hydrophobic tails face inwards, away
from the external aqueous environ-
ment.lipoic acidA vitamin of the *vita-
min B complex. It is one of the
*coenzymes involved in the decar-
boxylation of pyruvate by the en-
zyme pyruvate dehydrogenase. Good
sources of lipoic acid include liver
and yeast.lipolysisThe breakdown of storage
lipids in living organisms. Most long-
term energy reserves are in the form
of triglycerides in fats and oils. When
these are needed, e.g. during starva-
tion, lipase enzymes convert the
triglycerides into glycerol and the
component fatty acids. These are
then transported to tissues and oxi-
dized to provide energy.lipoprotein See lipid.lipowitz alloyA low-melting
(70–74°C) alloy of bismuth (50%), lead
(27%), tin (13%), and cadmium (10%).liquationThe separation of mix-
tures of solids by heating to a tem-
perature at which lower-melting
components liquefy.liquefaction of gasesThe conver-
sion of a gaseous substance into a liq-
uid. This is usually achieved by one
of four methods or by a combination
of two of them:
(1) by vapour compression, provided
that the substance is below its *criti-
cal temperature;
(2) by refrigeration at constant pres-
sure, typically by cooling it with a
colderÛuid in a countercurrent heat
exchanger;
(3) by making it perform work adia-
batically against the atmosphere in a
reversible cycle;
(4) by the *Joule–Thomson effect.
Large quantities of liqueÜed gases
are now used commercially, espe-cially *liqueÜed petroleum gas and
liqueÜed natural gas.liqueÜed natural gas (LNG)See
liquefied petroleum gas.liqueÜed petroleum gas (LPG)
Various petroleum gases, principally
propane and butane, stored as a liq-
uid under pressure. It is used as an
engine fuel and has the advantage of
causing very little cylinder-head de-
posits.
LiqueÜed natural gas (LNG) is a sim-
ilar product and consists mainly of
methane. However, it cannot be
liqueÜed simply by pressure as it has
a low critical temperature of 190 K
and must therefore be cooled to
below this temperature before it will
liquefy. Once liqueÜed it has to be
stored in well-insulated containers. It
provides a convenient form in which
to ship natural gas in bulk from oil
wells or gas-only wells to users. It is
also used as an engine fuel.
liquidA phase of matter between
that of a crystalline solid and a *gas.
In a liquid, the large-scale three-
dimensional atomic (or ionic or mo-
lecular) regularity of the solid is ab-
sent but, on the other hand, so is the
total disorganization of the gas. Al-
though liquids have been studied for
many years there is still no compre-
hensive theory of the liquid state. It
is clear, however, from diffraction
studies that there is a short-range
structural regularity extending over
several molecular diameters. These
bundles of ordered atoms, molecules,
or ions move about in relation to
each other, enabling liquids to have
almostÜxed volumes, which adopt
the shape of their containers.
liquid crystalA substance that
Ûows like a liquid but has some order
in its arrangement of molecules. Ne-
matic crystals have long molecules all
aligned in the same direction, butlipoic acid 328l