Lecture 2. Adsorption on the interphase of liquid-gas

Prepared by PhD Falfushynska Halina

Adsorption is a spontaneous processFor reaction or process to be spontaneous, there must be decreases in free energy of the system i.e. ΔG of the system must have negative value.Also we know, ΔG = ΔH – TΔSAnd during this process of adsorption, randomness of the molecule decreases which ΔS is negative. We can rewrite above equation as

Types of AdsorptionForces of attraction exist between adsorbate and adsorbent. These forces of attraction can be due to Vanderwaal forces of attraction which are weak forces or due to chemical bond which are strong forces of attraction. On the basis of type of forces of attraction existing between adsorbate and adsorbent, adsorption can be classified into two types: Physical Adsorption or Chemical Adsorption.Physical Adsorption or PhysisorptionWhen the force of attraction existing between adsorbate and adsorbent are weak Vanderwaal forces of attraction, the process is called Physical Adsorption or Physisorption. It takes place at low temperature below boiling point of adsorbate. As the temperature increases in, process of Physisorption decreases.

Chemical Adsorption or ChemisorptionWhen the force of attraction existing between adsorbate and adsorbent are chemical forces of attraction or chemical bond, the process is called Chemical Adsorption or Chemisorption. Chemisorption takes place with formation of unilayer of adsorbate on adsorbent. It has high enthalpy of adsorption

Physical Adsorption vs T and Chemical Adsorption vs T

Comparison between Physisorption and Chemisorption

Physisorption Chemisorption1.Low heat of adsorption usually in the range of 20-40 kJ mol-1

High heat of adsorption in the range of 40-400 kJ mol-1

2.Force of attraction are Van der Waal's forces

Forces of attraction are chemical bond forces

3.It usually takes place at low temperature and decreases with increasing temperature

It takes place at high temperature

4.It is reversible It is irreversible5.It is related to the ease of liquefaction of the gas

The extent of adsorption is generally not related to liquefaction of the gas

6.It is not very specific It is highly specific7.It forms multi-molecular layers It forms monomolecular layers

8.It does not require any activation energy It requires activation energy

Applications of Adsorption1. Charcoal is used as a decoloriser as it adsorbs the coloring matter from the coloured solution of sugar.

2. Silica gel adsorbs moisture from the desiccators.

3. Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidity of rooms.4. Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the air for breathing.

5 .Adsorption processes are useful in carrying out heterogeneous catalysis.

Natural sorbentsFabricate sorbents

chitin

Pectin

Cellulose

Factors affected AdsorptionTemperature. Adsorption increases at low temperature conditions.Adsorption process is exothermic in nature. According to Le Chatleir principle, low temperature conditions would favour the forward direction.PressureAs depicted by Adsorption Isotherm, with the increases in pressure, adsorption increases up to a certain extent till saturation level is achieved. After saturation level is achieved no more adsorption takes place no matter how high the pressure is applied.Surface Area. Adsorption is a surface phenomenon therefore it increases with increase in surface area.Activation of AdsorbentActivation of adsorbent surface is done so as to provide more number of vacant sites on surface of adsorbent. This can be done by breaking solid crystal in small pieces, heating charcoal at high temperature, breaking lump of solid into powder or other methods suitable for particular adsorbent.

The surface-active agent is called surfactant or a wetting agent.

Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid.

Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants. For example, organic acid and soap are active agent of water; salt is non-active agent of water.

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Molecules and ions that are adsorbed at interfaces Molecules and ions that are adsorbed at interfaces

are are

termed termed surface active agents,surface active agents, surf surfactants oractants or

amphiphileamphiphile

The molecule or ion has a certain affinity for both The molecule or ion has a certain affinity for both

polar and polar and

nonpolar solvents.nonpolar solvents.

Depending on the number and nature of the polar Depending on the number and nature of the polar

and and

nonpolar groups present, the amphiphile may be nonpolar groups present, the amphiphile may be

hydrophilic,hydrophilic,

lipophiliclipophilic or be reasonably well-balanced between or be reasonably well-balanced between

thesethese twotwo

extremes. extremes.

