Molecularlv Dehvdrated Sodium Phosphates

Another group, the crystalline metaphosphates, all have the saine nominal ..... and Smiths', can be used, but if the specific absorbence variee, this ...
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Molecularlv Dehvdrated Sodium Phosphates J

B. C HAFFORD,

J

Wkstraco Mineral Products Division,

Food Machinery and Chemical Corp., Carteret, N . J .

T h e synthetic detergents industry has been primarily responsible for the present status of development and understanding of the rnoIecularly dehydrated sodium phosphates, three of which have emerged as important components of cleaning compositions. In order of decreasing commercial tonnage, these include pentasodium triphosphate, tetrasodium pyrophosphate, and the various phosphate glasses. These materials serve as detergent builders by increasing the cleaning efficiency of organic surface active agents. Their ability to sequester alkaline earth metal ions and to deflocculate inorganic soils contribute to their usefulness in this respect. The corrosiveness of the phosphates toward certain metals can be inhibited, and their tendency toward hydrolytic reversion in aqueous solution can usually be con trolled by avoiding prolonged exposure to conditions causing accelerated hydrolysis. The hydrol?tic sensitiwity of the phosphates has not prevented their successful incorporation into excellent, modern washing products. OLECUL=1RLY dehydrated or complex phosphates have been of more than passing interest to chemists for a great many years. Only recently, hoivever, has there been any reasonably good scientific insight into the nature an?, structure of these compounds, which are used so extensively in synthetic detergent, manufacture. The primary function of the synthetic detergents is, of course, to clean. By stimulating interest in the complex phosphates used to augment their properties, they have certainly performed this function adinirably and cleaned away many of the misconceptions surrounding t,he real nature of the molecularly dehydrated sodium phosphate system. This system is not uncomplicated. Excellent reviem are available ( I , 15,16, 22), and, consequently, this paper identifies the individual members whose existence has been demonstrated and pinpoints those which have achieved considerable importance in the general field of detergency. To effect some simplification, the discussion is restricted to t'hose complex phosphates formed by complete molecular dehydration of niono- or disodium orthophosphate, or mixtures of the tIvo. lhDI\IDUAL COhlPLEX PHOSPH4TES

saine nominal composition, corresponding t o dehydrated monosodium orthophosphate. The naines of t'he individual metaphosphates have often been somewhat confusing, and it' is sometimes difficult to decipher just n-hich one is being referred to witlout soiiie knowledge of hoff it was prepared. Table I lista these compounds and briefly indicates their mode of preparation. Jladdrell salt is an insoluble material fornied vihen monosodium phosphate is dehydrated in the range of 230" to 400" C. It exists in t'wo distinct crystalline modifications called XaP03 I1 and I11 by Partridge (I,$). XaP03 I1 forme a t a higher tcniperature t'han S a P O l 111. hIaddrell salt ]!as been used as an abrasive cleaner in dental creams.

TABLE I.

CRYSTALLIi\E AIETAPHOSPHATES

Approx Ternhi. of Formation, O

JIaddreil salt SaPOz I1 (high temperature form) KaPOa I11 (low temperature form) Sodium trimetaphosghate YaPOa I Pl'aPOs I' (prepared from melt) XaPOa I" (prepared from melt) Sodium tetrametaphosphate (iYaPOa)&(prepared indirectly) Kuirol salt (SaPOl)z (prepared from seeded melt) ~~

The simplest compounds of the s j stein are tetrasodium pyiophosphate, obtained by dehydrating t x o molecules of disodium phosphate; and pentasodium triphosphate, or sodium tiipolyphosphate, deiived from two molecules of disodium and one of monosodium phosphate. The next member of the seiies is hexasodium tetraphosphate, equivalent to two dehydrated molecules each of niono- and disodium orthophosphate. This compound is not readily formed, except by indirect methods, and is only of theoietical interest. The pyro- and triphosphates are water-soluble, crystalline materials that exhibit a mildly alkaline reaction and have the ability to forin coordination complexes with the alkaline earth metals. Tetrasodium pyrophosphate exists in five separate crystalline modifications, but only one of these i s stable a t ordinary temperatures. Sodium triphosphate exists in two isomeric crystalline forms at ordinary temperatures, Another group, the crystalline metaphosphates, all have the 1938

