Liquid menbranes

1. INTRODUCTION
• LIQUID MEMBRANES: A liquid acting as a semipermeable
barrier between two fluid phases.
• Driving Force  Solute concentration gradient. (in most
cases)
• Mechanism  Solution-diffusion (sometimes also chemical
reaction)
• 3 arrangements: BLM, ELM and SLM

2. Classification of liquid membranes
according to transport mechanisms

3. TRANSFER EQUATIONS
Difusion and chemical reaction Common steps
• Solution-diffusion (with or
without chemical reactions) is a
commonly accepted mechanism
for the transport of a solute in
liquid membrane.
• Rates of chemical changes
and/or rates of diffusion may
control all liquid membrane
transport kinetics.
• Analysis of mechanisms and
kinetics of the chemical and
diffusion steps of the overall LM
transport system is needed to
find the rate-controlling ones.
• Diffusion steps in aqueous feed.
• Diffusion of the complex solute-
carrier in the LM phase.
• Partitions between aqueous
feed and organic LM phases at
feed -LM interface and
between LM and aqueous strip
phases at LM-strip interface.
• Kinetics of chemical
interactions with formation of
solute-carrier complex and
destruction of complex.

4. Diffusion transport regime
Steady state
No steady state• The diffusion flux Js (M,
g/cm2s) of the species is
defined as the amount of
matter passing perpendicularly
through the unit area during
the unit time
• In a steady-state permeation
experiment, the flux of a
species S through a membrane
of thickness h is related to the
concentration gradient
through Fick’s first law:
• When steady state cannot
be assumed, the
concentration change with
time must be considered.

5. Diffusion transport regime
SLM diffusion coeficient (Dm) Bulk diffusion coefficient (Db)
• The diffusional process
through a SLM is affected by
the porosity and tortuosity
of the polymeric support.
• Dm has to be corrected for
the membrane
characteristics to obtain the
bulk diffusion coefficient Db.
• The bulk diffusion
coefficient Db is derived by
Stokes-Einstein relationship:
and the Wilke-Chung relation:

6. Diffusion transport regime
Lag-time experiment Permeability experiments
• The time required for the complex to
diffuse across the membrane from
the feed phase to the receiving
phase, assuming dilute conditions.
• Db=τDlag
• At time t=0, a carrier which is
substituted with a chromophoric
group is added to the feed phase
([cf]=0). The carrier diffuses
through the membrane and the
increase of concentration in the
receiving phase ([cr]t) is
monitored by UV/Vis
spectroscopy.
• Db=Dmτ/ϵ

7. Chemical reactions’ kinetics regime
transport
• When one or more of the chemical reactions are
sufficiently slow in comparison with the rate of
diffusion to and away from the interfaces.
• Two series of chemical reactions mechanisms and
their kinetics have to be analyzed:
1. Solute uptake at the aqueous feed phase-organic LM
interface or partition and chemical interactions with
solvent exchange and formation of solute-carrier
complex
2. Solute release with chemical interactions between LM
and aqueous strip phases at LM-strip interface with
destruction of the complex

8. Chemical reactions’ kinetics regime
transport
Determination of activation
energy
1. Irreversible first-order
reactions:
2. Reversible first-order
reactions:
3. Series of first-order
reactions
• Activation energy of transport
gives information about the
rate-limiting step in the
transport process.
• Ea values below 20 kJ/mol are
accepted as indicative of pure
diffusion-limited transport. At
activation energies above 40
kJ/mol, chemical reactions do
play a role in the transport.

9. Mixed transport regime
• When both chemical reactions and film diffusion
processes occur at rates that are comparable, the
solvent extraction kinetics are said to take place
in a mixed diffusional-kinetic regime.
• Unless simplifying assumptions can be used,
frequently the differential equations have no
analytical solutions, and boundary conditions
have to be determined by specific experiments.

10. Emulsion Liquid Membrane Design
1. Membrane Film model: film with constant thickness.
a) Uniform flat sheet model: (planar geometry). Considers
resistance as a sum of resistances through phases.
b) Spherical shell model: mass transfer rate directly related to DC
across the film.
2. Advancing Front Model: irreversibility of stripping reaction.
Mass flux:
Flux of solute arriving at Rf:
Rate of reagent being consumed:
Mass balance for solute in Ep:
Mass balance for solute in Rf:
3. Reversible Reaction Model: reversible stripping reaction
(No reaction front).
Mass Balance for solute concentration in membrane portion and external
phase:

11. Bulk Liquid Membrane Design
1. Bulk Organic Hybrid Liquid Membranes(BOHLM):
Uncharged (hydrophobic) symmetric membranes as barriers.
• Overall permeability coefficients on KF/E and
KE/R:
• Overall mass balance:
• Mass transfer coeficient:
2. Bulk Aqueous Hybrid Liquid Membranes:
Charged (hydrophilic)symmetric membranes as barriers.
• Overall permeability coefficients on KF/E and
KE/R:
• Model equation:
• Overall mass balance:

12. Supported Liquid Membrane Design
The membrane phase is held by capillary forces in the pores of a microporous
polymeric or inorganic film. The inmobilized liquid is the membrane phase
and the film serves as a support for the membrane, which separates the
feed phase from the strip phase.
• Diving force: difference in concentration
• Mass transfer takes place due to difference in
chemical potential:
Variation of Chem.pot:
• Flux:
where the diffusion coefficient is:

13. EQUIPMENT
Chemical Composition of Liquid Membranes
• Diluents
Main components
(immiscible with other phases)
• Surfactants
Specially important in ELM
• Carriers / Tranporters
Only for Facilitated Transport

14. EQUIPMENT
Equipment for Emulsion LM (ELM)
• Emulsion: colloidal suspension
of a liquid in another immiscible liquid.

15. EQUIPMENT
Equipment for Bulk LM (BLM)
• Layered LM
• U-tube
• Bulk Hybrid LM  Supports (solid membranes)
• Organic (BHOLM) or Aqueous (BHALM)
• Configurations: FS, Sp, Tub, HF

16. EQUIPMENT
Equipment for Supported LM (SLM)
• Support  microporous solid membrane
• LM inside the pores
• Configurations:
• FS, Sp, Tub, HF

17. APPLICATIONS
Applications of Emulsion LM (ELM)
• Metal ion separations
• Separation of biomolecules
• Enzyme immobilization (not for separation processes)
(Most of them on laboratory scale)

18. Supported Liquid Membranes’
Applications
a) Analytical Applications

19. b) Applications on the Biotechnological and
environmental sciences:
c) Separation of stereoisomers:particularly enantiomers

20. Bulk Liquid Membranes’ Applications
• Metal separation
• Wastewater treatment
• Biotechnologies
• Drugs recovery-separation
• Organic compounds separation
• Gas separation