STUDIES IX T H E ADSORPTIOX FROM SOLUTION FROM T H E STAKDPOINT OF CAPILLARITY1-I BY
m.A.
PATRICK AND D.
c. J O N E S ~
It is the purpose of this paper to present data on the adsorption from solution involving various liquid (and a few solid) solutes and a large number of solvents differing widely in their physical and chemical properties in order to show the relation between the solubility of liquids in one another and the curvature of the Furface between the two liquids at equilibrium when the solubility is measured. That the above relation can be ascertained by a study of the adsorption from solution by silica gel, is readily understood when one remembers that silica gel i s a net work of capillaries formed by the juxtaposition of very small spherical particles of more or less hydrated silica3. These capillaries are obviously T‘-shaped, and therefore the magnitude of the radius of curvature of a liquid surface formed by an adsorbed liquid in these capillaries depends upon the amount of liquid adsorbed. As the capillary forces are stronger the smaller the capillary, a very small amount of liquid adsorbed means that the liquid surfaces are very concave. As an example, let us take a capillary system as above containing, say, water, and immerse this system in kerosene. By virtue of the fact that the water presents highly concave surfaces to the kerosene, the water’s solubility in the kerosene will now be less than if the water were allowed to dissolve in the kerosene from a plane surface. Now let us take the opposite case which is the baeis of the experiments subsequently performed. This time an “empty” capillary system is immersed in a saturated solution of water in kerosene. Here, some water, which wets the capillary, will “condense” or separate out in the capillaries, and the same equilibrium point as in the first case will be reached. That is to say, water is adsorbed from the solution by virtue of the capillary forces bringing about a phase separation. Thus, one observes the opposite state of affairs to that which has received the greater amount of attention. A study of the higher vapor pressure of small drops and the greater solubility of small particles is no more important than the opposite case we have here-that of a liquid in a small capillary presenting correspondingly a lower vapor pressure and a lower solubility. The following experiments also show that many of the generalizations in regard to adsorption from solution-such as those stated by Freundlich4 based on early work by van Bemmelen5 and others-will have to be greatly Contribution from the Chemical Laboratory of Johns Hopkins University. Sational Research Fellow. 3 Patrick and McGavack: J. Am. Chem. SOC. 42,946 (1920). “Kapillarchemie,” 259: Second edition. 6 “Die Absorption.”
17'. A. PATRICK AKD D. C . J O S E S
2
modified. For example, the idea that adsorption is greater from solvents having the higher surface tension is not borne out in the following experiments. In fact, this erroneous idea is responsible for the little work that has been done on adsorption by silica gel from liquids other than water. For, it was assumed that since there is very little adsorption from water-a liquid having a very high surface tension-there would be still less from organic liquids, which generally have much lower surface tensions. It will be shown, on the other hand, that there is great adsorption by silica gel from organic liquids. Experimental Preparation of Materials The silica gel was prepared in the usual manner from iodium silicate and hydrochloric acid.l After activation it contained 6.2% water. .A small amount of acid is usually present in gels prepared in this manner. In this case the hydrochloric acid was estimated by passing a stream of air over the gel heated in an electric furnace, and then through tubes containing KI and KIOS solution. The iodine liberated was determined a t various tempera tures. No acid came off below 700'C, and the total acjd content, released mainly a t 900'-1000' was but 0.14%. The formic acid was dehydrated by PzOb followed by distillation under diminished pressure.2 The purified acid had a M.P. of 8.3'c. The acetic acid was purified by several distillations in an all galss apparatus through a 6-column Young evaporator still-head. This acid was further dehydrated by fractional freezing until the M.P. was 16.6'. The n-butyric acid was dehydrated and d'stilled through a 12-column Young still-head. The acid finally obtained had a M.P. of - 5 . 