P

b

300-400 230-300

400-to0 500-525 375-525

... 5%

Sodium trimetaphosphate, KaP03 I, is formed a t calcination temperatures above about 400' C. It is a solub!e, crystalline, complex phosphate which does not have the ability to sequester the alkaline earths, presuinablg because of its ring structurc. It has two additional isomeric forms, prepared with some 2iffioulty by cooling a melt, and called 9izP03 I' and KaP03 I". Both of t,hese modifications tend t o revert to S a P 0 3 I on tempering at 300" to 600" C. Sodium trimetaphosphate readily hydrolyzes to triphosphate in the presence of alkali, and it might conceivably be valuable in specialized detergent applications where it might serve as a reservoir for the more valuable triphosphate. A t present, it is of little commercial impoittiii~e.

INDUSTRIAL AND ENGINEERING CHEMISTRY

VO!.

46, No. 9

-Synthetic Sodium tetrametaphosphate, which also has a ring structure, may be prepared by controlled hydration of phosphorus pentoxide, or indirectly through the formation of the copper salt. This compound may be hydrolyzed in the presence of alkali t o sodium tetraphosphate, which does not form directly from the orthophosphates. A final member of the crystalline metaphosphate series is Kurrol salt, a highly polymerized compound prepared by seeding a metaphosphate melt. Kurrol salt is difficult to synthesize and has no present commercial value. The remaining group of complex phosphates includes the sodium phosphate glasses, ranging from the metaphosphate composition at 69.6% phosphorus pentoxide, to a composition approaching 60% phosphorus pentoxide. These glasses are made by rapidly chilling a melt having the desired sodium to phosphorus ratio. 4 s the phosphorus pentoxide content is lowered below about 60%, it becomes increasingly difficult to prepare a clear glass, because of the formation of crystals of tetrasodium pyrophosphate embedded in a glassy matrix of more acid composition. The sodium phosphate glasses are characterized by high solubility in water and high calcium sequestering ability. They have attained considerable stature in the cleaning field, chiefly in the realm of hard surface cleaning. Jl’hile the manufacturers of the phosphate glasses probably have good reasons for claiming superiority of one glass composition over another in specific applications, this discussion considers them in one single category.

little meaning without some convenient yardstick to gage their magnitude. I t may be easier for the engineer to visualize this sodium triphosphate production level if it is indicated that the corresponding phosphorus would require continuous operation of more than fifteen 12,000-kw. electric furnace installations. Although a substantial amount of triphosphate is manufactured from wet-process phosphoric acid, the capital investment in phosphorus furnaces and conversion facilities sufficient to produce the 1953 output of sodium triphosphate might be of the order of $85,000,000. The relative importance of sodium triphosphate is readily demonstrated by t h e fact that this chemical comprises almost 60% of all of the inorganic phosphates produced for nonfertilizer applications. PROPERTIES IN DETERGENT APPLICATIOh S

Special properties of the molecularly dehydrated phosphates are important to the synthetic detergent producer. Sodium triphosphate, the most important of the group, is used primarily in heavy duty synthetic detergents. Tetrasodium pyrophosphate is chiefly used in cleaning applications as a soap builder and in industrial cleaning compounds, although it has enjoyed some use in synthetic detergents. Phosphate glass is used largely in the absence of surface active agents in machine dishwashing, or in dry mixes containing some surface active materials. SOOC

COMMERCIAL DEVELOPMENT

From the standpoint of detergency, the most important complex phosphates are sodium triphosphate, tetrasodium pyrophosphate, and phosphate glass, in order of decreasing rank. Sodium triphosphate probably was first prepared, in impure form, in 1848 by Fleitmann ( 5 ) in attempts to make polyphosphates from sodium phosphate melts. It remained a laboratory chemical until the last decade, with commercial production in this country starting in the early 1940’s. Tetrasodium pyrophosphate and phosphate glass were both well-established products by this time. Both had been described by Graham ( 7 ) in 1833. The pyrophosphate received considerable commercial impetus in the early 1930’s, when the soap manufacturers discovered that it is a builder superior to the trisodium orthophosphate previously uscd, and began adding it to their heavy duty soap powders. Phosphate glass had found considerable application in the water-conditioning field, and has been assuming increasing importance as a component of dry mixtures used for hard surface cleaning, as in dishwashing. The excellent water-softening capacity of the phosphate glasses is of particular importance in this service. Sodium triphosphate production increased a t a relatively modest rate for a few years after its commercial inception, until the outbreak of the revolution in cleansing compounds fomented by the synthetic detergents. When sodium triphosphate was chosen as the preferred builder for household washing products, its fortunes soared along with those of the synthetic detergents. This is shown dramatically in Figure 1, in which the yearly production levels of triphosphate, pyrophosphate, and glass are compared with the annual sales of solid synthetic detergents. Sodium triphosphate has become the most important of the complex phosphates, and its remarkable growth, starting around 1947, parallels the growth and acceptance of the synthetic detergents. The slow decline of soap sales, due to replacement with synthetic detergents, is common knowledge. Tetrasodium pyrophosphate has managed to maintain its annual tonnage despite this, through some use in synthetics and through industrial applications. Phosphate glass has shown a small increase in yearly production, with 8ome fluctuation from year to year. The major portion of the 468,000 tons of sodium triphosphate produced in 1953, worth some %75,000,000,was consumed by the synthetic detergents industry. Numbers of this size often have

