Hysteresis of Contact Angles in the System Mercury - Benzene - Water

13 Hysteresis of Contact Angles in the System Downloaded by UNIV OF MARYLAND BALTIMORE COUNTY on January 12, 2015 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch013

Mercury - Benzene - Water

ANTOINE M. GAUDIN and AUGUST F. WITT Massachusetts Institute of Technology Cambridge 39, Mass. Experiments with a water drop on mercury under benzene in absence of oxygen show an equilibrium contact angle of 119° ± 1° and absence of hysteresis. Under these condi tions the dynamic contact angles did not i n dicate any speed dependence. The same system when saturated with oxygen shows a hysteresis of 110°. pH dependence of hys teresis was found for oxygen-contaminated systems. In the pH range 0 to 4, the hys teresis is nil, increasing to 141° at pH 14. We conclude that contact angle hysteresis is ab sent in the pure three-phase system. H y s teresis when encountered is attributed to oxidation reactions occurring at the mercury surface. When a drop of a liquid is placed on a solid surface, the system will try to establish a state of minimum total surface free energy. D e pending upon the relative values of the three interfacial tensions i n volved, the contact angle, Θ, between the liquid and the solid phase will vary from 0° to 180°. The correlation between γ and θ is given by Young s equation T

^SG

-

^SL

=

>LG

C

0

S

θ

W

Rayleigh [4] found as early as 1890 that the magnitude of θ often depends upon whether previous to the measurement the liquid phase was advancing or receding over the solid phase. Sulman [5] in 1920 termed the spread between the maximum and minimum contact angles measured in the same system "contact angle hysteresis." Since then many systems have been found with hysteresis values up to 150°. The fact that hysteresis is often overlooked or neglected, added to the diffi culty of obtaining good reproducibility, renders the value of many pub lished data doubtful. 202

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

Downloaded by UNIV OF MARYLAND BALTIMORE COUNTY on January 12, 2015 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch013

73.

GAUDIN

AND

WITT

Hysteresis

in Mercury-Benzene-Water

203

Current theories to explain hysteresis of contact angles are p r i marily based on the concepts of surface roughness, surface hetero geneity, friction, and adsorption phenomena. Unintentional adsorption, or contamination—the result of inadequate experimental technique—is, however, the most frequent explanation. A s all systems involving solids are subject to the reasons indicated above for hysteresis, we chose the system mercury-benzene-water, which should be affected only by ad sorption phenomena, controllable under proper experimentation. A n additional advantage is the fact that all interfacial tensions involved can be measured. Materials Used A l l glassware was cleaned in hot chromic acid, rinsed with hydro chloric acid, and again rinsed with conductivity water. Water, specific conductance, Κ = 6 x 1 0 " o h m " per c m . Benzene, reagent-grade, chemically purified and submitted to zone refining (refractive index 1.5011, 20°) Mercury, triple distilled, total impurities less than 0.00009 weight %. It was redistilled and stored in a quartz vessel under nitrogen. Unless otherwise stated, water and benzene were saturated with purified nitrogen and saturated with each other. The atmosphere above the three-phase system under investigation was constantly flushed with nitrogen saturated with water and benzene. The nitrogen, prepurified (99.998% N , 0.002% 0 ) , was passed through activated copper, heated to 300° C , and passed through a concentrated KOH solution. This p r o cedure reduces the oxygen content to less than 5 χ 10" mole %. 7

2

1

2

4

Preparation of System and Experimental Setup In a borosilicate glass vessel with optically flat walls, mounted on the tilted stage of the microscope table, a drop of water is squeezed on to the surface of mercury from a glass capillary which is part of a micrometer buret. By means of a motor and chain drive connected to the micrometer, the volume of the drop may be increased or decreased at any desired rate. A n image of the drop is focused on a glass screen by means of the microscope tilted into horizontal position. T o reduce the heating of the system due to light absorption, a blue filter was placed between the microscope lamp and the reaction vessel. This filter arrangement was subsequently replaced by an electrical circuit automatically illuminating the system for 0.1 second, when the drop was photographed by a Bolex reflex camera. The desired lateral movement of the three-phase interline (Hg H 0 - C H ) is achieved by changing the drop volume. In continuously recording the volume change of the drop, the so-called "dynamic" a d vancing and receding contact angles are registered. The system (ex clusive of motor) is placed on a heavy steel plate equipped with rubber mounts to reduce extraneous uncontrollable vibrations. Vibrations i n duced by the motor are a constant factor in all experiments. This a r rangement also permits the measurement of "static" contact angles and the investigation of relaxation phenomena. One distinctive feature of this setup is that the speed of lateral movement of the three-phase interline changes continuously. With increasing drop volume, the speed 2

