THE ADSORPTION O F THE VAPORS O F CERTAIN DICHLORO HYDROCARBONS BY ACTIVATED CHARCOAL. IV J. N. PEARCE
AND
J. F. EVERSOLE
Physical Chemistry Laboratory, T h e State University o j Iowa, Iowa C i t y , Iowa Received November 17, 1993
In the previous papers reports have been made of studies of the adsorption of the vapors of methane and its chlorine derivatives ( 6 ) , and of ethyl, propyl, isopropyl, n-butyl, and tertiary-butyl chlorides (7). Since all of these adsorptions have been made on samples from the same stock of acid-washed, ash-free, steam-activated charcoal, the results should be comparable and significant. The extremely uneven nature of the charcoal surface gives rise to a great variation in the nature of the adsorption forces emanating from the surface atoms. The surface is composed chiefly of capillary pores, cracks, and fissures; the surface atoms may be those of unbroken benzene rings, exposed carbon chains, and projecting peaks of single carbon atoms. Obviously, the degree of unsaturation of the surface forces, and hence the intensity of the adsorption forces, will be greatest for the projecting atoms. The influence of the fine capillary structure upon adsorption is shown by the calculations of Magnus (3). Assuming that the attractive force exerted upon a molecule decreases with the sixth power of the distance from the surface element, he finds that a molecule situated in a hemispherical concavity of about equal size is attracted about three times more strongly than by a plane surface. The forces operative in the adsorption of vapors and gases at charcoal surfaces may be those of simple adsorption due chiefly to van der Waals forces, or they may be more of an electrical nature. The latter follows from the fact that the electrical field at a distance of 1A.U. from a surface atom is of the order of 1 X lo9 volts per centimeter. This field is capable of orienting and attracting vapor molecules possessing permanent dipole moments; it is capable not only of increasing dipole moments already present, but also of inducing dipoles in neutral molecules and quadripoles. The induced dipole will be greater, the greater the ease of deformation of the electronic orbits of the constituent atoms. In this way many nonpolar molecules may become as highly adsorbed as those of a highly polar nature. In the interpretation of the previous results we have assumed that the 383
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J. N. PEARCE AND J. F. EVERSOLE
attraction between the surface of the adsorbent and the vapor molecules is largely influenced by an attraction for some particular atom or group of atoms within the molecule adsorbed. I t has been assumed that the chlorine atom plays the dominant r61e in the adsorption of the alkyl chloride vapors. If our assumption is correct, we should expect that for any alkyl chloride series the number and position of the chlorine atoms in the molecule, as well as the shape and volume of the molecule, should be important factors in the intensity of the adsorption and in the relative amounts of the different vapors adsorbed under corresponding conditions. With the normal compounds, the chlorine atom should be attracted toward the surface of the adsorbent, with the hydrocarbon chain extending outward into space. The surface cover-power of all normal alkyl chloride molecules should be the same. On the basis of the assumption that adsorption takes place through the chlorine atom, the intensity of the adsorption should increase with the number of chlorine atoms in the molecule. If two chlorine atoms are present, the intensity and magnitude of the adsorption will depend upon their position. If the two are joined to the same carbon atom, the molecule will probably be adsorbed on a single point or elementary space, unless the points are sufficiently close together to attract the chlorine atoms separately. On the other hand, if the two chlorine atoms are attached to two different carbon atoms, the molecule should lie flat upon the surface. The surface covering power of these molecules will be greater than for those with the chlorine atoms attached to the same carbon atom. The present paper gives a brief statement of the results obtained in a study of the adsorption of the vapors of ethylene, ethylidene, propylene, and trimethylene chloride vapors. The dichloro compounds used were Eastman's products of highest purity. They were further purified by means of a special fractional distillation apparatus (2,8). The liquids finally used were fractions of 75 to 100 cc. collected over a range of 0.01" to 0.10"C. and having the correct boiling points. The apparatus and the technique employed were the same as that described in the previous papers (6, 7 ) . It should be stated that before use each sample of charcoal was first outgassed at 525"C., then flushed one or more times with the vapor to be adsorbed, and finally outgassed a t 525°C. DISCUSSION
The results obtained are collected in tables 1 to 4. In these x/mis the number of cubic centimeters of vapor (N.T.P.) adsorbed by one gram of charcoal under the equilibrium pressures p (in millimeters). The 0°C. and 40°C. isotherms were carried up to the saturation pressures of the
385
ADSORPTION O F VAPORS OF DICHLORO HYDROCARBONS
pure liquids a t these temperatures. The equilibrium pressures at the higher temperatures were limited to the saturation pressures at 50°C.,the temperature of the air bath. Each pressure recorded is the mean of three or four readings taken at 30-minute intervals during which the increase in pressure did not exceed the limits of accuracy of the large precision cathetometer. Duplicate series on entirely different samples of charTABLE 1 The adsorption of ethylene chloride vapor by charcoal at various temperatures At 0°C. ~~
x/m , . .
