How Does the Drying Agent in a Snack Bag Soak Up Moisture?
― A Grain Weighing One Gram Hides Walls Equal to Two Tennis Courts
Those small, clear beads in a bag of seaweed or rice crackers do not "soak in" water the way a sponge does. Instead, they stick water molecules, one at a time, to the walls of pores too thin to see. Unfolded and laid flat, those walls from just one gram would cover about two and a half tennis courts.
You open a bag of seaweed, and a small packet marked "Do not eat" falls out. Inside are clear beads that look like bits of glass.
They stay dry when you touch them. They do not swell up with water like a sponge. Yet the seaweed in the bag stays crisp for weeks.
So where does this tiny bead put all that water?
There are only two reasons it can soak up moisture
Inside, a bead is built like a sponge made of tiny grains stuck together. Add up the walls of all the gaps, and one gram has roughly 700 square metres of them. These walls attract water molecules and hold them fast.
The pores are tens of thousands of times thinner than a human hair. In pores this narrow, water vapour turns back into liquid and fills them even when the air is not fully damp. That is why the bead keeps taking in moisture at ordinary room humidity.
Both powers come from the bead's shape, not its material. Silica gel is made of almost the same stuff as glass and quartz. A glass cup does not soak up moisture, but silica gel does, because what is inside is built completely differently.
What is going on inside a bead?
Look at Figure 1. On the left is a bead from the bag. It is 2 to 5 millimetres across and looks like a plain glass ball.
The middle shows a close-up of the inside of that bead. Even smaller grains, a few nanometres across, cling together like a bunch of grapes. The gaps between them form pores like a maze.
The right zooms in even further on a pore wall. The wall's surface is packed with "hands" that get along well with water molecules. When a water molecule comes close, it is drawn to these hands and sticks to the wall.
What matters is the size of the walls. One gram of glass ball has a surface about the size of a fingertip. But split the same weight into a heap of tiny grains, and the surface grows tens of thousands of times over. It is the same idea as tearing paper into smaller and smaller bits: the total length of the torn edges keeps growing.
Why can it take water even from air that is not fully damp?
If water could only stick in a single layer on the walls, the amount it could take would be limited. Silica gel can take in more thanks to the second mechanism.
Normally, water vapour turns back into liquid only when the humidity reaches 100 per cent. Inside a very narrow pore, though, things are different. Water stuck to the facing walls tries to join up across the gap. So even when the humidity is below 100 per cent, water collects inside the pores first.
In Figure 2, try changing the width of the pore. The narrower the pore, the lower the humidity at which water fills it. The pores in silica gel are just narrow enough to begin filling at ordinary room humidity.
When a pore is 10 nanometres across, it does not fill until the humidity reaches about 80 per cent. Narrow it down to 1 nanometre, and it begins to fill even at humidity in the teens. Food-grade silica gel is said to be made with fairly narrow pores so that it can take water even from dry air.
Some drying agents for shoe boxes and closets have blue beads mixed in. These are "indicator beads", soaked with a chemical that changes colour when it takes up water. When they turn pink, it is a sign that the pores are almost full of water. Recently, more products use chemicals that change to a different colour instead.
Rice crackers and similar snacks sometimes contain a white powder or granules called "lime desiccant". It does not use pore walls. Instead, it takes up water by reacting chemically with it and turning into another substance. It gets quite hot during the reaction, so take care not to wet it or get it in your eyes. Check the label and throw it away following your local authority's rules.
Summary
Silica gel is the same material as glass, turned into a cluster of fine grains like a bunch of grapes. The wide walls folded up inside stick water molecules on one layer at a time. And inside the narrowest pores, water vapour turns back into liquid and collects even from air that is not fully damp. It is said to be able to store about a third of its own weight in water while still looking dry.
A drying agent works not through the "material" that absorbs water, but through the "shape" that traps it.
Hidden in one gram are walls equal to two tennis courts.
The way pore size changes how water behaves is also linked to why sugar and salt cake together. There, water gathered on the surfaces of grains is what sticks the grains to each other.
