Micelles 2

Flashcards about Micelle Formation

Which technique measures particle count in solution to detect micelle formation?

Osmotic pressure

How does osmotic pressure change below the CMC?

It increases linearly as unimers dominate.

How does osmotic pressure change above the CMC?

It plateaus as micelles count as one particle.

Which technique relies on surfactants adsorbing at interfaces to detect micelle formation?

Surface tension

How does surface tension change below the CMC?

Tension drops as unimers cover the interface.

How does surface tension change above the CMC?

Tension plateaus as excess surfactants form micelles in bulk.

Which technique measures ion mobility in solution to detect micelle formation?

Molar conductivity

Why does conductivity drop at the CMC for ionic surfactants?

Counterions bind to micelles, reducing the number of free ions to carry current.

Which technique relies on micelles solubilizing hydrophobic drugs above the CMC?

Drug solubilization

Which technique detects particles larger than single surfactants to detect micelle formation?

Light scattering

How does light scattering change below the CMC?

Minimal scattering due to unimers being too small.

How does light scattering change above the CMC?

Increased scattering due to micelle formation.

Is molar conductivity applicable to non-ionic surfactants?

No, it only works for ionic surfactants.

What does molar conductivity measure?

Charge mobility, not particle count (unlike osmotic pressure).

What carries the most current in a solution?

Small, singly charged molecules

What carries the least current in a solution?

Large, multicharged molecules

What happens to conductivity as the number of ions increases in a solution?

It increases.

When micelles are formed, approximately what percentage of K+ ions are bound to the micelles?

85%

Below the CMC, what happens to the concentration of unimers as surfactant concentration increases?

Concentration of unimers increases as surfactant concentration increases

Above the CMC, what happens to the concentration of unimers?

Concentration of unimers remains virtually constant.

Above the CMC, what happens to the concentration of micelles as surfactant concentration increases?

Concentration of micelles increases.

Approximately what percentage of counterions are associated with the micelle for ionic surfactants?

85%

What is the thermodynamic basis for micelle formation?

Chemical reactions always go to the lowest energy state

What two interactions need to be considered in micelle formation?

Interaction between water and hydrophobic tails, and water and hydrophilic head groups.

What is the structure of water?

An extensively hydrogen bonded structure, consisting of a lattice of temporary 3D ‘flickering structures’.

What happens to water molecules in contact with hydrocarbon chains?

1. cannot form hydrogen bonds between the hydrocarbon chain and water

2. Results in a water cage around the HC chain

3. Means reduction in entropy

4. Water in contact with the HC chain becomes more ordered

What is the hydrophobic effect?

When HC chains cannot interaction with water = water 'squeezes' the HC chains out.

Is the hydrophobic effect a repulsive force?

No, it is the driving force for micellisation.

How does micelle formation affect water molecules?

It allows water molecules to regain entropy (i.e. freedom) - HC chains are shielded in the core of the micelle

What is the driving force of micellization?

The hydrophobic effect

What is the mechanism of hydrophobic effect?

Water forms ordered cages around hydrophobic (HC) tails leading to entropy loss; to regain entropy water expels HC tails, driving them to cluster.

How do micelles fix the problem of the hydrophobic effect?

HC tails hide in the micelle core, eliminating the need for water cages, and water interacts only with hydrophilic heads; this results in entropy increase.

What is the result of micelle formation in terms of Gibbs free energy?

Gibbs free energy decreases making micellization spontaneous

What is the equation for free energy?

ΔG = ΔH - TΔS

What is the most important energy change in micelle formation?

Increase in entropy because water molecules regain their freedom upon micellization.

What percentage of the G value does entropy constitute?

90-95%

What role does the enthalpy change play in micelle formation?

A small part, approximately 10% of the overall free energy change.

Why is there an entropy change during micelle formation?

Due to loss of water structure around the hydrocarbon chain, increase in internal freedom of hydrocarbon chain inside the micelle core, and loss in high surface tension between hydrocarbon chain and water upon micellisation.

How do water molecules interact with surfactant headgroups?

They compensate for loss of interaction with other water molecules; polymer head groups can form hydrogen bonds with water and ionised head groups can be hydrated.

What stops micelles from growing and growing?

Head groups start to repel each other if brought in too close contact; further growth would lead to a 'hole' of water in the core, which is entropically unfavourable.

What is energy required to bring head groups together?

Repulsion between head groups

What is the driving force of micelle formation?

Water + hydrocarbon chain.

What is the opposing force of micelle formation?

Water + headgroups coming together

How do surfactants influence aggregation?

Hydrophobic effect is driving force; repulsive forces oppose aggregation.

Are aggregation number and micelle size smaller or larger for ionic surfactants compared to nonionic ones?

Smaller

Is the CMC larger or smaller for ionic surfactants, versus nonionic?

Larger

What does a lower CMC indicate about micelle size?

Larger micelles

What two opposite forces determine micelle characteristics?

Hydrophobic effect (driving force) and repulsive force between head groups (opposing force).

What two micelle characteristics are determined by the balance of hydrophobic effect and repulsive headgroup forces?

Aggregation number and CMC

What happens to aggregation number, micelle size, and CMC if the hydrophobicity of a surfactant increases?

All increase.

What happens to aggregation number, micelle size if the hydrophilicity of a surfactant increases?

All decrease.

What is reduced when counterions bind to micelles?

The number of free ions to carry current.

When does an increase in scattering occur in light scattering during micelle formation?

Above the CMC, when micelles form

With micelle formation, what happens when water interacts only with hydrophilic heads?

Entropy increases.

What happens to water interacting with hydrocarbons?

form H bonds