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Vol. 15, No. 3
T h e Value of Silicate of Soda as a Detergent1-I By William Stericker PHILADELPHIA
QUARTZCO., PHILADELPHIA,
PA.
URING the Civil Although silicate of soda is widely used in detergent processes, ~ ~ ~ i ~ little is known of its d u e therein. Detergent action appears to be War rosin became by the hydrolysis of the soap very scarce and based on ( I ) emulsification, ( 2 ) wetfing, (3) lubrication, ( 4 ) deflocwas the active agent in washhigh so that soap-makers culation, ( 5 ) solution, and (6) lathering. Alkaline salts are 1%. Donnan4had previously known to aid in washing. Since silicate of soda i s more expensive began to look for substit $ & ~ ~ ~ p $ ~ h e ~ ~ tutes. The rosin was used than some of these, i f must haoe certain adoantages if its use is to be parallel with the drop numbers of its solutions against justified. Experiments with unsaponifiable and saponifiable oils to give a firm, hard bar of the oil in question. The soap which could be rubbed show that certain silicates haw emulsifying power and fhat they aid practical importance of this on the clothes on a washemulsification with soap to Q greater extent than sodium carbonate WBS not recognized. In 1903 board without going to does. They also aid in lathering to ( 1 greater extent. Laundry Hillyer6 independently got tests showed that Na20.3.3Si02, either alone or wifh soap, could be results similar to Doman’s. pieces and wasting. It was On the basis of his experifound that silicate of soda2 used to wash oery greasy, dirty ooeralls. ments he concluded that would accomplish the same detergent action was “largely purpose. This material, or entirely t o be explained by therefore, became an important constituent of bar soaps, and the power * * * of emulsifying oily substances, of wetting and penetrating into oily textures, and of lubricating texture and a very large proportion of the “laundry>>soaps con- impurities so that they can be removed easily.” He thought all tain silicate in varying amounts. I n this country it is used these effects were explained by the low cohesion of the soap ~ 0 1 ~ . with soap, but in Europe mixtures of sodium carbonate and tions, together with their strong adhesion to oily matter which silicate of soda are sold widely under various trade names, resulted in a low interfacial tension. Jackson6 agreed that emulsification was an important factor, and are used in place of soap. but pointed out another which he said was equally important. I n view of its widespread use, it would be expected that He thought the dirt might cling to or be enclosed in the oily there would be considerable data on its value in detergent proc- globules of the emulsion or might go off in the soapy liquor. esses. This is not the case. A great many writers simply These particles exhibited Brownian movement or pedesis. Soiled fabrics immersed in soap solution were examined under the midismiss it as a filler and an adulterant without any evidence croscope. The soap removed the dirt from the fabric and in the form of experimentaldata O r even sonabl able arguments. started pedesis. Jackson thought this was due to the removal of It is true that silicate of soda is not soap and that it may be the grease. He claimed that the same results could be obtained used to give a bigger bar of so-called soap for a given price by long soaking a t room temperature, during which time action had a chance to operate, as could be obtained with than if the bar were pure soap, However, in return the this larger amounts of alkali, higher temperatures, and mechanical customer profits, because all kinds of soaps are cheaper since agitation. Both these investigators emphasized the oily nature of the the manufacturer can use oils which melt a t lower temperahowever, used lampblack from tures and give softer soaps. These soaps contain larger Soil on the fabric. which all fatty and oily materials had been removed. This quantities of sodium oleate which is a particularly valuable purified material had properties Tvhich were decidedly different If soaps made from these oils contained no from those of ordinary lampblack. It mixed easily with water silicate and were used on the washboard, they would dissolve to form a stable suspension but mas removed by passing through too rapidly and, hence, be wasted. On the other hand, Some filter Paper. If the suspension were made in very dilute soap it passed through the filter without even blackening it. writers have claimed silicate of soda has detergent power but solution, The black which had been filtered out of the water suspension they have been equally negligent about furnishing the data could not be readily removed from the paper with water but on which this conclusion was based. This lack of definite in- could with dilute soap solution. Similar results were obtained formation seems strange but probably resuits from the fact that with clay, hydrated silica, and ferric and aluminium hydroxides. It is evident that this action is entirely distinct from any effect of soda is variable material. Not only does the on the oily portion of the soil. ratio Of sodium oxide to silica vary from 2 : 3 to 1 : 4 in COmPickeringS has pointed out that solubility is another important mercial solutions, but, in addition, the silicates usually are factor in the cleansing process. He found that soap solutions in the form of so~utions which may vary in concentration. would dissolve large amounts of mineral oils or benzene. Some of the oils he used were carefully purified so that they were Experimenters who have set out to get definite information almost pure paraffin hydrocarbons, while others were ordinary preferred to work with materials of more definite composition. commercial grades of oil. For example, a 20 per cent solution of Most of the literature deals with the effect of silicate on potassium stearate dissolved 121 cc. of benzene. The benzene fabrics. There is so nluch confusionand so much obvious did not separate when the solution was diluted, showing that misinformation on this subject that the writer feels that he t h ~ ~ ~ & ! $ ~ ~ ~not~necessary ~ but was a sign cannot discuss it adequately until he has obtained more of useful properties. While the production of a foam or suds is not necessarily evidence that the material producing it is a experimental data.
