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Questions and Answers
Free energy is directly proportional to the volume of solids.
Free energy is directly proportional to the volume of solids.
False
Gas adsorption on solids is influenced by the chemical properties of both the adsorbent and the gas.
Gas adsorption on solids is influenced by the chemical properties of both the adsorbent and the gas.
True
Physical adsorption of gases on solids can be reversed.
Physical adsorption of gases on solids can be reversed.
True
Cohesive attraction between solid particles can increase the free energy.
Cohesive attraction between solid particles can increase the free energy.
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Chemical adsorption of gases is irreversible.
Chemical adsorption of gases is irreversible.
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Surface tension reduces free energy at the interface of liquids.
Surface tension reduces free energy at the interface of liquids.
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The shape of droplets is non-spherical to minimize surface area.
The shape of droplets is non-spherical to minimize surface area.
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Activated charcoal is an example of a gas adsorbent that can be used in gas masks.
Activated charcoal is an example of a gas adsorbent that can be used in gas masks.
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Hygroscopic materials become liquid upon adsorbing significant amounts of water vapour.
Hygroscopic materials become liquid upon adsorbing significant amounts of water vapour.
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Deliquescent substances must be stored separately from other preparations due to their tendency to form liquid when adsorbing water vapour.
Deliquescent substances must be stored separately from other preparations due to their tendency to form liquid when adsorbing water vapour.
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Adsorbing active agents onto inert solids can help improve powder flow and control the rate of release.
Adsorbing active agents onto inert solids can help improve powder flow and control the rate of release.
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Containers that hold tablets and powders can be impermeable even if they adsorb oxygen and carbon dioxide.
Containers that hold tablets and powders can be impermeable even if they adsorb oxygen and carbon dioxide.
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High affinity gas-solid interactions lead to increased adsorption until a certain limit is reached.
High affinity gas-solid interactions lead to increased adsorption until a certain limit is reached.
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For physical adsorption, an increase in temperature results in increased adsorption capacity.
For physical adsorption, an increase in temperature results in increased adsorption capacity.
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The porosity of a solid has no effect on the adsorption capacity of gases.
The porosity of a solid has no effect on the adsorption capacity of gases.
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Chemical adsorption is independent of changes in gas pressure.
Chemical adsorption is independent of changes in gas pressure.
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As the temperature increases, the amount of water vapor adsorbed by a solid decreases due to gas-solid affinity changes.
As the temperature increases, the amount of water vapor adsorbed by a solid decreases due to gas-solid affinity changes.
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Strong bonds facilitate physical adsorption rather than chemical adsorption.
Strong bonds facilitate physical adsorption rather than chemical adsorption.
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When gas pressure decreases, the amount of gas adsorbed for physical adsorption also decreases.
When gas pressure decreases, the amount of gas adsorbed for physical adsorption also decreases.
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A decrease in solid diameter results in an increase in the adsorption capacity of the solid.
A decrease in solid diameter results in an increase in the adsorption capacity of the solid.
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Chemical adsorption does not change with temperature fluctuations.
Chemical adsorption does not change with temperature fluctuations.
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Isotherm I indicates that there are gas-gas interactions present.
Isotherm I indicates that there are gas-gas interactions present.
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The affinity of gas with solid A is greater than that with solid C.
The affinity of gas with solid A is greater than that with solid C.
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Physical interactions are solely associated with Isotherms II and III.
Physical interactions are solely associated with Isotherms II and III.
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Desorption studies can help determine the type of chemical interaction that occurs.
Desorption studies can help determine the type of chemical interaction that occurs.
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Isotherms IV and V represent non-porous solids.
Isotherms IV and V represent non-porous solids.
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The affinity between gas and solid I, II, IV is greater than that for III, V.
The affinity between gas and solid I, II, IV is greater than that for III, V.
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Gas-gas affinities can be depicted by the steepness of the slope on the isotherm curves.
Gas-gas affinities can be depicted by the steepness of the slope on the isotherm curves.
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Chemical interaction occurs in the first layer while physical interaction occurs in subsequent layers.
Chemical interaction occurs in the first layer while physical interaction occurs in subsequent layers.
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Affinity of gas with solid B is less than that with solid A.
Affinity of gas with solid B is less than that with solid A.
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Isotherms II and IV indicate that gas-solid affinity is less than gas-gas affinity.
Isotherms II and IV indicate that gas-solid affinity is less than gas-gas affinity.
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Isotherm IV indicates that a solid is non-porous.
Isotherm IV indicates that a solid is non-porous.
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The adsorption capacity, Q, increases with an increase in Qk.
The adsorption capacity, Q, increases with an increase in Qk.
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Effective or specific surface area can be calculated using density of the gas and molecular weight.
Effective or specific surface area can be calculated using density of the gas and molecular weight.
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A good adsorbent must have a specific surface area less than 1 m²/g.
A good adsorbent must have a specific surface area less than 1 m²/g.
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Physical adsorption is characterized by a high affinity towards gas.
Physical adsorption is characterized by a high affinity towards gas.
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The mean particle diameter can be calculated using the density of the solid and the specific surface area.
The mean particle diameter can be calculated using the density of the solid and the specific surface area.
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To determine if a solid is porous, gas with cohesive bonds and low temperature should be used.
To determine if a solid is porous, gas with cohesive bonds and low temperature should be used.
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Isotherm III is associated with porous materials.
Isotherm III is associated with porous materials.
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A solid that produces Isotherm I during interaction with gas is likely to be highly porous.
A solid that produces Isotherm I during interaction with gas is likely to be highly porous.
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The adsorption isotherm can be plotted as pressure versus the amount of gas adsorbed per unit mass.
