SURFACE ENERGY OF MBRCURY.
2-55
[CONTRIBUTION FROM T ~ E KENT CISEMICALLABQRATORY OP THE UNIVERSITYOF CHICAGO. {
SURFACE ENERGY OF MERCURY AND THE ENERGY RELATIONS AT THE INTERFACE BETWEEN MERCURY AND OTHER LIQUIDS.1 BY WILLIAMD. WARRENSAND WARRBNW. SWING Received September 24, 1920.
Introduction. The measurement of interfacial tension offers a convenient method for determining the energy relations-free energy, latent heat, total energy of surface formation, and work. of adhesion--between liquids insoluble, or slightly soluble, in each other. White considerable is known concerning these relations in the case of liquids, but little is known about the energy relations existing between solids and liquids a t their interface. Such knowledge would be of special value in connection with problems concerning lubrication and the flotation of ores. It i s difficult to measure either the force or the work of adhesion between a solid metal and a liquid, but the adhesional work may be determined from measurements of surface tension provided the metal is in the liquid state. Among the metallic elements mercury and gallium2 have freezing points lower than the boiling points of ordinary liquids. Some of the low melting alloys also possess this characteristic. This paper gives the energy relations obtained by measuring the surface tension of mercury a t a-arious temperatures, the interfacial surface tension between mercury and various liquids and vapors a t z o o , and the interfacial surb'ace tension between mercury and 4 organic liquids at different temperatures between o o and 60
tnrface Energy Relations of ~ ~ in a ~~~~~~* r ~ ~ Siiacc- the values found in the literature3 vary by a hundred ergs per square centimeter, it was thought best to make a new determination of the surface tension of mercury in a vacuuni. Also it is necessary to know the surface tension of mercury a t several temperatures if the total energy relations a t the interfaces is to be calculated. A new form of apparatus was constructed for measuring ehe surface tension of merciiry 2% vacm. Fig. I represents the final form of the apparatus used. The carefully purified mercury was distilled from Flask A into XeserA similar papei on the energy reldtions a t the interface between water and. 01ganic liqsids will be presented in TEIISJOURNAL by Harkins and Cheng Por the pre Iiminary work ox mercury see the preceding paper by Harkins and Grafton. a We hope to work on the stirface erlergy relations of both liquid and solid galIium
in thi5 !abora tory Cenac, Ann chzm, [VIII]29, 298 j r g r 3 )
r
2540
~~ILLIAM D. HARKINS AND WARXGN
w.
EWING.
voir B after the system had been evacuated by means of a mercury condensation pump, with the pump still running. The spirals C, C' permitted the reservoir to be raised so that the mercury flowed over and dropped from the tip T. The height of B was adjusted so that a drop formed in :? minutes. Determinations were made in which the time of formation of the drop was varied between one and five minutes and it was found that the weight of the drop is independent of the time .tvhenever the period is greater than 1.5 minutes. When IO drops had fallen into D, the tube was sealed off and another IO drops collected. The sealed portions of the tube containing the mercury were broken and the mercury weighed in a weighing bottle. A constriction was made in the capillary tube a t E to control the speed of drop formation. In the diaP
Pig. I.-Apparatus
for the determination of the surface tension of mercury in a vacuum produced by a mercury diffusion pump.
gram F represents a ground glass joint and G a mercury seal. It was found necessary to have the portion of the capillary tube above the tip perfectly smooth and clean. Any roughness of the glass or the presence of a particle of dust caused the mercury column l o break apart a t that point, so that many tubes had to be tried before a satisfactory one was found. A 2-stage mercury condensation pump was used in producing the vacuum, and the system was evacuated for 2 hours before starting the collection of drops. The pressure was so low that no pressure could be detected by the use of a McLeod gage. The results obtained are given in Table I. The surface tensions, Col. 2 , are calculated from the equation y =: m g / 2 m $ ( ~ / v ~and ) the latent heats from the Clapeyron equation 1 - T A ~ / A T . The following results indicate that the Ramsay-Shields constant is 0.96 instead of the normal 2 . 1 2 for mercury, or the entropy of surface formation for the area occupied by a molecule is 0.0135 X 3[0-** ergs. I(-
SURPACG ENZRGY OP hlERCURY. TABLE
r.--TETi FREEENERGY,
LATENT FIEAT, AKin
2541
TOTALENERGY
OP SURPACE
FORMATION OF MGRCKJRY in Vacuo." (All values in ergs per sq. cm.) Entropy of surface formation = 0 . 2 2 ergs per degree per cm2. 1.
Temp.
O
C.
2. Free energy of surface formation.
3. Latent heat of surface formation.
5.
4.
