A. Introduction

Report on Colloid Chemistry, p. 289 (1922). 11 U s e r : Ibid. p. 277. 11 Spurr: Eng. Mining J., 123, 204-5 (1927). la Hall: “Master's Thesis”...
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A IIUE TO m w E s

xr (:RYS~AI,

FA(

BY iMUTUAL ORIENTATION' SL'OAT AN" ALAN

w. c. MtiNZIES

A. Introduction A rnotual orientation is any regular arranmnwnt Crf crystah of 0111: substance on a crystal of another. Typical examph,s nr(' iliostcited in Plates I lind z . 1x1 genernl, rnutuul orientntionu take place whenever the point3 in farm and diinension the ntit plancs of two erystds coinci to zllow :L mutual interacbion of the e

Tiir din'wence between the poink in tlie net planes permissible in tlrc formrtim of mutual orientations varies. In rare cases, as will be shown Inter, it rcmhcs as inucli as thirty percent. At first it is dificiilt to see how one aubstancc can orimtate mother undcr theso conditons. Friedel' has, howt explained this diffeulty hy postulating a n orientated rruclous. According to Friedi4 it is only necessary for enough orientated suhstance 1.0 be h i d down to form B nucleus. The nucleus then impresses its own orientation on suhseqiiently deposited substance. The forces in operation in the formation of any mutual orient.ation may be classified as follows: ( I ) the mut.ual attraction between the solute ions and the substrate ions, ( a ) the mutual attraction between the solute ions and thc

_________

* From the thesis of C. Allen Slost, presented in partial fulfillment ofthe requirement 61 the degree of Doctor of Philosophy. ' Friedel: "Le(.ow de Cryatallogrsphie," 527 (1916)

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solvent, (3) the mutual orientation between the oppositely charged solute ions, and (4) the mutual attraction between the solvent and substrate. Of these (I) and (3) are the more important. As will be shown later, (2) and (4) are important only when the differences between the parameter of the salt and that of the substrate approaches the limit at which orientation ceases, Following Lennard-Jones and Dent: force (I) may be further analysed into four parts: (1%)the direct electrostatic attraction of the charge on the ion by the valency charges of the substrate, (xb) the attraction between these ions and the dipole produced by the polarization of the attracted ion by the field near the surface, (IC)the force due to the polarization of the surface ions by the attracted ion, and (Id) the force of attraction known to exist between neutral atoms-conveniently termed the van der Waals’ attraction. Contrary to popular belief, the above writers show that the van der Waals’ attraction falls off much more slowly with distance than does the electrostatic attraction. At small distances the electrostatic forces are much greater than the van der Waals’ attraction. As the distance between the surface and the ion increases, it is the van der Waals’ attraction that predominates. It is thus seen that it is the van der Waals’ attraction that acts a3 the first agent in the adsorption of an ion. When the ion has approached sufficiently close, it is the strong electrostatic force which completes the final capture and fixes the ion in the space lattice. When cleavage pieces are used as substrates, orientated crystals are usually deposited on the corners and along the edges formed by the step-like layers of the cleavage surface. (See Plate I ) Such depositions may be regarded as definite proof that the more exposed portions of the surface are surrounded by a stronger field of force than the flat portions. Indeed, such depositions are now regarded as independent evidence substantiating the “active spot” theory of the catalytic surface? The present research is both a study of mutual orientation and an attempt to relate and apply it to the study of specific problems of surface chemistry. The varied nature of the problems investigated necessitates the division of the paper into sections. The purpose and objectives of each study will be outlined in the appropriate section.

B. Repetition of the Work of Royer on the Orientation of the Alkali Halides on NaC1, KC1, and PbS The purpose in repeating Royer’s work on the orientation limits of the alkali halides on NaCl, KCl, and PbS was, first, to gain a knowledge of the technique involved in performing orientation experiments and, second, to investigate certain differcnces between the work of Royer and Barker. Royel-‘ arranged the alkali halides of the sodium chloride arrangement in a series according to the lengths of their parameters. He then tried to Lennard-Jones and Dent: Trans. Faraday SOC.,24, 92-108 (1927). J. W. C. Frazer: Eighth Report of the Committee on Contact Catalysis, J. Phys. Chem., 34, 2134-5 (1930). Royer: Compt. rend., 29, 2050-2 (1925);See der, Friedel: loc. cit. p. 526. a

