---
title: "Define specific surface, diffuse layer"
subject: "Soil Mechanics"
chapter: 4
key: "semester_4/soil_mechanics:specific-surface-diffuse-layer-define"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/specific-surface-diffuse-layer-define
site: BCT Engineering
---

# Define specific surface, diffuse layer

Soil Mechanics · Chapter 4: Soil Structure and Clay Minerals

- Asked in: 2075 Bhadra, 1 marks
- Video: [3.4 Hydration of Clay Minerals, Diffuse Double Layer](https://www.youtube.com/watch?v=Rmn6TtORIPI) — dahaproject3
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/specific-surface-diffuse-layer-define
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-4

## Question

Define specific surface and diffuse double layer.

## Short answer

Specific surface is total particle surface area per unit mass or volume, increasing as particles get smaller (kaolinite about 15, illite about 80, montmorillonite up to 800 m²/g). Diffuse double layer is the layer of water and cations around a negatively charged clay particle, with cation concentration gradually decreasing with distance from the surface.

## Answer

- **Specific surface** is the total surface area of the soil particles per unit mass (m²/g) or per unit volume. It rises as particle size falls, e.g. kaolinite about 15 m²/g, illite about 80 m²/g, montmorillonite up to 800 m²/g.
- **Diffuse double layer** is the layer of water and positive ions (cations) around a negatively charged clay particle, in which the ion concentration decreases gradually with distance from the particle surface.

## Check yourself

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

1. Specific surface of soil is:
   A. Total surface area of particles per unit mass
   B. Surface area of the soil layer in the field
   C. Ratio of weight of water to solids
   D. Volume of voids per unit mass
   Answer: A. It is surface area per unit mass (m2/g) or per unit volume. (Key point: Specific surface)

2. Which mineral has the highest specific surface?
   A. Kaolinite (about 15 m2/g)
   B. Montmorillonite (up to 800 m2/g)
   C. Sand (about 0.01 m2/g)
   D. Illite (about 80 m2/g)
   Answer: B. Montmorillonite has particles that are very thin, giving up to 800 m2/g. (Key point: Mineral values)

3. As particle size decreases, specific surface:
   A. Decreases
   B. Becomes zero
   C. Rises
   D. Stays unchanged
   Answer: C. Smaller particles have more surface area per unit mass. (Key point: Trend with size)

4. In the diffuse double layer, ion concentration:
   A. Is highest at the free water
   B. Is uniform
   C. Increases with distance
   D. Decreases gradually with distance from the particle surface
   Answer: D. The cation concentration is highest near the negative surface and falls with distance. (Key point: Diffuse layer concentration)

## 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
- [Adsorbed water; diffuse layer formation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aadsorbed-water-diffuse-layer) — 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
- [Specific surface area and its effect](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aspecific-surface-effect-fine-soil) — Soil Mechanics, chapter 4
- [Two-layer profile stress diagram, 6 m sand](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asand-clay-profile-stress-diagram-capillary) — Soil Mechanics, chapter 6

## Related in other subjects

- [Double surface treatment materials and procedure](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Adouble-surface-treatment-material-procedure) — Transportation Engineering II, chapter 3