2 ' and a critical solution temperature3 with water of -3.5'. The carbon disulfide, chloroform, toluene and carbon tetrachloride were purified by distillation until the various fractions gave the same ternary critical solution temperatures4. (This method consists in finding the C.S.T. of three liquids, one of which is the one whose purity js desired, the other two (pure) are selected so as to give a convenient temperature. In the case of toluene, for example, the latter was added to an acetic acid water mixture of certain concentration. By observing the C.S.T. according to the method described by D. C. Jones, one can test the purity of the toluene, or, if the curves were originally determined and the impurity known, one can estimate the amount of impurity. Small amounts of substances, especially those very dissimilar in chemical nature from the liquid in which they are dissolved, cause a great alteration in the C.S.T.) Nitrobenzene was purified to constant M.P. of 5.9'. Similarly the benzene was finally obtained with a melting Patrick and McGnvack: loc. cit. 2D. C. Jones: J. SOC.Chem. Ind. 1919, 362T. 3 Faucon: Ann. Chim. Phys. 19, 84 (1910). 4 D. C. Jones: Ph.D. Dissertation, Johns Hopkins University (1921). 1
ADSORPTIOK F R O M SOLUTIOS
3
point of 5 -4 5 . ' The petroleum products were purified by chemical treatment and then by prolonged treatment with silica gel itself. All solvents were finally shaken for long periods with silica gel before using. The gasoline used in the experiments had a boiling point of 60'- 70'C and the kerosene a boiling point of 2 70' - 2 90°C. The experimental manipulations necessary to obtain the data for the adsorption isotherms were of a simple kind. I n general, a series of solutions of varying concentration and of definite volume ( 2 5 or j o cc.) were made up in bottles and a definite weight of gel was then added. The bottles were transferred to a shaker in an air thermostat maintained at 2 7'. It was found that 24 hours was sufficient to attain equilibrium. In certain cases the concentrations were found unchanged after six months from the values attained after 24 hours.
Methods of Analysis In a certain number of cases it was possible to estimate the concentration by some titration process, but this method is limited in scope and other methods of analysis had to be employed. For the estimation of various acids, titration by N/2o or N/IOObaryta was used. The iodine was estimated by very dilute thiosulfate solution. The Zeiss interferometer was used to determine sulfur in benzene, and nitrobenzene in petroleum, while ternary critical solution temperature methods (see above) were used to estimate sulfur in carbon disulfide, benzene in petroleum and n-butyl alcohol in benzene. Where this latter method is available and sufficiently delicate, i.e.-where sufficient difference in solubility exists, the method is convenient and necessitates only the simplest type of apparatus.l Expression of Results I n the accompanying tables C, expresses the initial concentration of the solution in mols per litre and C represents the equilibrium concentration in the same units. In all cases the ordinary formula weight is taken, i.e.-no association effects are considered. A represents the millimole solute adsorbed per gram of gel, and is in all cases found by dividing the amount of solute lost to the solution (as found by above analytical methods) by the weight of gel added. It is to be noted that the quantity A is strictly empiric; that is to say, no attempt has been made to take into account any effects produced by the possible adsorption of solvent or changes in volume brought about by the removal of the solute. However, when adsorption takes place in moderately dilute solutions, which is usually the case, the above two effects are so small that for all practical purposes the factor A can be regarded as equivalent to the a of the Freundlich adsorption equation I
a = KCll D. C. Jones: loc. cit.
