Water (H2O) consists of positive hydrogen and negative oxygen atoms. Bulk water however is neutral as these charges balance each other out.

The situation changes, however, near the surface of the charged capillaries

All building materials (brick, stone, mortar, glass, ceramics etc.) they all contain silica sand as their primary ingredient, which has a negative surface charge. This will attract opposite charges from the water - positive water and salt ions - which will result in an electrical charge distribution along the capillary walls known as the electrical double layer (EDL). In other words, the surface of the capillary walls charge up, resulting in a "battery effect".

The location of the electrical double layer (B) - at the boundary of a solid (A) and a liquid (C)

This thin nanometer thickness layer, the electrical double layer is responsible for the bonding of water onto the capillaries, which is electrostatic in nature and its intensity decreases exponentially with the distance from the capillary surface.

The attraction is strongest near the wall surface and decreasingly weaker further away from the surface. As a result, a thin water layer in the immediate vicinity of the surface, where the attraction is strongest, will not move but will be held tight against the capillary surface (fixed layer), while further away from the surface the water will gradually start flowing (mobile or diffuse layer). Hence the "double layer" name referring to its dual nature - a combination of a fixed and mobile sublayers.

Electrostatic charge distribution along the electrical double layer

Electrical double layers are formed in nature at ANY solid-liquid interface. The thickness of the electrical double layer is very small, typically less than 10 nm.

What we commonly refer to as "capillarity" or "capillary effect" is nothing else but the combined effect of various forces acting in the electrical double layer area. Although these forces are very small, on short distances they have a considerable effect, being significantly more powerful than gravity.

Due to the complexity of physical-chemical-electric processes occurring at molecular level, despite of all scientific advancements, many aspects of the double layer are still not fully understood but still being actively researched.

Summary

  • The surface of any solid submerged in water will acquire a surface charge (which is negative for most solids), which will result in a "battery effect" along its surface.

  • This nanometer thin dual-polarity layer developed at the boundary of any solid - liquid is known as the electrical double layer (EDL).

  • The electrostatic attraction of the oppositely charged molecules in the electrical double layer is responsible for the bonding of liquids to solids (adhesion forces).

  • There are a lot of forces at play in the EDL area, discussed in the next section.