ADSORPTION AT CRYSTAGSOLUTIOK INTERFACES*
IV. Macroscopic Ammonium, Cesium and Potassium Alum Crystals grown in the Presence of Dyes and other Foreign Materials. BY M. E. LASH A N D W. G. FRAXCE
Introduction In previous articles the growth' and solution of single copper sulfate crystals and the growth of potassium alum2 and ammonium alum3 crystals in the presence of dyes was studied. It was found that dyes modified the velocity of perpendicular displacement of the crystal faces and also modified the crystal habit. These effects were attributed to the specific adsorption of the dyes a t the crystal-solution interface. The present investigation is a continuation of the study of selective adsorption by crystal kaces using ammonium, potassium, and cesium alum crystals. The work consists of (A) the determination of the growth ratio of ammonium alum in the presence of two dyes and also for the pure alum using cubic seed crystals. (B) A comparison of the effects of various dyes upon the crystal habit of ammonium, potassium and cesium alum. (C) A study of the influence of the state of dispersion of dyes in alum solution upon their effectiveness in modifying the crystal habit. (D) An attempt to ascertain the thickness of the layer of dye adsorbed on the ionic planes of the crystal during its growth. (E) The measurement of the interfacial angles of a crystal containing adsorbed dye. Experimental A. Growth Ratio Measurements. (I) The Growth Ratio of Ammonium Alum as affected by Dyes. By growth ratio is meant the ratio of the perpendicular displacement of the cube faces to that of the octahedral faces of the alum crystal. I n the previous papers of this series of investigations it has been shown that the determination of this ratio affords a measure of the repressing effect of foreign substances upon the growth rate of the faces. The study undertaken required close control of the conditions of temperatiire and humidity and made use of a motion picture record of the growth of individual macroscopic crystals. The apparatus used was essentially that described by Keenen, Bennett and France4 and consisted of a large air thermostat and three mechanical units; ( I ) the thermal regulator, ( 2 ) the illuminating and optical system and (3) the automatic mechanism controlling the camera and illuminating system.
* In part from the diasertation presented to the Graduate School of The Ohio State Cniversity by M. E. Lash, August 1928 in partial requirement for the Ph.D. degree. * T. S. Eckert and W. G. France: J. Am. Ceramic SOC.,10, 579 (1927). 2 F. G. Keenen and W. G. France: J. Am. Ceramic SOC.,10, 821 (1927). 3 G. W. Bennett and W,G. France: J. Am. Cerarmc SOC.,11, 571 (1928). 4 Keenen, Bennett and France: J. Am. Ceramic Soc., 10, 435 (1927).
ADSORPTION AT CRYSTAGSOLUTION INTERFACES
725
The procedure followed was that previously developed in this laboratory and described in detail by Bennett and France. Ammonium alum crystallizes in cubo-octahedrons. The principal face is the octahedral and while cube faces are always present in the normal crystal, they are usually quite small. Frequently when foreign materials are adsorbed by a given face the adsorption retards the perpendicular growth, with the result that that area of such a face increases and often exceeds the principal faces in area. One can therefore usually determine whether or not a substance is adsorbed by observing if the habit of the crystals is changed when they are grown in the presence of the foreign material. Preliminary to the growth ratio measurements, qualitative tests were made. Small crystals of ammonium alum were secured to nichrome wires and allowed to grow in saturated solutions containing from one half to one percent of foreign material. Thirty-nine compounds which contain polar groups such as "2, OH, S03Na and COOH were found t o have no effect upon the crystal habit of ammonium alum. These compounds are given in Table I. A new series of eighteen dyes was tried in a similar manner; all but four were without effect, These are designated by the numbers 4, 11, 12 and 13 and their formulas are shown below. Dye No. 4. K a o 3 s c , - K
=
N17-K
=
X--(>H*
.
= N ~ - . H - < z >
I
SO&a Dye
KO.1 2 . HzK
Na03SDye
KO.13.
