negative charge of the other. However, depending on the lengths L of the
interacting dipoles, the most attractive interaction may often be when the
two dipoles are antiparallel to each other, with the positive region of one
dipole directly adjacent to the negative region of the other. This antiparallel orientation allows the molecules to draw closer together,
reducing the value of r 12 and thereby maximizing the attractive interaction energy.
In the purification technique of column chromatography, a solid material
containing many polar bonds, such as silica or alumina, is placed in a
vertical glass column and is referred to as the stationary phase. The liquid
mobile phase of solution to be purified is then flowed through the
column. Because dipole–dipole interactions between polar molecules in
the mobile phase and the polar surfaces of the particles in the stationary
phase slow the polar molecules’ descent, compounds in the solution flow
out the bottom of the column, or elute, in order of increasing polarity and
can thus be separated. This same technique works for ions in solution as
well because they are slowed by ion–dipole interactions. The polarity of
the solvent dictates the rate of movement of compounds through the
column. If a solvent is too polar, the stationary phase attracts the solvent
rather than the solutes and no separation occurs. However, if a solvent is
not polar enough, some more polar solutes may not make it all the way
through the column.
q +1
θ 1
θ 2
L 1
q +2
r 12
μ 2
μ 1
L 2
q –1
q –2
φ
Figure 5.5 A schematic depiction of the variables involved in a dipole–dipole
interaction. L is the distance between the centers of the two partial charges of either
dipole 1 or 2. r 12 is the distance between the midpoints of L 1 and L 2 . θ 1 and θ 2 are the
angles between L 1 and r 12 or L 2 and r 12 , respectively. ϕ is the angle of rotation
between dipoles 1 and 2. q + and q − are the partial positive or negative charges in
each dipole. µ is the dipole moment.
INTERMOLECULAR FORCES AND SELF-ASSEMBLY 141
interacting dipoles, the most attractive interaction may often be when the
two dipoles are antiparallel to each other, with the positive region of one
dipole directly adjacent to the negative region of the other. This antiparallel orientation allows the molecules to draw closer together,
reducing the value of r 12 and thereby maximizing the attractive interaction energy.
In the purification technique of column chromatography, a solid material
containing many polar bonds, such as silica or alumina, is placed in a
vertical glass column and is referred to as the stationary phase. The liquid
mobile phase of solution to be purified is then flowed through the
column. Because dipole–dipole interactions between polar molecules in
the mobile phase and the polar surfaces of the particles in the stationary
phase slow the polar molecules’ descent, compounds in the solution flow
out the bottom of the column, or elute, in order of increasing polarity and
can thus be separated. This same technique works for ions in solution as
well because they are slowed by ion–dipole interactions. The polarity of
the solvent dictates the rate of movement of compounds through the
column. If a solvent is too polar, the stationary phase attracts the solvent
rather than the solutes and no separation occurs. However, if a solvent is
not polar enough, some more polar solutes may not make it all the way
through the column.
q +1
θ 1
θ 2
L 1
q +2
r 12
μ 2
μ 1
L 2
q –1
q –2
φ
Figure 5.5 A schematic depiction of the variables involved in a dipole–dipole
interaction. L is the distance between the centers of the two partial charges of either
dipole 1 or 2. r 12 is the distance between the midpoints of L 1 and L 2 . θ 1 and θ 2 are the
angles between L 1 and r 12 or L 2 and r 12 , respectively. ϕ is the angle of rotation
between dipoles 1 and 2. q + and q − are the partial positive or negative charges in
each dipole. µ is the dipole moment.
INTERMOLECULAR FORCES AND SELF-ASSEMBLY 141