biological system A biological system is a complex Biological network inference, network which connects several biologically relevant entities. Biological organization spans several scales and are determined based different structures depending on what the system is ...

s, a double layer is the surface where two different phases of matter are in contact. Biological double layers are much like their interfacial counterparts, but with several notable distinctions. The surface of

biological cell The cell is the basic structural and functional unit of all forms of life. Every cell consists of cytoplasm enclosed within a membrane; many cells contain organelles, each with a specific function. The term comes from the Latin word meaning ...

s carry many different types of

chemical group In organic chemistry, a functional group is any substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...

s, each with a different

dissociation constant In chemistry, biochemistry, and pharmacology, a dissociation constant (''K''D) is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex ...

, causing them to have varying

electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

s at a

physiological pH Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a subdiscipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out chemical and ...

. This indicates that biosurfaces are chemically

heterogeneous Homogeneity and heterogeneity are concepts relating to the uniformity of a substance, process or image. A homogeneous feature is uniform in composition or character (i.e., color, shape, size, weight, height, distribution, texture, language, i ...

. This biospecific feature is typical for all biosurfaces, including

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

macromolecule A macromolecule is a "molecule of high relative molecular mass, the structure of which essentially comprises the multiple repetition of units derived, actually or conceptually, from molecules of low relative molecular mass." Polymers are physi ...

s and

s. In certain organisms, cells are covered with the

glycocalyx The glycocalyx (: glycocalyces or glycocalyxes), also known as the pericellular matrix and cell coat, is a layer of glycoproteins and glycolipids which surround the cell membranes of bacteria, epithelial cells, and other cells. Animal epithe ...

layer, which can be modeled as a

polyelectrolyte Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are t ...

layer with a volume spread electric charge. This means that the notion of a

surface charge A surface charge is an electric charge present on a two-dimensional surface. These electric charges are constrained on this 2-D surface, and surface charge density, measured in coulombs per square meter (C•m−2), is used to describe the charge ...

is located on certain flat surfaces. This does not apply; instead, the cell surface is a finite thickness polyelectrolyte layer with a volume charge. At

equilibrium Equilibrium may refer to: Film and television * ''Equilibrium'' (film), a 2002 science fiction film * '' The Story of Three Loves'', also known as ''Equilibrium'', a 1953 romantic anthology film * "Equilibrium" (''seaQuest 2032'') * ''Equilibr ...

, the relationship between these polyelectrolyte layers and a fluid bulk is called the

Donnan equilibrium Donnan can refer to: * Donnan (surname) * Donnan, Iowa, a community in the United States * Donnán of Eigg, Gaelic priest of the 7th century * Donnan equilibrium ** Donnan potential {{disambig ...

. The polyelectrolyte volume charge creates an equilibrated

electric potential Electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as electric potential energy per unit of electric charge. More precisely, electric potential is the amount of work (physic ...

known as the

Donnan potential Donnan potential is the difference in the Galvani potentials which appears as a result of Donnan equilibrium, named after Frederick G. Donnan, which refers to the distribution of ion species between two ionic solutions separated by a semipermeab ...

. Part of the Donnan potential is located inside of the polyelectrolyte layer, while the other part is associated with the external double layer located in the

dispersion medium Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with ''colloids'', heterogeneous systems consisting of a mechanical mixture of particles between 1 nm ...

. In another feature, the cells are not in an equilibrium with the fluid bulk. There is a constant

ion exchange Ion exchange is a reversible interchange of one species of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid. Ion exchange is used in softening or demineralizing of water, purification of ch ...

between living cells and a fluid. Consequently, there is a difference in electric potentials between the cell interior and a fluid bulk, known as the

transmembrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. It equals the interior potential minus the exterior potential. This is the ...

. This non-equilibrium potential affects the structure of the double layer.Dukhin, A.S. Colloids and Surfaces A, 73, 29-48 (1993)

Notes

General references

* Ohshima, H. (2006). ''Theory of Colloid and Interfacial Electric Phenomena'', Elsevier. * Duval, J.F.L. ''et al.'' (2005). ''Langmuir'', 21, 11268–11282. {{DEFAULTSORT:Double Layer (Biospecific) Physical chemistry Colloidal chemistry