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Introduction |
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Surface chemistry is the study of processes that occur at the interface of two bulk phases. The bulk phases can be of the type solid-gas, solid-liquid, liquid-gas and liquid-liquid. There is no interface between gases because of their complete miscibility. The bulk phase can be solutions or pure compounds. |
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Adsorption |
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Adsorption is a surface phenomenon. In this process, accumulation or concentration of a substance takes place at the surface or interface as compared to the bulk phases. This happens because the molecules of the bulk phase at the surface are attracted only from below and from the sides as compared to the molecule inside the bulk of the phase. In the interior, molecules experience forces of attraction from all sides. |
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Difference between Absorption and Adsorption |
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In the process of absorption molecules of one phase interpenetrate uniformly among those of another phase to form a solution with the second phase. In adsorption, molecules of one phase are present in higher concentration at the surface of the second phase. Water vapor is absorbed by calcium chloride while it is adsorbed by silica gel. In the same manner, ammonia is absorbed by water and is adsorbed by charcoal. |
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Types of Adsorption |
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Depending on the nature of attractive forces existing between the adsorbate and adsorbent, adsorption can be classified as:
i) Physical adsorption
ii) Chemical adsorption |
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Adsorption of Gases on Solids |
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Adsorption of gases on solids depends on the following factors:
i) Nature of the gas (adsorbate)
ii) Nature of the solid (adsorbent)
iii) Specific area of the solid
iv) Pressure of the gas
v) Temperature
vi) Activation of adsorbent. |
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Freundlich Adsorption Isotherm |
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At equilibrium, pressure Ps, x/m reaches its maximum value, that is, no further adsorption takes place even if the pressure is increased. A saturation state has been achieved. |
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Langmuir Isotherm |
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Langmuir adsorption isotherm is based on the following assumptions.
i) Adsorption does not proceed beyond monolayer coverage.
ii) All the sites available on the adsorbent surface are equivalent and the surface is perfectly uniform, that is flat. |
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Adsorption from Solutions |
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Adsorption of solutes from solutions also takes place on solids. Process of adsorption is used to reduce colors from solution. For e.g., raw sugar solution is decolorised by charcoal. Chromatographic separation of a mixture of compounds in solution happens due to the different adsorption tendencies of the compounds on to the solid phase. Chromatographic methods are used in the analysis of environmental pollutants as well as in the pharmaceutical industry. As mentioned earlier, activated carbon is used extensively in water and waste-water treatment. |
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Catalysis and its Types |
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A catalyst is a substance that increases the rate of a reaction by providing an alternate path of lower activation energy. Since catalysts are not consumed in a reaction, very small quantities of catalyst are required to enhance a reaction rate. |
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Nature of Solid Catalyst |
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Heterogeneous catalysis is extensively used in chemical industry. Hence, development, selection and application of solid catalysts forms an important area in surface chemistry studies. The nature of the solid catalyst has profound effects on heterogeneous catalytic reactions. Solid catalysts can be metals, semiconducting oxides and sulphides, insulating oxides and acids. These substances can be used in the pure form or in the form of mixtures. The catalysts can be crystalline, microcrystalline or amorphous. |
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Properties of Catalysts |
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Catalytic activity
The activity of a catalyst depends on the strength of chemisorption. To be active, the surface of the catalyst should be extensively covered by the adsorbate, that is, the chemisorption should be strong. However, if the strength of the adsorbent-adsorbate bond becomes too strong then the activity of the catalyst declines because other reactant molecules cannot react with the adsorbate or because the adsorbate molecules become immobilized on the surface. It has been observed that the catalytic activity increases from group 5 metals to group 11 with maximum activity shown by group 7-9 elements of the periodic table. |
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Zeolites - Shape-Selective Catalysis by Zeolites |
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Shape-selective catalysis are those reactions that depend on the pore structure of the catalyst and the size of the reactant and product molecules. In such reactions, zeolites are used as catalysts. Zeolites are microporous aluminosilicates of the general formula Mx/n [(AlO2)x (SiO2)y].z H2O where n is the charge of the metal cation, Mn+. M is usually Na+, K+ or Ca2+ and z is the number of moles of water of hydration, which is highly variable. The characteristic of zeolites is the openness of the [(Al2)O2]n framework. In this framework, some of the silicon atoms are replaced by aluminium atoms. Zeolites are found in nature and they are also synthesized for catalytic selectivity. Because of the three dimensional cage like structure, zeolites can be used as ion-exchange materials and selective adsorbents. |
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Enzyme Catalysis |
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Enzymes are macromolecules, usually proteins, produced in living systems, which act as catalysts in physiological reactions. The striking characteristics of enzymes are their catalytic power and specificity. Enzymes have immense catalytic power; they accelerate reactions by factors of at least a million. Most reactions in living systems do not occur at perceptible rates in the absence of enzymes. A simple reaction like hydration of CO2 is catalyzed by the enzyme carbonic anhydrase. The transfer of CO2 from tissues into the blood and then to the alveolar air would be very slow in the absence of this enzyme. The enzyme can hydrate 105 molecules of CO2 per second, which is 107 times faster than the unanalyzed one. |
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Colloids |
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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. |
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Types of Colloids |
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Colloids are characterized
a) according to the state of the dispersed phase and the state of dispersion medium.
