The Handbook of Soap Manufacture | Page 4

W.H. Simmons
explanation commonly accepted is that the alkali liberated by
hydrolysis attacks any greasy matter on the surface to be cleansed, and,
as the fat is dissolved, the particles of dirt are loosened and easily
washed off. Berzelius held this view, and considered that the value of a
soap depended upon the ease with which it yielded free alkali on
solution in water.
This theory is considered by Hillyer (Journ. Amer. Chem. Soc., 1903,
524), however, to be quite illogical, for, as he points out, the liberated
alkali would be far more likely to recombine with the acid or acid salt
from which it has been separated, than to saponify a neutral glyceride,
while, further, unsaponifiable greasy matter is removed by soap as
easily as saponifiable fat, and there can be no question of any chemical
action of the free alkali in its case. Yet another argument against the
theory is that hydrolysis is greater in cold and dilute solutions, whereas
hot concentrated soap solutions are generally regarded as having the
best detergent action.
Rotondi (Journ. Soc. Chem. Ind., 1885, 601) was of the opinion that the
basic soap, which he believed to be formed by hydrolysis, was alone
responsible for the detergent action of soap, this basic soap dissolving
fatty matter by saponification, but, as already pointed out, his theory of

the formation of a basic soap is now known to be incorrect, and his
conclusions are therefore invalid.
Several explanations have been suggested, based on the purely physical
properties of soap solutions. Most of these are probably, at any rate in
part, correct, and there can be little doubt that the ultimate solution of
the problem lies in this direction, and that the detergent action of soap
will be found to depend on many of these properties, together with
other factors not yet known.
Jevons in 1878 in some researches on the "Brownian movement" or
"pedesis" of small particles, a movement of the particles which is
observed to take place when clay, iron oxide, or other finely divided
insoluble matter is suspended in water, found that the pedetic action
was considerably increased by soap and sodium silicate, and suggested
that to this action of soap might be attributed much of its cleansing
power.
Alder Wright considered that the alkali liberated by hydrolysis in some
way promoted contact of the water with the substance to be cleansed,
and Knapp regarded the property of soap solutions themselves to
facilitate contact of the water with the dirt, as one of the chief causes of
the efficacy of soap as a detergent.
Another way in which it has been suggested that soap acts as a cleanser
is that the soap itself or the alkali set free by hydrolysis serves as a
lubricant, making the dirt less adherent, and thus promoting its
removal.
The most likely theory yet advanced is that based on the emulsifying
power of soap solutions. The fact that these will readily form emulsions
with oils has long been known, and the detergent action of soap has
frequently been attributed to it, the explanation given being that the
alkali set free by the water emulsifies the fatty matter always adhering
to dirt, and carries it away in suspension with the other impurities.
Experiments by Hillyer (loc. cit.) show, however, that while N/10
solution of alkali will readily emulsify a cotton-seed oil containing free
acidity, no emulsion is produced with an oil from which all the acidity

has been removed, or with kerosene, whereas a N/10 solution of
sodium oleate will readily give an emulsion with either, thus proving
that the emulsification is due to the soap itself, and not to the alkali.
Plateau (Pogg. Ann., 141, 44) and Quincke (Wiedmann's. Ann., 35, 592)
have made very complete researches on the emulsification and foaming
of liquids and on the formation of bubbles. The former considers that
there are two properties of a liquid which play an important part in the
phenomenon, (1) it must have considerable viscosity, and (2) its surface
tension must be low. Quincke holds similar views, but considers that no
pure liquid will foam.
Soap solution admirably fulfils Plateau's second condition, its surface
tension being only about 40 per cent. of that of water, while its
cohesion is also very small; and it is doubtless to this property that its
emulsifying power is chiefly due. So far as viscosity is concerned, this
can have but little influence, for a 1 per cent. solution of sodium oleate,
which has a viscosity very little different from that of pure water, is an
excellent emulsifying agent.
Hillyer, to whose work reference has already been made, investigated
the whole question of detergent action very exhaustively, and, as the
result of a very large number of experiments, concludes that the
cleansing power of soap is largely
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