Chemistry Reference
In-Depth Information
of the nature of the stress, however, the ultimate cause of bubble coalescence
will always be the same—the loss of liquid from the lamellar layer until a critical
thickness of 5-15 nm is reached and the liquid film can no longer support the
pressure of the gas in the bubble. As pointed out above, the loss or drainage of
liquid from the lamellae can be affected by a number of factors, including (1) a
high viscosity in the liquid, which will slow the drainage process and, in some
cases, have a dissipating or buffering effect on many types of mechanical distur-
bances; (2) surface rheological effects, which can retard the loss of liquid by a vis-
cous drag type of mechanism; and (3) the presence of repulsive electrical or steric
interactions across the lamellae, which can oppose drainage through the effects of
the disjoining pressure. The addition of surfactants to a foaming system can alter
any or all of these system characteristics and therefore enhance (or reduce) the sta-
bility of the foam. Surfactants will also have the effect of lowering the surface ten-
sion of the system, thereby reducing the work required for the initial formation of
the foam [Eq. (8.1)]. With the exception of bulk rheological effects, each of the
phenomena relating surfactants to foam formation and stability is discussed in
more detail below.
8.2. THE ROLE OF SURFACTANT IN FOAMS
For a liquid to form a foam, it must be able to form a membrane around the
gas bubble possessing a form of elasticity that opposes the thinning of the lamellae
as a result of drainage. Foaming does not occur in pure liquids because no such
mechanism for the retardation of lamellae drainage or interfacial stabilization
exists. When amphiphilic materials or polymers are present, however, their adsorp-
tion at the gas-liquid interface serves to retard the loss of liquid from the lamellae
and, in some instances, to produce a more mechanically stable system. Theories
related to such film formation and persistence, especially film elasticity, derive
from a number of experimental observations about the surface tension of liquids:
1. As given by the well-known Gibbs adsorption equation
i
¼
1
RT
ds
i
d ln a
ð
8
:
4
Þ
T
the surface tension of a solution will decrease as the concentration of the
surface-active material in solution increases (assuming positive adsorption)
up to its critical micelle concentration (the Gibbs effect).
2. The instantaneous or dynamic surface tension at a newly formed surface is
always higher than the equilibrium value; that is, there is a finite time during
which the amphiphilic molecules in the bulk solution diffuse to the interface
in order to lower the surface tension (the Marangoni effect). The two effects
are complementary, often discussed as the combined Gibbs-Marangoni effect,
