190
RASHAD I. RAZOUK
T H E HEAT O F WETTING OF PARTIALLY SATURATED CHARCOAL RASHAD I. RAZOUK Department of Chemistry, FacuZty o j Science, Fouad I University, Cairo, Egypt Receiaed February 7, 1040 I. INTRODUCTION
Few attempts have been made to measure the heat developed when a powder which has been partially saturated with the vapor of a liquid is immersed in the same liquid. Parks (7) has stated that glass wool nearly saturated with water vapor gives no further heat when dropped into water. Bellati and Finazzi ( 5 ) measured the heat of wetting of various samples of silica gel of different water contents and found that the heat evolved decreased with the rise in the water content of the gel. Ray and Ganguly (8) confirmed this observation and found further that the heat developed decreased linearly with the increase in the water content of the gel. Katz (6) found the same general behavior when animal charcoal of various water contents was immersed in water. Unfortunately, air was not excluded in those experiments and its presence might have hindered the complete wetting of the powder, while its displacement surely involved energy changes (9). Furthermore, different samples of various water contents were used in the above-mentioned experiments, but in order to get comparable results, it is best to use one and the same sample throughout. With the aid of the apparatus described in an earlier communication (9) these two difficulties could be obviated, and it has thus been possible to measure the heat developed on wetting the same piece of charcoal when i t contains various quantities of adsorbate, without exposing the charcoal to air. 11. MATERIALS AND TECHNIQUE
The charcoal used was one of the pieces described in a previous paper and there designated as charcoal IIb. It was a piece of willow-wood charcoal which had been treated with hydrofluoric acid and water and finally outgassed a t 560OC. Methyl alcohol was chosen as the wetting liquid on account of the high heat of wetting of charcoal by this liquid and the rapidity with which this heat is developed. In carrying out an experiment, the ampoule containing the charcoal was attached to an adsorption-measuring system similar to that described by Bangham, Fakhoury, and Mohamed (2). After outgassing the charcoal
HEAT OF WETTING OF PARTIALLY SATURATED CHARCOAL
191
in situ a t 300°C. to a pressure less than mm. of mercury, it was left to cool down, and then the ampoule was surrounded with a thermostat regulated a t 30°C. f 0.1". A definite quantity of methyl alcohol vapor was then admitted to the charcoal; when equilibrium conditions had been established, the prevailing pressure was recorded, and then the capillary neck of the ampoule was sealed off. The latter was then fitted in the calorimeter and the measurements were carried out as described in the earlier contribution. TABLE 1 The heat o j wetting of charcoal IIb containing various amounts of methyl alcohol at 30°C. ~
~~
EEAT OF WETTING
RELATIVE VAPOR PREBBURE
AYOUNT ADSORBED
PIP0
8
-H
mam per mam
calorie8 pcr gram
O.Oo0
O.oo00
0.002 0.008 0.017 0,040 0.079 0.173 0.273 0.372 0.521 0.621 0.760 0.790 0.884 0.972 1.OOO
0.0192 0.0391 0.0477 0.0673 0.0810 0.0963 0.1054 0.1117 0.1197 0.1239 0.1288 0.1300 0.1328 (0.1343) (0.1350)
17.0 12.5 9.8 8.9 6.9 5.5 4.0 2.8 2.3 1.7 1.3 0.7 0.6 0.3 0.1 0.0
* *
* The values in parentheses were obtained by extrapolation. In calculating the heat of wetting, a correction had to be applied as a result of the condensation of the vapor of methyl alcohol in the dead space of the ampoule. 111. EXPERIMENTAL RESULTS
The results of the experiments on the wetting of charcoal IIb with methyl alcohol a t 30°C. are shown in table 1. The first column gives the equilibrium relative vapor pressure ( p / p ~of) the alcohol to which the charcoal had been exposed, PO being the saturated vapor pressure of the alcohol at this temperature. The second column gives the amount adsorbed ( S ) a t the corresponding relative vapor pressure, expressed in grams of methyl alcohol per gram of charcoal. The last column gives the heat ( - H ) developed when 1 g. of the partially saturated charcoal ivas immersed in methyl alcohol a t 30°C.
192
RASHAD I. RhZOUK IV. DISCUSSION
The results given above are shown in the curve of figure 1, which represents the heat of wetting as a function of the amount of methyl alcohol already contained in the charcoal. It is clear that after the charcoal has adsorbed about 0.04 g. per gram the heat developed diminishes practically linearly with the amount adsorbed, and therefore the heat of adsorption per molecule becomes constant and is independent of the presence of other molecules. According to Bangham (I), the adsorption in this region is physical. Between s = 0.04 and s = 0.135 g. per gram, the differential heat of adsorption amounts to 3200 cal. per gram-molecule. This differential heat of adsorption may also be calculated from the adsorption isotherms at two different temperatures. If da g.-mol. are
S
S
L04slh
1 FIQ.2 FIG.1. The heat of wetting of charcoal IIb of various methyl alcohol contents. a4bsciesa, amount already adsorbed; ordinate, heat of wetting. FIG.2. The semi-log isotherms of niethyl alcohol on charcoal IIb at 0' and 30°C. Abscissa, log,^ p / p ~ ordinate, ; values of 8 . FIG.
