SI Phenomena

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  • Created by: LBCW0502
  • Created on: 25-02-18 15:30
Give examples of surface tension
Insects walking on water, duck feathers don't get wet, tents, clinical test for jaundice (powdered sulfur floats on the surface of normal urine/powdered sulfur sinks if the surface tension is lowered due to bile
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Why is this relevant to pharmacy?
Understand formulation of products: mix oil and water (emulsions/creams), stabilise solid suspensions, granulation, film coating on tablets, dissolution of tablets and capsules
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What is an interface?
Boundary between two phases e.g. liquid/solid, two solids or two liquids
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What is a surface?
Boundary between two phases e.g. liquid/gas, solid/gas
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Give an example of a cohesive force
Van der Waals (attractive)
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Give an example of a repulsive force
Kinetic energy
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Describe the forces in a gas
Kinetic energy > cohesive energy (no surface)
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Describe the forces in a solid
Large cohesive forces and an interface is formed
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Describe forces in a liquid
Intermediate. Interface formed but liquid can take up the shape of a container
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Describe features of surface tension
Each molecule surrounded by other molecules with attractive forces (cancel out). At the surface, there is an imbalance of imf due to no adjacent molecules in contact from above. Pulls surface molecules downwards (tension/skin effect)
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Give examples of surface tension
Spherical shape of droplets (forces pulling at surface/large volume leads to water losing shape and taking up shape of container). Mercury droplets (larger surface tension than that of water droplets) - inward compression/increase in free energy
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What is the definition of surface tension?
The force (in N) acting at right angles to any 1 metre along the surface of the liquid (force per unit length) - F/2l
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What are the units for surface tension?
N/m
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What is the definition of surface free energy?
The work required to increase the surface by 1 m^2 (energy per unit of area) - F/2l
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What are the units for surface free energy?
J/m^2
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Are surface tension and surface free energy numerically equal?
Yes - but not equivalent. Both have units of N/m but surface tension is usually in mN/m
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What are some of the considerations for surface tension?
Molecules in a liquid are not static but in dynamic equilibrium. Resident time of 10^-6 s at surface. Relative number of molecules vapour depends on temperature (higher temperature/more molecules). Surface is ~ one molecule thick
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Why does water have a higher surface tension than ethanol?
Water can form H bonds in two regions but ethanol forms a H bond in one region
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Why does mercury have a very high surface tension?
The cohesive forces are greater than the repulsive forces so a larger volume can be contained within a droplet
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What is interfacial tension?
The force (in N) acting at right angles to any 1 metre in the interface between two immiscible liquid phases e.g. oil and water
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Why do charged molecules dissolve?
They bond with water molecules (H bond and electrostatic interactions/anions and cations). Dissolution occurs when the ions are separated
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Why can't non-polar molecules dissolve in water?
They cannot form bonds with water molecules and cluster together. Phase separation. Minimises disruption of H bonds in water (there would be a large SA being disrupted). Only interface disrupted
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What aspects need to be considered in interfacial tension?
Molecules in bulk of liquid and those at the surface. Imbalance of forces at interface of liquid (accumulation of free energy at surface of liquid). Strong interfacial tension/high amount of free energy. Tension/wants to interact with bulk
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Why do ethanol and acetone not have interfacial tension values (measured against water)?
They are miscible in water and there is no phase separation (non-polar)
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What is Antonoff's rule?
With two mutually saturated liquids, the interfacial tension is the difference between the surface tension of the two liquids (this is an approximation)
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What is the work of adhesion?
The work required to separate an area of a liquid/liquid interface to form two separate liquid/air interfaces (theoretical concept) - column (application - coating tablets)
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What is the Dupre equation?
Sum of the two different surface tensions minus the interfacial tension (larger number for work of adhesion leads to stronger adhesion with water e.g. fatty acids, primary alcohols etc.) - units mN/m
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What is the work of cohesion?
The work required to separate a single (column of) liquid to create two liquid/air interfaces within the same (column of) liquid. How strong are the cohesive forces in the liquid/more difficult to create interfaces/higher surface tension
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What is the formula for the work of cohesion?
2 x surface tension (units of mN/m)
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What is the initial spreading coefficient?
Determines whether or not a liquid will spread on the surface. Wa - Wc. S>0 (liquid will spread), S
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Explain why a liquid would spread on a surface
There is a small alpha angle (small angle of contact). Wa > Wc. Gives positive initial spreading coefficient. Good spreading of liquid
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Explain why a liquid would not spread on a surface
There is a large alpha angle (minimise contact with surface). Wa < Wc. Liquid will not spread
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How can oil spread in water?
