Surface Chemistry Notes, Absorption, Adsorption, Application and Uses

The branch of chemistry that deals with the study of the nature of the surface and the phenomenon that takes place at the surface is called surface chemistry

The boundary between two or more phases that exist together is called the interface. Or A surface forming a common boundary between two things (two objects or liquid or chemical phase). The interface between matter and air or matter and vacuum is called the surface. The properties of the molecules forming the interface are different from those in the bulk that these molecules are forming an interface phase.

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Type of Interface

Several types of interface can exist depending on whether the two adjacent phases are in a solid, liquid, or gas state. Liquid gas interface. Liquid-liquid interface. Solid gas interface. Solid-solid interface.Solid-liquid interface. There is no gas-gas interface because gases always mix spontaneously.

Importance of the Interface in Pharmacy

(a) Emulsion formation and stability (b) Penetration of molecules through biological (c) Adsorption of the drug onto solid adjacent in a dosage form. (d) The dispersion of insoluble particles in a liquid medium to form suspensions.

Absorption (Chemistry)

It is a bulk phenomenon in which one substance enters into another substance and gets distributed throughout the body is called absorption. When distributed throughout the body of other substances it is called absorption. And a testing method of absorption.

Example:- Chalk in water, Tea biscuit, sponge put in water it takes up water the water enters the whole body of the sponge.

Absorbable and Absorbent 

Absorbable and Absorbent

Absorbate: Is the substance that is being absorbed. and Absorbent: The substance which absorbs

Adsorption (chemistry) and Type of Adsorption

It is a surface phenomenon the phenomenon of accumulation of a substance on the surface of a solid or liquid is called adsorption. The phenomenon of concentration of gas or liquid at a solid surface is called adsorption attainment of equilibrium in adsorption is instantaneous it is an exothermic phenomenon.

Adsorbent:- The substance on whose surface adsorption occurs is called adsorbent.

Adsorbate:- The substance that gets adsorbed on the solid surface is called adsorbate. The substance that deposits at the surface is called adsorbate.

Desorption:- The process of removal of an adsorbate substance from the surface of the adsorbent is called desorption.

Example of adsorption:- (i) Adsorption of gas by charcoal in this example gas is adsorbate and charcoal is adsorbent. (ii) Adsorption of water by silica gel.

Positive and Negative Adsorption

Positive adsorption:- When the concentration of adsorbate is more at the surface as compared to in concentration in the bulk phase such adsorption is called positive adsorption.

Negative adsorption:- When the concentration of adsorption is less at the surface as compared to its concentration in the bulk phase is called negative adsorption.

Sorption definition

The process in which both adsorption and adsorption take place simultaneously is called sorption. In some cases, both adsorption and adsorption occur together and are not distinguished in such cases the substance gets unformulated and distributed into the bulk of the solid but at the same time, its concentration is higher at the surface than in bulk such a phenomenon is called sorption. The sorption of gases by metals is known as occlusion. For Example:- dyeing of fabric.

Mechanism of Adsorption

Consider a molecule present in the bulk of the solid or liquid which is surrounded by many other molecules balance inter-molecular force attraction results there is no net pull on these molecules. However, molecules near the surface are being attracted by molecules below it only therefore surface molecules experience a resultant downward attractive force within solid or liquid surface molecules are not surrounded from all sides.

  • Molecules present in the bulk of liquid are a balanced force of attraction such molecules are stable and have low energy.  
  • Molecules present at the surface are an unbalanced force of attraction it is unstable and has high energy.

The adsorption occurs to balance the unbalanced force of attraction of the surface molecules. When any species comes in contact with the surface molecules the surface molecules hold the other species and balance their unbalanced force. After adsorption unbalances, unstable and high-energy molecules convert the balance to stable and low energy.

Type of Adsorption

Based on the force of attraction B/W, the adsorbate and adsorbent adsorption are classified into two physical adsorptions and chemical adsorption.

(i) Physical adsorption or Physisorption:- The type of adsorption in which the particle of adsorbate is held to the surface of the adsorbent by the physical force of attraction. Such van der Waals force is called physical adsorption or physisorption. The force holding gas molecules of adsorbate to the surface of the adsorbent is weak. Example: adsorption of hydrogen or oxygen on charcoal. Adsorption of nitrogen on mics. 

(ii) Chemical adsorption or Chemisorption:- When the molecules of the adsorbate are held to the surface of the adsorbent by the chemical forces or chemical bond is called chemisorption. In chemisorption the force which holds the molecules of adsorbate to the surface of the adsorbent is strong.

It is characterized by the high heat of adsorption is the amount of heat evolved when one mole of an adsorbate (gas and liquid) is adsorbed on the surface of an adsorbent.

Example of chemisorption: (a) Adsorption of oxygen by a surface of metals (b) Adsorption of hydrogen on nickel.  

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Difference Between Physisorption and Chemisorption

(i) In physisorption molecules are attracted to the surface simply by Vander Waals force. but in chemisorption molecules are attracted by a chemical bond.

(ii) The force that holds molecules on the surface is weak in the case of physisorption but strong in the case of chemisorption.

(iii) In the case of physisorption heat of adsorption is fewer lies below 20 to 40 Kj /mole but in the case of chemisorption heat of adsorption is greater and lies below 40 to 200 KJ/mole.

