Sorption of Water by Cellophane J
Vi L. SIMRIL
. ~ N DSHERMAN SMITH University of North Carolina, Chapel Hill, X. C .
G
,YCEROL-free, nonmoistureproofed cellophane is a desirable subject for research in the physics and chemistry of cellulose, for this material is pure regenerated cellulose in the form of a film which lends itself readily t o experimental treatment. Moreover, the highly standardized industrial process whereby cellophane is prepared f r o m mood p u l p y i e l d s a product which for uniformity and reproducible behavior surpasses the natural cellulosic materials, subject as they are to the vagaries of the growth process. As a preliminary t o a detailed examination of some of the physical properties of cellophane, a fairly comprehensive study of the cellophane-niuisture equilibrium was undertaken in the hope that some of the film properties which show marked dependence on moisture content might be correlated. Thermodynamic treatment of the data makes it possible to draw significant conclusions as to the mechanism whereby the moisture is held in the film.
ture in the total weight of the moist sample (the regain) plotted as a function either of the mater vapor pressure or the relative vapor pressure. For all celluloses the sorption isotherm is a double one; the moisture regain on adsorption is lower than that on desorption for the same sample a t the same water vapor pressure. This hysteresis effect was observed a t all temperatures and necessitated the following procedure: With the temperature of the thermostat adjusted, the chamber containing the suspended sample mas evacuated until continued pumping produced no further loss in weight. The weight so obtained was arbitrarily selected as the dry weight of the sample in equilibrium with zero water vapor pressure. Very slight variations in the dry weight were observed with a general increase as the temperature waslowered. With the dry weight established, a small increment of water vapor was added to the system, whereupon the sample gained in weight and a t length reached an equilibrium regain indicated by a constant extension of the quartz helix. The spiral extension, the mater vapor pressure, and the temperature were read in quick succession. I n this way successive points on the adsorption curve were readily obtained. It was possible to approach within a few per cent of the saturation pressure. The desorption isotherm was obtained by a reversal of the procedure just outlined, successive small quantities of water vapor being pumped from the manifold. The period required for the samples to reach equilibrium was variable. I n general, equilibrium was attained more quickly a t the higher temperatures and the higher moisture contents than a t the lower. Occasionally, insufficient data were obtained by one circuit of the sorption cycle. I n such cases i t was always possible immediately thereafter t o repeat the process and obtain still further points which fitted the curve equally well, indicating the essential reproducibility of the hysteresis. The method has a number of distinct advantages, Temperatures, moisture contents, and vapor pressures may be measured within a short space of time without disturbing the equilibrium. Moreover, with a single thickness of sample there is no danger of adventitious condensation of moisture such as may occur below saturation in a multiple, laminated
Four sorption isotherms at temperatures from 16.5’ to 49.5’ C. are determined for regenerated cellulose i n the form of cellophane. From them vapor pressure curves a t various equal moisture contents are constructed. Examination of the log p .us. 1/T plot shows that the net heat of sorption is not independent of temperature. AH, A F , and A S values calculated for the intermediate temperature range show a low entropy (high AS) for the water sorbed at low moisture contents. The entropy becomes progressively higher as saturation is approached. Variation i n shape of the entropy curves for adsorption and desorption points to an essential difference between the two processes. The entropy of the sorbed water on desorption is lower than that on adsorpkion at all moisture contents. The hysteresis loop is interpreted on the basis of the entropy of the sorbed water; the resulting theory is a n extension of that of Urquhart. Some conclusions regarding the mechanism of swelling are reached. The relation between moisture regain and relative humidity is discussed. Regain is shown t o be independent of temperature only when A H = 0.