4
W. A. PATRICK AND D. C. JONES
TABLEI Adsorption of formic, acetic and n-butyric acid from various solvents by silica gel. Formic acid from toluene e o C A 1.034 1.034 0.607 1.034 0.245
0.601 0.348 0.450 0.104 0.138
0.150
0.072
0.059
0.146
5.56 4.36 4.89 3.02 2.85 2.52 I .46
Formic acid from nitrobenzene 5.220 1.128 0.583 0.322
4.700 0.898 0.419 0.191
0.176
0.100
0.124
0.041
5.19 9-93 2.19 I ..63 1.19 0.80
Acetic acid from gasoline 2.980 1.370 0.678 0.362 0.198 0 . I35
2.660 1.116 0.441 0.195 0.053
3.91 3.56 3.14 2.44
0.021
2.00
2.25
Acetic acid from toluene e o C A 1.913 1.780 2.28 0.640 0.266 0.116 0.043
0,498 0.138 0.028 0.010
2.18 1.75 1.40 0.53
Acetic acid from nitrobenzene 3.550 1.483 0.713 0.241 0.068 0.055
3.171 I ,316 0.585 0.140 0.038 0.028
3.47 2.06 1.64 1.43 0.63 0.55
Acetic acid from carbon disulfide co C A 0.071
0.171 0.340 0.693 1.405 3.167 3.845 3.991 5.441 6.710 8.027 8.066 9.778 12.450 15.410
--
0.040 0.183 0.448 I . 115 2.853 2.893 3.385 4.628 5.970 7.205 7.475 9.170 11.925 15.320
1.18 2.15
2.48 3.26 3.90 3.95 3.82 3.55 3.55 3.03 2.36 2.38 2.IO I .23 0.38
Acetic acid from carbon tetrachloride 3.470 1.330 0.710 0.337 0.168 0.073 0.023
3.245 I . IO0
3.04 2.66
0.441
2.70
0 . I95
2.33 2.08 I .08 0.35
0.055 0.001
--
N-butyric acid from kerosene eo C A 1.358 0.547
1.120
0.131
0.353 0.156 0.068
0.021
0.002
0.272
4 . I3 3.41 2.09 2.07
0.67
N-butyric acid from gasoline 2.480 1.050 0.270
2.280 0.846 0.126 0.016
1.93 2.33 2.04 I . 70
0.126 0.054
0.0008
1. 2 0
0.015
0.0001
0.34
5
ADSORPTION FROM SOLUTION
TABLE I [continued)
N-butyric acid from toluene
Acetic acid from kerosene 2.086 1.055
0.698 0.361 0.178 0.085
1.786 0.745 0.445 0.130 0.008
4.74 4.54 3.81 3.13 2.13
0.001
1.25
.36
I . 130 0.086
I
0.113
0.029
0.022
0.73
I.2 5 0
I .oo
TABLEI1 Benzene from kerosene
Nitrobenzene from kerosene
C
c o
1.150
0.962 0.775 0.580 0.785 0.386 0.272
0.186 0.084
0.952 0.705 0.517
0,365 0.482 0.201
0.987 0.046
--
co
A 2.44 2.44 2.36 2.19 2.19 I .98 1.62 1.54 0.87
C
A
1.025
0.748
1.21
0.588
0.320
1.19
0.269
0.013
0.127
--
I . 23
0.86
TABLE111 Benzoic acid from carbon tetrachloride Benzoic acid from benzene 0.688 0,674 0.311 0.156
0.660 0.628 0.266 0.114
0.078
0.052
0.036
0.019 0.009
0.012
0.74 0.70
0.484 0.456
0.72
0.084
0.452 0.365 0.039
0.63
0.178
0.110
1.11
0.45 0.30
0.028
0.007
0.66
0.12
Benzoic acid from chloroform 1,340 0.307 0.146 0.073
Benzoic acid from kerosene
0.125
0.67 0.45 0.37
0.063
0.25
I
.300
0.275
1.29 1.19 0.92
0.058
0.024
0.008
--
0.93 0.48
TABLEITT
Iodine from kerosene
C
C, ,0128 '
0043
7
,001
.OI22
.0039 .oo16
A '0059 ,0024 ,0016
Iodine from carbon tetrachloride
CO '0954 .o40o .OI79
C ,0946 .0397 .01 78
A .0039 .0024
. 001 2
6,
W. A. PATRICK AND D. C. J O N E S
Preliminary Qualitative Experiments Before discussing the results given in the accompanying curves, it is necessary to describe a few qualitative experiments of a very simple nature which throw light upon the solubility relationships of liquids in connection with the adsorption phenomena. A definite weight of gel was added to a definite volume of liquid A in a graduate cylinder. A second liquid B, heavier than A, was then added until the gel was completely immersed in this liquid. In many cases it was found that liquid A was ejected from the gel to a greater or less extent and its volume could be read off above liquid B. In this way, water completely replaced kerosene. Xtrobenzene ejected I O cc. out of 11 cc. originally present of kerosene. Water replaced 4 cc. out of I O cc. of n-butyl alcohol. Formic acid ejected 9 cc. out of I O cc. of kerosene.
