Adsorption at the gas-solid interface

Adsorption from solutions

István Bányai,

University of Debrecen

Dept of Colloid and Environmental Chemistry

www.kolloid.unideb.hu

Short repetition

Lamgmuir and Langmuir-Blodget monolayers

4. lecture

Monolayer and polymolecular Langmuir – (Blodgett (LB)) layers

0 0

0 0A A kT

Liquid-expended films obey the equation of state (van der Waals equation)

Surface pressure

The surface pressure of the film is determined by measuring the force which must be applied to maintain the float at a fixed position on the surface (located optically) and dividing by the length of the float.

Gaseous films A kT

So far

4. lecture

There are four (?) main phases that can be identified from π~A isotherms

First order transitions between phases should appear as constant pressure regions in the surface pressure-area isotherm

4. lecture

Monolayers

• Summary – Insoluble monolayers are formed from amphiphiles that are

virtually insoluble in water. They can be spread on a water surface from drops of a solution of the amphiphile in a volatile solvent or by spontaneous spreading from crystals or drops of the bulk material. Surface balance is used to manipulate monolayers and is a component of various techniques for measuring monolayers properties.

– Monolayers exist in a number of phases, must of which can be compared to smectic phases. The major phases are the S, L and G. The hydrophilic part of each molecule anchors the molecule to the water surface and the hydrophobic part is in the air and may be disordered, or organized into hexagonal or rectangular patterns.

Molecular size, shape, conformation and packing can be determined

4. lecture

LB: transfer on a surface

2009.09.22 5. lecture

LB layers Hydrophilic surface

Tail-to-tail, head-to-head configuration

2009.09.22 5. lecture

LB layers hydrophobic

First layer tail-to-tail weak, head-to-head strong. Further layer types Y HH, TT, X HT, Z

TH. Single layer horizontal technique. Self-assembly.

Solid surface

Solids keep their shape A, so surface tension of solids can be decreased only by adsorption. Surface tension depends on the structure and the solid’s history too.

Solid surface has a structure. Solid surface is never homogeneous at a molecular level even if in case of a single crystal.

Hard sphere representation of surface features of single crystal

A crystal can be cut so that one face of the crystal makes up the surface of interest

An atom on the surface has less nearest neighbors, compared to the bulk. The loss of coordination increases the surface energy.

Basic definitions

Adsorption is the enrichment (positive adsorption, or briefly, adsorption) or depletion (negative adsorption) of one or more components in an interfacial layer.

The material in the adsorbed state is called the adsorbate, while that present in one or other (or both) of the bulk phases and capable of being adsorbed may be distinguished as the adsorptive. (In some cases of chemisorption adsorptive and adsorbate may be chemically different species (e.g. in dissociative adsorption).)

When adsorption occurs (or may occur) at the interface between a fluid phase and a solid, the solid is usually called the adsorbent (or substrate) ; for gas/liquid interfaces it may be in some, but not in all, cases useful to call the liquid phase the adsorbent

Chemisorption-Physisorption (IUPAC)

Chemisorption (or chemical adsorption) is adsorption in which the forces involved are valence forces of the same kind as those operating in the formation of chemical compounds. The problem of distinguishing between chemisorption and physisorption (see below) is basically the same as that of distinguishing between chemical and physical interaction in general. No absolutely sharp distinction can be made and intermediate cases exist, for example, adsorption involving strong hydrogen bonds or weak charge transfer.

Physisorption (or physical adsorption) is adsorption in which the forces involved are intermolecular forces (van der Waals forces) of the same kind as those responsible for the imperfection of real gases and the condensation of vapours, and which do not involve a significant change in the electronic orbital patterns of the species involved.

Chemisorption

(a) the phenomenon is characterized by chemical specificity, the adsorptive(s) may be altered (dissociation), may be not reversible

(b) the energy of chemisorption is of the same order of magnitude as the energy change in a chemical reaction between a solid and a fluid: thus chemisorption, like chemical reactions in general, may be exothermic or endothermic

(c) the elementary step in chemisorption often involves an activation energy; (activated adsorption), true equilibrium may be achieved slowly or in practice not at all.

(g) since the adsorbed molecules are linked to the surface by valence bonds, they will usually occupy certain adsorption sites on the surface and only one layer of chemisorbed molecules is formed (monolayer adsorption)

Physisorption

(a') the phenomenon is a general one and occurs in any solid/fluid system, (although certain specific molecular interactions may occur, arising from particular geometrical or electronic properties of the adsorbent and/or adsorptive);

(b') evidence for the perturbation of the electronic states of adsorbent and adsorbate is minimal;

(c') the adsorbed species are chemically identical with those in the fluid phase, so that the chemical nature of the fluid is not altered by adsorption and subsequent desorption;

(d') the energy of interaction between the molecules of adsorbate and the adsorbent is of the same order of magnitude as, but is usually greater than, the energy of condensation of the adsorptive;

Phisysorption

(e') the elementary step in physical adsorption from a gas phase does not involve an activation energy. (Slow, temperature dependent, equilibration may however result from rate-determining transport processes); in physical adsorption, equilibrium is established between the adsorbate and the fluid phase.

