Biomedical Engineering Reference
In-Depth Information
and possibly its morphology and functions, are dependent on the concentra-
tion and orientation of the species (fi brinogen, fi brin or protofi brils) present
on the surface.
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6.8 Intermolecular forces and interactions
The two intermolecular forces and interactions that are most relevant in
many biological studies are probably van der Waals and electrostatic.
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However, in some situations other forces, such as solvation forces, are also
important.
6.8.1 van der Waals (vdW) forces and interactions
Van der Waals forces exist among all kinds of atoms and molecules. The
origin of vdW force stems from the instantaneous dipole-induced dipole
interactions among adjacent apolar atoms and molecules, which can be
repulsive or attractive. Among polar molecules, there are three components
that contribute to the total vdW forces: the induction force, the orientation
force and the dispersion force. For neutral molecules, the last is the most
important. The one-sixth power of vdW forces suggests they drop substan-
tially over a long distance. It is regarded as a long-range force which exerts
its effect from far away (>10 nm) to submolecular distance (<1 nm).
To compute vdW forces in a medium, the solvent medium is treated as
a continuum. The polarizability and the dielectric permittivity of each par-
ticle and molecule, and the dielectric permittivity of the medium are then
functions of the absorption frequencies. The expression of the total vdW
interaction consists of contributions from zero and non-zero adsorption fre-
quency terms. The former includes the contributions from the orientation
and induction interaction energies, whereas the latter represents the disper-
sion interaction energy.
The vdW forces that act between macroscopic bodies and surfaces in a
solvent medium are relevant to the phenomena of protein adsorption. An
approximation of energy changes during the protein adsorption process sug-
gests that the vdW component is always negative, implying that attraction
always occurs between the protein and the adsorbent. However, the vdW
energy contribution is relatively small. Its value becomes more negative (i.e.
stronger attraction) if the protein becomes large, but it also becomes much
less signifi cant when the separation of protein from the surface increases.
To calculate vdW forces between two macroscopic bodies or a body
with a surface, individual interactions between atoms, one from each
body, can be evaluated pairwisely and then summed up over the whole
