ADSORPTIOX AT CRYSTAL-SOLUTION IKTERFACES
V, The Effect of Adsorbed Dye on the Lattice Size of Potassium Alum Crystals
F . G. FOOTE, F. C. BLAKE, A N D W . G. FRANCE
The previous papers' in this series of investigations have shown that the growth ratios of potassium and ammonium alum crystals are greatly modified when grown in the presence of dyes and other foreign substances. Thus potassium alum, when grown in the presence of diamine sky blue develops into perfect cubes having no octahedral faces. The cube faces alone are colored and the growth ratio [IOO]/[III]undergoes a change from 1.61 to 0.0 in a dye concentration of 0.1%. These effects have been accounted for on the basis of ( I ) the residual valency force fields of the crystal planes; ( 2 ) the interionic distances within the faces; and 13) the presence and distribution of polar groups in the adsorbed material. That the structure of the crystal planes is a determining factor in the adsorption process is evidenced by the fact that only those faces populated by ions of like charge were colored by the dyes and experienced a repression in their perpendicular displacements. Uryckoff2 has determined the crystal structure of potassium alum. According to his results, the cube form is made up of alternate layers of positive and negative ions, positive layers containing only potassium and aluminum ions, the negative layers sulfate ions. The octahedral form is made up of layers each of which contains both positive and negative ions. Spangenberg has suggested that those forms which are made up of atomic layers composed of all positive or all negative ions would have stronger external field than those forms whose layers contain both positive and negative ions? One would therefore predict that if a growing potassium alum crystal adsorbs dye at all, it would be adsorbed on the cube rather than on the octahedron planes. The precise way in which the adsorbed dye molecules are built into the alum lattice is unknown. I t is not unlikely that there is a definite orientation of the molecules accompanied perhaps by a slight stretching of the lattice. To determine whether or not the lattice constant is modified by the adsorption 1 France and hlcBurney: J. Am. Chem. SOC.,46, j40-44 (1924); Eckert and France: J. Am. Ceramic SOC., 10, 579-91; Keenen and France: 821-27 (127); Bennett and France: 11, 571-81 (1928). 2 Wyckoff: Am. J. Sci., 205, 209 (1923). 3 Spangenberg: Z. Kryst., 59, 375, 383 (1923); 61, 189 (1925).
ADSORPTION AT CRYSTAL-SOLUTION INTERFACES
2237
process the lattice constants of pure and dyed potassium alum crystals were determined. The dye has the following structure and colored the cube faces only.
XHz OH
OH NH2
The powdered crystal method of Debye and Sherrer' and of Hull2 was used and Blake's correction factol3 for sample penetration was applied. This affords a very accurate method of measuring lattice size. The samples were prepared for X-ray analysis by grinding in an agate mortar and passing thru a zoo-mesh screen. The sample tubes were small, glass, capillary tubes about 0.4 mm. in diameter and 7 cm. long. One half of each tube was filled with the sample and the other half with powdered
FIG.I
aluminum. One tube was filled with pure alum, another with dyed alum and a third with the dye. The cameras used were semi-cylindrical and had a septum thru the center. Each filled sample tube was placed in a tube holder so arranged that the tube could be oscillated thru a small angle on the axis of the camera. The film, backed by an intensifying screen, was held on the back of the camera by a strip of cotton webbing. Using this arrangement the film has two spectra side by side, one of the sample, the other of the Debye and Sherrer: Physik. Z., 17, 277 (1916). *Hull: Phys. Rev., (2) 10, 661 (1917). SBlake: Phys. Rev., ( 2 ) , 26, 60 (1925).