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orientate each of these salts in turn upon NaC1, KCl, and PbS as substrates. The results of Royer’s experiments are presented graphically in Fig. I . (See p. 2009). The horizontal lines crossed or touched by the broken arrows indicate the salts found to orientate on each substrate respecitvely. A study of the existing literature reveals the fact that Barkels had previously performed some of the same experiments in using the method of mutual orientations to show that the cubic salts, NH4Br, NH4C1, CsCl, CsBr, and CsI do not belong to the same isostructural series as the other cubic alkali halides. Royer in his experiments found t h t t all t h e salts whose parameters lie between and include those of LiCl ( j . 1 4 A ) and KCK (6.5 j A) were orientated on NaCl. Of those salts whose parameters exceed that of KCN none were found to orientate. Barker, on the other hand, found a number of salts of parameter greater than that of KCN to orientate on NaC1, but was uncertain about the orientation of some salts Royer found to orientate. A comparison of the results of Royer and Barker in these cases is given in Table I.

TABLEI Comparison of the Differences in Results obtained by Royer and by Barker in Certain Doubtful Cases of Orientation when using NaCl as a Substrate Parameter in A Salt Barker Royer

6.45

6.57

6.60

NaI M?

KBr

ill

M

Ir

RbCl KI ?*I? M Ir Ir

7.05

M = mutual orientation Ir = irregular I n the case of NaI on NaCl Barker was uncertain about orientation because the XaI was so hygroscopic that it took on water and streamed down the sides of the substrate. The growth of RbCl on NaC1 was indeterminate. However, YaCl is orientated by RbC1. Barker, therefore, concluded that RbCl ought to be orientated on SaC1. Both KBr and K I were found by Barker to orientate on NaCl with ease. RbBr was not found to orientate by either worker. If X I orientates on SaCl, it means that the limiting difference allowable between KaC1 and that of theornost widely spaced salt found to orientate upon it has been increased half an Angstrom unit, thus making the total difference between the parameter of the substrate and that of the orientated salt 2 j . 2 instead of 16.35 percent of the parameter of the former. Preliminary experiments made upon galena were generally unsuccessful until it was discovered that a film of grease upon the surface of the galena had a deleterious effect upon its ability to orientate those salts whose parameters 5

Barker: Mineralog. Mag., 14, 23j-2j7 (1906);2. Kryst. Mineral., 45, 1-67 (1908).

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differed very much from its own. The addition of a small amount of the base corresponding to the salt was found to be very helpful in removing the adsorbed grease. A more thorough discussion of the effect of grease on the orientating action of galena will be given in a succeeding section. ilfaterials. For SaC1, rock salt was used. This a t times contained orientated inclusions, thus making it necessary to inspect carefully all cleavage pieces before using them for substrates. KCl crystals of sufficient size were grown from saturated solutions of this salt containing just enough formamide to give clear cubes. The galena came originally from the mines a t Joplin, %Io. I t contained a great many Orientated halite inclusions as shown by the white precipitate obtained with a solution of the orientated substance and AgSOR. These inclusions made it very difficult to get clean, smooth, cleavage pieces. The salts used were the C.P. grade of commerce or better. Any specimens of unknown purity were purified hy repeated crystallization. Rubidium hydroxide was made by t.he double decomposition of rubidium sulfate and barium hydroxide, Procedure. A drop of an aqueous salt solution, usually saturated, was placed upon a carefully inspected fresh cleavage piece of the substrate and allowed to evaporate. The result was then viewed under a microscope. In the cases where the salts were deliquescent, as were LiC1, LiBr, KF, KaBr, and S a I , the substrate was placed in a small brass box with glass top and bottom. By this means evaporation was secured by passing a stream of dry or warm dry air over the specimen. In the case of the most hygroscopic substances like LiC'1 and S a I , both the solution and the substrate to which it was applied were heated in order to secure the salt in the form of anhydrous cubes. Saturated solutions of very soluble salts like KC1 and KI sometimes deposit in the form of a thick heavy crust which obscures the orientation underneath. This difficulty was overcome by diluting the solution slightly and applying i t in smaller quantity. In the case of galena a fresh cleavage piece was boiled in 3 solution of the salt made slightly alkaline with the corresponding base. This served the double purpose of removing traces of grease and securing rapid evaporation. Results. N a C l as Substrate. All salts between and including LiCl and KI were found to be orientated from aqueous solution with the exception of RbCl and RbBr. K C l as Substrate. The limits for the orientation of aqueous salt solutions were found to be the same as those determined by Royer. All salts between and including LiBr and RbBr were readily orienhted.