(f') under appropriate conditions of pressure and temperature, molecules from the gas phase can be adsorbed in excess of those in direct contact with the surface (multilayer adsorption or filling of micropores, capillary condensation)

Reading (Physical Chemistry Text) Even a gas like nitrogen, which is nonpolar and not very reactive , will adsorb to a surface under certain

conditions. When adsorption is occurs this is spontaneous which means that the free energy change, G is

negative . In most cases there is also a decrease in entropy, S, because the gas looses degrees of freedom.

Hence, to ensure that Delta G is negative, the enthalpy change, H, must be negative and large enough to

compensate the term of entropy.

Physical adsorption , because involves only the forces of molecular interaction, is very similar to the

condensation of a vapor to form a liquid. The enthalpy of the physical adsorption is

roughly (20kJ/mol) the same as the enthalpy of condensation (or it is usually not greater than few

times) and in many ways the adsorbed materials , especially when many layers have been adsorbed

, behaves like a two dimensional liquid. Reversibility : Physical adsorption can be reversed by simple

reducing the gas pressure and usually without raising of the temperature.

In chemisorption , on the other hand, a chemical bond is formed. Consequently the enthalpies of

the adsorption are much greater (~>80kJ/mol) than for physical adsorption. Furthermore, the

adsorbed atoms are localized at particular sites on the solid surface and only one layer of adsorbate

may be chemisorbed. Physical adsorption on top of a chemisorbed layer is possible if the conditions are

appropriate.

Volumetric method, gas adsorption has been measured by this method in which the change in volume of gas

during adsorption is measured directly.

Gravimetric methods a microbalance is just a very sensitive weighing device, usually to weigh nanograms.

0G H T S

Adsorption at the gas-solid interface

• A solid surface in contact with gas usually attracts an adsorbed layer of gas molecules

0G H T S

Adsorption isotherms: the amount of gas adsorbed as a function of the equilibrium gas pressure. Classification of adsorption isotherms based on their shapes

Even a gas like nitrogen, which is nonpolar and not very reactive, will adsorb to a surface under certain conditions. When adsorption is occurs this is spontaneous which means that the free energy change, G is negative.

Strong interaction type I or Langmuir, type II or BET, type IV or capillary condensation

Adsorbed amount are shown as volume, mass or the moles of molecules adsorbed per unit area, or as θ, coverage of surface vs. relative pressure p/p0

Adso

rbed

am

ount

or

cover

age

1m

bp

bp

1

m m

p p

b

2 2( / ) ( / ) / /m m Amol g m molecules N molecules mol specific surface area m g

Langmuir type I (for gas) isotherm

Assumptions: monolayer, uniform surface, adsorbed molecules do not interact with neighboring adsorbed molecules, adsorption-desorption dynamic equilibrium with ka and kd rate constant respectively.

m the amount is required for complete monolayer coverage monolayer capacity, and b, the sorption constant is defined as where E1 sorption „enthalpy”:

1exp /

a

d

kb

k E RT

rearranged into

Irving Langmuir Nobel price in 1932 A

dso

rbed

am

ount

or

cover

age

BET isotherm, type II Brunauer , Emmett, Teller (for gas)

0 0( ) 1 1 /m

Zp

p p Z p p

1exp /vZ E E RT

Assumptions: multi layer adsorption of gas, the Langmuir equation is applied to each layer, adsorption and desorption can only occur at exposed surfaces, at equilibrium, the distribution of adsorbate between the different layers is constant.E1, Ev are the enthalpy of adsorption of first layer and the evaporation of the adsorbate.

Net adsorption energy

Adso

rbed

am

ount

or

cover

age

Stephan Brunauer; Paul Emmett , Ede Teller

B monolayer capacity: m

Capillary condensation, type IV (for vapor)

Condensation occurs when the actual vapor pressure exceeds the equilibrium vapor pressure with porous solids

Zsigmondy: If surface of liquid is concave, radius r<0, the Kelvin equation shows that the equilibrium vapor pressure pr , may be significantly lower than p0 (≡ p∞). Thus condensation may occur pr /p0 <1, a phenomena known as capillary condensation. Expectations: porous solids, relative pressure is high, liquid wets the surface.

