INDUSTRIAL AND ENGINEERING CHE.\fISTRY
988
T ~ B L111. E SPECIFIC HE.4TS
.4T CONSTLVT VOLUME O F AIIXTURES O F M E T H A S E A S D CRUDE O I L
Mass Per Cent Methane as Fol1oaa:1 414 2 367 4 323 5 106
7 -
're nip.
0
7
6.06%
F. 0.465 0 469 0.491 0.495 0.476 0.478 0.485 0.510 0 515 0.498 0.492 0 500 0.530 0.535 0 519 0.505 0.516 0.549 0,555 0.540 0.518 0.531 0.569 0.575 0.562 0.532 0.547 0.589 0.595 0.583 a Specific heats reported for a specific volume of 0.035 cu. f t . per lb. 70 100 130 160 190 '70
__
0 458a 0.472 0.485 0.499 0 512 0 526
marked, indicating an approach to the critical composition a t these temperatures. The change in specific gravity with equilibrium pressure for a temperature of 160" F. is depicted in Figure 6; the curve for pure methane, near the bottom, was obtained by interpolation of published compressibility data (1). Figure 7 shows the variation in specific volume TT-ith composition for the two-phase portion of the system studied. The points shown are not directly determined experimentally but were read from smooth isothermal curves drawn through the experimental points on the pressure-volume plane. The agreement of the data with a linear relation b e h e e n specific volume and composition is considered to be within the absolute accuracy of the measurements. Figure 8 is a similar diagram for the region near bubble point on a larger specific volume scale. The curvature of the isobars in the condensed region is in contrast to the straight lines in the two-phase region. The partial specific volumes, VP,under constant pressure conditions, of methane in these mixtures at 160" F. is shown in Figure 9. The partial specific volume of the dissolved methane increases somewhat as the concentration of methane in the solution is increased. The fact that the par-
VOL. 28, NO. 8
tial volume of the methane in the two-phase region is practically a constant and equal to the specific volume of methane at the same temperature and pressure indicates that there is little t'ransfer of heavy components into the gas phase a t the pressures and compositions investigated a t this temperature. S o plots of the thermodynamic properties were made since they are similar in appearance to diagrams for other mixtures published earlier (5, 6).
Acknowledgment Financial assistance for t'his work was given by the American Petroleum Institute. The Union Oil Company of California furnished the analysis of the crude oil with the exception of the molecular weight determinations. J. E. Sherborne and W.R. Mendenhall carried out several of the laboratory measurements.
Literature Cited
(3) (4)
(7) (8)
(9)
Kralnes, H. M., and Gaddy, 1 ' . L., J . .Im. Chem. SOC.,53, 395 (1931). Sage, B. H., Backus, H. S.,and Lacey, W. N., IND.ENG.CHEX., 27, 686 (1935). Sage, B. H., and Lacey, W.N., Ibid.,26, 103 (1934). Ibid.,27, 1484 (1935). Ibid., 28, 249 (1936). Sage, B. H.. and Laces, V.N., Am. Petroleum Inst.. Production Bull. 216 (1935); Oil Weekly, 80, S o . 11, 31 (1936). Sage, B. H., Lacey, W. S . , and Schaafsma, J. G., IND.ENG. C H E U . , 26, 874 (1934). Sage, B. H., Mendenhall, W.R., and Lacey, W. N., Am. Petroleum Inst., Production BuZl. 216 (1935); Oil Weekly, 80, No. 13, 30 (1936). Sage, B. H., Schaafsma, J. G., and Lacey, W. N., IND. ESG. C H E U . , 26, 1218 (1934).
RECEIVED .Ipril 13, 1936
RESISTANCE OF SOLID SURFACES TO WETTING BY WATER S THE vaterproofing of
light-weight oven or knitted fabrics, it is generally essential to preserve the air porosity of the material. The waterproofness that can be effected is therefore definitely limited by the size of the openings, because water will readily pass through if the pressure behind it is sufficient to break the surface film across the openings. FTater will penetrate, however, at a much lower pressure or even against pressure, if it can spread over the surface of the threads from one face of the cloth to the other. The waterproofing of open fabrics, therefore, presents the problem of preventing this spreading of water over the thread surfaces. The desired effect is attained by depositing on the fabric some chemical substance that has of itself this ability to resist wetting. For practical reasons, preparations intended for use in waterproofing open fabrics commonly consist of emulsions. In these preparations the active water-repellent agent is combined with other ingredients whose presence is required to ensure the desired fluidity and stability in the emulsion, to provide proper pH control, to increase the permanence of the proofing effect, and to modify the appearance and feel imparted to the finished fabric. These auxiliary constituents may impair, or they may enhance, the effectiveness of the proofing treatments. The complexity of the problem thus presented makes it desirable to study carefully the rretting characteristics of materials selected for this use. M
ROBERT N. WENZEL Mellon Institute of Industrial Research, Pittsburgh, Pa. Alaiiy substances, some n idely different chemically, are knowi and have been used as water-repelling agents. It is obvious that they must possess this property in varying degree. Yet no attempt to compare water-proofing agents on the basis of a quantitative evaluation of this essential characteristic appears t o have been macle. Moreover, few of the experimental methods that have heretofore been applied in the investigation of wetting problems are at all adaptable to the study of the wetting of different solid substances by the same liquid. The explanation probably lies in the fact that, in most cases where wetting problems are of industrial importance, the solid itself is not subject to modification or control. Attention is therefore necessarily confined to the liquid phase, its wetting power being altered either by the use of liquids of different polarity or by the introduction of surface-active solutes. A survey of possible experimental procedures led finally to the direct measurement of contact angles by the tilting plate method as the most satisfactory for comparing a wide variety of solid materials. When a suitable apparatus and proper technic had been developed, this method was found to be rapid and precise, and to afford results reproducible
AUGUST, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
989
