This post is a re-post from Building Science Corporation and Joseph Lstiburek From SEPTEMBER 26, 2017.
It is just shocking, just shocking that some molecules have a positive electrical charge and a negative electrical charge. Water is a molecule with one oxygen and two hydrogen atoms held together with a covalent bond at an angle of 104.5 degrees (Figure 1) resulting in a permanent dipole (Figure 2). The polar nature of the water molecule causes magic and mystery to happen with its interaction with other materials.
Water likes to stick to itself and stick to other things…it sure is sticky. Hydrogen bonding, among other forces, causes the water molecule to cling to other molecules resulting in adsorption. This has consequences. Many materials “can capture water molecules from the air and localize them on their surfaces…such water is said to be in the adsorbed state”.
Adsorbed water is kind of like a 4th state of matter (Figure 3). Apologies to the physicists for what comes next…a breach of protocol for sure. Here goes. Adsorbed water is not quite a liquid. But it is a liquid…sort of. It has weird properties. It sticks to surfaces in “monolayers” (Figure 4). The number of monolayers is directly related to relative humidity. Let me explain why this is weird. Relative humidity is not absolute humidity or vapor pressure. The sheer number of water molecules in the air adjacent a surface is not as important as the relative amount of water molecules in the air adjacent the surface compared to the total amount of molecules that can be in the air adjacent the surface. This is nuts….but it is so.