Lecture 7: surface chemistry Flashcards
What is an interface?
An interface is where one phase meets another without miscibility
What is the phase that is always miscible with itself?
Gas
What creates surface tension?
- Molecules within the bulk of a liquid are subject ed to equal attractive forces in all directions
- The molecules at the liquid air interface experience unbalanced attractive forces
- There is a net inward force towards the bulk
- This force pulls the molecules of the interface together, resulting in interface tension
What is surface tension?
The force per unit length that must be applied parallel to the surface so as to counterbalance the net inward pull of the molecules at the bulk.
What is interfacial tension?
The force per unit length that exists between two immiscible liquid phases
Do stronger intermolecular forces lead to higher or lower surface tension?
Higher
For which molecules is the surface tension equal to the London dispersion force contributions to the surface tension and why?
Non polar molecules because London dispersion forces are the only attractive forces
What are the factors affecting surface tension?
- Temperature
- Salts
- Alcohol
How does temperature affect surface tension?
- Increasing temperature decreases the surface tension
- Surface tension is a measure of the energy needed to move a molecule from the bulk to the surface, so as temperature increases, the energy needed decreases since the intermolecular forces weaken
How does an increase in salt concentration affect surface tension?
- An increase in concentration of salts in an aqueous solution leads to an increase in surface tension
- Ion water interactions are much stronger than water water interactions
- For this reason salt ions remain in the bulk because it takes a lot of energy to get them to the surface
- Since they are in the bulk, the interactions between the water molecules get stronger, leading to an increase in surface tension.
Does the valency of salts affect the magnitude by which the surface tension of the solution is changed?
Yes
divalent and trivalent salts increase surface tension to a greater extent than monovalent salts
How does the concentration of alcohol in a solution affect its surface tension?
- Increasing the concentration of alcohol in a solution results in a decrease in the surface tension
- Alcohol molecules adsorb at the surface, and as the concentration increases, the water water interactions are replaced by weaker alcohol-alcohol interactions, and so the surface tension decreases
How does the hydrophobicity of an alcohol affect the extent to which it changes the surface tension?
- The mode hydrophobic (longer hydrocarbon chain) an alcohol is the more it lowers the surface tension
- Hydrophobic alcohols have a greater tendency to adsorb at the air water interface and disrupt the water water interactions
What are the 2 methods used to measure surface and interfacial tension?
- Capillary rise method (surface tension only)
- DuNouy ring method
How do you measure surface tension from the capillary rise method?
- Place a capillary tube into a liquid in a beaker.
- The liquid will rise in the tube to a height that is related to the surface tension of the liquid
How do you measure surface tension from the DuNouy Ring method?
???
What is work of cohesion?
The work needed to pull apart a volume of spreading liquid that has a unit cross sectional area
What is work of adhesion?
Work needed to separate two immiscible liquids that form an interface with a unit cross sectional area.
How do you calculate work of adhesion and cohesion?
Wc = 2 x surface tension
Wa = surface tension of liquid 1 + surface tension of liquid 2 - interfacial tension between the 2 liquids
How do you calculate spreading coefficient and how would you know if the liquid spreads based on this?
S = Wa - Wc
Spreading occurs only when Wa > Wc so S has to be positive
What do surfactants do to the surface tension of a liquid?
Surfactants are chemicals that lower the surface tension of a liquid. They do this by adsorbing at the air-water interface and forming a monolayer. This monolayer lowers the cohesive forces between liquid’s molecules, and as a result, the surface tension is lowered.
What is critical micelle concentration?
- It is the concentration above which micelles start to form
- At this point the surfactants have saturated the interface
How does the turbidity change beyong the cMC?
The turbidity increases significantly
What happens to the surface tension at CMC?
It remains relatively constant
It decreases sharply until around the CMC
What happens to the electrical conductivity at CMC?
Decreases beyond the CMC
What happens to the diffusion coefficient at CMC?
Decreases beyond the CMC
What are the pharmaceutical applications of micelles?
- To incorporate poorly water soluble drugs and excipients into aqueous solution
- To enhance drug stability in aqueous formulations
What is maximum additive concentration?
The maximum amount of solubilizate that can be incorporated into a given system at a fixed concentration
What does the HLB value tell us about the hydrophilicty/lipophilicity of a drug?
Molecules that have a higher polarity and greater hydrophilicity have a higher HLB value while lipophilic molecules have a lower HLB
What is the general HLB value of surfactants that form W/O emulsions?
HLB<10
What is the general HLB value of surfactants that form O/W emulsions?
HLB>10
What is the HLB value at which complete water solubility occurs?
HLB 7.3
Fill in the blank:
For a proper emulsification, the HLB value of a surfactant should be as close as possible to the value of the _____________.
Dispersed phase
How do you calculate the overall HLB value of the mixture when you mix multiple surfactants together?
You add the HLB values multiplied by their relative fractions
What is the difference between absorption and adsorption?
- Adsorption is the accumulation of one component at the surface of the other
- Absorption is the penetration of one component throughout the body of the other
What are the 2 types of adsorption?
- Physisorption
- Chemisorption
What is adsorption isotherm?
1.An adsorption isotherm is a graph that represents the variation in the amount of adsorbate adsorbed on the surface of the adsorbent with the change in pressure at a constant temperature.
2. To reach this equilibrium you have to place the solid and the solution in contact for a long time at constant temperature
What does the Langmuir adsorption equation show us?
The relationship between the concentration of the adsorbate on the surface of the adsorbent and the equilibrium concentration of adsorbate in the solution
For gaseous adsorbate, the concentration is replaced by pressure
What is the separation factor in the Langmuir adsorption model
The separation factor in the Langmuir adsorption model refers to a parameter used to characterize the selectivity of adsorption on a surface.
The adsorption process is favourable if 0 < RL < 1 and is unfavourable if RL > 1
How does the nature of the adsorbent affect solid-liquid adsorption?
not sure pls help to check
How does solubility affect solid-liquid adsorption?
The solubility of a solute has an inverse relationship with it’s adsorption.
How does pH affect solid-liquid adsorption?
- The pH of the solution affects the degree of ionization of the drug and its solubility.
- Unionized species, owing to their low solubility, display higher extent of adsorption compared to the unionized form
How does temperature affect solid-liquid adsorption?
- Adsorption is an exothermic process
- Increasing the temperature will decrease the extent of adsorption
What is contact angle?
Contact angle is the angle formed between the line tangent to a liquid droplet and the solid
surface over which it spreads.
How does contact angle determine the wettability for the liquid?
angle > 90 indicates low wettability (e.g., water on a plastic bag) while angle < 90 indicates good wettability (e.g., water on a glass surface)
What is the function of a wetting agent?
To lower the contact angle between the surface and wetting liquid.
What is critical surface tension and how do you calculate it?
Critical surface tension refers to the minimum surface tension required for a liquid to spread over a solid surface
Draw a graph and determine the point at which costheta is atleast 0