OH KH2
OH
("v\/
I-S03Na =
a - K = N - ( ) q Na03S \/\/-
S03Pl'a
726
hl. E . LASH A S D W. G. FRAKCE
TABLE I Compound added
Acetamide Hydroquinone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hexamethylene tetramine. . . . . . . . . . . . . . . . . . . . . . . . . . . Raffinose, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hippuric acid. . . . . . . . Leucine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alanine. . . . . . . . . . . . . . . . . . . . . . . . . Phenyl alanine .............................. Guanidine hydrochloride.. . . . Cocaine hydrochloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phthalimide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diphenylamine. . . . . . . . . . 2,4, Dinitro phenol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Propyl resorcinol . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benzidine . . . . . . . . .. .................. Picric acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P Magnesium phenol sulfonate., . . B Naphthalene sulfonic acid. . . . . . . . . . . . . . . . . . . . . . . . . P Toluene sulfonyl aniline.. . . . . . . . . . . . . . . . . . . . . . . . . . Sodium-p-toluene sulfonate. . . . . . . . . . . . . . . . . . . . . . . . . . Sulfanilic acid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Amino toluene 4 sulfonic acid.. . . . . . . . . . . . . . . . . . . . . Alpha naphthylamine chloride. . . . . . . . . . . . . . . . . Hydroxylamine hydrochloride. . . . . . . . . . . . . . . . . . . . . . . . Hydrazine hydrochloride. . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium-p-phenol azobenzene sulfonate. . . . . . . . . . . . . . . . Ortho aminophenol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A,cetoxime. . . .............................. Ethyl urethrane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ethyl-p-toluene sulfonate. . . . . . . . . . . . . . . . . . . . . . . . . . . . Benzidine sulfate. . . . . . . . . . . . . .................. 2 Naphthylamine 7 sulfonic aci ..................
Effect on Ammonium Alum Crystal X O
effect
11
12
1,
,,
11
19
11
11
,,
1,
11
12
11
!l
,, ,, ,, 3,
11
19
1,
,,
,, ,, >l
,I
11
>I
71
11
12
11
19
19
19
11
11
l!
,, ,, ,, ,, ,I
1,
1,
),
,, ,, j,
f,
,, ,,
,, ,f
1, ),
91
11
11
11
,, ,, ,, I,
l!
Ethyl chlorohydrin. . . . . . . . . . . . . . . . . . . . . . . . . . . Acetaldehyde. . . . . . . . . . . . . . .................. Methyl p-amidophenol. . . . . . . . . . . . . . .
........................
,, ,, 12
19
fl
,l 21
>, ,,
ADSORPTION AT CRYSTAL-SOLUTIOS I N T E R F A C E S
727
TABLE I1 Data (Summarized) Dye So. 4 Ratio
g. dye
Expt.
IO0
I .
0.;
8.9
0.06
0 . 0 0j o o
5.
1.4
i.7
0.18
0 .GO37 j
I11
per too c c .
not used-film
5.
fogged
2 .
2.6
5.5
2.2
j .o
0.47 0.54
0.002jo
6. 3.
4.0
5.2
0.95
0.00125
I '
3.5
1.17
0 .OOIOO
8.
5.5
3.0 3.6
Expt.
IO0
.53
I
o.00182
0 , 0 0 07
5
Dye S o . 13
I .
1.3
2 .
0.4
3.
2 . 1
4.
I
.6
I '
8.
12.2
6.3 7 .8 6.9
not used-crystal
5 .
6.
Ill
5.4 3.6 4.8
Ratio
g. dye per 100 cc.
0.11
0.002j
0.06
0 . 0 0 12.5
o
0.27
0.00080
0.23
0.000~0
irregular
5 . 2
I
.05
0.00030
2.3 3.2
1.57
0.00020
I
.so
0.00020
T A B L E 11 Data (Summarized) Pure Alum from Cubical Seed Crystals
Expt.