Tots! energy of surface formation.
TA 7".
480.3 60.1 540.4 0.00046 62.3 540.4 ...... I0 478.3 540.6 ...... 20 476.1 6 1 .j 66.7 540.9 ...... 30 474.2 48.9 540.2 ...... 40 471.3 71 . I 540.5 ...... 50 469.4 540.4 ...... 60 467. I 73.3 Instead of the value 480 3 dynes at oo as given here, Celiac found 460 or 467, according as the correction used was that determined by calculations by Lohnstein or by the experiments of Rayleigh. Cenac, Cantor (Wied. A m . ,1894, p. 423) and 0
Siedentopf (Diss. Gottingen, 1897) obtained the value 0.00046 as given above.
0.0005
for ____ instead of yo
Apparatus and Experimental Procedure. The interfacial tensions were measured by means of the drop-weight apparatus devised in this laboratory for measuring the surface tension of liquids in air, but in a slightly modified form. Since mercury does not wet glass, the drops form and fall from the circumfereiice of the bore and not from the circumference of the tube. Also, because OS the high density of mercury, it is not possible to cause the drops to form slowly enough if the mercury siphons through the tube under the influence of gravity, sh third difficulty, not met with in making similar measurements with water, is that the mercury surface adsorbs very readily any water in the organic liquid, the dry organic liquid dissolving enough moisture from the air in a few minutes to appreciably change the interfacial surface tension. To overcome the first of these difficulties a tip made from a metal which mercury wets was first tried, but it was found that with a metal such 8s platinum enough of the metal dissolved in the mercury to change its surface rension. Glass tips were, therefore, used in all measurements recorded. l'~ insure a sharp edge to the bore of the tube a t the end v7here the drop broke off, the bore was filled with Woods metal before grinding. All the tips used had a diameter oi 1.2 mm. to 1.4mm., and this was accurately measured. To control the speed of flow of mercury, a very small constriction (B, Fig. 2 ) was made in the siphon. After considerable practise it was fourid possible to make a constriction so small that with a head of mercury of 2 centimeters in the reservoir A, a drop weighing one g. would take 5 minutes to form a t the end of the tip C.. I n practise suction was applied
2.542
WILLIAM D. H.4RKINS AND WARREN W. SWING.
at El until the drop was so large that it would fall in about one minute after the suction was released. The constriction was so small that sulI I furic acid could be forced through under pressure only very slowly, and so small, too, that a small particle of dust was found to clog the bore completely, thus ruining the tube for further use. In purifying the liquids used great care was exercised both in drying the liquids and in E keeping them from contact with moist air during t h c experiment. It was found that the same porj."jg- ---Glass parts of apparatusfot the de- tion of liquid could not be l ~ e d termination of the surface tension (inter- for running a second set of drops, facial tension) between other liquids and since in changing the liquid from mercury For a figure showing the metal the weighing bottle (Fig*2, E) parts See TrrlsJ o u R N A t , 37, 1656-76 (1915) to another weighing bottle, in order to remove the mercury, enough moisture was taken up from the air t o change the interfacial surface tension markedly. Purification of Liquids. The liquids used were purified by the methods given below. ury.- Commercial mercury was distilled and this was then I' by electrolysis and repeated distillations according to the method recornmended by the Bureau of Stamdards.I ~ l were treated ~ with ~ freshly ~ burned o quickliine ~ €or ~ several days, c:iskilled OB,fractionally distilled and dried over metallic calcium for 2 weeks. The ~ ~ ~ ~ and Q also ~ alcohol-free ~ ~ b ether o were n ~purified by heating for several hours, in a flask containing mercury and fitted with a reflux condenser, until the black precipitate which appeared a t first no Imger iormed. They were then fractionally distilled and dried over llic sodium for z weeks. e k and undecylinie acids were distilled iuz vaczzo. ~~r~~~ disulfide and di-amyl amine were fractionally distilled and the distillates dried over freshly fused potassium hydroxide. Aniiiule was fractionally distilled 2 times, finally with zinc dust and hydrogen, and the colorless product was dried over potassium hydroxide. Acztom was fractionallv distilled and dried over calcium chloride. The kalogetz c o m p o u ~ d swere distilled and kept over mercury, redistilled and always used immediately after the redistillation. Btrr. Siandards, BaR, 4s X Q (1927).
2543
SURFACE ENERGY OF MERCURY.
