The Sorption of Alcohol Vapors by Cellulose and Cellulose Acetates

T H E SORPTION OF ALCOHOL VAPORS BY CELLULOSE AND CELLULOSE ACETATES*? BY S. E. SHEPPARD AND P. T. NEWSOME

The present paper deals with (a) the sorption equilibria of various alcohols with cellulose and cellulose acetates, (b) the rates of adsorption and desorption of alcohols by cellulose and cellulose acetates under definite conditions and (c) deductions regarding the sorption process and the fine and coarse structure of cellulose materials. All measurements were made with an enclosed silica spring balance at 3oOC. The cellulose materials used consisted of a series of primary and secondary acetates-primary acetates, extending from pure cotton linters (0% acetyl) t o cellulose triacetate (44.8% acetyl), and secondary (hydrolyzed) acetates decreasing from 44.8% acetyl to 35.5% acetyl. As will be shown later, it IS necessary to distinguish between adsorption by the “native” and by the “hydrate” crystalline forms of the various cellulose materials. The secondary acetates all possess the “hydrate” form and the primary acetates are more or less converted from the “native” form of the original cotton linters to the “hydrate” form depending on the degree of dispersion in the acetylating mixture (the permanent conversion to hydrate form is probably complete above 40y0 acetyl). Conversion to the hydrate form, expanded crystal lattice, results in an increased adsorbing capacity of pure cellulose and cellulose derivatives, as indicated by moisture adsorption experiments (see “The Sorption of Water S’apor by Cellulose and its Derivatives,” Part 1.l). The same result is also shown by the following alcohol adsorption data. The relative rates of adsorption of methyl and ethyl alcohol vapors a t the saturation pressure at 30%. are independent of the acetyl content, although the maximum adsorption varies greatly with the acetyl content (see Figs. I and 2 ) . However, the relative rates of adsorption of n-propyl and n-butyl alcohol vapors are definitely lower for the higher acetates (see Figs. 3 and 4). This indicates the existence of a difference in structure between the different acetates which was not apparent in the methyl and ethyl alcohol adsorption curves. A further discussion of this point will be given later in this paper. In general, the rate of adsorption of alcohol vapor by any primary, or secondary acetate decreases as we go up the alcohol series (see Figs. I , z , 3 and 4). The difference in the rate of adsorption of water and various alcohols up to n-octyl alcohol by a secondary acetate (38% acetyl) is shown in Fig. 5 . Higher alcohols are adsorbed more slowly than the lower alcohols because of lower vapor pressure and greater molecular size. *Presented a t American Chemical Society Meeting, Buffalo, dugust, 1931 Communication No. 494 from the Kodak Research Laboratories.

SORPTION OF ALCOHOL BY CELLULOSE ACETATE

2308

S. E. SHEPPARD AND P. T. NEWSOME

FIG.4


2309

R A T E O F ADSORPTION O F W A T E R A N 0 V A R I O U S &LCOUOLS 8Y CeLLULOIC ACCTATL (a* 7. A C E T Y L ) A T THE S A T U R A T I O N PRCSSURli3 AT S0.C.

*

The difficulty of maintaining a saturated atmosphere about the adsorbent increases with increasing molecular weight and lower vapor pressure of the alcohol. This is very important in a comparative study of adsorption rates a t the saturation pressures. The rate of adsorption of the higher alcohols varies greatly depending on the shape of the apparatus even in the absence of air. Molecules of the higher alcohols diffuse slowly through long lengths of connection tubing. I n all of the measurements recorded here the adsorbent was suspended from a silica spring balance in an air-free system not over 3 cm. above an area of 7 sq. cm. of liquid and the saturation pressure if known, was maintained as observed with a manometer. Figs. 6 and 7 show the maximum adsorptions a t the saturation pressures a t 3ooC. of water and alcohol vapors in per cent of the original dry weight and also in mole5 of the initially dry materials. The arrows in the figures indicate the direction of changing acetyl content (up to the triacetate and down again by hydrolysis). The variation of maximum adsorption with acetyl content shown in Fig. I can logically be carried only as far as 41.5% acetyl, since the triacetate and three secondary acetates do not belong to the same series-the latter data are included in order to show the variation in adsorption of the different vapors by the same acetate. Nevertheless, it is clear that secondary acetates have a much greater adsorbing capacity for water and alcohol vapors than primary acetates of the same acetyl content. The data of Fig. 7 show the variation in adsorbing capacity of a series of acetates all from the same batch. The water adsorption falls continuously

