BASE EXCHANGE OF MONTMORILLONITE
65
REFERENCES
(1) ABRAMSON,H. .4.: J. Phys. Chem. 36, 289 (1931). (2) BRACNING, J. F.: S. M. Thesis, Massachusetts Institute of Technology, 1940. (3) DANNENBERG, E. M.:S. B. Thesis, Massachusetts Institute of Technology, 1939. (4) FREUNDLICH, H., SCHMIDT, O . , AND LINDAW, G.: Kolloid-Beihefte 38, 43 (1932). (5) HAOSER, E.A,, AND LE BEAU,D. S.: J. Phys. Chem. 42, 1031 (1938). (6)HAUSER, E.A., AND LE BEAU,D. S,: J. Phys. Chem. 43, 1037 (1939). (7)HAUSER,E. A., AND LYNN,J. E.: Ind. Eng. Chem. 32, 659 (1940). (8) HAUSER, E.-4.,A N D REED,C. E.: J. Phys. Chem. 40,1169 (1936). (9) HAWSER, E. A,, AND REED,C. E.: J. Phys. Chem. 41, 911 (1937). (lo)HAUSER,E. A., AND SCHACHMANN, H.: J. Phys. Chem. 44, 584 (1940). (11)HOFMANN, E., AND BILKE,W.: Kolloid-Z. 77, 238 (1936). (12) HOFMANN, U.,ENDELL,K . , AND WILM,D.: Z. angew. Chem. 47, 539 (1934). (13) JENXY,H., AND REITEMEIER, R. F.: J. Phys. Chem. 39, 593 (1935). (14)KELLEY,W. P., AND JENNY,H . : Soil Sci. 41, 367 (1936). (15) MARSHALL, C. E.: J. Phys. Chem. 41, 935 (1937). (16)MWELLER, H.: J. Phys. Chem. 39, 743 (1935). (17)XORTHRWP, J. S . , AND KWNITZ, M.: J. Gen. Physiol. 7, 729 (1925).
BASE EXCHANGE OF T H E CLAY 1IINERAL MONTMORILLOXITE FOR ORGAXIC CATIOXS AXD ITS DEPEKDEXCE UPON ADSORPTION DUE TO VA?; DER WAALS FORCES' STERLING B. HEXDRICKS Bureau of Plant Industry, U . 5. Department of Agriculture, Washington, D . C . Received July PI, 1940
The clay mineral montmorillonite shows base exchange not only for positive inorganic ions, but also for large organic cations. Its nicotine salt, which is used as an insecticide, has been described in some detail (11). Gieseking (1) has prepared a number of organic salts of montmorillonite and has measured their characteristic (001) interplanar spacings. His results indicate that the replaceable positive ions are located between the individual hydrous aluminum silicate layers (7) of which the mineral is formed. Large organic ions are held more firmly by the clay than are inorganic positive ions, even including hydrogen ion. Their presence markedly alters the dispersibility of the clay and practically destroys the swelling of the lattice in water. Gieseking did not find any correlation between the (001) interplanar spacing of a montmorillonite salt and molecular dimensions of the organic cation.
* Presented a t the Seventeenth Colloid Symposium, held a t Ann Arbor, Michigan, June 6-8,1940.
