ADSORPTION

FAR 221/3

Department of PharmacyIsland College of Technology

2007

Objectives

Students should comprehend and understand and explain matters relating to this topic namely;

- Surface and interfacial tensions, together with the Gibbs free energies related to them- Process of adsorption through the formation of bonds- The different types of gas adsorption isotherms;

Freundlich Langmuir Brunauer, Emmet, Teller (BET)

- Determination or prediction of various physicochemical phenomena based on adsorption studies.

Factors affecting gas adsorption- Adsorption of dissolved solutes on solids- Different types of disolved solutes adsorption isotherms;

Langmuir Constant High affinity Slow

- Information gleaned from the adsorption of solutes onto the surfaces of solids.- Factors affecting the dissolved solute adsorption

Contents and Instructions

SURFACE FREE ENERGY AND ADSORPTION• Recap briefly the relationships between free energy, surface area and surface

tension• Describe the difference in free energy between the surface of a solid and the bulk• Describe the process that occurs at the interfaces of solid-air, solid-liquid and

solid-solid, including;An explanation of the adsorption of gas onto solidsAn explanation of the adsorption of dissolved solutes or solvent onto solids

Cohesive force that can exists between solid particles and their potential effect on the flow of powders

ADSORPTION OF GAS

The type of adsorption isotherms for gasExplain the Freundlich isotherm in detail, including the equation Explain the Langmuir isotherm in detail, including the equation and the assumptions. Mention the applicability for certain assumptions for chemisorptionExplain the Branauer, Emmet, Teller (BET) isotherm in detail, including equation and different types relating to the adsorption behavior and conditions. Mention the types which the equation is applicable to.Additional reading on the conclusions, conception and deviations from the three isotherms.

Application of adsorption isotherm for gas

Discuss the applicability of these isotherm in determining or predicting certain physicochemical characteristics. For examples;

- The solid-gas and gas-gas affinity, based on the isotherm curves. Explain that the affinity is proportional to the steepness of the curves and give examples to illustrate this. Compare by means of diagrams, the solid-gas affinity for the different types of BET isotherms.

- Physical or chemical interactions can be distinguished by performing desorption studies. Describe the involvement of physical and/or chemical interactions for certain isotherms. Elaborate on the presence or absence of gas-gas interactions (cohesive forces) and/or solid-gas affinity (adhesive forces) in the different isotherms.

- Solid characteristics such as; porosity. Discuss the presence or absence of pores and the degree of porosity based on studies with gas.

adsorption capacity effective or specific surface area. Define the relevant equations average particle diameter

- Choice of gas adsorption materials. Explain the factors that constitute good adsorption materials. Examples of the materials (activated charcoal, silica gel, calcium cloride, etc) and their behaviour.

- Other uses include;• prediction of rates for various processes. Give examples.• selection of appropriate product packaging. Elaborate.

Factor affecting gas adsorption

Describe and explain the factors that affect gas adsorption, including: Affinity

Types of bonding Solid characteristic Gas pressure Temperature

An example showing the effect of temperature on physical adsorption:

Adsorption from solutionThe types of adsorption isotherm for dissolved solute

Describe and explain the adsorption of dissolved solutes by solids, including its isotherms and outline a simple study to determine the extent of adsorption (involving two sets of solution, one acting as control and the other containing various quantities of solids) Describe the construction of the isotherm curves based on the results of the study (y axis: amount adsorbed/weight of solid, x axis: concentration of solution at equilibrium)

Outline the different types of adsorption isotherm for dissolved solutes onto solids (constant, High affinity, Langmuir and slow) and the factors that affect them

-23oC

0oC

80oC

↑ T, ↓Q.No interaction between NH3 molecules

Mass of NH3 adsorbed by 1g or charcoal

100oC

↑ temperature, ↓Q, ↓QkIsotherm altered. Interaction between the vapor molecules

Amount of water vapor adsorbed by charcoal

Another example:

30oC

100oC

P

P

Give in detail and discuss the characteristic of the different isotherms. Include the physicochemical interactions and the affinity between the components, and the conditions that affect them.

