Lecture 2. Adsorption on the interphase of liquid-gas Prepared by PhD FalfushynskaHalina
Adsorption is a spontaneous process For 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 – TΔS And during this process of adsorption, randomness of the molecule decreases which ΔS is negative. We can rewrite above equation as
Types of Adsorption Forces 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 Physisorption When 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 Chemisorption When 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 PhysicalAdsorptionvs T and ChemicalAdsorptionvs T
Applications of Adsorption 1. 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.
Fabricate sorbents Natural sorbents Cellulose Pectin chitin
Factors affected Adsorption Temperature.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. Pressure As 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 Adsorbent Activation 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.
Surface Active Agents • Molecules and ions that are adsorbed at interfaces are • termed surface active agents, surfactants oramphiphile • The molecule or ion has a certain affinity for both polar and • nonpolar solvents. • Depending on the number and nature of the polar and • nonpolar groups present, the amphiphile may be hydrophilic, • lipophilic or be reasonably well-balanced between thesetwo • extremes. • It is the amphiphilic nature of surface active agents which • causes them to be adsorbed at interfaces, whether these be • liquid/gas or liquid/liquid.
Surface Active Agents 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
Іsoterms of surface tension 1 – for solution of surface active substance, 2 – surface inactive substance, 3 – surface neutral substance
Critical Micelle Concentration(CMC) Minimum concentration at which surfactants molecules begin to form micelles
Effect of Surfactant’s structure on CMC Branched chain systems and double bonds raise CMC Since the chains must come together inside the micelles Length of hydrocarbon chain and polarity of Surfactants Increase in chain length of hydrocarbon facilitate the transfer from aqueous phase to micellar form cause decrease in CMC Greater interaction with water will retard micelle formation thus ionized surfactants have higher CMC in polar solvents than nonionic Surfactants. In polar solvents: Polarity of Surfactant molecules CMC Length of hydrocarbon chain CMC In non-polar solvents: Polarity of Surfactant molecules CMC Length of hydrocarbon chain CMC
Ionic SurfactantsAnionic Surfactants Carboxylates Soaps Alkyl benzene Sulphonates 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
CH3 N+ CH2COO- CH3 CH3 CONH (CH2)3 N+ CH2COO- CH3 Ionic Surfactants Amphoteric Surfactants Cocobetaine (CB) Cocoamidopropylbetaine ( CAPB)
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.
As surface or interfacial work is equal to surface tension multiplied by the area increment. The work of cohesion, which is the energyrequired 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 L Wa = γ L + γ S - γ LS
Problem Calculate 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.
Solution We 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.
Liquid Substrate Spreading Coefficient is The difference between the work of adhesion and the work of cohesion S = Wa - Wc = (γ L + γ S - γ LS ) - 2 γ L S = γ S - γ L - γ LS S = γ S – (γ L + γ 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 (γL + γ LS )is larger than YS , (S is negative) the substance forms globules or a floating lens and fails to spread over the surface.
SpreadingCoefficient, Sin Water at 20oC S = S = liquid B n-hexadecane 72.8-(30.0+52.1)= - 9.3 Drop on water surface n-octane 72.8-(21.8+50.8) = + 0.2 Spreading n-octanol 72.8-(27.5+8.5) = +36.8 Spreadingagainst inpurity
Factor affecting Spreading Coefficient Molecular Structural: • The greater the polarity of the molecule the more positive [S] as ethyl alcohol and propionic acid • Non polar substances as Liquid petrolatum have negative [S] fail to spread on water • For organic acids, as Oleic acid, the longer the carbon chain decrease in polar character decrease [S] • 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.
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 wetting Wetting 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. A wetting agent is a (surface-active) substance which promotes this effect.Three types of wetting can be distinguished: 1. Spreading wetting. 2. Adhesional wetting. 3. Immersional wetting. Spreading wetting In spreading wetting, a liquid already in contact with the solid spreads so as to increase the solid-liquid and liquid-gas interfacial areas and decrease the solid-gas interfacial area.
Wetting (A) and unwetting (B) solid by liquid Gas Gas Liquid Liquid θ θ Cos θ = 0÷1 Cos θ = -1÷0 А) B)
completewetting incomplete wetting nowetting Ө Ө Ө Ө Ө = 180° = 0° < 90° = 90° > 90° 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: Г – adsorption, mole/m2; С – concentrationSAA, mole/L; - surface tension of SAA, J·m/mole; Т – temperature, К; R = 8.314 J/(mole·К)