Experimental The cellophane samples were preconditioned by alternate saturation and drying until the internal strains induced by the casting process were removed. The “annealing” is effectively complete when the dry dimensions of the film become constant. Such treatment is necessary if the sorptive behavior of the film is to be reproducible. The samples were then suspended on quartz spiral balances in chambers in which measured pressures of water vapor could be developed by the direct injection of small amounts of liquid water. Chambers containing samples undergoing identical treatment mere attached to a common manifold ; this, in turn, mas connected to a manometer from which the vapor pressure could be read directly. The whole assembly was enclosed in a n air thermostat with a double plate glass window through which the differential heights of the manometer and the elongations of the spirals were read by means of a sensitive cathetometer. The temperatures of the experiments were at all times controlled with an accuracy of 1 0 . 2 ” c. Data on the equilibria between water vapor and a hygroscopic sorbing material such as cellulose are customarily expressed in the form of isotherms, with the percentage of mois-
226
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February, 1942
sample. Finally, the quartz helices are sufficiently sensitive to permit an accuracy of measurement within the limits of reproducibility of the sample behaviors. All data recorded here are average values for three samples. The individual figures were generally in good agreement and served chiefly to check any accidental errors in measurement. It was occasionally necessary to replace a single sample due to failure of one of the quartz spirals o r r e v i s i o n of t h e a p paratus. At no time, however, was a full set of samples replaced at once; hence there is adequate continuity throughout the data, which may therefore be accepted as data for a single, consistent material.
0 0
ADSORPTION DESORPTION
30
20
IO
b
1
I
I
I
I
20 40 60 80 100 W A T E R V A P O R P R E S S U R E (MM.HG)
0
FIGURE1. SORPTION ISOTHERMB FOR WATER SORBED BY CELLOPHANE
Results The cellophane-moisture equilibria were measured a t 16.5", 24.8", 35.9", and 49.9" C. Figure 1 shows the four isotherms with per cent regain plotted as a function of water vapor pressure. This is the basic plot on which the subsequent discussion is based. In Figure 2 the data for adsorption are replotted to show the dependence of the regain on the relative humidity. It is evident that for a given relative humidity, the regain is generally somewhat higher, the lower the temperature. The particular nature of the dependence of the regain on the temDerature will a m e a r later. From the data of Figure 1, water vapor pressure curves for the cellophanemoisture system a t 90 regularly increasing-moisture contents were plotted 80 as a function of the temperature. These conventional vapor pressure 70 curves are shown in Figure I 3. The desorption curves f 60 a r e displaced slightly 50 downward from the posicn tions of the adsorption cn curves as a result of hysW 40 E teresis. As the moisture a content of the film in30 creases, both the adsorpa < tion and the desorption 20 vapor pressures approach w the comparable vapor + < 10 pressure for liquid water. 3
90
80 ';j 70
2
J80 w
5 50
v)
cn
k? 4 0
a LT
30
a
> E 20
w I-
$10
I S 20 2 5 30 35 4 0 4 5 50
TEMPERATURE OC.
3.
FIGURE 2 . ADSORPTIONISOTHERMS FOR CELLOPHANE
Q
'
FIQURE
100
where H = total heat of wetting, calories/gram R = gas law constant, calories M = molecular weight of water PI, p z = water vapor pressures of film at absolute temperatures 2'1 and T,, respectively, for any particular moisture content
-
Discussion
; 40 €30 8 0 R E L A T I V E VAPOR PRESSURE
ployed for the calculation of heats of sorption. The total heat of sorption is given by the integrated form of the Clausius-Clapeyron equation :
e
VaDor Pressure data of the s o r i presented in Figure 3 may be em-
227
I 5 2 0 2 5 30 35 4 0 4 5 5 0 TEMPERATURE 'C.
VARIATION OF W A T E R VAPOR PRESSURE WITH
TEMPERATURE OVER CELLOPHAXE
VARIOUSMOISTURECONTINTS
AT
INDUSTRIAL AND ENGINEERING CHEMISTRY
228
If the term M ( T , - TI) In Pol is subtracted from the H of Eauation 1. the difference is the net heat of sorution over and abbve the heat of condensation of water:
Vol. 34, No. 2
sorptive power is lowered roughly 10 per cent, and the hysteresis loop is appreciably narrowed throughout most of the length of the curve. Such a progressive lowering of the sorptive capacity of the film indicates that cellophane is easily modified, in this regard at least, by very mild treatment.