2310


A C C T I L C O N T C I I T 0 7 C E L L Y L O % L ACCTITLSO

0 MLTHIL 4 L C O Y O L I CT*YL i *(-PROPYL ON-BUTIL

'I

(' 'I

FIG.6

with increasing acetyl content. The methyl and ethyl alcohol adsorptions rise continuously with increasing acetyl content while n-propyl and n-but'yl alcohol adsorpt'ions show maxima at 3 1.87~ acetyl. Werner and Engelmann2 measured the adsorption of ethyl alcohol by sheets of cellulose (cellophane) and cellulose acetates when immersed in a solution of alcohol wit,h 10% ether. A maximum adsorption ~ l " sfound at 37.7% acetyl. They also obeerved that the amount of water retained by the various sheets when immersed in liquid water and blotted dry decreased directly as the acetyl content increased from o to 44.8% acetyl. This is in fair agreement with our results on the sorption of water vapor by different cellulose acetates3 They point out relations between the variation of adsorption and solubility with acetyl content. The amount of alcohol adsorbed by primary acetates, when expressed as moles per gram of dry material, decreases continuously in going up the alcohol series from methyl to n-butyl. But when the same adsorption is

0

FIG.7


2311

expressed as per cent of the dry weight, this is not true because certain acetates adsorb more n-propyl alcohol than either methyl or ethyl alcohols (compare Figs. 6 and 7 ) . Approximately 50% of adsorbed water appears to be held by hydroxyl groups in pure cellulose and any decrease in hydroxyl content produced by acetylation results in a decreased moisture adsorption. But the alcohols, also possessing polar hydroxyls, behave quite differently. A decrease in hydroxyls occasioned by acetylation results in an increased adsorption of alcohols by primary acetates, except for n-propyl and n-butyl, by acetates containing more than 32'% acetyl, This behavior of the alcohols cannot be attributed to any gradual change of fine structure from native to hydrate form since both native and hydrate forms give, in general, the same results. (Compare our results with the native form to those of Werner and Engelmann using the hydrate form.) Neither is it entirely true that alcohols are held more strongly by acetate groups than by hydroxyls, since n-propyl and n-butyl alcohols show the reverse above 3 2 % acetyl. Our results4 show that increased dispersion as indicated by different viscosities does not increase the adsorbing capacity, since regenerated celluloses of low viscosity absorb less water than mercerized cellulose of high viscosity. However, Pringsheim, KuseFIQ.8 nack and Weinreb3 have found that the acetylation of cellulose, employing zinc chloride as catalyst, is accompanied by a decrease in particle size and that the conversion of primary triacetate into acetone-soluble secondary acetate involves a further disaggregation and theoretically, increased solubility in a given solvent will always be accompanied by increased adsorbability of the vapor of that solvent. Our results agree with this in that secondary acetates adsorb alcohols as well as water more strongly than primary acetates. Further measurements on the adsorption of different alcohols by cellulose materials of different crystalline structure, viscosity, chemical constitution, and external dispersity are necessary in order to clarify the problem. Such a study may also give an explanation of the wide divergence which has been found in the moisture regain of both dope and fibrous acetylated cellulose triacetates. Fig. 8 and Table I show the adsorption of the saturated normal alcohols up to and including n-octyl by the same secondary acetate (38% acetyl). The molar adsorbing capacity of this acetate, contrary to the behavior of primary acetates, does not fall in a regular manner as we go to the higher alcohols. However, as an approximation, the molar adsorption falls rapidly from water with no carbon atoms to butyl alcohol with four carbon atoms and then re-

