We've been studying the states of matter and phase changes. So far, we've looked at melting, freezing, vaporization, and condensation. They are all standard phase changes. There is a clear progression between solids, liquids, and gases. Today, we'll look at sublimation. It's a unique transition! Why? It's the change from a solid directly to a gas.
A Molecular Perspective on Sublimation
Like the other phase transitions, sublimation depends on kinetic energy and Van der Waal's forces. We start with a solid. So, we know that our molecules have a fixed arrangement, limited movement, and preset volume. Van der Waal's forces are dominant. The molecules cannot slide past one another. We end up on the opposite side of the spectrum. In gases, the kinetic energy overpowers Van der Waal's forces. The particles move freely about their container. For an ideal gas, we assume their collisions are perfectly elastic. They don't interact with one another, and there is empty space between particles. So, gases fill the volume of their container and are expandable and compressible. Overall, we are transitioning between extremes.
How do we transition between these two extremes? It all depends on the temperature and pressure. Most compounds that sublime have weak Van der Waal's forces. They almost exclusively experience dispersion forces. The classic example is solid carbon dioxide or dry ice. Because of the carbon-oxygen double bonds, we might expect carbon dioxide to be polar. It makes sense. There is a big electronegativity difference between those two elements. But, carbon dioxide has a linear geometry. So, the dipoles cancel. What's the result? Carbon dioxide only experiences dispersion forces.
For carbon dioxide to remain solid, we need unbelievably low temperatures. Its sublimation temperature is −78.5oC! That's frigid! Remember, the temperature is our measure of the average kinetic energy. What do such low temperatures indicate? They show how easy it is for the kinetic energy to overpower the dispersion interactions. The other option is to increase the pressure. Dry ice will remain solid at room temperature above 5.1 atmospheres. Remember, pressure measures the force that air particles exert on a substance. They push against the surface molecules and keep them together. So for carbon dioxide, we need over five times the pressure of our atmosphere.
So, let's put it all together! How does sublimation actually occur? We start with a solid at temperatures above the boiling point. The surface molecules absorb heat. But the temperatures are so high that they instantly transition to a gaseous phase.
Dye-Sublimation Printing
Do you have a case on your phone? Does it have an image on it? If so, the printing process may have used sublimation! We start by designing the image on a special transfer paper. But there's a trick! The ink is in the solid phase. How? We use the piezoelectric effect. When you apply a slight pressure to some crystalline solids, they build up a charge. We can use the electrostatic interactions to stick the dye to the transfer paper.
What about the rest of the printing process? Next, we spread the transfer paper onto a press. When we're ready to apply the image, the press begins to heat the ink. What happens when the press reaches the correct temperature? It pushes the paper against the material. The combination of high pressures and temperatures causes the ink to sublimate. Most of the materials are amorphous solids. So, they often have pores. The gaseous dye fills the material's pores. It gets stuck in the pores because of electrostatic attraction. When we remove the press, the ink rapidly cools. It undergoes deposition, returning to the solid phases.