where POIand Po2 are the vapor pressures of water at temperatures TI and TZ
It is apparent from the form of Equation 1 that if In p is plotted against 1/T, the slope of the resultant isostere is a direct measure of H : €1 (in calories) =
-
30
(slope of isostere x z5 2 0
With Briggsian logarithms this becomes:
)
2.303 X 1.987 18.016
(3)
W [r
t
A H , the net heat of sorption, is given by the difference between the slope of the isostere for any particular moisture content and that of the saturation isostere. I n Figure 4 log p is plotted as R function of 1/T for various adsorption moisture contents. It is immediately evident that the isosteres are not straight and that the departure from the linear is most marked a t the low moisture contents. The steepness of the slopes of the curves a t the lower temperatures indicates a high net heat of sorption in this range, while the isosteres tend to parallel the water isostere a t the higher temperatures, indicating that AH decreases raDidlv with ~" increasing temperature. 2 .o Stamm and Loughb o r o u g h (3) found that AH for the wetting of Sitka spruce was nearly I .E independent of temperature. On the other hand, Sheppard and hTetvsome ( 1 ) found a noticeI .c able temperature coefficient for A H for cellulose acetate films. The variability 0.F of AH for cellophane is possibly Q attributable t o 0 the essential 0 -I plasticity of this material, espe)T x i o 4 cially on prolonged treatment FIGURE 4. ADSORPTION ISOSTERES AT with water vapor VARIOUSMOISTURE CONTENTS a t the higher temwxatures. The order in which the four isotherms were obtained was 24.8", 35.9", 49.9", 16.5" C., with the measurements a t 16.5" following the treatment a t the highest temperature. It seemed probable that this treatment might have produced a permanent structural alteration in the film. To check this supposition, the 24.8" isotherm was redetermined (actually a t 25.0'). The comparison of the two isotherms is given in Figure 5. Some modification of the film is apparent. The
Z
W U
iT
2
IC
W A T E R V A P O R PRESSURE (MM.HG)
FIGERE 5. EFFECT OF INTERYEKING TREATMENTS ON THE 25 SORPTION ISOTHERM This alteration of the sorption does not, however, explain the sharp drop of the log p curves a t the low temperatures, particularly a t the low moisture contents, for a low value for log p results from high sorption. I n other words, if the 16.5" isotherm is actually low, as a consequence of the pretreatment of the film, raising it higher would only aggravate the effect noted a t low temperatures in Figure 4. One is forced to conclude, therefore, that AH is not constant for cellophane throughout this temperature range. The lack of constancy in A H effectively invalidates the graphical method for the estimation of AH from the isosteres of Figure 4 and, in fact, the use of Equation 2 in any form with the expectation of a high degree of accuracy. Over short temperature ranges, however, Equation 2 may be employed without gross error, with the assumption that the result so obtained is the AH for the mean of 7'1 and TI.On this assumption, AH values for various moisture contents were calculated directly from the data of Figure l, using the 24.8' and 35.9" isotherms. The resultant curves for AH of adsorption and AH of desorption a t 30.4" C. are shown in Figure 6, plotted as a function of the moisture regain in per cent. Extrapolation of the AH curves to zero moisture content gives a figure of the order of 800 t o 1000 calories for the net A H a t zero per cent regain. The adsorption and desorption curves should be convergent at this point. Similar maximum heats of wetting have been found by other investigators. It is especially noteworthy that a t all moisture contents the desorption curve lies above the adsorption curve, indicating that adsorption and desorption are essentially different processes. I n simplest terms, the results indicate that the heat required t o remove one gram of water from an infinitely large cellophane sample on desorption is greater than that required to place the gram of water in the sample a t the same moisture content on adsorption.
February, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
Free Energy Only a small fraction of the differencein the energy requirements of the two processes is attributable to the difference in free energy between the two. The free energy change is calculated from the expression, AF =
RT
Po In M P
(4,
229
cellophane; it corresponds to a relative humidity of about 65 per cent. At this moisture content, on adsorption, a machinecast sample of cellophane begins to free itself of strains imposed by drying under tension, as indicated by shrinkage in both the machine and the transverse directions. It is also a t this humidity that the film first becomes permeable to watersoluble gases.
where AF = free energy change on transferring 1 gram of water from pressure p to pressure Po Po = saturation pressure of water vapor p = vapor pressure of cellophane-moisture system at temperature T (in this case 303.6' K. or 30.4' CJ. The values for p may be obtained from either Figure 3 or 4.
