WordNet

to undergo a reversible or temporary breakdown of a molecule into simpler molecules or atoms; "acids dissociate to give hydrogen ions"
(psychiatry) a defense mechanism in which memory of an unacceptable act or impulse is separated from the emotion originally associated with it
a state of separation between persons or groups
the act of isolating something; setting something apart from others (同)closing off
a countrys withdrawal from international politics; "he opposed a policy of American isolation"
a feeling of being disliked and alone
separated according to race, sex, class, or religion; "separate but equal"; "girls and boys in separate classes"
a garment that can be purchased separately and worn in combinations with other garments
force, take, or pull apart; "He separated the fighting children"; "Moses parted the Red Sea" (同)disunite, divide, part
act as a barrier between; stand between; "The mountain range divides the two countries" (同)divide
make a division or separation (同)divide
come apart; "The two pieces that we had glued separated" (同)divide, part
go ones own way; move apart; "The friends separated after the party" (同)part, split
discontinue an association or relation; go different ways; "The business partners broke over a tax question"; "The couple separated after 25 years of marriage"; "My friend and I split up" (同)part, split up, split, break, break_up
divide into components or constituents; "Separate the wheat from the chaff"
independent; not united or joint; "a problem consisting of two separate issues"; "they went their separate ways"; "formed a separate church"
make clearly visible; "can this image be resolved?"
find the solution; "solve an equation"; "solve for x" (同)solve
the state of lacking unity
sorting one thing from others; "the separation of wheat from chaff"; "the separation of mail by postal zones"
the act of dividing or disconnecting
the social act of separating or parting company; "the separation of church and state"
coming apart (同)breakup, detachment
the space where a division or parting occurs; "he hid in the separation between walls"
the termination of employment (by resignation or dismissal)
separate or isolate (one thing) from another and place in a group apart from others; "the sun segregates the carbon"; "large mining claims are segregated into smaller claims"
someone who is or has been segregated
divide from the main body or mass and collect; "Many towns segregated into new counties"; "Experiments show clearly that genes segregate"
separate by race or religion; practice a policy of racial segregation; "This neighborhood is segregated"; "We dont segregate in this county"

PrepTutorEJDIC

(…から)…‘を'引き離す,分離する《+『名』+『from』+『名』》 / (化学で)…‘を'解離する
孤立;隔離,分離
…‘を'『分離』する』,引き分ける;(…から)…‘を'引き分ける《+『名』+『from』+『名』》 / (…に)〈一つの物〉‘を'『分割する』,分ける《+『名』+『into』+『名』》 / 〈人が〉別れる;別れて(…に)なる《+『into』+『名』》 / (…から)分離する,離れる《+『from』+『名』》 / 『離れた』,分かれた / (関係がなく)別の,異質な(different) / 個々の,個別の(individual)
…‘を'『決心する』,『決意する』 / 〈問題・疑惑など〉‘を'『解決する』,解明する,解く / 〈議会などが〉…‘を'議決する,票決する / (構成要素に)…‘を'分解する《+『名』+『into』+『名』》 / (…を)『決定する』,『決心する』《+『on』(『upon』,『against』)+『名』(do『ing』)》 / (…に)『分解する』《+『into』+『名』》 / 〈U〉《文》強固な意志,分屈 / 〈C〉決心,決意
〈C〉〈U〉『分離』,分割,別離・〈C〉(…の分離(分割)した箇所《+『of』+『名』》 / 〈C〉〈U〉(合意・判決に基づく)夫婦の別居
…‘を'分離する;〈患者など〉‘を'隔離する / 〈特定の宗派・人種・社会団体など〉‘を'差別する,差別待遇する / 分かれる,分離する / (特に人種で)差別する

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/12/18 21:42:10」(JST)

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For other uses, see Dissociation.

Dissociation in chemistry and biochemistry is a general process in which ionic compounds (complexes, or salts) separate or split into smaller particles, ions, or radicals, usually in a reversible manner. For instance, when a Brønsted-Lowry acid is put in water, a covalent bond between an electronegative atom and a hydrogen atom is broken by heterolytic fission, which gives a proton and a negative ion. Dissociation is the opposite of association and recombination. The process is frequently confused with ionization.

Dissociation constant

For reversible dissociations in a chemical equilibrium

AB A + B

the dissociation constant K_d is the ratio of dissociated to undissociated compound

where the brackets denote the equilibrium concentrations of the species.

Dissociation degree

The dissociation degree is the fraction of original solute molecules that have dissociated. It is usually indicated by the Greek symbol . There is a simple relationship between this parameter and the van 't Hoff factor . If the solute substance dissociates into ions, then

For instance, for the following dissociation

KCl K⁺ + Cl^-

As , we would have that

Salts

The dissociation of salts by solvation in a solution like water means the separation of the anions and cations. The salt can be recovered by evaporation of the solvent. See also: Solubility equilibrium

An electrolyte refers to a substance that contains free ions and can be used as an electrically conductive medium. Most of the solute does not dissociate in a weak electrolyte whereas in a strong electrolyte a higher ratio of solute dissociates to form free ions.