Carefully purified aromatic hydrocarbons ar;d ether contained an inmpurity, presumably a suliur compound, which caused a Slack precipitate to form in the presence of mercury. A-Eter refluxing several *hours with mercury, this impurity was removed.
e Xnterfacial Tensions at the Phase Boundary Between Mercury and Another Liquid. TABLEII.-THE INTERFACIAL TEKSION AND ADHESIONBGTWSEN MERCURY AND ORGANICBcr~s. (Liquids arranged in order ol adhesional work against mercury.) (Values in ergs per sq. em. Temperature zoo.) 2.
1.
3'
Free surface energy
Free interfacial energy
Of
against
4'
Adhesional work against mercury (W 4
liquid. mercury. Y H g S - 7 1
Acetone.. . . . . . . . . . . . . Hexane.. . . . . . . Ethyl e t h e r . . . . . . . . . . . . . . .
DLarnyl Amine. . . . . . . . . . .
Propyl alcohol.. . :Nitro-ethane. , . . :athyl alcohol (G) ~
...
Ti.
Yl.
2 6 ,x
390 378 379
18.4 21.8
21.7 24 6 23.7 34.9 22.4 2 7 .o
Carbon tetrachloride ( G ) ... 26.6 Toluene.. . . . . . . . . . . . . . . . . 29 .o 28.8 an-Xylene . . . . . . . . . . . . . . . . 29 .0 29 .O 2 7 .o . . . . . . . . . . 27 . I 30.6
. . . . . . . . . . 2 2 .a Octyl alcohol (G) . . . . . . . . . . 27 . 5 Methylene chloride (G) . . . . 26 .5 Nitrobenzene. . (6)........ 43.4 BtInylidene chloride (G) . . . . Carbon di .....
24.6 31.4 42.6
Aniline. .. .... Water (G . . . . . . . . . . . . . . . . 7 2 . 8 Oleic acid . . . . . . . . . . . . . . . . 3 2 . 5 28.2 38.7 Methyl iodide (G). . . . . . . . . 35 49.6
4.76 .Q
37.5 37 1
368 378 364 359 $2
359 357
--Ti.
5
Cohesiona! wark against itself. (WC)
271.
6.
7. Adhesional
DiiTeiSence. (4)-(5) S
against water. "/E,O
work
w'4--wc.
I12
52.2
60
116 I 19 123 130 I32 133
36 9 43 :5 43.5 4.9 . 2 47.4 69.8 44.8
79
134
40
75
73
79
44
81 85 63 89 90 93 89
144 146 147 148 148 149
54.0
38.0 58.0
90
1.51
97 97
53.3 58.0 57.6
ir;--ri
90
..... 96
..... 85
..... 56 67 67 64 6: 64 67 103 94 92 71 91
357 359 361 357 353
1:jx
54.0 54.3
r54
61.2
93
343
156
II0
352 31.' 3.50 337 3.36
157 165
45.6 55.0 53 .O
I12
86.8 49.2
I21
62.8
I 08
56
85.2
92
IJO
32
145.6 90 63 75
169 170
37 5 393 322
171 177 178 187 191
326
193
304
207
320
213 (952)
341
145.6 65.0 j6.4 77.4
70 99.3 952
Values marked (G) obtained by Dr. Z, 11.Crafton.
91
I02
$2
122
135 I 16
1.37 14
T
.....
.."..
84 178
8.
a544
WILLIAM D. I-IARHINS AND WARREN W. EWXNG.
Adsorption of Vapors on a Mercury Surface. Preliminary experiments were made on the surface tension of mercury in air satbrated with vapors of volatile liquids. The results are only approxiniate as the only precaution taken to control the concentration of the vapor was to let the apparatus stand for an hour a t 2 0 * *O.I with a Sew drops of the liquid in the bottom of the weighing bottle (Fig. 2 , E). Tn benzene vapor the surface tension was iound to be about 394, in ether vapor about 389 and in carbon disulfide vapor about 370 ergs er sq. cm. This shows a lowering of the free-surface eiiergy of mercury f 80 to 100 ergs per sq. cm. and gives values approximating those found in liquids. This indicates a s t m a g positive adsorption of these vapors on a mercury surface. An attempt was made to measure the adsorption of nonylic acid in water solution by mercury but the results were not concordant. The solution could not be kept from creeping up into the tube and thus displacing the mercury. The same difficiilty was met with in the case of some of the organic liquids. Monochloro-acetone in particular would displace the mercury for a distance up into the tube of from I to 2 em. This is probably caused by the attractive forces between the liquid and glass being stronger than the forces keeping the mercury in contact with the glass. The interfacial tension against benzaldehyde could not be determined because in it a white skin formed about the niercury drop and supported the drop until it became abnormally large. The skin then broke letting the drop Sal1 through. f the ~ ~ ~ ~Work. r ~ r n ~ ~ As has been slated, in all of this work the mercury was dropped down from the sharp inside edge of the capillary tube, or from what is called an open tip. This method, as was found by Karkins and Brown, is not so accurate as that in which w7hat is termed a closed tip is used, so the results should not be expected to attain the precirion better than 0.1% such as was obtained in work on water and other liquids in their work. The precision attained in the present work seemed to be of the order of x%-thab is, results on the same liquid agreed to within 1% even after long periods of standing, provided the liquid was kept carefully sealed in the interim, though opening the bulb to the ordinary air of the laboratory would often allow enough moisture to be absorbed to cause a lowering of several per cent. in the interfacial tension. However, such manipulation as this is inexcusable. wo important sources of error have been mentioned in the previous section. First, the organic liquid may creep up the capillary tip between the mercury and the well of the glass capillary, as was the case with ~ ~ n o c h l o r o " ~ c e tand o n ~with , an aqueous solution of nonylic acid. HOWever, in such cases the results obtained were altogether irregular aid
~
a
SURPACE ENERGY OF MERCURY.
