A SIMPLIFIED TECHNIC FOR T H E DETERMINATION O F CONTACT ANGLES AND ITS APPLICATION TO STUDIES ON WETTIXG’ W. W. BENTOX Division of Agricultural Biochemistry, University of Minnesota, Minneapolis, Xinne s o t a E R I C Kh’EES
AND
Received September 1 , 1937
During the progress of an investigation on the properties of tooth surfaces it was found desirable to obtain information regarding the relative wetting characteristics of these surfaces. The method used was a relatively crude “horizontal-plate” technic for determining the angle of contact in the system solid-liquid-air, At the start it was felt that only approximate values would be obtained, but subsequent studies indicated that surprising accuracy could be secured. The data reported indicate the accuracy to be expected and illustrate the adaptability of such a method to allied problems. The horizontal-plate method, as discussed and compared with other common technics by Bartell and Hatch (3), appears to have definite limitations. Its use is limited to the investigation of plane surfaces which in many instances may be difficult to obtain. Much difficulty is introduced where dual contact angles occur. For instance, the observed contact angle of water on galena varied from an “advancing” angle of 90” to a “receding” angle of 0”. The apparatus used for measuring the angle is rather elaborate and may not be readily accessible to many laboratories. Emphasis has been placed on the advisability of the determination being made in an atmosphere saturated with the liquid under observation. The angle formed has usually been calculated from the measured height and radius of the drop or from the measured radius and volume (8). Photographing the drop in contact with a solid and projection or enlargement of the image has made possible the direct measurement of angles by means of a protractor (14). Our apparatus for direct measurement of contact angle consists of an arc lamp, condensing lens system, adjustable stage, and a small picture frame, and is shown schen~aticallyin figure 1. A thin sheet of paper is fastened to the picture frame by means of thumb tacks, so that it lies flat 1
Paper S o . 1531, Journal Series, Minnesota Agricultural Experiment Station. 1195
1196
ERIC KK’EEN AND W. W. BENTON
against the glass. The measurements are carried out in a darkened closed room. The beam of light from the arc lamp is passed through the condensing lens across the plane surface to be measured. The plane surface is placed beyond the focus of the lens and a drop of liquid to be tested is placed on the surface in the path of the beam. The image of the drop is projected through the glass and paper where it may be outlined in pencil by hand. The paper may then be removed and the contact angle measured by means of a tangent meter. Depending on the size of the drop and its position between the focus of the lens and the picture frame, the magnification may vary from about 20 to 30 diameters. The condensing
TOP VIEW
- CONDENSING LE OF LIQUID --- DROP BLOCK OF PARAFFIN LIGHT BEAM
- SHADOW OF DROP W - PlCTURE FRAME G - GLASS
U FIG.1. The apparatus
lens system and carbon arc lamp from a Zsigmondy ultramicroscope system are admirably adapted to the method. For some work a microscopic stage adjustable in the vertical axis is adequate. Where small objects with only one plane surface are to be used their base may be embedded in a paraffin block mounted on a leveling holder adjustable in the horizontal plane. As recommended by Mack ( 8 ) , small drops delivered by the capillary point of a small pipet are used in order to insure the observed angle being the advancing angle rather than something in between this and the receding angle. It was found that drops varying in size up to 0.3 em. in diameter gave satisfactory results. Careful control of humidity or temperature is not essential. The drawing can be completed in less than thirty seconds after the drop has come in contact with the surface.
1197
DETERMINATION O F COSTACT AXGLES
Under such conditions evaporation can be neglected in the case of water and aqueous solutions, in fact, the contact angle shows no appreciable change over a period of two to three minutes. EXPERIJIESTAL
In order to test thoroughly the accuracy of the method a large number of determinations were made with the system paraffin-m-ater-air. Table 1 illustrates the results obtained. Two kinds of paraffin blocks were used; those termed “melted” were formed by melting paraffin (ordinary Parowax), allowing it to settle, and TABLE 1 Contact angles o j water on paru.87~ NVMBER E X I T . NO.
