chem 2360 c20c basic physical chemistry lectures 1 (1)

Surface and Colloid Chemistry

Lecturer: NIGEL JOHN

Monday

9am to 11am

Physical Chemistry Lab (FSA CL 4)

Adsorption of gases on solid.

1. Adsorption processes are critical in the operation of solid catalysts.

There are two main types of adsorption:

a. Physical adsorption (Physisorption)

b. Chemical adsorption (Chemisorption)

2. Physical Adsorption

a. Weak van der waal forces

b. Heat evolved ca. H20 kJmol-1

c. Physical adsorption is sometimes called van der waals adsorption.

d. Requires no activation energy

3. Chemical Adsorption First proposed by Irving Langmuir in 1916 a. Adsorbed molecules are held to surface by covalent forces/

bonds.

b. Heat evolved is comparable to chemical bonding H300-500 kJmol-1

c. Often requires an appreciable activation energy

4. Adsorption Isotherm

The amount of adsorption as a function of pressure at a constant temperature

There are a number of isotherms proposed, some empirical, others theoretical.

5. Langmuir Isotherm (Ideal Adsorption) Simplest theoretical model

Based on assumptions:

I. Chemisorption on perfectly uniform surface with no interaction between neighbouring molecules.

II. Heat of adsorption is independent of surface coverage.

Langmuir Isotherm

Model

Gas atoms or molecules adsorb onto surface and occupy single sites.

Let be the fraction of surface covered, (number of adsorption sites filled)

Let 1- be the fraction uncovered, (number of adsorption sites available)

is calculated by assuming that equilibrium is established.

rate of adsorption = rate of desorption

Langmuir Isotherm

rate of adsorption = ka P(1-)

ka rate constant of adsorption

P= pressure of gas

rate of desorption = kd

At equilibrium kaP(1-) = kd

Langmuir Isotherm

Graphically represented

At equilibrium

Rate of adsorption = rate of desorption

kaP(1-) = kd

therefore = kaP/ (kaP +kd)

divide top and bottom by kd

= KP/ 1 + KP where K = ka/kd

1- = 1/ [1+KP]

Langmuir Isotherm

Experimental Langmuir plot is normally P/V vs P,

derived from = KP/ (1+KP),

where = V/Vmono

Where: P/V = P/ Vmono + 1/KVmono

y= mx + c

6. Competitive Gaseous Adsorption

When two gas components compete for surface adsorption, e.g A and B

A = VA/Vmono =[KAPA]/[1+KAPA+KBPB]

B = VB/Vmono =[KBPB]/[1+KAPA+KBPB]

The equation for A reduces to one component system if PB= 0 or KB=0

KB = 0, means that species B is not adsorbed.

7. Enthalpy of Adsorption (or Isosteric enthalpy of adsorption) Enthalpy of adsorption at a fixed surface coverage H, found

from van't Hoff equation dlnK/dT = H/ RT2 V.H. equation [lnK / T] = Had/RT

2 [lnP / T] = -[lnK / T] = -Had/RT

2

with d /dT (1/T) = -1/T2, this expression rearranges to: [lnP / (1/T)] = Had/R Therefore a plot of lnP vs 1/T Slope =Had/R

Recap Define the following:

absorption

adsorption

physical adsorption

chemical adsorption

adsorption isotherm

Give an example of an adsorption isotherm

State the van't Hoff equation

Non- Ideal Adsorption.

