On the basis of the diffusion rate of liquid and solution through the animal membrane, they can be of two types. crystalloid and colloids. Those substances which can be obtained in the crystalline form, are easily dissolved and diffuse through the membrane are termed as crystalloid. For example, sugar, urea and salt.
While substances like starch, gum etc which cannot be diffused through the membrane easily are known as colloids. It is well known, that solutions are homogenous systems while suspensions are heterogeneous systems, i.e., they consist of more than one phase. In between the extremes of suspensions and solutions lies the colloidal system.
Colloids appear homogenous like a solution, but they consist of comparatively large particles of one substance dispersed throughout another substance (dispersion medium).
- In a solution, the particles are ions or small molecules.
- In a colloidal system, the dispersed phase may consist of particles of a single macro molecule or an aggregate of many atoms, ions or molecules.
- Although colloidal particles are larger than simple molecules, they are small enough to remain suspended.
- Colloidal particle sizes range from about 10Ao to about 1000 Ao i.e., 1nm to 1000 nm. It is well known, that solutions are homogenous systems while suspensions are heterogeneous systems, i.e., they consist of more than one phase. In between the extremes of suspensions and solutions lies the colloidal system.
Colloidal state is "the state of matter in which the size of constituent particles is in between 1 to 1000 nm, because of which they can pass through a normal filter paper but cannot pass through the animal or vegetable membrane."
The non-diffusibility of colloids compare to true solution is due to the difference in particle size. Since the constituent particles are bigger compared to true solution, they cannot pass through membrane.
On the other hand, the particles in colloids are smaller than the particles present in a suspension, hence they cannot precipitate like a suspension. The main differences between true solution, colloids and suspension are as follows.
||<10-9m or 1nm
||More than 1000nm
|Pass through the filter paper
||Pass through filter paper but not through animal membrane
||Cannot pass through filter paper or animal membrane; particles are visible
||Particles are invisible
||Particles can be observed by ultra-microscope
||Particles are visible
- The colloids can be differentiated from true solutions and suspensions by using their different properties. But the difference in properties is because of their constituent particles only.
- If the particles present in any solution are very small, they can easily pass through filter paper as well as any membrane, just like a true solution.
- But if the particles are big enough, they cannot pass through any membrane and filter paper as in suspension.
- If the particle size in intermediate in between these two extremes, they make colloids.
- The colloidal particles size is in the range of 1-1000 nm, so they can pass through the filter paper and but cannot pass through the animal membrane or vegetable membrane.
- While the particles size of true solution is less than 10-9 meter or 1 nm so particles can be pass through filter paper as well as any membrane.
- In case of suspension, the particles are bigger than 10-9 m, hence they settle down at the bottom of the beaker and separate out as a residue on filter paper.
As we know, in a normal solution, there is a substance dissolved in another substance. The substance which is present in a smaller amount is known as solute, and the other is termed as solvent. Hence a solute dissolved in solvent to make a solution. In the same way in colloids, the term solute is referred to as dispersed phase and solvent as dispersion medium. Hence colloids are also termed as colloidal solution or colloidal suspension.There are different ways to classify colloids. Colloids can be classified -
1. Based on physical state of dispersed phase and dispersion medium
- Based on physical state of dispersed phase and dispersion medium.
- Based on the nature of interaction between both phases.
- Based on the type of particles of the dispersed phase.
Out of the three physical states, dispersed phase and dispersion can be any one. Hence there are eight possible combinations of these three states of matter and each combination is named as a certain type of colloid.
2. Based on the nature of interaction between both phases
On the basis of interaction between both phase and medium, colloids can be two types.
3. Based on the type of particles of the dispersed phase
- Lyophilic colloids: In such type of colloids there is an affinity between dispersion phase and dispersion medium. For example, when gum or starch are mixed with a dispersion medium like water, they directly form colloidal sol. This sol is also called as colloidal sol or lyophilic sol. If water is present as a dispersion medium it is termed as hydrophilic sol. Lyophilic sol are reversible in nature i.e. precipitate can remix and convert to colloidal again. For such type of sol the viscosity is very high and surface tension is low. Lyophilic sols are quite stable due to affinity between dispersion phase and dispersion medium, and hence not easily coagulated.
- Lyophobic colloids: when there is no affinity between dispersion phase and dispersion medium, we cannot prepare sol by simple mixing but certain preparation methods required for that. Such type of colloids are termed as lyophobic colloids and if the medium is water then they are called hydrophobic colloids. Since they do not have affinity between phase and medium, they are unstable and easily coagulated. They are irreversible in nature and cannot re-form after precipitation. When a dispersion phase is added to dispersion medium to form lyophobic colloid, there will be no change in viscosity as well as surface tension. For example; when a substance like metal sulphide is mixed with dispersion medium, they form lyophobic colloids. If the dispersion medium is water, they called as hydrophobic colloids.
Third type of classification is based on the range of particle size of dispersed phase in given colloids. On the basis of particle size, colloids can be of three types.