66
STERLING B. HENDRICRS
The properties of these organic clay compounds give considerable information about the mechanism of ionic exchange, not only for montmorillonite but also for other substances, such as zeolites, permutites, and proteins. It is not the purpose here to discuss this question but rather to extend the experimental observations and to show how the results obtained depend upon molecular dimensions. METHODS AND MATERIALS
Previous work on the hydration of inorganic montmorillonite salts (5) showed that materials from various sources were similar in behavior; for this reason all the compounds described here were prepared from a single analyzed sample which was kindly furnished by Dr. P. G. Nutting of the U. S. Geological Survey.2 The free acid of this montmorillonite sample was prepared by treatment of the clay for short times (30 min.) with successive lots of dilute (1:50) hydrochloric acid in large excess; the excess hydrochloric acid was removed by washing with water. The base-exchange capacity of the clay, as determined by displacement of hydrogen ion with barium ion by successive leaching with a tenfold excess of barium chloride solution, was between 0.90 and 0.94 milliequivalent per gram. The organic compounds used in the work were either commercial materials that had been purified if necessary, or were prepared by standard methods. Dr. G. E. Hilbert supplied the purine compounds and nucleosides. Montmorillonite salts were made either by neutralizing the acid clay with the appropriate organic base or by treating it with a large excess of the organic salt in water solution. The solution was heated to boiling and then shaken for 30 min. The compound finally was separated by centrifuging and washed with water and alcohol to remove any excess amine, if an excess had been used. The nitrogen contents of the montmorillonite substituted ammonium salts were determined by Mildred S. Sherman of this laboratory by micro-Kjeldahl methods. The majority of the (001) interplanar spacings were measured on unoriented samples that had been dried over phosphorus pentoxide in a vacuum and then transferred to capillaries of Lindemann glass. Some of the samples, as indicated, were also examined a t 70 per cent relative humidity after standing a t that humidity (30°C.) for several days. After it was established that water sorption a t low humidities had little effect on the (001) spacings, some of the samples were examined a t less than 20 per cent relative humidity in unprotected plaques. All photographs were made with Ni K, radiation, using a preparation-to-plate distance of 5 cm. As a rule, only one order of reflection was present from (OOl), and since this was rather broad the measured spacings are probably accurate to 0.20 A. only
.*
* Sample B: Bee page
107 of U. S. Geological Survey Bulletin No. 878 (1937).
BASE EXCHANGE OF MONTMORILLONTE
67
Water sorption as a function of relative humidity a t 30°C. was determined for four montmorillonite salts by equilibrating weighed amounts of them at various relative humidities. These samples were then subjected to differential thermal analysis according to Le Chatelier’s method, as discussed in a previous article (4). RESULTS AND PRELIMINdRY DISCUSSION
b
Cursory examination of the results listed in tables 1 to 3 and of previous data published by Gieseking (1) shows that the (001) interplanar spacing depends upon the organic cation present in the clay, but in a manner that apparently is not determined by the structure of the ion. Presumably, then, the organic molcules are situated, a t least in part, between the alumino silicate layers. Maximum base-exchange capacity per gram of montmorillonite for small ions such as aniline is about 0.90 milliequivalent, which is not significantly less than found by displacement of hydrogen ion with barium ion,--namely, 0.90 to 0.94 milliequivalent per gram. Some base-exchange capacities in milliequivalents are as follows: benzidine, 0.91; p-aminodimethylaniline, 0.90; p-phenylenediamine, 0.86; a-naphthylamine, 0.85; 2,7-diaminofluorene, 0.95; and piperidine, 0.90. In some cases, such as for primary n-amylamine, @-naphthylamine, and o-phenylenediamine, the maximum base exchange was not attained, either because a rather acid salt was used or because the clay salt was hydrolyzed by washing before analysis. Extremely weak bases like the o- and m-nitroanilines did not form salts with montmorillonite. Large molecules like the alkaloids brucine (Strychnos group) and codeine (morphine group), even though they are stronger bases than aniline and benzidine, neutralize less of the hydrogen on the clay. Thus, codeine (4 milliequivalents to 0.5 milliequivalent of acid clay in 25 cc. of water) neutralized 0.63 milliequivalent of hydrogen per gram of clay, and brucine similarly gave 0.65 milliequivalent per gram. The difference, namely, 0.30 milliequivalent, between these amounbs and the exchange capacity of the clay represents the amount of hydrogen so covered by alkaloid molecules as to be unavailable for neutralizing other molecules (cover-up effect). The above interpretation was substantiated by the fact that three successive 10-milliequivalent lots of normal barium chloride solution displaced a total of only 17 milliequivalents of hydrogen ions from a gram of the brucine clay, which includes some displaced brucine which was titrated as acid. Thus the brucine, even under this drastic treatment, still covered 0.12 milliequivalent or almost 15 per cent of the total replaceablehydrogen without neutralization. The codeine clay behaved similarly, save that 0.15 milliequivalent of hydrogen remained after exchange with barium ion.