- Get a drying agent that contains blue indicator beads and split the beads between two small saucers. (You can also try step 3 with the drying agent from a bag of seaweed.)
- Put one saucer in an empty jar with a lid and close it. Leave the other out in a damp place such as a bathroom or kitchen. Every few hours, compare how the colours change.
- Drop one dry, clear bead into a glass of water. Hold your ear close, and you may hear small crackling sounds as the bead cracks.
The sound in step 3 is thought to come from water rushing into the pores all at once, with the heat and force that result making the bead creak. The beads are not edible, so try this out of reach of small children and throw them away afterwards. Do not try this with white powder lime desiccant.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from lower-secondary science to university courses
- Middle schoolCovered in lower-secondary science
- High schoolCovered in upper-secondary chemistry
- High school+Advanced upper-secondary material, or a textbook sidebar
- UniversityNot taught in high school; university courses (physical chemistry, surface chemistry)
- ResearchNot yet settled even at university level; what researchers are investigating right now
Middle schoolTerms: this phenomenon has a name
- Adsorption: molecules sticking to the surface of something. It is distinct from "absorption", where they soak into the interior.
- Silica gel: silicon dioxide (the same material as glass and quartz) made into a solid riddled with fine pores.
- Capillary condensation: water vapour turning back into liquid water inside very narrow pores, even when the humidity is below 100 per cent.
- Specific surface area: the surface area per gram. For silica gel it is said to reach several hundred square metres.
Middle schoolHigh schoolChecking with a formula: how many grams of water stick in a single layer on the walls?
First, we work out the weight of water when its molecules are packed in a single layer, with no gaps, across the walls. Then we compare it with the amount the gel can actually take. Finally, we compare that with the water in the air inside a seaweed bag.
| In symbols | m ＝ S × σ （σ ＝ m₁ ÷ a） |
| In words | Weight of water stuck on = area of the walls × weight of a single layer of water over 1 square metre |
| Where it comes from | It is simple counting: water molecules line up on the wall, each taking up its own patch of area. The weight of a one-layer film is the weight of one molecule, m₁, divided by the area a that each one occupies. |
What the symbols mean: m is the weight of water stuck on (in milligrams), S is the wall area of one gram of silica gel (in square metres), and σ is the weight, per square metre, of a film of water molecules one layer thick (in milligrams).
| Wall area of 1 g of silica gel | About 700 square metres (several hundred to 800, depending on the product) |
| Weight of a one-layer film of water | About 0.29 milligrams per square metre (assuming each molecule takes up about 0.1 square nanometres) |
| Amount of water it can actually take | About 35 per cent of its own weight in damp air |
| Water vapour that 1 litre of air at 25℃ can hold | At most about 0.023 grams |
| Area of one tennis court (doubles) | About 261 square metres |
| Weight of one layer of water (milligrams) | 700 × 0.29 ＝ 203 |
| Convert to grams | 203 ÷ 1000 ＝ 0.203 |
| Amount it can actually take (grams) | 1 × 0.35 ＝ 0.35 |
| How many times the one-layer amount? | 0.35 ÷ 0.203 ≒ 1.7 |
| How many courts is the wall area? | 700 ÷ 261 ≒ 2.7 |
| Water a 5-gram packet can take (grams) | 5 × 0.35 ＝ 1.75 |
| Water in 1 litre of air at 60% humidity (grams) | 0.023 × 0.6 ≒ 0.014 |
| How many litres of air's worth can the packet take? | 1.75 ÷ 0.014 ＝ 125 |
A single layer on the walls alone holds about a fifth of the gel's own weight in water. The amount it can really take is about 1.7 times that, and the difference is thought to be water that turned back into liquid inside the narrow pores (capillary condensation). And a 5-gram packet can take more than 100 times the water in the air inside the bag. That is how it keeps soaking up the moisture that seeps in through gaps in the bag, week after week.
