MOLECULAR ORIENTATION AT SURFACES OF SOLIDS
I. MEASUREMEKT O F CONTACT ANGLE AND T H E WORK OF ADHESION OF ORGANIC SUBSTANCES FOR WATER* BY A . H. NIETZ
The study of solid surfaces has been taken up only recently and has not yet been given the attention it deserves. Some practical applications of the study of wetting power have been made in the flotation process, largely with minerals, of course. A few organic substances have been studied by Adam and Jessop,‘ who have calculated the work of adhesion of several organic solids. I n recent years it has come to be realized that molecular W = TL+ Ts-TL, orientation at interfaces (polarity of solid surfaces) is intimately connected AD HE^^^^. with the phenomenon of adhesion and with the stability of colloids. The FIG.I interfacial tension of gelatin and toluene2 was shown in this Laboratory to support this View, and in order to obtain more quantitative information on this, Dr. Sheppard suggested to the author an experimental study of the “contact angle” of related organic compounds. The method used has been known for many years but was never applied to any extent so far as known until the work of Adam and Jessop (loc. cit). Young in 1809 and D ~ p i - 6in~ 1869 publiahed some of the relations for the contact angle of a liquid against a solid and the work of adhesion between the two. For the sake of clearness, the development of the method from the equation of Dupr6 and the work of Young is included here. If we have two bars, each I sq. cm., in cross-section, of liquid and solid respectively, represented by L and S in Fig. I , the work done in separating L from S is W = T L Ts - TLS where TI,and T s are the free energies of the surfaces which have appeared and T L s is the free energy of the interface which has disappeared. Consider next the relations for a drop of liquid L resting in equilibrium on a solid S (Fig. 2 ) . At the point B, we have an equilibrium of forces represented by the equation T ~=. TBL ~ T~ COS e where 0 is the angle of contact between the liquid and solid as shown. This
c1Is
+
+
*Communication KO.323 From the Research Laboratory of the Eastrnan Iiodak Company. J. Chem. SOC.,127, 1863 (1925). * S. E. Sheppardand S. S. Sweet: J. Am. Chem. SOC.,44, 2797 (1922). “Cohesion of Fluids,” (1805). “Theorie mecanique de la Chaleur,” p. 369 (1869).
A. H. S I E l Z
256
equation can also be derived from purely theoretical considerations. If we now substitute this equation in the Dupr6 equation, we obtain = T ~(I , ~pose) which is an expression easily applied to the work of adhesion. It is necessary to make but two measurements, TI,.$,the surface tension of the liquid in contact with the solid; and 8, the angle of contact of water against the solid. This is the basis of the experimental procedure.
w
+
Experimental Methods Two practical methods for the measurement of the contact angle have been used. One employs the rotating cylinder described by Ablett,’ and the other the tilting plate as used by Adam and Jessop and others. The former uses material coated on a silver-plated cylinder, the angle being obtained by adjusting the height of liquid until the liquid-air boundary is straight right up to the line of contact. In the second method a plate was coated with the material and then inclined until the liquid-air interFIG.2 face was perfectly plane, the angle being measured directly. The second method offers decided advantages and is sufficiently accurate for ordinary work. Differences due to unknown causes have been found between the two methods. Though some refinements in Ablett’s apparatus have been made and the accuracy increased, his results have not been reproduced in every respect for paraffin, and some mysterious differences occur with other substances when measured by the two method. Time has not permitted a complete investigation which should be made of the causes for these differences. An attempt was made to check hblett’s results with paraffin. The paraffin used represents a product of very high purity. It was prepared especially for this work by Dr. H. T. Clarke, the treatment consisting of repeated stirring with warm concentrated sulphuric acid, melting at about 100’for several hoirs in contact with metallic sodium, and final distillation in vacuo. The boiling point of the final product was z 1 8 ~ - 2 2 sat~ 4 mm. The results with this paraffin gave 109’ i 30‘ for the angle of contact in water, measured by Ablett’s method, and the same by the plate method. This high value in itself would indicate a high degree of purity, Le., freedom from unsaturated or polar compounds. .Iblett’s results were verified so far as two sets of values for clockwise and counter-clockwise rotation at various speeds are concerned. But it was not found possible to determine what he calls a “stationary angle,” that is, an angle when there is no relative motion of solid with respect t o liquid. It would seem next to physically impossible Phil. M a g , 46, 244 (1923).