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Licl

5.14

h

,488

8.66

1

70

t

2 FIG.I

PbS as Substrate. All salts between and including AgCl and RbBr were found to be orientated from aqueous solution. Of these KCN, KBr, RbCl and RbBr were not obtained as mutual orientations by Royer. The results obtained for all substrates used are summarized and contrasted with those of Royer in Fig. I .

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The coarse broken line and the continuous black line indicate the orientation limits established by Royer and by ourselves respectively. The sign, , indicates that the salt was not orientated from aqueous solutioon. The figures to the right of each arrow head represent the difference in Angstrom units and in percent between the substrate marked R and S and the first and last salt to be orientated.

\

Discussion of Results. From the figure it is seen that there is no difference between the results of Royer and ourselves in the case where KC1 was used as the substrate. This is, however not true of the other two substrates, PbS and KaC1. With PbS as a substrate, it is seen that we haye obtained the orientation of four additional salts, KCK, KBr, RbC1, and RbBr, thereby increasing the limiting difference between the parameter of P,bS and t$at of the most widely spaced salt orientated upon it from 0.48 A to 0.89 A. This increase may be attributed to the precautions taken to prevent contamination of the galena surface with grease. The results obtained by us for the orientation of the alkali halides on NaC1 agree with those of Royer for all salts whose parameters lie between and include those of LiCl and KCN. For the salts whose parameters are larger than that of KCY the results can best be represented by Table I1 in which the results obtained by Royer and by Barker are contrasted with our own.

TABLE I1 Comparison of the Results obtained by Royer and by Barker with our Results. XaC1 as Substrate Parameter in A. 6.46 6.57 6 60 7.05 KaI KBr RbCl KI Salt M? 11 M? M Barker M Ir Ir Ir Royer M ?\I Ir 11 Sloat and Menzies I r = irregular hl = mutual orientation From Table I1 it is seen that the results obtained by us on KaC1 agree with the results obtained by Barker except in the case of RbCl. In this case we were neither able to orientate RbCl on KaC1 nor confirm Barker’s ohservation that NaCl orientates on RbC1. The character of the deposit obtained in each case was very similar. KO really adequate explanation can be given as to the reason Royer did not get KBr and K I to orientate on NaC1. It is quite probable, however, that Royer’s method of applying a hot solution to a substrate, especially if the solution were concentrated, would most likely produce a thick heavy crust which would obscure any orientation underneath.

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Two other interesting cases which demand an explanation arise from the ) it has a fact that RbCl (6.60 A) is not orientated on NaCl ( ~ . 6 2 8 ~ 1although parameter on1 slightly larger than that of KBr (6.57 A) and from the fact that K I (7.05 ) is orientated on NaCl while RbCl and RbBr (6.86 are not orientated although their parameters lie closer to that of NaCl than does that of KI. Simple correlation of the facts observed with the values for the various salts of such quantities as lattice energy, energy of ion hydration or heat of solution in a saturated solution could hardly be anticipated in view of the fact that we are here dealing with the initial stages of formation of a monomolecular layer of one salt upon a different salt. Examination, however, of the available data on the radii of ions as given by Goldschmidte did show that the ratio of the radius of the anion to that of the cation is 1.2 I S for RbCl, 1.315 forRbBr, 1.473 forKBr,and 1.654forKI. Fromtheseitisseenthatthe ratios of the two orientated salts are larger than those of the unorientated. That these ratios do have a significance is further verified by several examples taken from the last-mentioned paper of Barker. This work shows that KC1 (a/c = 1.361) does not orientate on K I or R b I although KaBr (a/c = 2.000) does; and that RbCl (a,'c = 1.215) does not orientate on ",I or RbI although KCN (a/c = 1.457) does. I n each of these cases the parameter of the unorientated salt lies closer to that of the substrate than that of the unorientated salt, but in every case the unorientated salt has a lower ratio of the radius of the anion to cation than the orientated salt. The fact that K I is orientated by NaCl while RbCl and RbBr, although fitting the NaCl lattice more closely, fail to be orientated is thus not an isolated fact. I n the analogous cases also, a larger value of the ratio of the radius of the anion to that of the cation is found to be concomitant with greater susceptibility to orientation. Paraphrasing the latter statement, it might be said that the greater the lack of symmetry, or uniformity of field over the surface of a sphere enclosing the ion pair, the better the chance of orientation. An analogy may be drawn from a consideration of the relative temperatures of crystallization of liquids. Whether on not the forces causing crystallization be termed van der Waals' forces, when the external fields of force surrounding the molecule are symmetrical and uniform in azimuth, then, as in the case of sulfur hexafluoride, the freezing point is lower than it is in the general run of substances. This correlation between dissymetry and ease of orientation serves also to interpret the superiority of S a c 1 over KC1 as a substrate causing orientation.

