The Role of Silica-
Soluble Silicate
Cleansers JAMES G. VAIL Philadelphia Quartz Company, Philadelphia, Pa. ILICATE solutions, either alone or combined with soaps, are used as cleansers in a great diversity of ways. Household and industrial uses are included. Compositions have been chosen partly on the basis of empirical trial and partly as a result of systematic study but, broadly speaking, the art of cleaning is not, yet sufficiently understood to be called a science. Until this stage is reached, the attempt to evaluate particular reagents proceeds under a handicap. Cleansing is essentially complex because dirt is various and the surfaces from which it is to be removed are many and different. The literature of the past decade has added to the knowledge of several phenomena which are important. The deflocculation of solid insoluble particles, the wetting of both solid and liquid surfaces, the effects of hydrogen-ion concentration, the appropriate amounts of alkali, and cost factors have all come in for a share of attention, and a partial basis has been laid for a more rational choice of detergents to fit particular needs. The conventional view of soaps and alkaline compounds used for industrial cleaning has been to regard them as sodiuni or potassium salts without much emphasis on the effects contributed by the anions or the negatively charged colloidal micelles, as the case may be. Among the soluble silicates one hesitates to indicate the condition of the silica as silicate ion or colloid, although crystalline sodium metasilicate seems to be an orthodox chemicalcompound and solutions with four moles of silica to each mole of sodium oxide certainly contain colloidal matter. The lack of precise knowledge of constitution need not, however, prevent us from examining
Reading from top t o bottomWASHINGGAS METERSRETURNED WASHING
AFTER
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BAKERY PANS WITH SILICATE SOLUTIONS
ELECTROCLEANING WITH THE HELPOF SILICATE STARTOF MILK BOTTLES’ JOURNEYTHROUQH BATHSOF HIGHLY ALKALINESILICATE 2 94
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the role of silica with respect to knownattributesof detergents. Thus we may make useful comparisons among the different soluble silicates and between silicates and other inorganic salts. No attempt is here made to review all aspects of cleaning, even when they have been discussed in the literature, but rather to choose cases where the available data are sufficient to throw light on the influence of dissolved silica as a constituent of cleaning fluids. One result of the effort has been to indicate wide areas in which further work is needed. It is hoped that others may be interested to help supply the missing information.
Effect of Silica on pH The curves of pH against concentration for silicates of varying alkali-silica ratio are asymptotic to a limiting pH for each ratio. The highest level reached by NazO : 4Si02 a t any concentration is about 10.8, while metasilicate solutions reach pH 12 a t less than 0.2 per cent. By plotting pH against ratio of sodium oxide to silica, a family of curves is obtained with constant sodium oxide; it appears, therefore, that the amount of silica associated with a unit of alkali has a material effect upon the intensity of its action and also that for a given pH which may be desirable for a particular kind of cleaning, varying amounts of alkali may be used within the family of soluble silicates (Figure 1). Intensity of action is not completely indimted by hydrogenion concentration. The case is well illustrated by the behavior of alkaline cleaners toward tin and aluminum. Baker has studied a wide range of concentrations a t 60" C. (2). From his data we can plot the concentrations of alkali as carbonate, phosphate, and silicate which are tolerated for 2 hours without etching tin plate (Figure 2). This figure shows that silica in solution exerts a protective action different from the phosphate and carbonate ions, and varying largely according to the relative amount of silica. The silicates provide safety for the metal a t temperatures and concentrations of alkali effective for degreasing without the introduction of a poisonous inhibitor or an unstable reducing agent. The difference is clearly due to specific characteristics of the silica. Loss of weight from tin plate plotted against sodium oxide concentration shows the well-known fact that phosphate, carbonate, and metasilicate are less corrosive than equivalent normalities of hydroxide, that the different anions behave differently although all are able to cause injury to a tin surface
The polar ends of detergent molecules have perhaps had more than their share of attention. The anions of alkaline salts used in washing are here considered. Particularly, silica in cleaning solutions is shown to impart effects different from other anions, and differences between the soluble silicates are shown as functions of the alkali-silica ratio between NaSiOs and Na20:4Si02. Data from the literature are recalculated to show the influence of silica on pH, corrosion of soft metals, deflocculation, and the building effect by which the sudsing power of soap is increased.
in 24 hours a t 60" C. (Figure 3). When, however, the data for silicates of higher ratio are put upon the same scale, we see the effect of silica which makes it possible to work safely with 1 per cent sodium oxide without appreciable loss using a 1: 3 ratio, or to choose a lower ratio safely with less severe conditions of time or temperature (Figure 4). Such an operation as engineering a cleaning process for oily sardine tins can now be approached with something better than cut-and-try methods. Useful inferences can also be drawn by comparing these data with those of other workers who have found silicate solutions milder than other cleaners. For example, Zanker and Schnabel (17) showed that after two hundred cycles of boiling soap and soda, cotton fibers had lost 28 per cent of their initial strength, whereas in the presence of silicate of 1 : 2 ratio the loss was only 17 per cent.