6

6


ADVANCES IN CHEMISTRY SERIES

204


of movement of the interline decreases (and vice versa). A l l experiments, unless otherwise stated, were conducted with a constant rate of drop volume change of 0.0189 cu. mm. per second. Changes in the system were recorded with a 16-mm. Bolex reflex camera with time lapse attachment at a speed of 1 frame every 6 seconds. The films were developed and the contact angles measured with a protractor from enlarged pictures (Figure 1).

INTRODUCTION

Figure 1. Principles

of experimental setup

The cleaned reaction cell is closed with a ground-glass l i d and flushed with nitrogen for 20 minutes at a rate of 50 to 100 c c . per minute (STP). Without interrupting the flushing process, benzene (equilibrated with water) is introduced through the top of the cell. Next the rigorously cleaned microburet, which terminates in a capillary about 0.1 mm. in outside diameter, is filled with conductivity water, which is deoxygenated and saturated with benzene. The capillary is now i n troduced into the reaction vessel so that its end is about 8 m m . from the bottom of the cell. Mercury is then introduced until its surface is about 0.5 mm. from the capillary. After the capillary and the mercury interface have been focused on the glass screen, the system is ready for experimentation. Experimental

Results

Influence of Oxygen. E a r l i e r work by Bartell [1], as well as our preliminary experiments, indicates that the presence of even traces of oxygen in the system greatly influences contact angles and hysteresis. Oxygen is minimized by deoxygenating the whole system and constantly flushing it with purified nitrogen. In two independent experiments, mercury was introduced into the reaction vessel by two extremely different techniques. A . Mercury was introduced through a glass tubing which reached into the benzene phase. It thus did not come into contact with the gas phase above the benzene surface. B . Mercury was introduced by passing it in a fine spray through the gas atmosphere above the benzene surface. The formation of a water drop at a constant rate of volume change of 0.0189 c u . m m . per second, experiment A , produces a dynamic


13.

GAUDIN

AND

WfTT

Hysteresis

in

Mercury-Benzene-Water

205

advancing contact angle, 0 , of 119° constant from zero volume to the final volume of 3.4 cu. mm. (frame 30). 0 , the dynamic receding con tact angle, obtained when reducing the drop volume at the same rate, is identical with 0 down to a volume of 0.795 cu. mm. Over this volume range we observe absence of contact angle hysteresis. When the drop volume is further decreased, 0 decreases and reaches a minimum value of 106°, corresponding to a maximum hysteresis of 13° at the last frame (Figure 2). A second experiment on the same part of the A

R

A


R

1 1 1 I I M

Ί 1

1 1 11

1 1

-

ο ADVANCING CONTACT ANGLE

-

• RECEDING CONTACT ANGLE

M 0

ι

M 4

8

Figure 2.

ι ι ι ι ι ι ι ι ι ι 12

16 FRAMES

20

24

28

Contact angles in system Hg-C H -H 0 6

Mercury

-

6

2

introduced without contact with gas phase

mercury surface, done immediately after the conclusion of the first, showed identical results. In the portion of this experiment where no hysteresis can be observed, we may reasonably assume that the r e corded angle is identical with the equilibrium contact angle. This is confirmed by a calculation of 0 by means of Young s equation, based on interfacial tension measurements done by Bartell [2]. In experiment B , where mercury is introduced through the gas atmosphere, an entirely different behavior is encountered (Figure 3). 0 has now increased from 119° to 149° and hysteresis is observed over the full experiment, reaching a maximum value of 82°. A second experiment on the same surface showed a marked decrease of 0 to 134°, while 0 , the receding contact angle, is identical with the one measured in the first experiment. In these experiments the final part of the receding drop was filmed at a rate of 1 frame per second, in order to follow the variation of 0 better, while the rest of the experi ment was recorded at a rate of 1 frame per 6 seconds. Assuming that the increased hysteresis, Δ 0 , found in the second experiment is due to contamination of mercury by oxygen and other un identified gaseous components, we flushed the equilibrated liquids ( H 0 a n d C H ) with oxygen (99.5%) and introduced mercury as in experi ment A without bringing it in contact with the gas atmosphere above the 1

