In this article we will discuss about:- 1. Meaning of Colloid 2. Properties of Colloid 3. Types 4. Soil Colloids.
Meaning of Colloid:
Graham (1861) divided the soluble substances into two classes—crystalloid and colloids depending on their power to diffuse through vegetable and animal membranes. Crystalline substances such as sugar, salt and urea which in solution diffused rapidly through animal or vegetable membranes were known as crystalloids. The other class of substances like gelatin, starch, glue etc. found in amorphous state which exhibited little or no tendency in solution to diffuse through animal or vegetable membranes were called colloid (Gk., Kolla—glue, eidos—like).
Examples – Clay, egg albumin, gelatin, glue, clouds, fogs etc.
Colloid is referred as the dispersed system. The substance in solution is termed as the dispersed phase and the medium in which the particles are dispersed is called the dispersion medium.
Colloid exhibit a number of properties.
Out of which, the important properties are as follows:
Colloidal particles possess the power of adsorbing gases, liquid and even solids from their suspension. The adsorption of ions is governed by the type and nature of ion and the type of colloidal particles. In case of cations, the higher the valency of the ion, more strongly it is adsorbed. As for example, divalent ions like calcium (Ca++) and magnesium (Mg++) are held more strongly than monovalent ion like sodium (Na+) and potassium (K+). Due to this property, soil is able to hold water and nutrients and keep them available to plants.
(ii) Electrical Charge:
Colloidal particles have an electrical charge; some positive and some negative. Colloidal clay is negatively charged and thus attacks positively charged ions (cations).
(iii) Tyndall Effect:
Dust particles floating in air form a colloidal suspension. When a strong beam of light is passed through such a colloidal suspension, the particles become visible and they appear bigger than they really are. This is due to diffusion of light by colloidal particles. Such an effect is termed as ‘Tyndall effect’.
(iv) Brownian Movement:
Colloidal particles when seen under an ultra-microscope, the suspended particles are found to be in a constant zig-zag motion. This motion is called Brownian movement after its discoverer Robert Brown (1827). This phenomenon becomes less prominent as the particle size increases.
(v) Precipitation or Coagulation:
Colloidal solutions can be easily precipitated or coagulated by adding small amount of electolytes. ‘The higher the valency of the active ion, the greater is its precipitating action.
(vi) Osmotic Pressure:
The osmotic pressure of colloidal solution is very small. Because colloidal particles are aggregates of a large number of molecules.
(vii) Adhesion and Cohesion:
Clay particles possess the properties of adhesion and cohesion. The force of adhesion and cohesion is developed in the presence of water.
Soil colloidal particles possess the property of plasticity. Due to this property, clay colloid can be moulded in any shape.
Types of Colloids:
Colloids may be of two types as follows:
(i) Lyophilic or hydrophilic (water loving) Colloid- It will have the love for solvent on its surface e.g. gelatin, gum and certain other organic substances.
(ii) Lyophobic or hydrophobic (water hating) Colloids- It does not yield colloidal solution readily when brought in contact with water. It does not attack solvent on its surface. e.g. metal, metal sulphide, metal hydroxide and other inorganic substances.
Various kinds of colloidal suspension are possible.
A few examples are as follows:
(i) Solid in liquid – White of an egg, ink, clay in water (Dispersion of clay particle in water).
(ii) Solid in gas – Smoke in atmosphere.
(iii) Liquid in liquid – Milk (liquid fat in water)
(iv) Liquid in gas – Clouds, fogs in atmosphere.
Soil colloids are of two kinds as follows:
1. Inorganic Colloids or Mineral Colloids:
The inorganic colloids are often described as clay colloid. In a broad ways, two groups of clays are recognized—the silicate clays, so characteristics of temperate region and the iron and aluminium hydrous oxide clays found in tropics and semi-tropics. The clay fraction of the soil contains particles less than 0.002 mm in size. Particles less than 0.001 mm size possess colloidal properties and are known as soil colloid.
(i) Shape and Size:
The early students of colloids use to suppose that the silicate clays are more or less spherical in nature, but later studies have shown that the silicate clays are plate like in structure and formed a laminated layer. The actual size and shape of minerals are determined by mineralogical organizations, the conditions under which they have developed. The minerals may be mica like and definitely hexagonal, irregularly plate or flake like, lath shapes blades or rod-like.
