86
F. E . BARTELL AND KENNETH E. BRISTOL
WETTING CHARACTERISTICS O F SOLID SURFACES COVERED WITH ADSORBED FILMS F. E. BARTELL
AND
KENNETH E. BRISTOL
Department of Chemistry, University of Michigan, Ann Arbor, Michigan Received April 1 , 1959
Clean solid surfaces exposed to the atmosphere adsorb constituents of the atmosphere almost immediately. This adsorption alters the free surface energy of the solid and hence the wetting characteristics of the solid. The extent of alteration depends upon the constituent adsorbed and upon its concentration in the atmosphere. An alteration of the free surface energy of a solid is readily detected by measurement of the contact angle of a liquid upon the solid. We had observed that hydrophilic solids such as glass and quartz might give different contact angle measurements on different days. It seemed likely that the contact angle values were closely related to the degree or humidity of the atmosphere. Subsequent study showed this to be true. In the present investigation, clean hydrophilic surfaces and clean organophilic surfaces were treated with water vapor and with organic liquid vapors, respectively, so as to produce definite and controlled adsorption. Contact angle measurements were made with different liquids upon such surfaces. ' The method employed for measuring contact angles was the sessile-drop method. 1. MEASUREMENT OF CONTACT ANGLES ON HYDROPHILIC SURFACES
Apparatus I n order to measure, by the sessile-drop method, both the advancing and the receding contact angles of a given system, an apparatus was designed with which liquid could be added to the drop or removed from the drop very slowly. The solid surface used was of Pyrex or quartz made in the form of a tip, S, which fitted a ground-glass joint (figure 1). The horizontal surface of this tip was polished optically plane, and a small capillary extended from the center of this surface down through the solid. The tip was mounted in a cell, C, which had parallel plate-glass windows and was sealed by a ground-glass joint, E. The liquid, of which the contact angle was to be measured, was contained in a reservoir, R. This could be raised or lowered so as to control the direction of flow of the liquid through the syphon tube, T. The rate of flow was controlled by the accurately ground stopcock, A; also, the flow of liquid could be stopped when desired.
WETTING CHARACTERISTICS OF SURFACES
87
Control of water vapor concentration The amount of adsorption of water on the solid surface was regulated by controlling the concentration of water vapor in contact with the solid. Solutions (or solid materials) capable of maintaining a constant aqueous tension were placed in the bottom of the cell C, and the system was kept in an air thermostat a t 25°C. These materials were so placed in the cell that the surface of the tip was 2 or 3 cm. above the materials. The sub-
FIG.1. Apparatus for the measurement of angle8 of contact in atmospheres of controlled vapor concentrations
stances used to obtain the different aqueous tension conditions are given in table 1.
Method of making the measurements The solid was first treated with boiling benzene for 0.5 hr., was next heat-treated in a furnace a t 425"C., and was then slowly cooled in the furnace to remove strains. When cool it was removed from the furnace and quickly placed in the cell C, where the solid surface was allowed to
88
F. E . BARTELL AND KENNETH E. BRIBTOL
come to equilibrium with the gas phase in the cell. One hour was found to be sufficient for the hydrophilic surfaces to reach sorption equilibrium in the system a t 25°C. The organic liquid to be used was forced over through the syphon tube and allowed to form a drop on the surface of the solid. The rate of flow of the liquid was adjusted so that the drop grew very slowly (Le., approximately 5 min. were allowed for the drop to grow from a diameter of 1 mm. to 3 mm.). The contact angle formed with this advancing liquid-air interface against the solid will be referred to in this paper as the dynamic advancing angle. Photomicrographs of the drop were taken a t intervals as the interface advanced. The direction of the flow of the liquid was TABLE 1 Aqueous tension values of materials used in controlling the concentration of water vapor’ __ WATER VAPOR PREBBURE AT 25%.