68
STERLING B. HENDRICKS
Simiiar results were obtained by Gieseking for heptyltributylammonium iodide, which displaced only 0.80 milliequivalent of hydrogen ions from a clay having an exchange capacity of 0.94 milliequivalent per gram. I n the absence of corollary experiments, however, the difference might be TABLE 1 Interplanat spacings of montmorillonite as injluenced by humidity and combination ra'th various amounts of aromatic amines LILLTEQDIVALENlS COMPOUND
d(001) f 0.2 A.
or
NITBOOEN PEB QBAY
Aniline. . . . . . . . . . . . . . . . . . . . . . . .
12.8 14.2
0.62 0.91
@-Naphthylamine.. . . . . , . . . . . . .
13.5 13.5 13.5 15.1
0.25 0.40 0.68 0.66
a-Naphthylamine . . . . . . . . . . . . . .
14.4 15.4
0.85 0.85
o-Phenylenediamine . . . . . . . . , . .
12.4 12.6 12.4 12.4 12.9
0.11 0.30 0.71 0.71 1.37
m-Phenylenediamine.. . . . .
12.4 12.4 12.8 12.8
0.31 0.77 0.90 1.32
p-Phenylenediamine . . . . . .
12.4 12.4 13.9 13.9
0.82 0.82 1.72 1.72
p-Aminodimethylaniline . . . . . . .
13.5 13.5
1.80
13.0 15.2
1.00
Benzidine. , . . . . . . . . . . . . . . . . . . .
1.80 1.82
due to a factor influencing base exchange such that large organic molecules can not take part, as is known to be the case for crystalline zeolites and amorphous permutites. The question naturally arises whether or not both groups of an organic diamine will be effective in ne-itralizing an acid clay. This is answered
69
BASE EXCHANGE OF MONTMORILLONITE
TABLE 2 Interplanar spacings of wme diaminojluorene salts of montmorillonite YIWJEQUIVALEMIJ OF NITROGEN COMPOUND
2,5-Diaminofluorene. . . . . . . . . . .
Per pam of oby
Total in aolution
0.20 0.42 0.76
0.20 0.40 1.00 1 .MI 4.00
13.0 12.8 12.7 13.2 13.8 14.0
1.11
1.20
13.0 12.9 12.7 13.0 13.5 15.8
2,7-Diaminofluorene. . . . . . . . . . .
TABLE 3 Intervlanar svacinas . - of. some aliphatic and heterocyclic salts of montmorillonite d(OO1)~ COMPOUND
f 0.2
13.1 13.4 13.4
1 ~
1
SALT V8ED IN PREPABATION
0.85 (0.85)
Chloride Chloride
0.50 (0.50) 0.90 (0.90)
Free base Free base
3.75 (0.75)
Excess sulfate
3.43 (0.68)
Excess hydrochloride
0.65 (0.65)
Primary n-amylamine
Pi per idine . . . . . . . . . . .
MILLJEQQIVALBNW OF NITROGEN PER ORAY ( Y n w M O L S S OF BABE)
A.
Adenine. . . . . . . . . . . . .
12.4
Guanine. . . . . . . . . . . . . .
12.4
3-Methylcytosine.. . . .
13.0 12.8
1.56 (0.52) 2.45 (0.82)
Free base Free base
Adenosine . . . . . . . . . . . .
13.4
2.43 (0.49)
Free base
Guanosine. . . . . . . . . . . .
13.6
1.33 (0.27)
Free base
Codeine, . . . . . . . . . . . . .