17.701 20.611 20.76 114.25~117,71~124.68
At 40.00"C.
At 63,96"C.
At 79.50"C.
: :
'I
67.34~103.61~142.06~179.32 193.24 196.32 86.22; 88.831 90,61 91.01 92.43 92.73
A t 99.48%.
At 136.20"C.
coal showed that pressure equilibria are easily reproducible a t all temperatures and pressures. As with the monochloro vapors, the dichloro vapors decompose with measurable speed at about 180°C. In his development of a quantitative theory of adsorption, Langmuir (1) has deduced six relations, each applicable to definite limiting conditions. These relations satisfy the different possible configurations of the surface atoms and the number of molecules which each elementary space may ad-
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J. N. PEARCE AND J. F. EVERSOLE
sorb. The simplest of these involves the simple adsorpt,ion of vapor molecules on a plane surface possessing only one kind of elementary space, and each space capable of holding only one adsorbed molecule. For this condition the amount of gas adsorbed by one gram of charcoal is given by the relation,
_x -- -.
m
1
abp ap'
+
or
P 1 P x/m -ab+b
(1)
TABLE 2 T h e adsorption of ethylidene chloride vapor b y charcoal at various temperatures At 0°C. 0 . 2 5 0.501 1.50 4.86 10.90 22.55 38.25 49.50 57.92 66.00 70.66
$A::::::~2 8 . 8 0 57.481 8 5 . 5 4 9 5 . 1 2 ~98.87~103.05~107.8~110~48~113.66~116.48~120.5 At 40.00"C.
At 64.00"C.
At 79.50"C.
~~~
At 99.50"C.
~~~
~
~~
At 136.65"C.
Here, l / b is the slope of the straight line plot and l / a b is the intercept on the ordinate at zero pressure. Charcoal, however, is a highly porous solid with a variety of projecting points, crags, crevices, capillaries, and minute elementary surface spaces, all of which may be unlike. The relation which Langmuir has derived for amorphous adsorbents represents a complicated function whose exact form can not be determined with our present knowledge of the nature of charcoal surfaces. Since the plots of the log x l m against the log p show that the Freundlich
ADSORPTION OF VAPORS O F DICHLORO HYDROCARBONS
387
equation does not hold for any of these vapors, we have chosen the only alternative possible and have plotted our isotherms in accordance with equation 1. These isotherms for the four dichloro compounds are shown in figures 1 and 2. Within the pressure range indicated by the experimental points the adsorption relations for these vapors are satisfactorily expressed by the Langmuir equation. At lower pressures than those indicated the values of p ( s / m ) appear to diminish rapidly with diminishing pressure and the simple adsorption equation 1 no longer applies. TABLE 3 T h e adsorption of propylene chloride vapors b y charcoal at various temperatures At 0°C.
At 40.00"C.
At 6323°C.
At 79.20"C.
At 99.22"C. 15.261 40.681 85.28 61.331 66.921 At 136.20"C.
P xfm
I
2 031 7 3 3 ~16 651 35 231 67 601105 651136 60~138151 21 74 34 46 42 81 49 62 54 09 56 91, 58 62 60 35
NcBain and Britton (4)have studied the adsorption of nit.rous oxide, ethylene, and nitrogen by charcoal at different temperatures and under pressures up to 40 and 60 atmospheres. They find that equation 1 adequately expresses the adsorption relations throughout the whole pressure range. McBain, Lucas, and Chapman (5) find that a plot of ( p / p o ) / ( z / m ) against the relative humidity, p/po, gives a straight line throughout the pressure range studied for toluene, hexane, octane, decane, acetone, and acetic acid vapors. It appears to fail, however, for methyl alcohol
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J. N. PEARCE AND J. F. EVERSOLE
vapor a t low pressures, and the authors attribute the deviations to the presence of residual impurities. The study of the adsorption of vapors of the kind here used and the possibility of drawing definite conclusions as t o the influence of the structure and complexity of the vapor molecules upon adsorption is hampered by the fact that the study is limited t o temperatures a t which liquefaction is possible. This is particularly true at the lower temperatures employed a t which the forces operative in condensation may coexist along with those TABLE 4 T h e adsorption of trimethylene chloride vapor by charcoal at various temperatures At 0°C. p
x/m.. . . . . . . . . . . At 40.00"C. p . . . . . . . . . . . . . . . 0.101 0,151 0.351 3,621 8,301 15.75l 23.801 26,751 31.021 33.00 x / m . . . . . . . . . . . . 21.39; 42.90 63.18 78.71 81.77 85.08 88.23 89.24 90.51 92.00
1
At 63.85%.
p . . . . . . . . . . . . . . . 0.151 0,251 0.501 3.321 12.341 19,321 36.961 45.241 50.05~ x / m . . . . . . . . . . . . 27.641 44.57 57.72; 72.771 77.91 79.82 82.96 84.23 85.981
At 79.10"C.