BUB8TANCE
NO.
mm. of Hg
1
2 3 4 5 6 7 8 9 10 11 12 13 14
PZOS Anhydrous CaC12 Saturated solution of MgClz Saturated solution of Zn(NO& Saturated solution of Ca(N0a)z Saturated solution of KHdNOa Saturated solution of NHdCI KNO, Saturated solution of (NHI)zSO~ Saturated solution of KC1 Saturated solution of BaClt 14% water solution of concentrated HzSOd Saturated solution of K Z S O ~ 10% water solution of concentrated HZSOr Double-distilled water
+
0 (approximately) 0 (approximately) 7.6 9.5 12.1 14.7 17.1 19.3 20.5 21.5 22.5 23.0 23.4 23.8
* These data were obtained from the International Critical Tables. then reversed and the interface was allowed to recede and the drop, having a dynamic receding angle, was photographed. The drop was made to grow again until the previously wet surface was entirely covered; then as the liquid advanced further, the dynamic advancing angle was again observed. The flow of liquid was then stopped and the system was allowed to stand. The drop now was observed to advance very slowly and in 0.5 hr. it came to rest with the liquid forming a definite and reproducible contact angle, which in this paper is referred to as a “static advancing angle” (meaning that it has been formed after the forced flow of liquid was stopped and after the liquid was allowed to advance under the influence of surface forces only until no further change occurred). This drop also was photographed.
8s
WETTING CRARACTEHIBTICS OF SURFACES
In table 2 are given the data for the contact angles of acetylene tetrabromide on Pyrex and quartz surfaces over the range of aqueous tension values. Figures 2 and 3 are curves drawn by plotting aqueous tension values against the values of the contact angles. Those dotted portions of the curves representing measurements a t aqueous tension values higher than 23.8 mm. were obtained in the following manner: Double-distilled water was placed on the surface of the tip, which was then allowed to stand a t 25OC. in the closed cell containing an atmosphere saturated with water vapor. The system was allowed to TABLE 2 Contact angles of acetylene tetrabromide on Pyrex and on quartz i n systems having controlled water vapor concentrations AQUEODB T E N 8 I O N YALUE OF GAB PHABE AT 26%.
mm.
CONTACT ANGLE0 O N PYREX
CONTACT ANGLE8 O N QUARTZ
~-
w
@do*
8,o'
E&'
of H g
0 7.6 9.5 12.1 14.7 17.1 19.3 20.5 21.5 22.5 23.0 23.4 23.8 (saturation: (above saturation)
17' 17" 19" 21" 28" 36" 36.5" 36" 36" 38" 36.5" 38" 38
0 0 0 0 0 0 18" 18" 25" 25" 28 32" 35"
0 0 0 0 5" 11" 14.5' 18" 19" 26.5" 26" 29" 36" 36"
10"
6"
9"
19" 23 28" 30"
10" 10" 13" 14"
15" 18" 21 24.5"
31"
18"
26"
37.5" 38" 38" 38"
29" 34" 34" 34"
' '
1 ~
31" 36" 37" 37"
* The meanings of the symbols used in this paper are as follows: 8da refers to a dynamic advancing angle; ed, refers to a dynamic receding angle; e,. refers to a "static advancing angle." stand until the visible film of water had evaporated from the surface. In this condition the surface gave, with the organic liquid, a dynamic receding angle which was very nearly the same as the dynamic advancing angle, and was also nearly the same as the static advancing angle. Contact angles on Pyrex
In the case of Pyrex, the dynamic advancing contact angles varied between fairly wide limits as the concentration of water vapor was varied between the zero value and the value represented by an aqueous tension of about 19 mm. A very decided change of the dynamic advancing angle