FIG.I Adsorption of Acetic Acid
Discussion of Results Several outstanding facta will be immediately noticed upon an inspection of the curves: I . Contrary to the idea that very little adsorption would take place from organic solvents which have relatively low surface tensions, it is seen that adsorption does take place to a very marked extent, and that the amount adsorbed from these solvents bears no relation to their surface tensions. From Fig. I in the case of acetic acid, the greatest adsorption occurs from kerosene, and becomes less and less in the order CS2, gasoline CClr, C&b,C&, C6H&O2; while the surface tensions of these substances, respectively, are 26, 3 2 , 15, 2 5 , 29 and 43. 2 . In the case of the acids, including benzoic, the solvents are arranged in the same order.
ADSORPTION FROM SOLUTION
7
3. The petroleums, which have the lowest surface tensions, are the solvents from which the greatest adsorption occurs. 4. In general, adsorption of a solute increases as its solubility in the solvent decreases. For example, the adsorption of benzoic acid from the four solvents is in inverse order of its solubility in these solvents. Likewise formic acid, much less soluble in toluene than butyric acid, is much more strongly adsorbed. Iodine js adsorbed, to a small extent it is true, according to the same laws. The adsorption of sulfur in benzene and CS2 was also studied. I n the first case a small negative result was found, while with the second system no effect was observed. Again, nitrobenzene is adsorbed to a very great extent from kerosene, with which it is only partially miscible, while benzene, much
FIG.2 Addsorptionof Foimic and S- Butyric dcids
closer to kerosene in the solubility series, is adsorbed to a considerably less extent. The benzene-kerosene curve (Fig. 3) is peculiar in that it does not resemble the other curves,-in fact, it does not resemble an adsorption curve a t all, but rather suggests a chemical reaction between benzene and the gel. It is noticed that the system acetic acid-carbon disulfide (Fig. 4) was investigated throughout the entire range of concentration. One must remember, however, that the amount adsorbed per gram gel is reckoned by a change in concentration of the solution, and therefore in a 100% acetic acid or 1007~ CS2 solution, the term A is regarded as being equal to zero. By definition, therefore, a maximum of the curve must necessarily follow. Rut it is significant that the maximum occurs a t an equilibrium concentration of about 3 mols per litre, which is equivalent to about an 876 solution of acetic acid in carbon disulfide. After changing the concentration of the solution to the extent of this maximum value, why should the gel, beginning with an equilibrium concentration of be less and less effective in changing the concentration of the solution as the percentage of acetic acid becomes
Sx,
8
W. A. PATRICK AND D. C. JONES
greater? The cause of this lies undoubtedly in the phase compositions. Apparently there is a concentration range where the adsorbent does most work in altering the concentration of the solution. Before it reaches this point it is only the finer capillaries Le.-those that give opportunity for very great concave curvature, that can withdraw acetic acid from solution. After the maximum is passed, less and less work i s required of the adsorbent, because the exterior phase and the interior adsorbed phase are approaching one another in composition. In the case of adsorption of nitrobenzene from kerosene, we have another phenomenon which is not in agreement with the ordinary accepted ideas of adsorption. For here, not only has kerosene a very low surface tension, but nitrobenzene, dissolved in kerosene, tends to raise the surface tension of the latter. n’evertheless, nitrobenzene is adsorbed very strongly from kerosene.
FIG.3 Millimols adsorbed per Gram Gel. (a)
A rigid application of the Gibbs’ theory to the experiments under consideration here, would have to take into account not the surface tension of the solvent against air or vapor, but the interfacial tension between the solvent and the gel. Since, however, the gel contains water, and in all probability presents a water surface to the solvent, one has to do with a water-solvent interface. But even if this were not the case, and the system was comprised of, say, a silica-solvent interface, the Gjbbs’ generalization would still apply relatively; for, since solids present enormous surface tensions, the interface in question-silica-solvent-would be greater than either a water-solvent or air-solvent interface. Kow, assuming a water-solvent interface (since in studies with silica gel it is apparent that a water layer surrounds the silica particles), in the case of adsorption from kerosene, a water-kerosene interface exists. This is one of the largest known liquid-liquid interfaces. Since most liquids have a larger surface tension than petroleum (about 2 0 ) and a smaller tension than water
ADSORPTION FROM SOLUTION
9
( 7 j), this interface is reduced in two ways when a solute concentrates (i.e. is adsorbed) in the adsorbent. This partly explains the great adsorption observed in all cases from kerosene. However, from the facts discussed above, and from the large adsorption effects observed in some cases-3syG of gel weight-it is obvious that the Gibbs’ theory cannot entirely account for the adsorption in the case of silica gel,-nor do the generalizations of Freundlich largely apply. As above indicated, adsorption-at least in the case of silica gel-can best be accounted for by lowering of the solubility of a solute in its solvent due to the highly concave surfaces it (the solute) presents to the solvent when concentrated in pores of the adsorbent-which pores first of all the solute preferentially wets.