F. G. FOOTE, F. C. BLAKE, A S D W . G . F R A S C E
2238
aluminum standard. The cameras had a radius of approximately 16,;cms. The source of X-rays was a molybdenum Coolidge tube. The incident beam was collimated by passing thru two sets of adjustable lead slits. The beam was rendered monochromatic by passing thru a zirconium oxide filter. Zirconium oxide passes only the K-a lines of molybdenum. Exposures were for approximately thirty-six hours. Fig. I shows the arrangement of the apparatus. The alum spectra were rather faint but the lines could be measured with a fair degree of accuracy. The dye alone gave a series of lines identical
7-1
I
Aluminum
Dye
~
1
with the sodium chloride spectra, sodium chloride being present in the dye as an impurity. S o lines due solely to the dye were reco;ded. The lattice constant for potassium alum has been determined as 12.08h by T'egard and Schjelderup.' Fig. z is a diagram of the spectra showing the position of the lines. S o difficulty was experienced in assigning indices to the alum lines. The distances between corresponding lines on either side of the undeflected beam were measured. Sample penetration was corrected for by means of Blake's correction factor, 61 = cotO6O. The effect of film shrinkage was overcome by computing the camera radius from the aluminum spectra. Formulae. Computing camera radius.
x
sin 0 = __ 2 ' Qhkl
2L
4R = -
0
IF'egard and Schjelderup: Ann. Physik, (4) 54, 146 (1917)
2239
ADSORPTIOS AT CRYSTAL-SOLUTIOS I S T E R F A C E S
Computing lattice constant of potassium alum. 8‘ deg
=
I80
2L’
___
4R
where : plane indices. lattice constant for aluminum = 4.0438 A,. lattice constant for alum. spacing of planes of indices h,k,l. glancing angle (aluminum) glancing angle (alumn) wave length of 1 1 0 . K ~o . j ~ o oA”. radius of camera distance between corresponding lines of film (aluminum) distance between corresponding lines on film (alum)
h,kJ a, a’ dhki
e e’
x K 2L 21’
Data and Results.
TABLEI. Pure Alum Camera
e
Line
I
K
= 16.616 cms. Plane
az(A’)
3O
4;‘
26.4”
2 IO
12
006
2
4O
j
30,6”
211
I2
087
3
4O
46
9 ,4”
220
12.0;6
1
4
so
j
6‘
18’ 43.3‘1
6
6’
43’
7
bo
8 9 IO
7 O
31
7‘
14.1’’
3.3”
I:;[ [:;I
j3.6”
12.088 12.081 12.
I08
I 2 .o;;
12,131
IO0
39)
jI.2“
620
13’
j8‘
8.4”
733
Mean a, Greatest mean mror
12,132 12
,036
I 2 082 0 .j 2 7 ;
F. G. FOOTE, F . C. BLAKE, A S D W. G. FRANCE
2240
TABLE 11. Dyed Alum R = 16.696 cms. 2
Camera Line
3O
2
4'
3
4O
46' 6' 46'
4
so
j
5"
6
6' 6'
42'
10'
7 8
9 I3O IO
Plane
0
1
I7O
si.4" 58 9"
a"(A") 1 2 ,031 12.114
2 10 211
34.0"
12.0;s
j4.8"
12.062
40.9"
I2
12.102
40'
.3" 9 . j" j z . j"
59j
3 1,3"
1 2 .I O j
20'
5 0 . j"
12.024
3' 37' 18'
2
12.16j 1 2 . I12
Mean a Greatest mean error Lattice constant for pure alum Lattice constant for dyed alum
.ooj
12.078 0.66'1, 12.082
12
A'
078 A'
Conclusion Within experimental error, the lattice constants for pure and dyed alum are the same. It would appear therefore that adsorption of this dye by a growing potassium alum crystal does not measurably affect the plane spacings. I n the event that this result is supported by subsequent work, one would be justified in assuming that the dye is adsorbed interstitially rather than as individual planes or by replacement of the ions of the unit cell, The results obtained in the growth measurements also seem to warrant such an assumption. Further work with other alums and dyes is being undertaken. T h e Phystcal and T h e Chemzcal Laboratorzes, T h e Ohzo Stale L'na~erszly, Columbus, Ohzo.