for the dry area around it. Either value may be the greater, Because of their recognized practical but suppose that in this case the importance, wetting phenomena have wetted area has the lower spebeen receiving more and more attention cific energy and that therefore the drop tends to spread sponin textile technology. In this paper the taneously. Assume that it has author discusses critically the application not reached equilibrium, that its of wetting theory to the study of watershape is, say, approximately repelling agents. I t is shown that, conhemispherical. Spreading will trary to the usual assumption, the adhethen take place, and, as it does, both the wetted area under the sion tension, as defined in terms of chardrop and the free liquid surface acteristic interfacial tension values, is not over it will be thereby ina direct measure of wettability, but that creased. The former involves a the latter depends also definitely on the release of energy, the latter a physical condition of the surface wetted. consumption of energy. The difference is the net energy deThis fact must be recognized in all probcrease which determines the lems involving the wetting of solid surspeed of wetting under the imfaces. I t has an important bearing on posed conditions and is thus a the effectiveness of water-repelling mameasure of the wetting characterials deposited on the fibrous surfaces teristics of the solid. Sow it should be apparent that, for of textile fabrics. identically the same increase in A convenient and dependable experithe free liquid area at the upper mental procedure for measuring the wetsurface of the drop, a greater ting of different solids by the same liquid amount of actual surface is is described, and measurements on a comwetted under it if the surface of the solid is rough than if it prehensive list of water-repelling subis smooth. Consequently, for stances are presented. It is shown how the process involving the rough this method can be used to evaluate the surface, there is a greater net surface effect of aqueous waterproofing energy decrease to induce spreademulsions as well as nonaqueous watering, and the rough surface is wetted the more rapidly. proofing solutions. The same reasoning applies for a water-repellent surface, in which case the dry interface fundamental principles of has the lower specific energy. The drop will then spontaneously assume a more spherical form. Again, for an identical change in the shape of the drop, and therefore in the Theory of Wetting Action at Solid Surfaces area and total energy content of the free liquid surface, there The physico-chemical principles underlying wetting action will be more actual surface involved in the change a t the are to be found in standard texts (2, 12, 20) and have been solid-liquid interface if the surface is rough than if it is recently reviewed in papers dealing with scientific and techsmooth. The net energy decrease will thus be greater for nical problems in many different fields (4-7, 11, 13, 14, the rougher surface. Here the process considered is the 16-19, 21, 22). reverse of wetting, and the rough solid is therefore the more Whenever a process involves the wetting of a solid by a strongly water-repellent. liquid, three different interfacial boundary surfaces-in the In either case the effect of a roughened surface is to magnify present instance, solid-liquid, solid-air, and liquid-air-are the wetting properties of the solid. A solid substance with a involved. Wetting replaces an area of the solid-air interface positive wetting tendency will wet the more readily, the by an equal area of solid-liquid interface and is generally also rougher its surface. If the smooth surface is water-repelling, accompanied by an extension of the liquid-air interface. the roughened surface will be more strongly so. These surface relations vary with the conditions of the Because forces are more easily visualized than energy problem and may change progressively as wetting proceeds. values, it is conventional, in the analysis of wetting problems, As each interface has its own specific surface energy content, to defme force concepts (the interfacial surface tensions) as wetting, with its accompanying change in the extent of each numerically equal to the characteristic interfacial specific interface, results in a net decrease or increase in total surface energy values, and to deal with these forces as vector quantienergy. Wetting is thus a thermodynamic process, and the ties constant in magnitude and variable in direction. I t is magnitude of the free energy change involved determines in this translation from energies to forces that the importance whether or not wetting will proceed spontaneously, a t what of the physical condition of solid surfaces is likely t o be obrate and how far it can progress against the external forces scured. For when energy values, in dyne centimeters per that may be brought into play to resist it, or, alternately, how square centimeter, are replaced by forces expressed in dynes large an external force may be needed t o overcome the per centimeter length, it becomes important to bear in mind initial resistance to wetting. the fact that the ends of this centimeter length are actually a Consider a drop of water resting on a horizontal solid surcentimeter apart in space only if the surface involved is perface. The specific energy content of the solid interface will, fectly smooth. in general, be different for the wetted area under the drop than We must, then, recognize a distinction between the total
with a considerable degree of accuracy anywhere within the entire range of practically significant values. Measurements then soon revealed the fact that, for certain materials, the method of producing the surface-that is, its physical condition-had a much more pronounced effect on its water repellency than was to be explained on the basis of any conception of wetting resistance as a fixed property of chemical substances, comparable, for example, to the surface tension values in terms of which it is usually expressed. It is suggested that the observed effect of the physical condition of the surface can be simply and adequately explained without denying the specific character of interfacial tensions or rbsorting to the assumption of surface contamination. It',s only necessary to apply the fact that, within a measured unit area on a rough surface, there is actually more surface, and in that sense therefore a greater intensity of surface energy, than in the same measured unit area on a smooth surface. That this simple fact, while it can have no effect upon the specific surface energy values, does necessarily involve a proportional change in the wetting characteristics of the solid, will appear from the following brief consideration of the wetting action.
