1. Charles University in Pilsen - Biophysics dep - Surface Tension

2. Charles University in Pilsen - Biophysics dep - Surface Tension
Ability of the surface of a liquid to act like a thin, flexible film.
Practical Examples
Rain drop without air friction effect
Intermolecular hydrogen bonds
Surface tension at 20°C is 7.29 x 10-2 J/m2
Water strider + capillary action
Mercury drop
Intermolecular metallic (electrostatic) bonds
Surface tension at 20°C is 4.6 x 10-1 J/m2
Charles University in Pilsen - Biophysics dep - Surface Tension

3. Charles University in Pilsen - Biophysics dep - Surface Tension
A water molecule deep within the liquid is pulled equally from all sides
A water molecule at the surface lacks neighboring water molecules (toward the interface)
net force is not zero!
net force = zero
when all hydrogen bonds can be fulfilled cohesion takes place=stable state
This unequal attraction causes the water at the air-water interface to act as a cohesive surface.
and the liquid to shrink to the smallest possible surface area.
Charles University in Pilsen - Biophysics dep - Surface Tension

4. Charles University in Pilsen - Biophysics dep - Surface Tension
The smallest surface possible
high surface tension, in order to achieve the lowest possible energetic state.
Temperature matters?
Water molecule representative
low T
high T
Weaker intermolecular forces
Liquid is subject to an inward force of molecular attraction which is balanced by the resistance of the liquid to compression
Increasement of surface area
Lower Surface Tension
Charles University in Pilsen - Biophysics dep - Surface Tension

5. Charles University in Pilsen - Biophysics dep - Surface Tension
Surface Behaviour
Solid material - more dense than water
water’s surface
what happens?
material’s nature
hydrophilic
it has a surface to which water is attracted
hydrophobic
it has a surface to which water is “afraid”
Charles University in Pilsen - Biophysics dep - Surface Tension

6. Charles University in Pilsen - Biophysics dep - Surface Tension
hydrophilic
The adhesion of water to the surface of this material coats the surface of the object with water.
reduces the surface tension
causes the object to sink
hydrophobic
The unfavorable interactions between the water surface and the object make it difficult to wet the surface.
Charles University in Pilsen - Biophysics dep - Surface Tension

7. Charles University in Pilsen - Biophysics dep - Surface Tension!
Two forces now come into play - the energy it would take to overcome this repulsion and the force of gravity. g σ
If the force of gravity is strong enough, it will prevail and the object will sink
If the gravitational force is less than the surface tension then the object will float on the surface of the water.
Of course, the critical feature here is the amount of force per unit area -- put a needle into water end-on instead sideways and the needle will immediately sink Air force is negligeble! g σ
Charles University in Pilsen - Biophysics dep - Surface Tension

8. Charles University in Pilsen - Biophysics dep - Surface Tension
Surfactants
Surfactants can interact with water in a variety of ways, each of which disrupts or modifies the hydrogen bonding network of water.
Why?
Class of molecules which contain hydrophobic (non-polar) hydrocarbon "tails" and a hydrophilic (polar) "head" group.
"like dissolves like"
organize themselves into 3-dimensional spheres called micelles
which have a hydrocarbon core and sulfate groups around the outer surface.
Since this reduces the cohesive forces in water, this leads to reduction in the surface tension and our sulfur sinks.
Charles University in Pilsen - Biophysics dep - Surface Tension

9. Charles University in Pilsen - Biophysics dep - Surface Tension
Eg: Detergents
Without detergent, we can not remove an oily stain from clothing because grease and oil are large, non-polar, hydrophobic molecules.
However, the interior core of a micelle is quite greasy just like an oily stain. When we add detergent to our wash water, the oil or grease on our clothes can dissolve in the interior of the micelles and thereby go into solution.
Human body - lungs
The body uses surfactants to reduce surface tension in the lungs.
The human body does not start to produce lung surfactants until late in fetal development. Therefore, premature babies are often unable to breathe properly, a condition called Respiratory Distress Syndrome. -administration of artificial surfactants virtually eliminates this health problem
Charles University in Pilsen - Biophysics dep - Surface Tension

10. Charles University in Pilsen - Biophysics dep - Surface Tension
Measuring Methods
Capillary Action
water can move up a thin capillary, against the force of gravity - Surface tension "pulls" neighboring water molecules along.
The liquid climbs until the adhesive and cohesive forces are balance by the force of gravity
The liquid surface adjacent to the wall of the capillary tube and the tubing wall form an angle .
r = radius (m)
h = height of liquid column (m)
d = density of liquid (kg/m3)
g = acceleration due to gravity (m/s²)
 = (r h d g)/ 2 cos 
Charles University in Pilsen - Biophysics dep - Surface Tension

11. Charles University in Pilsen - Biophysics dep - Surface Tension
Du Noüy Ring method
A ring of thin wire is submersed but slightly under the surface plan
The ring is then gently pulled out of the water and the equilibrium state between surface tension and gravity is measured
If F is the force applied at the dynamometer and l the lengt of the wire curled, surface tension is calculated using the equation
 = F / 2l
Charles University in Pilsen - Biophysics dep - Surface Tension

12. Charles University in Pilsen - Biophysics dep - Surface Tension
Dropping method
Measurement of the surface tension of a fluid which is posteriorly related with an already known surface tension of a standard fluid
The less surface tension, the less mass of liquid will be droped
2 / 1 = m1 / m2
1- known liquid
2- unknown liquid
Charles University in Pilsen - Biophysics dep - Surface Tension

13. Charles University in Pilsen - Biophysics dep - Surface Tension
Surface tension - Scientificaly
measured in Newtons per meter (N·m-1)
represented by the symbol σ or γ or T
defined as
the force along a line of unit length perpendicular to the surface.
the energy required to increase the surface area of a liquid by a unit area.
Charles University in Pilsen - Biophysics dep - Surface Tension

14. Charles University in Pilsen - Biophysics dep - Surface Tension
Thank You
Charles University in Pilsen - Biophysics dep - Surface Tension

15. Charles University in Pilsen - Biophysics dep - Surface Tension
Surfactants
Energia interna vs T ????
Measurement
viscosity/surf tension
Curiosidade dos pulmoes
Charles University in Pilsen - Biophysics dep - Surface Tension