FIG.4 Complete Adsorption Isotherm HAC-CS2
KO exception has been found to the generalization that greater adsorption always follows lower solubility of the solute adsorbed in the solvent. The complete ejection of kerosene from kerosene-saturated gel by water may be taken as an extreme case. As an example of the application of the above mechanism, let us take the case of the adsorption of acetic acid from CSZ. On the introduction of the gel to such a system, the acid (which, in preference to the CS2, wets the gel) is adsorbed as a Gibbs’ layer owing to its lowering of the interfacial tension gel-CSz. The gel now fills up with a phase rich in acetic acid due to the marked concave curvature that this phase presents to the body of the solution; i.e.-a phase separation is induced by the presence of the capillaries. Exactly the same equilibrium position (if the concentration were chosen correctly) would be reached by first filling the capillaries with acetic acid, and immersing in CS2. I n other words, acetic acid, although miscible in all proportions with CS2 when the surfaces are plane, no longer is so if the cur-
IO
W. A. PATRICK AKD D. C. JOSES
vatures of the separating surfaces are sufficiently concave. This is merely applying to adsorption from solution our ideas, largely enunciated by Patrick and McGavack,l as to the mechanism of adsorption from a gaseous phase. I n this latter case, analogously, a gas is condensed in capillaries &e.-adsorbed) a t pressures lower than the saturation pressure at the temperature, due to the presentation of highly concave surfaces of liquefied gas-in the pores of the gel which is wetted-to the main body of gas. I n a few words, then, the state of affairs in general is as follows: liquids presenting highly concave surface$, can exist in contact with vapor above the critical temperature, and in contact with another liquid, well above the critical solution temperature. The above conception leads to some interesting conclusions concerning what is ordinarily termed solubility. Since acetic acid is adsorbed .from carbon disulfide-with which it is assumed to be completely miscible, it follows that, a t least under certain circumstances, acetic acid possesses a definite solubility in carbon disulfide. I n a later paper, it will be shown how “solubility,” considered from this standpoint, can be calculated from adsorption formulae. Furthermore, the above experiments indicate a possible method by which adsorption from solution may be calculated from adsorption from the vapor state and vice versa. This would be important and necessary in those cases where adsorption from the vapor or the liquid would be difficult or impossible to measure. For, neglecting implications involved in the so-called van Schroeder’s paradox-which has been largely disproved,2 to calculate adsorption from solution from measurements of adsorption from vapor, all that would have to be known would be the vapor pressure-composition curves of the two liquids involved, ( i e . how nearly the liquids obey Raoult’s Law) which would immediately give the equilibrium concentration (C) of the solution which is in equilibrium with the amount of solute adsorbed (A) either from the vapor or the liquid. It must be remembered, of course, that the phenomena of preferential wetting applies to the vapor state as well as to the liquid. Summary I . The adsorption from solution by silica gel has been investigated in the following systems: formic acid, acetic and butyric acid in a series of solvents; nitrobenzene and benzene from kerosene; benzoic acid and iodine from a series of solvents; and acetic acid from carbon disulfide throughout the entire range of concentration. 2. A discussion of the results is given, leading to the conclusion that adsorption by silica gel is due to a phase separation in the capillaries, caused by preferential wetting followed by the production of highly concave surfaces of solute which brings about, a lowering of the solubility of the solute in the solvent. J. Am. Chem. SOC.42, 946 (1920). Freundlich : “Kapillarchemie,” 925.