INDUSTRIAL AND ENGINEERING CHEMISTRY
990
or “actual surface” of a n interface and what might be called its superficial or “geometric surface”; the latter is the surface as measured in the plane of the interface. Where perfect smoothness is an acceptable assumption, as a t liquid-liquid or liquid-gas interfaces, actual surface and geometric surface are identical, but at the surface of any real solid the actual surface will be greater than the geometric surface because of surface roughness. This surface ratio will be here termed the “roughness factor” and designated by r: r = roughness factor
=
actual surface geometric surface
By definition, surface tensions, like specific energy values, are related to one unit of actual surface. But when water spreads over the surface of a real solid, the forces that oppose each other along a given length of the advancing periphery of the wetted area are proportional in magnitude, not to the surface tensions of the respective interfaces but to their total energies per unit of geometric surface. This must be true if surface tensions themselves are characteristic properties, unaltered by surface roughness. For if a solid, M , of surface tension x and water-solid interfacial tension y presents a surface so rough that its actual surface per unit geometric surface is doubled, then its energy content per unit geometric surface must also be doubled. T h a t surface can then be no different in wetting characteristics from the smooth surface of a solid, N , of surface tension 2x and water-solid interfacial tension 2y. In the latter case the surface forces in vector relation with the surface tension of the liquid a t the periphery of the wetted area are equal to 22 and 2y; and so they must be also for the roughened surface of solid M . This point is illustrated in the diagrams. The upper lefthand sketch of Figure 1 presents a plan view of the solid surface. Line mn is a segment of the periphery of the wetted area advancing from left to right. The solid surface is here considered to be ideally smooth, and consequently the energy cohtent of a measured unit area, abcd, will be exactly the specific energy of the interface. Its values before and after wetting are represented by force vectors SIand S12 which are considered to act on unit length ab of the periphery. Their vector sum, S1 - Slz, defines the adhesion tension, A , its negative value indicating its direction and marking it as a resistance to wetting. The upper right-hand sketch of Figure 1 is the customary diagram of the equilibrium relation between surface tension Sz of the liquid, the contact angle 8, and adhesion tension A , as given in the equation ;
‘rn
s,
cose
FIGURE 1 . VECTOR RELATIONS OF SURFACE FORCES (Aboue) Solid surface smooth.
(Below) Solid surface rough.
A
VOL. 28, NO. 8 SI COS 8
I n this presentation the three forces, 81,S1s, and Sz,must all be considered to act upon the same length of the intersection of the boundary surfaces-that ip, between a and 6. The lower diagrams of Figure 1 represent the way in which the situation changes if the surface is roughened. The specific interfacial energy a t the solid surface is concentrated within a smaller measured area, efgh. The solid-liquid and solid-air interfacial forces effective along the periphery from a to b are therefore magnified by the same factor, r, by which the surface contained within area abcd has been increased. But the liquid-air interfacial force effective between a and b remains, as before, the surface tension Sz. It must, therefore, act a t a greater angle (as shown) if it is to balance the increased resistance which the solid surface now opposes to the wetting process. By an exactly similar analysis, if the solid has a positive wetting tendency and S I is therefore greater than S12,the equilibrium contact angle, 8, would be less than go”, as would also the angle 8’; but since 8’ has the greater cosine, it would in this case be the smaller angle.
Effective Adhesion Tension Apparently, then, the second equation of the familiar expression, A = S1 - S I 2 = 8, COS e can apply only where all surfaces are perfectly smooth. For any real solid the cosine of the equilibrium contact angle, multiplied by the surface tension of the liquid, measures not the adhesion tension A , but the product rA. We should therefore write: r A = r(S1 - S12)= S,cos e This expression is of general validity because for smooth surfaces, r is equal to 1. The product rA may be termed the “effective adhesion tension” ; it is the value always measured experimentally. Unlike the adhesion tension, it does not have a fixed reproducible value dependent solely upon the chemical composition of the three contiguous phases. From factor r it derives a disposition to vary widely and to be extremely difficult to control. Bartell and Smith (8), discussing the activation of carbons by heat treatment, point out that the change in surface energy and adhesion tension may result from (1) a difference of crystal structure, (2) a difference in the amount of adsorbed impurities, and (3) the formation of a highly roughened or pitted surface. I n the revised formulation here proposed, a change in crystal structure is probably to be considered a change in A itself, a new form of matter having appeared. At the same time, it is to be recognized that, for a single crystal, the surface energy will be different on different faces (16, $0). Adsorbed impurities must reduce the total exposed surface and effect a corresponding change in surface energy, modified by the energy contributed by the exposed surface of the impurity, Regarding the effect of a roughened surface Bartell and Smith say: “It is conceivable that the interfacial tension of a highly pitted or highly roughened surface would be slightly different from that of a plane surface of the same solid.” More recently, however, Bartell and Hatch (7) preface a report on the wetting characteristics of galena with this statement: “Recent work has shown that the free surface energy of a solid, hence its wetting characteristics, may be altered much more easily than had previously been supposed.” The experience accumulated in the course of the present research, extending over the past two years, amply supports the opinion of these investigators that the exact surface condition of the solid is a highly important factor in determining wetting properties.
