Micelles 1

Flashcards covering the properties, classification, and behavior of surfactants and micelles.

What are micelles?

Colloidal, spherical (or near spherical) nanoparticles composed of 50-100 surfactant molecules with a size of ~5-10 nm.

Why do micelle solutions appear transparent?

Solutions appear transparent because micelles are smaller than the wavelength of visible light, preventing light scattering.

Why are ionic micelles generally smaller than nonionic micelles?

Ionic micelles are typically smaller due to stronger repulsion between head groups.

What are nonionic micelles made from and why are they larger?

Made from uncharged surfactants; larger because of weaker repulsive forces.

How are micelles formed from surfactants?

Micelles are clusters of surfactants held together by hydrophobic interactions, with tails inward and heads outward.

What is the dynamic exchange of surfactants in micelles?

A rapid process where individual surfactants may enter or leave the micelle.

What is the dynamic breakdown and reforming of micelles?

A slow process where micelles are constantly breaking down and reforming.

What does CMC stand for?

Critical micelle concentration - The concentration at which micelles are formed.

What happens to surfactants below the CMC?

Surfactants adsorb at the interface, reducing surface tension as hydrophobic tails point away from water and hydrophilic heads stay in water.

What happens to surfactants at the CMC?

The interface becomes saturated with surfactants, and additional surfactants form micelles in the bulk solution.

What happens to surface tension above the CMC?

Surface tension plateaus as micelles form instead of further adsorbing at the interface.

What is a surfactant?

An amphiphilic molecule with distinct hydrophobic and hydrophilic regions that accumulates at interfaces and self-assembles.

What are Type II Molecules?

They also adsorb at interfaces but cannot form micelles due to lacking charged/large enough head groups, and phase-separate at high concentrations.

What industries widely use surfactants?

Detergent, agrochemical, pesticide, paint, paper, food, and pharmaceutical industry.

In what formulations are surfactants present?

Creams, suspensions, and emulsions.

What can aggregated surfactants be used as?

Micelles and vesicles.

How can insoluble drugs be solubilized by surfactants?

By incorporating them into micelles.

How are surfactants classified?

Based on the charge on the surface-active part of the molecule.

What are the main types of surfactants based on charge?

Ionic, non-ionic, and zwitterionic.

What are the two types of ionic surfactants?

Anionic and cationic.

What is an anionic surfactant?

Negatively charged surfactant with a positive counterion.

Give an example of an anionic surfactant.

Sodium dodecyl sulfate (SDS) or sodium lauryl sulfate (SLS).

What is sodium dodecyl sulfate used for?

Preoperative skin cleaner and bacteriostatic action against gram-positive bacteria.

Why do counterions accompany charged head groups?

water cannot sustain isolated charges; counterions (e.g., Na⁺) always accompany charged head groups.

What is a cationic surfactant?

Positively charged surfactant with a negative counterion.

Give an example of a cationic surfactant.

Cetylpyridinium chloride.

What activity do cationic surfactants have?

Bactericidal activity.

What are the uses for Cetylpyridinium chloride?

Cleaning of wounds and contaminated utensils.

What is a non-ionic surfactant?

Uncharged.

Describe the structure of Brij surfactants.

Alkyl chain linked to polyoxyethylene glycol - e.g. dodecyl polyoxyethylene glycol

What is a zwitterionic surfactant?

Two charges, overall neutral.

Give an example of a zwitterionic surfactant.

Dodecyl sulfobetaine and lecithin.

Give an example of a drug that is a surfactant

Propanolol

Give examples of naturally occurring surfactants.

Bile salts, lecithin, synovial fluid, and lung surfactant.

What is osmotic pressure?

The pressure required to prevent the flow of water across a semi-permeable membrane from a region of lower solute concentration into a region of higher solute concentration.

What happens in the U-tube setup without applied pressure?

Water moves toward high solute concentration to dilute it.

What happens when external pressure is applied?

External pressure can prevent water flow, maintaining equilibrium.

What characterizes an isotonic solution?

It has the same osmotic pressure as bodily fluids.

What did McBain observe above a certain concentration of surfactants?

As if less material was present than first thought.

How do surfactants differ from normal solutes in osmotic pressure behavior?

Normal solutes increase osmotic pressure linearly, while surfactants form micelles above the CMC, reducing the number of free particles.

What happens to osmotic pressure of Normal solutes (e.g., glucose)?

Π always increases linearly.

What results from increasing CPP?

Transition from spheres to cylinders to bilayers to reverse micelles.

How can CPP be modified experimentally?

Add salt to ionic surfactants or change temperature for non-ionics.

What mechanism occurs when adding salt to ionic surfactants?

Salt screens head-group charges, reducing a0​, which increases CPP.

What is the effect of adding salt to ionic surfactants?

Spheres transform into worm-like micelles.

What behavior do cylindrical micelles exhibit?

Increases viscosity/elasticity.

What is the benefit of cylindrical micelles in shampoos?

Shampoos gain flow without being watery or too thick.

What is the formula for CPP?

V / (lc * a0)

What does V represent in CPP?

Volume of hydrophobic tail(s)

What does a0 represent in CPP?

Cross-sectional area of hydrophilic head group.

What does lc represent in CPP?

Length of hydrophobic tail.

How are reverse micelles formed?

In non-polar solvents(e.g. oil) , surfactants flip orientation, with heads clustering inward and tails extending outward.

What favors the formation of reverse micelles?

Large hydrophobic tails (e.g., phospholipids in oil).

What is the structure of cylindrical micelles?

Long, flexible tubes with ~nm diameter and µm length.

When does the CPP increase?

When head groups shrink or tails expand.

What is a Colligative Property?

Depends on the number of molecules present in water

Individual surfactants may enter or leave the micelles

Rapid process

Micelles are constantly breaking down and reforming

Slow process

True or False: hydrophobic = lipophilic

hydrophobic ≠ lipophilic

What does CPP stand for?

Critical Packing Parameter