MOLECULAR ORIENTATION AT BURFACES OF SOLIDS
257
to obtain this condition since in an apparatus of Ablett’s type ib would be necessary to add liquid at esactly the same rate as the advance of solid out of the liquid to prevent motion of solid into or out of the liquid. Ablett himself does not specify how these stationary angles could have been obtained. Merely allowing the cylinder to remain motionless and adding water would give a result in the %me sense as the solid advancing into the water, or, in the present case, counter-clockwise motion. I t is hoped later to make an at-
APPARATUS FOR MEASURING CONTACT ANGLE
FIG.3
tempt to obtain the necessary condition for a stationary angle, but it is very doubtful that it can easily be secured. Further reference is made below t o differences between the cylinder and plate method. For the present work the plate method was found more rapid and convenient, and the results are believed more reliable, a t least until the nature of certain discrepancies is better understood. A special form of apparatus was designed which is very convenient in use and eliminates to some extent the error involved in the motion of the solid in or out of the liquid. This is sketched in Fig. 3 . The slide S is an ordinary microscopic slide on which is coated a small quantity of the melted solid. The slide is held by a convenient clamp C so that it can be turned to make various angles with the water as the disk D is turned. The height of the liquid in the jar J is so adjusted and the clamp C is so placed that the line of contact L is on the axis of rotation of the disk D. This disc has a carefully graduated metal scale affixed to its surface in such a way that it can be adjusted. When an adjoining pointer indicates zero degrees, the slide S should be in a horizontal position as shown by a small spirit level placed on it. The disk is mounted to the chuck of a small jeweller’s lathe which permits smooth turning and accurate centering. By the use of this apparatus, therefore, the angle as measured is relatively free from the large errors due to the liquid advancing or receding from the solid. Added advantage can be obtained by placing the jar in a small outer tray with a draining tube so that by providing a suitable source of supply,
2 58
A.
n. NIETZ
pure water can be run continually into the jar J and overflow at the level of the line of contact so that the surface is automatically swept and renewed. The jar J is a rectangular one, preferably with plate glass sides front and rear, such as an absorption cell for photometric work. A glass is ruled with black lines at intervals of about 3 mm. and is cemented to the rear surface. In place of this, a single fine black wire, held around the jar by means of a rubber band on one side, may be used. A small mirror is placed in front of the jar J at an angle. The height of the liquid is so adjusted that it is just up to the axis of rotation. The slide S is then tilted until the image of the straight lines, seen in the mirror, reflected from the under side of the liquidair interface, appears perfectly straight right up to the line of contact. To facilitate this a ground glass and suitably bright electric lamp are placed on the far side of the jar. The substances used throughout were Eastman Synthetic Organic Chemicals and of as high punty as is commercially obtainable. I n several cases these were recrystallized. h sample of the substance to be used was melted in a small test tube and several drops allowed to crystallize on the end of a clean microscope slide. Great precautions, of course, had to be taken to assure the cleanliness of all the glassware used and the purity of the water. Potassium dichromate cleaning solution was used on the glassware and the surface tension of the water was checked before every determination. This was done at first by means of a du Koiiy tensiometer. I t was found, however, that this could be done more conveniently and just as accurately by placing a slide of pure paraffin of known contact angle in the apparatus and reading the angle. A very slight difference of surface tension or contamination of any sort could be very easily detected by the lowering of the angle. As previously inferred, the Ablett cylinder and the plate methods gave many widely divergent results, the cause of which is unknown and requires further investigation. Some of these discrepancies are shown in Table I.
