Drugs and the pharmaceutical sciences

6 Quigley
This inertial force depends directly upon the mass of the particle, and the square of its
velocity. It is, therefore, more important with heavier particles. The inertial force is
attenuated by the viscosity of the fluid, and is consequently influenced by temperature
which is inversely related to viscosity. For this reason it is less effective in liquid than in
gaseous contexts (Fig. 6).
Brownian Motion
Smaller particles, less heavy, are less influenced by inertia. However, they are more
affected by Brownian motion wherein they are vectored from the fluid pathway to the
pore surface by collisions with the fluid’s molecules. The result is retention of the
particles by the filter surfaces they impact.
At all temperatures above absolute zero the various sections of all molecules are in
constant motion; the various bonds being flexed, rotated, stretched, etc. The higher the
temperature, the greater the amplitude of the molecular motion. The significance of
absolute zero is that only at that temperature or below is all molecular movement frozen.
In their frenetic activity, the fluid molecules collide, perhaps repeatedly, with suspended
particles. The latter are directed to new directions of travel within the fluid stream. As a
result of their induced random and erratic movements, the particles have opportunities to
encounter pore surfaces. This is the nature of Brownian or diffusional interception. It is
favored by small size particles, and by the lower viscosities of the suspending fluids.
Adsorptive Interactions
The donating of electrons by one atom, possibly already part of a molecule, to another
one results in a strong bonding between the two atoms. This is the nature of the chemical
bond. The same occurs from the sharing of electrons between two atoms. The atomic
interactions resulting from electron sharing is called covalent bonding. By convention,
electrons each represent a full negative charge. Thus, possessing more electrons than in
its neutral state confers a negative charge upon an atom. Having fewer than the normal
number of electrons in its possession gives an atom a plus or positive electrical charge.
Matrix
thick porous
Depth straining
FIGURE 6 Inertial impaction.
Inertial
impaction
Surface
retention
“Tortuous
path” pore
Direction
of flow