D
THEBASIS OF DETERGENT ACTION In order to determine the function of silicate of soda in detergent processes, a consideration of the factors upon which they are dependent is necessary. Up to 1903 almost all the Received November 22, 1922. This term is used instead of sodium silicate t o indicate that not sodium metasilicate but commercial material which contains an excess of from 0.5 t o 3 equivalents of silica is meant. [Cf. J. G. Vail, THISJOURNAL, 11 (1919),1029.1 Jackson, Cantor Lectures of the Society of Arts, 1907; Hillyer, J . A m . Chem. SOC.,25 (1903),524.
detergent, housewives and laundry operators have learned that good detergents usually lather. They therefore demand a soap which will form suds. Shorter0 has pointed out that the pressure on the convex side of a bubble is greater than that on the concave side. Hence, when the bubble rests on a surface, it tends to become a plane and will wet the surface more readily. Suds also seem to lift the dirt out of the wash liquor, thereby prevent-
1 2
4
6 6
7 8 9
2. physik. Chem., 31 (1S99),42. J . A m . Chem. Soc., 25 (1903),511. Cantor Lectures, J . SOC.Arts, 55 (19OS),1101, 1122. Rec. trav c h i m , 28 (1909),120,424; 29 (1910),1. J . Chem SOC.,111 (1917),86. J . SOC.Dyers Colourzsts, 34 (1918), 136.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
ing redeposition. In the power waSher they act as a cushion which prevents injury to the materials which are being washed. Detergent power is dependent, therefore, on the following factors : emulsification, wetting, lubrication, deflocculation, (either in the presence or the absence of oily materials), solution, foaming, and perhaps others which have been overlooked or which are not yet known.
