The energy of water molecules is reduced as the water bonds chemically with other molecules and structurally via Van der Waal’s forces. As a result, water activity describes whether or not water will be available to support microbial growth, participate in chemical reactions, or cause physical changes.
This has caused some people to refer to water activity as a measure of “bound” and “free” water. Sometimes people go further, interpreting a water activity of .25 for example as 25% bound water and 75% free water.
This is not scientifically accurate, and can cause confusion.
The concept of “bound” water has multiple definitions. For example, it is frequently used to describe the process of physically restricting the flow of liquid water by creating a gel system. When the concepts of water activity and physical binding are confused, it can result in attempts to lower water activity by “binding” water using gels.
The gel does prevent water from flowing as a liquid, but doesn’t lower water activity. The water can still move easily as a vapor. In fact, gel-bound water may have a very high energy, or water activity, and be available for microbial growth and other reactions.
Instead of thinking of “bound” and “free” water, it is more accurate to think in terms of available energy. For example, water in a sample with a water activity of 0.25 has 25% of the energy of pure water.