Physics And Chemistry Basis Of Biotechnology - De Cuyper - tiera.ru

Biomimetic materials synthesis
Other inorganic particles synthesised in phospholipid vesicles include selenides [70],
mixed semiconductors [71], Al2O3 [72] and AI2O3 nano-composites with other metal
oxides/hydroxides [73], Au and Co metals [74], as well as magnetic iron oxides [75].
3.5.2. Layered surfactant assemblies
3.5.2. I. Surfactant monolayers and Langmuir-Blodgett films
Surfactant molecules, spread on a water surface, align themselves such that the ionic
(or polar) part of the molecule interacts with the water while the hydrocarbon tail is
oriented away from the surface. Compressing these molecules together at the air-water
interface forms an ordered two-dimensional monolayer of the surfactants. These
compressed arrays can be transferred, layer-by-layer, onto solid substrates. The
resulting films are called Langmuir-Blodgett (LB) films. It must be noted that LB films
can be formed not only from surfactants, the same technique may be used to create
arrays of fullerenes, polystyrene microspheres and other macromolecules and
supramolecular assemblies.
Compressed surfactant monolayers above an aqueous sub-phase, supersaturated
with respect to an inorganic solid, have been shown to induce oriented crystal
nucleation. It has been demonstrated for a variety of different systems that the
adsorption of an amphiphile with the appropriate head group at the air-water interface
can induce an oriented nucleation event from supersaturated solutions. Examples of
inorganic materials, for which such a molecular templating may be achieved include
three different phases of calcium carbonate [76, 77], barium sulphate [78, 79] and
calcium sulphate [SO]. In these systems there is a high correspondence between the
packing of the monolayer and the lattice of the nucleated crystal (geometric factor).
Ions adopt unique stereochemical conformations in the crystal lattice that can be
mimicked by the monolayer headgroups (stereochemical factor). The headgroup charge
results in an accumulation of ions at the interface (electrostatic factor). These three
factors are important for oriented nucleation although a degree of mismatch is tolerable.
For example, to synthesise calcium carbonate under surfactant monolayer,
monolayer film of amphiphile was spread onto supersaturated calcium bicarbonate
solution, and then compressed to an appropriate surface pressure. Crystallisation of
CaCO3 proceeded slowly, accompanied by CO2 evolution. Depending on the
experimental conditions (such as the amphiphile used and the concentration of calcium)
one or two of three CaCO3 phases (calcite, aragonite or vaterite) was formed. The
appearance of a certain CaCO3 phase were first explained by the lattice match between
the amphiphile headgroups and inorganic substrate, however, other effects such as the
promotional role of water molecules in the nucleation of a specific phase can not be
neglected. The concept of direct epitaxial growth of inorganic crystals on organic
templates was re-examined in the study of Xu et. al. [81]. The formation of crystalline
CaCO3 under amphiphilic porphyrin templates was shown to proceed through an
intermediate amorphous phase. The amorphous CaCO3 undergoes phase transformation
into the crystalline material with the orientation controlled by the porphyrin template.
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