Whatever, the size, the horizontal axis is much greater than the vertical axis. Each crystal lattice of a mineral is composed of several laminated or plate like units which are bound together with a varying degree of tenacity. If the units are bound loosely, then the crystal of minerals swells easily on wetting and shrinks after drying. Two types of structural units are basis in the layer lattice structure of most clay mineral, namely tetrahedral unit and octahedral unit.
(ii) Surface Area:
The silicate clay minerals exhibit tremendously high specific surface area due to the following reasons:
(a) Fineness of divided state.
(b) Plate like structure of the fine particles.
(c) In some cases due to the presence of internal surface area i.e. in between the crystal units.
(iii) Electronegative Charge and Adsorbed Cations:
The minute silicate clay colloid particles, referred to as micelles (microcells) is negatively charges. Consequently, thousands of positively charged ions or cations are attracted to each colloidal crystals. In this way, they form an ionic double layer.
The inner layer is formed by the colloidal particles itself, the surface of which are highly negative in charge and the outer ionic layer is made up of a swarm of rather loosely held cations which surround and in some cases penetrate the particles. Thus a clay particle is accompanied by an enormous number of adsorbed cations.
All cations may be absorbed by clay micelles, but certain cations are especially dominated under natural conditions. In humid region, the clay minerals are dominated by hydrogen (H+), aluminium (Al+++ ), and calcium (Ca++), next is magnesium (Mg++) and potassium (K+) and sodium (Na+) comes in last portion, But in arid and Semiarid region and under well drained condition, Clay minerals are dominated by Calcium (Ca++) and Magnesium (Mg++), next to these are Sodium (Na+) and Potassium (K+) and Hydrogen (H+) comes in last portion.
Structural Organization of Different Silicate Clay:
The most important silicate clays present in the soil can be grouped in Kaolinite, montmorillonite and Hydrous micro.
In the Kaolinite group, the important minerals are Kaolinite, hallosite, anauxite, dicksite and several others. Of all the members of this group, Kaolinite is of greatest importance in soils. Kaolinite minerals are plate-like in nature. The mineral crystal is composed of flake crystal unit. These units in turn are composed of alternate layer of alumina and silica sheet. Since every unit contains one each of silica and alumina sheets, this group of clay is said to have 1 : 1 type of crystal lattice occurs commonly in soils.
The most prominent member of the 1 : 1 type of clay mineral is Kaolinite. The two sheets of each Crystal units of Kaolinite are held together by oxygen atoms which are mutually shared by the silicon and aluminium atom in their respective sheets. The flat crystal units, in turn, are held together rigidly by oxygen-hydrogen linkages. Consequently the lattice is fixed and no expansion ordinarily occurred between the unit when the clay is wetted. The water and cations do not enter between the structural unit of the micelle.
The effective surface of kaolinite is thus restricted to its outer face. This is one of the reason for its low adsorptive capacity for cations. That is why the cation exchange capacity of this type of mineral is comparatively lower than the mineral of montmorillonite group. Kaolinite is a dioctahedral minerals, and in pure kaolinite, the Al atoms occupy the same positions in all the layers.
The Kaolinite crystals usually are hexagonal with clean cut edges. In comparison with montmorillonite particles, they are large in size, the diameter generally ranging from 0.1 to 5μ (microns) with the majority falling within 0.2-2μ. As the structural units are held together with tenacity, the particles of kaolinite cannot be broken down very easily into thin sheets. In contrast with other silicate group, the plasticity, cohesion, shrinkage and swelling properties of kaolinite are very poor.
The cation exchange capacity of kaolinite is 3-15 milliequivalent per 100 gm. The external and internal surface area is 30 to 35 sq. m per gm. and 7-10 sq m. per gm respectively. The total surface area is 37-45 sq m. per gm. General formula of kaolinite is Al2O3 2SiO2 2H2O. This clays are formed in humid, warm and well drained tropics and subtropics.