15.8 17.4
0.35 (0.35) 0.60 (0.60)
Free base Free base
Brucine. . . . . . . . . . . . . .
16.9
1.30 (0.65)
Free base
in the affirmative by two different results. Firstly, 0- or p-phenylenediamine salts containing less than 1 milliequivalent of nitrogen per gram of clay do not show an increase in d(001) when exposed to water vapor a t
70
STERLING B. HENDRICKS
70 per cent relative humidity (30°C.) compared with the dry material. Both a- and P-naphthylamine compounds swell a t this relative humidity. The presumption is that the diamine is holding the lattice together, even as the oxide linkages might limit the swelling of graphitic acid (6) (“tieacross” effect). Secondly, the base-exchange capacity of a benzidine montmorillonite containing 1.0 milliequivalent of nitrogen per gram was found to be not more than 0.26 milliequivalent for barium ion. Thus about 0.70 milliequivalent of hydrogen ions must have been neutralized by the 1.0 milliequivalent or 0.50 millimole of benzidine. In other words, about 50 per cent of the benzidine molecules formed salts with both amino I
I
I
1
x139A
(
Para-Phenylene Diamine
I5-
IO-
I
12 86
Diamine 0 5-
Diamine 0. 0
12 6A
1 00
I
5
10
5
Number of Washings
FIG.1. Hydrolysis of
0-,
m-,and p-phenylenediamines
groups. That the “cover-up” effect is negligible is shown by the fact that all the hydrogen ions of the benzidine clay are neutralized when a large excess of the free base is used. Hydrolysis of the three phenylenediamine clay salts is shown in figure 1. In these experiments 1.0 g. of the salt was shaken for 30 min. with successive 100-cc. lots of water. The nitrogen content of the clay was determined on a small sample after each washing. It is perhaps evident that the marked break in the hydrolysis is a t a nitrogen content only slightly less (10 to 20 per cent) than required for formation of the fully neutralized base. Moreover, the hydrolysis is in the expected order according to the basicity of the 0-, m-,and p-phenylenediamines.
71
BASE EXCHANGE O F MONTMORILLONITE WATER ADSORPTION
The nature of water sorption by the montmorillonite amines further supports the above interpretations. Differential thermal analysis curves for two p-phenylenediamine salts a t 40 per cent and 90 per cent relative humidities are reproduced in figure 2, together with a curve for the calcium salt hydrated a t 25 per cent relative humidity. Previous work has shown that four distinct steps are involved in the hydration of the calcium montmorillonite (5). These are (1) hydration of the ion with six molecules of water, (2) completion of the first layer of water molecules, which includes those required for ion hydration, (3) formation of a second layer of water molecules, and (4) capillary condensation. The maxima in the component curves (see figure 2) a t 240" and 190°C. are associated with the first and
1.72 rne.of N
.060g ,0459 0759
25 %
,0309
40 %
,1109.
,0309
90%
.OB69 ,0459.
40%
,1759 0459.
90%
FIG.2. Differential thermal analysis curves for the calcium and p-phenylenediamine salts of montmorillonite a t various relative humidities. The total weights of adsorbed water and amounts associated with particular components are printed below the curves.
