Drawing a fine line between adsorption and absorption can be difficult because the distinction between them is quite fuzzy. It’s not just the terms adsorbent and absorbent that sound quite similar—in many cases, materials that undergo one process also do the other.
When engineering desiccants like activated alumina, manufacturers have to keep in mind both processes. The reason being that desiccants must induce and sustain a state of dryness in its vicinity. To put it simply, the desiccant should be able to take in water and also retain it for an extended period of time.
With this in mind, let’s take a closer look at the difference between the two.
The Differentiating Factors
When distinguishing between adsorption and absorption, there are a few factors that can help differentiate between the two. You must take into account where the process occurs, what forces are involved between molecules, and how thick the film of water is.
Given that most adsorbed substances form a layer that’s only one molecular layer thick, we can conclude that desiccants undergo the process of adsorption. To help illustrate this conclusion, we’ll use the analogy of a sponge.
When you wring a damp sponge, some liquid will run out of it. However, there will come a point when you twist the sponge, and nothing comes out—yet the sponge remains damp. This layer of liquid coats every surface of the sponge at a microscopic level and adheres strongly to the surface. This is known as the adsorbed part.
Many materials, like the sponge, are highly porous, making their surface area much wider than it seems. Desiccants like silica gel beads also have tiny pores on the surface that increase its surface area and allow it to trap moisture inside. Moreover, the lower vapor pressure inside the bead allows water droplets to adhere more easily to the surface.
Consider the sponge example again. When you place a sponge in water and bring it out, it stops dripping after a while. However, the liquid is still visible in the pores that can be wrung out when squeezed. This portion represents absorption and has thousands of molecular layers.
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