mutually more or less soluble in each other. The chief differences between
hydrophilic and hydrophobic colloids are their degree of hydration and their
reaction to electrolytes. The particles of a hydrophilic colloid require the
addition of a large quantity of an electrolyte to bring about coagulation,
whereas the hydrophobic colloids are sensitive to, and coagulated by, very
small quantities of electrolytes. Gelatin, agar-agar, starch, and protein solu-
tions belong to the hydrophilic group; the metal sols belong to the hydro-
phobic group. There is no distinct line between the hydrophilic and hydro-
phobic colloids. Even the particles of the rather typical hydrophilic colloids
are not hydrated to the same extent. Thus an agar-agar sol is more strongly
hydrated than a gelatin one. Another way of expressing this is to say that
about 1 per cent of agar-agar will form a stiff gel, but more than 1 per cent
of gelatin is required for a stiff gel.
The charge on colloidal particles. That the micelles possess either a
negative or a positive charge is agreed, though the origin of the charge is
still disputed. For aqueous solutions the charge is most easily explained on
the basis of adsorbed ions. The charge may also come from ionization
of the micelle, or by electrification by contact with the dispersing medium,
in the same manner that a glass rod becomes charged when rubbed with
fur. If the charge on the micelles is reduced to practically zero, the col-
loidal system becomes unstable. The electrical charge is one important
factor in the stabilization of sols. One example in foods is the casein of
milk. When the electrical charge of casein reaches zero, the protein floc-
culates and is precipitated. Kruyt cites it as an example of a protein sol
that is not sufficiently hydrated to be stabilized by hydration alone, so that
it can exist when negatively or positively charged, but not when the charge
is neutralized.
Freundlich uses the term electrokinetic phenomena to designate certain
electrical properties of colloidal systems. He also states that these electro-
kinetic phenomena are closely associated with the physical properties of
interfacial tension, adsorption, colloidal stability, mutual precipitation, and
flocculation.
The theories that have been advanced to explain electrokinetic phenom-
ena are based upon the double-layer theory of Helmholtz. This theory is
that the micelle is surrounded by a double layer of ions, the inner layer,
which may be negative or positive, being closely adsorbed by the micelle,
and the outer layer, consisting of ions of opposite charge from those of the
inner layer, lying close to the micelles in the intermicellar liquid. If the
inner layer of ions is negative, the micelle is negatively charged, the outer
layer being positively charged. As the colloid passes through its isoelectric
point the charge of each double layer is reversed.
Effect o£ electrolytes upon hydrophobic colloids. When a hy-
drophobic colloid is coagulated by an electrolyte its electric charge is
removed. The amount of electrolyte required depends upon several factors:
(1) The manner of adding. More electrolyte is required if it is added in
10 RELATION OF COOKERY TO COLLOID CHEMISTRY
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mutually more or less soluble in each other. The chief differences between
hydrophilic and hydrophobic colloids are their degree of hydration and their
reaction to electrolytes. The particles of a hydrophilic colloid require the
addition of a large quantity of an electrolyte to bring about coagulation,
whereas the hydrophobic colloids are sensitive to, and coagulated by, very
small quantities of electrolytes. Gelatin, agar-agar, starch, and protein solu-
tions belong to the hydrophilic group; the metal sols belong to the hydro-
phobic group. There is no distinct line between the hydrophilic and hydro-
phobic colloids. Even the particles of the rather typical hydrophilic colloids
are not hydrated to the same extent. Thus an agar-agar sol is more strongly
hydrated than a gelatin one. Another way of expressing this is to say that
about 1 per cent of agar-agar will form a stiff gel, but more than 1 per cent
of gelatin is required for a stiff gel.
The charge on colloidal particles. That the micelles possess either a
negative or a positive charge is agreed, though the origin of the charge is
still disputed. For aqueous solutions the charge is most easily explained on
the basis of adsorbed ions. The charge may also come from ionization
of the micelle, or by electrification by contact with the dispersing medium,
in the same manner that a glass rod becomes charged when rubbed with
fur. If the charge on the micelles is reduced to practically zero, the col-
loidal system becomes unstable. The electrical charge is one important
factor in the stabilization of sols. One example in foods is the casein of
milk. When the electrical charge of casein reaches zero, the protein floc-
culates and is precipitated. Kruyt cites it as an example of a protein sol
that is not sufficiently hydrated to be stabilized by hydration alone, so that
it can exist when negatively or positively charged, but not when the charge
is neutralized.
Freundlich uses the term electrokinetic phenomena to designate certain
electrical properties of colloidal systems. He also states that these electro-
kinetic phenomena are closely associated with the physical properties of
interfacial tension, adsorption, colloidal stability, mutual precipitation, and
flocculation.
The theories that have been advanced to explain electrokinetic phenom-
ena are based upon the double-layer theory of Helmholtz. This theory is
that the micelle is surrounded by a double layer of ions, the inner layer,
which may be negative or positive, being closely adsorbed by the micelle,
and the outer layer, consisting of ions of opposite charge from those of the
inner layer, lying close to the micelles in the intermicellar liquid. If the
inner layer of ions is negative, the micelle is negatively charged, the outer
layer being positively charged. As the colloid passes through its isoelectric
point the charge of each double layer is reversed.
Effect o£ electrolytes upon hydrophobic colloids. When a hy-
drophobic colloid is coagulated by an electrolyte its electric charge is
removed. The amount of electrolyte required depends upon several factors:
(1) The manner of adding. More electrolyte is required if it is added in
10 RELATION OF COOKERY TO COLLOID CHEMISTRY