90
F. E. BARTELL AND KENNETH E.
Bmwor,
6 30 nr
Y)
2 25 9b
I
b 20 E
E
I 15 z
0
E
IO
v)
sa 35 0 0'
IO' 20' 30' 40' ACETYLENE TETRABROMIDE CONTACT ANGLES
5
FIG.2. The system Pyrex-air (water vapor)-acetylene tetrabromide 30
-6 I n
ry
% 25 6
X
k 20 Ei E
z
0 m
z
p
IO
DYNAMIC REC. ANGLES STATK ADV. ANGLES o DYNAMIC ADV. ANGLES
A 0
0 0'
IO'
20'
30'
40'
5(
ACETYLENE TETRABROMIDE CONTACT ANGLES
FIG.3. The system quartz-air (water vapor)-acetylene tetrabromide
WET'I'ING CHARACTERISTICS O F SURFACES
91
occurred when the aqueous tension values were raised from 15 to 19 mm. With aqueous tension values of 19 mm. and higher, the magnitude of the dynamic advancing angle remained practically constant. This variation of contact angle with water vapor concentration would account to a large extent for the differences in contact angles observed when no attempt is made to control the partial pressure of water vapor (figure 2). The dynamic receding angles were not measurable (i.e., were zero) from zero concentration of water vapor to a concentration represented by an aqueous tension value of about 21 mm. Above this concentration of water vapor, the dynamic receding angle became greater in magnitude as the concentration of water vapor was increased. The dynamic receding angle values approached the dynamic advancing angle value at the saturation pressure of water vapor. In an atmosphere having an aqueous tension value below 15 mm., a drop of the organic liquid, allowed to stand for a half hour, spread completely over the Pyrex surface, giving what is interpreted as a 0" static advancing angle. In atmospheres with aqueous tension values of 15 mm. or more, the static advancing angle values became larger as the concentration of water vapor was increased, and approached the value of the dynamic advancing angle a t the saturation pressure of water vapor.
Contact angles on quartz When quartz was used as the solid surface, the dynamic advancing angle of acetylene tetrabromide was found to have a value of 10" at zero concentration of water vapor. The angle was larger a t higher aqueous tension values, until a t 20.5 mm. it had a value of 31". A t a somewhat higher vapor pressure of water, the angle reached a value of 38". At the saturation pressure of water vapor, the value of the dynamic advancing contact angle was not greater than 38" (figure 3). For a given vapor pressure of water, the static advancing angle was always less than the dynamic advancing angle. In a system saturated with water vapor, the static advancing angle did not differ from the dynamic advancing angle by more than I". The dynamic receding contact angle of acetylene tetrabromide on quartz in a system dried with calcium chloride or phosphorus pentoxide had a magnitude of 6". This was somewhat surprising, since acetylene tetrabromide did not give a measurable receding angle on Pyrex until the vapor pressure of water had reached a value of 20.5 mm.
Thickness or completeness of water filmdependent on aqueous tension Such investigators as McHaffie and Lenher (9), Frazer (6), Lenher (8), and d'Huart ( 5 ) have shown that quartz and glasses adsorb increasing amounts of water as the aqueous tension values become greater. It is
believed by them that the solid adsorbs the water vapor as a fih whose thickness is governed by the concentration of water vapor in the gas phase (at constant temperature). These workers differ in their conclusions as to the number of molecular layers in the films, but they all state that the film must be multimolecular in thickness a t high humidities. Findings of these investigators, together with our data on the contact angle values, indicate quite conclusively that the magnitude of the contact angle depends upon the completeness and thickness of the adsorbed film of water molecules.
Measurements on hydrophilic surfaces other than Pyrex or quartz Surface properties of several insoluble hydrophilic minerals were next investigated. These crystalline minerals were freshly cleaved and placed in a cell containing an atmosphere saturated with water vapor a t 25°C.
TABLE 3 Contact angles of acetylene tetrabromide and alpha-bronwnaphthalene on s w t e m saturated with water vapor (at 26°C.)