AUGUST, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
991
Data recently published by Lee (17) on adhesion in relation to bituminous road materials are worthy of note in this connection. Lee placed drops of road tars on highly polished glass and stone surfaces under water and calculated interfacial tensions and contact angles from the dimensions of the drops after long immersion. H e found that differences due to various degrees of roughness of the solid surface were much greater than any differences due to the nature of the materials. Lee's data on the effect of the degree of polish of the solid surface are reproduced in Table I. From his values of cos cy, which corresponds to -cos 8 as conventionally represented, the values of 8 and of r given in the table were calculated, r being taken as 1.0 for the highly polished surface. Values of r for the roughened surfaces vary from 1.22. for a fair polish, to as high as 2.24. Concerning these measurements Lee says: These results show that the binder has much more difficulty in wetting a rough stone surface which is already in contact with water than in wetting a smooth surface. In addition to the frictional effect of the rough surface, water is presumably entrapped in the minute crevices of a rough surface and prevents the tar from wetting the solid. A striking example of this FOR COXTACT ANGLE FIGURE2. TILTISGPLATEAPPARATUS was found in the case of a limestone. A drop of highMEASUREMENTS aromatic tar was introduced under water onto a flat surface of this material having a ground finish. After remaining in position for a day, no contact had taken place beangle to 147" and greatly improve the effectiveness of this tween the limestone and the tar, and, on tilting the stone, the wax as a water-repelling agent. drop traveled across the surface of the stone without leaving a Factor r further accounts for the fact that many substances stain. Highly polished limestone, however, was wett,ed as easily show excellent water-repelling properties on textile fibers as granite. when they are but slightly effective on smooth surfaces. It The value of cos a that Lee gives for this high-aromatic therefore widens the field of choice for textile proofing comtar on highly polished limestone under water is 0.835. It pounds It means also that the success of a spotproofing requires a roughness factor of only 1.2 to increase this value treatment is determined by the physical characteristics of the to unity. The interfacial contact angle would thus reach total deposit, as actually laid down on the fabric, quite as 180". For any roughness factor beyond this, the effective much as by its chemical composition. adhesion tension would have a negative value, a resistance Experimental Procedure to wetting, greater than the whole of the tar-water interfacial tension. Stated otherwise, in order to effect the smallest The tilting plate method of measuring contact angles was conceivable area of contact between the ball of tar and the apparently first described by Huntington (15) who used it solid, more energy mould be required to replace the limestoneto measure contact angles against cleavage surfaces of blende mater interface by limestone-tar interface than could be and galena in studying the flotation of sulfide minerals.' In recovered from the tar-water interface destroyed. Unless different forms it has since been employed by a number of sufficient additional energy could be derived from the investigators (3, 9, 11, 13, 11,22). weight of the drop or were otherwise applied, wetting could The principle of the method may be stated briefly. If a not occur. Factor r thus offers a simple explanation of the flat plate is dipped into a liquid, the surface of the liquid as beharior observed by Lee. The cushioning of the drop by it approaches the plate will curve upward or downward to a film of water then appears as a consequence of the increased establish the angle of contact required by the several interresistance to wetting, rather than as its cause. facial surface tensions. If the liquid is water and the plate is coated with a water-repellent substance, the-contact angle will be greater than 90" and, when the TABLEI. EFFECT OF DEGREE OF POLISH OF SOLID SURFACE ON plate is vertical, the water surface will curve downWETTING PROPERTIES OF TARIN THE PRESENCE OF WATER (27) ward on either side of the plate. As the plate is Cos a or tilted, the curvatures will change to maintain the Nature of Solid Degree of Polish Tar -coss I 0 D~~~~~~ contact angle, the curvature of the water surface on Plate glass High polish Low-aromatic 0.289 1.00 106.8 the under side of the plate becoming less and less Ground surface Low-aromatic 0.647 2.24 130.3 138,2 and finally disappearing altogether. A t this point, High polish Plate glass High-aromatic 0.617 1 .OO Ground surface High-aromatic 0.889 1.44 152.8 where the water surface is flat and horizontal right Soottish whinstone High polish Low-aromatic 0.335 1.00 iyi:: up to the line of contact with the plate, the angle to Fair polish Low-aromatic 0.409 1.22 Ground surface Low-aromatic 0.573 1.71 125.0 which the plate is tilted measures the contact angle. If the plate is tilted beyond this position, the water surface on the under side of the plate takes the The influence of a distinct crystal habit in a waxlike material opposite curvature, rising as it meets the plate. on the excellence of its waterproofing effect is exactly similar. 1 It is interesting to note that Huntington reported widely different conThus, if a wax of fair water-repellency exhibits a contact tact angles for the same mineral, depending on the direction of cleavage, a angle of 115" when the measurement is made on a perfectly result ascribable t o different intensities of surface energy on different planes smooth surface, a roughness factor of 2.0 would increase the of the crystal lattice. ~~~
~
-
992
INDUSTRIAL AND ENGINEERIXG-CHEMISTRY
VOL. 28, NO.8
The apparatus developed for the present use is shown in Figure 2:
of the water surface is an integrated effect apparent for a distance of 1 cm. or more away from the plate.
