T H E SORPTION OF SODIUM HYDROXIDE ON CELLULOSE AND WOOD BT R. RICHARDSON
* AND 0. MAASS
This paper consists of data dealing with the effect of aqueous sodium hydroxide on cellulose, in particular with its sorption qualities. I t is presented in the form of plates of some results obtained a t the University of McGill.** In review, whereas cotton alkali sorption has been previously investigated, practically no results for wood cellulose are to be found in the literature, Wood cellulose was first used but it was natural t o extend the measurements to other types of cellulose. I n a practical way the related industries of mercerization, alkali cooking, and zanthating may make use of the results. Theoretically the subject is interesting from a standpoint of absorption in an abstract sense apart from the materials used. Since adsorption theory is in advance of absorption theory at presents experiments on absorption are particularly interesting. The sorption calculations were made from measurements of the change in concentration of the liquid solution before and after contact with a known amount of the solid phase. For purposes of cursory investigation and of time factor determinations, conductometric measurements were satisfactory and flexible and were first used. For detailed work titrometric and occasionally gravimetric methods were used. The samples for titration were secured in a weight pipette. The accuracy in analysis attained was such that the average deviation from the mean was I part in 4000 a t 40% alkali. Each determination of a concentration consisted of the average of 3 such analyses. Quantities were adjusted to produce a concentration change of 0.5%. The samples of cotton and “Celanese” were purified by cooking in a one percent alkali solution for 4 days. The purification of spruce was attempted as can be seen from one of the spruce plates. Plate number 9. The lignin is partially attacked, however, and results are not repeatable. The sample denoted as “Celanese” has suffered considerable alteration in purification and is not to be compared with the original commercial sample. The acetyl number of the purified “Celanese” is being estimated to determine the degree of alteration but this value is not yet available. A reaction period of I hour was agreed on for celanese above 45% where the downward trend of the curve for “Celanese” is due to the slow diffusion in such viscous solutions. There is a strong temptation to present nothing but facts and results since it is certain that with further investigation the interpretations given to certain
* Acknowledgment is hereby made of two scholarships granted t o one of us by the Canadian Pulp and Paper Association. ** This work is part of a research program of “Penetration Studies” being carried out under the direction of Dr. Maass in the Institute of the Pulp and Paper Association. It is also part of the research program of the Forest Products Laboratories, Montreal.
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of the curves must be changed. Such a course would be, however, dull. It is particularily requested that the deductions made at this time concerning the results be regarded as purely working hypotheses. Final conclusions cannot yet be hazarded. The use of the terms sorption, adsorption, and absorption are familiar enough. I n this article sorption is a general term. Adsorption refers to surface concentration on a solid structure. Absorption refers to internal concentration throughout the solid structure. The use of such terms as pseudo sorption and others is not so general. Anticipating criticism from their use they will be explained in some detail. It is obviously not expressing the facts to say that in a zs’% solution of alkali 0 . 2 g of NaOH is sorbed per gram of cotton since there must be some water sorbed or associated with the cotton contemporaneously. The hygroscopicity of cotton suggests that cotton in solution would not be bone dry. There must be two values, one for alkali and one for water, for example, 0.3 g NaOH plus 0.3 g HzO. Such true values have never yet been accurately obtained. The value 0.2 g NaOH is all that can be measured. It is a net result of the two true values. To express this fact it is perhaps permissible to use the term pseudo sorption to be applied to such a result as 0 . 2 g NaOH. It will be argued that the term is unnecessary since the true values have never been determined. I n rebuttal it may be pointed out that the literature abounds with citations of molecular ratios of compounds calculated from pseudo sorption curves by investigators who have not realized that such values are only useful by comparison and are not absolute in nature. The use of such a term would caution against the misuee of sorption values. This case is not the only example of errors contracted by the promiscuous use of sorption values. Consider the addition of salt to the solution of alkali. Until the sorption of any of the three liquid constituents is proved equal to zero it must be assumed that there is a true value for each, one for alkali, one for salt, and one for water. By analysing for alkali alone and considering mathematically that the salt is water a value is obtained which is denoted as the uni-pseudo sorption value of the alkali. However, by analysing for all three components, the net difference between the true values of alkali and the actual water alone can be calculated. I n this manner the bi-pseudo sorption of the alkali is obtained. The literature records only uni-pseudo values whereas bi-pseudo values are much better indicators of the true state of affairs. The mathematical definitions of these sorption terms are given.