Montmorillonite is most common clay minerals in soil. The most important mineral of montmorillonite group are montmorillonite, biedellite, nontronite and saponite. Of these, montmorillonite is the most important mineral in the soil. The flake like crystal unit of montmorillonite are composed of two silica layers and one aluminium layer. Hence its crystal lattice is 2 : 1 type. The alumina and silica are tenaciously bound to each other by mutually shared oxygen atoms. The structural units are loosely held by weak oxygen- oxygen linkage.
This types of minerals are expanding type i.e. they will swell on wetting and shrink on drying as the units are loosely bound. Due to the expansion, water moves between the crystal lattice. The montmorillonite crystals are easily broken down into very thin sheets. The average size of the montmorillonite crystals varies from 0.01 to 1.0 μ. Thus they are smaller than the average kaolinite crystal. This lattice is negatively charged and has adsorption capacity for cation 10-12 times more than that of kaolinite clays.
In this type of mineral, the properties such as swelling, shrinkage, cohesion and plasticity are also very high. The cation exchange capacity of montmorillonite is 80-150 milliequivalent (meq) per 100 gm. The internal and external surface area are 500-600 sq. m. per gm. and 80-150 sq. m per gm. respectively. The total surface area is 580-750 sq. m per 100 gm. The general formula is Al2 O3.4SiO2.5H2O. These clays are developed in arid regions, poorly drained and more of alkaline rocks.
iii. Hydrous Mica:
The important minerals of hydrous mica are illite and vermiculite. The structure of these clays are 2 : 1 silica and alumina type similar to the montmorillonite except that they have potassium ion holding adjacent layers. The potassium ion binds the illite units so tightly that water cannot penetrate between the layers freely. Thus it has moderate swelling. The properties like cation exchange capacity, swelling, shrinkage, cohesion and plasticity of illite are less than montmorillonite and more than that of kaolinite.
The average size of illite minerals varies from 0-2 μ. The cation exchange capacity of illite is 15-40 me. q. (milliequivalent) per 100 gm. The total surface area is 120-170 sq. m. per gm. The external and internal surface area are 50-70 and 70-100 sq. m. per gm. respectively. The general formula is 2AI2O3.5SiO2.5H2O.
Hydrous Oxides Clays of Iron and Aluminium:
Hydrous oxides are oxides containing associated water molecules. The most common examples are gibbsite (Al2O3.3H2O) and goethite (Fe2O3.H2O). In soil, gibbsite is probably the dominant aluminium oxide, goethite and limonite (Fe2O3.x H2O) are the most prominent iron hydrous oxides. The ‘x’ indicates that the quantity of associated water of hydration is different for different mineral. These clays have definite crystalline structures, the small particles may carry negative charges which attacks cations.
Cation adsorption is even lower than for kaolinite. Most of them are as sticky, plastic or cohesive as are the silicate clays. This account for much better physical condition of soils dominated by hydrous oxide.
The management of hydrous oxides clay of iron and aluminium are as follows:
(i) Organic manures should be added.
(ii) Ground rock phosphate or bone meal is to be recommended. The soluble phosphate should not be recommended as they get fixed. The organic acid releases PO4 from raw material and as soon as they release, they are absorbed by plants.
2. Organic Soil Colloid—Humus:
Organic colloid is represented by humus. The complex humus micelle is composed basically of carbon, hydrogen and oxygen rather than of aluminium, silicon and oxygen as are the silicate clays. The humus consists of a negatively charged anion called micelle. Each negative charge on the humic micelle attacks a univalent cation.
Humus is not as stable as clay and is thus somewhat more dynamic being formed and destroyed much more rapidly than clay. The major sources of negative charges are thought to be partially neutralized carboxylic (- COOH) and phenolic group associated with central units of varying size and complexity. The negative charge on a humic micelle depends on pH. The metallic cations like calcium (Ca++), magnesium (Mg+ +), potassium (K+) etc. are absorbed on the negative side.
As a result of which, these ions are not leached down from the soil and can be taken up by plant roots by the process of cation exchange. Humus absorbs large amount of water and swells up. The size of organic colloid is almost equal to montmorillonite. The cation exchange capacity (i.e. 150-300 meq. per 100 gm) and water holding capacity of organic colloid is much more than that of silicate clay. The soils that contain much organic colloid are fertile.