IO0
I11
Ratio
I .
11.2
j . 1
2.20
2 .
11.9 9.3 10.6
j .O
2
4.0
2.32
4.4
I
3. 4.
.38 .30
Deviation -5.1%
+ z .6 0
.o
$3.4
Mean of ratios 2.32 + 0.03 Average deviation 2 .8'3 Dye No. 1 2 repressed the perpendicular growth of the cube faces only slightly. These dyes were of a high degree of purity and it is believed that the effects observed are the result of the action of the dyes rather than of any impurities present, Such impurities as XaC1, alpha-naphthylamine, and diphenylamine which may have been present in small amounts were found t o have no effect on the alum even when present in concentrations as high as one half percent. Before being used for the groivth ratio measurements these dyes were further purified by repeated salting out of water solution and subsequent recrystallization.
728
M. E. LASH AND
W.
G. FRANCE
Dyes No. 4 and No. 13 only were used in the growth ratio determinations. Dye No. 1 2 could not be used because at the concentration necessary to affect the growth ratio, the limited amount of light which passed through the solution was insufficient to affect the photographic film. Dye No. 1 1 could not be used for similar reasons, particularly because the color of the dye in alum solution is deep red which rendered the transmitted light low in actinic value. The data and results obtained in these measurements are summarized in Table 11. The columns (100)and ( I I I ) represent the perpendicular displacements between parallel cube and parallel octahedral faces respectively. The results are also shown graphically in Fig. I . I (2) G r o w t h R a t i o of P u r e 0 -4mmonium Alum when grown from 0 Cubical Seed Crystals. The growth ratio for ammonium 0 alum has been found by Bennett Grams of Dye per 100 c c Solution and France to be 1.53. However it FIG.I might be expected that this ratio would be different if instead of a normal crystal with octahedral faces present, a seed crystal having only cube faces, was used. Due to the large area of the cube facescompared to that of theoctahedral faces, the velocityof their perpendicular displacement should be greater. Seed /a crystals for these measurements were obtain1m ed by mounting very small crystals of ammonium alum on copper wires by plunging t: 9, the heated end of the wire into the crystal. 0" 70 They were allowed to grow in a solution of the salt containing 0.01percent of dye No. 4. %W The resulting crystals were very nearly perca $3 fect cubes and weighed about 0 . 2 gram. Pa The data and results are summarized in 0 IO Table 111. It will be seen that the growth * O E 4 6 8 ID IZ /1 I6 18 ZV 22 ?4 Time tn H o u r s ratio Vloo/Vlll is 2.32. Fig. 2 shows the FIG 2 time rate of the perpendicular displaceGrowth Rates Of Ammonium *lum ment of the cube face for both cubical and normally shaped crystals. B. A Comparison of the Effects of Dyes upon the Crystal Habit of Ammonium and Cesium Alums. I n part (A) the effect of eighteen dyes upon the crystal habit of ammonium alum was determined. The procedure was repeated using potassium alum with the same results except for dye KO. I Z which was without effect. It was thought advisable to determine whether those dyes which modified the habit of ammonium and potassium alum, would also produce the same effects on cesium alum. Due to the low solubility of cesium alum only @ :
ADSORPTION AT CRYSTALSOLUTION INTERFACES
729
small crystals were obtained and no growth ratios were measured. The dyes were added t o the saturated alum solutions which were permitted t o evaporate slowly. The crystals formed were removed and examined with a microscope. Dye No, 13 which modified the habit of both the ammonium and potassium salts colored the crystals uniformly but did not change their shape. Dye No. 2 which repressed the perpendicular displacement of the cube faces of ammonium alum to a small extent, and which left potassium alum unaffected, had no effect upon the crystal habit of cesium alum, although the crystals were uniformly colored. Oxamine blue and dye No. 11, both of which were effective in modifying the crystal habit of ammonium and potassium alums behaved similarly with cesium alum. Bismarck brown which did not modify the crystal habit of ammonium alum, and which affected that of potassium alum was found to have a very pronounced effect upon cesium alum. The crystals produced were brown and nearly perfect cubes. C. The State of Dispersion of Dyes in Saturated