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Colloidal Dispersion

Many of the materials we get to see around us such as gases, liquids, pure solids and solid solutions along with liquid crystals but we also get to see many other forms where the matter is not uniform throughout and are considered to be heterogeneous. The progress in colloid science was stimulated by several important developments which made us think over these materials which are not considered as pure or in pure state.

Highly monodisperse lattices with small diameters can be formed of various polymers by emulsion polymerisation. Since colloidal dispersion also exhibit a reversible phase transition in presence of non-adsorbing polymer the addition of sodium polyacrylate to a suspension of polymer matter causes particles to attract one another due to osmotic pressure difference.

The development of direct measurement of interactive forces increased the accuracy of the measurement. The analysis was speeded up more with the advent of scanning electron microscope.

 

Colloidal Dispersion Definition

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Colloid science is a rich discipline with a number of excellent detailing that cover the basics of colloid science in much greater detail. Colloid generally refers to the dispersed phase of a two component system in which the elements of the dispersed phase are too small to be easily observed by an optical microscope and whose motion is affected by thermal forces.

When the continuous phase or the suspending medium is liquid then these materials do not settle down easily and neither can they pass through a membrane and hence it appears as soft mass like gel.

There are examples like gel, emulsions, food materials, biological system and many colour coatings. They can either be liquid, or solid particles dispersed in a gaseous, liquid or solid medium. They can also exist as gaseous materials dispersed in liquids and solids. Apart from these association colloids are typically micelles formed from surfactant or block copolymers.

The colloidal particles dispersed in liquids exhibit astonishing properties and there are dispersions such as colloidal gold sol prepared by Faraday almost a century ago can still be found and addition of salt can cause rapid irreversible flocculation. For many dispersion the physical state or the stability or phase behaviour can be altered dramatically by modest change in composition and this complex behaviour arise from different forces that act among the particles which determine the spatial distribution and dynamics. The Brownian motion and dispersion forces would flocculate Faraday’s gold sol unless the electrostatic repulsion between the particles exist.

Doublets and more complicated structures formed during flocculation can have lifelong existence as the Brownian movement is almost nil.

Another type of transformation occurs when ions are removed from electrostatically stabilised system. Polymer lattice in an electrolyte solutions are milky white fluids but dialysis eliminates the ions and leads to iridescence due to Bragg diffraction of visible light from an ordered structure.

Colloidal crystals contains defects and dislocation which permit flow as ordered structures at finite but lower shear stress.

 Colloidal system display complex rheological behaviour related to their thermodynamic non ideal characters. Macromolecules recover from extensions several times their equilibrium dimensions which makes the polymer solution very elastic.

Changing the relative magnitudes of colloidal forces has many dramatic effects. With aqueous lattices, the lowering of ionic strength may increase the viscosity substantially as electrostatic repulsion comes into play. And in case the flocculated sols assume disordered structures which deform elastically under little strains and fracture at higher strains will flow like liquids.

Sedimentation and Brownian motion of colloidal particles also reflect the balance of inter particle and hydrodynamic forces. 
 These leads to five distinct points:
  • Clear fluid at the top
  • A region of particles settling at initial dilute conc
  • A dense disordered region of particles and still settling freely
  • An iridenscent ordered region
  • An opaque disordered sediment
The complex behaviour reflects a combination of kinematic processes stemming from concentration dependence of the settling velocity and thermodynamic factors responsible for the order and disorder transition.

Brownian motion is equally sensitive to interparticle forces but effects are very subtle.

The microscopic theory establishes the origin of dispersion forces as the flocculating dipoles induced in non-polar molecules by absorption of photons from background radiation field. These dipoles generate transient fields with spectra centered about absorption frequencies which polarises the surrounding material on wavelengths. 

Objects interact through the fluctuating electromagnetic field that exist in the interior of any absorbing medium and extends beyond its boundaries in the form of travelling waves radiated into surrounding medium and standing waves that eventually decay away from surface. These materials are fully characterised by their dielectric permittiveness which are considered as the analogue of molecular polarising characters.

Colloidal Dispersion Experiment

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Muddy water that we get to see in the field is a dispersion of generally a very small clay mineral and other grains of minerals in an aqueous solution which contain small amounts of a variety of organic and inorganic materials. The dispersion might be either stable or unstable.
  • A tiny fragment of sodium hexa meta phosphate is dissolved in 1 litre of distilled water in two beakers
  • The presence of lumps needs to be broken carefully so that there is no pieces left and is smoothened out completely.
  • Clay becomes uniformly dispersed throughout the extremely weak sodium hexa meta phosphate solution.
  • The dispersion is milky and the particles remain suspended for many hours.
  • This stable colloidal dispersion the inter particle forces must show repulsion and this would confirm the lack of agglomeration and particle uniformity concentration.
  • Slowly one of the beaker is stirred with salt.
  • The dispersion now looks unstable and more granular which also shows particles swirling inside the liquid.
  • The effect of adding the strong electrolyte NaCl to the clay dispersion made the attractive forces acting between the crystals.
  • The net negative charge on each particle is balanced by electrolyte ions of positive charge, better known as counter ions decrease with the increasing distance outwards from particle surface, combine with positive charged counter ions and form an electrical double layer sheathing the particle.
  • As the particles of any one clay mineral species in a given electrolyte have the same double layer is very thick, the dispersion is then found to be stable.
  • At a higher electrolyte concentration the double layer becomes thinned allowing particles getting affected by weak attractive Van der Waal forces.
  • This finally result in particle agglomeration and flocculate in the beaker where salt was added.