High schoolHigh school+What the wall's "hands" really are, and the heat given off
High schoolThe surface of the silica gel wall is lined with "silanol groups", which are silicon atoms carrying an oxygen and a hydrogen. Water molecules bond to these silanol groups through hydrogen bonds. A hydrogen bond is a weak link between molecules, so heating makes the water let go, and the silica gel can be reused again and again. This is a major difference from lime desiccant, which takes water through a chemical reaction.
High school+When water vapour sticks to a wall, heat is given off, just as when water vapour turns back into liquid (heat of adsorption). When a dry bead is dropped into water, water rushes deep into the pores all at once, and that heat, together with the force of air being pushed out of the pores, can crack the bead. Humidity is "the ratio of the water vapour present now to the most the air can hold at that temperature", and it is called relative humidity.
UniversityThe Kelvin equation, and how surface area is measured (the BET method)
The calculation behind Figure 2 uses the Kelvin equation. Let r be the pore radius, γ the surface tension of water, V the volume of one mole of water, R the gas constant and T the temperature. Then the relative humidity at which a pore fills with water is, in natural-log form, −2γV ÷ (rRT). For water at 25℃, 2γV ÷ RT is about 1.05 nanometres, so a pore 2.4 nanometres across fills at about 42 per cent. In real pores, the radius is reduced by the thickness of the water film already stuck to the wall, so analysis uses a formula with this correction added.
The figure of "700 square metres per gram" for silica gel is found by adsorbing nitrogen gas at low temperature and using the BET equation (Brunauer–Emmett–Teller equation) to work out the amount in a single layer from the amount adsorbed. "Adsorption hysteresis", where the adsorption curve differs when humidity is raised and when it is lowered, is also known as a sign of capillary condensation. Pore sizes are sorted into "micropores" (under 2 nanometres) and "mesopores" (2 to 50 nanometres).
📖 Derivations and further reading: Kelvin equation (English Wikipedia) ／ Silica gel (Japanese Wikipedia)
ResearchWhat is still not clear
- How far is the Kelvin equation right in nanometre-sized pores? In pores only a few water molecules wide, the assumption that water is a smooth liquid breaks down. How narrow a pore the equation can handle is still being studied by experiments and computer simulations.
- Is water inside a pore the same as ordinary water? Water trapped in very narrow pores has been reported to change its freezing temperature and how easily it flows. The details of the mechanism are still debated.
- Materials that pull drinking water from the air In dry regions, research is advancing on materials that soak up water from the air at night and release it with the sun's heat by day. Researchers are searching for pore shapes that can pull more water from drier air.
In other words, what this article says is "an explanation of what is known so far". How water behaves inside very narrow pores in particular is a field where new discoveries keep coming.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Middle school | Science: "Water vapour and humidity", "Changes of state" | Humidity; water vapour turning back into water |
| High school | Chemistry: "Hydrogen bonds", "Silicon compounds" | Bonding between silanol groups and water; what silica gel is made of |
| High school+ | Chemistry: "Vapour pressure", "Changes of state and heat" | Relative humidity, heat of adsorption, why a bead dropped in water cracks |
| University | Physical chemistry, surface chemistry | Kelvin equation, BET equation, adsorption hysteresis |
| Research | Water in nanospaces, materials that harvest water from air | The research section |
| ― | Links to daily life | Keeping moisture out of food, shoes and cameras; how to dispose of drying agents properly |
- Wikipedia, "Silica gel" (Japanese edition)
- Wikipedia "Kelvin equation"
- Seiichi Kondo, Tatsuo Ishikawa and Ikuo Abe, Kyuchaku no Kagaku (吸着の科学, "The Science of Adsorption"), Maruzen
- S. J. Gregg, K. S. W. Sing, "Adsorption, Surface Area and Porosity", Academic Press
- National Consumer Affairs Center of Japan (国民生活センター), safety notice on accidents involving lime desiccants
※This article is a science explainer for general readers. The figures given are rough guides to help you understand the mechanisms. Drying agents are not edible. If one is swallowed or gets into the eyes by mistake, follow the product label and, if necessary, consult a medical institution or a poison information centre. Follow your local authority's instructions for disposal.