TABLE I Comparison of Contact Angle measured by hblett Cylinder and by Plate Methods Myristic acid Palmitic ” Stearic ” Ethylene glycol dilaurate ,, ” distearate Cellulose acetate ,, nitrate 2-4-6 trichlor aniline
out 60 54
53 i6 56 58
Cylinder
Plate I I j I11
106 123
75
--
25
/ I
15
62
55
MOLECULAR ORIENTATION AT SURFACES OF SOLIDS
259
While in none of these measurements by either method was the surface of the water swept, it is to be noted that several of the above substances do not contaminate a water surface in the ordinary sense (lower the surface tension). Ethylene glycol dilaurate and distearate, and stearic acid, do not affect the surface tension. Because of erratic results obtained in a few cases with the cylinder, and because of the greater convenience and ease of measurement with the plate method, as well as the general consistency of the results obtained, the latter has been preferred for the present at least, and the measurements dicussed below were all made in that way. By use of overflow a t the surface with the plate method as already suggested, the surface is swept continually, and if this is desired, the apparatus as illustrated is most convenient. However, unless contamination from a foreign substance is considered, and this is not likely under ordinary conditions of cleanliness, there is no particular advantage in sweeping the surface. Even though the material under examination lowers the surface tension by a large cos 8) are those amount, the values of T and 8 in the equation W = T ( I under such conditions. As long as the solid is in contact with water, a t any rate, sweeping will have no noticeable effect on the observed surface tension, unless, of course, the rate of solution is extraordinarily slow. If any foreign substance, however, is present , this will be swept away. The plate method is more susceptible to foreign contamination than the cylinder. For example, pure paraffin was coated on a cylinder and turned on a lathe to give a smooth surface, and the same material was coated on a slide and scraped. The contact angle measured by the two methods with these A very small fragment of cetyl alcohol specimens was in each case 108'. was then dropped on the water in each case. Cetyl alcohol lowers the surface tension of water 50%. The contact angle was lowered from 108' to 96' with the cylinder method, and from 108' to 72' with the plate. Yo reason is known for this difference between the two methods. Another objection to the cylinder method is the difficulty of measuring small angles. Cnless the top surface of the water-air interface is used for reflection, and this is poor, giving a very faint image, the angle cannot be determined if it is much under 60'. Since many substances give angles from 30' to 60' measurements on them are best made by the plate method.
+
Results Measurements here recorded for solids are, within limits of error, well in accord with other similar work for liquids and the more limited published results for solids. Harkins and his co-workers' have demonstrated quite clearly the existence of certain polar groups and their effect on the work of adhesion of liquids, as measured by the method of interfacial tension. Langmu? has also put forth views which have experimental verification and which explain the effects of polar groups in producing molecular orientation. ~~
*
J.Arn.Chem. Soc.,39,354(1917);42,700 (1920);43,35(1921). J . Am.Chem. SOC., 39, 1x48 (1917); Chem. Met. Eng., 15, 468 (1916).
a60
A . H. SIETZ
That the measurement for solids are comparable with results of Harkins for liquids is fairly well indicated by Table 11. Comparisons are made for the substances of similar constitution which could be found in the measurements by the two methods. The results for liquids are calculated from the values of the surface tensions of the two liquids and the interfacial tension. Those for solids are based on the measurement of contact angle and surface tension of water in contact with solid.
