5) Moleculer Structure Of Surface Tension Flashcards
Molecular structure of liquids
The molecules are close to each other
Large molecular forces of attraction exist
A characteristic property is fluidity
-the ability to be moved in a particular direction by relative small forces
The quasicrystalline structure of liquids
The molecules oscillate about an equilibrium node for a short period of time (relaxation time) and then jump to another node
Relaxation time:
- Nature of the liquid
- temperature
- is usually around 10^-13 seconds
The quasicrystalline structure of liquids 2
At the equilibrium node the molecules are arranged in a crystal like way
Away from the node the arrangement is usually lost
Ordinary liquids are isotopic and amorphous
Van der waals forces
The molecules of liquids interact with VDWF
- repulsive at small distances
- attractive at larger distances and decreasing away from molecule
Radius of action r- the smallest distance between 2 molecules at which the force is weak so it’s neglected
Sphere of molecular action- sphere centred at molecule with radius= to radius of action
Van der Waals forces 2
Radius r
The smallest distance between to molecules at which the force becomes small enough to be neglected
Intermolecular forces
DRAWING
Force on a molecule submerged deeper than r
The molecule is pulled equally in all directions by neighbouring liquid molecules. net force is 0
Intermolecular forces 2
The net force F_k on the molecule is not 0
Drawing
Cohesion pressure
P_k is the sum of cohesion forces acting on the unit area of all molecules
I’m water
P_k= 10^9 pa
Energy of molecules
Due to cohesion pressure the molecules on the surface have higher potential energy than other molecules
When molecule passes from surface to a layer in the bulk its kinetic energy increases and vice versa
Surface tension
The molecules on the surface interact with forces parallel to the surface =surface tension forces
Coefficient of surface tension
Alpha is the coefficient
F is the sum of surface tension
L is length along line
Alpha=f/L
Alpha is measured in N/m
Surfactant is additive that modified surface tension
Positive= reduced alpha( soaps)
Negative= increases alpha (salt)
The coefficient of surface tension 2
It also also be defined as the work needed to increase the area of the surface by one unit
Alpha = A/s
Alpha depends on:
The nature of the liquid
Temperature
Wetting
The curving of the free surface of the liquid at the boundary between liquid gas and solid
A liquid may wet a sold body but not wet another
Eg: water wets glass but not paraffin wax
Contact angle
The angle between the solid surface under the liquid and tangent to the free surface of the liquid
DRAWINF
Contact angle 2
The liquid is
Absolutely wetting
0 = theata
Wetting
0
