Toni L5 Flashcards
(23 cards)
Define surface tension
y=(δG/δA)ni,p,T
What is the meaning of surface tension
The surface tension aka interfacial tension describes how free energy of a system varies as the surface area changes
What does it mean is surface tension is greater than 0
if y>0 there is a tendency for the system to change towards a state of minimum surface area
What are the units for surface tension y
y has units of Jm-2 or Nm-1
What is the typical surface tension value of pure water
γ ≃ 77 mJ m−2
What reduces the surface tension
adsorption of organic molecules at the air-water interface will reduce the surface tension
Describe surface concentration graph and how we use it to calculate surface concentrations
The concentration of a solute at the interface is not necessarily the same as in the bulk solution. The difference between the two integrated across the interface is the surface concentration
Define surface concentration
This is a measure of the excess amount of solute at the interface compared to the bulk. It’s calculated by integrating the difference in concentration across the interface and is expressed in mol m⁻²
What can be determined if we change the position of the dividing surface
As we change the position of the dividing surface, if we assume the surface concentration for the solvent is 0, we can determine the concentration of the species
What are Gs and Ss
surface excess gibs energy
Surface excess gibs entropy
What do Gs describe
describe how the surface alone contributes to the system’s overall free energy per unit area
What happens as more of a surface active solute is added to solution
as more of a surface active solution is added to the solution, the surface concentration increases and the surface tension decreases
What are the approximations used in the Langmuir isotherm
1) all the binding/adsorption sites are equivalent
2) the adsorption cannot proceed beyond monolayer coverage
3) the probability that a molecule adsorbs is not influence by the occupation of neighbouring sites
What is a BET isotherm
The BET isotherm is a model used to describe how gas molecules adsorb onto a solid surface when multiple layers of molecules form.
The BET model allows for multilayer adsorption.
When does multilayer adsorption occur/ BET isotherms occur
If Molecules can adsorb onto a monolayer of their own kind. This is common at the solid/gas interface when the pressure (p) of an adsorbing gas is close to the saturated vapor pressure p0.
What does the Langmuir isotherm used for
The Langmuir isotherm is a model that describes how molecules adsorb onto a solid surface to form a monolayer
What do each letters of the Langmuir isotherm represent
r/rm = θ = Kc/(1+Kc)
r is the surface concentration of the absorbed species
c the concentration in the bulk phase
rm the maximum surface concentration for a monolayer molecules
θ Surface coverage
K the equilibrium constant for the adsorption reaction
What does a plot of 1/θ versus 1/c tell us
slope 1/K and intercept 1
What equation can be used if only r and not rm is known
1/r = 1/rm + 1/rmKc
What does a plot of 1/r versus 1/c give us
A plot of 1/r vs 1/c will be a straight line with slope 1/Krm and intercept 1/rm
How can you calculate surface tension from a graph
If you plot a graph of gibbs free energy against area, the gradient is the surface tension
In a closed system what are the changes in mole numbers due to
In a closed system any changes in the mole numbers when the surface area increases are due to transfer of material from the bulk to the surface
