1456
F. E. BARTELL, T. L. THOMAS, AND Y. FIJ
THERMODYNAMICS OF ADSORPTION FROM SOLUTION.I V TEMPERATURE DEPENDENCE OF ADSORPTION’ F. E. BARTELL, TUDOR L. THOMAS,’,’ AND YING FU Department of Chemistry, University of Michigan, Ann Arbor, Michigan Received Augwrt ##, 1060 INTRODUCTION
The temperature dependence of adsorption in solid-gas systems has been extensively investigated, whereas the temperature dependence of adsorption in solid-solution systems has not been thoroughly studied. Data published by Freundlich (5), Groves, Bowden, and Jones ( 8 ) , and Hartman, Kern, and Bobalek (11) for adsorption by solids from solutions at different temperatures have shown that adsorption decreases with increasing temperature, but the various factors. which may influence the adsorption have not been thoroughly discussed. It is the purpose of this investigation to study these various factora more fully. Adsorption is an exothermic process. One would expect, therefore, that ,an increase in temperature should cause a decrease in adsorption. This expectation has been amply verified by studies on the adsorption of gases. In adsorption from solutions temperature should affect not only the adsorption process but also the solubility of the adsorbate. Since the solubility of a substance determines its chemical potential, which in turn controls the adsorption, the solubility factor cannot be neglected in any investigation of adsorption from solutions. The relationship between solubility and chemical potential has been expressed by Lash Miller (12) as follows: “The greater the solubility of a substance S in any solvent the less the potential for any given concentration.” Since solubility and adsorption are different manifestations of the escaping tendency of the solute, adsorption should be greater if the solubility is smaller. This has been found to be the case (2, 13). The influence of temperature on adsorption from gaseous systems is due only to the exothermic character of the process, while in adsorption from solutions an additional factor, i.e., solubility, is superimposed on the normal temperature effect. If the solubility of the adsorbate increases with increase in temperature, then both effects work in the same direction and the adsorption is decreased. If, on the other hand, the solubility of the adsorbate has a negative temperature coefficient, then the normal temperature effect and the soluLility effect will work against each other, and the adsorption may show an increase or a decrease depending on which factor is predominant. Since the solubilities of most substances have positive temperature coefficients, the general impression that adsorption from solutions always decreases with increasing 1 The data in this paper are from a dissertation submitted to the Horace H. Rackham School of Graduate Studies by Tudor L. Thomaa in partial fulfillment of the requirements for the degree of Doctor of Philosophy, September, 1949. * Holder of the du Pont Fellowship, 1948-49. a Present address: Linde Air Products Company, Tonawanda, New York.
THERMODYNAMICS OF ADSORPTION FROM SOLUTION. I V
ooc . 25°C. 45oc.
1457
1.386moles/liter of solution 0.969 mole/liter of solution 0.863 mole/liter of solution
The result for 25°C. agrees essentially with those in the literature (3, 10). Adsorption determinations The adsorption flasks were provided with standard-taper mercury seals to prevent evaporation. The solutions were shaken mechanically with the adsorbents for 24 hr., the mixtures were centrifuged, and a portion of the solution was removed for analysis. For adsorption by graphite 0.5 g. of solid was used; for blood
1458
F. E. BARTELL, T. L. THOMAS, AND Y. FU
char, 0.25 g. The volume of the solutions was 10 ml. in every case. In recording the results (tables 1 and 2, figures 1 and 2) both the conventional x / m and the Gibbs surface excess are given, the latter being obtained from the expression x'/m = x / m ( l
+ N&/N,o.)
TABLE 1 Adsorplion of n-bulvl alcohol f r o m aqueous solution on graphite O'C.