Effect on Metals The dairy industry uses a large amount of aluminum equipment which must be scrupulously clean. Aluminum is sensitive to alkalies in varying degrees according to the acid radical. Figure 5 shows that a t 60" C. and 24 hours' exposure metasilicate is safe up to a concentration of l per cent sodium oxide, whereas the same sodium oxide as caustic, carbonate, or phosphate is harmful. The time and temperature are arbitrary; less severe conditions than those chosen for the comparison are usually sufficient for cleaning. With increasing relative amounts of silica, still higher concentrations of sodium oxide become safe, or, when occasion requires, more severe conditions of time and temperature can be chosen (Figure 6).
Bactericidal Effect Hall (7) found that metasilicate solutions have a greater bactericidal effect than other scdium salts. All the salts investigated increased the ability of caustic solutions to destroy bacteria. Of these the silicate was most effective. Again silica appears to have a specific character and function though its action has not been explained.
Deflocculation Most cleaning operations involve disposal of solid dirt which cannot be dissolved. Suspension in a liquid which is drained away and subsequent rinsing without redeposition
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LINES OF
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FIGURE 1. ALKALINITYSHOWNBY VARIATION OF PHIN SILICATE SOLUTIONS OF CONSTANT SODIUM OXIDECONCENTRATION
FIGURE 3. PROTECTIVE ACTIONSHOWNBY CONCENTRATION OF SODIUM OXIDEv8. Loss OF WEIGHT OF TINWHENSODIUM OXIDEIs COMBINEDWITH VARIOUSANIONS (FROM DATABY BAKER)
FIUURE 5. PROTECTIVE ACTIONSHOWN BY CONCENTRASODIUM OXIDEus. Loss OF WEIUHTOF ALUMINUM (FROM DATA BY WHENVARIOUSANIONS ARE PRESENT BAKER)
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comprise the ideal process of removal. Silicate solutions are good deflocculators. McDowell (9) studied the dispersion of clay with silicates from the point of view of the ceramic industries, but his data can be rearranged to show effects of
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FIGURE 2. PROTRCTIVE ACTIONSHOWN BY CONCENTRATION OF SODIUM OXIDETOLERATED BY TINPLATE FOR 2 Horns AT 60" C. WITHOUT ETCHING (RECALCULATED FROM BAKER)
FIGURE 4. PROTECTIVn ACTION SHOWN BY CONCENTRATION OF SODIUM OXIDEos. Loss OF WEIGHTOF TININ 24 HOURS AT 60' C. WHENSODIUM OXIDEIs COMBINED WITH VARYINQ AMOUNTS OF SILICA (FROM DATABY BAKER)
silica. By plotting viscosity of clay suspensions against ratio of sodium oxide to silica in the silicate, a family of curves of constant sodium oxide concentration is obtained which shows that the viscosity of a suspension of Tennessee ball clay is sharply reduced by increase of the silica ratio while alkali remains unchanged (Figure 7). Viscosity is a sensitive index of deflocculation; it bears a direct relation to the ability of the particles to resist the tendency to settle out of a fluid medium. Another series carried out with Florida kaolin shows that a unit of alkali is most effective as NazO:4Si02, while combined as hydroxide or carbonate the smaller amount of deflocculation is striking (Figure 8). Fall (6) worked with finely divided manganese dioxide. His results also are susceptible to reexpression to bring out the action of silica. His concentrations were nearer those likely to be encountered in laundry practice. A plot of his values for amount of solid suspended vs. percentage of silica in solution a t temperatures of 40 O and 75" C. for three alkali-silica ratios shows that the more siliceous solution suspends more dirt over a wider range of concentration than the more alkaline solution (Figure 9). From Fall also we can take the suspending power of sodium as a base and plot the differences in results with like amounts of titratable sodium oxide in other combinations, which gives clear indication of the speoXc effect of silica in this aspect of the cleansing process (Figure 10). The range of concentration over which the silicate solutions exert good deflocculating
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action in comparison with the same amounts of titratable sodium oxide in other combinations is indicated in Figure 11.
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Though unexplained, the action is not that of alkali but of silica (for example, see citation 16). Hypochlorite bleaching of cotton in the presence of silicate is less subject to iron stains and yields better colors with less danger of strength losses in laundering (3). These observations also rest on the properties of silica in soluble form.
