ADSORPTION AT CRYSTAL-SOLUTION INTERFACES. V I P
EFFECT OF STIRRING AND GROWTH RATESON THE HABITAND DYEADSORPOF ACID AND ALKALI TION OF ALUM CRYSTALS.INFLUENCE MEDIAON THE HABITOF ALUM CRYSTALS PHOEBE A. PAINE
AND
WESLEY G. FRANCE
Department of Chemistry, The Ohio State University, Columbus, Ohio Received June 5, 193.$
The present study is an extension of previous work carried out in this laboratory on adsorption at crystal-solution interfaces (2, 3, 4, 5, 10). Part I is concerned primarily with the effect of stirring on the habit of alum crystals, grown both from pure solutions and from solutions containing small quantities of dye. Part I1 deals with the influence of acids and alkalies on the habit of alum crystals. I. EFFECT O F STIRRING AND RATE O F GROWTH ON THE HABIT AND DYE ADSORPTION O F ALUM CRYSTALS
Observations were made of the habit of individual potassium alum crystals grown both from stirred and from unstirred solutions. Quantitative measurements of the change in the relative growth rates of the cube and octahedral faces caused by stirring and by the adsorption of diamine sky blue were made and expressed in terms of the growth ratio Vl0~/Vll1 where Vl0o and Vlll represent the relative rate of growth (i.e., perpendicular displacements) of the cube and octahedral forces respectively. The measurements were made by photographing the growing crystal every hour over periods ranging from twenty-four to forty-eight hours, using the apparatus and method described in previous papers. The growth ratios so obtained are given in table 1. The above work was carried out in a constant temperature room held at 30°C. in which the humidity was controlled by the presence of open jars of calcium chloride. Further studies were made a t about 23°C. (room temperature) with no effort to control the humidity. Potassium alum solutions containing varying concentrations of diamine sky blue were allowed to evaporate both with and without stirring and the resulting crystals analyzed colorimetrically to determine the amount of dye adsorbed. The results are given in table 2. 1 Presented in part before the Division of Colloid Chemistry a t the Eighty-sixth Meeting of the American Chemical Society held in Chicago, September, 1933.
425
426
PHOEBE A. PAINE AND WESLEY G . FRANCE
Ammonium alum solutions containing varying concentrations of crystal violet were allowed to crystallize by cooling solutions saturated a t 30°C. to room temperature (23°C.). The crystals were filtered off and the filtrate allowed to evaporate a t room temperature until another crop of crystals was obtained. The results of the colorimetric analysis of these crystals are given in table 3, in which the column headed “hot solution” refers to solutions saturated a t 30°C. and “cold solution” to those saturated at 23°C. From the results given in table 1 it is evident that the effect of stirring on potassium alum crystals grown both from pure solutions and from TABLE 1 Growth ratios of potassium alum crystals Vlrn/Vl11
SUBSTANCE
Stirred Unstirred -~
Pure potassium alum... . . . , . . , . , . . , . , . . , . , . . . . . . . . . . . . . . . . . . . . Potassium alum 0.002 g. of diamine sky blue per 100 cc.. . . . . Potassium alum 0.004 g. of diamine sky blue per 100 cc.. . . . . Potassium alum 0.006 g. of diamine sky blue per 100 cc.. . . . . Potassium alum 0.008 g. of diamine sky blue per 100 cc.. . .. .
1.75 1.66 0.75 0.79 0.45
+ + + +
1.61*
0.38 0.29 0.00
TABLE 2 Adsorption of diamine s k u blue by potassium alum crystals DIAMINE SKY BLUE PER CC. OF BOLUTION
100
PERCENTOFDYEADBORBED
I
Stirred
Unstirred
0,008 0.017 0.055 0.080
0.01 0.04 0.09 0.13
grams
0.002 0.004 0.006 0.008
solutions containing varying concentrations of diamine sky blue is to decrease the size of the cube faces, i.e., to increase the V1oo/Vlll ratio. Other workers have observed a similar decrease in the size of cube faces in the case of potassium alum grown from supersaturated ( 8 ) , rapidly cooling, or rapidly evaporating solutions (1). A consideration of Valeton’s theory (9) of crystal growth suggests a plausible explanation. Valeton assumes that the actual rate of growth of any crystal face depends on two factors: (1)the specific rate of growth of that face (i.e., perpendicular displacement) which is dependent only on its ionic character, and (2) the speed with which the ions can diffuse to the face, Since the specific growth rate of the cube face is very high (because the face is populated by ions of like charge),
ADSORPTION AT CRYSTAL-SOLUTION
INTERFACES. VI1
427
the actual growth rate is determined by the rate of diffusion to the face. Any condition (such as stirring or growth from supersaturated, rapidly cooling, or rapidly evaporating solutions) which increases the rate of diffusion will increase the actual growth rate of the cube face and hence the V1oo/Vlll ratio. The relatively greater si22 of the cube faces on crystals grown from unstirred solutions containing dyes than from stirred solutions can undoubtedly be accounted for in part by the same explanation that holds for crystals grown from pure solution. A second factor, smaller dye adsorption from stirred solutions as is shown in table 2, also is involved. The dye is adsorbed on the cube faces only, so that obviously the less color is adsorbed, the less the inhibiting effect on the growth rate of the cube faces, and the larger the V 1 ~ ~ / V ratio. ~ 1 l This lesser adsorption of color from stirred solutions can be explained by the increased rate of diffusion of the TABLE 3 Adsorption o j crystal violet by ammonium alum crvatals CRYSTAL VIOLET PER 100 CC. OF SOLUTION AT 23°C.