second of these steps, respectively. The maximum a t 160°C. and the maxima a t lower temperatures unresolved from it have been shown to accompany the third and fourth steps. p-Phenylenediamine montmorillonite salts in which either one (1.72 milliequivalents of nitrogen per gram of compound) or both (0.82 milliequivalent) amino groups took part in salt formation failed to show the highest temperature maximum mentioned above. Ionic hydration would hardly be expected and, as a matter of fact, the result further confirms the previous interpretation. The second maximum associated with hydration of the first layer other than involved in ionic hydration appears for both diamine salts. It is more prominent when both amino groups take part in salt formation (0.82 milliequivalent of nitrogen per gram of compound), since less of the surface is covered by organic molecules. The
72
STERLING B. HENDRICKS
amount of adsorbed water taking part in this process approaches the quantity expected from the extent of the surface and the probable surface covered by the pphenylenediamine ion. The expected amount for the salt in. which both amine groups are completely neutralized would be about 0.065 g. of water, corresponding to the covering of one-third of the available surface by the amine. The discrepancy between this amount and the observed quantity, 0.045 g., is not unexpected, since the shape of #e organic ion is probably such as to destroy the hexagonal-like arrangement of the water layer (5). The lower temperature maximum in the salt having only one amino group neutralized can be due to multilayer
I
25
50
75
I00
Relotive Humidity
FIG.3. Water sorption a i a function of relative humidity for several montmorillonite salts.
adsorption in capillaries formed by individual aggregates and adsorption on external surfaces, since the compound does not have a variable (001) spacing. It can be seen from table 1 and the discussion in the next section, that space is available between silicate sheets only for a single layer of water molecules in these salts. Results similar to those discussed above were obtained for benzidine salts of montmorillonite. Amounts of adsorbed water as a function of relative humidity a t 30°C. are shown in figure 3 for several montmorillonite salts. In general, the organic compounds are markedly less hygroscopic than are the inorganic ones. DiiTerential thermal analyses were made a t all indicated points. It will be of later use to point out that not
BASE EXCHANGE OF MONTMORILLONITE
73
more than 0.020 g. of water is associated with the 180-185°C. maximum for the brucine salt. MEASUREMENT OF VAN DER WAALS “THICKNES8” AS A METHOD FOR DETERMINATION OF MOLECULAR STRUCTURE
The nature of the salts must be discussed in more detail before the dependence of d(001) upon the structure of the organic cations is evident. Schematic drawings showing the structure of a hydrated aluminum silicate
FIQ.4a. Schematic drawing of a single layer of p-phenyleneammonium ions between the hydrous magnesium aluminum silicate layers of montmorillonite.
layer of the montmorillonite type are reproduced in figures 4a and 4b. Opposite faces of a layer {efined by planes passing through the centers of oxygen ions are about 6.6 A. apart normal to (001). Thus [d(001)- 6.61 A. is the space AA‘ occupied by the organic ion between oxygen centers. This includes the space required by the internal atomic arrangement of the molecule and the van der Waals interaction distances between its surface atoms and the oxygen ions on the silicate surface.
74
STERLING B. HENDRICKS
Organic cations are held to the surface, not only by the Coulomb forces due to the ionic nature of the compound but also by van der Waals forces between the molecule and the neighboring surface. The space AA' then is determined by the distances characteristic for such interaction and by
Surface of Silicate Layer Contains 0.- ions
FIQ.4b. Schematic drawing showing overlapping of molecular ions between silicate layers of montmorillonite. the particular orientation of the molecule. It might be expected that the molecule would be so oriented as to make the total force of attraction a maximum. In other words, the maximum number of atoms possible within the molecule would, consistent with the characteristic separation of the ionic portion, approach the oxygen surface of the silicate layer.
