Chemical Engineering July 2021 - 20

Facts At Your Fingertips
Adsorption bonding
Department Editor: Scott Jenkins
P
hysical adsorption phenomena
play a critical role in many
separation and purification
processes, including separations of
industrial gases, purification of gases,
moisture removal from fluids, water
purification and others. Adsorption
is a surface phenomenon in which a
fluid (gas or liquid) molecule attaches
to the surface of a material by physical
forces known as Van der Waals
interactions. This one-page reference
discusses the nature of Van der Waals
forces, and their role in adsorption
processes, with a focus on London
dispersion forces.
Van der Waals forces
Van der Waals forces is a general
term
that describes
the
relatively
weak (compared to typical covalent
bond strengths) intermolecular forces
that result from individual electrostatic
attractions between otherwise nonattracting
atoms and molecules. Van
der Waals interactions can be classified
into three categories, as follows:
Hydrogen bonds. Hydrogen bonds
form between H atoms - that acquire
significant positive charge from
being bound to electronegative elements
- and negatively charged lone
electron pairs on adjacent molecules.
The most important H-bonds form
with hydroxide and amide groups.
Hydrogen bonds have strengths
ranging from 5 to 50 kJ/mol.
Dipole-dipole interactions. Dipoledipole
interactions are the attractive
forces that arise when the partial positive
and negative charges that form
on polar molecules are attracted to
each other. These permanent attractive
forces act at close range.
London dispersion forces. London
dispersion forces refer to the electrostatic
attraction of spontaneously
formed transient temporary dipoles
that arise in non-polar molecules.
For adsorption phenomena, London
forces are the most important.
London forces
For a neutral molecule or atom, the
average charge distribution is zero,
but at any given instant, molecules
20
have asymmetric distributions
of electron
density. Temporarily
fluctuating dipole
moments can arise
from a brief shift of orbital
electrons to one
side of an atom or
molecule. In adsorption,
these transient
dipoles induce complementary
dipoles
in neighboring molecules
so that adsorbents
are attracted to
Unequal distribution
of electrons
Attractive force
Temporary dipoles
FIGURE 1. Transient dipoles resulting from spontaneous shifts in electron
densities among molecules give rise to attractive forces
adsorbate molecules and they attach.
These temporary induced dipoles are
known as London dispersion forces,
named for Fritz London, the German
physicist and Duke University professor
who described the intermolecular
forces of noble gas atoms in 1930.
London forces have the following
characteristics:
Non-chemical. London forces attract
molecules to each other, but do
not chemically change the molecules
Additive. The observed London force
is the sum of all the individual interactions
of the adsorbate molecule
and the neighboring molecules. In
adsorption onto activated carbon, for
example, the magnitude of the London
forces will be related to the density
of carbon within the vicinity of the
adsorbate molecules
Nonspecific. London forces are
present among all molecules
Temperature-independent. London
forces are unaffected by temperature,
and thus the adsorption
forces will be constant with temperature.
Adsorption capacities will still
be sensitive to the changes in vapor
pressure or solubility of the adsorbing
molecules that result from temperature
differences
Short-ranged. The magnitude of
the London force is sensitive to the
proximity of the adsorbate molecules
from the adsorbent material. London
forces can be considered negligible at
separations greater than about two
molecular layers. In activated carbon,
the adsorption forces are significant if
the gaps or voids within the pores of
the carbon structure (pore widths) are
less than four or five molecular layers.
Bond strengths
Bonding energies in adsorption range
from about 10 to 70 kJ/mol, much
lower than typical covalent bond
energies of 200 to 500 kJ/mol. The
adsorption bonding energy is high
enough for adsorption to occur, yet
low enough to allow the adsorbent to
be regenerated by removing the adsorbed
molecules.
Adsorption
The affinity of a fluid component for a
particular adsorbent depends on molecular
characteristics, such as size,
shape, and polarity of the adsorbent
material, as well as the partial pressure
or concentration in the fluid, and
the system temperature.
Commercial adsorbents are highly
porous, with pore surface areas
ranging from about 100 to 1,200
m2/g. The large surface area allows a
large amount of adsorption relative to
the weight of the adsorbent. A number
of factors can affect adsorption,
such as pore size distribution, molecular
size of the impurity species,
particle size of the adsorbent, pH of
the solution and others. Activated
carbon is one of the most complex
solid adsorbents, and is very versatile
because of its extremely high surface
area and micropore volume. n
Editor's note: Information for this column came from several
sources, including: Clark, J., Intermolecular Bonding, article
in Chemguide (www.chemguide.co.uk) 2017; Desotech,
Carbonology articles: What is adsorption?, www.desotech.com,
accessed June 2021; and others
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM JULY 2021
http://www.chemguide.co.uk http://www.desotech.com http://WWW.CHEMENGONLINE.COM
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