About half the length of a microscope slide, cut to 0.5-inch width, is coated with the solid to be tested. The liquid is held in a cell with ends and bottom of brass, the front and back sides consisting of microscope slides joined in place with balsam cement. The cell is supported on a mechanical stage that permits it to be moved horizontally or raised and lowered smoothly and steadily. A petrographic microscope is used in horizontal position, the cell being independently supported between the microscope tube and the revolving circular stage. The slide is clamped to the revolving stage in such position that the coated end dips into the water in the cell and its plane surfaces are parallel to the line of sight. The edge of the plate is thus presented to the observer. By revolving the microsco e stage, the plate is rotated about the line of sight as a center. fts angular position at any time can be read from the circular scale and vernier on the microscope stage. The plate holder is provided with a screw adjustment for centering the slide in the field.
For experimental work on finishes, this method has a distinct advantage over hydrostatic pressure tests on proofed pieces of standard fabric, because the numerical result obtained is not descriptive of only the few least resistant openings of the thousands included in the area under test. When using fibrous materials on the plate, however, the submerged area becomes wet through, more or less rapidly, and while several measurements can usually be made a t a single immersion, the readings cannot then be checked over the same area without drying. With the bonded metal-paper sheets, strips 10 to 15 centimeters long can be employed and tested a t either end; also, the wet portions can be cut away with metal shears to obtain further check readings. The method gives results accurate to within a few degrees. It can be used to measure angles up to and slightly beyond 170°, but it is difficult to take readings that close to the horizontal. Bearing in mind that the water repellency is proportional not to the contact angle but to its cosine, and further that 180' corresponds to the maximum useful value
FIQURE 3. CHANGEOF CURVATURES OF WATER SURFACES WITH ROTATION OF PLATE
TABLE11.
COXT.4CT
ANGLE MEASUREMENTS ON
PARAFFIN
WAX
It has been found satisfactory to use the microscope without either objective or eyepiece. Viewed in this manner, the appearance of the meniscus and the way it changes as the plate is rotated are shown in Figure 3. With proper illumination, a fine bright line appears near the bottom of the meniscus that is quite sensitive to changes in the position of the plate. By adjusting the forward edge of the plate to within 1 mm. of the cell wall, the water surface at that point is thrown upward and out of the way so as not to interfere with observation of the meniscus beyond. The plates may be coated by dipping into the melted sample at a suitably elevated temperature or by any other convenient means. In cases where the solid will not adhere to glass, or the coating loosens and peels off when submerged in water, brass strips of similar dimensions have been used. If necessary, several small holes can be drilled through the metal near the bottom edge and others well above the water level so that the coatings on the opposite faces of the plate will be tied together by the coating material itself and thus securely anchored to the plate. If sufficiently rigid, a plate molded of the material to be tested can be used. Tests on emulsions, to be applied from dilute baths, have been made possible by using half-inch strips cut from sheets fabricated by bonding heavy steel-plate paper of pure rag stock to both sides of a 28-gage steel sheet by means of a metallic adhesive (f0). These sheets can be dipped, run through squeeze rolls, and ironed dry exactly as a textile fabric would be; but being rigid, they can be used for the contact angle measurements. Finally, if desired, measurements can be made directly on strips of proofed textile fabric backed by a waterproof adhesive tspe and mounted on metal plates. Exact leveling of the apparatus is made unnecessary by taking two successive readings, the plate tilting in the one case to the left, in the other to the right. Half the difference then gives the angle from the vertical, and this value plus 90' gives the contact angle. Usually the operation is repeated a number of times for each determination. For concordant results, all settings must be made while the cell is very slowly raised. This procedure is essential, for there is apparently what corresponds to a frictional resistance to movement of the liquid-air interface across the surface of the solid. Thus Ablett (I),who used a horizontal cylinder coated with paraffin and dipping into water, found definite maximum and minimum contact angles, depending on the direction of rotation of the cylinder, and observed that these angles were equidistant from the value read with the cylinder stationary. Static readings, however, are most unreliable. The liquid may rest at any angle between two extremes (2). Moreover, in t,he study of waterproofing we are interested in the total force resisting wetting; and since this frictional force, if it may be called such, acts to assist the adhesion tension in opposing wetting, it is the maximum contact angle that becomes the important criterion. As the water level is raised, minute surface irregularities on the plate may cause the very tip of the water line to vibrate or advance with an irre ular motion. The individual threads of a mounted piece of fa%ric have the same effect. This action does not confuse the end point, however, because the main curvature
Date (1934)
Plate
-Reading-. Left Right
No. Temp. C.
June 1
1
June 2
1
June 4
..
