The Handbook of Soap Manufacture | Page 3

among which may be mentioned
that of Kingzett, who says that "Soap, considered commercially, is a
body which on treatment with water liberates alkali," and that of Nuttall,
who defines soap as "an alkaline or unctuous substance used in
washing and cleansing".
Properties of Soap.--Both soda and potash soaps are readily soluble in
either alcohol or hot water. In cold water they dissolve more slowly,
and owing to slight decomposition, due to hydrolysis (vide infra), the
solution becomes distinctly turbid. Sodium oleate is peculiar in not
undergoing hydrolysis except in very dilute solution and at a low
temperature. On cooling a hot soap solution, a jelly of more or less firm
consistence results, a property possessed by colloidal bodies, such as
starch and gelatine, in contradistinction to substances which under the
same conditions deposit crystals, due to diminished solubility of the
salt at a lower temperature.
Krafft (Journ. Soc. Chem. Ind., 1896, 206, 601; 1899, 691; and 1902,
1301) and his collaborators, Wiglow, Strutz and Funcke, have
investigated this property of soap solutions very fully, the researches
extending over several years. In the light of their more recent work, the
molecules, or definite aggregates of molecules, of solutions which
become gelatinous on cooling move much more slowly than the
molecules in the formation of a crystal, but there is a definite structure,
although arranged differently to that of a crystal. In the case of soda
soaps the colloidal character increases with the molecular weight of the
fatty acids.
Soda soaps are insoluble in concentrated caustic lyes, and, for the most
part, in strong solutions of sodium chloride, hence the addition of
caustic soda or brine to a solution of soda soap causes the soap to
separate out and rise to the surface. Addition of brine to a solution of
potash soap, on the other hand, merely results in double decomposition,
soda soap and potassium chloride being formed, thus:--

C{17}H{35}COOK + NaCl = C{17}H{35}COONa + KCl potassium
sodium sodium potassium stearate chloride stearate chloride
The solubility of the different soaps in salt solution varies very
considerably. Whilst sodium stearate is insoluble in a 5 per cent.
solution of sodium chloride, sodium laurate requires a 17 per cent.
solution to precipitate it, and sodium caproate is not thrown out of
solution even by a saturated solution.
Hydrolysis of Soap.--The term "hydrolysis" is applied to any resolution
of a body into its constituents where the decomposition is brought
about by the action of water, hence when soap is treated with cold
water, it is said to undergo hydrolysis, the reaction taking place being
represented in its simplest form by the equation:--
2NaC{18}H{35}O{2} + H{2}O = NaOH + HNa(C{18}H{35}O{2}){2}
sodium water caustic acid sodium stearate soda 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