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