Lecture 5 + 6 Flashcards
The boundary between 2 phases is generally described as
interface
when one if the phases is a gas or a vapor, the term ______ is frequently used
surface
The importance of interfacial phenomena in pharmacy:
Affect: - adsorption of drugs onto
solid walls in dosage forms
- emulsion formation and
stability
- suspension formatiin
- biological activities of some
drugs
define cohesive forces:
Cohesive forces: are the forces that exist between molecules of
one phase (i.e. between like molecules).
Define adhesive forces:
are the forces that exist between molecules of
two different phases (i.e. between unlike molecules).
Define miscibility
MISCIBILITY: occurs when cohesive forces are less than
adhesive forces, e.g. water and alcohol
Define immiscibility
IMMISCIBILITY: occurs when cohesive forces are more than
adhesive force e.g. water and oil.
Define Wettability
WETTABILITY: occurs when adhesive forces are more than
cohesive force e.g. water and solids
Define surface tension
SURFACE TENSION [y] is the force per unit length that must be applied
parallel to the surface to counterbalance the net inward pull and has the units
of dyne/cm.
Define interfacial tension
INTERFACIAL TENSION is the force per unit length existing at the interface
between two immiscible liquid phases and has the units of dyne/cm.
True or false;
Interfacial tension is less than surface tension because an adhesive forces,
between the two liquid phases forming the interface are greater than when a
liquid and a gas phase exist together
True
True or False;
If two liquids are completely miscible, interfacial tension exists.
False
True or False;
High surface tension reflects high intermolecular attraction force. So, increase
in hydrogen bonds or molecular weight causes increase in ST.
True
How can you measure the surface and interfacial tension?
1- Capillary rise method
2- Ring (Du Nouy) tensiometer
3- Drop weight / count method (Stalagmometer)
True or false;
Capillary rise method measures both Surface and interfacial tension.
False, surface tension only
capillary rise method (surface tension only)
When a capillary tube is placed in a liquid contained in a
beaker, it rises up to a certain distance.
* Because the force of adhesion is grater than the cohesion, So ,
the liquid continues to rise until the upward force (represented
by S. tension ) is balanced by the downward force of
gravity due to weight of liquid
y= 1/2 rpgh
Ring method (Du Noüy Tensiometer)
(Measure Surface and interfacial tension)
The principle :
* “ The force necessary to detach a platinum – iridium ring
immersed at the surface or interface is proportional to the surface
or interfacial tension “ .
* y = Dial reading in dynes x beta
* 2 x ring circumference
* beta is a correction factor [ radius of ring , radius of wire forming
ring ….]
* In effect, the instrument measures the weight of liquid pulled out
of the plane of the interface immediately before the ring becomes
detached
Drop Weight and drop volume method (count method)
(Measure Surface and interfacial tension)
If the volume or weight of a drop as it is detached from a tip of
known radius is determined, the surface and interfacial tension
can be calculated from:
* γ = Φ mg = Φ V pg
2 π r 2 π r
* Where,
* m = the mass of the drop
* V = the volume of the drop
* p = the density of the liquid, r = the radius of the tip
* g = the acceleration due to gravity, Φ = a correction factor