September 1954

Detergents-

800 70 0

2 600 e 5 500 U m

5 400 m

0 3

$ 300

.-@-

200 100

-

N%Pz O r

d

Glass

Figure 1. Phosphate Production in United States

Detergent Building. I n the presence of these phosphates the organic surfactants are able to clean better, or to clean equally well a t a lower concentration, than in their absence. There have been many attempts t o reduce this building effect to quantitative terms (9-la, 17, $0). The measurement of detergency, however, is an exceedingly complicated undertaking, and the results are meaningful only within narrowly defined limits, despite the fact that it is perfectly feasible to set up a precise test using a given set of conditions. Studies in the Westvaco laboratories, admittedly based on practically unrealistic tests in machines surh as the Launder-Ometer, using unrealistic artificially soiled test clothes, indicate that all three types of phosphate rank high in building action a t hardness levels to about 16 grains per gallon. Above 16 grains per gallon, phosphate glass exhibits decreasing effectiveness as a builder with increasing hardness, while the pyro- and triphosphates tend to maintain their excellent showing. This same general effect was exhibited when several different types of surfactants mere used, and may demonstrate a real advantage of the simpler phosphates over the glasses in very hard water. The tests were not designed to take into account the effects of repeatedly washing the same fabric. Alkalinity. Other things being equal, detergency increases

INDUSTRIAL AND ENGINEERING CHEMISTRY

1939

viith increasing alkalinity (19), but doniestic washing compositions are limited to about pIl 10 to 10.5 by the fear of undue harshness t o the hands and skin of the user. Triphosphate at pH 9.6 and pyrophosphate a t p€I 10.2 fit into this picture nicely. The phosphate glasses are less alkaline and require pH adjustment upward. Sequestering Ability. One of the out,standing properties of the coniplex phosphates is their ability t o sequcster the alkalinc earth metals by forming soluble, undissooiat)etl complexes. This comoles-forming ability may be nwasured in many ways. 9 simple test for calcium sequestration involves t,it,ratinga solution of the phosphate, adjusted to pFI 10, with calcium or magnesium chloride solut'ion (8). The end point, is the point of incipient prccipitation, when just enough alkalitie earth rnet,al ion is present to begin to precipitate the phosphate. Such a test is highly empirical; it is influenced strongly by factors such as concentration, temperature, and pH, and, consequently, its results are subject, to considerable uncertainty of interpretation. Kevertheless, it indicates that the phosphate glasses sequeeter from about 12 to 18% of t,heir might of caIcium, depending on their coniposit,ion and conditions of preparation. Sodium triphosphate sequesters about 10 or 11% o€ its nc4glit of calcium, and tctrasodium pyrophosphate, 4 to 5%. IVith magnesium, t,he results are very different'. Since magnesium triphosphate docs not precxipitatc under the t,est conditions, it is necessary t o add a sinal1 amount of precipitate to locate the end point. If sodium oleate is used for this purpose, the glasses sequester only ahout 2.0 to 3.S7c of their weight, of magnesium, compared t o 6.4y0for triphosphate and 8.3% for pyrophosphate. Since inoet natural waters contain both calcium and niagnesium, it is liltelj- that the triphosphate represents the best compromise if it is desired to sequester completely both hardness elenients. mining sequestering power may or Other inet,hods for d may not yield the same results. Also, many procedures for deteriniiiing water-softening ability depend on the presence of soap as an indicator, with the ciid point talien when enough calcium ions are present to destroy the sudsing ability of the soap. Thc variable and conflicting results obtained from the different, methods for determining sequestering or soft,enitig ability h a w been a source of some confusion to phosphate users. Probably much of the lack of agreement betaeen methods, after the usual variables of p l l , concentration, time, and t,emperature are eliminated, can be explained by considering the competit,iono l various chemical species in solution for the calcium ion. 4 f c v simple equilibrium calculat,ions demonstrate this effect and its impact on i,he results obtained. Toplry ( 8 2 ) drterniineci expcrimentally that 0.0073M calcium oxalate can exist in solution in equilibrium with 0.0163M sodium triphosphatc, at 25' C. and pH 9.5. Using the solubility produrt of calcium oxalate, 2.3 X lo-@,he easily calculates the dissociation constant of the assumed calcium complex