Application of adsorption isotherm for dissolved solute

Describe and explain the information from the adsorption of dissolved solutes onto solids, including:

• Affinity of dissolved solutes to solids• Elucidation of the characteristics of the solids (for examples, porosity,

surface area, particle diameter, etc)• Determination of an appropriate solids act as an adsorber (for toxins, drug

overdose, bile secretions, water, etc.)

and drug carrier (affinity of drug to the adsorbing solid can be reduced at the target site or at certain pH range in order to release the drug at a controlled rate.

• Compatibility of highly soluble drugs with insoluble or less soluble drugs during coadministration (possible reduction in adsorption and bioavailability due to the adsorption of the highly soluble drug by the insoluble drug). For example, the adsorption of dissolved lincomycin onto kaolin

Factors affecting adsorption from solution

Discuss the factors that affect the adsorption of dissolved solutes, includingConcentration of dissolved solutesTemperature (explain the influence on physical adsorption and the rate of chemical reaction)Surface area and average particle diameter (in terms of the influence on Q) porosity (in terms of the influence on Qk and the area between the adsorption and desorption curves)Affinity between dissolved solutes and solids, which in turn is affected by:

Solubility of solutes. Discuss in terms of bonding between dissolved solutes and its solvent being difficult to break; adsorption is inversely proportional to solubility b. Ionization of dissolved solutes An increase in the degree of ionization usually leads to a decrease in adsorption and vice-versa. Mention that the ionization process can effected by pH. For example:

(1)

(2)Minimum therapeutic concentration

Concentration of Lincomycin in the blood

(3)

time

Treatment (1) lincomycin on its own(2) Lincomycin given 2 hours after kaopectate(3) Lincomycin and kaopectate given together

In addition, mention the condition that leads to maximum adsoption for amphoteric substances c. Polarity or changes on the solid surfaces For example:At a pH whereby the solubility of atropine is lower than hyoscine, theory dictates that the adsorption of atropine is higher than hyoscine. On the other hand, at the same pH, all of the hyoscine exists in non-ionized form whilst 50% atropine is in ionized form. Therefore, theory dictates that the adsorption of hyoscine is higher than atropineAdsorption studies with magnesium trisilicate revealed that the adsorption of atropnie is higher than hyoscine, suggesting that the low solubility of atropine is more significant than the low degree of ionization for hyoscine. However, the high atropine adsorption was actually due to its positively charged ions being easily adsorpbed onto the negatively charged surface of magnesium trisilicate.

An additional example;

% ionized benzocaine

% benzocaine adsorbed by nylon (low polarity)

% non-ionized benzocaine

Can be influenced by pH

Other application of adsorption from solution studiesOutline other uses of dissolved solutes adsorption studies such as chromatography, purification process, packaging, blood dialysis and the determination of absorption and reaction sites.For examples,IV solutions can be stored in plastic begs or infused by using administration sets made from polyvinyl chloride (PVC). Up to 50% nitroglycerin can be lost when the drug solution is stored in PVC infusion bags for 7 days at room temperature. The drug could be recovered from the inner surface of the bags using a more non-polar solvents, methanol. Hence, the initial loss was attributed to adsorption rather than chemical degradation.

The tendency for adsorption will depend on the relative affinity of the drug for the hydrophobic PVC and the hydrophilic aqueous infusion medium. Other drugs that could be adsorbed onto PVC are diazepam, insulin and isosorbide dinitrate.

• Recap briefly the relationships between free energy, surface area and surface tension

W = γ . Δ AW = work (Nm) γ = Surface tension (nm-1) Δ A (m2)

Surface energy is minimum if surface tension is less, surface tension is less by reduce the surface area.

• Describe the difference in free energy between the surface of a solid and the bulksolid

Mg tris.

MgO

Polarity and charge strength Mg trisilicate> MgO>CaCO3

Therefore the affinity of the antacids toward the positively charged digoxin also follows in that order

% digoxin adsorbed by the antacids

CaCO3

Concentration of drug

Existence of free energy at surface of solid- At surface, attraction more toward bulk. à γ high

- At bulk, attraction is balanced in all directions. à γ lowAtom or molecules are close to each other and in compact arrangement. Cannot move freely like in liquids. γ reduction through surface contraction is not effective or significant.

Describe the process that occurs at the interfaces of solid-air, solid-liquid and solid-solid, including;An explanation of the adsorption of gas onto solidsAn explanation of the adsorption of dissolved solutes or solvent onto solidsCohesive force that can exists between solid particles and their potential effect on the flow of powders

i) Between surface of air and solid

Gas atoms or molecules, when very near or collide with surface of solid, can form à gas adsorption.