Symbols. a Bone dry weight of solid phase, gms. b Weight of solution used, gms. M Moisture content of the solid phase before the experiment, gms. HzO per bone dry gm. of solid phase. x, y, I, the original concentrations of alkali, salt, and water in the solution, yoby weight. x’, y’, z’, the final concentrations of alkali, salt, and water in the solution, yc by weight.
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AND 0 . MAASS
Case 1.
Let there be two liquid phase components, alkali and water, and it be desired to calculate the alkali as sorbed. Then the water is considered as unsorbed. Pseudo sorption of x (alkali). b (x-x’)-x’aM g,’g solid phase. a(rO0-x’)Case 2.
Let there be three liquid phase components, alkali, salt, and water, and it be desired to calculate the alkali as sorbed, and let the water plus salt be considered as unsorbed. Uni-pseudo sorption of x (alkali). b(x-x’)-x’aLI g/g solid phase. a( 100-x‘) The similarity of form t o case one explains in part why these values were first used in place of bi-pseudo results. Note that y and y‘ need not be known. Case 3.
Let there be three liquid phase components, alkali, salt, and water, and it be desired to calculate the alkali and salt as simultaneously sorbed. Then the water alone is considered as unsorbed. Bi-pseudo sorption of x (alkali). bxzl-bzx’-IooaMx’ g i g solid phase. Iooaz’ Bi-pseudo sorption of y (salt). byz‘-bzy‘-IooaMy‘ - g/g solid phase. Iooaz‘ Certain experimenters have erroneously considered hI equal to zero. If h l is small the error introduced is numerically small, as in the case of air-dry cotton. In other cases it cannot be ignored. The practice of neglecting M is likely to cause mistakes. Wood samples are usually presoaked in water so that equilibrium may be attained rapidly during the experiment and A4 in these cases is very large. A factor due to the solution of the solid phase in the liquid phase is neglected in these formulae but is sometimes appreciable. As a summary t o these formulae and mathematical considerations it may be generalized that if there are n liquid phase components, having chosen one to be considered unsorbed (unless otherwise stated this is usually the solvent), n-I simultaneous pseudo sorption values can be determined, one for each of the remaining liquid phase components. This is a much better procedure than choosing n-r components to be unsorbed and calculating t’he sorption of the nth component, even if this operation be repeat,ed until values for all components are obtained. To illustrate these calculations and show that too much reliance must not be placed on uni-pseudo sorption values the boxed sorption values for sodium
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hydroxide in Plate I may be compared. The addition of the salt has increased the sorption of alkali but the bi-pseudo sorption value shows that the increase is really not as large as would be concluded from the uni-pseudo value. The prefixes uni and bi refer to the number of liquid phase components which the mathematical procedure in each case permits having a simultaneous pseudo sorption value. The above discussion shows that the results which will be given in the following tables can be relied on only to give the relative changes of sorption caused by a change in the concentration of the alkali or due to a different sample of cellulose. These results cannot be used in their present form and in the light of present knowledge to calculate stoichiometric proportions, etc. While it is sometimes possible to conclude that compounds are formed it is impossible to calculate their formulae, I n the authors’ opinion no reliable and complete data for sorption of NaOH on cotton exists a t concentrations above 30%. It was found that only when
PLATE I Spruce Flake Sorption One hour, 2ooC, in sodium hydroxide, salt, water systems (A) Without salt. Equilibrium solution % NaOH
Pseudo sorption g NaOH/bone dry g
0.0474 4,562 0.0353 4.570 0.0411 4.348 4.465 0.0397 0,0399 4.508 Average4,591 0.0407 And by comparison with known adsorption values for another spruce sample :
(B) With salt. Equilibrium solution O/,NaOH-%NaCl
9.705 3 ,837 9.625 3.899 9.625 3.850 9.713 3.898 9.148 3.920 Average of last four tests3.9*
* Comparable
9.5 values.
Uni-pseudo sorption g NaOH-g NaCl /bone dry g
0.0480 0.0510
0.0546 0.0513 0.0568
=
o .0382 -0.0402 -0.0556 -0.0430 -0.0511 -0.048
Bi-pseudo sorption. g NaOH-g NaCl /bone dry g o .os05 0.0428
0.0498 0.0528 0.0497 0.0551
-0.0337 -0.0504 -0.0346 -0.0471 -0.042
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experimental determinations of the highest accuracy were made that reproducible results were possible. At high concentrations equilibrium is attained very slowly and with great experimental difficulty. Below 30% the data in the literature was used. Plate z illustrates the cotton isotherm a t room temperature. Rumbold’s’ very intensive examination at low concentrations has been linked by the work of Vieweg2 to the present examination of high concentrations. The curve is generally interpreted as being a combined effect of solid solution (adsorption) and of compound formation, the latter O.+OI
I
1
PLATE 2 Rumhold and Vieweg’s values were not determined at one hour and zo°C but are nevertheless comparable with the determinations as shown by the circles; equilibrium in these cases being attained after very short time intervals.
