Intermolecular Forces
Properties of Liquids: Viscosity and Surface Tension
Properties of Liquids: Viscosity and Surface Tension
Viscosity
The resistance of a liquid to flow is called its viscosity
- The greater the viscosity, the more slowly it flows
Measuring viscosity
- How long a liquid takes to flow out of a pipette under the force of gravity
- How fast an object (steel ball) sinks through the liquid under gravitational force
The Physical Basis of Viscosity
- Viscosity is a measure of the ease with which molecules move past one another
- It depends on the attractive force between the moleculees
- It depends on whether there are structural features which may cause neighboring molecules to become "entangled"
- Viscosity decreases with increasing temperature - the increasing kinetic energy overcomes the attractive forces and molecules can more easily move past each other
Surface Tension
By definition the molecules of a liquid exhibit intermolecular attraction for one another.
What happens to molecules at the surface in comparison to those in the interior of a liquid?
- Molecules in the interior experience an attractive force from neighboring molecules which surround on all sides
- Molecules on the surface have neighboring molecules only on one side (the side facing the interior) and thus experience an attractive force which tends to pull them into the interior
The overall result of this asymmetric force on surface molecules is that:
- The surface of the liquid will rearrange until the least number of molecules are present on the surface
- In other words the surface area will be minimized
- A sphere has the smallest surface area to volume ratio
- The surface molecules will pack somewhat closer together than the rest of the molecules in the liquid
- The surface molecules will be somewhat more ordered and resistant to molecular disruptions
- Thus, the surface will seem to have a "skin"
The "inward" molecular attraction forces, which must be overcome to increase the surface area, are termed the "surface tension"
Surface tension is the energy required to increase the surface area of a liquid by a unit amount
Water
- Intermolecular hydrogen bonds
- Surface tension at 20°C is 7.29 x 10-2 J/m2
Mercury
- Intermolecular metallic (electrostatic) bonds
- Surface tension at 20°C is 4.6 x 10-1 J/m2
Cohesive forces bind molecules of the same type together
Adhesive forces bind a substance to a surface
For example, attractive forces (hydrogen bonding) exists between glass materials (Silicon dioxide) and water.
- This is the basis of "capillary" action, where 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
1996 Michael Blaber