The extent of gas adsorption onto solids depend on:The chemical properties of adsorbent and gasTotal surface area of adsorbentTemperature and partial pressure of gas

ii) Between surface of liquid and solid

Solvent molecules, when touch the surface of solid, can form bonds à solute or solvent adsorptioniii) Between solid surfaces (cohesion)Solid powder or particles, when very near to each other or touch, can form cohesive attraction among particles and reduce a little of the free energy. à Practically cohesion between particles can reduce powder flow from the container through the punch of rotation tabletting machine

The type of adsorption isotherms for gas

If physical adsorption occur à weak à can be reversible (desorption). Desorption – occur by increasing the temperature or reduce gas pressure

If chemical adsorption/chemisorption occur à stronger à irreversible

Gas, it will be adsorbed by powderQuartz spring

Sample of granules or powder

Explain the Freundlich isotherm in detail, including the equation

Freundlich Adsorption Isotherm

xm

Chemical interaction

Physical interaction

P P PP

xm

xm

xm

xm

II III IV V

x

P

= k P1/n

m

xm

If P is increased will be increased, no limit nor plateauxm

Equation : = k P1/nxm

n and k are constants related to the rate of adsorption of a particular gas onto a particular solid surface, and is affected by temperature

Explain the Langmuir isotherm in detail, including the equation and the assumptions. Mention the applicability for certain assumptions for chemisorption

Assumes that: i) short bonds are involved ii) Only 1 layer of gas atoms/molecules adsorbediii) Adsorbed atoms/molecules remain at their positions.

log

log P

Slope = 1/n

log k

xm

Freundlich equation:log = log k + 1/n log Px

m

y =c+mx

- A limit existQ

Langmuir Isotherm

xm

P Po

iv) Rate of adsorption depends on the total number of vacant positionsv) Rate of desorption is related to the occupied positions.i – iv à suitable for chemical adsorptionv à unsuitable for chemical adsorption

Q = adsorption capacity = amount of gas required to form monolayer for covering the whole of solid surface

Q and a = constantsExplain the Branauer, Emmet, Teller (BET) isotherm in detail, including equation and different types relating to the adsorption behavior and conditions. Mention the types which the equation is applicable to.

BET Brunauer, Emmett & Teller Isotherms

P

Po

Langmuir Equation

slope = 1/Q

x

m

1

aQ

P

x

m

P

Q

1

aQ

= +

desorption rate

adsorption rate

a =

Additional layers and condensation in the pores will occur when P approaches Po (saturated vapor pressure)

Equation:

h = constant related to Ea (heat of adsorption) and Ec (heat of condensation)h = e–(Ea-Ec)/RT

Additional reading on the conclusions, conception and deviations from the three isotherms.

xm

P

(Po-P)

1

Qh

h-1

Qh= + x

P

Po

1

Slope =

P/Po

P(Po-P)

h-1 Qh

Qh

Suitable for types II and IV

Application of adsorption isotherm for gas

Discuss the applicability of these isotherm in determining or predicting certain physicochemical characteristics. For examples;

The solid-gas and gas-gas affinity, based on the isotherm curves. Explain that the affinity is proportional to the steepness of the curves and give examples to illustrate this. Compare by means of diagrams, the solid-gas affinity for the different types of BET isotherms.

Uses of adsorption studiesA) To determine or predict gas-solid and gas-gas affinities based on isotherm curves

Affinity is correlated with the steepness of slopea) Affinity of gas A with solid B > solid C > solid Db) No gas-gas interaction.

c) Affinity of gas with solid for I, II and IV type > III, V type

B

C

D

Gas A with solids B,C and D

xm

P

1. Isotherms II, III, IV, V à gas-gas interactions present2. Isotherm II and IV à gas-solid (cohesive) interaction > gas-gas (adhesive) affinity or interaction

3. Isotherm III and Và gas-gas (cohesive) affinity > gas-solid (adhesive) affinity or interaction

Additional layers and condensation in the pores will occur when P approaches Po (saturated vapor pressure)

Equation:

IV

II

I

V

III

No gas-gas interaction

h = constant related to Ea (heat of adsorption) and Ec (heat of condensation)h = e–(Ea-Ec)/RT

xm

P

(Po-P)