TABLEI1 Solids ( N e t s )
Liquids (Harkins)
P a r a f f i Oil Benzene Ethyl ether Isobutyl alcohol
47.8 66.5 79.2 94.3
Cyclohexanol Methyl hexyl carbinol Ethyl cinnamate Caprylic acid Undecylenic acid Di-isobutyl amine ((CH3)zCHCHz)zNH Dipropyl amine (CaH7)z N H Chlorobenmne
103.7
99.7 89.9 93.72 102.7
84.56
Pure paraffi Benzene, solid at I O Diphenyl ether Trichlorotertiary butyl alcohol Triphenyl carbinol Methyl cinnamate Caprylic acid Undecylenic acid Diphenyl amine
49.9 43 ' 5 77.2
102.9 111.7
111.0
66.2 70,0
85.4
93.68 68.46
1-2-4trichlorobenzene
62.6
TABLE111 Tralues of Work of Adhesion-Liquid Paraffias
Isopentane (CH3)2CH CH2CHa Hexane Octane Di-iso butyl (decane) Higher paraffis (about Cl6 Hal)
Phase
Harkins, Clark and Roberts: J. Am. Chem. SOC., 42, 700 (1920)
36.88 39.98 43.76 48.24 63.02
,Iromatic Hydrocarbons
Benzene Toluene o-xylene m-xylene pxylene Ethyl benzene (CeHdCHda) mesitylene pcymene
66.62 66.62 66.62 63.62 63.36 63.36 60.48
261
MOLECULAR ORIENTATION AT SURFACES O F SOLIDS
TABLEIT' General Data on Work of Adhesion Hydrocarbons
Hydrocarbons Di benzyl Stilbene Diphenyl Paraffi Benzene Naphthalene Anthracene Diphenyl methane Triphenyl methane Tetraphenyl methane
I
2
Halide Substitutions 1-2-4 trichloro benzene
97.6 75.7
0
&I MS
68 I08
0
0
31s
0
10s
22
o 9
v-
62 92 62 4s '5
0
O
0
0
?;s
3
4
M+
0
o
60 46 73 65 Si
Amines-Amides Diphenyl amine 2-4-6 trichloro aniline Acetamide
8 T'+ 9.2
0
0
Alcohols Myristil Cetyl Menthol Trichloro tert. butyl Triphenyl carbinol
Phenols fl-napht hol Resorcinol Thymol
-
118.8 94.7 99.6 49.9 43 ' 5 106.3 68.6 103,6 123.4 135.0
0
72
SI 88
Ketones Acetophenone Benzophenone
5
SO
Ethers Glyceryl phenyl Diphenyl
Aldehydes Trioxymethylene p-iso-butyraldehyde
4
3
-
17 0
-
w
N x
s
54.2 61.3 71.8 102.9 111.7
122
o
j
70
22
If
N
v
65 65
19 2.8
0
v
I1
MV
3s
I1
I2
11
25
o
6
85
I7
28
VM
80
0
5
ss
0
O
SN N
'5
4
I2
85.4 114.5 143.0
98
0
0
sx
62.6
V
v
34.5 75.6
83.4 100.0
128.5 137.8 65.6
262
A . H . NIETZ
TABLE IV (Continued) General Data on Work of Adhesion Hydrocarbons
Nitrogen Cpds. Azobenzene Hydrazobenzene Acids Caproic Caprylic Pelargonic a
P
”
Capric Undecylic Lauric Tridecylic a ”
Myristic Pentadecylic a
P
Palmitic Margaric a
’)
P
Stearic Arachidic Behenic Cerotic Erucic Cinnamic Hydrocinnamic Undecylenic Benzoic Esters Methyl cinnamate Ethylene glycol dicaprylate 11 ” dicaprate 93 ” dilaurate JJ ” dimyristate JI ” dipalmitate ” distearate Butyl carbamate Tristearin Miscellaneous Benzyl sulfide Zinc stearate Cmtile soap powdered ,9
2
3
5
64
0
0
52
1.1
2.8
104.5 118.;
I11
6
77
0
87.0 66.2 66. I 54.0 45. I 50.8 32.6 50.4 36. 5 32 . o 65.0 37.8 42.8 89.0
105
9
54.2
I 06
0
I11
0
56.8 46.7 47.9 40.8 46.2 127. 7 87.2
45 60 49 69 85 79 I11
P
)J
I
30 38 45 45 43 44 31 45 40
75 95 “5 73
31
I10
21
25
IIO
0
116 93 40 63 53 65
0
33 0
I7
70.0
40
2.8
93.6 111.0
8
15.7 -
0
-
9.5
45 84 96 123
IO
I IO
0
-
98 75 40
0
-
0
-
26
IIO
0
90 I35 68
0
I4
59.5
32.8 47.6 62.2 91. I 95.1 47.0
1.9 0
I
0
65
68.8
fs 53
s MV
72.8 21.3 -
MOLECULAR ORIEXTATION AT SURFACES O F SOLIDS
263
I n the case of benzene, caprylic acid, and undecylenic acid, measurements were made on both liquid and solid and in each case values for the liquid are higher. The general trend of the results for solids closely parallels those for liquids, however. Measurements of a series of solid homologues, or of solid compounds with similar terminal groups, show greater variations within the group than for a similar class in Harkins’ data for liquids. This is seen by comparison of Table IT‘ with some of Harkins’ values for related compounds. In Table I11 are given a few of the latter for hydrocarbons, which as a whole