Study Set Content:
But, >
Cohesive forces > Adhesive forces
The net effect (cohesion – adhesion = cohesion) inward force
pulls the molecules at the surface towards the bulk and
contracts the surface, such force results in what is called
surface tension
The molecules of a water droplet are held together by cohesive
forces, and the strong cohesive forces at the surface constitute
surface tension.
If cohesive force > adhesive force
Adhesive greater than cohesive
Cohesive greater than adhesive
If Adhesive force > cohesive force
Wetting
• Spreading
• Rising in the tube
The adhesive forces between water molecules and the walls of
glass tube are stronger than the cohesive forces lead to an
upward at the walls of the vessel and contribute to
capillary action
A capillary tube is placed in a liquid contained in a beaker, the
liquid generally rises up the tube a certain distance.
Capillary rise
For a liquid that adheres to glass, the energy is lowest when a thin film covers as much glass as possible. This leads to curvature of the
liquid inside
Means pressure beneath the meniscus is less than atmospheric pressure by about 2y/r where r is the radius of the tube
Curvature
The principle of the instrument depends on the fact that the force
necessary to detach a platinum-iridium ring immersed at the
surface or interfacial tension.
DuNouy Ring Method
(Hydrophilic-Lipophile Balance) the higher the HLB of an agent,
the more hydrophilic it is.
HLB
0 – 3
Anti-foaming
4 – 6
W/O emulsifying agents
7 – 9
Wetting agents
8 – 18
O/W emulsifying agents
13 – 15
Detergents
10 – 18
Solubilizing agents
Addition of Neutral salts
o Altering
ionic strength
Addition of water in moderate amounts
o Although not changing the pH, may cause a small
positive
or negative deviation