(iv) Physisorption is reversible but chemisorption is irreversible 

(v) Physisorption is usually multi-layer but chemisorption is mono-layer.

(vi) Physisprption is not specific in nature but chemisorption is specific in nature.

(v) Physisorption usually occurs at low temperatures but chemisorption occurs at all temperatures.

Factor Affecting Adsorption 

There are two things required for adsorption adsorbate and adsorbent following are some factors that affect adsorption.

(i) Nature of adsorbent 

Adsorption is a surface phenomenon so adsorption depends on the surface area of the adsorbent. The greater the surface area rate of adsorption will be. Adsorption will be high on rough surfaces as compared to smooth surfaces. Adsorption of activated charcoal is high as compared to charcoal.

(ii) Nature of adsorbate 

In more cases adsorbates are gases. Those gas adsorbates which can easily liquefy the adsorption phenomenon for such gases are high. Easily liquefiable gas means having a higher tendency to become liquid which in turn should have a greater inter-molecular force of attraction.

The higher the critical temperature of gas the more easily it will be liquefied and hence adsorbed to a greater extent.

When critical temperature decreases liquefied will decrease and adsorption of such gas also decreases.

Effect of temperature

Adsorption is an exothermic process so when temperature increases adsorption will be decreased.

The graph shows that when temperature increases adsorption will be decreased.

Effect of pressure

A dynamic equilibrium exists below the adsorption gas and the gas in contact with the solid as stated so an increase in pressure leads to an increase in adsorption and a decrease in pressure cause desorption.

 Adsorption Isotherm 

The relation between the amount of substance adsorption by the adsorbent and the equilibrium gas pressure at constant temperature is called the adsorption isotherm. The extent of adsorption is usually expressed as u/m where x is the mass of adsorbate and m is the mass of adsorbent.

The pressure at which the amount of gas adsorbed becomes equal to the amount of gas desorbed so that the extent of adsorption becomes constant even though the pressure is increased this state is called the saturated state and Ps is called saturation pressure.   

Freundlich adsorption Isotherm Equation

The amount of gas adsorbed per unit mass of the adsorbent with pressure at a constant temperature. Freundlich suggested an empirical equation. 

        x/m = Kp1/n

x = mass of adsorbate and m= mass of adsorbent, K= Proportionally constant, P= Pressure of gas, n/1= power of pressure which is 1 to infinity.

A saturation pressure amount of gas adsorbed becomes equal to the amount of gas desorbed.

Amount of gas adsorbed = Amount of gas desorbed  

At saturation pressure when pressure has increased no change will occur in the extent of adsorption.

At low pressure

  x/m = Kp1/n

 x/m = Kp1/1

 x/m = Kp1 at low-pressure graph almost straight.

 At high pressure

At high pressure, the graph becomes parallel to x

x/m = Kp▫ , x/m ∝Kp▫

x/m = constant

At intermediate pressure:- x/m will depend upon the power of pressure which lies below 0 to 1

x/m = Kp1/n

n can take any whole number value which depends upon the nature of adsorbate and adsorbent.

Verification of Freundlich Adsorption Isotherm equation

x/m ∝ P1/n

x/m = kp1/n  (taking log on both sides)

log x/m = log k + log P1/n

log x/m = log k + 1/n log P

Y = C + MX

Use of Freundlich adsorption isotherm

It is used to determine the extent of the adsorption of gases by solids at low pressure. It is also used for the adsorption of solute from solution and has been found satisfactory when the pressure in the equation is replaced by concentration C of solute. 

/m = KC1/n

log x/m = log k = 1/n log C.

Limitation of Freundlich adsorption isotherm

It is purely empirical. it has no theoretical basis. It is valid over a certain range of pressure (it fails at the pressure of gas). The constant K and n vary with temperature.

Langmuir Adsorption Isotherm

Langmuir in 1916 derived a simple adsorption isotherm based on theoretical consideration. It was named after him. It is applicable for chemical adsorption.

Assumption

(i) Solid surface is homogeneous and has a fixed number of adsorption sites.

(ii) Each site cannot adsorb more than one molecule i.e. the adsorption of molecules is confined to a mono-molecular longer.

(iii) The adsorption is considered an equilibrium process blow condensation of adsorbate molecules on the adsorbent and their desorption.

(iv) The adsorbed gas behaves ideally in the vapor phase.

(v) There is no interaction below the adjacent adsorbed molecules.

(vi) The surface of the adsorbent is uniform energetically.

Application of Adsorption

(i) A large number of industrial processes like the synthesis of ammonia, manufacture of sulfuric acid (H2SO4), alcohol, etc. are catalyzed reactions where reactants are adsorbed on the surface of a solid catalyst.  

(ii) Softening of hard water by ion exchange is based on the adsorption phenomenon.

(iii) surface-active agents are widely used in washing, paints, lubricants, etc.

(iv) Removal of coloring material from various types of solutions by charcoal.

(v) Many adsorptions molecular are being used in volumetric analysis. and separation of inert gases.

(vi) In curing disease some drugs are adsorption on germs. And use in chromatography analysis.

(vii) Most drugs function through.     

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