A weak electrolyte is a substance whose solute exists in solution mostly in the form of molecules, with only a small fraction in the form of ions. Simply because a substance does not readily dissolve does not make it a weak electrolyte. Acetic acid (CH₃COOH) and ammonium (NH₄⁺) are good examples. Acetic acid is extremely soluble in water, but most of the compound dissolves into molecules, rendering it a weak electrolyte. Weak bases and weak acids are generally weak electrolytes. In an aqueous solution there will be some CH₃COOH and some CH₃COO^- and H⁺.

A Strong electrolyte is a solute that exists in solution completely or nearly completely as ions. Again, the strength of an electrolyte is defined as the percentage of solute that is ions, rather than molecules. The higher the percentage, the stronger the electrolyte. Thus, even if a substance is not very soluble, but does dissociate completely into ions, the substance is defined as a strong electrolyte. Similar logic applies to a weak electrolyte. Strong acids and bases are good examples such as HCl, H₂SO₄, NaCl. These will all exist as ions in an aqueous medium.

Gases

The degree of dissociation in gases is denoted by the symbol α where α refers to the percentage of gas molecules which dissociate. Various relationships between K_p and α exist depending on the stoichiometry of the equation. The example of dinitrogen tetroxide (N₂O₄) dissociating to nitrogen dioxide (NO₂) will be taken.

N₂O₄ 2NO₂

If the initial concentration of dinitrogen tetroxide is 1 mole per litre, this will decrease by α at equilibrium giving, by stoichiometry, 2α moles of NO₂. The equilibrium constant (in terms of pressure) is given by the equation;

Where p represents the partial pressure. Hence, through the definition of partial pressure and using p_T to represent the total pressure and x to represent the mole fraction;

The total number of moles at equilibrium is (1-α)+(2α) which is equivalent to 1+α. Thus, substituting the mole fractions with actual values in term of alpha and simplifying;

This equation is in accordance with Le Chatelier's Principle. K_p will remain constant with temperature. The addition of pressure to the system will increase the value of p_T so α must decrease to keep K_p constant. In fact, increasing the pressure of the equilibrium favours a shift to the left favouring the formation of dinitrogen tetroxide (as on this side of the equilibrium there is less pressure since pressure is proportional to number of moles) hence decreasing the extent of dissociation α.

Acids

The dissociation of acids in a solution means the split-off of a proton H⁺, see Acid-base reaction theories. This is an equilibrium process, meaning that dissociation and recombination takes place at the same time. The acid dissociation constant K_a is an indicator of the acid strength: stronger acids have a higher K_a value (and a lower pK_a value).

Fragmentation

Fragmentation of a molecule can take place by a process of heterolysis or homolysis.

Receptors

Receptors are proteins that bind small ligands. The dissociation constant K_d is used as indicator of the affinity of the ligand to the receptor. The higher the affinity of the ligand for the receptor the lower the K_d value (and the higher the pK_d value).

UpToDate Contents

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Synthesis and characterization of novel amphiphilic copolymer stearic acid-coupled F127 nanoparticles for nano-technology based drug delivery system.

Gao Q, Liang Q, Yu F, Xu J, Zhao Q, Sun B.SourceSoutheast University, School of Chemistry and Chemical Engineering, Nanjing 210000, China.
Colloids and surfaces. B, Biointerfaces.Colloids Surf B Biointerfaces.2011 Dec 1;88(2):741-8. Epub 2011 Aug 18.
Pluronic, F127, amphiphilic block copolymers, are used for several applications, including drug delivery systems. The critical micelle concentration (CMC) of F127 is about 0.26-0.8wt% so that the utility of F127 in nano-technology based drug delivery system is limited since the nano-sized micelles c
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Time-resolved fluorescence biosensor for adenosine detection based on home-made europium complexes.

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In this protocol, the authors report a time-resolved fluorescence biosensor based on home-made europium complexes for highly sensitive detection of small molecules using adenosine as a model analyte. The fluorophore that used is europium complexes. Its signal can be measured in a time-resolved manne
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Japanese Journal

Density Functional Theory Investigation on the Dissociation and Adsorption Processes of N2 on Pd(111) and Pd3Ag(111) Surfaces

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Jpn J Appl Phys 50(4), 045701-045701-5, 2011-04-25
… The dissociation of N2 molecule on Pd(111) is most efficient if its center-of-mass (CM) is fixed on top of Pd atom while allowing the N atoms to dissociate on the hollow sites [hcp hollow--top--fcc hollow (h--t--f) configuration] with an activation barrier of 5.94 eV. …
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High-Rate Deposition of Amorphous Silicon Films by Microwave-Excited High-Density Plasma

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Dissociation Channels of c-C4F8 to CF2 Radical in Reactive Plasma

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Jpn J Appl Phys 50(3), 036203-036203-6, 2011-03-25
… Therefore, in the case of the dissociation via S4 and T4 excited states c-C4F8 may dissociate to two C2F4, and in the case of the dissociation via T1 excited state c-C4F8 may dissociate to four CF2 radicals. …
NAID 150000055412