SORPTION BY SILIC.4 GEL
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All previously reported photopotentials of silver-silver halide electrodes are complex functions of the above effect, plus the oxidizing action of free halogen or hypohalite on the metal, and hence almost impossible of exact interpretation or control. REFERENCES (1) (2) (3) (4) (5) (6) (7)
ATHANASIU, M. G . : Ann. Physik 4, 377 (1935). AUDUBERT, R . : Compt. rend. 179, 682 (1924). GARRISOK, A , : J. Phys. Chem. 28, 333 (1924). H . L.: J. Am. Chem. SOC.69, 416 (1937). KOLTHOFF, I. M., A K D SAXDERS, SANDERS, H . L . : Thesis, McGill University, Montreal, 1937. TUCKER, C. W . : J. Phys. Chem. 31, 1357 (1927). VANSELOW, W . , A N D SHEPPARD, S. E . : J. Phys. Chem. 33, 331 (1929).
APPLICATION O F T H E HYDROGEN-BRIDGE THEORY TO SORPTION FROM SOLUTION BY SILICA GEL1 ALBERT L. ELDER
. ~ N DROBERT
A. SPRINGER
Dcparlment of Chemislry, Syracuse University, Syracuse, New York Received Augwt 7 , 1939 111 this paper some data are presented on sorption from solution, ixivolving various solvents and solutes with silica gel as a sorbent. From the experimental facts an attempt has been made to draw some conclusions regarding the theoretical aspects of sorption phenomena. The principle of hydrogen bridging will be introduced as an explanation for the abnormalities observed in sorption work with silica gel.
EXPERIMENTAL
The manipulations necessary to obtain the data were very similar to those used by other investigators in the same field. In general, a series of acid solutions of varying concentration were made up in one of the organic solvents. Fifty cubic centimeters of each solution were introduced into a glass-stoppered flask and 5 g. of Patrick's silica gel (10 to 20 mesh) were added. The gel was previously washed five hundred times with distilled water in a Soxhlet apparatus and activated a t 400°C. The flasks were then transferred to a shaking machine kept a t 20°C. After 48 hr. of shaking, the flasks were allowed to stand in the bath until the gel had settled 1 This article is based upon a thesis submitted by Robert A. Springer to theFaculty of thc Graduate School of Syracuse University in partial fulfillmcnt of thc rcquiroments for the degree of Master of Arts, June, 1939.
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ALBERT L. ELDER AND ROBERT A. SPRINGER
out. Twenty cubic centimeters of the supernatant liquid were removed and titrated to phenolphthalein end point with base to determine the equilibrium concentration. DISCUSSION OF RESULTS
I n the accompanying figures the letter C expresses the equilibrium concentration of each solution in moles per liter. The letter A represents the milliequivalents of solute sorbed per gram of gel. Figures 1, 2, and 3 express a comparison of various acids in a certain solvent. Figures 4, 5, and 6 represent results obtained by using a single acid and various solvents. It is apparent that sorption of the acids diminishes as the number of carbon atoms in the acid increases, Le., acetic > propionic > crotonic > benzoic > palmitic. Also, sorption of the solute decreases with change in solvent: carbon tetrachloride > toluene > nitrobenzene > dioxane > water. I n most cases the amount sorbed from water is so small that a good comparison cannot be shown by graphical methods. Neither has all the work with dioxane been shown, for sorption was very poor. An abnormality is observed in the case of crotonic acid in carbon tetrachloride and toluene solutions. I n greater concentratipns it is less sorbed than propionic acid, but this is reversed a t low concentrations. It is very apparent that two questions must be answered to account for the sorption of organic acids from solvents. First, using the same solute, why is a greater amount sorbed from one solvent t h a n from another? Second, why is one organic acid sorbed in a greater amount than another from the same solvent? On the basis of the gaseous sorption theory, Patrick and Jones ( 5 ) explained sorption from solution. However, many of the results in this paper could not be accounted for by their deductions. Mention has also been made in the literature by Holmes and McKelvey (1) that sorption is a function of polarity. Nevertheless a good correlation between sorption and dipole moments cannot be made. Thus there must be something more fundamental which has not as yet been mentioned in the literature in connection with sorption. I n 1936 Huggins (2)published an excellent review on hydrogen bridging, in which he discussed the work of others and greatly extended the theoretical aspects of bonding. According to this theory, a proton can form a bond between two electronegative atoms. It is, therefore, an application of this association phenomenon that we wish to suggest as an explanation of sorption from solution by silica gel. Silica gel, activated so that i t contains about 9.0 per cent water, has been found to have a maximum sorptivity (4). If almost all of the water
945
SORPTION BY SILICA GEL
3
2 A
A
I
C
C
FIG.1. Adsorption of organic acids
FIG.2. Adsorption of organic acids
from carbon tetrachloride. 1, acetic acid; 2, propionic acid; 3, crotonic acid; 4, benzoic acid; 5, palmitic acid.
from toluene. 1, formic acid; 2, acetic acid; 3, propionic acid; 4, crotonic acid; 5, benzoic acid; 6,palmitic acid.