P E R C E N T O F D Y E ADSORBED
Hot solution
I
Cold solution
~rams
0.03 0.03
0.0035
0.04 0.04
0.0540 0.0550
0.0028
0.025 0.023 0,076 0.084
alum ions; since these ions are available, they will deposit on the crystal in preference to the foreign dye particles. It has previously been stated that the rate a t which ions can reach the cube faces of alum is increased when crystals are obtained by rapid cooling or evaporation as well as by stirring. It should follow, then, that less color should be adsorbed by crystals grown by cooling warm solutions than by those produced by slow evaporation at room temperature. The results given in table 3 show that this prediction is verified experimentally. 11. INFLUENCE O F ACIDS AND ALKALIES ON T H E HABIT O F ALUM CRYSTALS
Numerous workers have reported that alum crystallizes as cubes from alkaline solution ; other investigators have reported changes in the habit of alum crystals grown in solutions containing various acids. Saylor (7) states that the influence of acid and alkaline solutions “upon external crystal form has served as a key to the entire field of adsorption and has tied in with those examples where adsorption has actually been demonstrated. The new technique is absolutely general”; and, further, “Nega-
428
PHOEBE A . PAINE AND WESLEY G . FRANCE
tive 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.” H e has used as one of his examples the formation of large cube faces on alum crystals presumably grown from alkaline solutions or from other solutions containing readily adsorbable anions, The present study was made to determine the influence of acid and alkaline media on the crystal habit of potassium and ammoniumalum. The solutions were made up by adding the proper amounts of the foreign materials to saturated alum solutions. Individual seed crystals mounted on copper or nichrome wires were suspended in the solutions and allowed to grow over periods ranging from a few days to several weeks. The concentrations are expressed as the number of grams or per cent of foreign material per 100 grams of saturated solution. Up to 25 or 30 per cent concentrations of hydrochloric, nitric, acetic, sulfuric, and phosphoric acids the crystals showed the usual cubooctahedral form; the 110 face which frequently occurred in pure or slightly acid solutions tended to disappear at higher concentrations. Above 25 or 30 per cent, clumps of tiny crystals or needle-like formations formed in the nitric and phosphoric acid solutions, while crystal growth almost completely ceased in the other acids. Crystals of potassium alum grown in 1per cent sodium carbonate solution were similar to those grown from pure solution. When a 2 per cent sodium carbonate solution was used, some aluminum hydroxide precipitated; when this was filtered off and crystals allowed to grow in the clear filtrate, cubic crystals were formed and more aluminum hydroxide precipitated as the solution evaporated. Ammonium alum crystals in 1 per cent to 2 per cent sodium carbonate solutions were normal in habit. I n 3 per cent solutions aluminum hydroxide was precipitated; crystals grown in the clear filtrate were cubic and more aluminum hydroxide precipitated as the solution evaporated. The above results indicate that crystals grown from acid solutions do not show the enlargement of the octahedral faces predicted by Saylor’s “absolutely general technique.” Crystals grown in sodium carbonate solutions from which no aluminum hydroxide had precipitated showed no change in form; the cubic crystals obtained in solutions where some aluminum hydroxide had precipitated were not alum crystals but sodium or potassium sulfate. 111. SUMMARY
Stirring causes a decrease in the size of the cube faces of potassium alum crystals grown both from pure solution and from solutions containing varying concentrations of diamine sky blue. For a given concentration of
ADSORPTION AT CRYSTAL-SOLUTION
INTERFACES. VI1
429
dye, less color is adsorbed by crystals grown from stirred than from unstirred solutions, and by those obtained by rapid cooling than by slow evaporation at room temperature. An explanation is offered to account for these facts. No appreciable change in habit occurs when alum crystals are grown from acid solutions or from solutions containing sodium carbonate from which no aluminum hydroxide has been precipitated. REFERENCES (1) BUCKLEY: Z. Krist. 73, 449 (1930). AND FRANCE: J. Phys. Chem. 34,2236 (1930). (2) FOOTE, BLAKE, (3) FRANCE: Colloid Symposium Annual 7, 59 11930). AND FRANCE: J. Am. Ceram. SOC.10,821 (1927). (4) KEENEN (5) LASHAND FRANCE: J. Phys. Chem. 34, 724 (1930). (6) MARCAND WENK: Z. physik. Chem. 68, 104 (1910). (7) SAYLOR: J. Phys. Chem. 28,1441 (1928). (8) SHUBNIKOV: Z . Krist. 43, 433 (1914). (9) VALETON: Z. Krist. 69, 135, 335 (1924). A N D FRANCE: J. Phys. Chem. 36,2832 (1932). (10) WEINLAND
