colloid

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Colloids:

Colloids

Types of colloidal systems::

Types of colloidal systems: According to the interaction between particles of dispersed phase & those of dispersion medium: Lyophilic (solvent loving). Lyophobic (solvent hating). Association ( amphiphilic ).

1) Lyophilic colloids:

1) Lyophilic colloids Colloidal particles interact to an appreciable extent with the molecules of the dispersion medium (solvent loving ). Obtained simply by dissolving the material in the solvent ( due to the high affinity ).

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T he dispersed phase does not precipitate easily. The sols are quite stable as the solute particle surrounded by two stability factors: a- negative or positive charge b- layer of solvent If the dispersion medium is separated from the dispersed phase, the sol can be reconstituted by simply remixing with the dispersion medium. Hence , these sols are called reversible sols

2) Lyophobic colloids:

2) Lyophobic colloids Colloidal particles have very little or no attraction for the dispersion medium (solvent hating ). Colloidal particles: inorganic particles (e.g. gold, silver ,…) Dispersion medium: water . These colloids are easily precipitated on the addition of small amounts of electrolytes, by heating or by shaking. Less stable as the particles surrounded only with a layer of positive or negative charge Once precipitated, it is not easy to reconstitute the sol by simple mixing with the dispersion medium. Hence , these sols are called irreversible sols . Not obtained simply i . e. need special method for preparation

Methods to prepare lyophobic colloids::

Methods to prepare lyophobic colloids : A) Dispersion methods : Coarse particles are reduced in size by; 1 ) Ultrasonic generator 2) Electric arc. 3) Colloid mill

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1)Ultrasonic generator: Dispersion achieved by high intensity UG at frequency more than 20,000 cycles/second 2) Electric arc: Involves production of an electric arc within the liquid and dispersion achieved by intense heat generated by the arc so some metal of the electrodes dispersed as vapor then condense to colloidal particles . 3) Colloidal mill: • Material sheared between 2 rapidly rotating close plates. • Low efficiency & reduce the size of small proportion of particles only.

2) Electric arc::

2) Electric arc: Involves production of an electric arc within the liquid and dispersion achieved by intense heat generated by the arc so some metal of the electrodes dispersed as vapor then condense to colloidal particles.

B) Condensation methods::

B ) Condensation methods: Materials of sub colloidal dimensions are caused to aggregate in to particles with colloidal size range by; 1 ) Change in solvent. 2 ) Chemical reaction .

3. Association colloids::

3. Association colloids: Certain molecules or ions termed amphiphile (surface active agent SAA) are characterized by two distinct regions of opposing solution affinities within the same molecules or ions.

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At low concentration : amphiphiles exist separately (subcolloidal size) At high concentration : form aggregates or micelles (50 or more monomers) ( colloidal size )

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As with lyophilic sols, formation of association colloids is spontaneous , provided that the concentration of the amphiphile in solution exceeds the cmc . Amphiphiles may be 1. Anionic (e.g., Na. lauryl sulfate ) 2. Cationic (e.g., cetyl triethylammonium bromide ) 3. Nonionic (e.g., polyoxyethylene lauryl ether ) 4. Ampholytic ( zwitterionic ) e.g., dimethyl dodecyl ammonio propane sulfonate .

Comparison of properties of colloidal sol:

Comparison of properties of colloidal sol

Purification of colloids:

Purification of colloids a) Dialysis. b) Electro dialysis. c) Ultra filtration.

a)- Dialysis::

a)- Dialysis: Depend on difference in size between colloidal particles & molecular particles (impurities). Technique; use semi permeable membrane (e.g. collodion (nitrocellulose), cellophane). pore size of used semi permeable membrane preventpassage of colloidal particles & permit passage of small molecules & ions (impurities) such as urea, glucose, and sodium chloride, to pass through. A type of dialysis equipment; “ Neidle dialyzer” At equilibrium, the colloidal material is retained in compartment A, while the sub colloidal material is distributed equally on both sides of the membrane. By continually removing the liquid in compartment B, it is possible to obtain colloidal material in A that is free from subcolloidal contaminants

b- Electro dialysis::

b- Electro dialysis: Techbnique : An electric potential may be used to increase the rate of movement of ionic impurities through a dialyzing membrane and so provide rapid purification. A typical apparatus is shown in the figure. Application of electrical potential causes cations to migrate to the negative electrode compartment and anions to move to the positive electrode compartment, in both of which running water ultimately removes the electrolyte. Electrodialysis is carried out in a three compartment vessel with electrodes in the outer compartments containing water and the sol in the center compartment.

c) Ultra filtration::

c) Ultra filtration: Technique; Apply pressure (or suction) Solvent & small particles forced across a membrane while colloidal particles are retained. Pore size of the membrane can be increased by soaking in a solvent that cause swelling • e.g. cellophane swell in zinc chloride solution. e.g. collodion (nitrocellulose) swell in alcohol .

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