TREATMENT
COPTACT A N G L E
OF
REPLIC,TER
Highest Lonest 1 Mean value value value _ _ _ _ _ _ _ _ _ _ ~ degrees
degrees
degrees
108.0 108.0 108.0 109.0 106.0 105.5 106.5 108,O 109.5 112.0 115.0 112.0 115.0 113.0
103.5 103.5 102.0 103.5 100.0 104.5 100.5 105.0 105.0 107.0 107.0 107.0 108.0 106.0
105.5 105.0 105.6 106.3 104.3 105,.0 103.3 106.1 106,4 110.4 110.4 110.3 110.9 110.3
~
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Block.. . . . . . . . . . . . . . . . . . . . Block.. . . . . . . . . . . . . . . . . . . . Block.. . . . . . . . . . . . . . . . . . . . Block.. . . . . . . . . . . . . . . . . . . . Block . . . . . . . . . . . . . . . . . . . . . Block.. . . . . . . . . . . . . . . . . . . . Block., , , . , , , . , , , , , , , . . Block . . . . . . . . . . . . . . . . . . . . . Block., . . . . . . . . . . . . . . . . . . Slide. . . . . . . . . . . . . . . . . . . . . . Slide . . , . . , . , . .. S l i d e . .. . . . . . . . . . . . . . . . . . . . Slide. ........... Slide . . . . . . . . . . . . . . . . . . . ,
,
,
, , , ,
, , , ,
, ,
Melted “Katural” Melted Melted ‘Tatural” AIelted Melted “Satural” Melted
9 12 16 14 14 2 14
4 5 m 4
8 9 7 8
then pouring it into paper molds of approximately 1 cm.3in olunie After solidification one surface n-as shaved smooth with a microtome. The blocks termed “natural” were simply cut out of a paraffin block by means of a razor blade, and one surface was smoothed with a microtome. While the individual determinations on each block show variation the means agree closely, and the mean of 105.1” for all ninety determinations on nine blocks agrees with the value of approximately 105”, reported in the literature (2, 17, 1, 4). I t is somewhat divergent from the ralues of 108.0” and 109.8” reported by Mack and Lee (9), and 108” by Kietz (12). Another series was run using microscope slides which were dipped into melted paraffin and allowed to drain and solidify in air. The mean ~ a l u eof thirty-nine determinations of xvater on this surface was 110.4’. This
1198
ERIC KXEEX A S D W. W. BENTOS
compares favorabIy with the recent values of WenzeI (18) of 109.5' to 113.2' for paraffin on glass or metal slides. At a later date and with a different sample of paraffin we found an angle of 115' for paraffin on a glass slide. Table 2 compares our values with those recorded in the literature for several substances in contact with water. It is apparent that the proposed method gives results which compare very favorably with reported values. It is interesting to note that Bartell and Hatch (3) report that it is very difficult to obtain the advancing angle of 90" for water on galena v h e n using the horizontal-plate technic. We found it almost impossible t o get any value but 90" eren when using somewhat faulted surfaces or relatively large drops.
COXTACT ANQLE SUBSTASCE
~
TRE .4TMENT
Literature
Found pI-
~
I
~
.I
Galena.. . . . . . . . . . . . . . . . . . . Fresh cleavage surface Benzoic a c i d . , . . . . . . . . . . . . . "Chip" Stearic acid . . . . . . . . . . . . . , . . ' Son-polar ~
~
Palmitic acid
LITERA'PCRE REFERENCE
Son-polar
I ~
degrees
~
90.0 61.5 105.0
degrees
90 65 106
(3) (12) (12)
106.0
Evaluation of the u-ettiny capacity of soaps For thesc determinations the contact angles between paraffin and the sodium salts of a series of fatty acids were measured. The solutions employed were 0.01 molar throughout. The paraffin surface used was obtained by immersing a glass slide in hot paraffin and then allowing it to drain and solidify. The water-paraffin contact angle mas 115'. Figure 2 shows the results obtained. d progressive slight decrease in contact angle is evident with sodium butyrate, sodium caproate, and sodium caprylate. With increasing length of the carbon chain following the eight-carbon atom compound, sodium caprylate, there is a progressive rapid increase in wetting. The fact that this increase approaches a straight-line relationship becomes increasingly significant when it is seen that by extrapolation theoretical coniplete wetting of paraffin might be postulated Tyith a 0.01 molar solution of sodium palmitate. It is unfortunate that the insolubility and tendency to gel of both 0.01 molar sodium palmitate and 0.01 molar stearate prerent their uqe by this simplified technic. The first part of the curw corresponds roughly to the observed
DETERMINATION OF CONTACT ANGLES
1199
effect of adding the various sodium salts on lowering the surface tension of water ( 7 ) . I t has been emphasized (13) that in order for a surface to be wet by a liquid, reduction of the “solid-liquid” interfacial tension is of primary consideration in order to arrive at conditions such that the “solidair” surface tension shall be greater than the sum of the “liquid-air” surlace tension and “solid-liquid” interfacial tension. Data of Donnan and Potts ( 5 ) illustrate that the presence of the sodium salt of myristic acid in the aqueous phase brings about a pronounced reduction in the interfacial tension “paraffin-oil-water” over that induced by sodium laurate. Figure 2 gives a clear picture of the wetting characteristics of soap solutions. 12 0 110 100
90
80
Ln
g
70
0
W 60
P
I !J 50
-I
0
Q
5 5Q
40 30 20
0
NUMBER OF CARBON ATOMS
FIG,2. Contact angles between paraffin and the sodium salts of a series of fatty acids
Evaluation of the degree of polarity of surfaces Theoretically the angle of contact between water and a solid surface should give a quantitative measurement of the polarity of that surface. Adam and Jessop (2) attempted to produce polar surfaces on solids n-hose surface is normally not polar in nature, such as paraffin and palmitic and stearic acids, the degree of polarity being measured by the “water-solid” contact angle. Their efforts were not entirely successful. If a polar surface could be produced its presence should be easily demonstrated by means of the contact angle. Stearic and palmitic acids were chosen as substances whose surfaces might be transformed to the polar condition. THE JOURNAL OF PHYSICAL CHEMSTRY, YOL.
