Biology Reference
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Interparticle steric interactions appear owing to the noncharged
organic ligands on the surface of the nanocrystals.
24
Steric interactions
include the interactions between organic ligands and those between
ligands and solvent molecules. In a stable colloidal system, organic
surface ligands interact with solvent molecules, forming a stable
solvent shell on the surface of the nanocrystals (Fig. 13.1). When
two colloidal particles approach each other at close proximity, the
following effects take place: (1) Energy is consumed to break the
interactions between the ligands and the solvent molecules in the
solvent shells, which results in a repulsive interaction, also known
as solvophilic effects; (2) the entropy of solvent molecules increases
owing to the partial breaking of solvent shell, leading to an attractive
interaction; (3) the partial overlapping between organic ligands on
the approaching particles also leads to an attractive interaction; and
(4) the entropy of organic ligands increases owing to confinement
of their dangling chains, which results in a repulsive osmotic force
known as steric repulsion. The energy of these steric interactions
is dependent on the particle surface ligand density (N
) and the
o
effective interacting surface area (A
) between two particles, and can
i
be presented in the following form:
E
) (13.5)
where r is the sum of the particle radius and the thickness of the
solvent shell, E
= A
N
(E
- E
- E
+ E
) 2πrh
N
(E
- E
- E
+ E
S
i
o
SS
SE
LL
LE
o
o
SS
SE
LL
LE
stands for the per-ligand energy cost for breaking
the solvent shell, E
SS
is the per-ligand gain of solvent entropy, E
is
SE
LL
is the
per-ligand loss of surface ligand entropy. Overall, steric interactions
are a repulsive force between nanocrystals, and they are the major
stabilization force for noncharged colloidal nanocrystals.
the per-ligand energy gain due to ligand interactions, and E
LE
27
Figure 13.1
Schematic for nanocrystal dispersion and interactions: (1)
formation of solvent shell due to the interaction between
nanocrystal's surface ligands and solvent molecules, and
(2) interactions between two approaching particles. In
the illustration,
is a
characteristic overlapping distance after particle collision,
and
s
is the thickness of solvent shell,
h
0
R
is the radius of the nanocrystal.
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