INTERFACES AND SURFACTANTS Flashcards
What is an interface?
- When to insoluble phases meet they form an interface
- E.g. air and liquid, water and oil
Define surface tension
- Unbalanced forces at the surface lead to a net inward force of attraction on surface molecules that tends to minimise the area of the surface (surface tension)
What is surface free energy?
- Contraction of the surface therefore leads to a minimum free energy state and work is required to increase the surface area.
- The surface free energy is the ‘work’ required to increase the surface area by 1 m2
- Free energy and surface tension are numerically equal with units N m-1 (usually quoted as mN m-1)
Describe the methods of measuring surface tension
- Whilmey plate method: Thin plate can be wetted by liquid. Force acting on the plate is is proportional to surface tension
- Capillary rise: Measure how far up the tube liquid rises, with knowledge of its density, radius of capillary and gravity
When a substance dissolves does surface tension increase or decrease and WHY?
- When a substance dissolves in a liquid the surface tension (water = 72 mN m-1) decreases but never significantly increases
Draw out a surfactant
What is the surface activity of an surfactant dependent on?
- The surface activity of a surfactant depends on the balance between its hydrophobic and hydrophilic properties
What does it mean if a drug is amphilic
- The drug has both hydrophobic and lipophilic properties
- So acts as a surfactant
What is a wetting agent?
- They are commonly used to reduce surface tension of water and thus help water based solutions to spread
Surfactants adsorb at interfaces to form a layer one molecule thick, called a ….
Surfactants adsorb at interfaces to form a layer one molecule thick - a molecular monolayer
Why is the surface tension of water high?
- Therefore, water molecules are cohesive because they are strongly attracted to each other.
- Water has a high surface tension because hydrogen bonds among water molecules resist stretching or breaking the surface.
- Essentially due to strong intermolecular bonds
What is traubes law
- It states that for every extra CH2 group in a surfactant molecule, the surface activity approximately triples.
- So, shorter chain length molecules have less surface activity than higher chain length molecules.
Q.
What is the critical micelle concentration
How to calculate surface excess and what is it
- Surface excess is the conc of surfactants at the surface (mol/m2)
- Area per molecule is the space each molecule has at its surface
- where n is number of moles and A is surface area
How can this graph: surface tension again In C, be used to determine surface excess
- Surface excess () = - gradient of tangent / RT
(where R is gas constant and T is temperature) - Do gradient at conc just below CMC
What is each axis of the graph representing and what is this graph for
- Surface pressure
- Against area pre molecule
- Its used for studies of compression of monolayers of the three different types of monolayers
What are the three different types of monolayers and how does each one differ in terms of properties
- Condensed (solid) monolayer - these are closely packed
- Expanded (liquid) monolayer - occupy a larger area then condensed, highly compressible
- Gaseous monolayer - are separate and move about the surface independently
Label the points at which transition occurs between monolayer states
How can wettability be measured?
- The wettability of a solid material can be estimated based on contact angle they form with water.
- No direct way to measure surface tension (surface free energy).
Two types of adsorbtion
What is a detergent and what makes a good detergent
Medical and pharmaceutical applications