75
BASE EXCHANGE OF MONTMORILLONITE
This distance of approach would be determined by van der Waals radii for the various atoms (10). It is seen in table 1 that o-phenylenediamine montmorillonite has a constant (001) spacing of 12.4 to 12.6A. between a nitrogen content of 0.10 and 0.70 milliequivalent per gram of salt. Thus [d(00l) - 6.61 A. is 6.0 A., which is the van der Waals “thickness” between centers of oxygen ions for the o-phenylenediamine molecule. Some expected values for such a molecule lying flat upon the surface would be as follows: 2 X (1.4 1.2) A,, twice the s u of~ the van der Waals radii for oxygen and hydrogen (lo), or 2 X 3.34 A., which is the value of co for graphite. At higher o-phenylenediamine contents the value of d(001) increases somewhat, owing apparently to overlapping of molecules between some layers as shown schematically for p-phenylenediamine ions in figure 4b. Two complete layers of o-phenylenediamine would require d(001) to be 6.0/2) A. The observed 12.9 approximately 15.4 A., that is, (12.4 A. is an averaged result, which indicates that less than 25 per cent of the layers are probably doubled when 1.37 milliequivalents of amine per gram of clay are present. Results similar to those for o-phenylenediamine montmorillonite were obtained for the meta- and para-compounds. Benzidine shows almost complete formation of a double layer, d(001) being equal to 15.2 1.when only one amino group of each molecule is neutralized. An interesting consequence of the double-layer formation is the evidence that both surfaces of a montmorillonite layer form salts, a particular molecule being constrained to lie flat on the surface with which it has formed a salt. It is for this reason that a diamine apparently can act to tie two surfaces together when an amount insufficient for salt formation with only one group is present. In the aniline salt it is not unexpected that absence of the “tie across” results in partial double-layer formation a t lower concentrations of amine. Results presented in figure 5 for the heptyltributylammonium and the codeine salts of montmorillonite can now be discussed. I n the former case the value of d(001), as reported by Gieseking (l),is constant and is equal to 15.0 A. between an ammonium ion content of 0.30 and 0.70 milliequivalent per gram of clay. This may be interpreted as the region in which a single layer of substituted ammonium ions is being formed. Below 0.30 milliequivalent a single layer is not present between all silicate sheets, and above 0.70 milliequivalent double layers of substituted ammonium ions are present. In this case the double layer probably results from further van der Waals adsorption of the substituted ammonium salt, since d(001) changes without displacement of hydrogen ions. The characteristic van der Waals “thickness” between oxy en centers of heptyltributylamine therefore is about 8.4 A., (15.0 - 6.6) ., which is
+
+
d
I
I
I
I
I
I
/
l
/
I
/
I
Equivolence of Monimorillonife
-
-
.c ---------_-----
0
c
17.0-
y‘
0
e 16.0-
p--p--~ +Codeine ‘
~
Heptyl Amine (Gieseking)
-
t
I
(
I
~
,
Equivalence of Montmorillonite I (
/
/
,
,
I
BASE EXCHANGE OF MONTMORILLONITE
77
a plane P that is approximately perpendicular to the plane P’ of the other four rings. If the plane P’ is parallel to the silicate surface of montmorillonite, the nitrogen atom near the plane P is in position for salt formation with the bottom of the next silicate layer above. The distance normal to (001) between the nitrogen atom and the oxygen atom in the heterocyclic ring, which is probably the atom farthest removed along the normal, is 3.0 to 3.5 A. The expected van der Waals “thickness” between oxygen
FIG.6a. A schematic drawing showing the configuration of the heptyltributylammonium ion when held between montmorillonite layers.
FIG.6b. Structural formula of codeine (after Gulland and Robinson) centers or the space AA’ of figure 4a would be between 9.0 and 9.5 A., which is in agreement with the observed value, 9.1 A. The van der Waals “area” of a codeine molecule in the plane P’ would be about 150 1.2 Since the total surface per exchangeable ion in this not more than 0.45 particular montmorillonite sample is near 70 .&,z milliequivalent of codeine per gram of clay could be accommodated in one layer. This, probably somewhat accidentally, is near the upper limit of the constant d(001) region of codeine montmorillonite (figure 5). A similar argument holds for heptyltributylamine, where the exchange capacity of the clay is almost reached before a complete layer of molecules is present.
