Work, Energy, and Power
The States of Matter
Chemical Thermodynamics and the Equilibrium State
|The surface tension of a liquid depends on the internal forces that must be overcome to expand the surface area of a liquid, in other words, the work needed to pull molecules apart.|
Molecules on the surface have fewer neighbors, so the less the total surface area at the surface, the more molecules will have neighbors on all sides. Molecules in the interior are attracted from all directions, but on the surface, there are only attractions toward the interior of the liquid.
Imagine, to transform the liquid from lesser to greater total surface area, individual molecules in the liquid must be separated from mutually attracting neighbors as they take their place on the exterior. Viewing the droplet as a thermodynamic system, this change must represent an internal energy increase. The greater the strength of intermolecular forces, furthermore, the greater the required input of energy to increase the surface area.
Here is a good exercise to build your sense of chemical thermodynamics: imagine a droplet of some liquid on a table. Imagine the fluid drawing up into a sphere. Internal energy decreases as the polarities come together in the interior of the fluid. A small amount of heat must have flowed out if the internal energy decreased at constant volume. Next picture the fluid spreading out on the table. Internal energy increases as mutually attracting molecules to be separated as the surface area increases. A bit of heat must be flowing in.
Is the temperature changing in our imaginary transformation? No! The heat flows correspond to changes in the electrostatic potential energy component of internal energy.
Now imagine tides of energy flowing back and forth until the most probable situation comes about, the system reaches equilibrium. With strong intermolecular force, the energy available to flow in from the surroundings likely won't be enough to flatten the droplet out. The energy in the system comes into balance with the surroundings; the state of the droplet where that balance is reached depends on how strong the intermolecular forces are. If the forces are weak, the balance is reached with the droplet more spread out on the table. If the forces are strong (high surface tension) the balance is reached with the droplet more nearly a sphere.