2.2 Mixtures, Solutions, Colloids, Sols, and Gels
17
To prevent coalescence of the embedded particles in a fluid mixture, the particles
must have a net repulsive force between neighboring pairs, or sufficient kinetic
energy to overcome any weak attractive force. In general, mixed particles will have
electrical interactions with the background medium, and with other such particles,
just as atoms do. Over atomic scales, these interactions are dominated by charge
and quantum effects: ionic forces and charge sharing interactions. In addition, there
are van der Waals forces coming from charge-dipole, dipole-dipole, dipole-induced
dipole forces, and induced-dipole induced-dipole (dispersion) forces. Also, the net
interaction between mixed particles must include the effect of any media molecules
adsorbed onto the surface of the particles. For example, gold nanoparticles can be
put in water suspension if the gold particles acquire a charge through reagents such
as citrate ions or thiol that attach to the gold surface, making the gold particles repel
each other.
All nearby atoms and molecules will have short-range electrical interactions due
to the dispersion force. This force is always attractive, and is larger between similar
molecules than dissimilar molecules of the same molecular weight, accounting for
why identical macromolecules in solution, even with no charge or dipole moments,
tend to clump together and ‘congeal’. The dispersion force, even in the presence
of permanent dipole interaction, can dominate the attractive interaction of nearby
molecules, because the dispersion force grows with the size of the molecules, their
proximity (flat molecules can get closer together, making the average distance
between atoms in adjacent molecules smaller), the ease with which the electrons
in the molecule can shift position due to an external electric field (polarizability),
and the number of matching electron resonant frequencies. 10 One result is that
homogeneous mixtures of fluids, each with neutral molecules having no fixed dipole
interactions, still separate from each other over time, making the fluids largely
‘immiscible’. In contrast, the immiscibility of oil and water comes from the fact
that water molecules are polar, so that the water molecules attract each other with a
force larger than that between a water molecule and a non-polar fat molecule. We
say that oil is ‘hydrophobic’.
Note that soaps have molecules with a hydrophilic head by being negatively
charged, and a fatty hydrophobic tail. The tail will have van der Waals forces with
the oil. This means that soap molecules can form a water-attractive capsule around
oils, with the soap molecule tail immersed into the oil droplets. Van der Waals forces
between the fatty ends and other hydrocarbon chains will have enhanced attraction
because of the common frequencies of induced-dipole resonances. Small clusters
of lipid molecules in water with their hydrophobic ends pointing inward are called
‘micelles’, and the process of their formation is called ‘emulsification’.
The separation between mixed particles may be maintained by thermal fluctuations that tend to scatter the particles. These dispersive effects may act against local
interactions or any long-range forces such as gravity or external electromagnetic
fields. The thermal scattering and diffusion of the mixed particles in low concen10 Resonance is defined in Sect. 5.12.
17
To prevent coalescence of the embedded particles in a fluid mixture, the particles
must have a net repulsive force between neighboring pairs, or sufficient kinetic
energy to overcome any weak attractive force. In general, mixed particles will have
electrical interactions with the background medium, and with other such particles,
just as atoms do. Over atomic scales, these interactions are dominated by charge
and quantum effects: ionic forces and charge sharing interactions. In addition, there
are van der Waals forces coming from charge-dipole, dipole-dipole, dipole-induced
dipole forces, and induced-dipole induced-dipole (dispersion) forces. Also, the net
interaction between mixed particles must include the effect of any media molecules
adsorbed onto the surface of the particles. For example, gold nanoparticles can be
put in water suspension if the gold particles acquire a charge through reagents such
as citrate ions or thiol that attach to the gold surface, making the gold particles repel
each other.
All nearby atoms and molecules will have short-range electrical interactions due
to the dispersion force. This force is always attractive, and is larger between similar
molecules than dissimilar molecules of the same molecular weight, accounting for
why identical macromolecules in solution, even with no charge or dipole moments,
tend to clump together and ‘congeal’. The dispersion force, even in the presence
of permanent dipole interaction, can dominate the attractive interaction of nearby
molecules, because the dispersion force grows with the size of the molecules, their
proximity (flat molecules can get closer together, making the average distance
between atoms in adjacent molecules smaller), the ease with which the electrons
in the molecule can shift position due to an external electric field (polarizability),
and the number of matching electron resonant frequencies. 10 One result is that
homogeneous mixtures of fluids, each with neutral molecules having no fixed dipole
interactions, still separate from each other over time, making the fluids largely
‘immiscible’. In contrast, the immiscibility of oil and water comes from the fact
that water molecules are polar, so that the water molecules attract each other with a
force larger than that between a water molecule and a non-polar fat molecule. We
say that oil is ‘hydrophobic’.
Note that soaps have molecules with a hydrophilic head by being negatively
charged, and a fatty hydrophobic tail. The tail will have van der Waals forces with
the oil. This means that soap molecules can form a water-attractive capsule around
oils, with the soap molecule tail immersed into the oil droplets. Van der Waals forces
between the fatty ends and other hydrocarbon chains will have enhanced attraction
because of the common frequencies of induced-dipole resonances. Small clusters
of lipid molecules in water with their hydrophobic ends pointing inward are called
‘micelles’, and the process of their formation is called ‘emulsification’.
The separation between mixed particles may be maintained by thermal fluctuations that tend to scatter the particles. These dispersive effects may act against local
interactions or any long-range forces such as gravity or external electromagnetic
fields. The thermal scattering and diffusion of the mixed particles in low concen10 Resonance is defined in Sect. 5.12.