1

.x)

C. An Apparent Correlation of Mutual Orientation with the Dielectric Constant of the Solvent Experiment showed that very soluble salts like KCl, LiCl, LiBr, NaBr and NaI could be advantageously deposited from organic solvents such as methyl and ethyl alcohol, acetone, and furfural. These experiments sug6

Goldschmidt: Trans. Faraday SOC., 25,

282 (1929).

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gested that organic solvents might be used in investigating borderline cases of salts which barely fail to be orientated from aqueous solutions. In such cases the attraction of the solvent for the solute and for the substrate might be expected now to show itself as a factor of greater influence. In the so-called polar liquids, for example, the solute ions are pictured as surrounded by an envelope of solvent which must be squeezed out when the anhydrous salt is laid down upon its substrate. This work of dehydration should be smaller in liquids which are less polar and of lower dielectric constant. The following paragraphs give the experimental facts for various solvents in borderline cases. Khen the solute is deposited from the vapor phase, the solvent may be regarded as a vacuum.

Materials For this purpose, the same series of salts and substrates was used as in t,he preceding studies, namely t>healkali halides on NaC1, KCl and PbS. Since no test can be made unless the salts are not orientated from water, only those salts of larger parameter than that of the most widely spaced salt orientated from water were used, KBr and K I on SaCl excepted. All the salts used were of the same purity as those of the preceding section. As solvents furfural, ethyl alcohol, methyl alcohol, and acetone were used. Procedure. The method used for the deposition of salts from organic solvents was the same as that used for aqueous solutions in section B. A new procedure is involved, however, in the deposition of ammonium salts from the vapor phase. In order to sublime salts upon a substrate, a flat bottomed test tube inserted through a rubber stopper was placed in another test tube provided with a side arm. The substrate was fastened upon the bottom of the inside tube by a copper wire. After exhausting the apparatus through the side arm the ammonium salt was volatilized by the application of gentle heat to the bottom of the outer tube. The inner tube served as convenient means of regulating the temperature of the substrate where this was necessary. Results. The results of these experiments are tabulated in Table 111. From Table 111 it is seen that the ability of a substrate to orientate those ealts whose parameters lie without the limit of those orientated from aqueous solution would seem to be related to the dielectric constant of the solvent, as is not unreasonable. The orientating ability of a given substrate becomes greater as the dielectric constant of the solvent decreases. The success which characterized the deposition of salts whose parameters lie beyond the orientation limit for water is almost lacking in the case where PbS was used as a substrate. This may be ascribed to the presence of traces of foreign bodies which foul the galena surface by preferential adsorption. KH41 does not orientate on galena from the vapor phase because the H I vapor attacks the galena surface.

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TABLE I11 Substrate, KaC1.5.628 Dielectric Constant -+

81

hledium

6.j 7 A 6.60 6.86 7.05 7.24 7.32

7.05 7.24 7.32

7 .OS 7 24 '