show far less divergence than the hydrocarbons, in Table IV. Table IV gives complete data on contact angle and work of adhesion for i o organic solids. Values given are for the following: Column I Contact angle, 0 70reduction of the surface tension of water caused, T R (’olumn z % reduction of the contact angle of pure paraffin (‘olumn 3 Column 4 notes on spreading Work of adhesion, W Column j Column 2 is expressed as a percentage and represents the lowering of surface tension of water in the presence of solid. Column 3 shows the percentage lowering of the contact angle of pure paraffin against water in the presence of the solid in question. The data on spreading are only roughly quantitative and are expressed as F‘ violent 31 moderate S slight X none
It is at once evident from Table 1V that in each chemical class there is a rather wide range of values for the work of adhesion, W. There is also no tendency for any class to assume a particular value. This indicates of course that other factors than the mere presence of the better known polar groups influence the attraction for water. It is unfortunate that it is difficult to secure more comparisons, as can be done for liquids, between series of related compounds. Many of the substances needed are not solid at ordinary temperatures, and a great many are very difficult to prepare in any state of purity Consequently, it will require time to secure even a moderate amount of data. Xevertheless, the figures of Table IV afford some interesting information. Discussion for the present will be confined to work of adhesion only. Following are some of the comparisons which may be found in the table. Benzene 43 5 Benzoic Acid 93 6 The carboxyl group causes a great increase in the work of adhesion, as expected. The measurements for benzene were made in the solid state, in water at IO. Naphthalene 106 3 &Naphthol 128 5
264
A. H.NIETZ
The increase here is appreciable] though not large. A single phenolic hydroxyl group on the large naphthalene molecule already containing polar double bonds would probably not be able to exert much effect. Two phenolic groups in the benzene ring, however, produce a very great change. Benzene 43 5 Resorcinol ( I :3OH) '37 8 .A single phenolic group with an additional methyl group in thymol produces only a moderate increase. Benzene
43.5
C H (c'H3)2 The -C1 radicle is shown as polar from the values for trichlorobenzene. Benzene 1-2-4 trichlorobenzene
43 5 62.6
The alcoholic hydroxyl appears to have greater attraction for water than the carboxyl group. This is a t least the case for two of the higher hydrocarbons] as shown by 54 2 Cetyl alcohol 61 3 JIyIistyl alcohol Myristic acid 32 0 Palmitic acid 42 8 These values are put forward with reserve, since they seem inconsistent and these higher solid alcohols are purified with difficulty. The effect of additional polar groups is again seen in values for two ethers. Glycerol phenyl ether CH20H.CH OH.('H20C'6Hj Diphenyl etlier CeH ,OCeH a
97.6 75.7
Some interesting result,s are afforded by the following nirasurements on substituted methanes. The value for tolurne is taken froni Harkins' results for liquids but is inserted for comparison. Benzene (solid) 13 5 Toluene i liquid-Harkins) 6 6 . 6 Diphenyl methane 103.6 Triphenyl '' 123.4 Tetraphengl '' 110.0 The last valuc for trtraphenyl methane is not exact, the result lying between 40.0 and 145.6, The tetraphenT1 niethnne was w r y kindly furnished by Dr. Edward Mack, Jr. I
MOLECULAR ORIENTATION AT SURFACES OF SOLIDS
265
The differences between each member and the next of this series are Benzene 2 3 .I
Toluene* 37 .o
Diphenyl methane* 19.8