By carrying Topley's approach a little further, it, can he calculated t,hat t,he triphosphate has sequestered 4.97c of its Jq-eight of calcium under these cquilibriuin conditions, as compared t,o the 10 to llyo found in the absence of oxalate. If t'he oxalate concentration is halved, the calcium sequest'ered increases to 13.7%~and if it, is quartered, it increases t,o 8.310. I n view of this striking effect of the solubility product of the calcium salt of the competing anion, and of the concentration of that anion, it is easy tmosee why the various methods for calcium sequestering ability or water softening powder do not agree with each other. The sequestering ability of the complex phosphates might be more meaningfully defined, under a given set of conditions, in terms of the free calrium ion concontration which they allov,7 to exist in solution a t varying calcium to phosphate ratios. 1940

The ability of the phosphates to sequester calcium and nitrgncsium appears t,o have little relation to their power t o a synthetic det,ergeiits in removing soil from fabrics, judging from the similar building effects of the various phosphates as oppowd to their variable sequestering capabilities. sequestration inthy be much more important in proventing the accumulation of deposit's of insoluble calcium and magaesium salts over scvcral ivashing cycles and in diminishing puccipitation of the hwitncss elements during the rinse cyclc. If so) this property is of cousiclerable importance t o the vashing ponder manufacturer. Deflocculating Ability. The molecularly dehydrated sodium phosphates demonstrat'c a niarlicd ability to deflocculate or pcptize suspensions of many difticultly soluble inorganic compounds. This property finds use wherever it. is desirable to decrease ilie settling rate of such materials, or to decrease the viscosity of heavily loadcd suapensions, and it is likely that it also plays itii important part, in tmhccleaning process, both in prevent,ing redvposition of inorganic soils and in increasing the ease v i t h Tvhich they are removed from fabrics. It is difficult to formulate any general rules governing this properij-, other than t o observe that the phosphates are generally effeetivc on inorganic, but not on organie, materials. An unpuhlislied study of the peptization of a variety of minerals by severe1 complcs phosphat,es revcds 110 orderly effects. A given phosphat,c might show excellent deflocculating p o m r for one clay and yet be one of the poorer dcflocculants for another. A comparison oi sodium triphosphate with carboxymethy1~ellulo~c wit,h respect to their ability to prcvent soil redeposition in solt water illustrates that those t\vo components complenient each other in a built synthetic dctcrgent. The triphosphate is crcelleiit for preventing reriepoPitioii of an iron oxide soil, but it actually contributes to iiicreascd rcdeposition of a carbon black soil. (larboxymethylcellnloec., on t'he ot'her hand, shows exactly the reverse effect', while conibinations of the t\ro demonstratc improved redeposit,ion characteristics over those observed n-ith either component alonc. Redeposition is report,ed t o vary niarliedly with the riatui~ooF the soil and substrate (3, 4,I S ) , as corroborated by these findings. Corrosion. A disadvant,ageous propcrty of the complex phosphates is t,lieir corrosiveness towayd certain metals, partioulaily the so-called whit,e metals such as copper-nickel alloy, and d u ' m. Studies v i t h copper-nickel indicate that cmropion incs v i t h increasing sequestering ability and decreasing 111-1. Corrosion is not much of a problem for detergent manufacturcrs t'oday, as t,he use of silicates ( 1 8 ) and more recently develo~~cd organic inhibitors tends to control it adequately in most washing applications. IIYI~O LYmc I