Alum Solutions. A Zsigmondy slit ultramicroscope was used to examine the dyes both in pure water and saturated alum solutions in order to determine their state of dispersion. The solutions contained about 0.003 percent of dye. In all of them, the presence of colloid particles was revealed. The number of particles, however, varied widely among the different solutions. Dyes showing only a few particles were diamine sky blue, dye No. 4, dye No. 13, ahd anthraquinone green, while oxamine blue, Bismarck brown and dye No. 11 showed a great many; others were intermediate. In connection with these observations the sign of the charge of the colloid particles present in the water solutions was determined. Of the dyes diamine sky blue, oxamine blue, anthraquinone green, Bismarck brown, No. 4, No. 5 , KO.11, No. 12, and No. 13, all were negative except Bismarck brown which was positive. There appears to be no relation between the colloidal content or the sign of the charge of the particles and the action of the dyes on the growing crystals. The process of ultrafiltration was applied to alum solutions containing dyes. Very dense collodion filters removed all the color, less dense filters removed only a part of the color but practically all the colloid particles as shown by examination in the slit ultramicroscope. This indicates that the dyes may be both molecularly and colloidally dispersed. D. The Relation between the Weight of Dye adsorbed and the Area of the Adsorbing Surface. The shape of the curves in Fig. I suggests the possibility that at small values for the growth ratios, the dyes may be adsorbed in monomolecular layers. The following procedure was used in order to obtain information regarding this point. (I) Five crystals of ammonium alum were selected, uniform as t o size and symmetry. ( 2 ) The area of the cube faces was determined by use of a microscope equipped with a mechanical stage. (3) The crystals were mounted on nichrome wires and allowed to grow in ammonium
i30
M. E. LASH AND TV. G. FRANCE
alum solution containing 0.01 per cent of diamine sky blue for eight hours a t 30.0 C. + 0.2’. (4) The crystals were removed from the solution, dried and weighed. (j) The area of the cube faces was again measured as in ( 2 ) . (6) The cube faces of the crystals were then very carefully scraped with a razor blade until no more dye was visible. ( 7 ) The remnants of the crystals were weighed. This weight, taken from that obtained in (4) gave the weight of alum scraped off the cube faces. (8) The alum and dye scraped off the cube faces were dissolved in water and the weight of dye determined colorimetrically. Area of cube faces before growth 119 o m m 2 Area of cube faces after growth 430 6 ” Wt. of alum scraped off cube faces o 1928 g. Wt. of dye adsorbed o 0000468 g. Diamine sky blue, molecular weight 9 9 2 .
~ ~ of dye adsorbed. 1 0 = ~ ~ 28.6 X 1 0 molecules The work of IT. H. and R. L. Braggl indicates that the benzene and naphthalene molecyles are flat and that t$ former covers an area of approximately 36 sq A., and the latter 54 sq. A. Assuming that the two naphthalene and two benzene groups of the diamine sky blue molecule lie esse?tially in one plane, the area covered by these groups would be 180 sq. A. The area cov2red by the attached groups is not known but is perhaps less tha? 180 sq. A. For the purpose of calculation it will be assumed to be 1 2 0 sq. A, making theototal area assumed to be covered by the diamine sky blue molecule 300 sq. A. Mul$plying 300 sq. by the total ?umber of dye molecules adsorbed, 300 sq. A X 28.6 X IO'^ = 8 j8 X 1 0sq~ -1. ~ the total area covered by a monomolecular layer of the dye. It remains to compare this area with the total area of the ion planes present in the cube faces of the crystals during their growth, that is, the total area of the surface on which the dye mas adsorbed. This total surface area was computed from the above data together with the information given by Kyckoff* on the crystal structure of the alum. The volume scraped off the crystal is 0.1928 g. + 1.645 (the density of alum) = 0.1172 cm3 or 1 1 7 . 2 mm3; and the perpendicular displacement of the cube faces during growth is therefore 1 1 i . 2 mm3 t 430.6 y m 2 = 0 . 2 7 mm. As there are six planes of ions in a distance of 12h perpendicular t o the cube face in the alum crystal, the number of ion planes on which the dye
(0.0000468/992) X 6.06 X
K.