Oil is sufficiently polar to adhere to water more strongly compared to bonding between water molecules (oil clumps together if it is non-polar)
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Which factors affect surface tension?
Temperature, pressure, time and solute
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How does temperature affect surface tension?
As the temperature increases, the surface tension decreases due to increase in kinetic energy, more repulsive forces, H bonds become weaker, interface becomes weaker
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What is the critical temperature?
The temperature at which the liquid is lost e.g. 100 degrees Celsius for water (evaporates)
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How does vapour pressure affect surface tension?
As the vapour pressure increases, the surface tension decreases (less dramatic effect than temperature/important is aerosolised drug delivery)
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How does time affect surface tension?
Surface tension is an orientation effect and takes time to reach equilibrium. Proteins used to affect surface tension (high concentration/short time to equilibrium, low concentration/long time to equilibrium) - e.g. BSA
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How long does a pure liquid take to reach equilibrium?
0.003 seconds
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Why do solutions take longer to reach equilibrium?
Due to preferential orientation of molecules at the surface. Concentration-dependent
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How do different types of solute affect surface tension?
Type I solutes increase surface tension slightly. Type II solutes decrease surface tension. Type III solutes decrease and then stabilise surface tension
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Give examples of type I solutes
Simple electrolytes, glucose. Ionic bonds. Van der Waals forces. Dissolution. Hydration
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What is the surface tension of water?
72 mN/m
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Describe features of type I solutes
Heavily hydrated. Negative adsorption of solute at air/water surface due to solute-solvent attractive forces > solvent-solvent attractive forces. Concentration of solute at surface < concentration in bulk. Surface deficit of solute. Cohesive forces
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Describe features of type II and type III solutes
Non-polar and polar regions (type III have larger hydrophilic head groups). Concentration at surface > concentration in bulk. Surface excess/positive adsorption/minimise free energy
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Describe the orientation of type II and type III solutes at the air-water surface
Non-polar region in air (minimise interaction with water) and polar region in aqueous phase.
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Why do type II and type III molecules reduce surface tension?
Reduced forces of attraction between water molecules. Little attraction between solute molecules. Adsorption at interface, weaker cohesive forces
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Describe features of type II solutes only
Unionised organic molecules e.g. organic acids, alcohols, amines (co-surfactants)
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Describe the effect of a series of organic acids on the surface tension of water
Increase in HC chain, less soluble, larger decrease in surface tension, stronger the repulsion to water, higher positive adsorption at air/water interface, stronger effect of surface tension (reduction in cohesive forces). Achieve minimum energy
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Can this trend be seen in another homologous series?
E.g. alcohols, amines
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What is Traube's rule?
The molar concentration required to produce an equal lowering in surface tension (in dilute solution) decreases by approximately 1/3 (3-fold) for each addition CH2 group in a homologous series (divide by 1/3 or x 3) - applies to type II/III solutes
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Describe features of type III solutes
Can either be ionised organic molecules (e.g. sodium salts) or molecules containing a water soluble polymer as a head group (repeat same hydrophilic group to make it more polar)
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What is the critical concentration?
Threshold concentration when surface tension levels off and there is an increase in surfactant concentration (formation of micelles)
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Outline the general structure of a surfactant
Hydrophilic head and hydrophobic tail
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Describe what happens at concentrations below the critical concentration
Type III molecules are at the air/water surface. Concentration at surface > concentration in bulk (surface excess/position adsorption) - orientation to minimise free energy. Monomers present (constant)
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Describe what happens at concentrations above the critical concentration
Molecules start to aggregate to form micelles (hydrophobic core and hydrophilic surface). Micelles in equilibrium with monomers
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What is the critical micelle concentration?
Concentration at which aggregates start to form (spherical micelle usually formed/other structures include rods and bilayers)
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What are the general distinctions between type II and type III molecules?
Both possess hydrophilic and hydrophobic regions. Similar hydrophobic chains. Difference in nature of head group
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What distinguishes a type II molecule from a type III molecule?
Type II has unionised head groups. No aggregation (no CMC). Co-surfactants or oil-soluble surfactants. Type III have ionised head group. Polymeric head group. Aggregate. Surfactants or water-soluble surfactants
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Is it possible to convert type II molecule into a type III molecule (and vice versa)?
Yes e.g. alter pH to change type II molecule to type III molecule
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Understand formulation of products: mix oil and water (emulsions/creams), stabilise solid suspensions, granulation, film coating on tablets, dissolution of tablets and capsules

Card 3

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What is an interface?

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Card 4

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Card 5

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