Pyrex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quartz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gypsum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mica.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluorite . . . . . . . . . . . . . . . ...................... Celestite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36' 37" 37.5' 39' 37.50 38'
80Eid.3
in
36'
36.6" 36.3'
The system was allowed to stand so that the solid surface could come to sorption equilibrium with the gas phase. Drops of organic liquids were placed on the solid by means of a freshly drawn glass pipet. The height and diameter of the drops were measured with a microscope, and the contact angles were calculated by means of the formula tan 3 8 = 2h/d Table 3 gives the results obtained using acetylene tetrabromide and alpha-bromonaphthalene drops on the surfaces exposed to an atmosphere saturated with water vapor a t 25°C. The drops were placed by pipet rather than by the more complicated syphon system, since the experiments on glass and quartz had shown that in atmospheres saturated with water vapor the advancing and receding angles had practically the same value, hence the method of placing the drop made little difference.
WETTING CHARACTERISTICS OF SURFACES
93
Adsorbed f i l m masks the adhesive forces of the solid The data in table 3 show that acetylene tetrabromide on each of the hydrophilic solids gave a contact angle which in each case was the same, within experimental error, and had an average value of 37.5". Alphabromonaphthalene also gave contact angles on the different solids which were of the same value for the different solids, namely, about 36" (the values were practically the same as those obtained with acetylene tetrabromide). Since these solids have different wetting characteristics in the dry condition, the fact that the contact angles on the different filmcovered solids were the same for a given liquid shows that the surface of each solid had been altered so that each had the same wetting properties. A t high humidities, the hydrophilic solids must sorb a film of water which is thick enough to mask the adhesive forces of the underlying solid and to give surfaces having energy values peculiar only to an adsorbed film of oriented water molecules. If the apparent surface energy of the solid is due entirely to the water film or layer, this film must be continuous even under the drop of the organic liquid. During the period of the experiment, apparently no stripping of the water film occurred.
Persistence of the water film That adsorbed water is strongly held by a hydrophilic surface has been demonstrated repeatedly by experiments showing the high temperatures necessary to remove all of the adsorbed water from glass and quartz surfaces. Bunsen (4)found that glass must be heated to about 500°C. before it will give up all of its sorbed water. Langmuir (7) found that when glass was heated as high as 450"C., 22 cc. of water vapor was given off per 10,000 sq. cm. of surface. Bartell and Wooley (3) found that by heating Pyrex and quartz to 425°C. for 1 hr., these solids were made less hydrophilic and attained a condition such that further heating did not affect their wetting characteristics. This heat treatment probably caused the removal of water which had been sorbed by the Pyrex or quartz, and so made them less hydrophilic. In the present investigation experiments were made to determine whether such a drying agent as phosphorus pentoxide could remove the adsorbed water from Pyrex and quartz a t room temperature. Pyrex and quartz surfaces were allowed to stand for 1 hr. in air saturated with water vapor at 25°C. These surfaces were then placed in a closed cell containing air dried with fresh phosphorus pentoxide and allowed to stand 24 hr. At the end of this time, the dynamic advancing angles of acetylene tetrabromide on these surfaces were measured. The values of these angles were about 20". As has been reported in this paper, Pyrex and quartz surfaces which have not been exposed to water vapor have dynamic
94
B. E . BARTELL AND KENNETH E. BRISTOL
advancing contact angles with acetylene tetrabromide of lo", while the presence of adsorbed water on the Pyrex and quartz surfaces causes the dynamic advancing angle of acetylene tetrabromide to have values higher than 10". It appears, therefore, since the measured angle was greater
FIG.4. Coating apparatus
than lo", that adsorbed water must have been left on the Pyrex and quartz surfaces even after the surfaces suood over fresh phosphorus pentoxide. 11. MEASUREMENT OF CONTACT ANGLES ON ORGANOPHILIC SURFACES
In the investigation of the wetting characteristics of organophilic solids upon which films of organic liquids had been adsorbed, the contact angles of water drops upon such solids were measured. Crystals of organophilic
WETTING CHARACTERISTICS O F SURFACES
95
solids were cleaved and an attempt was made to drill a small capillary from the bottom of the crystal to the smooth top surface. Such drilling was not successful, because the surface immediately around the hole was roughened in the process, and the roughened surface did not permit of uniform spreading of the water drop. Pyrex or quartz tips (figure 1 ) coated with organophilic solids by sublimation in a high vacuum were therefore used.
