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JOHN H. BLOMQUIST AND WESLEY G. FRANCE
ADSORPTION AT THE CRYSTAL-SOLUTION INTERFACE.
XI11
AN ELECTRON-DIFFRACTION INVESTIGATION OF CRYSTAL SURFACESOF PURE SODIUMBROMATE AND SODIUM BROMATE WITH ADSORBED DYES' JOHN H. BLOMQUIST AND WESLEY G . FRANCE Department of Chemistry, The Ohio State University, Columbus,Ohio Received August 4 , 19.43 INTRODUCTION
The nature of the growth process of cystals is of deep interest to the colloid chemist, since a thorough knowledge of this process is necessary for the complete understanding of the formation and stability of colloidal systems produced by crystallization methods. In this laboratory attention has been directed towards adsorption a t the crystal-solution interface and to the effect of this adsorption on the growth forms of the crystal (4). One of the questions which such a research program must answer is that of determining the precise way in which the impurity is held in the crystal. One approach is through x-ray studies (4f, 4h, 4j, ll),and a second is that of electron diffraction. Since electrons are much more easily scattered than are x-rays, electron diffraction is particularly adapted to a study of surface structure. From the position of reflection spots the lattice constant can be calculated, and from their relative intensities, some information about the orientation of the molecules of impurity in the surface lattice should also be obtained. APPARATUS
An electron-diffraction camera has been recently designed in this laboratory by Dr. P. M. Harris and built in the instrument shop by Mr. John F. Beta. It was assembled and used in the course of this investigation. The basic principles are similar to those of cameras used in other laboratories. Electrons are accelerated by a potential of about 25,000 volts from a hot cathode toward a collimating system of two pinholes 0.1 mm. in diameter placed about 5 in. apart. The collimated beam is reflected from the surface of the crystal to be studied, and the resultant diffraction pattern is recorded on a photographic plate. The effective camera length is about 25 cm. The distinctive feature of this camera is the crystal holder. This was designed so that it could be used with crystals of non-conductors which might otherwise charge up in the beam and distort the diffraction pattern. The crystal holder was constructed so as to move the crystal back and forth continuously in the beam and yet maintain a constant azimuth of the electron beam on the crystal. In this way the various portions of the crystal that might have become charged, discharged themselves while put of the beam, and thereby produced an undistorted single reflection pattern. 1 Presented a t the Kineteenth Colloid Symposium, which was held a t the University of Colorado, Boulder, Colorado, June 18-20, 1942.
ADSORPTION AT THE CRYST.4L-SOLUTIOS INTERFACE.
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PROCEDURE
The first step was to determine the wave length of the electron waves for a constant voltage applied to the cathode tube. This was done by taking transmission photographs of thin gold film. Interpretation of these patterns in the light of the known structure of gold permitted the calculation of the wave lengths of the electrons which produced the patterns. A constant voltage was used throughout all of the diffraction experiments, and calibration pictures were taken both before single-crystal experiments were started and after they were completed. KOsignificant difference was found in these calibration pictures. Typical calibration data are those of plate 62-2 (see table 1). Here C is the constant but unknown voltage, D the diameter of the rings, L the camera length, 0 the diffraction angle, n the order number, and X the wave length of the electrons. The main problem was to study the surfaces of large inorganic crystals grown from pure solutions and from solutions containing various dyes. Sodium bromate was chosen because of the ease with which large crystals could be grown and because the dyes which modify the crystal habit have been investigated (4m). TABLE 1 Typical calzbratzon data Plate 62-2: gold; C, volts; camera length, 25.411 cm.
1. , . . ,. .., . .) .. : . . .’ ..
.i
Average.. .
1.672 1.942 2.739
3.233
~
~
0.03290 0.03821 0.05390 0.06362
1
I
......