7.32

39

32

25

water

furfural

methyl alcohol

ethyl alcohol

hl Ir Ir

?tl

11 hf >I

AI

;If

M

Ir Ir

Ir Ir

KBr RbCl RbBr XI KHJ RbI

M Ir Ir

KI NHJ RbI

Substrate, KCl. Ir Ir Ir M Ir hl

6.28 Ai M hf

Substrate, PbS. Ir Ir Ir Ir Ir Ir

5.97

KI NHJ RbI

Insol JI hl Ir Ir

M Ir Ir Ir

hI XI

21

I

acetone vapor

X.S.S. N.S.S. X.S.S. ?rl Ir M

hl

hl

hl

hf Ll M

Ir Ir Ir

hl Ir Ir

Ir

11

M

M = mutual orientation I r = irregular Insol = insoluble N.S.S. = not sufficiently soluble for experimentation. I n Fig. 2 the results obtained in this section are added to and contrasted with those obtained by Royer and by ourselves with aqueous solutions. Lines and numbers which occur both in this figure and in Fig. I have exactly the same significance as those of Fig. I . The fine broken lines extending down from the solid arrows represent the increase in the limiting difference between the parameter of the substrate and that of the most widely spaced salt orientated upon it brought about by using solvents of low dielectric constant rather than water. For the substrate, PbS, this increase is 3.19 percent and for KC1 7.33 percent. It may be said, however, that the orientating limit has not been reached since R b I orientates on KC1 from all solvents except water. A much larger limit would probably be found if alkali halides of larger parameter were available for experimentation. I n the case of the substrate, NaCl, the use of organic solvents has increased the limiting difference between the parameter of NaCl and that of the most widely spaced salt orientated upon it 4.81 percent above that obtained by us for aqueous solutions, and 13.69 percent above that obtained by Royer for the same solvent. KaC1 is, therefore, shown to have the ability to orientate a salt whose parameter is 30.06 Dercent larger than its own.

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I

FIG.z

D. The Effect of Grease on the Orientating Action of Galena The Hydrophobia of Galena As previously stated some difficulty was encountered in getting the alkali halides to orientate on galena. When a drop of an aqueous solution of sodium chloride was placed upon a galena substrate, it stood upon the galena very much like mercury on glass. On evaporation of the water the salt was de-

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posited in the form of a miniature crater. Any attempt to remove the excess of the solution simply resulted in the diminution of the area covered by the solution. Since galena is said to be hydrophobic,’ it was natural to suppose that the solution did not wet the galena on account of some inherent quality of the galena. To overcome the tendency of the solution to stand upon the galena in drops an attempt was made to use solvents of lower surface tension such as methyl and ethyl alcohol or part alcohol and water. I n this case the solutions spread well enough, but scarcely any orientation resulted. This was probably due to the preferential adsorption of traces of higher alcohols or other foreign bodies in alcohols. Contrasted with this strange behavior of the sodium chloride and other alkali halides in plain aqueous solution, aqueous ammonia solutions of XgC1 and AgBr readily yielded orientations on galena. This together with the fact that Langmuil.8 has found that the angle of contact of galena is increased when the galena surface is contaminated with grease led to the conclusion that orientation was prevented by the presence of grease. The previous experiment with aqueous sodium chloride solution was then repeated with the addition of a few drops of dilute sodium hydroxide. The substrate was then boiled in this solution, removed, and allowed to cool. On evaporation of the solution a film of closely-spaced perfectly-orientated sodium chloride crystals was deposited. This meant that the negative results previously obtained were most likely due to grease. Accordingly all glassware was carefully cleaned in hot chromic acid solution, rinsed, and placed in covered containers. All handling was done with tongs. Special care was taken to prevent grease films from the fingers from reaching the glass. Although these precautions were taken to prevent grease contamination of the solutions through the glassware and water, little improvement was seen in the orientation of the sodium chloride or the spreading of the solution. It was noticed that standing solutions still showed the presence of grease a t the liquid-air junction as evidenced by the poor capillary rise. Since all possible sources of contamination had been removed except from the salt itself, it was finally concluded that the contamination was due to grease in the sodium ~ h l o r i d e . ~ The sodium chloride was then heated to fusion to drive off all traces of organic matter. Solutions, neutral to phenolphthalein, were prepared from this salt. These solutions spread well and in nearly all cases crept over the surface of the galena in much the same fashion as water upon clean glass Freundlich: “Colloid and Capillary Chemistry,” p. 160. Langmuir: Trans. Faraday SOC.,15, 62 (1920). It was later learned that du Nouy had encountered somewhat similar difficulties in attempting to measure the surface tension of rabbit blood serum in a physiological salt solution. Initial surface tension vaned from 63 to 74 dynes. This variation was traced to the presence of grease in the “C.P.” sodium chloride. du Nouy: “Surface Equilibria of Biological and Organic Colloids,” p. 30.