Redeposition of Dirt The redeposition of solid dirt of various kinds has been studied by Carter (4). It is well known to laundrymen and housewives that clean and heavily soiled fabrics must not be washed together lest dirt lifted from one be deposited on the other. This action is different from deflocculation. The silicates protect against redeposition, apparently as a function of the silica. The solutions higher in silica ratio gave whiter fabrics when exposed to most varieties of solid dirt. Caustic and carbonated alkalies have no ability to prevent redeposition under the conditions studied, and phosphates much less than silicates. The mechanism of silica behavior in this respect has not been adequately explained, but this does not prevent the use of its peculiar properties in washing practice. This factor is one of the reasons for consistently better color in commercial laundering in the presence of soluble silicate.
Wetting Power Silicate solutions have good wetting power against oils, including those free from saponifiable matter (1, 14), against glass (I),and many metals (11). Comparison of the behavior of caustic solutions and silicate solutions used for washing milk bottles or for removing mayonnaise from glass leaves no doubt that the presence of silica improves the result, but the data are not available to give satisfactory quantitative
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Bleaching The behavior of silicate solutions with peroxides in bleaching is unique. The silica in solution prevents loss of oxygen from alkaline baths which would otherwise rapidly depreciate.
FIGURE 8. DE~FLOCCULATION OF FLORIDA KAOLIN, WITH SODIUM OXIDE AS CARBONATE, CAUSTIC, AND SILICATE,vs. VISCOSITY (RECALCULATED FROM: MCDOWELL)
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FIGURE 10. DEFLOCCULATION SHOWN BY PLOTOF RELATIVE SUSPENSION OF MANGANESE DIOXIDE BY UXIT AMOCNTSOF SODIUM OXIDE IN VARIOUS COMBINATIONS us. SUSPENDING POWER OF SODIUM HYDROXIDE AS BASE LINE (FROM FALL'S DATA)
expression. We must, for the moment, be content with the knowledge that glass washed in caustic comes out of the rinse with standing drops of water, whereas with silicate the mater flows out into an even film. Wetting power of silicate solutions against oil and glass has been shown by the work of Baker ( I ) and Peterkin (IO). Grayson has discussed the use of silicate solutions in dairy cleaning from a practical and economic standpoint (6).
Silicates and Soaps Up to this point we have not considered the relations between silicates and soaps. There was a time when the low cost of silicate compared to soap was a major reason for its use. The economies of cleansing procedures are still important, but the user of silicates need no longer think in terms of adul-
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FIGURE 11. DEFLOCCULATION AS INDICATED BY THE SUSPENDING POWER OF SODIUM OXIDE IN VARIOUSCOMBINATIONS, SHOWING THE EFFECT OF SILICAON AMOUNTAND RANGE
teration; he has a sounder approach in terms of work done per dollar expended. Alkalies increase the ability of soap to form suds. Baker ( I ) , working with dilute sodium stearate a t 60" C., showed that the amount of sudFi increased to maxima a t different pH values. Replotting his data to a basis of sodium normality, the maxima occur more nearly a t the same concentrations. The superior effect of silica is still more evident (Figure 12) when the sudsing action induced by sodium hydroxide is taken as the base, and the effect of the other ions is considered (Figure 13). Deflocculation of manganese dioxide by soap was found by Fall to reach an optimum a t a concentration of 0.2 per cent solids; silicate functions best a t about a tenth of this amount. Comparing similar normalities of sodium, we find that silica imparts the ability to make good suspensions over a relatively wider range of concentrations than does soap (Figure 14). Silicated soaps thus give not only more suspending power per unit of cost, but greater protection against unfavorable conditions.
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FIGURE12. SUDSFORMED ON AN 0.0088 PER CENTSODIUM FIGURE 13. VOLUMEOF SUDSFORMED BY VARIOUSBUILDERS STEARATE SOLUTION us. UNITS OF ALKALI, SHOWING ANION PLOTTED WITH THE AMOUNTO F SUDS FORMED BY CAUSTIC AS A EFFECTS (RECALCULATED FROM BAKER) DATA) BASELINE (BAKER'S
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Soap and silicate mixtures ( 1 , 14, 15) are good emulsifiers. Qualitative ob? servation of such 2 processes a s p: making asphalt I emulsions (8, 22, I S ) leads to the impression that 2k soluble silica 3GfI c o n t r i b u t e s to the r e s u l t s obCY tained. Here again is a field for further v, study. $ Under the conditions here reviewed the silica is in solution and can readily be CONCEIVTRATIOIV IN 56 NA=0 rinsed away after FIGURE 14. DEFLOCCULATION 9s SHOWN its work is done. BY CONCENTRATION OF SODIUM OXIDEvs. Laundries often SUSPENDING POWER ON MANGANESE DIfind it possible to OXIDE AT 40” AND 75” C. WHENSODIUM OXIDEIs COMBINED AS SOAPS, SILICATE, rinse less when using soap built AND CAUSTIC (RECALCULATED FROM FALL) with m e t a s i l i cate, and fabric shows no gain in ash after many cycles. Contrary results reported in certain older literature can only have come from the use of conditions which do not arise in modern American washing practice. Silicate solutions in washing practice, then, are alkaline compounds useful in some cases as such, but more often
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exhibiting distinctive properties as a function of the silica they contain. The freedom from stoichiometric boundaries within wide limits gives them a range of characteristics which further distinguish them from other alkalies used in cleansing processes. I n the interest of brevity, no attempt has been made to point out all the relationships to be derived from the graphs. They summarize extensive data and contain evidences worth careful study by those interested in detergent technic. Much additional work is needed to comprehend adequately the mechanism of cleaning in general, and in particular the advantages to be gained from the use of soluble silicates.
