Mar., 1954
ADSORPTION OF ALIPHATIC ALCOHOLS AND ACIDS
21 1
T H E ADSORPTION OF ALIPHATIC ALCOHOLS AND ACIDS FROM AQUEOUS SOLUTIONS BY NON-POROUS CARBONS2t3 B Y ROBERT s. HANSEN AND
ROY
P. C R A I G 4
Contribution No. 183 from the Institute for Atomic Research and Department of Ch.emistry, Iowa State College, Ames, Iowa' Received August 81, 1965
Isotherms have been determined for the adsorption a t 25" of the homologous primary normal aliphatic alcohols from ethanol to heptanol-1 and the homologous normal aliphatic acids from acetic acid to heptanoic acid from aqueous solution by the three non-porous carbons Spheron-6, Graphon and Acheson Graphite DAG-1. The marked progression of amount,s adsorbed at a given concentration on ascending a homologous series becomes almost non-existent if amounts adsorbed are compared at given absolute activity of the or anic solute, and at low absolute activities (less than 0.1) of solute the amounts adsorbed at a given solute activity appear to f e independent of osition in the homologous series. With non-porous adsorbents, relative adsorption of members in a homologous series is tterefore determined primarily by solute activity; this principle furnishes not only a rational basis for Traube's rule but also for inversion of Traube's rule.
Introduction The regular increase in adsorption of aliphatic acids, a t given concentration, on ascending the homologous series, has long been known, both as regards adsorption a t solution-air5 and solutionsolid6 interfaces. More recently, Hansen, Fu and Bartell,' investigating the adsorption of slightly soluble organic compounds from their binary aqueous solutions by a carbon black and several graphites, found that the adsorptions of valeric and caproic acids were congruent functions of reduced concentration of acid (concentration divided by saturation concentration) on each of the above adsorbents. Adsorptions of n-butyl and n-amyl alcohols were nearly congruent functions of reduced concentrations on the same adsorbents. Adsorptions of aniline and phenol were congruent functions of reduced concentration on the carbon black, but differed markedly only the graphites. Hansen, Fu and Bartell pointed out that the work required to remove solute from a solution as pure solute depended only on the absolute activity of the solute, and that, if the adsorbent acted on the same functional group, congruency of adsorption isotherms in a homologous series when plotted as functions of reduced concentration (closely paralleling absolute activities) was not surprising. The purpose of the present work was to investigate the dependence of adsorption on activity over more extensive homologous series ranges, including both completely and slightly miscible liquid pairs. Adsorbents were limited to non-porous carbons so that results would not be affected by steric factors. Experimental The adsorbents Spheron-6, Graphon and DAG-I were selected for use. Spheron-6 is a pellatized medium-processing channel carbon black, produced by Godfrey L. Cabot, Inc. Graphon is made by partially graphitizing Spheron-6 in an induction furnace at 3200'. These two adsorbents (1) Work was performed in the Ames Laboratory of the Atomic Energy Commission. (2) Based in part on a dissertation submitted by Roy Phillip Craig in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Iowa State College, Ames, Iowa, December, 1952. (3) Presented a t the 123rd National Meeting of the American Chemical Society, Los Angeles, California, March 18, 1953. (4) Dow Chemical Company, Rocky Flats Plant, Boulder, Colorado. ( 5 ) J. Traube, Ann., 266, 27 (1891). (6) H. Freundlich, Z . phzlsik. Chem., 61, 385 (1907). (7) R. S. Hensen, Y . F u and F. E. Bartell, THIBJOURNAL, 53, 760 (1949).
