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INDUSTRIAL AND ENGINEERING CHEMISTRY
Apparently the effect of light is nearly independent of the method of exposure. Similar results are noted whether the acetate is exposed in solution, as films, or powdered. Changing the atmosphere in the flasks also had little effect on the amount of degradation. However, the carbon-oxygen-carbon bonds between CsH1105 groups cannot be broken without introducing other atoms. From the results of the sucrose exposures it is obvious that water is necessary for the breaking of the bond between the C6groups in the case of this single sugar since no hydrolysis took place when thoroughly dried sucrose was exposed. The cellulose acetates used were dried in desiccators, but the handling and preparation of the solutions and films unavoidably allowed a certain amount of moisture to be picked up from the air as well as the mater remaining after desiccation. It is not Dossible t o dry cellulose or cellulose acetate completely in a desiccator. If light does Promote the depolymerization of the long straight chains by hydrolysis,
Vol. 35, No. 7
the increase in copper number can be readily understood as one new aldehyde end group being formed each time the chain is split. Therefore, these results indicate that light is, in effect, a catalyst promoting the hydrolysis of the cellulose acetate chains in a manner similar to its action on sucrose solutions. LITERATURE CITED
(1) Barthelemy, Pulp Paper Mag.,.30,421 (1930). (2) Braidy, Reu. ~ 6 %mat. . color., 25, 35 (1921). ( 3 ) Heyes, J . SOC.Chem. Ind., 47,90T (1928). (4) Kneoht and Thompson, J.SOC.Dyem Colourists, 36,255 (1920). ( 5 ) Kraemer, J. Phus. Chem., 39, 153 (1935); Ross, Stockwell, and Hill, P u l p Paper Mag Can., 27,473 (1929). (6) Rinse, IXD.ENQ.CHEM., 20, 1228 (1928). (7) Sohwalbe, Ber., 40, 1347 (1907). T~~ first paper in this series appeared in February, 1943, page 214. Abstracted from the Ph.D. thesis of c. c. Winding, University of Minnesota.
Sorption of Water Vapor by SOAP CURD JAMES W. RiIcBAIN AND WILL WIN LEE Stanford University, Calif.
Anhydrous soap, pure or commercial, takes up to 1 or 2 per cent of water according to a sorption mechanism of physical type. Except for sodium oleate, the curd or supercurd then suddenly forms a hemihydrate, which again takes up water more rapidly (up to 10 or 12 per cent) according to a sorption law until another phase forms. At low temperatures these new phases are
REVIOUS communications have dealt with those vapor pressure relations of soap systems that are best discussed from the standpoint of the phase rule and of phase diagrams. The present paper discusses the sorption of water vapor by soap systems containing only solids. At ordinary temperature anhydrous soap curd fibers can exist in several unstable crystalline forms, alpha and other monoclinic forms, as well as in the stable monoclinic y form. On heating, another form of crystalline curd fiber appears which we have named "supercurd". For example, sodium stearate supercurd exists as a stable crystalline phase between 90" and 117" C. and sodium oleate supercurd between 38" and 65" C. Just above this temperature the first of the three waxy translucent crystalline forms, entitled "subwaxy" soap, is stable (3, 26, 26). The curd, supercurd, and subwaxy phases will be discussed with regard to their vapor pressure relations before they have taken up sufficient water to yield any liquid or liquid crystalline phase. The data are of general interest and significance for all colloidal powders, fibers, and gels; in the solid soap systems,
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higher hydrates; that is, they are masses of crystalline fibers containing much larger amounts of water. The higher hydrates readily revert to hemihydrate when the relative humidity falls by 15 per cent or less. The sorption curves, through a high value of l / n exceeding unity, unmistakably show the presence of capillary liquid in pores and interstices of the fibrous mass.