78
STERLING B. HENDRICKS
Results for piperidine listed in table 3 show that the van der Waals “thickness” between oxygen centers of the molecule is about 1.0 b. greater than that of the aromatic diamines. While this result can be explained by the known structure, it is somewhat smaller than might be expected, say 2.0 A. However, this larger expected distance probably does not correctly take into account the interaction of the hydrogen atoms of the piperidine molecule with the oxygen ions of the neighboring silicate layers. STRUCTURE OF FLUORENE, PURINE BASES, AND NUCLEOSIDES
There is really not much doubt that chemical evidence is adequate for assigning a coplanar arrangement of all atoms of fluorene, except the hydrogen atom in the 9-position. A simple argument, for instance, is the fact that mild oxidation of the 2,5- and 2,7-diamine derivatives leads to highly colored semiquinone forms indicative of an aromatic structure and consequently of coplanar benzene rings. However, in the only crystal
GWNINE 2-AMiXO-6-OXYPURINE
GLJANINE 2.AUINO-6-HYDROXYPURlNE
ADENINE b -AMINOPURINE
FIG.7a. Structural formulas for guanine and adenine
structure uTork (8) on fluorene and its derivatives, the conclusion is reached that the planes of the benzene rings make an angle of about 20’ with one another. The van der Waals “thickness” b2tween oxygen centers of both 2 ,5- and 2,7-diaminofluorenes is about 6.2 A., i.e., (12.8 - 6.6) A. (table 2), as would be expected for a molecule in which the benzene rings are coplanar. Moreover, the 2,7-diaminofluorene montmorillonite in which only one amino group takes part in salt formation has d(001) equal to 15.8 b., correspondin to complete double-layer formation with an over-all “thick3.0 R. ness” of 6.2 !. Montmorillonite salts of the monoacidic purine bases adenine and guanine, possible formulas for which are shown in figure 7a, containing 0.75 and 0.69 milliequivalent of base per gram of compound, both have d(001) equal to 12.4 b. (note table 3). The van der Waals “thickness: between oxygen centers of each molecule therefore is about 6.0 A. (5.8 A.), corresponding to a coplanar arrangement of the atoms. The structural for-
+
BASE EXCHANGE OB MONT,MORILLONITE
79
mula of the guanine cation, therefore, corresponds more closely to the enol than to the keto form (9). Various structures, of course, contribute to the stable state of the ion. The pyrimidine base 3-methylcytosine, the formula for which is also shown in figure 7b, necessarily has the keto form and therefore can not have a coplanar atomic arrangement. The (001) spacings of its montmorilloonite salts containing 0.52 and 0.82 milliequivalent of base are 12.9 A,, whicli is significantly greater than that of adenine and guanine montmorillonites. The additional van der Waals “thickness” between oxygen centers, 0.5 A., is about the expected amount corresponding to the atomic arrangement shown in figure 7. A small difference like this can only be trusted if comparison is made between similar compounds. Thus, while both 2,7-diaminofluorene and 3-methylcytosine haye a T an der Waals “thickness” between oxygen centers of about 6.3 A , , the former can safely be said to have a coplanar atomic arrangement while the latter does not.
FIG.7b. Structural formula of 3-methylcytosine and a possible spatial configuration of the compound.
Montmorillonite salts of the nucleosides adenosine and guanosine, whirh are the glycosides 9-adenine d-ribofuranoside and 9-guanine d-ribofuranoside, respectively (2) (figure 8a), were also examined a t nitrogen contents which should be adequate for $ngle-layer formation; Values of d(001) for both are approximately 13.5 A, which is about 1.0 A. greater than the spacings of adenine and guanine montmorillonites. The molecules therefore h$ve a van der Waals “thickness” between oxygen centers only about 1.0 A. greater than would be required for a strictly coplanar atomic arrangement. This is surprisingly small, but nevertheless is entirely consistent with a possible spatial atomic arrangement for the nucleosides. The atomic arrangement required for adenosine is shown in figure 8b. Here the plane P of the ribose ring must be approximately parallel to the plane P’ of the adenine molecule. Moreover, the hydroxyl groups as well as the primary alcohol group must be approximately in the plane P‘. It does not appear likely that the configuration around the glycosidic C--?T bond is greatly altered by the constraint introduced upon adsorption. Without further discussion it can perhaps be seen that such organic salts
80
STERLIIUG B. HENDmCKS
play an important part in determining the properties of the widely occurring soils containing clays of the montmorillonite type. They also serve as a model for the aci&c and basic derivatives of proteins and for the fixing of dyes in proteins. An approach is ala0 indicated for the study of adeorption from solution in ita dependence upon molecular structure and the nature of the adsorbent.