Adsorption Isotherms

Ideal adsorption Non- Ideal Adsorption

Langmuir Isotherm

Freundlich Isotherm

Tempkin Isotherm

BET Isotherm

1. Deviation from Langmuir Isotherm This is due to the following possible factors:

Surface is not uniform

Interactions between adsorbed molecules

Adsorption of more than one layer (multilayer)

Sites are not equivalent

Steric hindrance of neighbouring sites

Non- Ideal Adsorption Isotherms Freundlich Isotherm

Tempkin Isotherm

BET Isotherm

2. Freundlich Isotherm An empirical adsorption isotherm that afterwards was backed by

theory. = c1P

1/c2 where c1 and c2 are constants. a) Take into account that the enthalpy of adsorption becomes less

negative as surface coverage, , increases ; i.e. most energetic sites are occupied first. b) Can be derived theoretically by assuming that -Had log , i.e. c2ln = -Had

/ RT + constant c) Applies to chemisorption and physisorption

3. Tempkin (or Slygin-Frumkin) Isotherm Empirical isotherm = c1ln c2P c1 and c2 are constants a) Can be derived theoretically by assuming that -Had

i.e. = c1 Had

/ RT + constant

b) Applies to chemisorption

4. BET Isotherm Langmuir isotherm assumes monolayer adsorption and does not

allow for the possibility of multilayer adsorption. Brunauer, Emmett and Teller proposed a multilayer isotherm

called BET isotherm

= V/Vmono = cz/ [(1-z){1-(1-c)z}] where z = P/P* P* = vapour pressure of a macroscopically thick layer of pure

liquid on surface, Vmono

4. BET Isotherm = V/Vmono = cz/ [(1-z){1-(1-c)z}]

c e(Hd -Hvap)/RT = constant

Hd = enthalpy of desorption

Hvap= enthalpy of vapourisation

4. BET Isotherm

BET isotherm normally expressed as:

z/(1-z)V = 1/cVmono + (c-1)z/ cVmono

plot of z/(1-z)V vs z,

gives a slope = (c-1)/ cVmono and

intercept = 1/ cVmono

Adsorption Isotherms Recap Langmuir Isotherm, = KP/ 1 + KP

Freundlich Isotherm, = c1P1/c2

Tempkin Isotherm, = c1ln c2P

BET Isotherm, = cz/ [(1-z){1-(1-c)z}]

NB: = V/Vmono

5. Surface Processes e.g. Dissociative Chemisorption

Physisorption is normally the precursor state for chemisorption.

Dissociation takes place as molecule moves into chemisorption.

Note 1:

Rate of surface coverage by adsorbate depends on ability of substrate to dissipate its energy as thermal energy.

5. Surface Processes e.g. Dissociative Chemisorption Physisorption is normally the precursor state for chemisorption. Dissociation takes place as molecule moves into chemisorption.

Note 1: Rate of surface coverage by adsorbate depends on ability of

substrate to dissipate its energy as thermal energy. Note 2: Molecules are often mobile on surface; a vital feature of solid

surface catalytic activity.

6. Catalytic Activity at Surface Catalysis provides an alternative reaction path with a lower

activation energy.

Mechanisms of surface reactions occurs in 5 main steps:

1. diffusion of reactants molecule to the surface

2. adsorption of gases on the surface

3. reaction on the surface

4. desorption of the product

5. diffusion of desorbed products into main body of gas

6. Catalytic Activity at Surface Slowest steps are usually adsorption or desorption in

heterogeneous reactions. e.g. Langmuir - Hinshelwood Mechanism A + B P e.g. oxidation of CO on platinum = kAB where = rate of reaction between mobile adsorbed A and B. A = [KAPA]/[1+KAPA+KBPB] B = [KBPB]/[1+KAPA+KBPB] = (kKAPAKBPB) / (1+KAPA + KBPB)

2

Surface Processes Recap Physisorption is normally the precursor state for

chemisorption

Molecules are often mobile on surface; a vital feature of solid surface catalytic activity.

Colloids

Colloids Classification of colloids (phase, solvent)

Preparation Methods

Purification of Colloids

Stability of Colloids

1. Colloids Comes from Greek for glue like.

A dispersion of small particles of one material in another, which does not separate on long standing.

In general, they are aggregates of numerous atoms or molecules.