.I
p . . . . . . . . . . . . . .. ' 0 , 1 5 ~0.351 0.901 2.231 7.181 15,411 36.071 47.571 50.15
x/m.. .........
25.908 42.93 54.35 63.93 69.99 73.32 76.74 77.94 79,151 1
. .I
At 99.50"C.
..I
At 136.20%.
p.. . . . . . . . . . . 0 . 3 0 ~ 0.751 2,381 6.651 19.321 40.731 50,44~ x / m . . . . . . . . . . . 24.07 41.38 55.71 65.27 72.03 75.52' 77.35
p . . ......... 0,801 3.271 8.671 18.331 35.581 49,951 x l m . . . . . . . . . . . . . 20.36 36.42 48.381 56.43 61.95 64.56
of adsorption. The orientation and adsorption of the vapor molecules is opposed by thermal agitation. Since the amount of vapor adsorbed diminishes as the temperature is raised, there should be for each vapor a critical temperature above which adsorption is impossible. A satisfactory comparison of adsorption magnitudes on a given surface can only be made when the thermal energies of the vapor molecules are equal. Although the critical adsorption temperatures may not be identical with their critical temperatures, we have assumed nevertheless that the
ADSORPTION O F VAPORS OF DICHLORO HYDROCARBONS
389
boiling points of the liquids are corresponding adsorption temperatures. We have assumed also that the variation of the adsorption with the temperature is linear through the narrow interval between the experimental isotherms just above and below the boiling points. In this way we have
1Pbrl
FIG.1. THE LANGMUIR ADSORPTIOXISOTHERMS FOR ETHYLENE AND ETKYLIDENE CHLORIDE VAPORSAT VARIOUSTEMPERATURES
calculated the isotherm data of table 5 for the vapors of all of the chlorine substituted compounds studied thus far. The natural boiling point isotherms plotted from these data are shown in figure 3. The relative amounts of the different vapors adsorbed a t their boiling points should be significant.
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J. N. PEARCE AND J. F. EVERSOLE
A study of figure 3 shows certain definite regularities. Under an equilibrium pressure of 1 mm. ethyl chloride is more highly adsorbed than is methyl chloride, and both are more highly adsorbed than n-propyl and nbutyl chloride. Of the latter two the propyl chloride is the more highly adsorbed. At still lower pressures the order of increasing adsorption is methyl, ethyl, n-propyl, n-butyl. This is exactly the order of their respec-
Pln-I
FIG.2. THELANQMUIR ADSORPTIONISOTHERMS FOR PROPYLENE AND TRIMETHYLENE CHLORIDE VAPORSAT VARIOUSTEMPERATURES
I
!I FIG. 3. TEE NATTJRAL
I
I
I
I
I
1 i jo 1 ; 1 #! Id,,.,
,I.,*....
W,‘?
P,,.,,
DERIVATIVES OF T H E HYDROCARBONS AT THEIR BOILING POINTS
ISOTHERMS O F VARIOUS C H L O R I N E
PHATIC
ALI-
tive molecular weights, boiling points, and dipole moments. At higher pressures the order of adsorption is exactly reversed; methyl chloride is the most highly adsorbed and butyl chloride the least. The isopropyl and tertiary-butyl chloride vapors, as we might expect, from their greater surface covering power, are less adsorbed a t all pressures than the corresponding normal vapors. Chloroform is more highly adsorbed than methyl chloride below 1.3 mm.
ADSORPTION OF VAPORS OF DICHLORO HYDROCARBONS
391
and more highly adsorbed than methylene chloride below 17 mm. From the trend of the isotherms we may infer that carbon tetrachloride is more highly adsorbed than chloroform a t very low pressures. At higher pressures, however, the order of increasing adsorption is that of decreasing molecular volume, namely, CC14 < CHC4 < CHzClz < CH8C1. Ethylidene chloride is more highly adsorbed than ethylene chloride up to 19 mm. Above this pressure they are adsorbed to practically the same extent a t all pressures. The greater adsorption of the lower boiling ethylene chloride a t the lower pressures is interpreted as being due to the fact
PlW,",
FIG.