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leaving a deposit of small perfectly orientated crystals such as previously shown in Plate I . There still remained a few instances in which the grease-free sodium chloride solution did not spread readily. It may be suggested that these were due to a film of adsorbed air since galena left in dry air becomes still more difficult to wet.l0 This explanation is, however, quite unlikely, first because Edser“ has observed that air is not measurably adsorbed upon galena, blende, or quartz, and second because no difference could be detected in the wettingororientating properties of such galena when it was cleaved in air or under the sodium chloride solution. Any grease not yet accounted for must occur with the natural galena. How then can the presence of this grease be explained? Any solution of this problem will have to be sought in the history of the galena and in the manner in which it was deposited. I n the mines around Joplin, Mo., from whence came the galena that was used in this study, the galena is found above the lighter zinc sulfide. The presence of bitumen of petroleum origin has led J. E. S p u r P to suggest that the galena was carried to the top by oil fiotation. It would be natural to expect that some of the petroleum had remained in the ore in much the same fashion as the sodium chloride that has left the ore full of inclusions. (See p. 2008). Indeed, in view of the strong affinity of the so-called hydrophobic sulfides for greasy bodies, i t would not be surprising if some of these were found to have grease built into their space lattice in much the same fashion as certain crystals are found to be colored by dyes. Additional evidence which shows that the hydrophobia of galena is not an inherent property is found in the authors’ observation that pure water spreads readily over lead sulfide prepared by heating lead and sulfur and in the observation of H a l P who found that artificially prepared MoSn is readily wetted by pure water in spite of the fact that the natural MoS: is said to be the most hydrophobic of sulfides.

F. The Orientation of Salts by Single Crystals of Silver Many theories of the metallic state have been put forward; of these, many have been discarded, and none is universally accepted. We here describe some new experimental facts which bear upon this problem. Materials. For this study silver, a non-oxidizing face-centered cubic metal whose parameter is 4.07 A, was selected as the substrate. For salts the bodycentered cubic alkali halides, NH&l (3.866 A), NHIBr (4.047 A), CsCl (4.11 A), and CsBr ( 4 . 2 9 A) were selected. Comparison of the space lattice of each type as shown in Figs. 3a and 3b reveals the fact that the (100) face of ‘Ol?dser: Brit. Ass. Adv. Sci., Report on Colloid Chemistry, p. 289 11 U s e r : Ibid. p. 277. 11 Spurr: Eng. Mining J., 123, 204-5 (1927). l a Hall: “Master’s Thesis” Gettysburg College, (1929).

(1922).

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the silver is very similar in form and dimension to the corresponding face of the ammonium bromide, excepting the extra atom in the face of the silver lattice. It was not thought that this would present any experimental djfficulties in view of the wide differences under which mutual orientations have already been shown to occur. There are no face centered cubic salts available which are suitable for this investigation. Single crystals of silver were grown by suspending a copper wire in a two percent solution of silver nitrate. Crystals grown by this method were not very large. However, patient search revealed a number of good cube faced crystals that could be used as substrates.

FIG.3.h Ag atoms

FIG.3 B SH4ion 0 Br ions

Ethyl alcohol was selected as the preferable solvent, first, because it has been shown that the orientated crystals are deposited more readily from solvents of lower dielectric constant, second, because it is an organic solvent in which all four of the salts are soluble, and third, because it is a solvent in which the bromide ion does not react readily with the silver, Procedure. A drop of alcoholic solution of the salt was placed upon a silver crystal, allowed to evaporate, and the result viewed under the microscope. Results. S H 4 B rand CsCl were orientated on silver. SH,Cl and CsBr were not. The difference in parameters tolerated by the silver is rather smalloas compared with that allowed by salts. In the case of NH4Br it '," 0.023 A or 0.56 percent of the parameter of the silver. For CsCl it is 0.04 4 or 0.98 percent of the parameter of the silver. This small difference of parameter is in striking contrast to the findings with a salt substrate. It must be remembered that the postulated charges on the ions of the dissimilar net planes, metal and salt respectively, which by their contact determine the orientation are in the present instance all respectively of like charge, a state of affairs that was not encountered in the case of salt upon salt. Again, insoluble contamination on the silver surface

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would be more prejudicial to orientation than in the case of a soluble salt substrate, in whose case contamination is less likely. Furthermore, salt substrates may be richer than metallic silver in Smekal cracks favorable to orientation. Because of the observed necessity of such a close coincidence in atomic spacing of salt and silver, it might be plausibly argued that in this case the mutual attraction that directs orientation was attributable entirely to polarization of uncharged silver atoms by approaching charged ions of salt. Again, it is true that innumerable purely covalent compounds crystallize from their melts or from solution, in which process the molecules of solid, containing no charged atoms, are able to orient correctly like molecules from the liquid phase as these approach and settle down to their appropriate regular positions in the growing crystal. This happens for other than the closed-packed arrangements, for which little directive force would be required. In this connection, however, two remarks are pertinent : first, no examples of orientation between crystals of electrovalent and covalent substances have been reported in the literature; second, we were unable to induce NaF or KF t o orientate upon carborundum or naphthalene to orientmateupon KaC1, KC1, PbS, or CaC03 (calcite) although there was quite close agreement between the parameters of certain of these substances in a t least one dimension. Since, therefore, the lattice structure of XH4Br and of CsCl are well established, the simplest interpretation of the observed orientation is to assign to the atoms of the silver crystal positive charges, not necessarily of effectively unit value, which serve to align the negative ions which form net planes in the salts.

