Chemistry Reference
In-Depth Information
indicating that it is the effect of benzene in water that controls the spreading (or
nonspreading) in this system. The interfacial tension of water-benzene is
unchanged throughout because it inherently includes the mutual saturation process.
Situations like that for benzene are very general for low-surface-tension liquids
on water. There may be initial spreading followed by retraction and lens formation.
A similar effect can in principle be achieved if a third component (e.g., a surfactant)
that strongly absorbs at the water-air interface, but not the oil-water interface, is
added to the system. Conversely, if the material is strongly adsorbed at the oil-
water interface, lowering the interfacial tension, spreading may be achieved
where it did not occur otherwise. This is, of course, a technologically very impor-
tant process and will be discussed in more detail in later chapters.
For normal use, it is assumed that the values of s represent equilibrium satura-
tion values. The antifoaming materials reduce the strength of the surface film by
reducing the lateral van der Waals interactions between adsorbed molecules due
to branching in the hydrophobic tail. They may also be made to lie flat in the sur-
face by the inclusion of several hydrophilic groups along the chain, by placing the
hydrophile in the middle of the chain, and by using the smallest number of methy-
lene groups in the chain consistent with the necessity for limited solubility.
In summary, it can be said that the various aspects of foam formation and per-
sistence are related to the actions of surfactant molecules and additives at the var-
ious interfaces in the system, coupled with the rheological characteristics of the
system, including the dilational viscosity of the interfacial layers and the bulk rheo-
logical properties of the system. Depending on whether foam is wanted, the choice
of surfactants and additives for a formulation must address all of those factors in the
context of the system being prepared and its end use.
8.7. LIQUID AEROSOLS
Mists and fogs are colloidal dispersions of a liquid in a gas. They may therefore be
regarded as being the inverse of foams. The interactions controlling their stability,
however, are not generally the same as those involved in foam stabilization, because
most mists and fogs do not possess the thin lamellar stabilizing films encountered in
foams. In fact, the stability of liquid aerosols is usually more dependent on fluid
dynamics than on colloidal factors, as illustrated below.
8.7.1. The Formation of Liquid Aerosols
Liquid aerosols may be formed by one of two processes, depending on whether the
dispersed system begins as a liquid or undergoes a phase change from vapor to
liquid during the formation process. In the first case, since the dispersed material
does not change phases, the aerosol is formed by some process that reduces the par-
ticle size of the liquid units. To this class belong spray mists such as those formed at
the bottom of a waterfall or by ocean waves (impact), mists produced by vigorous
agitation (mechanical breakup), and those formed by some direct spraying or