A

A

R

2

6

6



206

Θ

Ί — I — I — 1I — I — 1

n

I

1

1

—

1

1

—

τ

r

EXPERIMENT

1

1

I

r-

ΊΓΊΓ

EXPERIMENT Π

140

120


100

80

60

Y

ο •

ADVANCING CONTACT ANGLES • OF EXPERIMENTS I 8 2

• •

RECEDING CONTACT ANGLES OF EXPERIMENTS I a 2

40

20

1

1 1

1

I

1

ι ι ι

1 1

12 16 FRAMES

Figure 3.

20

ι ι ι ι 24

28

FRAMES

Contact angles in system Hg - C H - H 0 6

6

2

Mercury introduced in fine spray through gas phase

benzene. Hysteresis now was found to be 110° as compared to 13° for the "oxygen-free" conditions (Figure 2) and 82° for the slightly con taminated system (Figure 3). When the oxygen-saturated, equilibrated liquids were flushed for 20 minutes with purified nitrogen, hysteresis dropped to 80°. Further flushing for 4 hours reduced hysteresis to 38°. Flushing with nitrogen reduces the oxygen content in the system, but no assurance can be given that it removes all oxygen. Influence of pH. In a series of experiments the behavior of the sys tem was further investigated, when conductivity water was replaced by diluted hydrochloric acid or sodium hydroxide of varying concentration. Acidic pH Range. The influence of the acid is twofold. There is a slight decrease of contact angles (concentration-dependent) with i n creasing acid concentration, and hysteresis is absent over a wide range. Table I and Figure 4 show results obtained 6 minutes after contact of the benzene phase with the mercury. In consecutive experi ments on the same part of the mercury surface, it was found that after some time a varying degree of hysteresis can be observed at all acid concentrations (Table II). The period over which no hysteresis is de tectable is a function of pH. Identical results were obtained when ex periment at pH 2 was performed in the inverted system (benzene drop on mercury under water). 0 and 0 were 117° and hysteresis was n i l . Basic pH Range. In the pH range from 7 to 14 slight concentrationdependent increase of 0 occurs, and hysteresis observed in the whole basic pH range increases with increasing base concentration. While the advancing contact angle increases to a maximum of 141°,the reced ing contact angle decreases steadily and reaches a zero value at pH 13. A

R

A


73.

OAUDIN

AND

W/TT

Hysteresis

in Mercury-Beniene-Water

Table I. Effect of pH on Contact Angle Hysteresis in Mercury-Benzene-Water System


PH 0 1 2 3 4 6 7 9 10 11 12 13 14

93 111 116 118 119 119 119 126 129 130 132 134 141

2

4

93 111 116 118 111 104 98 83 80 52 38 0 0

6

8

Nil Nil Nil Nil 8 15 21 43 49 78 94 134 141

10

12

14

pH

Figure 4. pH dependence of contact angle hysteresis Table II. Effect of pH on Time Interval Over Which No Hysteresis Can Be Observed ο

θ

Ρ

0 1 2 3 4

Period of No

equil'

Hysteresis, Min.

93 111 116 118 Hysteresis

45 34 25 24


207


208

Figure 5, A and B, shows in sequence the behavior of a water drop at pH 14. In Figure 5, A , the abscissas are the successive volumes of the drop, but in Figure 5, B, the abscissas represent the diameter of the three-phase interline. The ordinate s in both A and Β are the ob served contact angles. Figure 5, B, shows that the drop expands with constant 0 = 141° to an interfacial diameter of approximately 1.0 mm. At this point the volume increase is stopped. During the 18 seconds that elapsed before the drop volume reduction was started, the system showed a slight relaxation phenomenon, which increased the threephase interline diameter, and slightly decreased 0 , while the drop volume remained constant (square dots in 5, B). When the drop volume was reduced, 0 steadily decreased to zero while the interfacial area, H g - H 0 , remained constant. When 0 reached zero, a thin film r e mained on the mercury surface.