Triphenyl
"
16.6
Tetraphenyl '' These indicate roughly a steady constant increase for each phenyl group added, the increase becoming less as the molecule becomes larger. They also show again the effect of the double bonds of the benzene ring. The di-and tri-phenyl methanes show a result on cooling in contact with water which is not understood at present, it being mentioned only as a matter of record. This peculiar effect is illustrated by the following: Cooled in .In Cooled in Contact Rith Water
Diphenyl methane
i"'e
IT
Triphenyl methane
(w
93 O 104 620
6i
46'
153' 7.9
123.4
Stearic acid / e 106" 96O (IT j z . 8 6j 2 (See Table V also) The substituted methanes give results in the opposite sense from most substances cooled in contact with water, as, for example, stearic acid. That is. instead of the water producing increased work of adhesion through orientation of polar groups, it has produced less. I n the case of triphenyl methane especially the change is tremendous, work of adhesion being reduced to approximately one-fifteenth by cooling on water. Conditions causing results in this direction might be the leaching-out of a very highly polar impurity, which we believe improbable, or an adsorption effect. At any rate we have not yet been able to investigate the matter further. Other results with aromatic hydrocarbons are the following. Benzene 43 5 Naphthalene 106 3 Anthracene 68 6 The increase from benzene to naphthalene is seemingly accounted for by the addition of the second nucleus and the double bonds it contains. The decrease shown by anthracene over naphthalene might be explained by assuming some sort of folded configuration for the naphthalene molecule, but this is not borne out by the findings of Bragg' as to the x-ray crystal structure. *The value for toluene IS out of h e , since, as previously stated, a liquid gives a hlgher result than the same substance In sohd form. Proc Phys. Soc , 3 4 , 3 3 (1921);35, 167 (1923).
266
A. H. NIETZ
Consequently we are again confronted by facts difficult to explain. It seems possible that in some of these cases consideration should be given to the structure of the crystal aggregate rather than to the individual molecule alone. Another case in which configuration may play a part is: Diphenyl 99.6 113.7 Naphthalene AlackL has shown that diphenyl has a “collapsed” or folded structure. Thr molecule consists of two benzene nuclei face to face. Consequently the residual valences are probably quite completely satisfied within the molecule itself. Naphthalene has a more extended configuration, and four of the double bonds are effective in increasing the work of adhesion.
I
1
l l
E’IG.
4
A comparison offering further difficulties is the following: Diphenyl ether Benzophenone ”
99.6 75 7 100.0
These results have been repeated and verified, though absolute values cannot be duplicated as a rule. The decrease from diphenyl to the ether must be due to a change in configuration of the molecule caused by the introduction of the oxygen atom, If this is the explanation, we are not able to describe the nature of the change as yet, The increase from the ether to the ketone is rather marked, but further data are required before any conclusions can be drawn. This increase is probably analogous to the increase in solubility in water of acetone over ether. A comparison which a t present seems to offer some difficulties in the wa). of satisfactory explanation is 123 4 Triphenyl methane (C6&)3 CH ” carbinol (C&)3 COH Ill i
It would seem very difficult to explain the higher value for the hydrocarbon, unless we should go so far as to assume that by means of steric hindrance 1
