Adsorption: Basics, Classification & Monolayers

Adsorption Basics

• Adsorption is an interfacial phenomenon involving gas/solute deposition on a solid.
• The material providing the adsorption site is the adsorbent.
• The adsorbed solute is the adsorbate.
• Acetic acid (adsorbate) and charcoal (adsorbent) exemplify solid-liquid interface adsorption.

Classification of Adsorption

• Adsorption is either physical or chemical.
• Physical adsorption involves reversible interactions between adsorbent and adsorbate, linked to van der Waals forces.
• Chemical adsorption involves the irreversible attachment of adsorbate to adsorbent through chemical bonds.
• The degree of adsorption relies on the following factors:
• Quality/physicochemical properties of both adsorbent and its surface area.
• Quality/physicochemical properties and quantity of the adsorbate.
• Pressure and temperature.

Monolayers at Liquid Surfaces

• Adsorbed materials divide into two groups: those forming "soluble" monolayers and those forming "insoluble" films.
• This division hinges on the adsorbate's solubility in the liquid subphase.
• Amyl alcohol forms a soluble monolayer on water, while cetyl alcohol forms an insoluble film on the same sublayer.

Insoluble Monolayers and Film Balance

• The thickness of films can be computed from the film's area and the spreading liquid's volume.
• Film thickness equals the length of vertically standing molecules when closely packed on the surface.
• The area available to molecules (cross-sectional) can be calculated if the molecular weight and density are identified.

Thickness of Oil on the Pond

• Benjamin Franklin placed 5 cm³ of fatty acid oil on a half acre (2 x 107 cm²) pond.
• Assume the acid, having a molecular weight of 300 and a density of 0.9 g/cm³, formed a condensed monomolecular film.
• The length of the fatty acid molecule was 25A, and its cross-sectional area was 22A²/molecule.

Cross-Sectional Area

• The area of cross-section per molecule is MS/VρN:
• M = molecular weight of spreading liquid
• S = surface area covered by the film
• V = volume of the spreading liquid
• ρ = density
• N = Avogadro's Number
• Langmuir, Adam, and Harkins, among others, quantitatively studied films spread over a liquid surface (usually water) in a trough.
• The film can be compressed against a horizontal float using a movable barrier.
• The force on the float is measured using a torsion wire arrangement, like a DuNuoy Tensiometer.
• The apparatus employed is a film balance.

Film Balance

• Surface pressure (π) is the compressive force per unit area on the float.
• This is the surface tension difference between the pure substrate (γo) and the film-covered substrate (γ).
• π is equal to (γo - γ).
• The substance under study is dissolved in a volatile solvent such as hexane and placed on the substrate surface.

Film Experiment

• The liquid spreads into a film and the volatile substance is allowed to evaporate.
• Then the movable barrier is moved to various positions in the direction of the float.
• The area of the trough that is available to the film at each position can be measured, and the corresponding film pressure can be read from the torsion dial.
• The film pressure is then plotted against the area of the film or, more conveniently, against the cross-sectional area per molecule in A².

Phase Changes

• A variety of phase changes are observed when an insoluble film is spread at an interface and then compressed.
• G= two-dimensional gas
• L₁-G = liquid phase in equilibrium with two-dimensional gas
• L₁= liquid expanded or two-dimensional bulk liquid state
• I= intermediate state
• L2 = condensed liquid state
• S= two-dimensional solid state
• The film collapses when compressed by excessive force.
• The cross-sectional area per molecule of the closely packed film at zero pressure is obtained by extrapolating the linear portion of the curve to the horizontal axis.

Film Balance Significance

• Film balances are considerably significant in biological systems
• Some proteins molecules unfold from a spherical configuration into a flat film when spread on the surface of the film trough.
• The unfolding and biological activity can be studied in relation to one another.
• The sizes/shapes of steroids, hormones, and enzymes can be investigated; as well as their interaction with drugs.

Adsorption on Solid Surfaces

• Active centers exist on the surface of adsorbents, which facilitate adsorption and removal of foreign molecules.
• pH levels impact the surface charge, as well as the solution. Acidic compounds absorb best at lower pH levels.
• It is typically an exothermic reaction; as such, higher temperatures lower the extent of adsorption.
• Low-temperature conditions favor the reaction.

Adsorption Defined

• Adsorption accumulates molecules on a surface when in contact with air or water.
• Absorption molecules of a phase interpenetrate uniformly to form a solution.
• Adsorption is a surface phenomenon
• In adsorption (moves from liquid to solid), the process is rapid at the beginning and slows down near equilibrium. Absorption takes place at a uniform rate.