The ability of clay mineral species to engage in cation exchange is its cation exchange capacity which is considered to be a distinctive and measureable property. 

The significance of cation exchange is that any change in the pore fluid chemistry of a muddy sediment is likely to cause an alteration in physical properties of sediment as the extent and strength of electrical double layer depend partly on electrolyte.

Colloidal Dispersion and Suspension

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Colloids in a suspending medium are a type of mixture and suspension often refers to the mixtures where the dispersed phase particles are greater than colloidal size. Colloids occur naturally and colloidal motion are usually studied with the help of pollens and spores. The medieval period stained glass produces brilliant colours and almost everyone in alchemy were aware of the colloidal gold. 

Colloidal particles by virtue of the small size have a very large specific surface area and surface effects are very critical or important in colloidal suspension. Suspended particles in a liquid leads to charging of the surface either by surface acids or bases or by adsorption of free ions as its very typical in clays. These can also be done with the help of surfactants or polyelectrolytes.

Unless special precaution are taken the colloidal particles carry electrical charge and the presence of dissociated chemical groups on the surface of adsorbed ions and of free counter ions and added salt ions leading to a complex, structured electrostatic layer in solution near the particle surface.
Two colloidal particles interact when they approach closely enough and their respective electrostatic fields overlap and therefore colloidal interaction due to electrostatics is not simply electrostatic repulsion but a dominant force contributor acting between identical charged particles arising from excess osmotic repulsion due to excess number of ions in surrounding double layer.

Colloidal phase behaviour can become complicated by kinetic effects such as aggregation, coalescence, gelation and glass formation. Colloidal suspension is never realised in real term because the Van der Waal attractions are always present.

What Makes a Colloidal Dispersion Stable?

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A colloidal dispersion comprise a collection of small particles, droplets or bubbles of one phase having at least one dimension between about 1 and 1000 nm while the dispersed in the second phase.

Either or both phases may be in gas, liquid, solid or super critical phase states. Matter of colloidal size just above dimensions and overlapping with emerging field of nanotechnology exhibit properties which differ from those of constituent atoms or molecules.

The stability of dispersion can be defined as the constancy of number of particles in a unit volume and it may change as a result of aggregation, sedimentation and chemical reaction. These mechanisms are very important as many instabilities result in significant variations in properties and behaviour of dispersions.

The motion of dispersed particles is dominated by Brownian motion which usually overcome the particle sedimentation caused by gravity leading kinetic stability. The dispersion stability is strongly affected by nano particles aggregation and the stability dispersion may become worse with time due to aggregation.

Properties of Colloidal Dispersion

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The colloids exhibit few properties which are unique in characteristics. Properties like Tyndall effect, colligative properties, Brownian movement and electro osmosis are specific to only colloids.

Tyndall effect:
In around 1869 Tyndall observed that after light was allowed to pass through colloidal solution, the path through which the light has traversed gets illuminated. This occurs because light is scattered by the particles that are present in colloidal solution. 

The intensity of this illuminated scattered light depends upon the dispersed phase and dispersion medium refractive index. The more the difference between these two phase and medium, the brighter the light intensity.

Colligative property
In colloids the colligative properties depend completely upon the number of moles of solute present in a given solvent mass. As the colloidal particles are aggregation of molecules, the colloidal dispersion have lower value of osmotic pressure, lower depression in freezing point and lower elevation of boiling point.

Brownian movement
The continuous collision between colloidal particles and also the molecules of dispersion medium results in a zig zag movement which is popularly known as Brownian movement. These happens mainly because the particles are in constant motion and after each collision the kinetic energy of these molecules are exchanged with neighbouring particles.

Electrophoresis 
The colloidal particle movement which occurs due to the effect of electrical field is termed as electrophoresis. When colloidal particles are introduced to an electric field, the colloidal particles migrate to electrodes which are oppositely charged. These particles get neutralised once they reach the respective opposite charged electrodes.

Electro osmosis 
Once the electrophoresis has taken place in a colloidal system, the dispersion medium begins to move in an electric field and the dispersed particles are segregated by suitable membrane or means, and this phenomenon is better known as electro osmosis.

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