The adsorption isotherm can be plotted as pressure versus the amount of gas adsorbed per unit mass.
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Study Notes
Adsorption
- Adsorption is a surface phenomenon where molecules of a substance (adsorbate) accumulate on the surface of another substance (adsorbent).
- Adsorbates can be gases, liquids, or solutes.
- Adsorbents can be solids or liquids.
- Excess free energy at the surface of a liquid can be reduced through surface tension.
- Droplets and bubbles are spherical because surface area is minimized.
- At the surface of liquids, attraction is more towards the bulk (high free energy).
- In the bulk of liquids, attraction is balanced in all directions (low free energy).
- For solids, atoms or molecules are closely packed in a compact arrangement, and they do not move freely like liquids.
- G reduction by surface contraction isn't efficient for solids.
- Gas atoms/molecules can bond with the surface of a solid and form gas adsorption.
- Solute or solvent molecules can bond with solid surfaces, forming solute or solvent adsorption.
- Solid powder or particles close to each other attract each other cohesively, reducing free energy.
- Cohesive attraction between solid particles can reduce powder flow.
Learning Outcomes
- Students will be able to explain the thermodynamic reason for adsorption.
- Students will be able to differentiate between isotherms and the factors that give rise to them.
- Students will be able to relate adsorption to pharmaceutical applications.
Adsorption of Gas onto Solids
- The extent of gas adsorption depends on the chemical properties of the adsorbent and gas, the total surface area of the adsorbent, temperature, and the partial pressure of the gas.
- Physical adsorption is reversible (desorption).
- Chemical adsorption is irreversible.
- Desorption can be achieved by increasing temperature or reducing gas pressure.
- A relationship between the total amount of gas adsorbed onto a solid with pressure at equilibrium, at constant temperature results in an adsorption isotherm.
- Isotherm plot gives the amount of gas adsorbed against pressure at equilibrium.
- Freundlich isotherm: x/m = kP1/n.
- Freundlich equation: log(x/m) = log k + (1/n) log P.
- Langmuir isotherm: x/m = QaP / (1 + aP).
- Langmuir equation: P/ (x/m) = (1/Qa) + (P/Qa)
- Assumptions of the Langmuir isotherm: all adsorption sites are equivalent, only one layer of gas atoms/molecules is adsorbed, adsorbed atoms/molecules don't interact with each other, occupation probability is independent of neighboring occupancy, rate of adsorption depends on the total number of vacant positions and rate of desorption is related to the occupied position.
B.E.T. (Brunauer, Emmett & Teller) Isotherms
- Different isotherm types (I, II, III, IV, V) represent different adsorption behaviors related to the pressure-dependent amount of adsorbate.
- These types depend on the affinity between gas and adsorbent.
- Type I adsorption has a high affinity, low gas-gas interaction and low porosity.
- Type II adsorption has relatively medium affinity, medium gas-gas interaction and medium porosity.
- Type III adsorption has low affinity and prominent gas-gas interaction and high porosity.
- Type IV adsorption occurs when the adsorbent has pores and moderate affinity.
- Type V has low affinity and prominent gas-gas interaction.
Factors Affecting Gas Adsorption
- Affinity (gas-solid): High affinity leads to isotherms I, II, IV; low affinity to isotherm III, V. Gas-gas interaction influences the isotherm type.
- Type of bond: Strong bonds (covalent or ionic) result in chemical adsorption; weak bonds (van der Waals forces) lead to physical adsorption.
- Properties of solid: Pore size, surface area, and diameter are key. Higher porosity, smaller diameter, and larger surface area indicate suitability for adsorption.
- Gas pressure: Increasing pressure generally increases the amount of adsorbed gas up to a limit.
- Temperature: Adsorption is often favored by lower temperatures, but also dependent on the type of adsorption (physical or chemical).
Adsorption of Solute by Solids
- Solvent molecules move freely due to Brownian motion and can collide/bond with a solid surface.
- Adsorption isotherms relate solute concentration in solution to the amount of adsorbed solute on a solid's surface.
- Studies involve preparing solutions, using a UV spectrophotometer to measure concentrations, and performing experiments in duplicate (with and without solid).
- The difference in concentrations then yields the absorbed amount of solute.
Isotherms and Their Types
- Isotherm type is based on the interactions between the solute/solid and solute/solute.
- Isotherm characteristics are affected by the solute's affinity and interaction (chemical or physical).
- Isotherm types (L, C, H, S) reflect different degrees of adsorption based on interactions between the solute and solvent.
Properties of the adsorbent solid
- The degree of porosity of a solid often correlates with the maximum amount adsorbed and the width of curves.
- Adsorption capacity (Q) can be derived from isotherms, showing the maximum amount of gas/solute to form a monolayer.
- Surface area (Sw) depends on Q and can be calculated using data from the isotherm.
Applications of Adsorption
- Suitable adsorbents for various purposes (e.g., gas masks, water purification, drug formulations) are identified based on their characteristics.
- Adsorption can be used to predict the rates of dissolution and bioavailability of various drugs.
- The presence or absence of gas-gas interaction relates to isotherm shape.
- The affinity between solute-solid dictates how easy or difficult the absorption process is.
- Adsorption can change the physical stability of a solid. A soluble substance absorbed by another powder substance will result in reduced bioavailability.
- The presence of a suitable/compatible adsorbent leads to higher drug release and efficient bioavailability.
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Description
This quiz explores key concepts related to the adsorption of gases on solids, including the influence of chemical properties, physical and chemical adsorption processes, and the behavior of materials like activated charcoal. Learn about the principles of free energy, surface tension, and the characteristics of hygroscopic and deliquescent substances.