56.54’ 15.65’ 132.55’ 149.21’
1 ~
~
0.01645 0.01911 0.02692 0.03176
’
I
111
200 220 311
1
I
0.07730 0.07778 0.07749 0.07794
./ 0.07763
The dyes used were from the series of isomeric acid and basic monoazo dyes prepared and analyzed in this laboratory by Professor W.R. Brode and his students (3, 6). Using the technique developed by the previous workers (4) in this laboratory, large individual crystals of sodium bromate were grown from solutions of pure sodium bromate in water, from solutions containing the dye designated as M-aSH2 which is strongly adsorbed and which changes the normal tetrahedral habit of sodium bromate to octahedral, from solutions containing the dye P-aOH-3 which is adsorbed weakly and does not modify the crystal which is not adsorbed habit, and from solutions cont’aining the dye P-p”z-7 by the sodium bromate. When the crystals were large enough, one was removed from the solution and dried carefully with filter paper. It was then correctly mounted on the crystal holder and its face was rinsed with clean dry benzene. At all times care was taken to avoid touching the crystal face with the hands. Eastman Contrast Lantern Slide Plates mere used to record the diffraction pattern. Exposures were usually long ones at low currents. The plates were developed in deep trays filled with E.K. D-11 developer.
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JOHN E. BLOMQUIST AND WESLEY 0. FRANCE
The type of reflection pattern was always the same. It is the pattern of spots and lines from a cubic crystal (13, page 146; 14). The spots are Laue spots which appear for certain settings of the crystal. The lines are Kikuchi lines. These occur at the intersection of the photographic plate with planes which make the Bragg angle on either side of a net plane of the crystal. The Kikuchi lime system was not extensive enough to allow an interpretation of the surface structure based only upon their positions. The analysis of the Laue spot system requires fitting indices to the spots from a consideration of the geometry of the spot pattern. The angles between the normals to the planes corresponding to TABLE 2 Summary of the data
1 Number of plates.. .......................... Number of spots.. ........................... Average h/ap.. ............................... Highest A/ac. ................................ Lowest h/ao.. ................................
Standard deviation. .........................
a o , ..........................................
SODIUM =OMATE
W I I E TEX ADBOXIZD DYUI
5 34 0.01158 0.01219 0.01103
4 17 0.01137 0.01195 0.01082
O.ooo33
o.oO035
6.70
6.83
m r n (-Tw)
3 14 0.01166 0.01223 0.01110 0.m9 '6.433
m r n(
2 12 0.01158 0.01200 0.01120 0.00021 6.70
O ~ n o )
10.0
10.0 5.1 5.6 3.7 8.9
2 2 7 9
the indices selected must check with those observed on the photographic plate. After the selection of the indices h, k, and 1, the Bragg equation 2sine A ( 1)
& - l / h 2 + kz +
?t
may be used to calculate A/Q, and the value of the wave length, A, being known, the lattice spacing may be determined. In addition to the calculation of the lattice constant for the surface lattice, a calculation waa made of the relative intensities expected for the observed spote, using Mott's formula (12). I# = I f ( @ ) I* (2) where
f(e)
2
- F(~)ICEC*~
= 2m[Z v'
(3)
ADSORPTION AT THE CRYSTAL-BOLUTION INTERFACE.
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Here Is is the intensity a t angle 8, c the charge, na the maas and I the velocity of the electrons, z the atomic number, and F(8) the atomic scattering factor for x-rays. [z F(8) was Calculated from the structure proposed for sodium bromate by Hamilton (8). The agreement between the calculated and observed values in most instances was not very close. Somewhat better than average b t a are those of plate 51-2 (see table 3). The intensity data were therefore not especially useful. This situation is doubtless due to the fact that the calculations are based on ideal conditions, whereas one knows that the electron beam waa actually inhomogeneous but does not know the relative intensities of the components, and one also knows that the calculated intensities are baaed upon total reflection within a minute or so of Brag's angle, whereas the half-intensity width must have been of the order of 15' to 20'. There seems to be no published intensity data for reflection which agree with Mott's law (13, page 86).
-
DISCUSSION OF RESULTS
The lattice constant observed for the surface lattice of pure sodium bromate agrees with that determined for the crystal by x-ray methods. The table of data (table 2) shows that the values of A/& for sodium bromate dyed with M-aNHz overlap to some extent those of pure sodium bromate. Therefore, to determine the significance of the differences between the averages which were used to calculate the lattice constants, a statistical treatment of the data must be used. For this purpose that of Goulden waa selected (5). Accordingly it waa assumed that the value of A/& for the dyed crystal was the same as for the pure crystal and the probability, P, that chance accountedfor the scattering of the individual values about the two average values 0.01158 and 0.01137 was calculated. The probability P is obtained from tables of the t function, where t is calculated from the equation t =
in which from
and
I Xl - x*1/82
.& are the two average values of
(4) A/%,
and S, is calculated
where N1 and N z are the respective numbers of observations entering into the corresponding averages, and X I and XZare the individual observations. When the data for pure sodium bromate and sodium bromate with adsorbed M-aNHn are substituted in the equation, the value of t is 2.15. Looking in the tables for a total number of observations corresponding to 50, one finds that the probability that chance could account for the two groups of values of A/& is only 0.04 or 4 per cent. The assumption that the lattice constant for the surface of the dyed crystal is really the same as that of the surface of the pure crystal must therefore be an unlikely one. Similar calculations for the values obtained for pure sodium bromate and sodium bromate grown from a solution with P-j3NHz-7 give a t value of 0.93.