as in many of the systems just mentioned, the following are simultaneously present: typical adsorption, formation of genuine compounds or hydrates, adsorption on the latter, and condensation of the unsaturated water vapor in all capillary pores and interstices of appropriate dimensions. 4 experiments were carried out with the purest single T g p s used in previous work (18, 27, 28). The McBainBakr spiral silica spring with platinum bucket was employed. Owing t o the discovery that silica springs can stretch when exposed t o heated water va.por, it was obligatory to begin all critical experiments with anhydrous soap, the water being frozen in the lower part of the tube with solid carbon dioxide or liquid air. Furthermorc, it is necessary after a series of experiments t o return to this point t o check for stretching of the spring. Thirty-five extended isotherms were presented in abbreviated form in another publication (14); the present paper is confined t o a more detailed examination of the initial portion of few typical isotherms. Those isotherms are plotted in the customary manner as x/m, the number of grams
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INDUSTRIAL AND ENGINEERING CHEMISTRY
of water sorbed per gram anhydrous soap, against p/p,, the relative humidity or actual pressure p divided by the vapor pressure of saturated vapor p . a t that temperature. To avoid duplication, reference should be made to those isotherms for further evidence supporting some conclusions drawn in the present communication. All isotherms begin with a smoothly rising curve representing soap containing steadily increasing but slight amounts of water until, a t about 40 to 50 per cent relative humidity, not more than 1 or 2 per cent of water has been taken up by the soap. (Even waxy sodium palmitate a t 145' C. behaves similarly.) Then at a definite value between 40 and 70 per cent relative humidity, with curd and supercurd but not with the subwaxy and waxy phases, there is a sudden break and the composition goes over to about 97 per cent soap, corresponding to a crystalline hemihydrate containing 0.5 mole of water to 1 mole of soap. Thereupon the curve continues with increasing slope; water is more and more rapidly taken up until 10 or 15 per cent is incorporated in the soap mass before any phase change occurs. Throughout the region described, the phenomena may be grouped together as sorption except for the break corresponding to the rather sudden formation of hemihydrate. This is shown by the example chosen for Figure 1 and the further illustrations in Figure 2, where the data are tested by the classical empirical sorption isotherm, x/m = k p W where x / m = weight of water taken up by 1 gram soap p = relative humidity
When plotted as logarithm x/m against logarithm p , this equation gives a straight line. Since the data (represented by circles on Figures 1 and 2) fall on straight lines for the anhydrous curd and then for the hemihydrate, it is a necessary conclusion that they refer to sorption. An example is given of one isotherm for sodium laurate, where the break to hemihydrate did not appear. The Langmuir isotherm is usually less accurately followed. Most of the data fall on two straight lines-one for sorption of water vapor by otherwise anhydrous curd or supercurd, and the other for sorption of water vapor on the hemihydrate which appears in the higher ranges of relative humidity. The figures include curd, supercurd, and subwaxy crystalline forms of the different soaps; but sodium oleate is distinguished by the fact that even curd and supercurd seem to form no hemihydrate. The isotherm for subwaxy sodium oleate is the same for hydration and dehydration. Where no hemihydrate has been formed for subwaxy and waxy soaps, the isotherms are reversible. Such reversibility must be ascribed to the flexibility of the curd fibers which produce only a semirigid structure.
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Figure 1. Isotherm Showing Sorption of Water Vapor by White Opaque Crystalline Fibers of Sodium Palmitate as Supercurd and Hemihydrate at 90' C. with Transition to Hemihydrate at 72 Per Cent Relative Humidity € I hemih drate (NaP.1/2 AZO)containing 3.14 per eent HaO; N