Literature Citations Baker, C. L., IND.ENQ.CHEM.,23, 1025-32 (1931) Ibid., 27, 1358-64 (1935). Carter, J. D., Ibid., 18, 248-52 (1926). I b i d . , 23, 1389-95 (1931). Fall, P. H., J. Phys. Chem., 31, 801-49 ( 1 9 2 7 ) . Grayson, F., Food Ind.,7, 231-2, 281-2 (1935). Hall, J. R., dissertation, Iowa State Coll., 1930; Iowa State Coll. J . Sci., 5,3 3 9 4 1 (1931). (8) Halvorsen, A. L., U. S.Patent 1,995,346 (1935). (9) McDowell, S. J., J. Am. Ceram. SOC.,10, 225-37 (1927). (10) Peterkin, A. J., and Smith, L., U. S. Patents 1,929,933 and (1) (2) (3) (4) (5) (6) (7)
1,929,934 (1933).
(11) Pinner, W. L., Philadelphia Quartz Co., Bull. 465 (1931). (12) Rouault, E., British Patent 421,269 (1933). (13) Smith, P. R . , U. S. Patent 1,989,775 (1935). (14) Stericker, W., IXD. ENQ.CHEM.,15, 244-8 (1923). (15) Vincent, G. P., J. Phys. Chem., 31, 1281-1315 (1927). (16) Weber, I. E., Melliand Tettile Monthly, 3 , 652-4, 753-4 (1931). (17) Banker and Schnabel,. “Die Beeinflussung der Waschwirkung von Seife und Seifenpulver durch Wasserglasfullung,” Berlin, Springer, 1917. RECEIVEDNovember 21, 1935. Presented before the Division of Industrial and Engineering Chemistry a t the 13th Midwest Regional Meeting of the American Chemical Society, Louisville, K y , October 31 to November-2, 1935.
Influence of “STEARINE”on Heat Polymerization of Sardine Oil
QT
HIS paper deals primarily with the “stearines” (the higher melting mixed, saturated, and unsaturated, triglycerides) of sardine oil which separate by crystallization when the oil is chilled, and their eflect upon the heat polymerization of the oil. Much of the theory involved is applicable also to the vegetable drying oils because the paint oils, such as linseed, perilla, hempseed, etc., also contain saturated, unsaturated, and mixed triglycerides. For example, linseed oils contain from 6 to 11 per cent of saturated acids ( I ) , perilla about 12 per cent (chiefly palmitic), soy-bean oil about 10 per cent, and hempseed oil about per cent.
Effect of Refrigeration on “Stearine” Content I n a bulletin (3)describing refrigeration of sardine oils and a simple specification for checking the degree of winterization, a curve (Figure 1) showed that it was necessary to specify a chill test of at least 12 hours to be assured of an oil that was free of “stearine” from a practical standpoint. Fish-oil “stearine” i s not accurately defined. Pure stearin is the triglyceride of stearic acid, but in commercial parlance
OTHO M. BEHR Vegetable Oil Products Company, Inc., Wilmington, Calif. the mono-, di-, and triglyceride combinations of mixed, saturated, and unsaturated triglycerides which solidify or crystallize from the oil a t various low temperatures are termed “stearines.” The saturated acids in these mixed glycerides of sardine oil are composed mostly of palmitic, myristic, and stearic acids. Weighed portions of “stearine” obtained from the filter press in the refrigeration plant were added to a thoroughly winterized oil which had a chill test of 41 hours. The curve shown in Figure 1 is the average of several determinations. This curve indicates that it is of little practical value to winterhe an oil to stand up beyond a standard chill test of 12 hours, but that it is vitally important to clear the oil of “stearine” to this extent. Based on these data, this company