Key Definitions and Characteristics

• Adsorbent: A solid (also known as a substrate) that provides a surface for adsorption.
• It should have a high surface area with proper pore structure and size distribution and has adequate mechanical strength alongside thermal stability.
• Adsorbate: A gas or liquid substance that is adsorbed on a solid.
• Surface coverage (θ): The degree to which a solid surface is covered by adsorbed molecules.
• It can be either complete or partial (θ = 0 to 1).
• Adsorption Heat: Usually exothermic.
• Physisorption (Physical Adsorption):
• Van der Waals' forces are forces of attraction.
• Features low enthalpy.
• It is best observed low temperatures, and is not specific. Since it not specific mult-molecular layers arise, it can undergo a reversible reaction..
• Chemisorption (Chemical Adsorption)
• Chemical Bonds as forces of attraction, with high enthapy.
• This process is best under high temperatures, its is highly specific. It generally froms a monomolecular layer, and it can undergo an irrreversible action.
• Temperature significantly impacts this; a gas that undergoes physisorption may transition to chemisorption with raised temperatures.

Key Applications

• Key applications include the adsorption of gasses, liquids, and solids:
• The removal of toxic gases using gas masks.
• Odour adsorption.
• Decolorization of liquids and as antidotes in oral intoxication.
• Fixation of volatile oil for scent removal and separations using liquid chromatography.

Factors Affecting Adsorption for Gases

• Adsorption of gases on solids depend on various factors:
• Temperature and pressure.
• The nature and activation of the adsorbent.
• Specific area of the solid.
• The nature of the adsorbate.
• Easily liquefiable gases are adsorbed more readily that elemental gasses.
• As solute solubility increases, adsorption decreases.
• High solubility is increased with molecular size, and polarity. virtually every solid surface can adsorb solutes. Solids with a porous structure are potentially good adsorbents.
• Presence of Other Solutes:
• Competition for the limited number of binding sites.
• Competitive Adsorption: if there is more than one solute competing the isotherm can be adapted.

Factors Continued...

• Surface Area of the Adsorbent: A surface phenomenon surface area increase improves adsorption.
• Specific Area of The Adsorbent: Area available for adsorption per gram.
• Solids: Porous are a good consideration, they should be large enough to allow entry.
• Activation of Adsorbent:
• Provides more vacant sites on the surface.
• Activating the surface requires
• Breaking solid crystals and lumps.
• Heating charcoal at high temperatures.
• Pressure - Adsorption Isotherm: With increasing pressure, as gas adsorption on the solid increases, there may be a saturation level
• The higher the concentration, the higher the amount adsorbed consumption.

General Requirements of Absorbents

• Should be abrasion resistant, thermally stable and have small pore diameter for higher expose surface.
• Here are three classes of absorbents:
• Hydrophilic + Polar - e.g., silica gel and zeolites. (Oxygen Based).
• Hydrophobic + Non-Polar - e.g. activated Carbon based compounds.
• Polymer based compounds.
• Activated Charcoal is a highly porous, mocro-crystalline material, used for extensive adsorption.
• Silica Gel traps silicon dioxide and moisture.

Adsorption Isotherms

• They show the association between the amount of adsorbate (x) that physically adsorbs to a surface of adsorbent (m).
• The equilibrium pressure is constant and physical. If these are left in contact long enough, it will equalize.

Freundlich Isotherm

• Freundlich Isotherm Equation.
• y = x/m = KP1/n
• y: Mass of Gas vs Adsorbent
• K and 'n' are constants.
• In this saturation P, the adsorption doesn't occur, because limited quantities exist. and doesn't increase with absorption.

Logarithmic Equation

• The logarithmic form yields a straight line. The constant is the line's intercept and slope.
• For solid/Liquid change "P" is replaced by "C" on these surfaces.
• Equilibrium concentration are the main component here (mg/100ml) by the amount.

Langmuir Adsorption Isotherm

• An equation based on the theory for gasses that adsorb onto an active surface, one molecule thick.
• Center occupancy is expressed '0' and then is expressed from 1 to form a layer.
• the condensation is proportional to 1 '0' and constant pressure.

Langmiur Equation Explained

• The rate will evaporate surface and becomes proportional to surface.
• They will, at equilibrium equal one another and the fractions are then replaced.
• Mass gasses are absorbed per pressure at consant temperature is the adsorbent.

Specific Surface

• By formula is now known as Langmuir in formula y.
• Specific Surface provides an overview for drug particle dissolution rates.
• The Solid/Liquid interface provides a base for a reaction from a solution for certain compounds.
• c/y helps equal the mass solution vs its concentration.