1048
JOHN H. BLOMQUIST AND WESLEY G. FR.4h‘CE
This corresponds to a probability of 0.35 or 35 per cent. The assumption that the lattice constants for the two crystals are the same is therefore reasonable. This statistical treatment seemingly indicates that crystals of pure sodium bromate and of sodium bromate grown from solutions containing the dyes P-aOH-3 and P-BNH2-7 have the same surface lattice constant and that this constant is differentfrom that for sodium bromate crystals grown from solutions containing M-aNH2. Two different explanations for this apparent change of lattice constants seem possible. Either the pure and dyed crystals actually do have different surface lattice spacings, or the observed difference between the latice constants may be attributed to a refractive-index effect due to an inner potential. A stretching of the lattice at the surface of the dyed crystal has not been observed by the workers who used x-rays (4f, 4h, 4j, 11). Miles (11)did point out that there was reason to believe that the concentration of any impurity in a growing crystal would be much greater at the surface than in the interior of the crystal. If one admits the possibility that the dye is concentrated in the surface layers and that this concentration of dye is responsible for stretching the lattice by a small amount, one must concede that the dye is arranged in a regular way in the surface, or, in other words, one must postulate some sort of solid solution. There does not seem to be any other mechanism which would account for the observed regularity of the diffraction pattern. Solid solution may take place in two ways (9). The mechanism of the simpler of the two ways is a more or less regular substitution in the space lattice of the solvent by the solute. For inorganic salts the solute must be similar to the solvent, especially with respect to its volume and its electrical characteristics. It is difficult to reconcile the large dye molecule and the regular sodium bromate lattice with such a mechanism. The second type of solid solution is interstitial rather than substitutional. In this case the solute atoms do not fit into the space lattice of the solvent but are crammed in between the solvent atoms, causing the enlargement of the unit cell of the solvent. Only a few cwes of this type of solid solution have been reported among the salts (7). It is ditficult to see how a large dye molecule could be adsorbed interstitially in the sense in which “interstitial solid solution” is used above. One is therefore a t a loss to explain the observed ditrerences in the lattice constants by a solid solution of the dye molecule in the bromate lattice. Attributing the difference to an inner potential (1, 2; 13, page 155) offers another alternative. It is to be noted that all of the plates were interpreted without assuming such a potential. The results obtained with this assumption agree well with the known structure of sodium bromate. It may be, however, that all of the plates for pure sodium bromate could be reinterpreted by assuming an inner potential of about 18 volts. This could not be verified because of the great difliculty involved in assigning indices to a h u e spot pattern when a refractive-index effect is present, unless all of the spots lie on the line from the central spot perpendicular to the shadow edge. This was not the case. A small additional positive potential of about 5 volts would be sufficient to account for
ADSORPTIOS AT THE CRYSTAL-SOLUTION
IXTERFACE.
