Adsorption 5th Lect

8/8/2019 Adsorption 5th Lect.

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Adsorption

The free energy at the surface of the solid and liquid is due to the

resultant inward force of attraction acting on the molecules in the

surface . The molecules in the surface have unsatisfied forces

which will attract molecules of other substances . This will result

in a higher concentration of a substance at the interface than in

the bulk phase and the phenomenon is known as adsorption .

Adsorbent is a solid in which adsorption takes place .

Adsorbate is adsorbed substance .


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Adsorption at Liquid Interface

Certain molecules or ions are partitioned in favor of interface .

Their concentration at the interface then exceeds their

concentration in the bulk of the liquid leading to a decrease in

surface tension and surface free energy of the system . Such

phenomenon is termedpositive adsorption .

A solute that causes an increase in surface tension is known as a

surface inactive agent, e.g. glycerin , sugar and sodium chloride .

They are partitioned in favor of the bulk leading to negative

adsorption and a corresponding increase in surface free energyand surface tension .


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Gibbs Equation

The addition of a solute to a solvent may change a surface tension .

Solute that reduce surface tension are known as surface active agent .

A surface active agent is adsorbed or oriented at the surface or

interface so that its concentration is greater than in the bulk of the

solution . Gibbs quantitatively expressed the adsorption at an

interface as : c d

= - --- -----

TR dc

is, surface excess, the difference in conc. of the solute in thesurface layer and the bulk solution in mole cm2 .

T is the absolute temperature .

R is the gas constant .

d / dcthe rate of change of surface tension with conc., c


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HLB System

In dispersion of mutually insoluble liquid e.g. water and oil, a

surface active agent is adsorbed at the interface as a result of an

equal balance of polar and non polar groups . If a molecule is too

polar, it will dissolve in the polar liquid and exert no effect on the

interfacial tension . If the molecule is very non polar, it will dissolve

in the non polar liquid and exert no effect on interfacial tension .

The hydrophilic lipophilic balance, orHLB system, is useful

expressing the ratio of hydrophilic and hydrophobic characteristics

of a surface active agent . Surface active agent with a lowHLB areoil soluble while that of highHLB value indicates water solubility .


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Estimation ofHLB Value

The HLB of a number of polyhydric alcohol fatty acid esters, such

as glyceryl monostearate may be estimated by using the formula :

HLB = 20 ( 1 - S / A )

In which :

S is the saponification number .

A is the acid number of the fatty acid .


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Adsorption at Solid Interface

It occurs either from an adjacent liquid or gas phase .

Solid / gas interface :

The degree of adsorption of a gas by solid depends on :

1. Chemical nature of the adsorbent ( material used to adsorb the

gas ) and the adsorbate ( material is being adsorbed ) .2. Surface area of adsorbent .

3. Temperature .

4. Partial pressure of the adsorbed gas .

The types of adsorption are :

1. Physical adsorption ( Van der Waals adsorption )a. Associated with Van der Waals forces ( weak interaction

between a solid and vapor ) .

b. Reversible .


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Adsorption at Solid Interface ( Cont.. )

The removal of the adsorbate from the adsorbent being known as

desorption . A physically adsorbed gas can be desorbed from a

solid by increasing the temperature and reducing the pressure .

2. Chemical adsorption ( Chemisorptions' )

The adsorbate is strongly associated to the surface of solid by a

primary chemical bonds, which is irreversible unless the bonds

Are broken . A surface compound is formed and no more than amonomolecular layer is adsorbed .


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Adsorption Isotherm

The relation between the amount of gas physically adsorbed on a

solid and equilibrium pressure or concentration at constant

temperature yields an adsorption isotherm .

Freundlich isotherm :

Y = x / m = KP1/n

whereY is the mass of gas x adsorbed per unit mass m of

adsorbent .

P is the pressure .

K & n are constant .Logarithmic Form : Log x / m = log K + 1/n log P

When plotting log x / m against P, it yield a straight line with an

intercept equal to log K and slope equal to 1/n .