have been described by Beebe, Biscoe, Smith and Wendell and surface oxygen complexes on S heron-6 have been discussed very recently in papers by inderson and Emmett.@ DAG-1 is a deflocculated artificial graphite produced by the Acheson Colloids Company. All adsorbents were heated in a vacuum ( l o + mm.) for 24 hours at 1000' to eliminate volatile impurities; the adsorbents were then stored in Mason jars until used, so that the probability of surface oxide formation9 must be acknowledged. Surface areas of the adsorbents were determined by the Brunauer-EmmettTeller technique to be 114.0 meters2/g. for Spheron-6, 78.7 meters2/g. for Graphon, and 102.4 meters2/g. for DAG1. All water was redistilled from alkaline permanganate solution. All alcohols and acids used were central fractions from distillation through a 30-plate vacuum jacketed Oldershaw column at reflux ratio 10: 1, except for heptanol-1, caproic acid and heptylic acid where a reflux ratio of 20: 1 was used. Except for butanol-1 (commercial grade), hexanol-1 and caproic acid (Eastman Kodak Co. practical grade), all starting materials were reagent grade. Prior to distillation the ethanol was purified by the method of Lund and Bjerrum'o and a water insoluble impurity of the butanol1 was removed tediously by dissolving the butanol in water, heating to separate a butanol-rich phase, and drying this phase with anhydrous magnesium sulfate. Boiling ranges of fractions used did not exceed 0.02" for ethanol, butanol-1 and pentanol-1. Other boiling ranges were: propanol-1, hexanol-1, heptanol-1 and propionic acid, 0.1"; acetic and n-butyric acids, 0.3"; n-valeric and n-heptylic acids, 0.5'; n-caproic acid, 2'. Adsorption isotherms were determined according to the techniques described by Hansen, Fu and Bartell7 with minor modifications as described by Hansen, Hansen and Craig.11 Experimental mole fractions of organic solute in completely miscible systems were converted t o absolute activities using activity data obtained in this Laboratory'*; the activity data for the systems ethanol-water and propanolwater were in substantial agreement with data published by Dobsonl3 and Butler, Thomson and MacLennan,14 respectively.
Results Results are presented graphically in Figs. 1-6, in which the quantity VAC/m (V is the number of ml. of solution added tom grams of adsorbent, AC is the solute concentration decrease due to adsorption in mmoles/ml. ; the quantity VAC/m is equivalent (8) R. A. Beebe, J. Biscoe, W. R. Smith and C. B. Wendell, J . Am. Chem. Soc., 69,95 (1947). (9) R. B. Anderson and P. H. Emmett, THIBJOURNAL,66, 753, 766 (1952). (10) H. Lund and J. Bjerrum, Ber., 64B,210 (1931). (11) R . 8. Hansen, R. D. Hansen and R. P. Craig, THISJOURNAL, 67, 215 (1953). (12) R. S. Hansen and F. A. Miller, to be submitted for publication. (13) H. J. E.Dobson, J . Chem. Soc., 127,2871 (1925). (14) J. A. V. Butler, D. W. Thomson and W. H. MacLennan, ibid., 674 (1933).
ROBERTS. HANSENAND ROYP. CRAIQ
212
Fig. 1.-Adsorption
Vol. 58
of fatty acids from aqueous solution on Spheron-6.
I.2
1.1 0 n-BUTY
1.0 .9
r'
R IC-----
-
ORA P HON
0 n-HEPTYLIC
.a
(3
.7 W P
.6
9
.5
-1
0
zJ *
;.3 z - .2 .I
0
-. I -.2
0
10
.20
.30 ACTIVITY
Fig. 2.-Adsorption
40 OR
.SO .60 .?O R E D U C E D CONCENTRATION.
(15) E. A. Guggenheim and
N. K.Adam, Proc. R o y . SOC.(London),
(16) R. S. Hansen. THISJOURNAL, 65, I195 (1951).
.so
1.00
of fatty acids from aqueous solution on Graphon.
to the product of the specific surface area and the surface excess of solute in the Guggenheim and Adam co"ention'6''6) is plotted against absolute solute activity, in the case of ComP'ete'Y soluble solutes, or reduced concentration (concentration divided by saturation concentration) in the case of slightly soluble solutes. Tabular experimental data can be obtained from one of us (RSH) on request. Except as otherwise indicated by elongated symbols the experimental reproducibility was from 0.01 to 0.02 mmole/g. 8 1 3 9 , 218 (1933).
.EO
Discussion The similarity of adsorption isotherms in the homologous series when considered as functions of absolute solute activity is marked, especially a t low activities. This similarity is emphasized further in Fig. 7, in which the adsorptions of aliphatic acids on S~heron-6are compared a t low concentrations ( 7 N and a t low activities (7B); the latter figure indicates that c"ngrl1ency of adsorption isotherms for members in a homologous series (expressed as functions of activity) is a t least a limiting law a t low activities. A similar conclusion can be reached for adsorption at the water-air interface, and hence for
.