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1049
the effect observed with the dyed crystal. This would be attributed to the oriented dye molecules in the surface layers of the crystal. The absence of a refractive-index effect for crystals is perhaps to be attributed to the presence of minute rough projections on the face of the crystals. The electrons pass through these projections and are diffracted, but the fact that their angle of incidence to the surface of these projections may be greater than aboat 5’ would mean that the refractive effect is so small as to lie within the experimental error of measuring the diffraction angle. Most of the work done on inner potentials of crystals has been with cleavage faces of naturally occurring minerals. There is no experimental basis for comparing the results of this research with the results of other workers on inner potentials, since none of their work has been concerned with crystals grown from solution under ordinary laboratory conditions. It is to be recorded that etching has been shown to destroy the refractive effect (10, 15). In the light of these reasons advanced for the absence of the inner potential effect in crystals grown from solution, it is difficult to see how the presence of the dye molecules could be responsible for the production of such an effect. In order to further the solution of this problem, it seems desirable that an electron-diffraction program be undertaken which will investigate the inner potential effect observed with crptals of naturally occurring salts and with crystals of the same salts grown from solution under laboratory conditions. The use of Tillman’s method (15) makes possible the simultaneous determination of the inner potential and the lattice spacing at the surface of the crystal. Piecision x-ray methods must also be used to measure the lattice spacings of pure and dyed crystal interiors as a function of distance from the surface, that is, the conipnrison of x-ray ponder photographs when the powder is scraped from the surface of the crystal and when the powder is obtained at various levels in the interior. These x-ray data, together with new electron-diffraction data from Tillman’s method, should be helpful in choosing a satisfactory explanation. Such a program is planned at this laboratory. SUMM.%RT
1. An electron-diffraction camera adapted to the study of surface structure
was used in this research. This features a new-type moving crystal holder adaptable to studying the surfaces of non-conductors. 2. Data are presented nhich indicate an apparent change in the lattice constant of sodium bromate crystals grown in a solution of a dye which is adsorbed by the sodium bromate selectively so that its crystal habit is modified. Yo lattice change was observed for crystals grown from solutions of dyes which did not change the habit of sodium bromate. 3. It is pointed out that this apparent lattice change may be due either t o an inner potential effect or to the formation of a solid solution of the dye in the surface lattice of the salt. 4. Methods are indicated which should allow one to choose between these alternatives.
1050
JOHN H. BLOMQUIST AND WESLEY 0. FRANCE
The authors wish to acknowledge their indebtedness to Professor P. M. Harris for his work in the design of the camera and for his helpful guidance during its construction and subsequent use; thanks are also due Professor W. R. Brode for furnishing the dyes used and Mr. John F. Beta for the construction of the camera. REFERENCES
(1) BEECIIINQ:Electron Diflraction, p. 54. The Chemical Publishing Company, New York (1939). (2) BETHE:Ann. Physik 81, 55 (1928). (3) EBERHART, D. R.:Dissertation, The Ohio State University, 1935. (4) (a) FRANCE AND MCBURNEY: J. Am. Chem. SOC.46,540 (1924). (b) ECKERTAND FRANCE: J. Am. Ceram. Soc. 10. 579 (1927). (c) KEENENAND FRANCE: J. Am. Ceram. SOC.10, 821 (1927). (d) BENNETTAND FRANCE:J. Am. Ceram. SOC.11, 571 (1928). (e) LASEAND FRANCE: J. Phys. Chem. S4,724 (1930). (f) FOOTE, BLAKE,AND FRANCE: J. Phys. Chem. 84, 2238 (1930). (9) FRANCE:Colloid Symposium Monograph I, 59 (1930). (h) WEINLAND AND FRANCE: J. Phys. Chem. 36, 2832 (1932). (i) PAINE AND FRANCE: J. Phys. Chem. 39, 425 (1935). (j) DAVISAND FRANCE: J. Phys. Chem. 40,81 (1936). (k) FRANCE AND DAVIS:J. Phys. Chem. 40, 177 (1938). (1) RIQTEBINK AND FRANCE: J. Phys. Chem. 44, 1079 (1938). (m)FRANCE AND WOLFE:J. Phys. Chem. 46, 395 (1941). (n) FRANCE AND WOLFE:J. Am. Chem. 500. 88, 1505 (1941). (5) GOULDEN: Methods of Statistical Analysis, p. 40. John Wiley and Sons, Inc., New York (1939). (6) GRIFFITH,M. E.: Dissertation, The Ohio State University, 1934. (7) HAM: 2. Krist. 91, 144 (1935). (8) HAMILTON: Z. Krist. 100, 104 (1938). (9) H A V I Q H ~ ~MACK, S T , AND BLAKE:J. Am. Chem. SOC. 47, 29 (1925). (10) HOPKINS:Phil. Mag. 11, 820 (1936). (11) MILES: Trans. Roy. SOC.(London) AS6, 125 (1935). (12) Mom: P m . Roy. SOC. (London) A M , 658 (1930). (13) THOMSON AND C O C H ~ ~ N Theory E: and Practiw of Ekctmn Diflraction. The Macmillan Company, New York (1939). (14) THOMSON: Phil. Mag. 18,840 (1934). (15) TILLMAN: Phil. Mag. 18. 858 (1934).