They pass through most filter paper

Detected by light-scattering, sedimentation and osmosis

2. Classification of colloids (A) Colloids are classified according to the two phases

involved. Sols

Solid in liquid (e.g gold in water)

Solid in solid (e.g gold in glass)

Aerosols Liquid in gas (e.g fog and many sprays)

Solid in gas (e.g smoke)

Emulsion Liquid in liquid (e.g milk)

2. Classification of colloids (B) Colloids can be classified according to "solvent

liking" Lyophilic (solvent attracting) Lyophobic (solvent repelling)

Example Lyophobic colloids metal sols

Lyophilic colloids

contain -OH groups that are able to form H-bonds gels, (semi - rigid mass of lyophilic sol in which all dispersion medium has be

absorded by sol particles.)

3. Preparation Methods Chemical precipitation,

e.g a precipitate of AgI on the presence of a peptizing agent (e.g Potassium Iodide)

Emulsions,

formed by shaking two components in the presence of an emulsifying agent (emulsifer) Emulsifer

Soap Surfactant Lyophilic sol

e.g Milk is an emulsion of fats in emulsifying agent, casein (a

protein containing phosphate groups)

4. Purification of Colloids Often purified by dialysis or electro dialysis (faster

technique)

5. Stability of Colloids The principal feature of a colloid is the very large surface area dispersed phase.

From thermodynamics dG = VdP - SdT + d change in surface area, surface tension at constant P and T dG = d Hence dG = -ve, for decreasing surface area, therefore colloids are not thermodynamically stable. Stability is due to kinetic stability.

5. Stability of Colloids Kinetic Stability

Colloid particles have long-ranged interactions:

Interactions between two clusters of atoms decrease as 1/r2 as opposed to 1/r6 between individual atoms.

Kinetic stability is imparted by factors that opposed the long-range dispersion attractions.

e.g Protective film on surface of colloidal particles Pt sol in H2O -Pt(OH)3H3 (protective film)

Emulsifier

Electrical charge

5. Stability of Colloids Kinetic stability

Double layer = stern layer + diffused layer

Two important regions of charge

First adsorbed layer of counter-ions is called the stern layer.

Immobile layer of ions that stick tightly to surface of colloids, Surface of shear

The electrical potential at this layer is called the zeta potential () or electrokinetic potential.

6. Colloids at high ionic strengths As counter-ions concentration increase the charge at

surface is progressively neutralised. When the surface is neutralised the colloid particles

flocculate as attractive van der Waals interaction becomes dominant.

Basis of Schultze-Hardy rule:

Hydrophobic colloids are flocculated most efficiently by ions of opposite charge type and high charge.

e.g. Al3+ ions in alum are used in septic tanks.

Offices hours Wednesday

10am to 12 noon


Colloids, Surface Tension and Surfactants

Colloids Classification of colloids

(phase, solvent)

Preparation Methods

Purification of Colloids

Stability of Colloids (large surface area dispersed phase)

1. Stability of Colloids The principal feature of a colloid is the very large surface area

dispersed phase.

From thermodynamics dG = VdP - SdT + d change in surface area, surface tension at constant P and T dG = d Hence dG = -ve, for decreasing surface area,

2. Surface Tension ()

I. Molecules at the surface of a liquid are attracted inward due to intermolecular attractions.

This creates a force on the surface which tends to minimise the surface area (), called the surface tension.

II. If the surface is stretched, surface tension increases.

NB: Surface tension is sometimes called surface energy.

3. The effect of solute addition on surface tension Addition of solute to liquids usually decreases the

surface tension.

These solutes are called surface active agents or surfactants.

These solutes accumulate at the interface and modifies the surface tension.

4. Interfacial Area I. Three physical states exist: Solid, liquid and gas

However when two phases are in contact with each other, an interfacial area results.

II. The interfacial area has different properties than the two bulk phases

5. Surface Excess Concentration of solute (J) How do we quantify the surface tension effect? I. Consider two phases and in contact, consisting of J

components, each one in nJ amount. II. If nJ components are uniformly distributed throughout

each phase up to the interface (or boundary). Total free energy G, G = G() + G()

5. Surface Excess Concentration of solute (J)

In practise, it is found that nJ components are not uniformly distributed.

