92168.pdf

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