2312


mains constant at a value of 0.004 to 0.00 j moles per gram of acetate. This behavior might be expected if we assumed that butyl alcohol just ceased to penetrate the cellulose units, and that from this alcohol onward, we have pure surface adsorption with the hydroxyls of the alcohols attached t,o the cellulose. Since the cross section of the alcohols remains the same with increasing number of carbon atoms, the molar adsorption would also remain constant, assuming uniformity of exposed surface and complek covering by the alcohols with the formation of a unimolecular layer. A calculation of the total adsorbing surface of a gram of cellulose acetate may be made as follows: According to N. K. Adam6 the area occupied by the -CHpOH group when oriented on a surface is 21.7 X 10-16 sq. cm. Since we have 0.0045moles of adsorbed alcohol, the total area occupied is 0.004j x 6.06 X 1oC3X 21.7 X = j,918,000sq. em. per gram.

TABLEI Adsorption of Various Vapors at Their Saturation Pressures at 3oOC. by the same Cellulose Acetate (Hydrolyzed-380jo acetyl) Vapor

I\Iethyl alcohol Ethyl alcohol n-Propyl alcohol n-Butylalcohol n-Amylalcohol n-Hexyl alcohol n-Heptyl alcohol n-Octyl alcohol Water Acetic Acid

Boiling V.P. Point mm. Hg. "C a t 30'C

Moles

Moles Per Gram V.P.

0.0080

0.0000j

Mol. % iibsorbed Wt. Adsorption per gram

64.5 160.0 32.03 aj.6 78.5 78.8 46.05 38.1 97.8 27.6 60.06 37,2 117.7 74.08 29.4 9.5 5.5 88.10 41.9 137.9 I 55 .8 102.11 51.5 175.8 116.12 56.8 194 130.14 50.1 IOO 31.82 18.02 16.5 118.1 20.6 60.03 144.9

,00827 ,00618 ,00397 ,00476

.ooorog ,000224 .000417 .0008j8

.oojoq

,00489 ,00385 ,00918 .o261j

.000288 .oorz7

From a consideration of compression forces and t'hickness of adsorbed water layer Stamm' calculated the total adsorbing surface of wood material to be 3 IO,OOO sq. cm. per gram. The above value for cellulose acetate is probably high because of some formation of polymolecular layers of adsorbed alcohol. The last column of Table I shows the adsorption in moles per gram of cellulose acetate divided by the vapor pressure of t,he adsorbate. The vapor pressures of n-hexyl, n-heptyl and n-octyl alcohols were not available. From methyl to amyl alcohols the above quantity doubles Cor each additional CH? group. The maximum adsorption at the saturation pressure at any temperature is independent of the absolute value of the vapor pressure of t'he liquid under consideration. The energy changes involved arc dependent only on the relative vapor pressure. Thus, the work which a molecule does in evaporating from a liquid surface is 4 = K T In P, where 4 is the work, K the Boltamann constant, T the absolute temperature and P the relative vapor pressure. Also, the amount of work done in reversibly transferring one mole of vapor from the


23'3

interior of the free liquid to a point on the adsorption surface is K T In P, where again P is the relative vapor pressure. It seems that the difference in pressure influences only the rate of attainment of equilibrium a t the saturation pressure. DewaP found that in general, the gas with the highest boiling point was most strongly adsorbed, while Schmidt and Hintelerg found apparently the reverse, that is, the adsorption on charcoal of a number of vapors at their 180