C
C
FIG.3. Adsorption of organic acids
FIG.4. Adsorption of acetic acid from
from nitrobenzene. 1, acetic acid; 2, propionic acid; 3, crotonic acid; 4, benzoic acid.
various solvents. 1, carbon tetrachloride; 2, toluene; 3, nitrobenzene; 4, dioxane. 3
3
2
2 A
A
I
I
0 C
C
FIG.5 . Adsorption of propionic acid from various solvents. 1, carbon tetrnchloride; 2, toluene; 3, nitrobenzene.
from vsrious solvents 1, carbon tetrschloride; 2, toluene; 3, nitrobenzene.
FIG.6. Adsorption of crotonic arid
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l L B E R T L. ELDER AND ROBERT A. SPRINGER
has been driven out, the structure is changed and the gel is of no use as a sorbent. For this reason, it has been assumed by investigators (3) that one is not dealing with silicic anhydride containing small amounts of water, but with silicic acid itself. Substantially, this means that hydroxyl groups are present. In a gel containing 9 per cent of water by weight, there is approximately one molecule of water for every three molecules of silicon dioxide. Assuming that the water adds to the silicon dioxide with formation of hydroxyl groups, this would permit two hydroxyl groups for every three silicon dioxides. Since it is practically certain that the structurc. of the gel contains capillary openings which held the water before activation, it follows that many of these hydroxyl groups are necessarily on the surface of the capillaries. Yet it cannot be inferred that all the hydroxyl groups are on the surface, since there are so many of them; rather, they must be distributed throughout the network structure (otherwise, complete dehydration would not alter the structure). Thomas (6), in his theory of colloidal systcms, suggests that the particlw are aggregates of molecules held together by covalent linkages in a regular pattern. The coagulation of silicic acid, followed by dehydration, would allow the hydroxyl groups to occupy definite positions in the silica-gel structure. The fact that hydroxyl groups are present makes hydrogen bridging possible. The following points indicate that these bridges should be fairly stable. Silicon is a fairly metallic element and does not attract electrons of other elements very strongly. In silicic acid the electrons joining the silicon and oxygen will lie close to the oxygen. This would necessarily make the oxygen highly electronegative : thus it would have a strong attraction for protons and should therefore lend itself readily to hydrogen-bridge formation. From the above discussion, it can be seen that hydrogen bridging is possible with silica gel. The following mechanism is suggested: A pair of electrons from an oxygen in the gel structure is bound to the labile hydrogen of the organic acid. In turn a pair of electrons from the oxygen of the carboxyl group is associated with the hydrogen in the hydroxyl group of the gel. This latter bridge is not as strong as the first one, because the hydrogen of the hydroxyl group in the silica gel is so tightly bound to its oxygen. Consequently, the bridge from the oxygen of the carboxyl group to the hydrogen of the silica gel is very unsymmetrical, whereas the bridge from the hydrogen of the carboxyl group to the oxygen of the silica gel should be fairly symmetrical. A planar representation of this is: -Si
*H-o \
C-CHa
SORPTION BY SILICA GEL
947
An attempt must be made to interpret the experimental results on the basis of the proposed theory. However, as will be shown in detail later, hydrogen bridging alone will not suffice to account quantitatively for all of the experimental results. It is necessary to attribute part of the effect to molecular size, and also to take into account the relative extent of formation of hydrogen bridges by solvent and solute. The association of carboxyl groups with the hydroxyl groups of the gel will account qualitatively for the sorption. The quantitative results can also be accounted for to a certain extent. The tendency to form hydrogen bridges is in the following order: formic > acetic > propionic > palmitic. This is also the order of extent of sorption (figure 2). In degree of formation of hydrogen bridges, benzoic acid fits into this series between formic and acetic acids, but in extent of sorption it lies between propionic and palmitic acids. This discrepancy may be explained by assuming that molecular size and shape are factors in the sorption. Benzoic acid, being a bulky molecule in contrast to the linear aliphatic acids, is more difficultly sorbed. It may also be pointed out that the entire series, benzoic acid included, is in the order of molecular size. This factor may bc even more important in determining the extent of sorption than the relative tendency to form hydrogen bridges. This idea receives some support from the behavior of crotonic acid. At the higher concentrations in all solvents, crotonic acid is less sorbed than propionic acid, which fits in with the order of molecular size but not with the order of tendency to form hydrogen bridges. In respect to formation of bridges, crotonic acid isvery nearly equal to acetic acid. However, a t low concentrations in solvents which themselves are only slightly sorbed by silica gel, such as toluene and carbon tetrachloride, crotonic acid is more highly sorbed than propionic acid (figure 6). It seems reasonable to assume that the relative tendency to form hydrogen bridges would be more important a t low concentrations, where only the more available surfaces of the gel are involved and molecular size is not such an important consideration. This reversal of extent of sorption does not occur in nitrobenzene solution (figure 3), where crotonic acid is less sorbed than propionic acid a t all concentrations. This anomalous behavior will be explained in the following paragraph. Hydrogen bridging has been offered to account for differences in sorption of various organic acids by silica gel from any one solvent. Differences in behavior of the same acid in various solvents must now be explained. This is obviously an effect of the solvent, and can be accounted for on the same basis as the differences in sorption of different acids. For if one solvent is more sorbed by silica gel than another, then obviously an acid dissolved in the first solvent will be less sorbed than one dissolved in the second, owing to greater saturation of the gel by the first solvent. This liypothcsis is fully borne out by the experimental results. Sorption for