A

A

R

R

2

VOLUME (ARBITRARY UNITS)

θ INCREASING DROP VOLUME

RELAXATION

o—o-o—ο—o-o—o-o—c>o-ommjf

140

DECREASING DROP VOLUME

100

60

20

J

L

J_

0.24 0.48 0.72 0.96 1.20 DIAMETER OF CIRCULAR THREE-PHASE INTERLINE (mm)

Figure 5. Behavior of H 0 2

A. B.

drop at pH 14

Contact angle vs. volume Contact angle vs. diameter of three-phase interline

A new experiment done immediately after the first one, on the mercury surface covered with the film left behind from the first ex periment, showed a readvancing contact angle of zero. The water drop spread with zero contact angle over this film and was stopped at its end. At this constant diameter the advancing contact angle increased until it again reached its value in the first experiment. When this value


73.

GAUDIN

AND

W/TT

Hysteresis

in

Mercury-Beniene-Water

209

was reached with a further increase of the drop volume, the circular three-phase interline increased its diameter, while 0 remained con stant at 141°, the value of the angle in the first experiment. At pH 14 when the connection between the film remaining from the first experiment and the glass capillary was broken, the film remained unchanged at the same location of the mercury surface. Readvancing contact angles were zero even after several minutes. At lower pH values, however, the system behaved differently. At pH values less than 13, 0 decreased but never reached the values of zero recorded at pH 14. When between experiments the water drop was completely sucked back into the capillary and the next experiment started about 30 sec onds afterwards, 0 had again the same value as in the first experiment. Speed Dependence. A l l experiments done with the controlled drop volume method are inherently accompanied by a steadily varying speed of lateral movement of the three-phase interline. A s the rate of volume change remains constant throughout each experiment, the lateral move ment of the three-phase interline decreases with increasing drop v o l ume and increases with decreasing drop volume. For our experiments with a maximal drop volume of 3.4 cu. mm., a drop volume change of 0.0189 cu. mm. per second, and a 0 of 119°, the speed (computed over 6 seconds) varied from0.3 χ 1 0 " to 0.1 mm. per second. In these experiments 0 is independent of the rate of movement of the three-phase interline over the whole speed range. Not so with 0 . The maximum hysteresis depends not only on the velocity of movement of the interline when the drop is shrinking, but also on the duration of contact at maximal diameter, and the magnitude of the maximal drop volume. Table III, for example, illustrates the effect of changing the duration of contact at maximal diameter. A


A

A

2

A

R

Table III. Variation of 0 with Time of Contact of Water Drop with Mercury Surface (at pH 9.3) Time of Contact at Maximal Size, Sec.

θ

6 180 300 600

Q

0 A

'

129" 127 128 128

Δ0 °

0 R

'

99 79 68 40

30 48 60 88

Discussion of Results Oxygen greatly influences the contact angle and hysteresis. In its presence the reaction 4Hg + 0 + 2 H 0 = 2Hg^ + 4 0 H " takes place. The oxidation according to Gatty and Spooner [3] proceeds step wise, the first step being the formation of "oxide patches" which have almost free surface mobility. These oxide islands grow in size and finally cover the whole mercury surface as a continuous film, the s u r face of which no longer has the properties of liquid mercury. The sta bility of this deposit is significantly influenced by the nature of the electrolyte in contact with it. While basic solutions favor film stabil ity, acidic solutions assist in dissolution of the deposit. Our experi ments thus indicate that in the pure system H g - C H - H 0 there is no 2

2

2

6

6

2



210

hysteresis of contact angles. Hysteresis can be observed only when a new phase is formed on the mercury surface. As long as we have a clean mercury surface and thus free mobility of the surface atoms, we can expect instantaneous readjustment to equilibrium conditions upon lateral movement of the three-phase interline and no hysteresis.


Acknowledgment The authors express their gratitude to the National Science Foundation and its officers for their support of this research. Literature Cited (1) Bartell, F. E., J. Phys. Chem. 56, 453 (1952). (2) Ibid., p. 532. (3) Gatty, O., Spooner, E., "Electrode Potential Behavior of Corroding Metals in Aqueous Solutions," p.111,Clarendon Press, New York, 1938. (4) Rayleigh, Lord, Phil. Mag. 30, 397 (1890). (5) Sulman, H. L., Trans. Inst. Mining Met. (London) 29, 44-138 (1920). Received March 26, 1963


Hysteresis of Contact Angles in the System Mercury - Benzene - Water

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