IT. H. and FT. L. Bragg: “X-Rays and Crystal Structure,” Chapter 14. (1923).
* Wyckoff. Am. J. Sei., (j)205, 209
ADSORPTION AT CRYSTAL-SOLUTION INTERFACES
73‘
was adsorbed is (0.27 x 10’ +- 1 2 ) X 6 = 13.5 X 1 0 5 and the mean area of the planes is (430 mm2 119 mm2) t 2 = 2 7 5 mm2. Therefore the total area of the adsorking surface is 2 7 j mm2 X 13.j X 105 =, 3 7 I 2 X 105 mm2 = 3712 X 1 0 ~ 9sq. A, or in round numbers 3700 X 1019 sq. A. The area covered by the dye molecules themselves has been shown above to be 858 X IO'^ sq. A. A comparison of these areas shows that the area of the surface on which the dye was adsorbed is about 4300 times that covered by the dye if present as a monomolecular layer. It is realized that the errors in this measurement are necessarily great both because of the lack of adequate technique and because more information is needed regarding the size and shape of the dye molecule, its arrangement in the crystal, and the state of aggregation of the dye in the solution. Therefore no claim is made for the accuracy of the results. The results indicate in a qualitative way that this adsorption is perhaps not in the form of a continuous mono-molecular layer a t the ionic planes. The thickness of the layer of dye adsorbed by crystal powders has been studied by Psneth and Vorwerk’ and Paneth and Radu2. In the case of the adsorption of Ponceau 2 R by lead sulfate, their measurements indicate that when the crystals have taken up a maximum of dye, the surface is covered by a monomolecular layer. Bancroft and Barnett3 in studying the adsorption of methylene blue by lead sulfate find that the amount of adsorption is a function of the pE of the solution and that Paneth’s method of determining the total adsorbing surface on the assumption that the dye molecules are present as a monomolecular layer, is in error. However these cases are perhaps not strictly comparable with the work presented in this paper since the adsorption here is by growing crystals whereas Paneth and Bancroft are considering the adsorption by crystal surfaces already developed. E. The Effect of Dye Adsorption on the Interfacial Angles of Ammonium Alum. A two-circle goniometer was used to measure the angle between the cube and the octahedral faces of an ammonium alum crystal grown in the presence of 0.005 percent dye S o . 13. The normal value for this angle is j4’ 44’. The angles measured were j4’ 4 2 ’ ; jq0 38’; 54’ 39’.
+
Discussion The results obtained in this investigation will be considered on the following basis : adsorption of foreign materials by growing crystals is dependent on ( I ) the nature of the residual valence force fields in the crystal faces; those faces having alternating planes of ions of like charge exhibiting a much greater adsorbing power than those with a checkerboard arrangement of the positive and negative ions; ( 2 ) the interionic distances in the crystal lattice; and ( 3 ) the presence and spatial arrangement of polar groups in the molecules of the adsorbed material. Paneth and Torwerk: Z . physik. Chem., 101, 480 (1922). physik. Chem., 101,488 (1922). Bancroft and Barnett: Colloid Symposium Monograph, 6,73 (1928).
* Paneth and Radu: Z.