Sorption Entropies Values for A F as calculated from Equation 4 are plotted in Figure 6; in addition, corresponding curves for AS are shown. They are derived from the relation: AH
- AF
= TA8
(5)
Ai3 here represents the net entropy change over and above the entropy of condensation of water vapor to liquid water. The general shape of the entropy curves confirms the supposition that water a t low moisture contents is held in cellulose with great tenacity. The low entropy (S for water in the film is equal to S for free liquid water minus AS) a t low moisture contents indicates a rather perfect state of organization for the water molecules. A S for the fusion of water at 30" C. is approximately 0.3 entropy units. This figure is exceeded below 16 per cent regain for desorption and below 10 per cent regain for adsorption. Such tightly sorbed moisture is certainly not liquid water. The smoothness of the curves denies the existence of actual stoichiometric hydrates or a t least denies the separate existence of any such hydrates. AS for desorption is consistently greater than Ai3 for adsorption. The entropy of the sorbed moisture on desorption is thus lower than on adsorption. This points clearly to the essential difference between the two processes. Through some mechanism, water is more tightly bound, at the same moisture content, as a consequence of the further hydration and dehydration of the film.
The Sorption Process I n terms of the usual theory of the sorption of moisture by cellulose, assuming the adsorption of water molecules on the exposed hydroxyl groups of the cellulose together with condensation in the structural interstices, the observed entropy difference may be accounted for in either of two ways: (a) The extent of the primary sorptive surface (i. e., the number of exposed hydroxyl groups) may be greater for desorption than for adsorption; or (b) the interstitial space may be redistributed with an increase in the proportion of water condensed in small pores. It can be shown that the decrease in entropy produced by transferring water from a plane surface to the concave surface of water condensed in a capillary is inversely proportional to the radius of curvature of the concave surface. Thus an equalization of the pore sizes within the film, involving a transfer of water from large to small pores, would effectively decrease the average entropy of the water condensed in the film. A driving force for this transfer is the difference in the vapor pressures of water in the large and small capillaries. It is noteworthy that the AS curves are nearly parallel until a moisture content in the neighborhood of 13 per cent is reached. This moisture content is otherwise critical for
PERCENT R E G A I N
FIGURE6. 30.4' C.
A H , AF,
AS FOR ADSORPTION AT DATAAT 24.8 O AND 35.9 ")
AND
(FROM THE
The convergence of the A S curves is clearly due to a gradual decrease in the slope of the AS-adsorption curve. The entropy of the adsorption water (entropy for liquid water-AS), which has been rising steadily, now rises less rapidly and reaches a maximum a t about 18 per cent regain. It seems logical t o suppose that the structural modification responsible for this restriction of the entropy is one which both augments the primary sorptive surface and also brings about an equalization of the pore space within the film. It is difficult to imagine one process without the other. The development of gas permeability and film plasticity both suggest such a rearrangement. The interstitial equalization probably continues, with gradual enlargement of all of the pores, until saturation is reached. On desorption the situation is quite different. There is no tendency for the pore sizes to differentiate and the space distribution achieved on adsorption persists to relatively low moisture contents. Moreover, the augmented primary sorptive surface developed by the adsorption process also persists until the structure collapses on dehydration. As a result the desorption entropy is lower than the adsorption entropy a t all moisture contents. On the basis of this evidence, the sorption process may be briefly summarized as follows: Starting with dry film, moisture is adsorbed a t a very low entropy on the exposed cellulose surfaces throughout the film. At about 13 per cent regain, when the initially available surface is saturated and the original interstices are a t least partially filled with moisture, a structural rearrangement begins which develops new cellulose surface and equalizes the interstices in which water is held by capillary condensation. This is accompanied by a general swelling of the film structure which continues to saturation. On desorption the adsorption process is reversed except that
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
2 30
the spatial distribution produced on adsorption persists to lower moisture contents, with the result that the desorption entropy is always higher than the adsorption entropy. This account of the sorption cycle for cellulose does not differ materially from the theory proposed by Erquhart ( 3 ) on the basis of the observed shape of the sorption isotherm. The theory is fortified, however, by the observed entropy differences for adsorption and desorption, and the various stages in the process are clearly marked. Moreover, the mechanism suggested for the transfer of water from large to small capillaries satisfactorily accounts for the swelling which continues to saturation. The development of new sorptive surface due to the uncoupling of hydroxyl contacts on adjacent chains is probably completed at a regain of about 30 per cent, whereas swelling continues gradually to a saturation value in the neighborhood of 60 per cent.