202.3 204.3 203.6 203.6
19
155.7 156.5 156.4 155.5
201.4 202.7 201.4 201.6
1
30
154.5 154.3 154.6 153.7
199.9 201.8 201.3 199.9
June 4
2
30
159.5 159.5 159.6 158.0
197.7 199.5 197.6 198.4
June 4
3
30.5
157.3 159.0 156.3 157.7
198.9 197.0 197.4 198.0
111.
111.5 112.6 113.3 113.f 112.8 112.8 113.1 112.5 113.0 112.8 112.7 113.5 113.4 113.1 113.2 109.1 110.0 109.0 110.1 109.5 110.8 109.0 110.6 110.2 110.2
rA = 72 COS e Dynes/cm
a
C.
-Contact Angle--. Plate Plate 1 2 Average -Deorees-
31 29 29 30 28 27 29 26 27
108.7 110.1 113.3 106.8 123.7 124.8 110.2 111.9 102.7 140.0
25 28 26
112.1 103.1 107.2 119.1 101.5 120.4 108.0 131.4 124.4 129.9 107.7 123.6 108.0 107.9 112.6 118.4 120.4
..
.. .. .. ..
28 26 28 25 26 28 26 26 27 25
111.8 109.9 113.0 108.3 122.0 125.8 112.8 108.4 103.1 139 6
110.2 110.0 113.2 107.6
122.8 125.3 111.5 110.2 102.9 139.8
.
-27 0
-27. D
-28 4
-24.0
-24.9
ANGLEMEASUREMENTS ON WAXES WAXLIKEPRODUCTS
Para5n wax, amorphous Ozokerite Ozokerite: gdk!ed Bees wax Carnauba wax, crude Carnauba wax, refined Candelilla wax Spermaceti Japan wax Montan wax. crude waxes:
I
43.0 45.3 46.7 47.3 Average 45.7 46.2 45.0 46.1 Average 45.4 47.5 46.7 46.1 Average 38.2 40.0 38.0 40.4 Average 41.6 38.0 41.1 40.3 Average
+
CONTACT
Temp.
J K
0(0/2) 90
------Degrees------
159.3 159.0 156.9 156.3
TABLE
Difference, D
AYD
Effective Adhesion Tension Dunes/cm. " , -24.9 -24.6 -28.4 -21.8 -39.0 -41.6 -26.4 -24.9 -16.1 -55.0
AUGUST, 1936
INDUSTRIAL ,4ND ENGINEERING CHEMISTRY
993 i
of the effective adhesion tension for waterproofing purposes, the exact values of readings beyond 170' have very little significance.
SOAPS ON PAPERSURFACE, FROM CARBON TABLE VI. METALLIC TETRACHLORIDE SOLUTION Concn. of Soh. Grams/liter 4.8 7.5 2.4 4 0
Experimental Results Table I1 gives data obtained with ordinary crystalline paraffin wax and illustrates the degree of consistency in the contact angle readings. The plates were of glass or metal and were coated by dipping. Readings taken a t different times on the same plate are likely to be in better agreement than show that a readings on duplicate plates. The results considerable variation in room temperature had no great effect on the readings Table I11 presents the average readings on each of duplicate plates of other waxes and waxlike products. In a few cases initial transient readings were obtained, higher than the final constant values. Thus Japan Fax gave initial readings of 107' to 111' C., which dropped to relatively constant values a t 103" and to still lower values around 99.5" after soaking the slides for 15 minutes in water. Carnauba wax gave initial transient values around 130" and constant values a t 125' C. I n the case of the wax listed as commercial synthetic wax K, which gave initial transient readings a t 115' and final values a t 107' C., the change was due to solution of the sample or some constituent thereof in the water, inasmuch as the same plate again gave high initial readings on changing the water in the cell. Measurements recorded on gums and certain other materials are summarized in Table IV. As indicated. some of these materials were deposited from solvents.
Mg stearate Ca stearate Ba stearate Th stearate
-Contact Initial value -Degrees128.4 113.6 118.9 108.4
Angle--. .4fter ironing
164.3 128.3 150.2 147 4
P
TABLE \'IT.