~ ~ V II O R

The complex phosphates are subject to hydrolytic t l c ~ w n i position in aqueous solution, a i d this is accentuated by hcat ~rnd by the presence of excessive acidity or alkalinit,y. Data apliear in the literature (a, 6,25). Hydrolysis is negligible enough to be unimportant in the servire intended Cor most synthetic dctergent,s. In manufacturing a spray-dried detergent, ho\vcvc:r, it is possible for considerable hydrolysis to occur, although conditions can usually I x controlled so that hydrolysis in solution is not an appreciable factor. By far tht. most stable of the phosphat,es under considerat,ion is tetrasodium pgrophosphatc. Aqueous solutions a t ordinary t,eniperat,uresshox little reversion over a period of many months, and even boiling solutions rct,ain a high proportion of their pyrophosphate content for a conriderable period. Sodium triphosphate is less stable, and consjderable hydrolysis occurs with boiling for an hour or two, although its solutions are reasonably stable below about 80" C. It is lass tolerant of free caustic, which accelerates its rcvcrsion. Triphosphate in solution hydrolyzes by splitting off ono pliosphoruu %.tom,forming one niolecule each of ortho- and pyrophosphate. The pyrophosphate so formed exhibits its own exccllcnt

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol. 46,No. 9

-Synthetic stability. Phosphate glass tends to show a still lower older of stability and hydrolyzes to trimeta-, tri-, and orthophosphate, the exact route depending on the composition of the glass and the conditions impressed on the solution. Solution hydrolysis presents few problems under most use conditions, but in synthetic detergent manufacture it is necessary to know and allow for the stability of the phosphate involved. In addition to reversion in solution, the further behavior of the complex phosphates in the presence of water or water vapor, and the stability of the phosphate hydrates, are areas of considerable interest to detergent manufacturers, particularly when spraydrying processes are involved. All of the phosphates under consideration are hygroscopic, but the glasses are the worst offenders in this respect. The glasses take up Rater R hen exposed to water vapor until they form solutions. Tetrasodium pyrophosphate and sodium triphosphate absorb water somewhat more slowly, forming the deca- and hexahydrates, respectively, without melting but with caking to a hard mass. The pyrophosphate decahydrate is a more or less conventional hydrate. It may be crystallized easily from aqueous solutions, the crystals tend to melt a t elevated temperatures, and the hydrate water may be driven off without destroying the original anhydrous material. None of these statements may be made about sodium triphosphate hexahydrate, and the two crystalline modifications of anhydrous triphosphate differ markedly in their affinity for water. The hydration of sodium triphosphate is not reversible, and there is negligible water vapor pressure over a mixture of the hydrate with the anhydrous salt. I t is possible, however, to determine the vapor pressure required to effect hydration by equilibrating anhydrous samples in the presence of varying pressures of water vapor. This "association" pressure is shown in Table I1 for the two forms of triphosphate, a t 25' and 50' C. Also included is the vapor pressure of a saturated solution of the hexahydrate. Conditions can be impressed under which XajPaO," I will hydrate, while KasP80toI1 will not.

TABLE 11.

REQUIRED T O HYDRATE SODICM TRIPHOSPHATES I AXD I1

m A T E R VAPOR PRESSURE

Vapor Pressure, Mm. HE at25' C. 500 c. h'asPa0m I SasPsOio I1

Association Pressure 10 12

NanPaOlo (satd. soln.) Water

Vaoor Pressure 23 24

40 64

66 93

Sodium triphosphate hexahydrate is unusual in that its water cannot be driven off without converting the triphosphate to ortho- and pyrophosphates (15, Wl), giving rise to the irreversible nature of its hydration. At temperatures to about 80" C. the hydrate is stable, even a t low pressures. Between 85' and 120" C., oven drying causes loss of water and destruction of the triphosphate molecule. Splitting of one phosphorus atom from the chain would be expected to yield an equimolecular mixture of pyro- and orthophosphate. Actually, of the order of 1.5 molecules of pyrophosphate appear for each molecule of orthophosphate. At a temperature of about 120" C., the intimately associated ortho- and pyrophosphates formed when the hydrate is decomposed begin to recombine to form triphosphate. This reconstituted triphosphate appears in the residue in increasing amounts as the drying temperature is increased above 120" C. The instability of the triphosphate hexahydrate obviously imposes a tower temperature limitation in processing spraydryed products containing sodium triphosphate. Assume that the dryer feed is prepared by adding anhydrous triphosphate to a concentrated slurry of synthetic detergent and auxiliary components. A portion of the phosphate, perhaps of the order of September 1954