Srn

m w s or ADSORCT~ON o r I.,UIL~IOP(

WITCR

VAPOR aT THE

CRLIIURCLil15*C

.*

./

I

FIQ.g

saturation pressures was inversely proportional to the molecular volumes at the boiling point. However, the two general laws are applicable to entirely different portions of the adsorption isotherm and are not contradictory (see Pearce and Johnstone).'O Cellulose acetate (13.7% acetyl) adsorbed 1 2 . 3 % benzyl alcohol at the saturation pressure. Fig. 9 shows the rate of adsorption of water vapor by sodium chloride and by potassium chloride at the saturation pressure at 25' C. The rate curves are practically straight lines up to 130% adsorption. Before this point had been reached both salts had formed liquid solutions. These curves are introduced in order to show the type of rate curves to be expected in the adsorption of a solvent vapor by a solid. The adsorbed water is also rapidly removed from remaining sodium chloride by evacuation at room temperature-only 0.77~ after I hour and none after 15 hours. Fig. I O shows the rate of adsorption of n-heptyl alcohol by a secondary cellulose acetate (37.2570 acetyl) both in precipitated and in sheet form. The sheet adsorbs alcohol very slowly compared with the precipitated acetate. It is obvious that the exposed surface is an important factor. That the influence of molecular size of adsorbed vapor is also very great is seen when we consider that either Precipitated or sheet cellulose acetate adsorbs water vapor at the same rate and to the same extent.

6. E. SHEPPARD AND P. T. KEWSOME

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In general, we may conclude that the adsorption of non-solvent vapors is increasingly influenced by external surface, the greater the size of the vapor molecules. I t is practically certain that the rate of adsorption of all of the alcohols by precipitated acetates will vary depending on the state of aggregation.

T \ M C 0%

L_t_

5

nourn5

,

,

,

,

,

,

,

,

,

,

, D

Fio. IO Rate of adsorption of N-Heptyl .41coho1 by Cellulose Acetate (37.25?, Acetyl) in Precipitated and Sheet Form,. A Precipitated 0 Sheet

TABLE IT Volume of Adsorption Space of Cellulose Acetate Adsorbate

Water Methyl alcohol Ethyl ” n-Propyl ” n-Butyl ” n-Amyl ” n-Hexyl ” n-Heptyl ’’ n-Octyl ”

x/m

0.165 , 2 j6 ,381 ,372 ,294

,419 ,515 . j68 ,501

d

0,998

x/m/d

,792

0.165 ,323

,789

,483

,804 ,810 ,817 ,817 (zz°C.)

,463 ,363 ,513 ,628 ,696

,827

.60;

,820

x/m = grams of vapor adsorbed per gram of acetate at the saturation pressure at 30°C. d = density of llquld adsorbate a t zo°C. x/m/d = volume of adsorbed vapor.

Table I1 shows the volumes of the different alcohols adsorbed by a given secondary cellulose acetate (38% acetyl). The volume is not constant. Coolidge” found that the volume of a number of vapors adsorbed on charcoal at the saturation pressure at 0°C. varied only from 0.424 to 0.494cc. per gram of charcoal, and he suggested that the adsorbent presents a fixed volume rather than a fixed surface. But cellulose materials being elastic gels, show neither a fixed volume nor a fixed surface.


231.5

rUTE OF ABeyHLPnON OF O l F M E W T AlCQllOW 61 K t T A T C BASE AT SATURATION PPWWC AT 90%

FIG.II

44

PATE OF REMOVAL OF DlFFEPDlT ALCOHOLS FPOM SATUFATED ACETATE BAS%

W EVACUITION A T M

A

M M AT 50.C

METHYL

8 - ETHYL

C

~

lv PPOPlL

FIQ.12

r

2316

5. E. SHEPPARD AND P. T. NEWSOME

they are more slowly removed from it independently of the total amount adsorbed a t saturation. This behavior is to be expected because of the lower vapor pressure and greater molecular size of the higher alcohols with the consequent lowering of the diffusion rate. I t should again be noted that all of the water adsorbed by a piece of sheet can be completely removed by the above evacuation procedure in about 30 minutes, whereas the lowest alcohol requires 5 hours.

