---
title: "Adsorbed water; diffuse layer formation"
subject: "Soil Mechanics"
chapter: 4
key: "semester_4/soil_mechanics:adsorbed-water-diffuse-layer"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/adsorbed-water-diffuse-layer
site: BCT Engineering
---

# Adsorbed water; diffuse layer formation

Soil Mechanics · Chapter 4: Soil Structure and Clay Minerals

- Asked in: 2078 Poush, 1+3 marks
- Video: [Absorbed water and Adsorbed water in soil ? Difference between Absorption and Adsorption.](https://www.youtube.com/watch?v=sM9Sj6e32hQ) — CiViL PEdiA
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/adsorbed-water-diffuse-layer
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-4

## Question

What do you mean by adsorbed water? Describe double diffuse layer formation.

## Short answer

Adsorbed water is held around clay particles by surface electrical attraction; near the surface it is dense and nearly solid, giving plasticity and cohesion. Negative clay attracts cations and dipolar water; strongly held cations form adsorbed layer, diffused cations form diffuse layer, together the double diffuse layer. Thickness increases with lower electrolyte concentration/valence and higher dielectric constant/temperature.

## Answer

#### Adsorbed water
Adsorbed water is the water held around a clay particle by the electrical attraction of the particle surface. Near the surface it is very strongly attracted, has a density higher than ordinary water and behaves almost like a solid. It is only partly removed by oven drying at 105 °C, and its influence decreases with distance from the surface. It gives clay its **plasticity and cohesion**.

#### Formation of double diffuse layer
1. A clay particle has a **negative charge** on its surface because of isomorphous substitution and broken bonds at the edges.
2. In water, the negative charge attracts **cations** (Na$^+$, Ca$^{2+}$, K$^+$) and dipolar water molecules. The cations are also hydrated.
3. The cations nearest the surface are held strongly, forming the **adsorbed (Stern) layer**.
4. Because of thermal energy, the cations further out are free to diffuse. Their concentration falls gradually with distance until it equals that in the free pore water. This is the **diffuse layer**.
5. The negative surface charge and the diffuse cloud of positive ions together form the **double diffuse layer**.

```text
 particle  adsorbed   diffuse layer
 -----|+ + +|+  +   +    +      (free)
 - - -|+ + +| +    +      +
 conc of cations decreases ->
```
**Thickness** increases as the salt concentration and cation valence decrease, and as the dielectric constant of the water and the temperature increase. Thick layers give more repulsion, dispersion, swelling and higher plasticity.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Adsorbed water is:
   A. Free water that drains by gravity
   B. Capillary water in sand
   C. Water held around clay particles by electrical attraction, behaving almost like solid
   D. Water vapour in air voids
   Answer: C. It is strongly held near the surface and only partly removed at 105 C. (Key point: Adsorbed water)

2. The negative charge on clay arises from:
   A. Cation exchange only
   B. Isomorphous substitution and broken bonds at edges
   C. Water pressure
   D. Chemical weathering of sand
   Answer: B. These two give the net negative surface charge. (Key point: Cause of charge)

3. Why do cations farther from the surface form a diffuse layer?
   A. They are repelled by water
   B. Thermal energy lets them diffuse while attraction weakens, so concentration falls with distance
   C. They are chemically bonded to the surface
   D. They are anions
   Answer: B. Thermal energy allows diffusion; concentration falls to the free-water level. (Key point: Diffuse layer formation)

4. Layer thickness increases with:
   A. Decrease in dielectric constant
   B. Increase in salt concentration
   C. Decrease in salt concentration and cation valence
   D. Increase in cation valence
   Answer: C. Lower concentration and valence expand the layer. (Key point: Thickness)

## Related in Soil Mechanics

- [Double diffuse layer in clay](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adouble-diffuse-layer-explain) — Soil Mechanics, chapter 4
- [Define specific surface, diffuse layer](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aspecific-surface-diffuse-layer-define) — Soil Mechanics, chapter 4
- [Diffuse layer; most swelling mineral](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adiffuse-layer-swelling-clay) — Soil Mechanics, chapter 4