stearate

The actual reaction which occurs has been the subject of investigation
by many chemists, and very diverse conclusions have been arrived at.
Chevreul, the pioneer in the modern chemistry of oils and fats, found
that a small amount of alkali was liberated, as appears in the above
equation, together with the formation of an acid salt, a very minute
quantity of free fatty acid remaining in solution. Rotondi (_Journ. Soc.
Chem. Ind._, 1885, 601), on the other hand, considered that a neutral
soap, on being dissolved in water, was resolved into a basic and an acid
salt, the former readily soluble in both hot and cold water, the latter
insoluble in cold water, and only slightly soluble in hot water. He
appears, however, to have been misled by the fact that sodium oleate is
readily soluble in cold water, and his views have been shown to be
incorrect by Krafft and Stern (_Ber. d. Chem. Ges._, 1894, 1747 and
1755), who from experiments with pure sodium palmitate and stearate
entirely confirm the conclusions arrived at by Chevreul.

The extent of dissociation occurring when a soap is dissolved in water
depends upon the nature of the fatty acids from which the soap is made,
and also on the concentration of the solution. The sodium salts of
cocoa-nut fatty acids (capric, caproic and caprylic acids) are by far
the most easily hydrolysed, those of oleic acid and the fatty acids from
cotton-seed oil being dissociated more readily than those of stearic
acid and tallow fatty acids. The decomposition increases with the amount
of water employed.

The hydrolytic action of water on soap is affected very considerably by
the presence of certain substances dissolved in the water, particularly
salts of calcium and magnesium. Caustic soda exerts a marked retarding
effect on the hydrolysis, as do also ethyl and amyl alcohols and
glycerol.

_Detergent Action of Soap._--The property possessed by soap of removing
dirt is one which it is difficult to satisfactorily explain. Many
theories, more or less complicated, have been suggested, but even now
the question cannot be regarded as solved.

The 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
s