Electro osmosis ,colligative propertries of colloids ,electrokinetic properties.double layer theory

electro osmosis.that topics deals with colloids and their one of the colligative properties that is electro kinetic property.under the electrokinetic colligative property of colloids consist 2 properties mainly electrophoresis and elecoosmosis.the electro osmosis have several application properties.the electroosmosis is mainly deals with the charge of colloidal system and their movements opposite charges.electrical double layer theory.

1. Electro-osmosis

2. COLLOIDS “ Finely divided particles of any substances with diameter lying within 10-2000 A⁰ range dispersed in a medium constitute.” A colloidal system consist two phases 1.continuous phase or dispersion medium 2.discontinuous phase or dispersed or suspended phase Dispersed phase may not necessarily be a solid always .it may be a liquid or even a gas as well. Dispersion medium may be gas or a liquid or even solid.

3. Different colloidal systems Dispersion medium Dispersed phase Examples Gas Liquid Clouds ,mists, fogs Gas Solid Smoke, volcanic dust Liquid Gas Foams ,whipped cream Liquid Liquid Emulsions ( milk, cod liver oil) Liquid Solid Starch ,proteins, arsenic sulphide ,gold Solid Gas Adsorbed or occluded gases Solid Liquid Jellies, gels, cheese Solid Solid Coloured precious stones ,rock salt

4. True solutions or molecular solutions : when a few crystals of a water soluble substances, like sugar or sodium chloride are added to water ,they dissolve to give a homogenous solution. The particles of a solution are invisible and do not settle on standing. Suspension : if an insoluble substance like barium sulphate or clay or sand ,is added to water the particles are large enough to visible ,if not to a naked eye at least under a microscope. They settle down standing for some time. Colloidal solution : in between these two extremes, lie the particles which are bigger than molecules but are too small to be visible even under a most powerful microscope.

5. Electrical properties of colloids The most important property of colloidal dispersion is that the particles carry an electric charge. All particles in a given colloidal system carry the same charge & dispersion medium has an opposite and equal charge , the system at a whole being electrically neutral. The presence of similar charges on colloidal particles is largely responsible for stability of system. Mutual force of repulsion between similarly charged particles prevent them coagulation or aggregation when they come closer.

6. The origin of these charges may be either external or internal When the charges are acquired by adsorption of positive or negative ions from the medium is called external Charges arise due to active acidic or basic groups present in the inherent structure of macromolecule which remain exposed on the surface ,in such a case the origin is said to be internal .

7. The origin of charges on colloidal particles The origin of charge on colloidal particles has not been completely understood. Frictional electrification : the origin of charge on the colloidal particles may be due to frictional electrification.it is believed due to rubbing the dispersed phase particles and with those of dispersion medium result in some type of charge on colloidal particles of the sol. this view is not satisfactory. Electron capture by colloidal particles: it is believed that the colloidal solutions prepared by Bredig’s arc dispersion method acquire a charge by electron capture, this view is not valid in all cases. Dissociation of surface molecule : the dissociation of surface molecule leads to electric charge on colloidal particles of the sol.

8. Eg: consider aqueous solution of soap C₁₅H₃₁ COONa C₁₅H₃₁ COOˉ + Na+ here in the cation passes in to the solvent while the anion have a tendency to form negatively charged aggregates due to weak interaction forces present in the long hydrocarbon chains. These aggregates are of colloidal dimensions & are negatively charged. This is not valid for colloidal solution of non electrolytes such as clay, smoke etc….

9. 4 . Presence of some acidic or basic groups in colloidal solution : Colloidal particles may acquire electric charge due to the presence of certain acidic or basic groups in colloidal solution. # protein molecules give rise to formation of colloidal solutions. Thus the particles of protein sol either have positive or negative charge depending upon the pH of medium. but in the case of an iso - electric point the protein will exist in as uncharged molecules.

10. 5. Selective adsorption of ions from solution : colloidal adsorb preferentially positive or negative ions present in the dispersion medium. When two or more ions are present in the dispersion medium the selective adsorption of ions common to the colloidal particles take place resulting in the formation of positively charge or negatively charged particle in colloidal solution. In simple words ,the electric charge on the colloidal particles essentially originates by the selective adsorption of ions common to the colloidal particles from the dispersion medium. “Commonly believed , the charge on the colloidal particle is due to preferential adsorption of either positive or negative ions on their surface.”