indicated by the horizontal reaches. At low concentrations the cotton curve is in sharp contrast with the curves for wood and “Celanese.” The latter curves obey Freundlich’s adsorption law. If the cotton curve is considered to be concave then the wood and “Celanese” curves are convex below ten percent sodium hydroxide. The theory of the cotton curve is based on applications of the phase rule. Dalton’s law of partial pressures may also be applied. In Nernst’s Theoretical Chemistry it is cited as Dalton’s law of absorption and as such applies to the absorption of gases in liquids. In this case it is extended to the distribution of alkali between the liquid solution (water) and the solid solution (cotton). Except for aberrations from the laws of ideal solutions a straight line relationship between sorption and concentration would indicate absorption. This rule for absorption takes the place of Freundlich’s relation 1
J. Am. Chem. SOC.,52, 1013 (1930).
* Ber., 40, 3876 (1907).
SORPTION OF SODIUM HYDROXIDE ON CELLULOSE AND WOOD
PLATE 3
0.2
02
01
ai
ao
on PLATE 4
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PLATE j
for adsorption. The phase rule indicates the same general type of curve for absorption as Dalton’s law. If compounds are formed the phase rule predicts a step-like diagram. Experimentally the cotton curve appears to be a mean. Perhaps this may be considered somewhat similar in type to the Pd-HZ sorption curve. The term solid solution is here used as a partial synonym for absorption, to forcibly denote that the effect is internal in nature and to recall simultaneously that the curves have some theoretical basis in the phase rule and Dalton’s law. The other laws of solid solutions may or may not be observed. It will require further work t o settle these points. The precise configuration of alkali
PLATE 6
SORPTION OF SODIUM HYDROXIDE ON CELLULOSE AND WOOD
307 I
PLATE7
I
IS
, CLLANES€ JORPTION 207.
00
'
0
kb
t-' O /
I
-1.5
IBRlUfl SOLUTI1N IL
PLATE 8
, % NaOH. 6
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PLATE 9
--
PLATEIO
SORPTION O F SODIUM HYDROXIDE ON CELLULOSE AND WOOD
30j3
and water in the cellulose crystal is not suggested. The term internal concentration may be substituted, or the original term absorption even may be considered of sufficient strength. A lucid portrayal of facts is desired more than anything else. Suggestions of more appropriate terms would be welcomed. It should not be forgotten that the generally accepted explanation of the horizontal portions of the step-like cotton diagram as being due to compound formation may be entirely fallacious. The step-like diagram may be possibly due to a greater adsorbing area being produced by swelling actions occurring a t critical concentrations. This viewpoint would eliminate absorption and attribute all curves to the effect of adsorption but on varying surface areas. The remaining plates show the effect of the source of the cellulose on the sorption curves. Below approximately IO% for wood and 30% for “Celanese” the type of curve obtained is likely due to adsorption. These curves obey Freundlich’s adsorption isotherm as shown by the straight line logarithmic plots of these curves below the critical concentrations just named. Plates number 6 and 8. Work on cellophane is progressing. It is planned to complete this curve and obtain curves for rayon and viscose silk from cotton. The cotton curve at intermediate concentrations is to be reconsidered especially with regard to purified samples of various origins. When this work is finished it is hoped the interpretations of the curves will be more conclusive. The planned work would include desorption measurement. A few of these have been made already. It should be pointed out from Plate I O that all these curves can be approximately represented by straight lines. Each curve can be replaced by one straight line as a first approximation. The constant sorption differentials or tangents of each of these lines are given. Spruce, on a basis of 44.5 % cellulose = 0.010g NaOH per g cellulose per % of conc. change in the solution Cotton 1’ 1) - 0.070 g ” cellulose Celanese ’1 JJ 1, = 0.066 g ” Spruce, on a basis of total wood content = 0.048 g NaOH per g wood per % of conc. change. These relations may eventually evolve a substantial theory. Various theories of adsorption and combination have been put forward by many workers in this field to explain alkali-cellulose affinity. Which name adsorption or combination is applied to this affinity is of little import as these terms merge into each other. The effect dealt with is certainly on the border line. When this work is finished it is hoped the interpretation of the curves will be more conclusive. McGW Untversity, Montreal, Canada.