1

Qh

h-1

Qh= + x

P

Po

1

Slope =

P/Po

P(Po-P)

h-1 Qh

Qh

Suitable for types II and IV

Isotherms II, III,IV and V à involved physical interaction only or chemical interaction for the first layer and physical interaction for successive layersCan be determined through desorption studies

B) Types of interaction Physical Chemical (only form monolayer)

Isotherm I physical interaction or chemical interaction

Can be determine through desorption studies

P

xm due to strong attraction

(chemically interaction), can not reverse as previously)Physical interaction

Chemical interaction Q

C) Properties of the solidi) Porosity

Isotherms I (physical or chemical)- cannot establish whether porous or not. To determine, must study with gas that has cohesive bonds and conducted at low temperature (physical adsorption can occur easily and additional layers can be formed easily)- if obtainIsotherms II or III à non porousIsotherms IV or V à porous

P P P P

xm

xm

xm

xm

II III IV V

P

Qk

Q

xm

Degree of porosity correlated with Qk and area between adsorption and desorption curvesii) Adsorption capacity, QQ à amount of gas to form a monolayer. Can be determined from the graph of isotherm (P vs x/m) or graph of Langmuir equation (P vs P/(x/m))Q↑ à adsorption capacity ↑

iii) Effective or specific surface areaa) If Q in mol/g:

Sw = Q N AN = avogadro no

A = 3.464 ( )

M = Molecular mass of gasΡ = density of gasb) If Q in ml/g

iv) Mean particle diameter

ρ = density of solid

D) Selection of gas adsorbent materialsGood adsorbent (based on information A à C)

Q N AVmSw = Vm = molar volume (22.4 l/mol)

6D = ρSw

4ρ√2N

M

- Sw > 1 m2/g (d decrease , Q increase)- Porous (Qk increase, area between curves increase)

- Affinity toward gas high (value of a increase)- Reversible (physical adsorption) so that solid can be re-used

So a suitable solid is one that produces isotherm IV when interacting with gasExamples of gas adsorbentsActivated charcoal à in gas masks & packets put together with solid preparations

E) Other Usesa) To predict the rates of:

i) Dissolution of soluble drugsii) Absorption & bioavailability of drugs

iii) Reactioniv) Cohesion between powder particles

Physical or chemical process ~ Sw ~ 1/db) Selection of product packaging Product such as tablets, granules, powder, capsules, if found to adsorb O2, CO2, water vapor, can undergo chemical changes, oxidation & reduction, and physical changes to affect dissolution, cohesiveness etc.

à Result in changes in: reducing the potency, powder flow

Silica gel, alumina

Hygroscopic i.e. remains solid even though adsorbed a lot of water vapor (40% weight of adsorbent)

CaCl2, Phosphorus pentoxide

Deliquescent i.e. becomes liquid if lots of water vapor is adsorbed It must be kept separate from preparations

à Must be stored in containers with have been proven to be impermeable and put together with packets of suitable adsorbent

Factors affecting gas adsorption

Affinity

1. Gas-solid à high àin I, II, IV type à low à in III, V type2. Gas-gas à none à I à present à II, III, IV, V gas-gas affinity > gas–solid affinity

C. Properties of solidi) Diameter ↓; Sw ↑, Q↑.ii) Porosity ↑; Qk ↑, area between curves↑

D. Gas PressureP ↑ à x/m ↑ until a certain limit à Q or QkP ↓ à x/m ↓ for physical adsorption à x/m remain at Q for chemical adsorption

B. Type of bondStrong à chemical adsorptionWeak à physical adsorption Determine with desorption

studies

E. Temperature T ↑à physical adsorption ↓ T ↓à physical adsorption ↑ x/m for chemical adsorption

constant, only rate changes

Adsorption from solution

The types of adsorption isotherm for dissolved solute

Describe and explain the adsorption of dissolved solutes by solids, including its isotherms and outline a simple study to determine the extent of adsorption (involving two sets of solution, one acting as control and the other containing various quantities of solids)

Describe the construction of the isotherm curves based on the results of the study (y axis: amount adsorbed/weight of solid, x axis: concentration of solution at equilibrium)

Example of the effect of temperature on physical adsorption:

-23oC

0oC

80oC

↑ T, ↓Q.No interaction between NH3 molecules

Mass of NH3 adsorbed by 1g or charcoal

100oCP

↑ temperature, ↓Q, ↓QkIsotherm altered. Interaction exists between the vapor molecules

Amount of water vapor adsorbed by charcoal

Another example:

30oC

100oC

P

Adsorption of solute by solid

Solution

Solvent one or more solutes

Move freely through brownian motion and can collide/touch the surface of solids, and if affinity exists, can form a band and adsorb physically or chemically.