M. E . LASH AND W. G. FRANCE
732
The adsorption of foreign materials by crystals has generally been regarded as very highly specific in character. This specificity of action may be attributed in part to items ( 2 ) and ( 3 ) in the paragraph above. If adsorption were due only to the presence of polar groups, all substances containing them should be adsorbed. Such is by no means the case. The configuration of the molecule or the distribution of the polar groups is most likely a contributing factor. I n support of this idea, one experimental fact may be cited; dyes No. 1 2 and No. 13 are isomeric, they are identical except for the relative position of the two S03Na groups in the naphthalene nucleus. Dye No. 13 was adsorbed by ammonium alum a t a very low concentration while dye No. I 2 was adsorbed but slightly a t a very high concentration. Korbsl discovered that sodium chloride when crystallized in the presence of formamid was octahedral in form, whereas the formamid had no effect on the crystal form of potassium chloride. Since the two salts have identical structures, the specificity of adsorption is most likely due either to the kind of ions present or to the interionic distances in the crystal lattice. Similar behavior has been observed in the comparison of the effect of dyes upon ammonium, potassium and cesium alums. Thus while Bismarck brown is not adsorbed by ammonium alum, it is adsorbed to a moderate extent by potassium alum, and very markedly by cesium alum. Oxamine blue and dye No. I I are adsorbed by all three salts; dye No. 1 3 is adsorbed by ammonium and potassium alum but not by cesium alum; dye KO.1 2 is slightly adsorbed by ammonium alum but not by the other two. This preferential adsorption may be due in part to the difference in the size of the ammonium, potassium, and cesium ions, or to the distances between the ions in the crystal lattice. The ultramicroscopic examination of the solutions of dye and alum showed that particles of colloidal dimensions were present in all the solutions. Some dyes appeared to be dispersed very largely in the colloidal state and others only slightly so. No relation between the apparent state of dispersion of the dye and its effectiveness in repressing the growth rate of alum was observed. The sign of the charge of the colloid particles does not indicate whether or not a dye will be adsorbed by any given crystal. It would seem then that adsorption of the dyes by the alum crystals is largely independent of the colloidal state and therefore must be dependent upon the molecular or ionic dispersity. The curves in Fig. z show the more rapid growth of the cube faces of ammonium alum when a cubic seed crystal was employed in place of a normally shaped crystal. It should be noted that Loir? observed that cubic crystals of alum grew three times as fast as octahedral crystals when both were suspended in pure alum solution. This more rapid growth of the cube faces is to be expected on the basis of the theory of crystal growth advanced by Niggli3 in which he states that the velocity of the perpendicular displaceKorbs: 2. Krist., 43, 451 (1907).
* Loir: Compt. rend., 92,
1166 (1881).
Niggli: 2. anorg. allgem. Chem., 110, 55 (920).
ADSORPTION AT CRYSTAL.-SOLUTION INTERFACES
733
ment of a crystal face depends on the thickness of the layer of crystallographically unsatisfied ions. The thickness of this layer is maximal for vicinal faces and hence they rapidly disappear. The cube face of the alum crystal has a higher growth velocity than the octahedral; hence if the cube faces are the only ones present, as in the case of the cubic crystals, their perpendicular growth will be more rapid than normal until they are reduced to their usual area or disappear entirely. From the comparison of the area of the adsorbing surface of the crystals with that which the adsorbed dye could have covered as a continuous monomolecular layer, it is apparent that such a layer is most likely not present in the cases here studied. It is of course recognized that there are large errors involved in the measurements such as the assumption that the dye is molecularly dis[I:? persed, whereas ultramicroscopic examination shows that some particles of colloidal y dimensions are present, and also the uncertainty of the size of the dye molecule and ,, area of the ionic planes. The adsorbed .i molecules or ions of the dyes probably pro$6 ’‘ a a a ’9 a 2’ 23 24 25 duce fields of force around them due to the no8 unsaturation of the polar groups. These FIG.3 force fields may inhibit growth over the en- Growth Rates of Ammonium Alum tire range of their forces which would exO f Dye No. I 3 tend laterally over considerable area and vertically through several layers