, ,,

0

2 1 1ln r L L

m

p V V

p RT r RT r r

Adsorption, desorption

hysteresis

Wetting, hysteresis , surface

roughness, microscopic chemical

heterogeneity of solid surface,

drop size effect, molecular

reorientation, and penetration

into the pores)

Pore size distribution can be calculated from pr/p0~r

Underground storage water, water budget!! (plough) The smaller capillaries store the higher amount water

Adso

rbed

am

ount

or

cover

age

hysteresis

non-spherical

Adsorption from solutions

Non-electrolyte adsorption Adsorption of strong electrolytes

Component

adsorption From dilute

solution

Equivalent or molecular adsorption

Ion exchange adsorption or non-equivalent

Neutral

surface

Non-

neutral

surface

polar

surface

apolar

surface

Empirical

rules

Excess

isotherms

Electrical

double

layers

Adsorption at low solute concentration

For low solution concentrations adsorption isotherms generally have a form similar to the type I isotherms

, ,m ma a p c

Similar likes similar

Every system seeks to achieve a minimum of free energy. Emprical rules.

0( ), /

V c ca mol g

m

Adsorbent

Components Bulk

a a

2009.09.23 5b. lecture

Adsorption at low solute concentration For low solution concentrations adsorption isotherms generally have a form similar to

the type I isotherms of gas-solid adsorption and can be fitted either by Langmuir equation (or Freundlich) isotherm equation.

The Langmuir equation may be written as follows:

where am is the monolayer capacity, c is the equilibrium concentration of solute and b is constant. The monolayer capacity can be estimated either directly from the actual isotherm or indirectly by applying the linear form of the Langmuir equation.

A plot of c/a against c must be a linear line with a slope of 1/am The surface area of the solid (usually expressed as square meters per gram) may be obtained from the derived value of am provided that the area occupied by the adsorbed molecule on the surface is known with reasonable certainty.

where NA is Avogadro’s number and fm is the cross-sectional are of an adsorbed molecule. Recall that the surface area determined by this method is the total area accessible to the solute molecules.

1/

ma ca

b c

1

m m

c c

a a ba

mAmsurface NaA f

, ,m ma a p c

Adsorption at low solute concentration

.

1/

ma ca

b c

1

m m

c c

a a ba

mAmsurface NaA f

c, mol/l

a,

mo

l/g

y = 29773x + 2.0978

R2 = 0.9744

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0 0.0002 0.0004ce, mol/l

c/a

, g

/l

2009.09.23 5b. lecture

Adsorption at higher solute concentration Composite adsorption isotherms for adsorption on solids from

binary liquid mixtures

For a mixture of two miscible liquids, A and B, the adsorption isotherms of one component is complicated by the competitive adsorption of the other component. The apparent adsorption isotherm that is determined by measuring the change in concentration of one component (the ‘solute’) is often called the composite adsorption isotherm or sometimes the isotherm of composition change. The apparent surface excess of component B is given by:

s s b

B B B A Bn n n n x

Where ns is the total amount at the surface whereas n is the surface excess amount.

2009.09.23 5b. lecture

Adsorption at higher solute concentration Composite adsorption isotherms for adsorption on solids from binary liquid

mixtures

U-shaped

S-shaped

CHCl3 from CCl4 on to charcoal

Excess adsorption isotherms-

apparent adsorption

Specific

surface

excess

amount

molar fraction of component(1)

x1,a azeotropic composition

One components concentrate at the

interface region and the other is depleted

Similar likes similar

Chromatography

– Adsorption Chromatography – Partition Chromatography – Size (Molecular) Exclusion

Chromatography (SEC) – Ion Exchange Chromatography – Affinity Chromatography

What is chromatography? Chromatography is a separation method .

The components to be separated are distributed between two phases: a stationary phase bed and a mobile phase which percolates through the stationary bed. The equilibration between the mobile and stationary phase may based on adsorption, partition, size exclusion, ion exchange or special affinity. The smaller the affinity a molecule has for the stationary phase, the shorter the time spent in a column.

1 11

1 1 2 21m

b p

b p b p

1 2b b selectivity

Basic expression for chromatography:

G, L, S, R,C

Types of Chromatography

Summary Adsorption chromatography utilizes a mobile liquid or gaseous phase that is adsorbed onto the surface

of a stationary solid phase. The equilibration between the mobile and stationary phase accounts for the

separation of different solutes. Partition Chromatography This form of chromatography is based on a thin film formed on the surface of a solid support by a liquid

stationary phase. Solute equilibrates between the mobile phase and the stationary liquid. Ion Exchange Chromatography In this type of chromatography, the use of a resin (the stationary solid phase) is used to covalently attach

anions or cations onto it. Solute ions of the opposite charge in the mobile liquid phase are attracted to the

resin by electrostatic forces.

