Adsorption from solutions - unideb.hu

2009.09.23 5b. lecture

Adsorption from solutions

Levente Novák & István Bányai,

University of Debrecen

Dept of Colloid and Environmental Chemistry

http://kolloid.unideb.hu/

2009.09.23 5b. lecture

Adsorption from solutions

Non-electrolyte adsorption Adsorption of strong electrolytes

Composite

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

Similar likes similar

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

Adsorbent

Components Bulk

a a

2009.09.23 5b. lecture

Adsorption from solutions

Non-electrolyte adsorption Adsorption of strong electrolytes

Composite

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

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:

hydrophobic - hydrophilic

2009.09.23 6. lecture

Electrical double layer

Levente Novák & István Bányai,

University of Debrecen

Dept of Colloid and Environmental Chemistry

http://dragon.unideb.hu/~kolloid/

2009.09.23 5b. lecture

Adsorption from strong electrolytes: electric double layer

Non-electrolyte adsorption Adsorption of strong electrolytes

Composite

adsorption From dilute

solution

Equivalent or

molecular

adsorption

Ion exchange

adsorption or non-

equivalent:

substituion

Neutral

surface

Non-

neutral

surface

polar

surface

apolar

surface

Empirical

rules

Excess

isotherms

Electrical

double

layers

Adsorption of strong electrolytes from aqueous solutions

Stoichiometric or equivalent Non-stoichiometric or ion exchange

Neutral surface Non-neutral surface

apolar

polar Ashless charcoal , lyotropic series (Ionic charge and size, determine the attraction to surface: Al3+ > Ca2+ = Mg2+ > K+ = NH4

+ > Na+. For anions: CNS-> (PO43- , CO3

2-) > I- > NO3- > Br- > Cl- > C2H3O2

- > F- > SO42- )

Ionic crystal from its solution at a specific concentration

Electric double layer

?? Which ion goes into the inner layer? (this gives the sign of the surface charge)

Every solid surface will be charged in water

Anion-, cation

Charged surface- electric double layer

Surface Charge Density. When a solid emerges in a polar solvent or an electrolyte solution, a surface charge will be developed through one or more of the following mechanisms:

a./Dissociation of surface charged species (proteins COOH/COO-, NH2/NH3

+)

b./ Preferential adsorption of ions. Specific ion adsorption: Surfactant ions may be specifically adsorbed.

c./ Charged crystal surface: Fracturing crystals can reveal surfaces with differing properties. d./ Isomorphous replacement: e.g. in kaolinite, Si4+ is replaced by Al3+ to give negative charges.

http://www.dur.ac.uk/sharon.cooper/lectures/colloids/interfacesweb1.html#_Toc449417608

solvation

Electric double layer

One layer of counter ion closely adsorbs on charged surface Double layer forms. Which is the preferential adsorption of ions?

• The related ions

• The ions which form hardly soluble or dissociable compounds with one of the ions of the lattice

• The ion of larger charges

• H+ or OH- (any surfaces )

Example: AgCl crystal AgNO3 or KCl solution

AgCl crystal KBr, or KSCN solution

From NaCl, CaCl2 solution : Ca2+

The free acid or base adsorb stronger than the other electrolytes, because the mobilities and specific charge of H + and OH- are larger.

http://www.dur.ac.uk/sharon.cooper/lectures/colloids/interfacesweb1.html#_Toc449417608

2009.09.23 6. lecture

Charge determining ions

0 ( )F C/m2 C/mol mol/m2

Ele

ctro

phore

tic

mobil

ity

When silver iodide crystals are placed in water, a certain amount of dissolution occurs to establish the equilibrium:

AgI = Ag+ + I-

The concentrations of Ag+ and I- in solution from the solubility product is very small (Ksp= aAg+ aI- =10-16). The slight shifts in the balance between cations and anions can cause a dramatic change in the charge on the surface of the crystals. If there are exactly equal numbers of silver and iodide ions on the surface then it will be “uncharged”.

Charge determining ions

=0, p.z.c.

<0

>0

AgI in its own saturated solution is negatív!

cAg+=cI- =8.7x10-9 mol/l

negative

positive

pAg+NTP = 5,3 (AgI)

cAg+>310-6 mol/l

c(Ag+)<310-6 mol/l

The , C/m2 surface charge density changes from positive to negative or vice versa. Zero-charged surface is defined as a point of zero charge (p.z.c.) or zero-point charge (z.p.c.).

The surface

concentration

of charge

determining

ions , mol/m2

2009.09.23 6. lecture

Potential-determining ions It seems that the iodide ions have a higher affinity for the surface and tend to be preferentially adsorbed. In order to reduce the charge to zero it is found that the silver concentration must be increased by adding a very small amount of silver nitrate solution about to: cAg+>310-6 mol/l

The charge which is carried by surface determine its electrostatic potential. For this reason, ions (adsorb, remain etc.) make the charge are called the potential-determining ions.

