The phase-volume theory. If spheres of the same diameter are packed
as closely as possible, one sphere will touch 12 others and the volume the
spheres occupy is about 74 per cent of the total volume. Thus if the spheres
or drops of the dispersed phase remain rigid it is possible to disperse 74
268 EMULSIONS
parts of the dispersed phase in the continuous phase ; but if the dispersed
phase is increased to more than 74 parts of the total volume, a reversal
of the emulsion will occur. However, the dispersed phase does not remain
rigid in shape but the drops flatten out where they come in contact with
each other, nor are all the dispersed particles the same size (see Figs. 27
to 30), so that it is possible for the dispersed phase to consist of from 1 to
99 per cent of the emulsion.
Hydration theory o£ emulsions. Fischer and Hooker state that
hydrated colloids make the best emulsifiers. Fischer states the emulsifying
agent, by w^hich a permanent emulsion is obtained, invariably "proves to be
a hydrophilic colloid when water and oil emulsions are concerned (a
Ij^ophilic colloid of some sort when other than aqueous mixtures are under
consideration). Put another way, oil cannot permanently be beaten into
water, but only into a colloid hydrate."
Fischer and Hooker have found albumin, casein, and gelatin to be good
emulsifying agents. Casein when not hydrated, i.e., at its isoelectric point,
is a poor emulsifying agent, but hydrated casein, i.e., acid or alkali casein
is a good emulsifying agent.
Fischer states that all permanent emulsions can be explained on the basis
of hydrated or lyated colloids. He says that when water changes to a
colloid hydrate, its physical constants change ; and these include, among
others, surface tension, viscosity, and adsorption. The treatment of the
colloid, such as freezing or heating, or the addition of substances which
alter the water-holding capacity of the colloid may crack the emulsion or
lessen its emulsifying ability.
Interfacial films. Clayton in discussing "Foods as Colloid Systems"
describes interfacial films as follows: "As early as 1840, Ascherson ob-
served 'that coagulation in form of a membrane occurs inevitably and
instantaneously when albumin comes into contact with a liquid fat.' — Any
solute which lowers the interfacial tension between oil and w^ater will
necessarily accumulate at that interface, and in the case of certain proteins,
notably albumen, the act of adsorption leads to a change in the physical
character of the emulsifying agent, this being 'precipitated' as a fibrous or
membrane-fibrous solid, no longer soluble in its original solvent. The ex-
istence of such interfacial membranes was verified by Ramsden and other
investigators."
In reading the various theories of emulsion one is impressed with the
similarity of many factors.
Oriented wedge theory. This theory for the manner in which emul-
sions are stabilized has been developed from the work of Langmuir and of
Harkins. It is based upon the concept that the molecules of the emulsifier
orient themselves in the interface between the dispersed and continuous
phases, forming a wedge, the curvature of which determines the size of the
dispersed phase. Fuller accounts may be found in Clayton's book and in
the articles of the authorities mentioned above.
THE TYPE OF EMULSION FORMED 269
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The phase-volume theory. If spheres of the same diameter are packed
as closely as possible, one sphere will touch 12 others and the volume the
spheres occupy is about 74 per cent of the total volume. Thus if the spheres
or drops of the dispersed phase remain rigid it is possible to disperse 74
268 EMULSIONS
parts of the dispersed phase in the continuous phase ; but if the dispersed
phase is increased to more than 74 parts of the total volume, a reversal
of the emulsion will occur. However, the dispersed phase does not remain
rigid in shape but the drops flatten out where they come in contact with
each other, nor are all the dispersed particles the same size (see Figs. 27
to 30), so that it is possible for the dispersed phase to consist of from 1 to
99 per cent of the emulsion.
Hydration theory o£ emulsions. Fischer and Hooker state that
hydrated colloids make the best emulsifiers. Fischer states the emulsifying
agent, by w^hich a permanent emulsion is obtained, invariably "proves to be
a hydrophilic colloid when water and oil emulsions are concerned (a
Ij^ophilic colloid of some sort when other than aqueous mixtures are under
consideration). Put another way, oil cannot permanently be beaten into
water, but only into a colloid hydrate."
Fischer and Hooker have found albumin, casein, and gelatin to be good
emulsifying agents. Casein when not hydrated, i.e., at its isoelectric point,
is a poor emulsifying agent, but hydrated casein, i.e., acid or alkali casein
is a good emulsifying agent.
Fischer states that all permanent emulsions can be explained on the basis
of hydrated or lyated colloids. He says that when water changes to a
colloid hydrate, its physical constants change ; and these include, among
others, surface tension, viscosity, and adsorption. The treatment of the
colloid, such as freezing or heating, or the addition of substances which
alter the water-holding capacity of the colloid may crack the emulsion or
lessen its emulsifying ability.
Interfacial films. Clayton in discussing "Foods as Colloid Systems"
describes interfacial films as follows: "As early as 1840, Ascherson ob-
served 'that coagulation in form of a membrane occurs inevitably and
instantaneously when albumin comes into contact with a liquid fat.' — Any
solute which lowers the interfacial tension between oil and w^ater will
necessarily accumulate at that interface, and in the case of certain proteins,
notably albumen, the act of adsorption leads to a change in the physical
character of the emulsifying agent, this being 'precipitated' as a fibrous or
membrane-fibrous solid, no longer soluble in its original solvent. The ex-
istence of such interfacial membranes was verified by Ramsden and other
investigators."
In reading the various theories of emulsion one is impressed with the
similarity of many factors.
Oriented wedge theory. This theory for the manner in which emul-
sions are stabilized has been developed from the work of Langmuir and of
Harkins. It is based upon the concept that the molecules of the emulsifier
orient themselves in the interface between the dispersed and continuous
phases, forming a wedge, the curvature of which determines the size of the
dispersed phase. Fuller accounts may be found in Clayton's book and in
the articles of the authorities mentioned above.
THE TYPE OF EMULSION FORMED 269