adsorbed by 1 g. of charcoal already containing a g.-mol. of adsorbate at the equilibrium pressure p and the absolute temperature T, then the heat evolved is given by the Clausius-Clapeyron equation
- dQ = RT'(d
log,p/dT), .da
(1)
on the assumption that the adsorbate behaves as a perfect gas when in the gaseous state and R is the universal gas constant. The total heat emitted when the adsorbent containing a g.-aol. of the adsorbate becomes saturated with the vapor of the latter is therefore equal to
where a0 is the saturation value for the adsorbate.
HEAT OF WETTING OF PARTIALLY SATURATED CHARCOAL
193
Now if the adsorption takcs place from the liquid phaw and not froin the vapor phase, the heat evolved will be
-H
=
-Q
+ (ao - u ) L
(3)
where L is the molecular latent heat of vaporization of thc liquid and is equal to - L = RT2(d log,p,/dT) (4) where p , is the saturation vapor pressure a t T. Hence we have
-N
=
RT2/;
= RT2
log.p/dT),.da
-
(a,
- a)RT2(d log,p,/dT)
/ti log.?/dT),.da P
(5)
Equation 5 gives the heat developed when an adsorbent containing It is usually called the "net heat of adsorption." The isotherms of the vapor of methyl alcohol on charcoal I I b were determined a t 0" and 30°C., using an apparatus and technique very similar to that described by Bangham, Fakhoury, and Mohamed (2). The results are shown graphically in the curves of figure 2, in which the amount adsorbed in grams per gram of charcoal is plotted as a function of the logarithm of the relative vapor pressure. Except for the early stages of adsorption, the scmi-log isotherms at 0" and 30°C. are straight and parallel to each other. This indicates the constancy of the heat of adsorption between s = 0.05 and s = 0.135 g. per gram of charcoal. In this range, the calculated net heat of adsorption amounts to 3250 cal. per gram-molecule, a d u e which is in excellent agreement with that directly obtained by experiment. It is interesting to note that the form of the curve of figure 1 indicates that no measurable heat is liberated when a pierr of charcoal already saturated with the vapor of methyl alcohol is immersed in the liquid a t the same temperature. This is rather surprising, for the non-development of heat in this case might mean that the process of wetting the vaporsaturated surface of charcoal is not accompanied by any change in the total surface energy of charcoal. But there is some evidence that this process is usually accompanied by a decrease in the surface free energy Thus it has been found that when a charcoal is first saturated with the vapor of methyl alcohol and then is brought into contact with the liquid, the latter will rise in the macrocapillaries of the charcoal (3). This is possible only if the process is accompanied by a decrease in the free energy. Furthermore, it has been predicted from mcasurenients of the expansion of charcoal 011 the sorption of vapors of methyl alcohol that a slight but a g.-mol. of the adsorbate is immersed in the same liquid.
194
RASHAD I. RAZOUK
definite amount of heat should be evolved on the wetting of the vaporsaturated charcoal (4). I t is believed that the absence of a measurable heat of wetting of the sat'urated charcoal is due to the very slow generation of heat, which makes its measurement by the usual method of mixtures unreliable. Indeed, this very slow development of heat is expected from the freeenergy changes. For, according to Bangham and Razouk (4),the driving force of the wetting process when the clean surface of charcoal is brought into contact wibh methyl alcohol,-namely, the free-energy decrease,amounts to 231 ergs per a t 3OoC., while that of bhe wetting process of the vapor-saturated surface of charcoal is only 16 ergs per cm.2 a t the same temperature, that is, only about one-fifteenth of the original value. V. SUMMARY
1. Measurements have been made of the heat developed when a piece of wood charcoal containing various amounts of methyl alcohol is immersed in the same liquid without exposing the charcoal to air. 2. The differential heat of wetting is found to be constant and equal to the calculated net heat of adsorption for the later stages of adsorption. 3. No measurable heat is developed when the saturated charcoal is plunged in the liquid adsorbate a t the same temperature.
The work described in this and the preceding paper was done in the Chemistry Laboratories of the Fouad I University of Cairo a t the time when Dr. D. H. Bangham, the Director of the British Coal Utilisation Research Association, was holding the Chair of Physical Chemistry. The author wishes t o thank him for his constant interest and advice during the above work. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9)
BAXQHAM: Trans. Faraday SOC.33, 805 (1937). BAXQHAM, FAKHOURY, AND MOHAMED: Proc. Roy. SOC.(London) 138, 162 (1932). BAWQAAM AND RazonK: Trans. Faraday Soc. 33, 1463 (1937). BANQHAMAND RazonK: Proc. Roy. SOC. (London) 166, 572 (1938). BELLATI AND F m a z z ~ :Atti. ist. Venezia 61, 4 (1902). I