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Adsorption isotherm for gas on a solid

A . Amount ofx of gas adsorbed per

unit mass m of adsorbent plotted

against the equilibrium pressure .

B. Log of the amount of gas adsorbedper unit mass of adsorbent plotted

against the log of the pressure .

According to freundlich equation,

the amount adsorbed increases

indefinitely with increasing pressure

or conc. At high pressure, this is not

a fact and the equation does not apply .


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Langmuir Adsorption Isotherm

Langmuir derived an equation based on the kinetic consideration

that adsorption is dependent on the rate of evaporation and the

rate of adsorption of the adsorbate .

P / y = P / ymax+ 1 / bymax

Where y is the mass of gas adsorbed per gram of adsorbent at pressure

P and at constant temperature .

ymax is the mass of gas that one gram of the adsorbent can adsorb

when monolayer is complete .So that a plot ofp / y against p is a straight line from which ymax & b can

be calculated from the slope & intercept .


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Solid Surface Interface

If the pressure term, P, is replaced by concentration, C, the

Freundlich isotherm applies to the adsorption of a solute from

Solution by such a way :

x / m = KC1/n

Log x / m = log K + 1/n log C

K is the rough measure of relative adsorptive capacity for a given

adsorbate .


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Solid Surface Interface ( Cont.. )

Langmuir Equation :

C /x/m = 1 / bymax + C / ymax

Whereas C is the conc. of the drug remaining after equilibrium is

attained .

Ymax is the maximum amount of the drug which can be

adsorbed per gram of adsorbent .

b is the affinity constant and may be used as a measure

for relative affinity of the sorbate to sorbent .


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Factors Affecting the Amount adsorbed

1. The surface area of adsorbent

The amount adsorbed will be proportional to the surface area of

the solid and this may be increased by using the solid in fine state .

2. The temperature

Physical adsorption is an exothermic process, so an increase inthe temperature will decrease the amount adsorbed .

3. The partial pressure of adsorbed gas or the concentration of thesolution

4. Affinity between adsorbent and adsorbate

Adsorbents differ in their ability to adsorb substances . Largemolecules are more readily adsorbed than small ones . According

to Gibbs equation the greater the surface tension reduction the

greater surface excess .


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Factors Affecting the Amount adsorbed ( Cont.. )

5. Effect of the solvent

The adsorption of methylene blue from aqueous solution by

charcoal is greater than from a solution in ethyl alcohol . Since the

reduction of the surface tension by methylene blue is less in ethyl

alcohol than in water, the surface excess will also be less .

6. pH of the solution

The adsorption of non-electrolytes is not greatly affected by the

pH value of the solution . If the solute is electrically, charged, the

pH of the solution affects the degree of dissociation of the soluteand the undissociated form is more readily adsorbed than ions

formed from it .


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Application of Adsorption

1. The removal of objectionable odors from rooms & foods .

2. The operation of gas mask .

3. The measurement of the dimension of particles in a powder .

4. Decolorizing solutions .

5. Adsorption chromatography .

6. Detergency and wetting .

7. Adsorption of toxins from the G.I.T.


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Adsorption is not always a desired process

1. Aluminum hydroxide particles when administered orally with

oxytetracycline will adsorb the antibiotic on their surface so that

the antibiotic is not as readily available for systemic absorption

from the G.I.T.

2. Quaternary ammonium compounds e.g. cetylpyridinum

chloride, have been used as oral antiseptics in the form of troches .

in troches using talc as a lubricant, the cetylpyridinum chloride is

adsorbed on the talc and its antiseptic activity is reduced .

3. The antimicrobial activity of preservatives appears to be reducedin the presence of magnesium trisilicate in particular benzalkonium

chloride, chlrohexidene, phenyl mercuric nitrate, and

methylhydroxy benzoate .

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Adsorption 5th Lect