Difference between Surface tension and Interfacial tension 1

[manvin]

Hi
I want to know what is the difference between surface tension and interfacial Tension. Can any body suggest me some book explaining these concepts.
Thanks.

 

[m777182]

Imagine a drop of a liquid(it could be a rain drop falling towards the earth).Look at a molecule in the inside of the drop: it is surrounded by the same specia from all sides so it is in a completely balanced field of forces.
Now look at a molecule in the border layer:there are only forces from the inside acting upon it, attracting it towards inside.In terms of physical chemistry it is said that the liquid on the surface has some excess free energy compared to the bulk.If you want to increase the surface of the drop you should perform work against those attractive (cohesional) forces.
Work is proportional to the increase of the surface and the coefficient is called surface tension.In this simpified model we assumed that molecules of air does not attract molecules of water from the inside of the drop.
In reality there will allways be an interface between two phases, any combination by two of liquid, gas or solid if there is an excess of free energy of the surface layer. If it is not present, the two phases will mix completely like alcohol and water.
There are many books about surface tension; I am using Adamson's Physical Chemistry of surfaces, J Wiley 1990 5th ed ISBN 0-471-61019-4.
m777182

 

[25362]

In answer to parts of manvin's query, and in addition to m777182's clear presentation, other names for surface tension (ST) commonly in use are: interfacial force, interfacial tension, surface tensity.

ST is quantitatively a force that appears to act across a line of unit length on the liquid surface. The greater the surface tension, the greater the force that may be applied to equalize the forces experienced by all the molecules in a liquid sample.

The ST of water is about three times higher than that of most other commonn liquids; this is a consequence of water's strong hydrogen bonds.

The very high ST of mercury, more than six times that of water, is traced to the strength of the bonding between its atoms.

When a small amount of water is poured unto a waxed surface, it forms nearly spherical beads. Because the attractive forces between the water molecules are greater than those between the water and wax molecules, there is a net inward attractive force on the water molecules at the surface toward the bulk of the liquid.

When a liquid has a low ST (and therefore weak intermolecular attractions), it easily wets solid surfaces. Wetting is the spreading of a liquid across a surface. Gasoline, for example, is composed of nonpolar hydrocarbon molecules that only attract each other by London forces. Weak attractions within the liquid are readily overcome by attractions to almost any surface, so gasoline spreads to a thin film.

Water wets glass because the surface of the solid contains lots of oxygen atoms, so part of the energy to expand the water's surface area is recovered by the formation of hydrogen bonds to the glass surface. When the glass is coated by a film of oil or grease, conditions differ.

ST decreases with increasing temperature.

ST is responsible for wet sheets of paper sticking together, for the ability of water to support the mass of a steel sewing needle and of some insects that are more dense than water itself, as if water had a flexible "skin". "Fire polishing" removes rough edges of laboratory glass tubing because of the tendency of the heated (liquid) glass to form a rounded surface. Among other rhings ST allows to fill a water glass above the rim.

Laundry detergents contain chemicals which are surface active agents (surfactants) that drastically lower the ST of water, making the water "wetter" and allowing the detergent solution to spread more easily across the surface to be cleaned.

In the absence of gravity or any other external resistances such as that of air on a falling raindrop, a sample of water would form a sphere, for then the maximum number of molecules are in the interior where they are surrounded by neighbors. Thus, the most stable situation is one in which the surface area is minimal as in a sphere.

Gravity tends to flatten drops, air resistance gives rain drops their teardrop shape.

In gravity-free environments as in an orbit space shuttle, the shape of liquid droplets is governed by ST alone. Tiny (0.01-mm) perfect spheres that have been formed in space are commercially available and are used to calibrate particle sizes for powdered pharmaceuticals.

Another observable effect of intermolecular forces is capillarity; but this is a another subject altogether.

To manvin, any book on General Chemistry could serve your purpose.