flexiblefullpage -
billboard - default

Adsorption: The Fourth State of Water (7 Minutes of BS)

Dr. Joe Lstiburek explains the fourth state of water: ADSORPED water, with a D. It exists on surfaces as tiny monolayers that fill pores and cause mold. It also explains why expansive soils expand.
October 17, 2023

Today, Dr. Joe Lstiburek of buildingscience.com is back to about water that doesn’t fit in a glass—the fourth state of water, ADSORPTION.

There are actually four phases of water:

  • Liquid

  • Solid

  • Vapor

  • and adsorped.

Adsorbed water—with a D—as opposed to absorbed, with a B.

ad·sorp·tion /adˈzôrpSH(ə)n/

"Adsorb water occurs when water molecules actually stick to a surface; think of golf balls sticking to a surface. You have one layer, a monolayer of adsorb water…

A monolayer of water is a layer of water one molecule thick.

and the second layer of water molecules sticks to the first layer, but they're actually sticking through the first layer to the surface.

And so they're not as tightly stuck as the first layer. And then you have a third layer and a fourth layer and, a fifth layer, a fifth monolayer, and we call this adsorb water.

And as the relative humidity increases, the layers of adsorb water increase.

Wait. Why?

Nobody knows why.

Vapor Pressure?

This has nothing to do with vapor pressure.

Is it relative humidity?

This is relative humidity. We don't know why that happens. We just know that it freaking does.

Now, as the number of monolayers increases, water flows from more of those layers to less of those layers; we call that process surface diffusion.

A deeper meaning for diffusion

This is different than the slippery layer on my back deck, where shade and moisture have harbored microscopic organic growth of tiny banana peels, which is why they’re slippery.

OK, that’s not true, but this is: It actually does happen inside the deck boards, on the surface of the wood cells where water used to be when the tree was growing.

Water covers the outside of the cells as a monolayer and additional monolayers can build up until the adsorbed water is back to back and belly to belly. This adsorb water in or on wood—or any organic material—is potential mold food.

Let me tell you about mold. Mold needs liquid water, yet mold can grow in environments where the relative humidity is 70 to 80%; where the hell is the liquid water?

In the water pipes?

What happens is that the water molecules stick to the surface. You have pores on the surface.

You have big pores, and you have small pores. The small pores get filled up with adsorb water before the big pores do, and just before they get completely filled, there's a depression. There's a curvature. They call that curvature—a meniscus after the Greek moon.

And Lord Kelvin showed that. You have a significant drop in vapor pressure when you have a curved surface, and that leads to capillary condensation in the pores.

A deeper meaning for condensation

Condensation is when water turns from vapor to liquid. We usually talk about condensation as a response to a dropping temperature. This form of condensation is caused by dropping pressure in the presence of porous material, like my deck or your drywall.

The capillary condensation provides the liquid that allows mold to grow on paper-face gypsum board at 70 and 80% RH, whereas it's not gonna grow on glass because it's not as porous until you get a hundred percent.

So, high humidity allows moisture to deposit itself in pores on a surface. It builds up layers until it almost fills the pore, at which point it becomes a monopuddle.


There's another thing that water does; it likes to stick to hydroxyls.

Not those old oreo-cookie knockoffs

—OH units.

An OH unit is a single oxygen and a single hydrogen bonded together.

Adsorption is important for soils engineering

This is a big deal with soils engineering. Expansive soils are a problem if they change moisture content.

Because clay is like a big hydroxyl layer cake.

So you want the expansive soil to either be always wet or always dry.


make it so that the water molecule can't stick to any hydroxyl.

Problems with clay soils happen when the soil gets wet because the soil molecules actually get bigger, causing slabs to shift and doors and windows to stop working. One solution is usually to keep them from getting wet, which can be a challenge.

So one of the guys that taught me this was nicknamed Dr. Dirt, and he was famous because he saved the Nellis Air Force Base’s runway.

Nellis Air Force Base is outside of Las Vegas and has one of the longest runways in the world—18,000 feet long. It can take a space shuttle.

It was wobbly-jobbly, and Dr. Dirt put a small drip irrigation system under the runway that made the soil always the same moisture content. Brilliant.

He fixed one of the Las Vegas casinos by, instead of a drip irrigation system, injecting sugar into the ground, and the sugar bonded to the hydroxyls, so the water molecules weren't able to get there.

That I call that a very sweet solution.

This is amazing stuff. I mean for, you know, simple solutions. I remember when I was flying back with him—wicked sense of humor. We were dealing with a subdivision failure in the mid-1980s with a production builder in Denver, Colorado. And then, you know, the soils were...


...and the engineers wanted to vent crawl spaces and vent the area under a basement slab to try to dry out the moisture, and there was no way of making that work. So Dr. Dirt proposed putting a polyethylene skirt. Eight feet wide around the entire perimeter of the foundation, out 18 inches below the dirt so the water would be drained away.

And then, I put in a continuous polyethylene ground cover. He called it a diaper; “Young Joseph, you're putting in a plastic diaper, but you need a skirt with that diaper.”

God, I love that guy.

And we love you, Dr. Joe.

This is Dan Morrison reminding you that you get paid for what you do and what you know. And while there’s only so much you can do, information is infinite.

7 Minutes of BS is a production of the SGC Horizon Media Network

catfish1 -