Geoscience Reference
In-Depth Information
considered as the asymmetrical distribution of the first electron of the H atom
induced by various electronegative atoms.
Ligand exchange processes involve replacement of one or more ligands by the
adsorbing species. In some instances, the ligand exchange process can be regarded
as a condensation reaction (e.g., between a carboxyl group and a hydroxyl alu-
minum surface). Under some conditions, ligand exchange reactions are very likely
to be involved when humic substances interact with a clay material.
The protonation mechanism includes Coulomb electrostatic forces resulting
from charged surfaces. The development of surface acidity by the solid phase of
the subsurface offers the possibility that solutes having proton-selective organic
functional groups can be adsorbed through a protonation reaction.
The p bonds occur as a result of the overlapping of p orbitals when they are
perpendicular to aromatic rings. This mechanism can be used to explain the
bonding of alkenes, alkylenes, and aromatic compounds to subsurface organic
matter.
London-van der Waals forces generally are multipole (dipole-dipole or dipole-
induced dipole) interactions produced by a correlation between fluctuating induced
multipole (principal dipole) moments in two nearly uncharged polar molecules.
Even though the time-averaged, induced multipole in each molecule is zero, the
correlation between the two induced moments does not average to zero. As a
result,
an
attractive
interaction
between the
two
is
produced
at
very
small
molecular distances.
The van der Waals forces also include dispersion forces that arise from cor-
relations between the movement of electrons in one molecule and those of
neighboring molecules. Under such conditions, even a molecule with no perma-
nent dipole moment forms an instantaneous dipole as a result of fluctuations in the
arrangements of its electron cloud. This instantaneous dipole polarizes the charge
of another molecule to give a second-induced dipole, resulting in a mutual dipole-
dipole attraction. All molecules are subject to attraction by dispersion forces
whether or not more specific interactions between ions or dipoles occur. Although
the momentary dipoles and induced dipoles constantly change positions, the net
result is a weak attraction. When many groups of atoms in a polymeric structure
interact simultaneously, the van der Waals components are additive.
Chemisorption denotes the situation in which an actual chemical bond is
formed between the molecules and the surface atoms. A molecule undergoing
chemisorption may lose its identity as the atoms are rearranged, forming new
compounds that better satisfy the valences of the surface atoms. The enthalpy of
chemisorption is much greater than that of physical adsorption. The basis of much
catalytic activity at surfaces is that chemisorption may organize molecules into
forms that can readily undergo reactions. It often is difficult to distinguish between
chemisorption and physical sorption, because a chemisorbed layer may have a
physically sorbed layer deposited above it.