M g stearate Sn stearate Ba stearate Th stearate Cd stearate Zn stearate A1 stearate
___
SoaPs ON PaPER SURFACE, ISOBUTYL ALCOHOLSOLUTION
bkALLIC
-Contact Plate 1 -Degrees145.9 151.6 152.5 158.8 162.0 162.9 169.4
AnglePlate 2
140.6 149.1 157.2 159.4 165.2 170.1 170 8
Av. 143.2 150.4 154.8 159.1 163.6 166.5 170.1
Effective Adhesion Tension Dynes/cm. -57.6 -62.6 -65.2 -67.2 -69.1 -70.0 -70,9
__
of the metallic soaps in chlorasol (a mixture of three parts ethylene dichloride and one part carbon tetrachloride). These cards were then air-dried overnight, cut into strips, and tested a t a number of different points. The number of areas tested and the extent of variation of the readings are indicated. The two more dilute solutions were applied five times with intermediate air-drying. With carbon tetrachloride as the solvent, the initial values obtained were lower but were increased in every case by pressing under a hot iron. The data are shown in Table VI. Results here for calcium stearate TIBLE Iv. CONT.iCT -4NGLE MEASUREMENTS ON MISCELLANEOCS are comparatively poor. In a similar test, using a solution containing 5.2 grams per liter in chloMATERIaLS --Contact A n g l e - Effective rasol, calcium stearate gave final values of 147.1' Plate Plate Aver- Adhesion Solvent 1 2 age Tension and 147.9". Tin stearate prepared from stan--Degrees-Dunea/cm. nous chloride and sodium stearate (the latter Cellulose acetate Acetone (Molded strips) made from a commercial triple-pressed distilled Raw rubber Benzene 59.5 5 5 . 5 57.5 38.7 stearic acid) was found sufficiently soluble in boilSynthetic resin (Coated paper) 86.2 4.8 G u m tragacanttt Hot nater 109.7 106:1 iOi:g -22. I ing isobutyl alcohol for coating on glass plates. G u m arabic copal Htohtywater E l alcohol l;f:: 1;:;; This coating gave initial values below 120' after Gum guaiac E t h y l alcohol 64.3 67.4 65.8 -29.5 air-drying overnight, but the values increased Gum kauri Isobutyl alcohol 87.0 79.1 83.3 8.4 Gum elemi Isobutyl alcohol 63.4 66.9 65.2 30,2 a.ftercontact with water, check readings near the Rosin damma: Gum E t h y l alcohol i!:: i;:: end of the plate being higher than readings taken Ester gum Isobutyl alcohol 63.9 64.8 64.4 31.1 a t points where the surface had not been previStearyl alcohol None 49.2 51.0 50.1 46.2 Stearone Sone 134.4 131.9 133,I -49. 2 ously immersed. Values approximating 140'were Monohydroxystearone Dihydroxystearone None 1;::; thus obtained. Light ironing, without sufficient Erucone None 115.4 114.8 115.1 -30.5 heat to impair the coating, which had a tendency Stearone 90, rosin 10 None 106.2 106.2 106.2 -20.1 Stearone 90,p a r a 5 n . 10 None 124.6 122.0 123.3 -39.5 to flake off on cooling, raised the contact angle i:;: E:: beyond 150'in every case, givingvaluesof 151.5', Stearone 90, synthetic mixed ketones wax 10 10 Sone Stearone 50, shellac wax 50 Xone 155.4 154.8 155.1 -65 3 152.4', and 159.0" on three different plates. --_ _. The results of Table VI1 were obtained by dipping the paper-surfaced plates into solutions The most varied results were obtained with metallic soaps. in hot isobutyl alcohol and drying in a desiccator overnight This fact will be apparent on inspection of Tables V, VI, and before testing. Subsequent pressing with a hot iron gave no VII, which give the results found on metal-bonded paper improvement in these results. surfaces. Of particular interest are a number of different values obtained for aluminum stearate. Deposited from solution on Paper Surfaces, it afforded results shown in Tables V and VII, T ABL E V. METALLIC SOAPSON PAPER SURFACE, FROM CHLORAvarying rather widely. Similarly deposited on glass, it gave SOL SOLUTION Concn. of Contact Readings No. of Av. values nearly as high, averaging 150.3', 159.4', and 151.2' Soh Angle Averaged Deviation on three plates. But when molded into half-inch strips by Groms/Ziter Degrees Degreea hydraulic pressure a t 5000 pounds per square inch (352 kg. AI palmitate 25 154.8 14 1.1 per sq. cm.), it gave consistent readings averaging only 113.4' A I stearate 25 157.6 6 3.9 Mg stearate 5.6 140.6 6 0.9 where the plate was smooth, waxy, and transparent, and A I palmitate 2.5 148.8 6 1.8 119.2' where it was white, chalky, and opaque. -~ Rlost of the waxes gave the same values on glass whether In the experiments of Table V, test cards, 10 X 2.5 inches deposited from a solvent or from t'he melted wax, but there were a few notable exceptions. All gave high values when (25 X 6.4 cm.), were treated on squeeze rolls with solutions
:::: :::;:::: :;A A:;!;
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;:::: :::;: ;:::; ;::,;
INDUSTRIAL AND ENGINEERING CHEMISTRY
994
deposited on paper surfaces. Some examples are presented in Table VIII. Residual traces of nonaqueous solvents were eliminated as a cause of this behavior by the fact that it wae also observed with ether after long aeration and with absolute alcohol. Those waxes that gave higher values on glass, when deposited from a solvent, reverted to the lower values if the plates were placed in an oven for a few minutes to fuse the coating. There was no appreciable alteration in the appearance of the deposit. TABLEVIII. EFFECTO F METHODO F PREPARINQ PLATES CONT.4CT ANQLE MEASUREMENTS O F w.4XES
Beeswax Paraffin Spermaceti Carnauba wax Synthetic wax A Synthetjc wax H Synthetic wax P
From Melted Sample Origi- Same plate nally retested reptd. after 6 mo. 108 113.6 110 110.5 110 113.8 125 129.2 112 116.6 131 129.8 118 123.8
ON
From CClr S o h . On glass On paper
125.0 110.7 l27:3 164.9 131.6 136.2
155.-2 162.4 149.6 153.1 158.0 150.4 149.2
VOL. 28, NO. 8
2. An improved apparatus for comparing the wetting characteristics of solid surfaces is described. The method applies the tilting plate principle for the direct measurement of contact angles. This method is capable of accurate and reproducible results provided the readings are taken dynamically. It should have a wide range of applicability. 3. Contact angle measurements on a number of waxes, metallic soaps, gums, and certain related compounds are reported. 4. A technic has been devised for comparing the water repellency imparted by dilute waterproofing baths to paper and textile surfaces. For this purpose, contact angle measurements have the advantage that they are not influenced by the mechanical strength and uniformity of the fabric. 5. The large amount of surface per unit area of fibrous materials plays a prominent part in the effectiveness of waterrepelling applications, as does any tendency of the deposit itself to give granular or microcrystalline, rather than smooth, coatings.
