Solvent Swelling Demonstrations

When a solid is typically dissolved into a liquid, the solid breaks up into individual molecules or ions which disperse throughout the liquid as much as possible, forming a solution. But what if the molecules in the solid are large enough to be all tangled up with each other? The liquid disperses into the solid as much as it can, but the solid still remains largely intact. The solid does swell, however, as it absorbs liquid. This process is called solvent swelling. Perhaps the most common example of this phenomenon is pasta swelling in hot water as the water molecules slip in between the starch molecules of the pasta. Here are some other examples of solvent swelling:

ABOVE LEFT: The superabsorbent polymer powder from a disposable diaper is placed in a Petri dish that contains a shallow pool of water (with green food coloring added). ABOVE RIGHT: The powder solvent-swells as it absorbs water (elapsed time: 10 min). These photographs were taken of the image on an overhead projector screen.

ABOVE LEFT: Silicone caulk that has cured overnight on a glass plate. ABOVE RIGHT: The cured caulk solvent swells in contact with toluene (elapsed time: 1 hour).

ABOVE LEFT: Segments of rubber bands. The one on the left has been solvent swelled by soaking in toluene for 10 min. ABOVE RIGHT: Segments of rubber bands. The one on the left has been solvent swelled by soaking in cyclohexane for 10 min.

More information:

Campbell, Dean J.; Rupe, Sarah B. Solvent Swelling and Optical Rotation Demonstrated on the Overhead Projector J. Chem Educ. 2000 77 876.

Ross, Joseph H. Demonstration of solvent differences by visible polymer swelling J. Chem Educ. 1983 60 169.