J. Am.Chem. SOC.,47, 2468 (1925).
MOLECULAR ORIENTATION AT SURFACES OF SOLIDS
267
the hydroxyl of the carbinol is prevented from exercising any effect. This would be the case if we had a configuration such as shown in Fig. 4. Any such explanation must of course be put forward with reserve, and requires additional evidence. Some information concerning the effect of double bonds is available from Table IV. Below are tabulated the values for six pairs, the double-bonded compound being the second in each case. Undecylic acid Undecylenic acid Hydrocinnamic acid Cinnamic ”
so.8 70.0
87.2 127.7
Stearic acid Oleic acid (Harkins)
56.8 89.62
Behenic acid Erucic acid
47.9 46.2
Hydrazo benzene AZObenzene
118.7
Dibenzyl Stilbene
118.8
104.5 94.7
In the first three cases the increase due to the double bond is considerable. In the last three there is a decrease. This would seem to fit in with the theory proposed a short time ago by T. M. Lowry‘ that double bonds are of two kinds, The first he calls the non-polar, and the second the semi-polar. The non-polar bond consists of two co-valences and the semi-polar of one covalence and one electro-valence. At any rate we should feel quite safe in saying that there is a wide distinction between the double bonds of the first three and of the last three pairs. Just why any such bond should show what is apparently a decrease over the saturated compound is not clear. It is hoped that later evidence will throw light on the matter. Spreading The spreading of substances, particularly solids on liquids, as shown by the well-known case of camphor particles placed on water, has long been the subject of speculation. A complete and satisfactory explanation is however lacking. Various theories have been proposed. most of which have been associated with the effect of substances that spread on the surface tension of the liquid. It seems likely that this explanation is not sufficient for in certain cases of violent spreading the reduction of surface tension is very slight and other factors such as solubility and volatility affect the spreading. Trichloro tertiary butyl alcohol serves as a good example of some of these effects. When a small amount is first added to water the surface tension seems unaffected. After a few minutes the surface tension is gradually lowered until after about depending on condia half hour the decrease may amount to as much as 137~~ tions. If the experiment is conducted in a small vessel which may be closed air tight, it will be found that as long as relatively small amounts are used and the vessel left open, violent motion will continue. If much larger amounts of solid are used, motion will cease as the water becomes saturated, or it will also stop if the cover is put on and the air becomes saturated (apparently) with the vapor of the substance. In the latter case slight motion is resumed Lowry: J. Chem. SOC., 123,
822
(1923);Bull., 39,203 (1926).
268
A. K. NIETZ
if the cover is again removed, though the degree of motion is very much less than before. Consequently, it seems certain that solubility, and to a much less extent, volatility, are factors in spreading. Table IV does not contain sufficient data to warrant any conclusions, since it shows only that in most cases of spreading the surface tension is lowered. Substances which show no spreading do not reduce the surface tension. The violence of spreading does not seem to be proportional to the surface tension lowering. Effect on Orientation of Conditions during Cystallization While a substance is crystallizing, heat and humidity have a large influence in determining the nature of the exterior face of the solid; in fact it is for this one reason, if for no other, that different samples of the same substance give different values. To insure reproducibility would require careful control of conditions during crystallization. It is rather surprising, in a way, that even in the solid phase some molecules are able to turn with such ease and to present an oriented arrangement. These facts are illustrated rather nicely by Table V which is practically self-explanatory, representing a series of experiments in which the humidity was varied considerably. The work of adhesion for stearic acid varies from 39.7 where crystallization took place in perfectly dry air a t low pressure, to 13I . o when crystallization occurred in air saturated with steam a t about 35'. Experiments G, H, J and K show that TABLEV Effect of Conditions during Crystallization on Orientation Stearic Acid
A. €3.
C. D. E. F. G. H.
J. I
