Page 277 - Introduction to Colloid and Surface Chemistry

266 Emulsions and foams

eventually break. Emulsions with a fairly uniform droplet size will
be less prone to this effect.
6. High viscosity A high Newtonian viscosity simply retards the
rates of creaming, coalescence, etc. If a weak gel network is
formed by, for example, dissolving sodium carboxymethyl cellulose
in an O/W emulsion, genuine stability might ensue. However, the
overall Theological properties of such an emulsion may not be
acceptable.

Emulsifying agents and emulsion type

The type of emulsion which is formed when a given pair of immiscible
liquids is homogenised depends on (1) the relative volumes of the two
phases, and (2) the nature of the emulsifying agent.

1. Phase volume The higher its phase volume, the more likely a*
liquid is to become the dispersion medium. However, the liquid
with the greater phase volume need not necessarily be the
dispersion medium.
If the emulsion consisted of an assembly of closely packed
uniform spherical droplets, the dispersed phase would occupy 0.74
of the total volume. Stable emulsions can, however, be prepared
in which the volume fraction of the dispersed phase exceeds 0.74,
because (a) the droplets are not of uniform size and can,
therefore, be packed more densely, and (b) the droplets may
be deformed into polyhedra, the interfacial film preventing
coalescence.
2. Nature of the emulsifying agent Alkali-metal soaps favour the
formation of O/W emulsions, whereas heavy-metal soaps favour
the formation of W/O emulsions. O/W emulsions in the middle
concentration region stabilised by alkali-metal soaps can often be
broken, and even inverted into W/O emulsions, by the addition of
heavy-metal ions.

Several theories relating to emulsion type have been proposed.
The most satisfactory general theory of emulsion type is that
originally proposed for emulsions stabilised by finely divided solids
(see Figure 10.1). If the solid is preferentially wetted by one of the
phases, then more particles can be accommodated at the interface if
the interface is convex towards that phase (i.e. if the preferentially