Dependence of Regain on Relative Humidity There is a time-honored rule in the textile industry to the effect that the regain of a sample is independent of the tem-
Vol. 34, No. 2
perature, provided the relative humidity remains constant. Equation 2 can be written alternately in the form,
For any given regain, p,/Pot and p,/Pol are the relative vapor respectively. If, a t conpressures a t temperatures T zand TI, stant regain, p , / P o ~= p,/Pol as required by the rule, it follows from Equation 6 that 4H = 0. Figure 4 shows that the rule is approximate for high moisture contents and high temperatures where the isosteres are effectively parallel to the saturation isostere.
Literature Cited (1) Sheppard, S. E., and Newsome, P. T., J . Phys. Chem., 36, 930 (1932). (2) Stamm, A. J., and Loughborough, W. K., Ibid., 39, 121 (1935). (3) Urquhart, A. R., J . TeztiZeInst., 20, T125 (1929).
PRESSXTED before t h e Division of Cellulose Chemistry a t t h e 102nd Meeting of t h e AMERICAN CBEMICALSOCIETY, Atlantic City, N.J. V. L. Simril held a Du P o n t Cellophane reeearch grant i n 1939-40.
Adsorption y Strontium Salts of Traces of Caustic oda lutions WILLI-431 E. CALDWELL' AND CHARLES A. BOYD Oregon State College, Corvallis, Oreg.
RUDE caustic soda solutions as produced by diaphragm electrochemical means contain iron compounds as an impurity. I n the rayon industry caustic soda solutions are used in the production of the viscose solution. It has been shown that the viscosity of the viscose is markedly changed by the presence of impurities, particularly metallic compounds; the size and quality of the thread produced are thus affected. Therefore it is important that the iron impurity be removed from the caustic soda used in the manufacture of rayon. Treatment to remove iron from caustic soda solution may tend to remove other deleterious metal impurities also. This paper is limited to the study of the removal of iron impurities from caustic soda solutions. It is recognized that other metallic impurities, such as manganese, nickel, and copper, have a marked effect on the production of viscose, and the presence of the latter must be more rigidly controlled than is the presence of iron. In some industrial processes (3) iron impurity is removed by agitating finely ground strontium sulfate and strontium carbonate ores in a 50 per cent solution of the raw caustic soda, Upon the settling of the ore through the solution, i t is found that the iron impurity has been concentrated in the ore and that the caustic soda solution is comparatively low in iron concentration. Although strontium ores have been employed in purifying
C
1
Present address, Chemical Warfare School, Edgewood Arsenal, R'fd.
caustic soda solutions for a number of years, little is known concerning the mechanism involved. The purpose of this xork is to study the form in which the iron occurs as a n impurity in commercial caustic soda, the nature of the mechanism by which iron impurities are removed from caustic soda solutions by strontium ores, and the factors influencing this removal.
Occurrence of Iron i n Crude Caustic Soda Several mechanisms may be postulated for the removal of iron impurities from caustic soda by strontium ores, depending upon the form in which the iron occurs as the impurity. If it is present as a hydrated iron oxide suspended in the solution, the removal may be attributed to an adsorptive process or to a mechanical sweeping by the ore particles settling through the solution. However, if it is present in some soluble form such as a ferrate or ferrite ion, the possibility of mechanical sweeping may be disregarded. Before purification the crude caustic solution is highly colored, usually purple. Earlier researchers, investigating the higher valence forms of iron, described the violet and purple solutions obtained when iron salts in concentrated alkali were treated with strong oxidizing agents. Fremy (4) reported the color of potassium ferrate in alkali solution as being red-violet; this ferrate was produced by passing chlorine gas through a concentrated solution of potassium hy-