The coating process The coating apparatus (figure 4) was similar to that described by Bartell and Hatch (2). The heating coil, made of No. 26 gauge tungsten wire, was in the shape of a cone with the apex pointing downward. Small crystals of the solid to be sublimed were placed in this coil. Since many of these substances broke up into finer particles when heated, the surfaces to be coated were held in position a t the side of the coil (see figure 4). This method of coating gave a smooth, mirror-like surface, free from roughness such as would have been caused by falling particles of solid. Preliminary to coating, the tips were placed in the coating chamber and the system was evacuated. When the pressure had been reduced to mm. of mercury, the current was started through the tungsten heating coil. .Just enough heat was used to vaporize the mineral slowly. If the vaporization was too rapid, most minerals did not give smooth shiny coatings. I t was also found that if the surface to be coated was allowed to become too hot, owing to heat radiated from the coil, a spongy deposit was formed. For this reason the walls of the coating chamber were cooled with ice water during the coating process. It was found to be unnecessary to flush out the apparatus with nitrogen before evacuation to mm. of mercury, since surfaces formed after such precaution was taken had the same wetting characteristics as the others.
Solids used The organophilic solids upon which measurements were made included galena (PbS), cinnabar (HgS), sphalerite (ZnS), antimony metal, and stibnite (SbsSs). Measurements were also made on surfaces of stibnite which had been altered by oxidation. The unoxidized stibnite surface was designated as “stibnite 1”. Some “stibnite 1” samples were heated in air a t different temperatures and for different lengths of time, and gave surfaces indicated as follows: “Stibnite 2”: “stibnite 1” heated a t 180°C. for 3 hr. “Stibnite 3”: “stibnite 1” heated a t 200°C. for 3 hr. “Stibnite 4”: “stibnite 1” heated a t 220°C. for 2 hr. Considerable difficulty was encountered in oxidizing the stibnite coatings simply by heating in air. No appreciable oxidation occurred until the‘temperature was over 21OoC., and i t was found that if the tempera-
96
F. E . BARTELL AND KENNETH E . BRIBTOL
ture was allowed to rise above this point, the stibnite coating usually would loosen from the glass tip. In order to effect oxidation a t a lower temperature, a little nitrogen dioxide gas was introduced with air into the furnace tube where the tip was being heated. After a few minutes, this nitrogen dioxide was blown out of the furnace tube by a stream of air, and the heating was then continued in air. The presence of the small amount of nitrogen dioxide caused the oxidation to begin a t a lower temperature, but the oxidation progressed too rapidly if nitrogen dioxide was allowed to remain in the furnace tube after the first few minutes. After the stibnite surface had become slightly oxidized, further oxidation would occur slowly in pure air even a t a comparatively low temperature. Using this method, other oxidized stibnite coatings were prepared from “stibnite 1” and are indicated as follows: “Stibnite 5”: “stibnite 1” heated in nitrogen dioxide and air a t 18OoC. for 5 min., then heated in air a t 205OC. for 2 hr. “Stibnite 6”: “stibnite 1” heated in nitrogen dioxide and air a t 180°C. for 15 min., then heated in air a t 210OC. for 3 hr. “Stibnite 7”: “stibnite 1” heated in nitrogen dioxide and air a t 180°C. for 10 min., then heated in air a t 18OoC. for 2 hr. The original “stibnite 1” mirrors were brown in color. The oxidized stibnite surfaces were a silvery gray. The galena mirrors were gray, a little darker than a silver mirror. The cinnabar films were pale yellow and transparent. The antimony mirrors had a bright silvery appearance. The sphalerite films were colorless and transparent; when they were peeled off the glass, they looked like little sheets of mica.