-0-
*Dp(oyuE
Fro. 88. Structural formules of guanosine and adenosine 9'
Fro. 8b. Schemetic drawing showing the probable configuration of the adenosine ion between layera of montmorillonite. The internal molecular thickneM of the ion is about 1.6 A. and the majority of the atoms probably lie in or near the planes P end P'
SUbi?dARY
A number of organic salts of the clay mineral montmorillonite were prepared and values of their interplanar cleavage spacings, d(001), were measured a t various organic cation contents. Regulta obtained show that the organic cation is held to the surface of the silicate layers of the m i n e d not only by the Coulomb forces between the ions but also by van der Waals attraction of the molecules to the surface. Values of d(001) de-
SH.4PES O F CLAY PARTICLES
81
pend upon the structure of the organic cation and the manner in which it is adsorbed upon the silicate surface. This dependence is elaborated into a method for the determination of molecular “thickness” for van der Waals adsorption. The structures of several molecules were studied by this method. It WM shown that fluorene and the purine bases adenine and guanine are “plane” molecules. A structure also was found for the nucleosides guanosine and adenosine, in which the plane of the ribofuranose ring is approximately parallel to that of the purine base. REFERENCES (1) GIESEKING,J. E.: Soil Sci. 47, 1 (1939). (2) GULLAND, J. M., AND HOLIDAY, E. R.: J. Chem. SOC.1936, 765. (3) GIJLLAND,J. M., A N D ROBINSOS,R.: J. Chem. SOC.128, 980 (1923). (4) HENDRICKB, S. B., AND ALEXANDER,L. T.: Soil Sci. 48,257 (1939). (5) HENDRICKG, s. B., NELSON,R. A., AND ALEXANDEE, L. T.:J. Am. Chem. SOC. 62, 1457 (1940). (6) HOFMANN, U.: Ergeb. exakt. Naturw. 18, 229 (1939). (7) HOFMANN, U.,ENDELL,K., AND WILM,D.: Z.Krist. 88,340 (1933). (8) IBALL, 3.: Z. Krist. 94, 397 (1936). , (9) LEVENE,P. A., AND BASS,L. W.: Nucleic Acid. The Chemical Catalog Company, Inc., New York (1931). These authors write the keto formula for guanine, as does also T. B. Johnson in Organic Chemistry (H. Gilman, Editor), Vol. 11, p. 1003. John Wiley and Sons, Inc., New York (1938). (10) PAULING, L.: The Nature of the Chemical Bond, pp. 174-8. Cornel1 University Prese, Ithaca, New York (1939). A discussion of van der Waals radii. (11) SMITE,C. R.: J. Am. Chem. SOC.66, 1561 (1934). (12) STOSICK,A. J.: J. Am. Chem. SOC.61, 1127 (1939).
STUDIES IN T H E DEGREE OF DISPERSION OF T H E CLAYS. IV
THESHAPESOF CLAYPARTICLES” * C. E. MARSHALL Department of Soils, Missouri Agricultural Ezperiment Station, Columbia, Missouri Received J u l y 3, IS@
The importance of particle shape was emphasized in the first paper of this series (6),which was concerned chiefly with the accuracy of the twolayer method for the centrifugal mechanical analysis of the clays. Since then many papers have been published on the crystal structures of the 1 Presented a t the Seventeenth Colloid Symposium, held a t Ann Arbor, Michigan, June 6-8, 1940. Contribution from the Department of Soils of the Missouri Agricultural Experiment Station, Journal Series No. 706.
’
