Liquid-gas and liquid-liquid interfaces 69a sol of a highly insoluble substance, such as silver iodide hydrosol, this
phenomenon will be of little consequence, since both large and small
particles have extremely little tendency to dissolve. In a sol of more
soluble material, such as calcium carbonate hydrosol, however,
Ostwald ripening occurs to such an extent that it is not possible to
prepare a long-lived dispersion with particles of colloidal dimensions
unless a stabilising agent, such as gelatin or a surfactant, is
incorporated.
Variation of surface tension with temperature
The surface tension of most liquids decreases with increasing
temperature in a nearly linear fashion (some metal melts being
exceptional in this respect) and becomes very small in the region of
the critical temperature, when the intermolecular cohesive forces
approach zero. A number of empirical equations have been suggested
which relate surface tension and temperature, one of the most
satisfactory being that of Ramsay and Shields:
(4.5)where M is the molar mass of the liquid, p is the density of the liquid,
x is the degree of association of the liquid, rc is the critical
temperature and A: is a constant.
Measurement of surface and interfacial tensions^2 '^43 '^44 '^144The many methods available for the measurement of surface and
interfacial tensions can be classified as static, detachment and
dynamic, the last of these being used to study relatively short time
effects. Static methods usually offer a greater potential for accurate
measurement than detachment methods (especially when solutions of
surface-active agents are involved)^43 , but detachment methods tend
to be the more convenient to operate. With careful experimentation
and exclusion of contaminants (especially surfactants), it is usually
possible to measure surface tensions to an accuracy of 0.01 to 0.1 mN
m"^1. It is unwise to use water which has been in contact with ion-
exchange resins.