THEFUNCTION OF ALKALIES~~
It is generally recognized now that alkalies aid in washing, although for certain delicate fabrics it may not be advisable to use them. Hillyers found that the presence of an excess of fatty acid in soap decreased its emulsifying power but was not able to note any increase with an excess of alkali. Krafft,ll however, found that a little alkali increased the emulsifying power. Elledge and Isherwood12 confirmed Krafft. Briggs and Schmidt,l3 using benzene in place of kerosene, also found that alkalies aided in emulsifying if used in small amounts. Jackson6 noticed that limited concentrations of alkali promoted pedesis. Since Spring’ obtained similar results, this was not due to the action on the oily portion of the soil. Shortergfound that both the fabrics andmost kinds of soil were negatively charged and that small amounts of alkali increased these charges, thereby increasing the natural repulsion. Certain kinds of soil are acid-e. g., the perspiration from almost all portions of the body. The alkali reacts with the acids present to form readily soluble sodium salts. In some dirt fatty acids may be present and be saponified so that an addition of the alkali actually means the addition of soap. Chevreul“ found that acid soaps are difficultly soluble. If the free acids are not neutralized, they will react with the neutral soap to form some acid soap. Furthermore, when the materials are rinsed, the concentration of soap becomes very small, thereby increasing the tendency to form acid soaps by hydrolysis which may be counteracted by the presence of a little alkali. I n addition to aiding soap in the ways enumerated above, alkalies may prevent its waste. A large proportion of the water used in this country is hard. There are large districts in which it has a high degree of hardness. If pure soap is used with such waters, a considerable amount is wasted, so far as washing is concerned, by reason of the formation of insduble calcium and magnesium soaps. Furthermore, the latter are detrimental in detergent processes since they form water-in-oil emulsions which are difficult to remove from the fabric and, therefore, are liable to form spots. Large laundries avoid any such difficulties by softening their water. In the home and even in some of the smaller laundries hard water is used. At least partial softening may be obtained by the addition of alkalies, which may be added separately previous to the addition of the soap or with the soap. The most convenient way is to use a bar containing both soap and alkali. The latter has another function in that it prevents soap from becoming rancid. However, very small amounts are sufficient for $his purpose. The Department of Chemical Engineering of the Laundrpmer’s National Association believes strongly in the policy of buying soap and alkali separately, but, for the reason just stated, permits the addition to flake soap of up to one per cent of silicate of soda, which is particularly effective for this p ~ r p 0 s e . l ~ “Alkalies” is used here in the sense of alkaline salts rather than of caustic alkahes. The Laundryowners’ National Association prefers the use of the word “sodas” which i t believes is a more accurate description. Because it is more common usage, the former term is used here. 1 1 2 p h y s i k . Chem., 36 (1902), 371. 12 THIS JOURNAL, 8 (1916), 793. 18 J . Phvs. Chem , 19 (19151,210 14 “Recherches chimiques sur les corps gras d’origine animale,” Paris, 10
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Although there seems to be no question that alkalies aid in washing and may have other valuable functions in soap, the amount which it i s desirable to use is not so easily determined. With silks and wools, where there is a tendency for the alkaline materials to weaken or otherwise deteriorate the cloth, it seems wise to use very little or none. Jackson6 showed that the effect of the different alkaline salts on these fabrics varied greatly so that it might be permissible to use some but not others. Very fine linens also require special consideration. For ordinary linens and cottons, which constitute the most of the materials washed, the addition of alkali seems desirable. The Department of Chemical Engineering of the Laundryowner’s National Asso~iation’~ recommends the use of one part of soda ash to three parts of soap in washing goods of this class. This proportion is based on the use of softened water of so-called zeyo hardness. It would seem advisable to use a larger proportion of alkali when hard water is used. SILICATEOF SODAAS
AN
ALKALI
The silicates of soda are alkaline salts, but they are more expensive than other alkaline salts-e. g., sodium carbonate, from which they are made. They therefore must offer some advantages over the others in order to compete with them. Their advantages in the manufacture of soap are sufficient to justify the soap-maker in paying the difference in price. One of these, for example, is that they willnot “bloom”-i. e., form crystalline excrescences on the surface of the soap. The properties of solutions of commercial silicates of soda in which there is more than one equivalent of silica are very similar to those of solutions of soap. Both belong to the class which McBain16 calls “colloidal electrolytes.” If this investigator is correct in his theory that this structure has an important bearing on the various factors in detergent action, it would seem that silicates of soda should possess this action to some extent. It may be that the differences between soap and silicate solutions occur just in those properties on which the action is dependent, although it seems unlikely. The essential difference between silicates and other alkaline salts is that the former form solutions with colloidal properties. Certain colloids have been found to have a beneficial effect on the washing process,17 and it is well known that they possess properties which are factors in detergent power. For example, they emulsify oils and deflocculate solid materials such as occur in dirt. The results which follow were obtained in an attempt to answer the questions: I-Does silicate of soda possess detergent powers? 2-Are there advantages to be secured by its use as an alkaline salt which compensate for its greater cost? They deal, for the most part, with only two of the factors which have been enumerated as having fundamental effects on detergent power; hence, they are not conclusive. Further experiments are now under way to extend this investigation to include other factors.
