Encyclopedia of Chemistry

emulsifying agent Any material having both
HYDROPHILICand HYDROPHOBIC characters acting to
stabilize an EMULSION.

emulsion Droplets of a liquid substance dispersed in
another immiscible liquid. Milk and salad dressing are
emulsions. These are examples of colloidal systems in
which both phases are liquid.
See alsoCOLLOID.

enantiomer One of a pair of molecular entities that
are mirror images of each other and nonsuperimpos-
able.

enantioselectivity SeeSTEREOSELECTIVITY.

encounter complex A COMPLEX of MOLECULAR
ENTITIES produced at an ENCOUNTER-CONTROLLED
RATE that occurs as an intermediate in a reaction
MECHANISM. When the complex is formed from two
molecular entities, it is called an encounter pair. A dis-
tinction between encounter pairs and (larger) encounter
complexes may be relevant in some cases, e.g., for
mechanisms involving PRE-ASSOCIATION.

encounter-controlled rate A RATE OF REACTION
corresponding to the rate of encounter of the reacting
MOLECULAR ENTITIES. This is also known as DIFFUSION-
CONTROLLED RATE, since rates of encounter are them-
selves controlled by diffusion rates (which in turn

depend on the viscosity of the MEDIUMand the dimen-

sions of the reactant molecular entities).

For a BIMOLECULAR REACTIONbetween solutes in

water at 25°C, an encounter-controlled rate is calcu-

lated to have a second-order rate constant of about

1010 dm^3 mol–1sec–1.

See alsoMICROSCOPIC DIFFUSION CONTROL.

endergonic reaction A chemical reaction that con-

sumes energy rather than releasing energy. Endergonic

reactions are not spontaneous because they do not

release energy.

endogenous Originating internally. In the description

of metal ion COORDINATION in metalloproteins, the

term endogenousrefers to internal, or protein-derived,

LIGANDs.

ENDOR See ELECTRON-NUCLEAR DOUBLE RESO-

NANCE.

endothelium-derived relaxing factor(EDRF) The

factor originally described as EDRF is NO., produced

by a specific P-450-type of ENZYMEfrom arginine upon

response of a cell to a biological signal (molecule). Dif-

ferent types of cells respond differently to the presence

of NO..

See alsoCYTOCHROMEP-450.

endotherm A warm-blooded animal, one in which

the internal temperature does not fluctuate with the

temperature of the environment but is maintained

by a constant internal temperature regulated by

metabolic processes. Examples include birds and

mammals.

endothermal reaction A chemical reaction in which

heat is absorbed.

endothermic The state of being warm-blooded or

producing heat internally. In chemistry, a reaction

96 emulsifying agent

Emulsion. Droplets of a liquid substance dispersed in another
immiscible liquid