11. Electrical double layer Consider a spherical colloidal particle with a negatively charged surface. This polyelectrolyte will tend to attract the positive ions from the dispersion medium around it self. As a result the macromolecule will soon be surrounded by an ion atmosphere. a region in which statistical preference for ions of opposite sign. Based on this concept Helmholtz develop correct & formulated idea of rigid double layer . With more contribution from Gouy,Debye & Huckel a modified theory was proposed to explain the charge distribution around a macromolecule, According to electric double layer theory “ the ions of charge, opposite to that of the surface concentrate & tend to orient as immobile layer on surface.as a distance from the surfaces increases mobility of the ions increased & the ion layers become more & more diffused”

12. According to modern views “when a solid is in contact with a liquid a double layer of ions appears at the surface of separation” One part of double layer is fixed on the surface of solid. This is known as fixed part of the double layer. The second part consist of a diffuse or a mobile layer of ions which extends into the liquid phase. This layer consists of ions of both the signs but its net charge is equal and opposite to that on the fixed part of double layer. In colloidal system ,the ions carrying the opposite charge of counter ions, that present more in diffuse portion of the double layer, giving a net opposite charge to this layer.

13. Zeta potential The existence of charges of opposite signs on the fixed & diffuse parts of the double layer leads to the appearance of a difference of potential between the two layers. This difference of potential between the two layers is known as zeta potential or electro-kinetic potential. ζ D --- dielectric constant of dispersion medium U --- mobility of colloidal particle η --- viscosity  

14. Flocculation of colloidal solutions The zeta potential decreases when an ion of opposite charge to that of the colloidal particle is adsorbed. This reduces the mutual repulsion between the similarly charged colloidal particles.as a result , the colloidal particles easily come closer to one another to form a bigger aggregates. This phenomena lead to changing colloidal state to suspended state is known as coagulation or flocculation of colloidal solution.

15. Electro-kinetic properties of colloids Since the solid particles & the liquid medium carry opposite charges ,when an electric field, the particles & the liquid will migrate on opposite directions. In an experiments particle can move but not the medium --- Electrophoresis. Medium can move but not the particles ---- Electro osmosis

16. Electro osmosis Electro osmotic flow ( EOF) (synonymous with electro osmosis or electro endosmosis ). The movement of dispersion medium under the influence of an electric field keeping the disperse phase stationary is called electro osmosis. Electric osmosis is one of the electric properties of colloidal system. Colloidal system is electrically neutral . Consequently logical to conclude that the charge of disperse phase must be balanced by an equal and opposite charge on the dispersion medium.

17. Here the movement of dispersion medium occurs towards one of the electrodes in an electric field and solid disperse particle remain stationary. Electro osmotic flow is most essential component in chemical separation techniques. Electro osmotic flow can occur in natural unfiltered water as well as buffered solution.

18. history Electro osmotic flow was first reported in 1809 by F.F.Reuss ( empirical society of naturalists of Moscow). He showed that water could be made to flow through a plug of clay by applying electric voltage. Clay is composed of closely packed particles of silica & other minerals & water flow through the narrow spaces between these particles. Any combination of electrolyte & an insulating solid would generate electro osmotic flow. Flow speeds are typically only a few millimeter per second.

19. The electro osmotic flow is caused by Coulomb force , induced by an electric field on net mobile electric charge in a solution. Because the chemical equilibrium between solid surface & an electrolyte solution typically leads to the interface acquiring a net fixed electrical charge.it mainly interface the electrical double layer or Debye layer . When an electric field is applied to the fluid ( usually via electrode placed at inlets & outlets ) the net charge in the electrical double layer is induced to move by the resulting coulomb force. The resulting flow is termed as Electro osmotic flow .

20. Working of electro osmosis (1) Observing the electro osmosis by a fixing gel material dispersed phase in a U-tube , separating two portions the liquid dispersion medium. The whole tube is now filled with solvent. Thickened form of gel cannot move but it can allow the solvent molecule travels through it. When an electric connection is made between the two arms of the U-tube through a pair of electrodes the solvent is seen to rise in one of the arms. If the solvent is water, the water will flow through gel towards the negative pole because the water acquire positive charge.( porous gel membrane behave as an set of capillary tubes). During the flow of water through the gel there is a preferential adsorption of OHˉ ions by the walls of the capillaries leaving H+ ions to the charge the water molecular positively.

21. (2) A simple apparatus for studying electro osmosis The colloidal system placed on the central compartment A which is separated from the compartments B & C filled with water, by the dialyzing membranes M & M’. The water in the compartments B & C also extends to the side tubes T & T’. When a potential difference is applied across the electrodes held close to the membranes in the compartments B & C. If the particle carry positive charge ,the water will have negative charge .then move towards anode and hence water level rise. Or if particle carry negative charge water will move towards the cathode.

22. applications Electro osmotic flow commonly used in microfluidic device, soil analysis & chemical analysis

23. Thank you by anand p p