2545
erratic, with a variation as large as 25%. The very regular results ob.. t s h e d in connection with the data presented here give a reasonable basis to the assumption that this error was avoided In order to do this, however, it was neces4do sary to use a small tip, with EL diameter less than 2 mm. in 47s order to give a bulging drop, so that the mechanical forces 470 cause the mercury to press tightly against the edge 46s of the tip. Second, there was no evidence of the formation $ oh a quasi7soli~film except 380 with benzaldehyde, where the results were etatirely erratic 375 and abnormally high, and in the case of the halogen de- 2 370 rivatives, which gave very consistent and low results. 2 365 In d?other cases the mercury .E! ~ e ~ a extremely ~ r ~ e bright, ~ g60 and consistent results were E] 8 obtained Attempts to ob- 8 tain the interfacial tension by ld 355 3 dropping the liquid upward m through the mercury failed 35O since with the experimental 345 arrangements at hand it: was found impossible to control 340 the drop sufficiently well. The results obtained ita vacuo are believed to be the most ac335 curate thus far obtained with oo 10' 20' 30" 40% 50' 6c C mercury. Any small differTemperature. the data of Fig. 3.--The free surface energy (or surface tension) enceS of mercury in e a c m and a t the interface with the preceding and of the other liquids, present paper are not due to the drop weight method as employed, but to the later removal of iinpurities sinal1 traces of which may change the values greatly. Discussion of Results. The data presented in the tables of this and of the preceding paper indicate that the adhesional work between a mercury surface and the surface of an organic liquid is always greater than that between the organic
4 5
-8
2546
WILLIAM D. HARKINS AND WARREN W. EWING.
mwLD 0
C r ) N
Nm b*W
N N
mc
SURFACE ENGRGY OF MERCURY.
251.9
stance and water, and also greater than that between the organic liqlaid and itself, or the cohesional surface work. A second point of interest is that for about half of the substances investigated. ihe di$erence between the adhesional work against mercury and that against water is nearly constant, and between 80 and 90 ergs. This is true for such liquids as the paraffin hydrocarbons, benzene, toluene, as d xylenes, carbon tetrachloride, chloroform, and nitrobenzene, so the work of attractzon is by EO means entirely speci$c. On the other hand the adhesional work is specifically high toward water in the case of the alcohols, water itself, organic acids, acetone and ether, while that toward niercury is very high in the case of the compounds containing iodine, bromine, and sulfur, and somewhat high for oleic acid, which indicates that the specific effects are very marked. The values for W A-WCor of S, the spyeadixg coeficient, are all large, which would indicate that if what may be termed the Neurnan triangle principle is applicable to spreading all of these liquids should spread on a pure mercury surface. That they often do not spread is not surprising, when it is considered that a mercury surface in ordinary air is always covered by a film oi water and other vapors, as may be seen by consulting the preceding section of this paper. It will be seen that this spreading CQefficient increases with the temperalure for zso-butyl alcohol, secondary octyl alcohol, and benzene, but decreases for octane. Table I11 shows that the adhesional work decreases rapidly with increase of temperature, while the tola1 adhesiovzal evzergy zncreases.l The latent heat of the interface beiween mercury and another liquid decreases rapidly as the temperature increases, while the latent heat of most crrdinary surfaces increases with the temperature, and the total energy of interface formation also decreases, though not so rapidly. Since the halogen and sulfur atoms in organic compounds show a specifically high attraction for the mercury surface, it is to be expected that a t such an interface the molecules should be oriented in such a way that i-he halogen or sulfur atoms are turned toward the mercury. CRTCAGO, ILL.
It i.; evident that at higher temperatures the adhesional energy must also de('reace.