41,
NO.
9
1200
ERIC KNEEN AXD TV. W. BESTOX
Polar surfaces were obtained by the follom-ingprocedure: Glass slides were very carefully cleaned and dried without contamination. The fatty acid was carefully melted on the glass surface and by tilting allowed to run down to one end. It was then permitted to solidify in this position, giving a lump of material. By loosening one edge with a razor blade the mass may be lifted from the glass, the under side being a smooth plane surface well adapted to the horizontal-plate method. Table 3 illustrates the results obtained. The value for highly polar benzoic acid is inserted for comparison. It is obyious that the surfaces of stearic and palmitic acids which mere originally in contact with the glass are both highly polar, the higher value for palmitic acid being probably attributable to traces of impurities present in it. The “non-polar” surfaces are those of the “air-acid” when the melted acid solidifies on a glass slide. While the drop of water did not appreciably wet the polar stearic acid surface on contact it started to spread immediately, the equilibrium angle being reached in about fifteen seconds. TABLE 3 Contact angles o j polar and non-polar surfaces against water SUBBTAXCE
’
SURFACE
1
CONTACT ANGLE
degrees
Stearic a c i d . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stearic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Palmitic a c i d . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Palmitic acid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benzoic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,
Son-polar Polar Kon-polar
II
Polar
j
~
i
105.0 64.0 105.0 71.3 61.5
Wetting properties of cholesterol towards water During investigations on the electrophoresis of sterols, llloyer (11) concluded that some of the phenomena observed must be due to changes in wetting of the crystals. T e accordingly investigated this phenonienon by contact-angle technic. Two types of cholesterol surfaces were used, one being formed in contact with glass in order to form a “polar” surface. h different surface was formed by allowing melted cholesterol to solidify in the hollow of a “hanging drop” slide. In the latter case a plane surface was secured by shaving the upper surface with a clean sharp razor blade. Readings were made on the “chip” (surface formed in contact with glass) immediately and at the end of a twelve-hour period. The mean angles were 76” and 75.8”, respectively. This surface shou-ed no tendency to wet more during the three minutes of contact permissible before evaporation distorted the angle. In the case of the shaved surface an entirely different behavior was observed. The drop spreads slowly over the surface and may or may not reach equilibrium before the end of
1201
D E T E R V I N S T I O S O F CONTACT A S G L E S
three minutes. The mean value of a number of determinations made at the end of a two-minute interval from time of contact was 60". Since cholesterol appears to wet more on standing in contact with water, the same shaved surface of cholesterol was immersed in water for t w n t y four hours, dried in the air, and re-read at various time intervals. Table 4 shows the results obtained. The whole surface of this cholesterol mass appears to be remarkably uniform in its wetting as measured immediately on contact. However, the speed of wetting varies rather pronouncedly with different surface areas. Of most significance is the rapidity with which the contact angle decreases. n'hile it has been observed that certain surfaces tend to become more wetted on standing in contact with water (lj),this phenomenon has been considered to be the result of rather TABLE 4 The increase in wetting (change in contact angle) of a shaced cholesterol surface while in contact with water PLACE O N THE SCRFACE
I i
82.0 81.0
1
16.0
!
