Detergency of Alkaline Salt Solutions I-Initial
and Available Alkalinity
FOSTER DEE SNELL,r130 Clinton Street, Brooklyn, N . Y. T H E PRIlWARY field of detergency of alkalirie used f o r ecaluation of detergency are reviewed. salt solutions is as builders with soap. A very The compounds studied have decreasing total dilute soap solution is mainly crystalloidal. I n alkalinities, as shown by p H or Cox values on more concentrated solutions, such as 0.1 per cent, 0.033 per cent solutions, in the following order: which are more efective cleaners, a reasonable sodium hydroxide, sodium orthosilicate, sodium amount of the soap is present in the f o r m of colloidal metasilicate, trisodium phosphate, sodium silicate micelles. B y a qualitatine application of the law of 1 z 1.58 (anhydrous), sodium carbonate, sodium mass action, the effect of a soap builder would be silicate 1 ~ 3 . 8 6(anhydrous), modiJied soda, and to cause the formation of more colloidal micelles, borax. T h y have decreasing available alkalinities which probably explains the increased detergency above p H 10.0, as shown by electrometric titrations when such builders are added. Another function of of 0.66 per cent solutions, in the following order: the builder is to neutralize acidic ingredients com- sodium hydroxide, sodium orthos il icate, sodium monly present in dirt. Since the p H of a neutral metasilicate, sodium silicate 1 ~1.58 (anhydrous), commercial soap is 10.2 in a 0.1 per cent solution. sodium carbonate, alkaline trisodium phosphate, anything more acidic than that is acid to the soap sodium silicate 1~3.86(anhydrous), mod$ed soda, solution. Some soap builders-notably silicatesand borax. hace negative radicals which, under suitable condiFor rational comparison of the relative value of tions, are also colloidal. soap builders, it is essenlial that the p H be translated For the purpose of discussion, the dirt dificult to into Con. remove is considered to consist of microscopic and T h e conclusion reached is that, owing to the submicroscopic particles coated with oil. These are necessity of neutralizing acidic dirt, a buffered too small for a detergent solution io displace the oil builder is essential. This rules out sodium hydroxJ l m and, therefore, behave like oil droplets. I m - ide. T h e most effective builders, so f a r as alkalinily portant factors are wetting power, deflocculating is concerned, were .found to be sodium orthosilicate power, and emulsifying power. Methods previously and sodium metasilicate in that order of prpference.
T
0 OBTAIN the highest efficiency from soap in commer-
and simple ions, the amount of each changing with varying conditions. Soap solutions are alkaline, owing to hydrolysis. For most concentrations the amount of hydroxyl ion present is approximately 0.001 N , but in very dilute solutions, although hydrolysis increases, the alkalinity decreases. Even in the presence of excess fatty acid over that equivalent to the alkali, a soap solution remains alkaline so that the other product of hydrolysis must be an acid soap, separating as a finely suspended solid. Excess alkali remains almost entirely in the free condition, showing that basic soaps are not formed. The degree of hydrolysis increases rapidly as the homologous series of fatty acids is ascended. Under laundry conditions hydrolysis occurs to the extent of about 10 per cent. An acetyl sulfonic acid soap, in which hydrolysis is impossible, behaves in most respects like potassium stearate, so that the major properties of soap solutions are believed to be due to the soap itself rather than to any products of hydrolysis. The law of mass action may be assumed to hold qualitatively in soap solutions, since the addition of common ions to the solution has the effect of producing more colloidal ionic micelles. Such an effect has been obtained experimentally, although the stages which may occur in going from the ionic to the colloidal condition are problematical. When a common ion is added to a n ordinary salt solution, the effect may be followed semi-quantitatively, and, since the cause and effect are similar in colloidal solution, parallel equilibria are suggested.
cial use, experience dictates that an alkaline agent be added. I n the laundry industry this is known as a soap builder. The widespread use of such builders and the relatively unsatisfactory results when a builder is not used, indicate the importance of such added alkalies. Commercial soap builders are salts of sodium hydroxide with weak acids. I n a few cases the hydroxide itself is used. By far the most common soap builders are the salts with carbonic acid. These are either the commercial normal carbonate, known as soda ash, or variable mixtures with the bicarbonate, known as modified sodas. Other important builders are trisodium phosphate, sodium silicates varying in alkali content, and borax. Caustic soda itself is popular in some sections. Admixed with soda ash it is known as Special Alkali. There are numerous proprietary compounds on the market composed of mixtures of these salts, with or without addition of soap.
NATUREOF SOAPSOLUTIOSS An aqueous soap solution may be primarily crystalloidal or colloidal, according to concentration. In very dilute solutions it is mainly crystalloidal, but in more concentrated solutions heavily hydrated ionic micelles or aggregates are formed which exhibit the osmotic effect of a n ordinary colloid. Although soap in aqueous solution may be called a colloidal electrolyte, this state is in equilibrium with neutral colloidal particles, that is, aggregates of molecules, simple molecules, 76
January, 1932
I S D U S T R I A L AND E N G I N E E R I N G CHEMIbTRY
The following representation of the equilibrium in a sodium palmitate solution is intended to indicate as many of the factors concerned as possible. Sodium palmitate is selected for illustrative purposes. The reactions and equilibria with the oleate and stearate are believed to be parallel. I n a sodium palmitate solution the negatively charged palmitate cluster contains more than simple palmitate ions. The free fatty acid combines with sodium ion or soap molecules to some extent, forming acid soaps of varying composition which may coexist in solution with free sodium hydroxide: Na+
+
Na,P, (in which x
ir
< y)
2IP
d 0H
.]I
SaHP2
The ionic micelle is thought t o contain some colloidal undissociated soap, so that the> micelles of sodium and potassium soaps are not identical as they would be if made up simply of the negative ion. A formula for the ionic micelle of sodium palmitate may be written (;\;aP),~(P-),.(H~O),, indicated above as Na.Py (x