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the exact hemihydrate composition until very low relative humidities are reached. Although the evidence is incomforming hemihydrates proceeds with retention of almost plete, it appears that some hydrate water is progressively lost in accordance with the conclusion . that a chemical compound exposed on a surface containing a volatile constituTABLEI. EMPIRICAL CONSTANTS FOR THE SORPTION ISOTHERM ent must form and decompose ac-Supercurd-Hemihydratecording to a sorption law (11). At very Soap k 11% k l/n Y S u b kw a x y - lln low humidities desorption becomes much NaP (90' C.) + 60% NaCl on bas!e of: greater until complete dehydration occurs. Total solids 0.98 X 1 0 - 4 1.1 4 . 6 X 10-10 4.2 ....... ... .. . Any subsequent rehydration then follows Soa alone 1.9 x 10-4 1.1 9.2 x 10-10 4 . 2 ....... NaP 1 .8 X ........ . ..... .... ,, the original sorption curve for anhydrous NaP 90" C C:{+ 2% NaCl 1.1 x 10-4 10-4 1.0 1.2 5.4'X'ici-7 216 NaMyr 90° C.) 0 . 6 X 10-4 1.4 1.9 X 10-7 2 . 8 soap. NaMyr [IOO'' C.) 0.89 x 10-4 1 . 3 ...... .._ g.i"x'i6-a $16 NaOl (90e C.) .. ..... ... ...... ... 0.24 x 10-4 1 . 5 Our data are in agreement with the findNaL (65' CJ5 1.0 x 10-4 1 . 3 ings of Katz (6) and Kratz (7), who obThe value8 given are for curd. served LI sorption isotherm with anhydrous soap taking up less than 3 per cent water up
I N CONTRAST, dehydration of all the other soaps after
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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LOG /j Isotherms for the Taking Up of Water Vapor by Originally Anhydrous Soap Systems
H = compoaition hemihydrate3 SIB = subwaxy phase. Straight lines on this logarithmic graph of x / n a against p indicate sorption.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
to 50 per cent relative humidity. The ensuing sorption was termed “swelling” by Kratz (7) and Lederer @,IO). Sorption is always accompanied by swelling-for example, in the case of charcoal taking up carbon dioxide (.%?)-but typical swelling is not mere sorption. The first sorption line in the lower part of each diagram (except that at 65” C., Figure 2) is followed by a flat vertical portion of constant relative humidity until the next phase is reached, whether hemihydrate, neat soap (as for sodium oleate at 90” C.), or subwaxy (as for sodium myristate at 100” C.). I n the last case the subwaxy form intervenes before hemihydrate can form because the subwaxy extends over this field at this temperature. Such phases as subwaxy and neat are readily distinguished from each other and from curd by their visual appearance. One of the most significant features is that, although the Langmuir sorption formula and the logarithmic formula are approximately obeyed, there are deviations from the usual adsorption relation: The slope is too steep, the exponent l / n exceeds unity instead of being less, and the usual sorption curves have an opposite curvature to that generally found. I n other words, sorption increases more rapidly than pressure. This behavior always indicates the presence of free capillary liquid in pores of greater diameter, which possibly increase in size by swelling as sorption increases (11). The important point in porous and fibrous systems is that, with a wettable solid for each value of relative humidity, all pores below a definite size must fill with liquid-that is, capillary water ([I). This capillary water becomes an essential part of the gross composition of the solid mass. On a phase diagram this gross composition, and not the supposed or actual composition of the fiber or solid structure itself, constitutes the end of the tie line for any heterogeneous equilibrium. The capillary liquid here may take the form of dilute soap solution or liquid crystalline soap solution, depending upon the temperature and the solubility of the particular soap. For partial wetting-that is, for a finite contact angle-larger pores will fill a t a given low relative humidity. Values of the empirical constants k and l/n for the sorption isotherm are given in Table I. Sorption of water by supercurd appears to decrease with increase in molecular weight; that is, at higher relative humidities higher soaps are less hydroscopic than lower soaps. This agrees with the findings of Stiepel and of Godbole but not of Martin Fischer. However, owing t o the differing values of the exponent l / n , this relation is reversed at low humidities. The same complication applies to the temperature coefficient which is slight, corresponding to a physical type of exothermic sorption. The empirical isotherm holds also for subwaxy soap. I n this respect subwaxy and curd are similar, confirming