Surface Chemistry Flashcards
What is an interface?
The boundary between two phases
What is the definition of interfacial tension (γ)?
Force per unit length at an interface
How does interfacial tension arise?
Molecules at the surface experience different intermolecular forces compared to molecules in the bulk
How does interfacial phenomena affect drug dissolution in the GI tract?
Interfacial tension between drug particle and fluid within the body forms a diffusion layer. Diffusion layer makes it harder for the drug particle to dissolve.
How does interfacial phenomena affect drug recrystallisation?
Interfacial tension between nuclei and recrystallisation fluid creates a diffusion layer. This means other drug molecules can’t join the nuclei. The higher the interfacial tension, the thicker the diffusion layer and the smaller crystals will be formed.
What is surface tension?
Interfacial tension between a liquid and a gas
How does surface tension arise?
Molecules in the bulk have cohesive forces acting in all directions but surface molecules can only for cohesive forces between molecules that are below or adjacent. The force within the bulk pulls the surface molecules inwards, contracting the surface.
What is the definition of surface tension?
The force per unit length that exists at the surface of a liquid
Which liquid has the highest surface tension (excluding mercury) and why?
Water has the highest surface tension due to the strong H bonds between all of its molecules
How does temperature influence surface tension? What does this mean for experimental practice?
Increasing temperature decreases surface tension in a linear fashion. This means measurements must be taken at a constant temperature
Name three methods of measuring surface/interfacial tension
The Du Nuoy Tensiometer (Ring Method), Wilhelmy Plate Method and Capillary Rise Method
Give a brief explanation of how the Du Nuoy Tensiometer (Ring Method) works.
A platinum ring is placed at an interface (either between two immiscible liquids or at the surface of a liquid). The upwards force on the ring required to detach the ring from the liquid can be used to calculate the surface tension.
Give a brief explanation of how the Wilhelmy Plate method works.
A platinum plate is lowered into a liquid (or two immiscible liquids if measuring interfacial tension). When the downwards force exceeds the surface tension, the liquid will separate from the plate.
Give a brief explanation of how the Capillary Rise Method works.
A glass tube is inserted into a liquid at a contact angle of 0. The liquid then wets the capillary wall and rises up the tube. The height the liquid rises to can be used to determine the surface tension
Why may someone use the WIlhelmy Plate or Ring Method over the Capillary Rise Method to measure surface/interfacial tension?
They give a more accurate reading. Capillary rise method cannot measure interfacial tension.
Why may someone use the Capillary Rise Method over the Wilhelmy Plate/Ring method?
It is much cheaper
True or False? Spreading can only occur over two immiscible liquids
True
What does S have to be in order for spreading to occur. Explain.
S must be greater than or equal to zero in order for spreading to occur. This means the adhesive forces between the two liquids must be stronger than the cohesive forces within liquid B
Give an equation to determine the spreading coefficient of a liquid
S = γA - (γB + γA/B)
What is the definition of cohesion?
The forces present within one liquid
What is the work of cohesion?
The work required to separate a column of liquid to form two L/G interfaces
What is the definition of adhesion?
Forces between two liquids
What is the work of adhesion?
The work required to separate a L/L interface to form two L/G interfaces
What is S initial?
The spreading coefficient before mutual saturation occurs
What is S final?
The spreading coefficient after mutual saturation occurs
Explain the relationship between spreading and polarity
As the polarity of an oil decreases, the spreading coefficient decreases.
Explain the relationship between contact angle and wetting.
The higher the contact angle, the poorer the liquid is at wetting
Give an equation the relates to wetting
Wa = Wsl = γlv ( 1 + cos (contact angle))
True or False? Wetting only occurs when forces of adhesion are less than forces of cohesion
False
What is the critical surface tension (γc)?
Surface tension at which cos (contact angle) = 1
Give an example of a materia; with low γc. What does this mean in terms of spreading and wetting?
Plastics. They are no easily wetted.