Detergents-

25%, dissolves and yields the thermally susceptible hexahydrate on subsequent spray drying. Thc major proportion of the phosphate is carried in undissolved form as a slurry. This solid phosphate tends to hydrate in the crutcher, but considerable control over the rate of hydration can be achieved through the proper choice of phosphate and other factors. Hydration is retarded by a high aolids rontent and elevated feed temperatures, although temperature must be held below the point a t which the dissolved phosphate begins to hydrolyze a t an appreciable rate. By employing conditions resulting in a low rate of hydration, it is possible to feed a considerable proportion of the phosphate to the tower in anhydrous form, thus reducing thermal decomposition by reducing the amount of hexahydrate present. If the tower tempeiature is less critical and it is desired 1 o avoid anhydrous triphosphate in the product, conditions may be adjusted to yield complete hydration; this usually involves some sacrifice of the high solids content of the feed. Analysis of actual spray-dryed materials has indicated that it is possible to obtain both anhydrous triphosphate and hexahydrate in the final product and to vary the extent of triphosphate decomposition from a few per cent to practically complete degradation to pyro- and orthophosphate. SUMMARY

Of the many individual molecularly dehydrated sodium phosphates, sodium triphosphate has assumed prime commercial importance as a builder for synthetic detergents. Tetrasodium pyrophosphate has continued to be used as a conventional soap builder, and the phosphate glasses have found their greatest detergency outlet in the field of hard-surface cleaning. The properties that make these phosphates uniquely advantageous in synthetic detergent formulations include their ability to sequester the calcium and magnesium ions found in natural waters, their ability to deflocculate and suspend insoluble, inorganic soils, the mild alkalinity of the triphoaphate and pyrophosphate, and, most important, their ability to increase the cleansing power of organic surface active materials. Properties that may pose problems and may require close definition before these phosphates can be most effectively utilized include their corrosiveness toward certain metals, their susceptibility to hydrolysis in aqueous solution, their hygroscopicity, and the thermal instability of the triphosphate hexahydrate. The continued preference for sodium triphosphate as a builder for the synthctic detergents must be taken as evidence that the advantageous properties of this phosphate outweigh its disadvantages, including problems engendered by the unusual nature of its hydrate. The utilization of the molecularly dehydrated phosphates in detergent applications has been aided b j the recent rapid growth of knowledge of the properties and structure of these interesting compounds. Conversely, the accumulation of this increased knowledge has been stimulated, in no small measure, by the acceptance of the phosphates in the synthetic detergents industry. ACKNOWLEDGMENT

The author is grateful to the Westvaco Mineral Products Division for permission to publish this paper and wishes to thank R. J. Fuchs and W. A. Tidridge, of the Westvaco Research Department, for valuable assistance in preparing some of the material. LITERATURE CITED

(1) Audrieth, 1,. F., arid Hill, 0. F., J . Chem. Edzrc., 25, 80 (1948).

(2) Bell, R. N., IND. ENG.CHEM.,39, 136 (1947). (3) Carter, J. D., Ibid., 23, 1389 (1931). (4) Carter, J. D., and Stericker, W., Ibid., 26, 277 (1934). ( 5 ) Fleitmann, Th., and Henneberg, W., Ann., 65, 304 (1848). (6) Friess, S. L., J . Am. Chem. Soc., 74, 4027 (1952). (7) Graham, T., Phil. Trans., 133,253 (1833).

INDUSTRIAL A N D ENGINEERING CHEMISTRY

1941

(8) Hafford, B. C . , Leonard, F., and Cummins, R. W.,Ind. Eng. Chem., Anal. Ed., 18, 411 (1946). (9) Harris, J. C., ,477~.Dyestuf Reptr., 37, 266 (1945). (10) Harris, J. C., Oil & Soap, 23, 101 (1946). (11) Harris, J. C., and Brown, E. L., Ibid., 22, 3 (1945). (12) Xersberger, A . B.. and Neidig, C. P., Chem. Eng. Xews, 27,

1646 (1949). (13) Nerrill. R. C , and Getty, E., J . Am. Oil Chemists Soc., 26, 5

(1949). (14) Partridge, E. P., Hicks, V., and Smith, G. U7., J . Am. C h n . Soc.. 63, 454 (1941).