ADSORPTIONOF ALIPHATICALCOHOLSAND ACIDS
Mar., 1954
213
1.2 +ACETIC---d PROPIONIC---
I---
1.0
-
.9
-
DAG-I
on-BUTYRIC-----
K n -VALERIC an-CAPROIC
I'
8-
a
.l-
c)
0 n- H E P T Y L I C
,s
K
p"
.6-
2
.5
/'
-
A
0
1
4
" / , Y
L---
A \
I
'\
\
-
\
\
1 ++-. 51
.I
-
c
-
d f+&++
0 -I
0 0
-
'0 -
-2
ACTIVITY
Fig. 3.-Adsorption
OR
REDUCED
CONCENTRATION.
of fatty acids from aqueous solution on DAG-I.
APPRCPANCL -I
1.0
OBUTANOL
-I
--------
X P E N T A N C L -I---_
surface tensions of aqueous solutions of aliphatic alcohols and acids; this has been strongly implied by Ward. l7 Inversions of Traube's rule in aqueous solution may well occur with porous adsorbents; this probability has been demonstrated with especial clarity by Kiselev and co-workers18who found that the maximum adsorption volume was nearly constant over the homologous series of fatty acids and (17) A. Ward, Trans. Faraday Soc., 41, 399 (1946). (18) A. Kiselev, Zhur. Fit. Khim., 2 3 , 452 (1949). Translation by E. Rabkin available as TT-237, National Research Council of Canada, Ottawa, 1951.
aliphatic alcohols in their adsorption from CC14solution by porous alumina-silica gel. Even in these cases, the initial slopes would be expected to be substantially constant when adsorption isotherms are plotted as functions of activity. There appears to be no definitive work on adsorption of aliphatic alcohols and acids from hydrocarbon solutions by non-porous adsorbents, but an inversion of Traube's rule mould clearly be expected in such cases, for the absolute activity coefficient at, low solute concentrations would decrease monotonically on ascending the homologous series.
214
ROBERT S. HANSENAND ROYP. CRAIG 1.P
-
1.1
-
--
t ETHANOL--A P ROPANOL-I-
VOl. 58
GRAPHON
OBIJTANOL-I---------
%PE NTA NOL-I
----
OHEXANOL-I U H E P T A NOL-I
--
.
-
-.I
:t 0
10
.30
.tO
ACTIVITY
Fig. 5.-Adsorption
.40 OR
.so REDUCED
.ao
.TO
.eo
.w
1.00
CONCENTRATION.
of normal alcohols from aqueous solution on Graphon.
1.2 1.1 I .o
.9
-8 0 HEPTANOL-I--
i
.7
K
A
a
: .5 ; .4 -I
; .3 -I
.2
5
.I
0 -.I
-.2
0
.IO
.20
.30
ACTIVITY
Fig. 6.-Adsorption
OR
.40 REDUCED
.50
.60
.70
.80
.so
1.00
CONCENTRATION.
of normal alcohols from aqueous solution on DAG-1.