EMULSIFYING POWER Hillyer5 found that sodium silicate did not emulsify a sample of cottonseed oil which had been washed with dilute sodium hydroxide solution until free of fatty acids. He found that the drop number against this oil was 15 in comparison with 9 for water, 10 for sodium hydroxide, and 167 for sodium oleate. Unfortunately, he fell into the error, common among investigators who are not acquainted with 18 J . SOC. Chem. I n d , S 1 (1918), 249T: J. Chem. SOC. ( L o n d o n ) , 116 (1919), 1279; J. Am. Chem. Soc., 42 (1920), 426. 17 Shorter, J. SOC. Dyevs Colouvists, 36 (1920), 299.
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In a third series the total concentration of emulsifier was held a t 0.5 per cent and the amounts of soap and silicate were varied 0.1 per cent a t a time. At room temperature 0.5 per cent soap alone and 0.4 per cent soap with 0.1 per cent silicate gave the best results. After heating the latter was better, as less oil had broken out of the emulsion. The sample containing 0.5 per cent silicate alone gave a good underlying emulsion, but the cream broke to free oil. Good results were obtained with 0.1 per cent soap plus 0.4 per cent silicate, which was somewhat better than soap alone. Carbonate and silicates of varying ratios were compared in a fourth series. Each sample contained 0.4 per cent soap with 0.1 per cent of one of the materials mentioned above. At room temperature Na20.2.4SiOz gave the best emulsion, but, on heating to 80°, Na20.3.3Si02 gave the best, and then Na2C03. In general, there was less free oil when silicate was present than when soap alone was used. Na20.3.3SiO~ was more effective than NazCOa. At room temperature I\’az0.2.4SiOz was better than either. PURIFIED MINERALOIL-The results with a highly purified LUBRICATING OIL-Sufficient water was added to the required mineral oil, “Acto” (U. S. P.),were not the same as with oils of oramount of a 5 per cent solution of the various materials tested to dinary purity. The emulsions formed with silicate were less bring it to 12.5 cc., and then 0.3 cc. of “neutral brown” lubricating stable. oil was added.21 The mixture was shaken vigorously for 30 sec., Series of tests in which the percentage by volume of oil varied allowed to stand 5 min., and then placed inan oven a t 80’ C. for an from 10 to 90 were run with solutions containing 0.5 or 5.0 per hour and a quarter. At the end of that time that oven was turned cent ATa20.3.3Si02. The solutions were shaken 30 sec. and aloff and the samples allowed to remain in it to cool. Their condition was noted a t each step. The materials tested were Na2C03, lowed to stand 10 min. In no case was there any oil-in-water emulsion, but with 70 to 90 per cent oil there was a little water Na20.1.7Si02, Na20.2.1SiO2, Naz0.2.4SiOs, NazO.3.3SiO2, and emulsified in the oil. Similar water-in-oil emulsions were obNa20.3.9SiO~. Each was used a t concentrations varying from 0.2 to 1.0 per cent by increments of 0.2 per cent. The emulsions tained with sodium oleate by Sanyal and J0shi.~4 This action was intensified when the samples were heated to 80” C. before formed creamed out on heating or standing. Pickering found that emulsions made with soap acted in a similar way.22 These shaking and then held a t that temperature. Then with 5 per creams would readily mix with the solution or with water, show- cent silicate as little as 20 per cent oil gave a slight emulsion of ing that the oil in them was still emulsified. But free oil, which water. The best emulsions were obtained with 70 to 90 per cent oil as before. The more concentrated solution of silicate emulsiwould not readily mix in, formed from them in varying amounts. In some cases part of the emulsion did not cream out but re- fied more readily. Neither Na20.3.9Si02nor Na20.2.1Si02would mained in the underlying liquid. Under the optimum conditions emulsify 20 per cent “Acto.” With smaller amounts of oil some emulsification took place almost all the oil was emulsified by silicate. The two most siliceous silicates were the best emulsifying with silicate alone. Naz0.3.9Si02was better than Nas0.3.3SiO2, which was better than sodium carbonate. These emulsions were agents in this series of exDeriments. Thev were about eaual ai&oueh less of the emuls;on creamed ou