FIG.13 Sorption of Ethyl Alcohol by Acetate Base at 30'C

Fig. 13 shows the maximum adsorption of ethyl alcohol vapor at different relative vapor pressures at 30'C. by a sample of the same sheet as used above. We have a typical S-shaped isotherm similar in all respects to the moisture adsorption isotherm. A calculation of the pore radius using the Kelvin equation gives t h e 9 t r i b u t i o n curve shown in Fig. 14 with a Fost probable pore radius of 4.4 -4.U. in fair agreement with the value of 5 . j A.U. obtained from the moisture adsorption isotherm. Some information may be gained from a comparison of the alcohol and water adsorption data. Alcohol adsorption by cellulose and cellulose acetates is much more profoundly affected by chemical composition and mechanical structure than water adsorption and hence may yield more conclusive evidence regarding the mechanism of adsorption by such materials. The fact that water is adsorbed just as rapidly and t o the same extent by a cellulose acetate in either sheet or precipitated form and that all of the water adsorbed at the saturation pressure may be removed by evacuation at room temperature


23'7

within IO minutes indicates that the mechanical structure offers no impediment to the diffusion of water molecules in such a material. Neither is there any indication here of pronounced chemical attraction between cellulose acetate and water although, since the moisture adsorption does vary with acetyl content, specific chemical forces must play a part. On the other hand, even the smallest alcohol molecule has difficulty diffusing in and out of the cellulose acetate material. Specific chemical or polar forces are insufficient to attract strongly any of the alcohols, yet a large molecule of a solvent vapor

FIG.14

is strongly attracted and adsorbed. The attractive force overcomes the resistance to diffusion and the material swells and finally dissolves in the adsorbate. On desorption the last traces of this vapor are removed more sIowly than adsorbed alcohols because of the stronger attraction between adsorbent and adsorbate and also increasing resistance to diffusion due to collapse of pore space. summary

The rates of adsorption of the normal saturated alcohols up to n-octyl by cellulose and a series of primary and secondary cellulose acetates were determined a t the saturation pressures a t 3oOC. The rates of adsorption and desorption of methyl, ethyl, n-propyl and n-butyl alcohols by a certain cellulose acetate sheet were determined at 3ooC. as well as a complete isotherm of the adsorption of ethyl alcohol by the same material at 3oOC. Alcohol adsorption by primary acetates increases with increasing acetyl content and n-propyl and n-butyl alcohols give maxima at 3 2 % acetyl.

2318

S . E. SHEPPARD AND P. T. NEWSOYE

Secondary acetates show greater adsorbing power than primary acetates. I n general, higher alcohols are found to be adsorbed and desorbed more slowly than the lower alcohols. Also, higher alcohols are adsorbed more slowly by cellulose acetate in sheet form than in precipitated form. It is found that the adsorption of different alcohols expressed in moles per gram of cellulose material decreases rapidly with increasing number of carbon atoms and becomes approximately constant at n-butyl.

1

2

a 4 5

References S.E. Sheppard and P. T. Newsome: J. Phys. Chem., 33, 1817 (1929). K. Werner and H. Engelmann: Z. angew. Chem., 42, 438 (1929). S. E. Sheppard and P. T. Newsome: loc. cit. S. E. Sheppard and P. T. Sewsome: loc. cit. H. Pringsheim, W. Kusenack and K. Weinreb: Cellulosochemie, 9, 48 (1928).

N. K. Adam: Proc. Roy. SOC.,101A, 452-72 (1922). J. Stamm: J. Phys. Chem., 33, 398 (1929). 8 J. Dewar: Chem. News, 94, 173, 185 (1906); 97, 4, 16 (1908). 9 G. C. Schmidt and B. Hinteler: 2. physik. Chem., 91, 103 (1916). 10 J. N. Pearce and H. F. Johnstone: J. Phys. Chem., 34, 1260 (1930). I I A . S. Coolidge: J. Am. Chem. SOC.,48, 1795 (1926). 8

7A.

Rochester, AT. Y., May 14, 1932.

The Sorption of Alcohol Vapors by Cellulose and Cellulose Acetates

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