F. Crystal Habit and Mutual Orientation When crystals of barium nitrate are grown from a solution saturated with methylene blue, cubes instead of octahedra are formed.’4 This is caused by the strong adsorption of the dye,upon the cube faces of the crystal as is verified by the fact that the cube faces are preferentially colored by the syncrystallization of the dye with the barium nitrate. Since the material modifying the crystal, crystallizes with it, GaubertI5 believes the change in crystal habit in all such cases is caused by orientated adsorption. h case exactly the converse of the barium nitrate one is found in the modification of the crystal habit of sodium chloride. When crystals of this salt are grown from a saturated solution containing urea, formamide, glycocoll, or a base, octahedra instead of cubes are formed. Saylor16 has shown that both cases of modification are due to the adsorption of anions on the octahedral faces and cations on the cube faces. Since the cube faces of sodium chloride are stable anyhow, modification of this salt takes place only when anions are preferentially adsorbed on the octahedral faces. I n the case of barium nitrate the octahedral faces are stable and li

lj

Gaubert: “Le Facies des Cristaux,” p. 13 (1911). Gaubert: Compt. rend., 180,378-80 (192j). Saylor: J. Phys. Chem., 32, I15j (1928).

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modification takes place only when cations are preferentially adsorbed on the cube faces. Methylene blue is a true basic dye and is, therefore, adsorbed upon the cube face as a true cation. Although urea, formamide, and glycocoll are non-ionizing compounds they are consistently adsorbed as if they were anions. In view of the fact that a close relation exists between the modification of crystal habit in both of the cases cited above and that Gaubert attributes change in crystal habit by syncrystallization to orientated adsorption, it occurred to us that it might be possible to observe the orientation if it were of the type studied in this paper. It was, therefore, decided to try urea and formamide on the octahedral face of sodium chloride. The adsorbed material, if orientated, should manifest itself in the form of regularly arranged crystals. Procedure. Clear octahedral crystals of sodium chloride were grown by allowing an almost saturated solution containing one-tenth its volume of formamide to evaporate slowly in a beaker. A drop of urea solution applied to the substrate quickly formed a crust which completely obscured the surface of the sodium chloride. To prevent this, as well as to allow the needle-like crystals of urea to grow a t an angle t o the surface, the substrate was placed in a small glass dish and covered with a saturated solution of urea. Ethyl and methyl alcohol and aqueous solutions were tried. As formamide is a liquid a t ordinary temperatures the substance was applied to the substrate out in the open when the outdoor temperature was below the freezing point of this substance. Results. Unorientated crystals of urea were obtained. No crystals were observed in the case of formamide on account of the fact that traces of water lowered its freezing point below the temperature used. With each substance, however, there appeared upon the faces of the substrate a large number of triangular etch figures whose apices pointed toward the base of the octahedral face that contained them. Etching took place only when urea and formamide were present. Xone was observed when plain solvent alone was placed upon the substrate. Discussion oj Results. The results indicate that the change in crystal habit brought about by the presence of the urea or formamide is not due to orientated adsorption. If such adsorption took place, urea grown upon the orientated layer should take the same orientation. This is not borne out by the experiment.

H. Miscellaneous Orientations NaCl, KCI, PbS, and CaC03 as Substrates. At ordinary temperatures NHaCl (3.86 A) and NH4Br (4.04 have the caesium chloride arrangement. Above 184.3' and 137.8" C. they have the sodium chloride arrangement. At 250' the parameter of NHaC1is 6.53 b and

a)