(15) (16) (17) (18) (19) (20)

Quimby, 0. T., Chem. Revs., 40, 141 (1947). Raistrick, B., Sci. J . Roy. Coll. Sci.,19, 9 (1949). Reich, I., and Snell, F. D . , IND.ENG.CHCX.,41, 2797 (1949). Robinson, E. A , , Soup Sanit. Chemicals, 28, 34 (1962). Strain, B., U. S.Patent 2,486,922 (Nov. 1. 1949). Stupel, H., Fette 21. Seifen, 56, 209 (1954). (21) Thilo, E., and Seemann, H., 2. anorg. Z L . alZgenz. Chem., 267, 65 (1951). ( 2 2 ) Topley, B., Quart. Revs. (London). 3 , 345 (1949). (23) Katsel, Die Chonie, 55, 356 (1942). RECZIVED for review 3 I a r c h 2 5 , 1954.

J4Y C . HARRIS, 31. R. SULLIVAN,

AVD

ACCEPTEDJuly 13, iQ54.

L. E. WEEKS

Chemical Research D e p a r t m e n t , Monsanto Chemical Co., D a y t o n 7 . Qhio

T h e relationship between whiteness (or reflectance) and retained soil is an important one. Rlost soils for fabrics are comprised of graphite or carbon black, for which no quantitative relationship with reflectance has been established. The present paper reports a n investigation of this relationship, utilizing a quantitative turbidimetric estimation of graphite from which the cellulosic fabric has been removed. This technique is not suggested as a substitute for reflectance wash test measurements but was designed to provide the yuantitative relationship already- mentioned. HE evaluation of surface active agents has been attempted from perhaps as many viewpoints as there are individual properties of the mateiials. Certain chemical, physscochemical, and physical properties are readily measured, but some of the methods have not been universally accepted. One property of the surface active materials that accounts for perhaps the largest usage is cleansing ability, but evaluation of this property remains perhaps less 1% ell standardized than many others. There are many laboratory machines (9)designed for control of soil removal in detergency tests, but the factor of measurement of washed soiled cloths is still under scrutiny. Though reflectance methods for estimation of soil removal are recognized as pobsessing shortcomings, these are perhaps most widely used, probablv because cleaning is recognized as effective nhen an agreeable degiee of TT hiteness has been obtained visually. Removal of soil from a fabric suiface has been nieasuied by Bacon and Smith ( 1 ) as a function of the mechanical work involved. The six general variables listed by them are detel-gent, mechanical force, time, temperature, ease of soil removal, and soil suspension and iedeposition. They applied the Kubellia and AIunk ( 5 ) equation to this problem

where

K = coefficient of reflectivity S = coefficient oi light scattering R = observed reflectivity for monxhrozatic light and calculated the per cent iemoval of black from their soiled fabric through ita use

R I S for soiled fabric K l S for soiled fabric

- K / S for scoured fabric - K / S for unsoiled fabric

loo=

per cent black removed

(2)

Their equation for detergency, which a t that time was still under investigation, was written as

where S K

per cent soil removal a constant ?I = constant slope (' = concentration F = force applied T = time = =

The equation applieh to the linear portions of their logarithmic detergency curves, but these linear portions fall below ordinary use-concentration conditions for most detergents. Since reflectance measurements as a calculation of brightness regained are used rather generally by industry, Reich, Snell, and Osipow (6) investigated the Kubelka-AIunk equation aB applied to soiled fabrics. They concluded that reflectance of soiled cloth can be correlated to the amount of soil present by the equation log

r(l - R)2

i2R

-$"'I

(1 - -___

= n log

G

+ constant

where

I? = reflectance of soiled cloth R' = reflectance of clean cloth G = amount of soil present However, they indicate that n (the slope) during washing is frequently 1 (and the equation becomes the Kubellca-Nunk) but that during artificial soiling (and agglomeration of soil) it has a value between 0.65 to 0.7. Hart and Conipton (4)demonstrated that the Kubelka-Munk equation frequently fails when applied to reflectance systems because of particle distribution, orientation, and specific absorbence. Provided that the specific absorbenee of a pigment, for the fabric system is essentially constant, Equation 2, 13ac0~ and Smiths', can be used, but if the specific absorbence variee, this must be determined. Hart and Cornpton calculate change in specific absorbence

(K/S)T = (K/S)WP f (K/S)RS f ( K / S ) B 1942

(4)

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

(5)

Vol. 46, No. 9