O F VARIOUS C H L O R I N E DERIVAHYDROCARBONS AT T H E I R BOILING POINTS
4. THELASGMUIR ADSORPTIONISOTHERMS TIVES O F THE A L I P H a T I C
that the two chlorine atoms are attached to the same carbon atom. Thus, the attraction for the vapor molecule is increased, and since the radical extends outward, more molecules may be adsorbed. Ethylene chloride is less adsorbed than propylene chloride a t pressures below 2.5 mm.; it is less adsorbed than trimethylene chloride below 1.7 mm. The latter, in turn, is less adsorbed than propylene chloride below 2.5 mm. Above the pressures indicated the order of adsorption is: ethylene > propylene > trimethylene. This is exactly the order to be predicted from the respective surface covering powers of the three vapors on
392
.
J. N. PEARCE AND J. F. EVERSOLE
the assumption that the adsorption is due to the attraction of the surface forces for the chlorine atoms. While we have discussed the adsorption with respect to four series of chlorine substituted vapors, the relative adsorbability of the vapors of the different series may be easily noted from figure 3. I n general, it may be stated that for any one of the series of vapors, the higher boiling, heavier molecules are most highly adsorbed a t low pressures. The adsorption of these quickly approaches a maximum and 'then increases but slightly with further increase in pressure. At relatively high pressures the more simple the molecule is, the greater is the tendency of the adsorbent to adsorb additional vapor as the pressure is increased. We have plotted the boiling point isotherms corresponding to the simple adsorption equation of Langmuir. These are shown in figure 4. Because of the approximate equality of the adsorption magnitudes of the ethylene and ethylidene chloride vapors, and of those of propylene and trimethylene chloride, it was necessary to displace the plots upward by definite space intervals. Within the limits indicated by the experimental points, the Langmuir equation for simple adsorption on a plane surface adequately expresses the adsorption for the chlorine compounds studied. At lower pressures the values of p / ( x / m ) fall rapidly below the straight line plot. This is not surprising since the Langmuir equation (1) applies only to adsorption on a plane surface. Charcoal surfaces are exceedingly irregular; they are made up of points, edges, and capillaries, and the atoms of each possess adsorption forces of widely varying intensity. Upon exposing the gas-free charcoal surface to a vapor it is the more intense forces emanating from the surface atoms that are first saturated. Only when these points have become covered with vapor molecules will the adsorption approximate in nature that on a plane surface. Then and only then should the simple adsorption equation apply to the adsorption of vapors on charcoal surfaces. The heats of adsorption of the four dichloro vapors studied in this research were calculated from the slopes of the isosteres. The heats of adsorption obtained are: ethylene chloride, 11,500 calories; ethylidene chloride, 10,965 calories; propylene chloride, 13,160 calories; trimethylene chloride, 13,700 calories. Since the heats of adsorption of these vapors have not been determined experimentally, we can only state that the above values are probably of the right order of magnitude. I n general, however, it has been found that heats of adsorption calculated from the isosteres are usually lower than those experimentally obtained. SUMMARY
The adsorption of the vapors of ethylene chloride, ethylidene chloride, propylene chloride, and trimethylene chloride on charcoal has been studied
ADSORPTION OF VAPORS OF DICHLORO HYDROCARBONS
393
a t several temperatures between 0°C. and 136°C. Within the pressure range employed, the adsorption magnitudes are satisfactorily expressed by the simple Langmuir equation for adsorption on plane surfaces. Both the natural and the Langmuir isotherms have been plotted for these vapors and for the chlorine derivatives previously studied a t temperatures corresponding t o the boiling points of the pure liquids. The Langmuir equation applies for the pressure range studied. The Freundlioh equation does not hold for any of the vapors. For any one series the amount of vapor adsorbed a t very low pressures is always greatest for that vapor having the greatest molecular weight, the largest molecular volume, and the highest boiling point; it is least for the simpler, lower boiling liquids. Similar relations obtain with increase in the number of chlorine atoms within the molecule. At higher pressures the order of the adsorption magnitudes is exactly reversed. The influence of the number and position of the chlorine atoms has been determined.. The heats of adsorption of the vapors have been calculated from the slope of the isosteres. REFERENCES (1) LANGMUIR: J. Am. Chem. SOC.40, 1361 (1918). (2) LOVELESS: Ind. Eng. Chem. 18, 826 (1926). (3) MAGNUS: Z. anorg. allgem. Chem. 166, 221 (1926). (4) MCBAINAND BRITTON:J. Am. Chem. SOC.62, 2198 (1930). ( 5 ) MCBAIK,LUCAS,AND CHAPMAN: J. Am. Chem. SOC.62,2668 (1930). (6) PEARCEAND JOHNSTONE: J. Phys. Chem. 34, 1260 (1930). (7) PEARCEAND TAYLOR: J. Phys. Chem. 36, 1091 (1931). (8) PETERSA N D BAKER:Ind. Eng. Chem. 18, 69 (1926).
