Balancing of hydrophobic effects and head-group hydration (temperture might influence orientation of hydrophobic chain → making it less hydrophobic)

What happens with non-ionic ethoxylated surfactants when temperature increases?

Increasing temperature leads to two phases. This is an exception.

Very low Krafft point (not relevant for applications) - we consider it not be have a Krafft point

Applications of anionic surfactants

Used in soaps, detergents and as stabilizers.

• Form stable films (good foaming agents)

• Sensitive to high ionic strengths (since they are charged)

• Sulphate/Sulphonate surfactants are less sensitive to Ca2+ than the fatty acids are. If we have hard water it will be less sufficient, forming an insoluble salt of calcium ions and two surfactants

However, irritant to the skin.

”Bath tub ring”

Applications of nonionic surfactants

As stabilizers in many applications

Less sensitive to high ionic strength and pH

Less efficient at high T

Often efficient at lower T (compared to anionic surfactants)

Form less rigid films than ionic surfactants (lack of electrostatic repulsion)

Useful for low foaming applications.

Applications of cationic surfactants

Cationic surfactants interact strongly with biological membranes → toxic.

– Pesticides and bactericides

Mineral ore flotation

Asphalt – (positive charged bitumen emulsions and negative gravel)

Hair conditioners – reducing static electricity and frizziness (hair is negatively charged → frizzy)

Anti-caking agents – preventing liquid bridges in powders through water-wetting prevention (make cationic → hydrophobic)

Anti-corrosion agents – preventing water wetting of steel surfaces

Drug delivery/gene therapy – form complexes with anionic macromolecules