of planes of ions. The break points in the curves (Fig. I ) in which the growth ratios are plotted against dye concentration, may be thought of as representing not a monomolecular layer of adsorbed dye, but a “saturated surface” in which the adsorbed dye exerts an almost completely inhibitory influence upon the further perpendicular displacement of the cube faces. That there is certain concentration of dye a t which this “saturated surface” can be formed was indicated by two determinations of the growth ratio of ammonium alum in the presence of o.oooz percent of dye No. 13. The curves in Fig. 3 show the distances between the 1 1 1 faces plotted against the corresponding distances for the I O O faces, which was the method followed in determining all the gror+*h ratios. At the very low concentration of o.oooz percent no adsorption took place and the growth ratio has the value of that of pure alum or 1.53. As the solution evaporated, the dye became more concentrated and after reaching a certain value was able to form a “saturated surface” with the result that the perpendicular growth of the cube face was quite suddenly checked as shown by the abrupt break in the curve. The results obtained from the study of adsorption of dyes by ammonium, potassium and cesium alums are in harmony with the theory previously advanced ( I ) that adsorption by a growing crystal is closely connected with its structure. ( 2 ) that adsorption will take place on those faces populated by ions of like charge and hence with strongest residual valency force fields. ( 3 ) @
&;? gE:;le
734
M. E. LASH AXD W. G . FRANCE
that the interionic distances within the faces, and (4) the presence and spatial arrangement of polar groups in the adsorbed material are also factors, On the basis of the determination of growth ratios, and the weight of adsorbed dye together with the area of the adsorbing surface, it may reasonably be concluded that there is a minimum concentration of dye in the solution at which the dye is adsorbed to form a “saturated surface,” and that the dye is present in this “saturated surface” in an amount many times less than that required by a continuous monomolecular layer. The conclusions drawn in this work are not in agreement with those of Saylor’ who studied the modification of crystal habit by foreign substances. Drops of saturated solutions containing foreign substances were permitted to evaporate a t room temperature on microscope half slides. The development of the crystals was observed through a microscope on the stage of which the half slides were placed. The following quotation, J . Phys. Chem., 32, 1446 is illustrative of his views. “Using acid and alkaline solutions, their influence upon external crystal form has served as a key to the entire field of adsorption and has tied in with those examples where adsorption can actually be demonstrated. This new technique is absolutely general; it applies to all crystal systems; it applies t o crystals growing from solvents other than water; and it can be applied to crystals growing from a melt if the chemistry of the melt is understood sufficiently.” From this the conclusion is drawn that if a given habit results from growth in an acid medium it is due to the adsorption of hydrogen ions and therefore all readily adsorbed cations will produce the same form. If another habit is favored when the crystal is grown in an alkaline medium, this is due t o the adsorption of hydroxyl ions and therefore all readily adsorbed anions will produce this form. The statement is also made p. 1455 that “Negative ions are adsorbed on the octahedron faces of the alkali halides and barium nitrate, on the cube faces of potassium alum, and on the pyramids of sodium nitrate. The other principal faces adsorb positive ions.” The results obtained in this laboratory certainIy are not in harmony with the above stated generalization. In all the cases studied in which adsorption and modification of the crystal habit occurred, the growth ratio varied in such a way as to indicate adsorption only by those faces populated by ions of like charge, If the other principal faces adsorbed any material a t all the effect on the repression of the perpendicular displacement of such faces was negligible in comparison to the effect on the other faces. I n an attempt to reconcile the results of Keenen and France2on potassium alum and dyes with his generalization, Saylor decides on the basis of some diffusion experiments that diamine sky blue and Bismarck brown are not C. H. Saylor: Bancroft’s “Applied Colloid Chemistry,” 2nd. Ed., p. 198; Colloid Symposium Monograph, 5, 49 (1928); J. Phys. Chern., 32, 1441 (1928). Keenen and France: J. Am. Ceramic SOC., 10, 821 (1927).