Size exclusion chromatography also known as gel permeation or gel filtration, this type of

chromatography lacks an attractive interaction between the stationary phase and solute. The liquid or

gaseous phase passes through a porous gel which separates the molecules according to its size and shape.

The pores are normally small and exclude the larger solute molecules, but allows smaller molecules to

enter the gel, causing them to flow through a larger volume. This causes the larger molecules to pass

through the column at a faster rate than the smaller ones.

Affinity Chromatography is the most selective type of chromatography employed. It utilizes the

specific interaction between one kind of solute molecule and a second molecule that is immobilized on a

stationary phase. For example, the immobilized molecule may be an antibody to some specific protein.

When solute containing a mixture of proteins are passed by this molecule, only the specific protein is

reacted to this antibody, binding it to the stationary phase. This protein is later extracted by changing the

ionic strength or pH.

Reading

Retention • The retention is a measure of the speed at which a substance moves in a

chromatographic system. In continuous development systems like HPLC or GC, where the compounds are eluted with the eluent, the retention is usually measured as the retention time Rt or tR, the time between injection and

detection. In interrupted development systems like TLC the retention is measured as the retention factor Rf, the run length of the compound divided by

the run length of the eluent front

Purification, separation

(industry, biotechnology,

gas mask)

The smaller the affinity a

molecule has for the stationary

phase, the shorter the time

spent in a column.

Reading

Thin Layer Chromatography

TLC chamber for development e.g.

beacher

with a lid or a closed jar

after ~5

min

after ~10

min

after

drying

Solvent = mobile phase, layer stationary phase

Reading

Thin Layer Chromatography

Developed TLC plates: We can examine the plate under UV light to see the components as dark spots against a bright green-blue background.

Unknowns can be identified using Rf values, fluorescence in UV light, or comparing with reference spots.

Place a small amount of solvent (= mobile phase) in the container. The solvent level has to be below the starting line of the TLC, otherwise the spots will dissolve away. The lower edge of the plate is then dipped in a solvent. The solvent (eluent) travels up the matrix by capillarity, moving the components of the samples at various rates because of their different degrees of interaction with the matrix (=stationary phase) and solubility in the developing solvent. Non-polar solvents will force non-polar compounds to the top of the plate, because the compounds dissolve well and do not interact with the polar stationary phase. Allow the solvent to travel up the plate until ~1 cm from the top. Take the plate out and mark the solvent front immediately. Do not allow the solvent to run over the edge of the plate. Next, let the solvent evaporate completely.

Reading

Reading!: Proteins Purification and Characterization

http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/Protein_Properties/protein_purification.htm

Nelson & Cox, Lehninger Principles of Biochemistry

Size Exclusion chromatography

Note how the small red spheres pass into the channels in the beads, whereas the large blue spheres do not. Thus, the small spheres have a longer "distance" to transverse than the large spheres to get to bottom of column, which means that a larger volume of solvent must pass through the column before the red spheres are eluted.

Reading

Ion Exchange Chromatography

Ion exchange resins have charged groups covalently

attached to the stationary phase (adsorbent, matrix), either

positive or negative. Obviously, if ionizable groups are

weak acids or bases, the pH of the buffer determines the

charge state of the matrix.

Proteins bind to the matrix by electrostatic interactions.

Strength of these interactions depends on

net charge on the protein (a function of buffer pH

and the nature of the ionizable groups on that protein,

reflected in the pI of the protein), and

salt concentration of the buffer (high salt

concentrations reduce the interaction and can be used

to elute the proteins by competing with the protein

groups for binding to the charged groups on the

matrix).

The higher the net charge on the protein at the pH of the

environment on the column, the more tightly it sticks to an

oppositely charged matrix, and the higher the salt

concentration required to elute it from the column.

The further the "working pH" is from the isoelectric

point (pI) of a protein, the greater the net charge on the

protein, and the more tightly it will stick to an ion

exchanger of opposite charge.

By proper choice of eluting buffer (often a gradient with

increasing salt concentration, or changing the pH),

specific proteins can be eluted from the column and

separated from other proteins in the mixture.

Reading

Affinity Chromatography

a more specific adsorbent in which a ligand specifically recognized by the protein of interest is covalently attached to the column material When a mixture of proteins is passed through the column, only those few that bind strongly to the ligand stick, while the others pass through the column. Protein of interest is eluted with a buffer containing the free ligand, which competes with the column ligand to bind to the protein, and protein washes off (with bound ligand).

Reading

Affinity chromatogphy

• Biotin is bonded to pegilated surface. Streptavidin –biotin non-covalent bond is strong (4 docking) (10-15 M)

36