Isotherms of solute adsorption onto solids

Graph relates the concentration of solution at equilibrium with the amount of solute adsorbed onto the surface of one unit mass of solid, at specified temperature.

Studies

a) Prepare solutions of various concentrations (from the lowest to near saturation)b) Determine the UV absorption (or other methods) to obtain a standard curve.c) Each concentration is put into 2 different container; one set without solid and

another set with solid of certain mass.

d) Allow equilibrium to be reached à concentration does not change after adsorption by container (set 1) and container and solid (set 2)e) i) determine the concentration in set 1 à initial concentration, and calculate initial amount in solution ii) determine the concentration in set 2 à concentration after adsorption, and calculate amount left in solution

Amount adsorbed = initial amount – amount left

Draw graph to determine isotherm

Set 1

Set 2

Amount adsorbed/ mass of solid

Concentration at equilibrium (concentration of set 2)

Determination of interaction and affinity of solutesa) Isotherm L

i) affinity of solute-solid > solute-solvent and solute-soluteii) Forms monolayer before additional layersiii) No competitioniv) Chemical or physical interaction (need desorption studies)

Amount adsorbed/ mass of solid

Equilibrium concentration (C)

Types of isotherms

S (slow)

L (Langmuir)

H (High affinity)

HC (constant)

Types of isotherms is affected by:Affinity (high, low)Interaction (chemical, physical)

Physical properties of solid (d, Sw, porosity)

Affected by competition solubility IonizationSurface charge

∆ pH

b) Isotherm CAdsorption occur easily because:

i) Affinity of solute-solid>solute-solvent and solute-soluteii) Surface area or place for adsorption to occur very big àdifficult to fill

rate of adsorption constant at the major part of the curve àe.g. fibrous materialsiii) Presence of competition does not greatly affect except steepness of slope and

Q may be a little lower

c) Isotherm HAdsorption is very easy

i) Affinity of solute-solid >>> solute-solvent and solute-soluteAt low initial concentrations, all of the solutes is adsorbed, none left in the

solution (equilibrium concentration O à amount adsorbed high)Solute prefer to be at interface than solution àSurfactant (surface active agent)Changes when : - affinity reduces - competition present - chemical, solubility changes

d) Isotherm SAdsorption low or difficult at low Ceq due to:

i) Affinity between solute-solid< solute-solvent << solute-solute, when adsorption has occurred, additional layers can form easily even though the monolayer has not formed or

ii) Chemical interaction between solute-solid, but places limited. Physical interaction of solute with those already adsorbed occurs easily or

iii) Competition from other types of solute presentPlaces for chemical or physical bonds to form limited (party filled by other

solutes already). Physical interaction of solutes with those already adsorbed occurs easily

Changes when:- Affinity changes- Type of interaction changes- Competition reduces

Uses of adsorption studiesA) Affinity of solute-solid

Can be predicted from the initial gradient of the slope of the curve

B. To determine characteristics of adsorbentQ à to determine Sw and dQk à total number of pores. Porosity or volume ≈ area between adsorption and

desorption curvesIsotherm C à total places or area for adsorption very high, unlimited

C. To determine a suitable adsorbent1. To adsorb;a. Toxin – diarrhoea eg. Kaolinb. Excess active substance – poisoning eg charcoal

. Bile secretions – to reduce cholesterol eg cholestyramined. Fats – eg chitosane. Water

diarrhoea à to solidify eg. PolycarbofilConstipation à to soften eg methylcellulose

ii) To carry active substances:- Adsorbent should be able to adsorb active substance in great quantities during

preparation

- Need to release the active substance at a particular location or pH. Active substance adsorption affinity at the site of absorption should reduce so that the active substance can be released at the required rate.

θ = thetaΦ = phiα = alpha Γ = gammaπ = piγ = gammaρ = rhoη = eta