Chemistry Reference
In-Depth Information
microelectronic substrates affords new opportunities for generation of fundamental
chemical property information and evaluation of new organic thin-film semiconduc-
tors as microelectronic components and devices.
4.6.6 b
I l I p I d
m
e m b r a n e S
(blm
S
)
Before the advent of the electron microscope, the structures of biological membranes
were not fully understood at the molecular level. However, it was realized that bio-
logical cells were contained by some kind of a thin
lipid membrane
. In order to
analyze this in more detail, experiments were conducted as follows. Lipids were
extracted from biological cells and compressed on the Langmuir balance, and the
value of area per molecule was estimated (ca. 45 Å 2/molecule). Knowing the diam-
eter of the cells and from the amounts of lipids (and the area/molecule data), the
conclusion was reached that the cell membranes were composed of
bilayer of lipids
(
bilipid membrane or BLM
). This was one of the most important results in the his-
tory of biological cell membranes. Later, of course, these results were confirmed
from x-ray diffraction data and other scanning probe microscopes (SPMs) (Birdi,
1999, 2002a).
4.7 veSIcleS and lIPoSomeS
It was mentioned that ordinary surfactants (soaps, etc.), when dissolved in water,
form self-assembly micellar structures. Phospholipids are molecules like surfactants;
they also have a hydrophilic head and generally have two hydrophobic alkyl chains.
These molecules are the main components of cell membranes. Lung fluid also con-
sists mainly of lipids of this kind. In fact, usually, cell membrane are made up of two
layers of phospholipids, with the tails turned inward, in an attempt to avoid water.
The external membrane of a cell contains all the organelles and the cytoplasm.
Phospholipids, when dispersed in water, may exhibit self-assembly properties
(either as micellar self-assembly aggregates or larger structures). This may lead to
aggregates that are called liposomes or vesicles. Liposomes are structures that are
empty cells and that are currently being used by some industries. They are micro-
scopic vesicles or containers formed by the membrane alone, and are widely used
in the pharmaceutical and cosmetic fields because it is possible to insert chemicals
inside them. Liposomes may also be used solubilize (in its hydrophobic part) hydro-
phobic chemicals (water-insoluble organic compounds) such as oily substances so
that they can be dispersed in an aqueous medium by virtue of the hydrophilic prop-
erties of the liposomes (in the alkyl region).
A certain type of lipid (or lipid-like) molecules are found that when dispersed in
water tend to make self-assembly structures (Figure 4.13). Detergents were shown to
aggregate to spherical or large cylindrical-shaped micelles. It is known that if egg
phosphatidylethanolamine (egg lecithin) is dispersed in water at 25°C, it forms a self-
assembly structure, which is called liposome or vesicle.
A liposome is a spherical vesicle with a membrane composed of a phospholipid and
cholesterol (less than 50%) bilayer. Liposomes can be composed of naturally derived
phospholipids with mixed lipid chains (such as egg phosphatidylethanolamine) or
Search WWH ::
Custom Search