x-ray data to the effect that both are crystalline and fairly closely related. T H E question of the exact hydration of curd fibers is an incompletely solved problem, partly because a comparatively small reduction of relative humidity suffices to produce a dehydration of all the higher hydrates down to hemihydrate. However, there is no doubt of the reality of hydration, since its existence is demonstrated by many measurements with solutions of inert reference substances. With the significant exception of glycerol, these are rejected by the hydrate water, increasing the concentration of the resulting mother liquor ( I , 8, 12, 16, 17). A similar conclusion resulted from previous studies of the vapor pressure of water (4, 12, 16), dilatometric measurements of freezing curds ( I @ , microscopic and ultramicroscopic examination
787
(2, go), and centrifuge (IS), and again by x-ray observations (22, 23, 24). As found in 1925 in McBain’s laboratory and shown in subsequent publications (3, 29), glycerol appears to be practically interchangeable with water in that its distribution between neat soap and other liquid phases is proportional to the water content of each phase. The isotherm for commercial toilet soap base a t 40” C. is sufficient in itself to prove the existence of hydrates containing at least 30 or possibly 40 per cent water. This temperature is too low for the existence of either neat or middle soap, but only for an ordinary, comparatively dilute soap solution lying well to the aqueous side of the phase diagram. Nevertheless, there is a flat in the isotherm extending from about 88 per cent soap at about 85 per cent relative humidity to somewhere between 70 and 60 per cent soap before the pressure curve rises to a much higher value a t about 50 per cent soap. This is definite proof that other hydrated solid phases intervene between hemihydrates and ordinary soap solutions, because the latter have only one fixed high humidity at one temperature. These data agree with previous indications in the literature. For example, according to Lederer (9, IO), a milled soap (85 per cent soap) shows 87 per cent relative humidity, whereas a household soap (70 per cent soap) exhibits 96.5 per cent relative humidity and a 63 per cent soap shows 97.5 per cent relative humidity. Hence, a t ordinary temperatures only a nearly saturated atmosphere affects commercial soap appreciably, allowing it to become moist or to %weat’’. Even in a hot climate, sweating of soap does not begin below 85 per cent relative humidity (6). LITERATURE CITED
Bennett, H. B., J . Chsm. SOC.,125, 1971 (1924). Darke, W.F., McBain, J. W., and Salmon, C. S., Proc. R o y . Soc. (London), A98, 395 (1921). Ferguson, R. H., Oil & Soap, 14,116 (1937). Ferguson, R.H.,and Vold, R. D., Ibid., 15,181 (1938). Goswami, M. N., Choudhury, A. R., and Basak, K. K., Indian Soap J . , 7, Q(1940). Katz, J. R., Kolloidchem. Beihefte, 9,1 (1917). Kratz, E.,2.deut. dl- u. FettAnd., 44,25 (1924). Laing, M. E.,J . Chem. SOC.,119,1669 (1921). Lederer, E.L., 2.anpew. Chem., 39,1007 (1926). Lederer, E.L., 2.deut. dl- u. Fett.-Id., 46,519 (1926). McBain, J. W.,“Sorption of Gases and Vapors by Solids”, p. 185,Chap. XI1 and XVI, London, Routledge, 1930. McBain, J. W., Bull, H. I., and Staddon, L. S., J. Phys. Chena., 38, 1075 (1934). McBain, J. W., and Ford, T. F., J . A m . Chem. SOC.,62, 866 (1940). McBain, J. W., and Lee, W. W., Oil & Soap, 20, 17 (1943). McBain, J, W.,and Martin, H. E., J. Chem. SOC.,119, 1369 (1921). McBain, J. W.. and Salmon, C. S., Ibid., 119,1374 (1921). McBain, J. W.,and Taylor, M., Ibid., 115, 1300 (1919). McBain. J. W..Vold. R. D.. and Frick. M.. J . Phus. Chem.. 44. 1013 (1940). McBain, J. W.. Vold, M. J., and Johnston, 5. A.. J . A m . C h m . SOC.,63,1000 (1941). Maclennan, K., J . SOC.Chem. I d . , 42, 393T (1923). Meehan, F.T., Proc. Roy. SOC.(London), 115A, 199 (1927). Piper, 5. H.,and Grindley, E. N., Proc. Phys. SOC.(London), 35, 5 (1923). Thiessen, P. A., and Spychalski, 2. physik. Chem., A156, 435 (1981). , Thiessen, P. A., and StaufF. J., Ibid., A176,397 (1936). Vold, R. D., J . A m . Chsm. SOC.,63,2915(1941). Vold, M. J., Macomber, M., and Vold, R. D., Ibid., 63, 168 (1941). Vold, R. D., Reivere. R., and McBain, J. W., Ibid., 63, 1293 (1941). (28) Vold, R. D., and Vold, M. J., Ibid., 61, 808 (1939). (29) Wigner, J. H.,“Soap Manufacture”, p. 116, New York, Chemical Publishing Co.,1940.
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