ZST. ~
C
C
millimrlcs/
milli-
moles/
lilcr
17.3 43.2 67.4 105.0 194.3 284.6 472.6 682.8 840.4
-
Xb!m
Xk/m
__
millimoles/
millimole$/
milliwwIes/
C/CO
__ --__ lif"
0.0125,0.077'0.078 0.0312,0.19210.193 0.0486i 0.2741 0.275 0.075810.28810.290 0.1402'0.313 0.350 0.2055i0.363'0 373 0.3410'0.394~0:410 0.4925'0.432'0.456 0.6063~ 0.532~ 0.571
17.3 44.1 69.0 104.0 193.5 281.7 169.6 377.9 337,3
gram
C
0.0179 0.075 0.075 0.0456 0.172 0.173 0.07141 0.237 0.238 0.1074'0.297 0.300 0.2oOo 0.349i0.356
mtllimolcsl
millimules1 gram
liler
gram
gram
17.9 0.0207 0.065.0.066 44.8 0.0520 0.159'0.161 70.1 0.0814 0.218 0.220 105.3 0.1222 0.277 0.280 194.2 0.2355 0.334 0.341 282.8 0.3280 0.3W1 0.409 168.2 0.5433 0.481 0.501 073.7 0.7805 0.60610.641 527.0 0.9600,0.8081 0.866
Adsorption of n-butyl alcohol f r o m aqueous solution o n blood char
I-
'C. C
!
2.5-c.
Ib/m
Xb/m
C
milli-
milli-
lilt,
moles! #ram
millimolerl
miliimalcsi liler
C/CI -I-__-
males f
grom
2.4 2.8 0.0020 1.12 1.12 5.0 13.0 29.7 92.9 169.5 343.8 548.2 716 0 10.5170'6.0601 6.443 I 719 6
I
45T. c/co
I
rb/m
j
milli-
moles/ tram
I
I
xL/m millimoles/ gram
~
0.6390'6.265 6.560
0.8134)6.350 6.745
(where N represents the mole fraction, i, the partial molal volume, and the subscripts a and b refer to the solvent and the solute, respectively), as suggested by Guggenheim and Adam (9). The xb/m values were calculated from the change in concentration due to the adsorption process, and the d / m values were Nb%/N,fi,), obtained by multiplying x b / m by the appropriate value of (1 iwhich is a factor dependent on the equilibrium concentration corresponding to the xb/m value.
THERMODYNAMICS OF ADSORPTION FROM SOLUTION. IV
1459
DISCUSSION
Effect of temperature Inspection of the curves indicates that the effect of temperature on the adsorption from solutions is a function of concentration. A t lower concentrations
FIG.1. Adsorption of n-hutyl alcohol by graphite: 0 , 0°C.;
a ,25°C ; Q, 15°C.
the adsorption decreases with rising temperature; at higher concentrations, the reverse is the case. This phenomenon can be easily understood from the previous discussion. Since the effect of temperature consists of two factors, the normal effect due to the exothermicity of adsorption and the solubility effects, the second factor must be weaker at lower concentrations, and what is observed is mainly due to the operation of the first factor. Since adsorption is an exothermic process, it must decrease with increasing temperature if no other factor intervenes. When
1460
F. E. BARTELL, T. L. "EOMAB, AND Y. FU
the concentrations become greater, the second factor becomea greater also. Since increasing temperature causes a decrease of the solubility of the alcohol,-in other words, an increase in the chemical potential of the adsorbate at the same concentration,-the adsorption should then increase with decreasing solubility, i.e., with increasing temperature. Thud the two effects oppose each other. As the solubility effect becomes higher and higher with increasing concentration, it soon overtakes the normal effect and the adsorption is increased.
8.0 7.0
e
6.0
v)
lj5.0
0
5- 4.0 1
5 3.0
X
2 .O I.o
0 .4 .5 .6 .7 .8 .9 1.0 RELATIVE CONCENTRATION FIQ.2. Adsorption of n-butyl alcohol by blood char: 0 , OOC.;, 0 , 2 5 ' C . ; O , 45°C.
0
.I
.2
.3
Standard energy changes For thermodynamic studies of adsorption from solutions, it is desirable to be able to determine the standard changes of free energy, A F O , and heat content, AH". Alexander and Johnson (1) proposed to calculate AFO by the following equation: AFo = -RT In (r/Lcbu,k) where t is the thickness of the adsorbed layer. In this equation it is assumed that the adsorbed molecules behave ideally. It has been shown in previous publications from this laboratory (6, 7) that the activity coefficient of the adsorbate on the solid is far from unity. Hence this equation cannot be applied. It has been
1461
THERMODYNAMICS OF ADSORPTION FROM SOLUTION. I V
pointed out (6) that when the adsorption isotherm is linear, the adsorbates behave ideally. Therefore the limiting slope of the isotherm a t zero concentration will give the equilibrium constant K for the process alcoholin solution
* a1COhOledsorbed
after the x / m values have been converted into concentrations. In order to determine K , it is necessary to know the thickness and the area of the adsorbed layer. The area of the layer is the same as that of the solid, which can be obtained by the B.E.T. method. The thickness was taken as 6.6 .&.and was obtained on the assumption that the adsorbed molecules are oriented on the solid surface and that the cross-sectional area of the molecule is the same as that in the TABLE 3 Standard energy changes
Adsorption of butyl alcohol by graphite
1
'C.