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that of SH,Br is 6.90x. By suspending a hot substrate in the vapor of these halides by means of the apparatus already described, both were found to orientate on SaCl and KCI. The differences between the parameten of liHI(:l and N€I,Rr and that of NaCl are rg.yj and 22.48 percent and between the same pair and KCI 3.96 and 9.97 percent re~pectively. AgCN in aqueous ammonia solution also orientates on both NaCl and K(X. The parameter of AgCX in unknown. Assuming, RS is usually the case, that bromide and cyanide ions have the same radius, the parameter of AgCN may be taken as equal to that of A& On this assumption the difference between the parameten is 2 . 5 7 percent for NaClas sobstrsttr and 8.1 percent for KCI. Aqueous solutions of a number of salts u’ere tried upon galena in order to determine if there xias any orientation. Of these KC103 and KCIO, were found to orientate. The crystals of KCIOa (See Plate 3) p ~ s e s two s types of orientation. One form is thc mirror image of t.he ot,her. The orientation of KCIO, is similar to that of KCIO.. KCIO, is monoclinic and KC10, is orthorhombic. The p a r a m e t ~ r sof KClOaare a, 4.647; b, 5.jRs; c, 7.085 and ” o l K f l O * a , 8.8j;b, 5.66;c, i.zjArrspecOrientation of KClOa on Galena t.ively. The st:cnn:l prsrnet,er of each salt is Seen to approach most nearly tt that of PbS ( j . g j 6). The cubic salt, NaC103 (6.55 A) is not, orientatfd on PbS. There are no indisputable case^ of t.hc orizntation of a solid cov;tlmt substance on an electrovalent crystal. Orientations between the same type are known. It, therefore, becomes of interest to know whether clectrovalmtcovalent orientations take place. For this purpose a number of organic compounds wcre sublimed upon the four subst,rates mentioned at the beginning of this seetion and in addition an attempt was made to orientate 5 number of salts upon the covalent substrate, carbonindum. Calcite is rhombohedral hexagonal. Its parameter is 6.36 A. The arrangement of the calcium and carbonate ions in the oleavage plane of calcite is very similar to that of the cleavage plane of SnCI. Kaphthalerre, quinone, and pdichlorobenzenc were sublimed upon NaCI, KCI, PbS, and (.‘aC0~. Kvne were found to orientate although the difference between the b ptlrameter of naphthalene and that of KCI is only 6.28 percent and that between the c parameter of quinone and NaCl is only 0.~45percent. Carborurndum as a Subslrale. Carborundum is hexagonal. It crptallizes in three types. The a parameter of all of these is given RS 3.09 A while the c parameter is 37.9, 1 5 . 1 i , and 10.10A respective1 for types I, 11, and HI. Of the sodium chloridc yrangement KBr (6.57 XaC1 (5.628 A), K F (5.33 and N a F (4.62 a),

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and of the caesium chloride arrangement NH4Cl (3.866 A), and R b F (3.66 were tried, but none were found to orientate. In view of the above facts, it is extremely doubtful whether orientation takes place between electrovalent and covalent compounds.

summary The investigation of the discrepancies between the results of Royer and Barker for the substrate, IGaC1, confirms the results of the latter except for the orientation of RbCl on YaC1. The four salts, KCN, KBr, RbCl, and RbBr, of larger parameter (2). than any found by Royer to be orientated on PbS, were found to be orientated when precautions are taken to prevent' the contamination of the galena surface with grease. (3). The orientating ability of a substrate is correlated with the ratio of the radius of the anion to that of the cation. ( 4 ) . The limiting difference between the parameter of NaC1 and that of the most widely spaced salt orientated upon it is found to be 2 5 . 2 percent of the parameter of KaC1 instead of 16.4 percent; for PbS as a substrate it is shown to be 14.9 percent instead of 8 . 2 percent, as found by Royer. ( 5 ) . Mutual orientations are very readily deposited from organic solvents. (6). Mutual orientations can be obtained from a solvent of lower dielectric constant when a solvent of higher dielectric constant fails to yield an orientation. ( 7 ) . Samples of galena were found to contain throughout their mass orientated crystals of halite. (8). The hydrophobia of galena is not an inherent quality, but is due to the affinity of galena for grease. (9). NH,Br and CsCl orientate on metallic silver. The significance of this new type of orientation is discussed. (IO). Change in crystal habit is not due to orientated adsorption. (11). Certain solutions which are known to cause change in crystal habit are shown to produce characteristic etch figures upon the octahedral faces of sodium chloride. (12). It has been found possible to deposit mutual orientations from the vapor phase. (13). High temperature forms of KH,Cl and KH4Br were found to orientate from the vapor phase on KaC1 and KC1. ( 1 4 ) . Kotwithstanding close agreement in atom spacing, no mutual orientation was found between substances one of which was of the electrovalent and the other of the covalent type. (I).

Frick Chemacal Laboratory, Princeton Unitmersity, Princeton, N . J .