Measurement of the water contact angles A tip with its coating was placed in the holder B (figure 1) and put in position in the cell C. After the tip came to constant temperature a t 25”C., water was forced over onto the surface from the reservoir R. The water drop was formed and allowed to stand, and the static advancing angle was photographed. The liquid-air interface did not advance after the water supply was shut off. Thus i t appeared that the static advancing angle was the same as the dynamic advancing angle. Photographs were also taken of receding drops provided a measurable receding angle was formed. Data obtained from water contact angle measurements on the pure organophilic surfaces are given in columns 2 and 3 of table 4. These data show that sphalerite, galena, cinnabar, and antimony metal surfaces have quite different wetting charttcteristics, and that the wetting characteristics of the stibnite mirror surfaces may be changed by oxidation treatments.
97
WETTING CHARACTERISTICS OF SURFACES
Formation of films on organophilic solids When the surface S (figure 1) is coated with an organophilic solid and is then placed in the cell C , containing a small amount of organic liquid a t 5O-8O0C., and allowed to cool, organic vapors will be sorbed by the solid and will form a complete film on the organophilic surface. If a suitable length of time is allowed for the visible layer of condensed organic liquid to evaporate from the solid, a uniform adsorbed film will be left on the surface. TABLE 4 Contact angles of water o n pure organophilic surfaces and on organophilic surjaces covered with organic liquid films DECALIN PILYE
8.0
~~
e.,
__ - _ _ Stibnite 1 . . . . . . . . . . . . . . . . . . . 80" 0" 75O Stibnite 2 . . . . . . . . . . . . . . . . . . . 79 0" Stibnite 3 . . . . . . . . . . . . . . . . . . . 75" Stibnite 4 . . . . . . . . . . . . . . . . . . . 64" Stibnite 5 . . . . . . . . . . . . . . . . . . . 70" 0' 78" Stibnite 6 . . . . . . . . . . . . . . . . . . . 55" 75" 0" Stibnite 7 . . . . . . . . . . . . . . . . . . . 39 61" 0" Galena, . . . . . . . . . . . . . . . . . . . . . 0" 77" 88" Cinnabar. . . . . . . . . . . . . . . . . . . . 113" Sphalerite . . . . . . . . . . . . . . . . . . . 46" Antimony metal, . . . . . . . . . . . . 55" __
I 1 94" 92"
1
84" 56"
Persistence of films o n organophilic surfaces Experiments were made to determine the relative degrees of adsorption of different types of organic liquids on the different organophilic solids. I t was found that liquids with a strong permanent dipole, such as acetylene tetrabromide and alpha-bromonaphthalene, could not be removed by unaided evaporation a t 25°C. in an atmosphere free from vapors of the adsorbate. But when non-polar or but slightly polar liquids, such as decalin, n-octane, or mesitylene, were used as the adsorbate, the film of organic liquid disappeared within about a half-hour.
Measurement of water contact angles o n adsorbed filmsof organic liquids o n organophilic surfaces After the organic liquid vapors in the cell had come to sorption equilibrium with the organophilic solid on the surface S, a water drop was caused to form on the solid surface. As in the previous experiments, the drops were photographed as the liquid advanced, remained fixed, or receded.