- Multimolecular colloids: When a large number of small particles (dispersion phase) aggregate to form a large size molecule or particle having the size in the colloidal range, it known as Multimolecular colloids. Here, the atoms or particles are held together by weak van der Waals force of attraction. For example; gold sol contains particles of various sizes having several atoms. Another example is sulpher solution.
- Macromolecular colloids: In this type of colloids, there are macromolecules like polymers which act as the dispersion medium. These macromolecules when dissolved in suitable dispersion medium, form a solution in which the molecules of substances i.e. the dispersion phase have size in the colloidal range. Polymers like starch, proteins, cellulose are formed from such type of colloids. These colloidal solutions are quite stable and resemble true solutions in many respects.
- Associated colloids: These colloidal solutions are also known as Micelles. In these colloids, when a dispersion phase dissolves in dispersion medium at low concentration, they behave as normal strong electrolyte. But as the concentration increases, they show colloidal state properties due to the formation of large particles which are aggregates of small particles present in solution.
At high concentration, the van der Waal force of attraction is strong between dispersion phase and dispersion medium. These aggregate particles called as micelles and the temperature associated with the formation of micelles is called as kraft temperature (Tk). The concentration at which the micelles formation starts is called as critical micelle concentration (CMC). The best example of associated colloids is soap and synthetic detergents in water.
Different examples of colloids are as follow.
|Type of colloid
||Colored glass, gem stone
||Paints, cell fluids, muddy water, ink, detergent, rubber
||Cheese, butter, jellies
||Milk, hair cream, Mayonnaise, cosmetic lotion, lubricant
||Fog, mist, cloud, insecticide spray
||Pumice stone, foam rubber, marshmallow, styrofoam, insulation, cushioning
||Froth, whipped cream, soap lather
Purest colloids are the finest colloidal products with the highest level of effectiveness by using colloidal silver or other colloidal metals. Purest colloids are called as meso colloids. These colloids consist of meso particles which have a large surface area compared to colloids produced by other methods. The process to develop meso particles is known as meso process.
In meso process, at constant concentration of metal particles, as the particles decreases, the particle surface area increases. With increase in the surface area, the chemical reactivity of particles increases as more surface area is included in the reactions. Hence, with increase in the surface area of particles, the effectiveness of colloidal solution increases.
The surface area of particles is written as cm2
/mL and expressed in cm2
/mL. The colloids with high purity are produced by using mesoprocess. In this process the metal is converted into a form of its individual atoms which further converts into particles consisting of nine atoms each.
These particles are dissolved uniformly in pure deionized water to produce a colloidal suspension. Due to small negative electric charge imparted to each particle which is known as zeta potential, the particles remain suspended in water indefinitely.
Hence there is no need to add any binder or additive to maintain the stability of meso colloids. They are simply pure water and pure metal. For example, a colloidal solution of silver = contains two different forms; silver nanoparticles and silver ions.
In meso silver there is 80% mesoparticles and 20% silver ions, hence the total silver concentration becomes 20%. There are many types of purest colloids produced by using meso process like
- Meso silver
- Meso gold
- Meso copper
- Meso platinum
- Meso palladium
- Meso iridium
- Meso titanium
- Meso zinc
- The main difference between crystalloid and colloids is their ability to diffuse through the membrane.
- Crystalloids pass through filter paper as well as through animal membrane while colloids cannot pass through animal and vegetable membranes due to large particle size.
- The same concept is applicable in intravenous fluid also which exists in between venous. These fluids can be classified on the basis of their ability to cross the membrane or barriers which separate different body fluid compartments like between intra vascular and extra vascular (interstitial) fluid compartments.
- Since colloidal particles are large enough to pass the diffusion barrier , they are infused in to the vascular space and show a great tendency to stay in intra vascular space to enhance the plasma volume compared to any crystalloid.
- For example; with 5% albulmin which is a colloid fluid, the plasma expansion is nearly twice compared to the equivalent volume of isotonic saline.
- The large solute particles which cannot move freely across barriers between fluid compartments can create a force which draws water into the large solute compartment.
- This force which works opposite to the hydrostatic pressure is known as the colloid osmotic pressure (COP) or oncotic pressure.
- The ability of each colloid fluid to expand the plasma volume is directly proportional to the colloid osmotic pressure COP. Hence, the higher the magnitude of colloid osmotic pressure, the higher the volume expansion will be. The colloid osmotic pressure of plasma is 25 mm Hg. If the oncotic pressure of a colloid fluid is greater than the oncotic pressure of plasma, the plasma volume expansion exceeds the infused volume.
- For example;25% albumin solution has oncotic pressure of 70 mm Hg and a plasma volume expansion is 4 to 5 times than the infused volume.
- The other examples of colloid fluids are hetastarch (A synthetic colloid available as a 6% solution in isotonic saline) , pentastarch ( Low-molecular-weight-derivative of hetastarch) and dextrans ( Glucose polymers produced by a bacterium incubated in a sucrose medium).