b) nature of interaction between dispersed phase and dispersion medium.
c) types of particles of the dispersed phase. |
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Mechanism of Micelle Formation |
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Micelles are formed by specific molecules which have lyophilic as well as lyophobic ends. Ordinary soap which contains sodium stearate (C17H35COONa) forms micelle in water. The stearate ion has a long hydrocarbon end that is hydrophobic (because it is nonpolar) and a polar carboxyl group (COO-) that is hydrophilic. |
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Cleansing Action of Soaps |
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The cleaning action of soap occurs when oil and grease are absorbed into the hydrophobic centers of soap micelles and are washed away. |
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Coagulation or Flocculation |
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Coagulation is the process by which a colloid precipitates out of a solution. The precipitation is brought about by induced aggregation. For e.g., an iron (III) hydroxide sol can be made to aggregate by addition of an ionic solution. A positively charged particle of iron (III) hydroxide gathers a layer of anions around it. The thickness of this layer is determined by the charge on the anions. The greater the magnitude of the negative charge, the more compact the layer of charge. For e.g., phosphate ions gather more closely to the positively charge iron (III) particle than do chloride ions. |
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Interaction between Colloids-Gold Number |
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It is known that lyophobic sols are unstable. They are however stabilized by adding stabilizers. When two lyophobic sols are mixed together, they may coagulate by mutual neutralization of electric charges or by destroying each others stabilizers. A lyophobic sol may be protected by the addition of a lyophilic sol. |
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Preparation of Colloidal Sols |
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Since lyophilic sols are quite stable, they can be easily prepared by shaking the lyophilic substance with the dispersion medium.
Examples are: Colloidal sols of gum, starch, gelatine and egg albumin. |
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Purification of Colloidal Sols |
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The colloidal solutions prepared by various methods usually contain electrolytes and other soluble substances as impurities. These impurities if not removed can destabilize the sols. |
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Properties of Colloidal Sols |
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Colloidal sols are biphasic in nature. It consists of the dispersed phase and the dispersion medium. In the colloidal solution, each particle is contained within its own surface boundary and therefore has a separate existence from the dispersion medium. |
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Emulsions |
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Emulsions are colloidal solutions in which both the dispersed phase and dispersion medium are liquids. Emulsions are broadly classified into two types.
i) Oil-in-water emulsion
ii) Water-in-oil emulsion. |
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Preparation of Emulsions |
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Emulsification is the process of making emulsions. Emulsions are made by shaking the dispersed phase and dispersion medium vigorously and then stabilizing the emulsion with an emulsifier. Most often soaps and detergents are added to stabilize emulsions. Stabilization is obtained by the coating of the drops of an emulsion by the stabilizer. This prevents the drops of the emulsion from combining together and separating out as a separate layer. Other common stabilizing agents are proteins, gum and agar. |
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Applications of Colloids |
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Colloids have uses in our daily life as well as in various industrial processes. Some of the applications where colloids are present are listed below.
1) Pharmaceutical industry makes use of colloidal solution preparation in many medicines. A wide variety of medicines are emulsions. An example is Cod Liver Oil.
2) Paint industry also uses colloids in the preparation of paints. |
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Conclusion |
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Adsorption is a surface phenomenon. In this process there is accumulation of a substance on the surface of a solid or a liquid as compared to the bulk of the solid or liquid. The substance which is adsorbed is called the adsorbate and the substance on to whose surface the adsorbate is adsorbed is called the adsorbent. In physisorption, adsorbate is held to the adsorbent by weak van der Waals' forces, and in chemisorption, the forces of attraction between the adsorbate and the adsorbent are chemical bond forces. All solids have the potential to adsorb gases. The extent of adsorption of a gas on a solid depends on the nature of the gas, nature of the solid, specific area of the solid, pressure of gas, temperature at which adsorption occurs, and activation of the adsorbent. Adsorption isotherm describes the relationship between the extent of adsorption and pressure of gas for a given temperature. Adsorption isobar gives the relationship between the extent of adsorption and temperature for a given pressure of gas. |