It is the amphiphilic nature of surface active agents It is the amphiphilic nature of surface active agents

which which

causes them to be adsorbed at interfaces, whether causes them to be adsorbed at interfaces, whether

these be these be

liquid/gas or liquid/liquid.liquid/gas or liquid/liquid.

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A surfactant molecule is depicted schematically as a cylinder representing the hydrocarbon (hydrophobic) portion with a sphere representing the polar (hydrophilic) group attached at one end.The hydrocarbon chains are straight because rotation around carbon-carbon bonds bends, coils and twists them.

Sodium Lauryl Sulfate molecule

How does surfactant work?

How Surfactants Work

Іsoterms of surface tension

1 – for solution of surface active substance,2 – surface inactive substance,3 – surface neutral substance

The Influence of Surfactants on Surface Tension

Critical Micelle Concentration

(CMC)

Minimum concentration at which surfactants molecules begin to form micelles

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Effect of SurfactantEffect of Surfactant’’s structure on s structure on CMCCMCBranched chain systems and double bonds raise CMC Branched chain systems and double bonds raise CMC

Since the chains must come together inside the micellesSince the chains must come together inside the micelles

Length of hydrocarbon chain and polarity of SurfactantsLength of hydrocarbon chain and polarity of Surfactants

Increase in chain length of hydrocarbon facilitate the Increase in chain length of hydrocarbon facilitate the

transfer from aqueous phase to micellar form cause transfer from aqueous phase to micellar form cause

decrease in CMCdecrease in CMC

Greater interaction with water will retard micelle formation Greater interaction with water will retard micelle formation

thus ionized surfactants have higher CMC in polar solvents thus ionized surfactants have higher CMC in polar solvents

than nonionic Surfactants.than nonionic Surfactants.

In polar solvents:In polar solvents:

Polarity of Surfactant molecules CMCPolarity of Surfactant molecules CMC

Length of hydrocarbon chain CMCLength of hydrocarbon chain CMC

In non-polar solvents:In non-polar solvents:

Polarity of Surfactant molecules CMCPolarity of Surfactant molecules CMC

Length of hydrocarbon chain CMCLength of hydrocarbon chain CMC

Surfactant Behavior

Structures of Micelles

Types of Surfactants

Ionic Surfactants Anionic Surfactants

Carboxylates

Alkyl benzene Sulphonates

Soaps

LABS

FAS

Ionic Surfactants Anionic Surfactants

Sulfosuccinate

Diester

Sulfosuccinate

Monoester

Ionic Surfactants: Uses Anionic Surfactants

• Cleansing Formulation– Shampoo– Hand wash– Bath gels– Tooth Paste– Soaps & Detergents

Cationic Surfactants(1)

Cationic Surfactants(2)

Important Property Substantivity

Cationic Surfactants

Ionic Surfactants Amphoteric Surfactants

N+

CH3

CH3

CH2COO-

CONH(CH2)3N+

CH3

CH3

CH2COO-

Cocobetaine (CB)

Cocoamidopropylbetaine ( CAPB)

Nonionic Surfactants

Nonionic Surfactants

Water Solubility of NonionicsReason

Interface Tension and Spreading

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When a liquid is placed on the surface of other liquid,

it will spread as a film if the adhesion force is

greater than the cohesive forces.

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As surface or interfacial work is equal to

surface

tension multiplied by the area increment.

The work of cohesion, which is the energy

required to separate the molecules of the

spreading liquid so as it can flow over the sub-

layer=

Where 2 surfaces each with a surface tension =

γ L

The work of adhesion, which is the energy

required to break the attraction between the

unlike molecules=

Spreading occurs if the work of adhesion is

greater than the work of cohesion, i.e. Wa > Wc

or Wa - Wc > 0

Wc = 2 γ Wc = 2 γ LL

Wa = γ Wa = γ LL + γ + γ SS - γ - γ LSLS

ProblemCalculate the work of adhesion of water on four solids, where the equilibrium contact angles are 30o, 60o, 120o, and (a hypothetical) 180o. The surface tension of the air/water interface is 72 mN m–1.