0 25 45
61.6
I
hcd./nolc
kcal./wwlc
-2.24 -2.43
-0.16 -1.47
Adsorption of butyl alcohol by blood char 0 25 45
477 412 390
-3.34 -3.54 -3.76
-0.94 -0.52
The results so obtained are given in table 3. Even though this method of calculation is more rigorous than the method of Alexander and Johnson, no great accuracy can be claimed, for the following reasons. In addition to the uncertainty involved in the assignment of the thickness and area of the adsorbed layer, which is common both to this and to the method of Alexander and Johnson, the present procedure depends on the accuracy of the adsorption data a t the lowest concentrations, which are usually the least accurate. The values tabulated indicate only the order of magnitude. I t is worth noticing that while AH may be as high as about ten times AF in the adsorption of vapors (4),this is not the case in adsorption from solutions.
1462
A . G. KEENAN, R. W. MOONEY, A N D L. A . W O O D SUMMARY
1. The effect of temperature on the adsorption of butyl alcohol from aqueous
solutions by graphite and blood char has been studied. At lower concentrations, adsorption decreases with increasing temperature; a t higher concentrations, the reverse is true. I t has been concluded that the temperature effect has two factors: ( 1 ) the “normal” and (2) the solubility effects. At higher concentrations the second factor predominates and overbalances the first factor. 2. A method for the calculation of changes of standard free energy and heat content of the adsorbate in adsorption processes has been proposed and applied to the present systems. This method consists in obtaining the equilibrium constant K from the limiting slope of the adsorption isotherms a t zero concentration. The results so obtained indicate that the absolute value of AH’ is not higher than that of AFO, as in the case of adsorption of gases. REFERENCES (1) ALEXANDER AND JOHNSON: Colloid Science, Vol. 1, p. 72. Oxford University Press, London (1949). (2) BARTELL A N D Fn: J . Phys. Chem. 33, 676 (1929). THOMPSON, AND MACHENNAN: J. Chem. Soc. 1933, 674. (3) BUTLER, Fu, A N D BARTELL: J . Am. Chem. Soc., in press. (4) DOBAY, (5) FREUNDLICH: Z . physik. Chem. 67, 448 (1907). A N D BARTELL: J . Phys. & Colloid Chem. 62, 374 (1948). (6) F u , HANSEN, (7) Fu, HANSEN, AND BARTELL: J . Phys. & Colloid Chem. 69, 1141 (1949). (8) GROVES, BOWDEN, A K D JOSES: Rec. trav. chim. 66,645 (1917). AND ADAM: Proc. Roy. Soc. (London) A139, 218 (1933). (9) GUGGENHEIM (10) HANSEN, Fu, ASD BARTELL: J . Phys. & Colloid Chem. 63, 769 (1949). KERN,A N D BOBALEK: J. Colloid Sei. 1, 271 (1946). (11) HARTMAS, J. Phys. Chem. 1, 653 (1897). (12) MILLER: A N D JOSES: J. Phys. Chem. 29, 1 (1925). (13) PATRICK
T H E RELATION BETWEEN EXCHANGEABLE TOSS AND WATER ADSORPTION O N KAOLINITE .4. G . KEENAX,’ R . W . hIOOSEY,1
A ~ L. D
A . WOODS
Department of Chemistry, Cornell Cnzversity, Ithaca, S e w York Received September 8 , 1950
The adsorption of mater by soils and clays has long been of interest to n-orkers in fields where these materials are utilized. In addition t o some early work (2, 19, 23, 24) on soils and soil colloids of rather indefinite composition, the adsorption of water on various relatively pure clay minerals has in recent years been Present address. Department of Chemistry, Champlain College, State University of New York, Plattsburg, New York. Present address: I n service, U. S. N. 3 Present address: Box 126, Dayton 1, Ohio.