We can calculate the surface potential on the crystals of silver iodide by considering the equilibrium between the surface of charged crystal and the ions in the surrounding solution:

0 ln ln PZC

kTa a

ze

Where 0 the electrostatic potential difference (arising form the charge bearers ions,) at the interface (x=0), shortly surface potential;

a the activity of the ions and

apzc the activity of the ions at zero-charged surface in the solution.

0 lnkT

aze

9

0 6

8.7 1025.7 ln 150

3 10mV

0 ( )F

surface potential of silver iodide

crystals placed in clear water

AgI in its own saturated solution is negative!

cAg+=cI- =8.7x10-9 mol/l

2009.09.23 6. lecture

Variable charges surfaces, pzc

0 2.303( ) ~ 60pzc

kTpH pH mV pH

ze

added ion [Ag] -loga Ψ0 mV

- 1.3E-5 4.9 -20

1×10-4 Cl- 1.8E-6 5.7 -72

5×10-5 Ag+ 5.0E-5 4.3 14

1×10-4 Ag+ 1.0E-4 4.0 32

0 ln ln PZC

kTa a

ze

pAg+NTP = 4.54 (AgCl)

KspAgCl =1.8 x 10−10

0 60 ( ),pzcpAg pAg mV

surface potential of silver chloride crystals placed in clean water is -20 mV

2009.09.23 6. lecture

The structure of the electrical double layer

The model was first put forward in the 1850's by Helmholtz. The interactions between the ions in solution and the electrode surface were assumed to be electrostatic in nature and resulted from the fact that the electrode holds a charge density which arises from either an excess or deficiency of electrons at the electrode surface. In order for the interface to remain neutral the charge held on the electrode is balanced by the redistribution of ions close to the electrode surface. Helmholtz's view of this region is shown in the figure

The overall result is two layers of charge (the double layer) and a potential drop which is confined to only this region (termed the outer Helmholtz Plane, OHP) in solution. The model of Helmholtz does not account for many factors such as, diffusion/ mixing in solution, the possibility of adsorption on to the surface and the interaction between solvent dipole moments and the electrode.

Helmholtz model

2009.09.23 6. lecture

Helmholtz model (positive surface)

/V

x (indiv.u.)

surface potential

As simple as it is not valid It couldn’t explain the electrophoretic mobility and the effect of electrolyte.

0

+

0 conts x

2009.09.23 6. lecture

Gouy-Chapman model

/V

x (indiv.u.)

surface potential

1/

1/ the thickness of the DL;

depends on ionic strebgth:

~I1/2!!!!!

0

0 exp x

The ions would be moving about as a result of their thermal energy

0 exp x

+

e eze ze

kT kTn n n n

the electrostatic potential for different salt concentrations at a fixed surface charge of -0.2 C/m2.

The potential distribution near the surface

at different values of the (indifferent)

electrolyte concentration for a simple

Gouy-Chapman model of the double

layer. c3>c2>c1. For low potential

surfaces, the potential falls to 0/e at

distance 1/ from surface.

The concentration of ions as a distance of negative surface

It couldn’t explain the charge reversal.

0 exp x

2009.09.23 6. lecture

Stern model In the Stern model the ions are assumed to be able to move in solution and so the electrostatic interactions are in competition with Brownian motion. The result is still a region (compact) close to the electrode surface (10 nm) containing an excess of one type of ion (Stern layer) but now the potential drop occurs over the region called the diffuse layer where the Gouy-Chapman treatment still applies.

model of compact Stern

layer

Details of Stern layer

2009.09.23 5b. lecture

1. Dehydrated cations form the charge: internal H-layer

2. Hydrated anions rigidly bonded: outer H-layer

3. 1 and 2 give S-layer (dashed blue line)

4. Dashed red line shows the shear plane: zeta-potential

5. In the diffuse layer still we have anion excess

2009.09.23 6. lecture

Stern model

xSt

xSt

exp ( )St stx x

Plane of shear

: a Debye Hückel parameter

1/ the thickness of DL

e eze ze

kT kTn n n n

x

x

Finite ion size, specific ion sorption,

compact layer

The whole DL is electrically neutral.

the electrostatic interactions are in competition with Brownian motion in the diffuse layer (G-Ch treatment)

2009.09.23

Stern model, charge reversal

/V

x (indiv.u.)

surface potential

d

Stern-p.

plain of shear

potential

PO43-

0

St

compact Stern layer

+

0

01

Kn

Kn

exp Sze

KkT

Electrostatic term depends on z

chemical term ≥0

2009.09.23 6. lecture

Stern model, charge enhancement

/V

x (indiv.u.)

surface potential

dStern-p.

plain of shear

potential

cationic surfactants

0

St

exp SzeK

kT