98
F. E . HARTELL AND liENNE:TH E. HRISTOL
Columns 4 and 5 of table 4 give the results obtained with water drops on acetylene tetrabromide films adsorbed on the different organophilic solids. These data show that the static advancing angles of water on acetylene tetrabromide films on coatings of the stibnites, galena, and cinnabar all have angles between 75" and 78". The surfaces which failed to give an advancing angle of this magnitude were films of acetylene tetrabromide on those solids which, in the absence of organic vapors (i.e., in the pure state), gave static advancing angles with water of less than 40" to 50" Such solids were "stibnite 7" and sphalerite. When these solids were covered with an adsorbed film of acetylene tetrabromide, the water static advancing angles were much lower than the 75" to 78" value which was common to the other solids. A notable fact was that the water dynamic receding angles on these film-covered surfaces were 0", or the same as the water dynamic receding angles on the pure "stibnite 7" and sphalerite. This indicates that the sorbed organic liquid film was removed from these solids by the advancing water interface. On those solids from which water did not remove the acetylene tetrabromide film, the film apparently masked the wetting characteristics of the underlying solid and the advancing contact angle measurement was a measurement of the wetting characteristics of the acetylene tetrabromide film. Columns 6 and 7 and columns 8 and 9 of table 4 give the results obtained from water contact angle measurements on adsorbed films of alphabromonaphthalene and decalin on different organophilic surfaces. Alphabromonaphthalene films on "stibnite 1," galena, and cinnabar gave surfaces on which water has a static advancing contact angle of about 79". The dynamic receding angles were 30" to 60". Adsorbed films of the nonpolar liquids decalin, mesitylene, and n-octane mere a130 formed on the above three solids, but decalin was the only one which gave a surface on which a stable advancing water angle formed. Water on the decalin films on the three solids gave an advancing contact angle of about 92". With the three solids both alpha-bromonaphthalene and decalin films appear to have masked the underlying solid so completely that the advancing water angles are a measure of the wetting properties of the films rather than of the solids. Measurements of water contact angles on the organic liquid films on the ('stibnite 1," galena, and cinnabar surfaces were repeated using new tips with new deposited mirrors, and it was found that the advancing angles gave good checks. The receding water angles varied between fairly wide limits, however, and gave values anywhere between 30" and the advancing angle value. The probable cause for the differences observed in the receding angle values was difference in the thickness or the completeness of the adsorbed films of organic liquid in the different ex
WETTING CHARACTERISTICS OF SURFACES
99
periments. If it had been possible to control the thickness of the films of organic liquids adsorbed on the organophilic solids, as it was possible to control the thickness of the adsorbed water film of the hydrophilic solid, the receding angles probably would have given better agreement.
Surface tension of adsorbed films The surface tension of adsorbed water or organic liquid films can be calculated, using the well-known Young equation (IO), if it be assumed that the interfacial tension existing between the liquid drop used and the liquid film under it is the same as the interfacial tension, 8 2 3 , between the two liquids in bulk (admittedly, such an assumption is questionable). If the surface tension of the water film is designated as S:,and the surface
P ER FILM
ID FIG.5 . Drop of organic liquid on surface covered with water
WATER DROP
-ORGANIC LIQUID FILM FIG.6. Drop of water on surface covered with organic liquid
tension of the organic film as S:,the following equations can be set up ( S 3 = surface tension of water; SZ= surface tension of organic liquid) :
S: =
823
+ Sz cos e,,
(see figure 5 )
(1)
S: =
523
+ S3 cos e,,
(see figure 6)
(2)
Since &, Sz, and S3 are all known, S i and S: can be calculated by using the cosines of the appropriate static advancing angles. Using the acetylene tetrabromide or alpha-bromonaphthalene values of e,, in saturated water vapor from table 2, S: is calculated to be about 7'7' dynes. This is the same value as that indicated by the empirical equations of Bartell and Bartell (10) to be the value of the surface tension of a complete film of oriented water molecules.
100
F. E. BARTELL AND KENNETH E . BRISTOL
If the value of 77 dynes is correct it can be seen from equation 1 that only those organic liquids whose 8% SZvalues are greater than 77 dynes will give measurable angles on water films. Table 5 gives a list of St and Szsvalues for six liquids. Carbon disulfide is the only liquid in this group which gave a measurable angle on the water film, and is the only Sz3) value is greater than 77 dynes. liquid of this group whose (Sz The other liquids spread completely over the surfaces. Only mutually insoluble liquids can be used for such tests. Liquids which show any appreciable solubility in water give anomalous results.