ADSORPTION AT CRYSTAL-SOLUTION INTERFACES
735
in true solution but that naphthol yellow, quinoline yellow, methyl violet, and methylene blue are. He states: "The dyes which are in true solution do not alter the crystal form of potassium alum." Relative rates of diffusion were determined by placing saturated alum solutions containing the dyes in test tubes fitted with rubber stoppers carrying glass tubes 3 mm. in diameter and 2 0 crns. long. These tubes were sealed off a t one end, filled with saturated alum solution and immersed open end down in the dye alum solutions in the test tubes. The progress of the diffusion was observed by the upward movement of the color boundary in the tubes. Potassium permanganate was used as a standard for molecular dispersion. After standing two days in a quiet dark place the potassium permanganate, methyl violet, naphthol yellow and methylene blue had diffused to the tops of the tubes. Since 2 0 crns. diffusion in 48 hours seemed excessively high, the experiments were repeated by Rlr. A. H. Burkholderl in this laboratory. The first results obtained were unsatisfactory due to the fact that small crystals formed in the tubes, when the temperature of the room dropped slightly. When these became detached from the walls they fell through the tube and stirred up the solution in the test tubes. The difficulty was avoided by saturating the pure alum solutions, placed in the small tubes, about 2 degrees below room temperature. This resulted in a concentration difference which accelerated the diffusion velocity, however, under these conditions the stirring effect was eliminated and the maximum diffusion observed in five weeks was but 15 crns. Diffusion constants were calculated and found to be 19, 2 2 , 1 2 , 4 and 18 for naphthol yellow, methylene blue, quinoline yellow, diamine sky blue, and Bismarck brown respectively. The use of these constants as a criterion for molecular dispersion would lead one to conclude that both Bismarck brown and naphthol yellow are similarly dispersed, a conclusion not reached by Saylor. Diamine sky blue has a molecular weight about three times that of either naphthol yellow or Bismarck brown and its diffusion velocity would therefore be expected to be less than that of the other two dyes. If Saylor's generalization is correct then one would expect that all baiic dyes forming readily adsorbed cations would produce the same habit in the same crystalline substance. This is not the case when lead nitrate is grown in the presence of the basic dyes Bismarck brown and methylene blue. Both are adsorbed but the former produces octahedra and the latter cubes. From these considerations it would appear that no simple rule has yet been found that enables one to predict just what foreign materials will be adsorbed by any one crystalline substance. However the result? obtained thus far in this laboratory seem to warrant the conclusion that if a foreign substance is going to be adsorbed by a given crystal, the adsorption, resulting in a modification of the crystal habit, will occur at those crystal planes having the stronger fields of force. Such plane? will in general be populated I 11.S.Dissertation, O.S.C., ".Idsorption a t Crystal Solution Interfaces," A . H. Burkholder, June 1929.
M. E. LASH AND W. G. FRASCE
736
by ions of like charge rather than by mixtures of like and unlike. I n general those substances containing polar groups will be more readily adsorbed than non-polar compounds. summary Growth ratios have been determined for (a) ammonium alum in the presence of two dyes; (b) pure ammonium alum when grown from cubic seed crystals. 2. The effect of eighteen dyes and thirty-nine simpler organic compounds upon the crystal habit of ammonium alum has been determined. 3 . The effect of certain dyes upon the crystal habit of ammonium, potassium and cesium alums has been compared. 4. The state of dispersion of dyes in water and in alum solution has been investigated by the use of the ultramicroscope and the ultrafilter. 5 . The sign of the charge of the colloidal particles in dye-water solutions has been determined. 6. The possibility of the dye being adsorbed as a monomolecular layer has been considered, and an approximate determination of the thickness of the layer has been made. 7 . Interfacial angles of an ammonium alum crystal on which dye was adsorbed have been measured. I.
The Chemical Lahoratories oj The Ohio State Uniziersity.