948
ALBERT L. ELDER AND ROBERT A. SPRINGER
any acid from the following solvents decrease8 in the order carbon tetrachloride, toluene, nitrobenzene, dioxane, and water (figure 4). This is also the order of increasing sorption of the solvent by silica gel, according to the hydrogen-bridge theory. Furthermore, the sorption of crotonic acid from nitrobenzene bears out the hypothesis of hydrogen bridging. It has been mentioned that in nitrobenzene solution crotonic acid is less sorbed than propionic acid a t all concentrations, while in carbon tetrachloride and toluene the crotonic acid is more sorbed at low concentrations and less at high concentrations. The low sorption a t low concentrations in nitrobenzene may be explained by the assumption (which appears quite reasonable) that sorption of the nitrobenzene itself by the silica gel produces exactly the same effect as would a higher concentration of the acid. If this be so, it is possible that the crotonic acid might be more highly sorbed at very low concentrations. This range has not been investigated. In a discussion of this paper with Dr. Maurice L. Huggins of the Eastman Kodak Company, it was suggested that there be indicated more specifically the competition effects produced in the system, i.e., bridging between solute molecules, between solvent molecules, and a competition of solute molecules for solvent molecules. The structure of the gel presents the possibility of a partial bridging between its hydroxyl groups. When a solution is put in contact with the gel, in addition to the bridging effects produced in the solution and within the gel itself, there is a gel-solvent and gel-aolute competition set up. It is evident then that the amount sorbed by the gel is dependent on the equilibrium set up between these different bridging effects. This in turn depends on the strength of the different bridge formations. It is evident also that the breaking of the bridges in the gel, permitting other bridge formations, is of some consequence. SUMMARY
1. Data have been presented on the sorption of formic, acetic, propionic, crotonic, benzoic, and palmitic acids from solutions in carbon tetrachloride, toluene, nitrobenzene, dioxane, and water by use of silica gel. 2. An hypothesis has been advanced to account for the results obtained, based on the formation of hydrogen bridges. 3. A semi-quantitative explanation of the results depends on a t least three factors: ( a ) the relative extent of hydrogen bridging by the different acids; ( b ) the size and shape of the acid molecules; and (c) the relative extent of formation of hydrogen bridges by solute, solvent, and gel. 4. The presence of an unsaturated linkage in an aliphatic acid gives rise to certain abnormalities in the sorption, which afford further proof of the correctness of the hydrogen-bridge hypothesis.
COMMUNICATION TO THE EDITOR
949
REFERENCES (1) HOLMEB, H. N., A N D MCKELVEY, J. B.: J. Phys. Chem. SI, 1522 (1928). (2) H U ~ G I N M. B , L.: J. Org. Chem. 1, 407 (1936). (3) MAGNUB, A., A N D KIEFFER, R.: z. anorg. Chem. 179,215 (19%). (4) MCGAVACK, J., A N D PATRICK, W. A. : J. Am. Chem. SOC.42,946 (1920). (5) PATRICK, W. A., A N D JONES,D . C.: J. Phys. Chem. !29, 1 (1925). (6) THOMAB, A.: Lecture given before the Syracuse Chapter of Sigma Xi, in 1939.
COMMUNICATION TO T H E EDITOR PHOTOCHEMICAL REDUCTION OF FERRIC IRON BY OXALIC ACID
The paper on “The Photochemical Oxidation of Oxalic Acid Sensitized by Ferric Ion,” published by R. Livingston (J. Phys Chem. 44, 601 (1940)),contains a statement which I should like to support by additional evidence. The results on the photochemical reduction of ferric ion by oxalic acid, as published in 1928 by E. Mencke and myself (Z. Elektrochem. 34,598 (1928))must be discarded. In December, 1936,I repeated this investigation with Dr. H. Hoch in Vienna. We found the permanganate titration not reliable enough and adopted the titration with titanium trichloride, used by Allmand and Young (J. Chem. SOC.1931,3079). Our experiments showed (1)that the velocity of the reaction was approximately proportional to the first power of the light intensity, which result was in agreement with the findings of Allmand and Young but contrary to those of Kornfeld and Mencke, who stated a dependence on the square root of the light intensity, and (3) that the reaction proceeded until the end, regardless of the presence of ferrous ions. This result also agreed with the findings of Allmand and Young and disagreed with those of Kornfeld and Mencke. For external reasons the investigation was not completed, and publication of the results was postponed. Since I have not been able to resume the investigation I should like to clarify the issue now, in connection with Dr. Livingston’s paper. GERTRUDKORNFELD. Research Laboratories, Eastman Kodak Company, Rochester, New York.
