Thin-film

Thin film

A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....

 composite

Composite material

Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...

 membranes

Artificial membrane

An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A...

 (TFC or TFM) are semipermeable membrane

Semipermeable membrane

A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".The rate of...

s manufactured principally for use in water purification

Water purification

Water purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose...

 or water desalination systems. They also have use in chemical applications such as batteries

Battery (electricity)

An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

 and fuel cell

Fuel cell

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...

s.

Essentially, a TFC material is a molecular sieve

Molecular sieve

A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a...

 constructed in the form of a film from two or more layered materials.

Membranes used in reverse osmosis

Reverse osmosis

Reverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...

 are typically made out of polyamide

Polyamide

A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,...

, chosen primarily for its permeability to water and relative impermeability to various dissolved impurities including salt ions and other small, unfilterable molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s.

History

The first viable reverse osmosis membrane was made from cellulose acetate as an integrally skinned asymmetric semi-permeable membrane. This membrane was made by Loeb and Sourirajan at UCLA

University of California, Los Angeles

The University of California, Los Angeles is a public research university located in the Westwood neighborhood of Los Angeles, California, USA. It was founded in 1919 as the "Southern Branch" of the University of California and is the second oldest of the ten campuses...

 in 1959 and patented in 1960. The current generation of reverse osmosis (RO) membrane materials are based on a composite material patented by FilmTec Corporation in 1970. FilmTec's FT30 membrane is known as a polyamide

Polyamide

A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,...

 thin film composite membrane.

Structure and materials

As is suggested by the name, TFC membranes are composed of multiple layers. Membranes designed for desalination use an active thin-film layer of polyimide

Polyimide

Polyimide is a polymer of imide monomers. The structure of imide is as shown. Polyimides have been in mass production since 1955...

 layered with polysulfone

Polysulfone

Polysulfone describes a family of thermoplastic polymers. These polymers are known for their toughness and stability at high temperatures. They contain the subunit aryl-SO2-aryl, the defining feature of which is the sulfone group. Polysulfones were introduced in 1965 by Union Carbide...

 as a porous support layer.

Other materials, usually zeolites, are also used in the manufacture of TFC membranes.

Applications

Thin film composite membranes are used in

• Water purification;
• as a chemical reaction buffer
  Buffer solution
  A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...
  
   (batteries and fuel cells);
• in industrial gas separations.

Limitations

Thin film composites membranes typically suffer from compaction effects under pressure. As the water pressure increases, the polymers are slightly reorganized into a tighter fitting structure that results in a lower porosity, ultimately limiting the efficiency of the system designed to use them. In general, the higher the pressure, the greater the compaction.

Surface fouling: Colloidal particulates, bacteria infestation (biofouling).

Chemical decomposition and oxidation.

Performance

A filtration membrane's performance is rated by selectivity, chemical resistance, operational pressure differential and the pure water flow rate per unit area.

Due to the emphasis on flow rate, a membrane is manufactured as thinly as possible. These thin layers introduce defects that may affect selectivity, so system design usually trades off the desired flow rate against both selectivity and operational pressure.

In applications other than filtration, parameters such as mechanical strength, temperature stability, and electrical conductivity may dominate.

Saturation.

Active research areas

Nano-composite membranes (TFN). Key points: multiple layers, multiple materials.

New materials, sythetic zeolites, etc. to obtain higher performance.

Fuel-cells.

Batteries.

See also

Langmuir adsorption
Maxwell-Stefan diffusion

Maxwell-Stefan diffusion

The Maxwell–Stefan diffusion is a model for describing diffusion in multicomponent systems. The equations that describe these transport processes have been developed independently and in parallel by James Clerk Maxwell for dilute gases and Josef Stefan for fluids...

Fighting Fouling with Cross linked Polyamide-Urea Membranes http://www.trisep.com/x20_sub4.htm