38.0
1
76.0
long periods of contact. Obviously with surfaces such as cholesterol time intervals of minutes rather than hours or days are significant. The wetting capacity of leaf surfaces
It has been recognized by various workers (16, 10, 6) that the ability of insecticidal and fungicidal solutions to wet leaf surfaces is of prime importance in considering the effectiveness of sprays. Xttempts to apply the contact-angle technic have not been conipletely satisfactory. With the cooperation of Rlr. J. H. Pepper of the Montana Experiment Station a method was developed which has given satisfactory results. Briefly, the technic consists of mounting small sections of leaves on the plane surfaces of small paraffin blocks. The paraffin surface is lightly smeared with a nitrocellulose adhesive and the freshly cut leaf segment placed on this and gently pressed down so as to give a rigid, non-curling surface. Contact angles with the desired solution are run immediately. Since the entire procedure may be performed very rapidly it is believed that such
1202
ERIC KNEEN AND W. W. BENTOK
factors as leaf drying or reaction with the cement may be neglected. By this means the behavior of one kind of leaf toward various solutions or the behavior of various kinds of leaves toward the same solution may be easily and quantitatively determined. It is obvious that this technic is limited to relatively smooth surfaces. Cabbage and ivy leaves were used successfully in an investigation of their wettability by water and by certain arsenicals. T h e wetting properties of tooth surfaces
For this study extracted human teeth were used. A plane surface was obtained by grinding with paper disks and a dental drill. Just sufficient grinding was done to obtain the smallest usable plane surface, care being taken that a surface of enamel was retained without exposing the dentine. The surfaces were cleaned thoroughly before use with soap and mater and repeated rinsings. A4fterdrying in air they were inserted into the holder TABLE 5 Angles given by various solutions in contact with a tooth surface
1
SOLUTION
I
CONCEKTRATION
I
Water, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium caprylate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium oleate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sodium lauryl sulfate. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I
0.01 molar 0.01 molar 0 . 0 1 molar
1
CONTACT ANQLE
1
degrees
~
,
81 .o 74.0
53.0 30.0
in such a manner that the plane surface was horizontal, and contact angle measurements made according to the usual procedure. Table 5 shows typical results with this type of surface. It would appear that the measurement of contact angles is well adapted to investigations dealing with the ability of solutions to wet tooth surfaces. SCMMARY
1. A simple, adequate horizontal-plate technic for determining contact angles in the system solid-liquid-air is described and its accuracy demonstrated. 2. Applications of this method are illustrated and discussed with reference to ( a ) the evaluation of the wetting capacity of soaps, ( b ) the evaluation of the degree of polarity of surfaces, ( c ) the wetting properties of cholesterol toward water, ( d ) the wetting capacity of leaf surfaces, and ( e ) the wetting properties of tooth surfaces.
DETERMINATION O F CONTACT AKGLES
1203
REFERESCES (1) ABLETT, R . : Phil. l\lag. 46, 214 (1923). (2) ADAM,K.K., ASD JESSOP, G.: J. Chem. Soc. 127, 1863 (1925). (3) BBRTELL, F. E., AND HATCH,G. B.: J. Phys. Chem. 39, 11 (1936). (4) COGHILL,W.H., ~ S DASDERSOS, C. 0 . : J. Phys. Chem. 22, 249 (1918). (5) DONNAS, F. G., ASD POTTS, H. E . : Kolloid-Z. 7, 208 (1910). (6) GREES, E. L.: J. Phys. Chem. 33, 921 (1929). (7) LOTTER?JOSER, A., 4 N D TESCH,W.: Kolloid-Beihefte 34, 339 (1931). (8) h I h C K , G. L . : J. Phys. Chem. 40, 159 (1936). (9) RIACK,G. L., AXD LEE, D. A , : J. Phys. Chem. 40, 169 (1936). (10) MOORE,W.: Technical Bulletin S o . 2 , Minnesota Agricultural Experiment Station (1921). (11) MOYER,L. S.: J. Gen. Physiol. 19, 87 (1935). (12) KIETZ,A . H : J. Phys. Chem. 32, 265 (1928). (13) KCTTALL,IT’. H.: J. Soc. Chem. I n d . 39, 67T (1920). (14) O’KAXE,IT. C. E r A L . : Bulletin 39, S e L Y Hampshire Agricultural Experiment Station (1930). (15) POCKELS, TON4.:Kolloid-Z. 62, 1 (1933). (16) STELLWAAG, F.: Z. angew. Entomol. 10, 163 (1924). s.11.:z . physik. Chem. 148, 227 (1930). (17) T A L M U D , D., A N D LUBMAN, (18) WESZEL, R. S.: I n d . Eng. Chem. 28, 988 (1936).