The quantity VAC/m is a surface excess and not Ihe total quantity of adsorbed solute in the adsorption region (this latter region being admittedly a rather ill-defined but none the less suggestive concept). Even if the concentration of solute as a function of distance from surface depended only on activity for all members in the series, isotherms for completely miscible homologs would necessarily deviate negatively from those of the slightly soluble homologs at higher activities, and indeed would be expected to do so in a marked way at activities which were lower the lower the position of the solute in the series. The general behavior of both alcohol
and acid adsorption isotherms is in accord with this observation. The rather marked deviation of the butyric acid adsorption isotherm from the other fatty acid adsorption isotherms should be noted; it presumably arises from an inconsistency in plotting which is difficult to remedy quantitatively but which is readily explained qualitatively. The higher fatty acids are of limited solubility; the abscissa in these cases is the reduced concentration, and -RT In c/co is very nearly the free energy required to concentrate the solute to saturation, hence also to solute saturated with water, rather
Mar., 1954
PYROLYSIS OF GRAPHITIC ACID
than to pure liquid solute. The concentrations of organic solute in the two cases would be quite similar (this is because the organic component generally has a much larger molar volume than water) but the organic solute saturated with water is thermodynamically more easily obtained, and can therefore be obtained at greater distance from the surface and by weaker adsorptive forces. In the completely miscible systems, especially such systems as butyric acid-water in which the next member of the series is incompletely miscible, the activity-concentration diagram shows an initial rapid rise in activity with concentration (Henry's law), reaches a plateau in which activity changes little with concentration, and finally increases again with concentration approaching Raoult's law as a limit. It is most probable that substances such as butyric acid are concentrated by adsorption to a part of the plateau region of the activity-concentration diagrams, rather than to pure acid. Since the upsweep of activity with concentration is not sharp it is difficult to select this part unambiguously; a value of 0.73 for the plateau point would cause the butyric acid adsorption values to agree well with the other fatty acids up to a reduced activity of 0.90 and would not be inconsistent with the activity-concentration diagram. Similar comments apply to propanol, and, to a lesser extent (because plateau regions in the activity-concentration diagram are less pronounced) to propionic acid, acetic acid and ethanol.
215
MOLES P E R LITER ADSORPTION OF FATTY A C I D S F R O M A Q U E O U S S O L U T I O N O N SPHERON-6
I_lm( I
Fig. 7a.-Adsorption
i,50[
APROPIONIC o n-BUTYRIC xn-VALERIC .n-CAPROIC D n-H EPTY LIC
U
.40
ACTIVITY
1 1 SPHERON-
of fatt acids from aqueous solution on SpKeron-6.
OR
R E D U C E D CONCENTRATION.
Fig. 7b.-Activity
or reduced concentration.
PYROLYSIS OF GRAPHITIC ACID BY EITARO MATUYAMA Faculty of Engineering, Yamaguchi University, Ube, Japan Received August 66, I963
Thermal decomposition of graphitic acid (prepared by the mixture of graphite with sulfuric acid, nitric acid and potassium chlorate) was studied with a thermo-balance and by the X-ray powder method. Observation of weight decrease with a constant rate of heating, revealed several rather sharp changes in the decomposition rate, which correspond to the rapid decrease of inter-layer spacing observed with X-rays. Under condition of constant temperature, the decomposition proceeded without any discontinuity, proportionally to the log of time. A series of microphotometer curves of the diffraction patterns at various stages of decomposition up to 1800"are shown. These figures illustrate the process of graphitization of the lamellar compound, i e . , the decrease of inter-layer spacings and the reorientation of mutual layer planes. With a specially designed Debye-Scherrer camera which permits direct comparison of the compound with Ceylon graphite, slight expansions or contractions of the layer planes of the order of 0.5% were observed; these changes are due to variation in the length of carbon-carbon bond and t o the displacement of carbon atoms from the layer planes in the formation of the lamellar compound and in the intermediate stages of the decomposition. Results obtained with other lamellar compounds are described briefly.
Introduction Considerable work has been done on the carbonization of carbon compounds and the graphitization of carbons such as carbon blacks, charcoals and cokes. All these carbons are in intermediate stages between the so-called amorphous state and the perfect graphite crystal. Some carbons are in a state which is readily transformed to graphite at elevated temperatures, while others do not easily undergo this change to a regular crystalline form. The structure of carbons and the different rates of crystal growth have been studied by
Hofmann,' Riley,2 Warren,' Bacon14 Franklin6 and others; however, all their starting materials were in rather complicated states to begin with. On the other hand, a series of compounds, the graphitic acids or oxides, is known in which carbon atoms are linked aromatically in planes as in graphite, and between them atoms or molecuIes (1) U. Hofmann and D. Wilm, 2. E'lektrochem., 42, 504 (1936). (2) H. F. Blayden, H. L. Riley and A. Taylor, J . A m . Chem. Soc., 62, 180 (1940). (3) J. Biscoe and 8.E. Warren, J . A p p l . Phys., is, 364 (1942). (4) G . E. Bacon, Acta Cwst.,3, 137 (1950). (51 R. E. Franklin, ibid., 4, 253 (1951).