Acknowledgment The advantages and limitations of contact angle measurements are well illustrated by the comparison, given in Table IX, of contact angle and hydrostatic pressure test results obtained on two different fabrics proofed by identically the same treatment and on the same fabric proofed by different treatments. The contact angle measurements show that the two fabrics, after being treated with the same aqueous waterproofing emulsion, have equally repellent thread surfaces. This result is not shown by the hydrostatic pressure method because of the difference in the construction of the fabrics. The second preparation applied to the same silk fabric is a nonaqueous solution of waxes capable of proofing the thread surfaces equally well, but it is seen to impart much less resistance to the passage of water under pressure. This difference is to be explained by effects on thread dimensions and air porosity. TABLEIX. CONTACT ANGLE MEASUREMENTS ox PROOFED FABRICS Fabric Openings/sq. om.
Aqueous emulsion Nonaqueous s o h .
Contact Angle Silk Rayon 2700 1600 7 - Degrees-167.8 169.0 169.3 ...
Hydrostatic Pressure Silk Rayon 2700 1600 C m 01uater 17 5 9 5
7.4
,
.
The hydrostatic pressure tests of Table IX were obtained with an apparatus essentially that of Barr (6) but with certain improvements in technic to be described in a later paper.
Summary 1. It is inherent in the fundamental theory of wetting action that the wetting properties of a solid substance should be directly proportional to the roughness of the surface wetted. This fact does not appear to have been clearly recognized. Variations in the wetting properties of a solid, when encountered in experimental data, have usually been attributed to some form of surface contamination.
The research described in this contribution was conducted on Mellon Institute’s Industrial Fellowship on Textile Finishing, which is sustained by the Onyx Oil & Chemical Conipany, Jersey City, N. J. The author is grateful to this donor for cooperation during the investigation and also for permission to publish this paper. The bonded metal sheets used in the experiments were supplied through the courtesy of A. W. Coffman of hlellon Institute.
Literature Cited Ablett, R., Phil. Mag., 46,244 (1923). Adam, N. K., “Physics and Cheniistry of Surfaces,” Oxford Univ. Press, 1930. Adam, N. K., and Jessop, G., J. Chem. Soc., 127, 1863 (1925). Baker, C. L., IND.ENQ.CHEM.,23, 1025 (1931). Barr, G., Dept. Sci. Ind. Research, 2nd Rept. of Fabrics Coordinating Research Comm., 1930, 113-39. Bartell, F. E., et al., Colloid Symposium Monographs, 5, 113 ( 1 9 2 7 ) ; 10, 267 (1932); Ih-D. EXQ.CHEM., 19, 1277 (1927); 20,738 (1928); J . A m . Chem. Soc., 56, 2205 (1934); J.P h y s . Chem., 34, 1399 (1930). Bartell, F. E., and Hatch, G. B., Ibid., 39, 11 (1935). Bartell, F. E., and Smith, C. N., IND.ESG. CHEM.,21, 1102 (1929).
Bosanquet, C.H., and Hartley, H., Phil. Mag., 42,456 (1921). Coffman, A. W., Chem. & Met. Eng., 3 9 , 1 4 4 (1932); U.S. Patent 1,862,332 (June 7, 1932). English, L. L., Illinois State Nat. Hist. Survey, Bull. 17, .4rt. 5 (1928). Freundlich, H.,“Colloid and Capillary Chemistry,” tr. by H. S. Hatfield, London, Methuen &- Co., 1926. Green, E. L., J. Phys. Chem,., 33,921 (1929). Harkins, W. D., Colloid Sumposium Monographs, 6, 17 (1928). Huntington, A. K., Trans. Faraday Soc., 1, 345 (1906). Kovalerski, I. I., Papier, 38, 139 (1935): Pulp & Paper Mag. Can., 36, 547 (1935). Lee, A. R., J. Soc. Chem. I n d . , 55, 2 3 T (1936). Mack, C., IND.ENG.CHEM.,27, 1600 (1935). Nuttall, W. H., J. Soc. Chem. Ind., 39, 67 (1920). Rideal, E. K., “Introduction to Surface Chemistry,” 2nd ed., London, Cambridge Univ. Press, 1930. Stellwaag, F., 2. angew. Entomol., 10,163 (1924). Sulman, H. L., Trans. Znst. Mining Met., 29,4 4 (1919). RECEIVED 3Iarch 27, 1936.
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