24 Questions
What are amphiphilic molecules composed of?
Hydrophobic and hydrophilic groups
What is the term for the hydrophilic part of an amphiphilic molecule?
Headgroup
What is the critical micelle concentration (cmc)?
The concentration at which micelles start to form
What happens to the amount of non-micellar solved amphiphile when the concentration exceeds the cmc?
It changes only slightly
What is the minimum number of monomers required to form a micelle?
50
What is the term for small aggregates of amphiphilic molecules?
Micelles
What is the purpose of micelle formation?
To decrease water/hydrocarbon contact
What happens to the micelle structure when the concentration is below the cmc?
Micelles do not exist
What happens when the amphiphile concentration is increased beyond a certain point?
The system turns over to 'reverse' or 'inverted' micelles
What is the characteristic of an aliphatic chain in a surfactant molecule that allows it to fit into a micelle?
It must be flexible
What is the term for the temperature below which a crystalline amphiphile is almost insoluble in water?
Kraft-point
What is the difference between normal and reverse micelles?
The location of the hydrophilic solvent
What is the effect of increasing the aliphatic chain length on the Kraft-point?
It increases the Kraft-point
What is the condition necessary for a surfactant molecule to form micelles?
The temperature must be above the Kraft-point
What is the role of the hydrophilic solvent in normal micelles?
It separates the hydrophobic core from the hydrophilic surrounding
What is the shape of the micelle in which the hydrophilic solvent is inside the aggregate?
Reverse spherical micelle
What is the concentration range of lyotropic liquid crystals?
Between the isotropic solution and the crystalline surfactant
What is the shape of the micelles that can lead to a nematic phase?
Rod-like or disc-like
What is the difference between lyotropic and thermotropic liquid crystals?
The temperature and concentration of the surfactant aggregates
What is the term for the phase where the preferred axes of rod-like or disc-like micelles arrange parallel to each other?
Nematic
What is the symbol for the nematic phase of rod-like micelles?
𝑁!
What is the characteristic of a biaxial nematic phase?
Long-range orientational order of two preferred axes
What is the characteristic of chiral lyotropic phases?
A helical structure of the director distribution
What is the result of introducing an increasing amount of order to a surfactant/solvent system?
A development of lyotropic liquid crystalline structures
Study Notes
Micelles
- In normal micelles, the polar headgroups of the internal surfaces separate the hydrophobic core of the aggregate from the hydrophilic surrounding.
- As the amphiphile concentration increases, the water content of the solution may become too small to separate discrete micelles, leading to the formation of "reverse" or "inverted micelles.
- In reverse micelles, the hydrophilic solvent represents the core of the micelle, and the hydrophobic chains and eventually added oil take the part of the continuous medium.
Micelle Formation
- Below a well-defined temperature (Kraft-point), the crystalline amphiphile is almost insoluble in water, and no micelles can form.
- The Kraft-point (T) increases with the aliphatic chain length.
- Only above T, a sufficient number of free monomers can exist in the solvent to exceed the critical micelle concentration (cmc).
Amphiphilic Molecules
- Amphiphilic molecules are composed of hydrophobic (lipophilic) and hydrophilic (lipophobic) groups.
- The hydrophilic part of the molecule is usually called the "headgroup", while the hydrophobic part is called the "tail".
- Examples of amphiphilic molecules include sodium decylsulfate.
Micelle Structure
- Micelles do not exist below the critical micelle (formation) concentration (cmc).
- Micelles are small aggregates with a finite size, typically consisting of 50 up to several hundred monomers.
- The setup of a micelle is a cooperative phenomenon that takes at least a minimum number of monomers (about 50) to sufficiently reduce the water/hydrocarbon contact by aggregation.
Lyotropic Liquid Crystals
- Lyotropic liquid crystals exist in a concentration range between the isotropic solution and the crystalline surfactant.
- They are mesophases like thermotropic ones, but with essential differences between the two classes of liquid crystals.
- The structure-building elements of lyotropic liquid crystals are anisometric micelles or quasi-micellar aggregates.
Lyotropic Liquid Crystalline Structures
- Nematic phases of rod-like micelles are denoted as N, while N or N refers to disc-like micelles.
- Both nematic phases exhibit long-range orientational order of only one preferred axis.
- If the two other aggregate axes are not equivalent, it is possible to get these axes ordered, too, resulting in a biaxial nematic (N).
This quiz covers the behavior of micelles and the effect of increasing amphiphile concentration on the system.
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