G G() + G() It is found that G = G() + G() + G() G() is called the surface Gibbs function i.e. G() = G - {G() + G()} similarly nJ() = nJ - { nJ() + nJ()} nJ() is called excess surface concentration of solute. Normally nJ is expressed as nJ per unit area called the surface excess (J) J = nJ()/ = interfacial area Both nJ() and J can be +ve or -ve, when +ve, it is called accumulation, when -ve, it is called deficiency.

6. Gibbs Surface Tension Equation Relates change in surface tension () with solute concentration (C), Points: I. When solute is transferred to interface, there is a lowering of free

energy (G()) G() = nJ() dJ Where J is called the chemical potential II. This lowering of free energy (G()) is equivalent to the lowering of

the surface tension ()

G() = -d = surface area = change in

6. Gibbs Surface Tension Equation Hence, nJ()dJ = -d ............................................eqn. 1 Rearrange: nJ()/ = -d/dJ nJ()/ = J J = -(d/dJ)T ..........eqn.4 Gibbs Adsorption Isotherm Given that J = J

+ RTlnCJ Therefore dJ = RTdlnCJ ..............................................eqn.6 Substitution of eqn. 6 in eqn. 4 J = -d/ RTdlnCJ = -1/RT (d/dlnCJ)T

6. Gibbs Surface Tension Equation Re: dlnx/dx = 1/x from mathematics dlnx= dx/x , dlnCj= dC j/C j J = -1 CJ (d/dCJ)T RT Therefore J = - CJ /RT (d/dCJ)T another form of Gibbs Adsorption Isotherm When J = surfactant molecule, S S = +ve therefore d/dCS = -ve i.e. Surface tension () decreases with increase in surfactant solute (S)

concentration.

Surfactants

1. Surfactants ( Surface active agents) Compounds that lower surface tension of a liquid

Used as emulsifying agents, detergents, wetting agents etc.

I. Structure

All surfactants have two regions:

a) Lyophobic (or hydrophobic) tail group normally hydrocarbon chain

b) Lyophilic (or hydrophilic) head group

1. Surfactants II. Classification

Surfactants are classified according to its HEAD group. a) Anionic, e.g. Sodium dodecyl sulphate b) Cationic, e.g. Hexadecyl trimethyl ammonium bromide,

(CTAB) c) Nonionic, e.g. Hexaoxyethylene mono hexadecyl ether d) Ampholytic, e.g. N-dodecylalanine

2. Critical Micellar Concentration (CMC) a) At low concentration in aqueous solution,

surfactant molecules orientate themselves at the air/water interface.

2. Critical Micellar Concentration (CMC) As surfactant concentration increases,

a certain well defined concentration is reached where the physical properties of solution changes,

critical micellar concentration.

2. Critical Micellar Concentration (CMC) As surfactant concentration increases,

a certain well defined concentration is reached where the physical properties of solution changes,

critical micellar concentration.

This change is associated with aggregation of molecules into micelles.

2. Critical Micellar Concentration (CMC) b) Measurement of CMC

Micelle formation can be monitored via surface tension or conductivity experiments.

2. Critical Micellar Concentration (CMC) b) Measurement of CMC

Micelle formation can be monitored via surface tension or conductivity experiments.

Typical aggregate no. ca. 100 molecules

2. Critical Micellar Concentration (CMC)

3. Krafft Temperature or Point For any particular surfactant, there exists a

temperature, at which it is highly soluble,

Krafft temperature or point.

At temperature below Krafft point micelles will not form.

e.g. CTAB (Cetyl trimethyl ammonium bromide)

Krafft temperature ca. 30C.

Offices hours Wednesday

10 am to 11 am


chem 2360 c20c basic physical chemistry lectures 1 (1)

Documents