SolutionWe can use the Young–Dupré equation to calculate the work of adhesion per unit area of contact between water and the solid. For the first solid (θ = 30o):

Using the same procedure for the other solids.We note that the work of adhesion falls to zero when the contact angle is 180o, but emphasize that such angles are never observed as it would imply that there is no interaction between the liquid and solid surface.

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Spreading Coefficient is The difference

between the work of adhesion and the work of cohesion

S = Wa - S = Wa - WcWc = = ( (γ γ LL + γ + γ SS - γ - γ LS LS ) - ) - 2 γ 2 γ LL

S =S = γ γ SS - - γ γ LL - γ - γ LS LS

S =S = γ γ SS – ( – (γ γ LL + γ + γ LS LS )) Spreading occurs (S is positive) when the surface

tension of the sub-layer liquid is greater than the sum of

the surface tension of the spreading liquid and the

interfacial tension between the sub-layer and the

spreading liquid.

If (γ(γ LL + γ + γ LSLS ) ) is larger than YS , (S is negative) the

substance forms globules or a floating lens and fails to

spread over the surface.

Liquid Substrate

Spreading Coefficient, Sin Water at 20oC

liquid B n-hexadecane 72.8-(30.0+52.1) = - 9.3 Drop on water surfacen-octane 72.8-(21.8+50.8) = + 0.2 Spreadingn-octanol 72.8-(27.5+8.5) = +36.8 Spreading against

inpurity

S =

OWOAWA S =

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Factor affecting Spreading Coefficient

Molecular Structural:o The greater the polarity of the molecule the more

positive [S]

as ethyl alcohol and propionic acid

o Non polar substances as Liquid petrolatum have negative

[S] fail

to spread on water

o For organic acids, as Oleic acid,

the longer the carbon chain decrease in polar character decrease [S]

o Some oils can spread over water because they contain polar

groups

as COOH and OH

Cohesive forces:Benzene spreads on water not because it is polar but

because the cohesive forces between its molecules

are much weaker than the adhesion for water.

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Application of Spreading coefficient in pharmacy

The requirement of film coats to be spreaded over the

tablet surfaces

The requirement of lotions with mineral oils to spread on

the skin by the addition of surfactants

Contact angles and wettingWetting is the displacement from a surface of one fluid by another. It

involves, therefore, three phases, at least two of which must be fluids.

The following account will be restricted to wetting in which a gas(usually air) is displaced by a liquid at the surface of a solid. Awetting agent is a (surface-active) substance which promotes thiseffect. Three types of wetting can be distinguished:1. Spreading wetting.2. Adhesional wetting.3. Immersional wetting.Spreading wettingIn spreading wetting, a liquid already in contact with the solid spreadsso as to increase the solid-liquid and liquid-gas interfacial areas anddecrease the solid-gas interfacial area.

Wetting (A) and unwetting (B) solid by liquid

А) B)

LiquidGas

Gas

Liquid

Cos θ = 0÷1 Cos θ = -1÷0

θθ

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.

 

nowetting

incomplete wettingcompletewetting

= 0° <90°    =90°   >90°   =180°ӨӨӨӨӨ

γ s– γsL< 0 γ s – γsL ≈ 0 γ s – γsL> 0

Surface active agent concentration dependence of the adsorption describes by

Gibb’s equation:

dC

d

RT

CГ

dC

d

Г – adsorption, mole/m2;С – concentration SAA, mole/L;

- surface tension of SAA, J·m/mole;

Т – temperature, К; R = 8.314 J/(mole·К)

Concentration dependence of surface tension

1, 3 – Henry’s direct dependence:σ0- σ = КС2- Shyshkovski equationσ = σ0 – a ln (1+ bc)

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3 3