+
+
TABLE 5 SI and 8 1values ~ for a h liquids OXQANIC LIQUID
SI1
(Sa
+ Sad
~.
Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toluene, . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrobenzene. . . . . . . . . . . . . . . . . . . . . . n-Octane . . . . . . . . . . . . . . . . . . . . . . . . . .
28.2 27.9 43.3 21.3
34.8 35.9 25.3 50.6
........
63.0 63.8 68.6 71.9 70.7 79.7
TABLE 6 Calculated values of surface tenszons of adsorbed organzc lzquzd $lma (Sa = 76) Stl
LIQUID
8..
Acetylene tetrabromide Alpha-bromonaphthalene Decalin
* These values were taken from the International Critical Tables. From equation 2 the surface tension of some organic liquid films can be calculated if the cosines of the appropriate values of e,, from table 4 are used. Table 6 gives the calculated values of the surface tension of some adsorbed organic liquid films. The polar liquids acetylene tetrabromide and alpha-bromonaphthalene give calculated values of S l considerably higher than the corresponding values of Sz for the liquids in question, as was the case for the polar water film. The non-polar liquid decalin gives a value for surface tension of the liquid film practically the same as the surface tension of the liquid itself. SUMMARY
Using dynamic advancing and receding contact angles, as well as static advancing contact angles, as measures of the wetting properties of solid surfaces, it has been shown that hydrophilic solids in atmospheres of high
THEORY O F CELLS WITH LIQUID-LIQUID
JUNCTIONS
101
humidity, and extremely organophilic solids in atmospheres saturated with organic liquid vapors, adsorb films upon their surfaces sufficiently thick to mask the original surface properties. Less organophilic solids adsorb organic films, but these films may be stripped away by water advancing upon the surface. The values of the surface tensions of some films adsorbed upon some solids have been calculated. In order to make the calculation, one assumption had to be made: namely, that the interfacial tension between a liquid drop placed upon a film is the same as the interfacial tension between the two liquids in bulk. The calculations indicate that the surface tensions of films of polar liquids are higher than the surface tensions of the polar liquids, but that the surface tensions of films of non-polar liquids are the same as the surface tensions of the non-polar liquids. REFERENCES (1) BARTELL AND BARTELL: J . Am. Chem. SOC.66, 2205 (1934). (2) BARTELL AND HATCH: J. Phys. Chem. 39, 11 (1935). AND WOOLEY: J. Am. Chem. SOC.66,3518 (1933). (3) BARTELL (4) BUNSEN:Wied. Ann. Physik 24, 321 (1885). (5) D’HUART:Compt. rend. 180, 1594 (1925). (6) FRAZER: Phys. Rev. 33, 97 (1929). (7) LANQMUIR: J. Am. Chem. SOC. 38, 2283 (1916). J. Chem. SOC. 1926, 1785. (8) LENHER: (9) MCHAFFIE AND LENHER: J. Chem. SOC.127, 1561, 1565 (1925). (10)YOUNG:Trans. Roy. SOC.(London), p. 65 (1805).
ON QUASI-REVERSIBLE CONDUCTION AND GALVANIC CELLS WITH LIQUID-LIQUID JUNCTIONS F. 0. KOENIG Department of Chemistry, Stanford Universitg, California Received February 4, 1939
I. INTRODUCTION THERMODYNAMIC AND KINETIC METHODS IN THE THEORY O F LIQUID-LIQUID
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The problem of liquid-liquid junctions, like many problems in physical chemistry, has been attacked theoretically by both thermod-ynamic and kinetic methods. The thermodynamic method was first applied to this problem by Helmholtz (11) and has been further developed chiefly by Nernst (20), Henderson (13), Lewis and Randall (18), Taylor (23), and Guggenheim (4, 5 , 6, 9). The kinetic method was originated by Nernst
