THREE FCSDAXESTAL TYPES OF WETTISG.’ ADHESIOS T E S P I O S AS THE 1IEASUIIE O F DEGREE O F R E T T I N G BY H . , J , O S T E R H O F AKD F. E. B.iRTELL
In an earlier paper from this laboratory it was pointed out that the term? “wettability” and “netting power”? have, heretofore, been inadequately defined.3 d s a result, each of these terms has been used by different authors to represent quite different things. Controversies and differences of opinion have arisen n.hich undoubtedly would not have occurred had there been better understanding as t o the significance of the terms used. In earlier papers4 a method for the measurement of the adhesion tensions of solids against liquids has been described. I t has been pointed out that adhesion tension data which can be expressed in terms of dynes per centimeter can be used for the determination of the degree of wetting of solids by liquids. I t is the purpose of this paper to reconcile, if possible, the different views on “wetting”j and to show that adhesion tension furnishes the most convenient and appropriate measure of the degree of wetting. In general the conflicting views relative to “wetting” or “non-wetting” can be classified as follows: I-Wetting of solid by liquid occurs only in case the angle of contact is zero degrees. z-Wetting occurs only when the angle of contact is less than 90 degrees. g-JT-etting occurs when the angle of contact has any value less than 180 degrees. In the latter two cases wettabz’lity is considered as being dependent upon the magnitude of the contact angle. I t is assumed by Richards and Carver,6 by Bosanquet and Hartley’ and by .Idam* that a zero contact angle exists for those cases in which a liquid wets a solid. According to Suttallg “the ability of a liquid to wet a solid surface, i.e., t o give an even, continuous film over it is dependent upon three surface tensions . . .” For a liquid to wet, the surface tension of the solid must be greater than the sum of the surface tension of the liquid and Presented a t the Detroit Meeting of The American Chemical Society, September 1927. These two terms though similar in meaning are not synonymous; one signifies the “wettability” of a solid by a liquid and the other the “netting power” of a liquid for a solid. 8 Colloid Symposium Monograph ( 1 9 2 7 ) . ‘Ind. Eng. Chern., 19, 1 2 7 7 , Nov. (1927’1;Z. physik. Chem., 130, 715 (Cohen-Festband) (1927). This term has been loosclv employed to refer to true “wetting,” to “wettahility,” “wetting power” and to .‘degree of wetting.” J. .4m. Chem. Soc., 43, 827 (1921). ’Phil. Mag., 6,42, 456 (1921). 6 Science Progress, 83,442 (192;). Fifth Brit. Ass. Adv. Sei. Report on Coll. Chem., 38 (1923).
I 400
H. J. OSTERHOF A S D F. E. BARTELL
the interfacial tension of solid against liquid. This is tantamount to saying that the angle of contact must be zero degrees. Itideallo observes “For liquids which wet solids the angle is very nearly zero and may be taken as actually equal to zero without serious error.” Edser’l states, “If there is no attraction between the molecules of the liquid and those of the solid, 0, (contact angle), = 180°, . . . . I n the case of mercury in contact with glass, experiment shows that 8 is equal to 140’ . . . consequently there is some attraction between the mercury and the glass molecules.” And further, “If the liquid wets the inside of the tube,
0
=
00.
’’
According to hblrttlY“Others assume that a liquid “wets” a solid when the angle of contact is less than 90°, and does not “wet” it when the angle is greater than 9 0 O . ” I n his discussion of advancing and receding contact angles, he expresses the novel but at the Pame time questionable view that true wetting is due to absorption or imbibition of the liquid by the solid, that a measure of this is given by the variation of the contact angle, and that the degree of wetting is indicated by the range of differencp between the advancing and receding contact angle. It is believed by him that this represents “the degree to which the surface energy of the solid is affected on coming into contact with the liquid.” Freundlich13 gives as the limit for non-wetting a n angle of contact of I So0, while for complete wetting it is 0’. If the angle of contact is 180” and “if the adhesion tension, taken as negative, is greater than the surface tension of the liquid, then we have no wetting.” S c h ~ m a c h e r ’uses ~ the term wetting to indicate limited wetting when the contact angle is between oo and 18oO, and complete wetting when the contact angle is zero degrees. ?;uttal115 in one paper makes a similar use of this term. Bancroft’@and others are inclined to omit the question of contact angles in their consideration of the phenomenon of wetting. Bancroft contends that wetting should be explained on the basis of adsorption. Excerpts picked somewhat at random from his articles and text are: “If a liquid wets a solid, it is adsorbed by the solid, forming a liquid film on the surface of the latter. . . . For a liquid to )vet, a solid in the presence of air, the liquid must be adsorbed more strongly than the air and must displace it. . . . Water wets glass, mercury does not. . . , hlercury does not wet glass because air is adsorbed more strongly than mercury by glass. . . . Gum arabic adsorbs air strongly and is not wetted readily by water. . . . The floating of pieces of metal on water is due to the slowness with which water wets them.” “Surface Chemistry,” 7 (19261. “General Physics,” 304. 1* Phil. Mag., 46, 244 (1923). 13 “Capillary Chemistry,” I j 7 . l 4 J. Am. Chem. Soc., 45, 2 2 j j (1923). J. SOC.Chem. Ind., 39, 67 (1920). “ M e t . Chem. Eng., 14, 631 (1916); J. Ind. Eng. Chem. 13, 88 (1921); “Applied Colloid Chemistry,” 74, 92 (1926). IC l1
T H R E E FLXDAMESTAL TYPES O F W E T T I S G
1401
Similar differences of opinion are encountered when one attempts to make a comparison of the various methods which have been employed for the measurement of the relative wetting power of a series of liquids.”
Types of Wetting In view of the existing lack of agreement, if not in ideas, certainly at least in terminology, it is distinctly worth while to attempt a more precise definition of wetting. Since it appears that in many cases the apparent disagreement is more in the form of expression than in fundamental ideas, it is our intent to propose, not a new system of nomenclature, but a reclassification of existent terms. Stated as a perfectly general definition “wetting” is that phenomenon which occurs when a solid phase and a liquid phase come in contact in any manner so as to form a solid-liquid interface. Only the free surface energies need be considered, since the processes are usually carried out isothermally and the latent heat of surface formation may be disregarded. For practical purposes, however, this concept of wetting is too general to be of any considerable value. Consequently, let us postulate that more than one type of wetting will be involved in the formation of a solid-liquid interface. The type of wetting is conditioned by the kind of free surface energy change which occurs in the entire system, the work which is performed by it, or the manner in which this change is brought about.’* From this standpoint, there are three, and only three, different and distinct types of wetting. They may be designated as adhesional wetting, spreading wetting, and immersional w e t t i n g , the terms originating in accord with the mechanical process whereby the wetting takes place. Fmploying the customary symbols19 (Fig I ) , we shall discuss in detail these types of wetting, and attempt to justify their existence and necessity. The applicability of these distinct types seems apparent on consideration of the work involved in the processes of adhesion, of spreading, and of immersion. Adhesional Wetting. When a unit plane solid surface supported parallel to a unit plane liquid surface is lowered toward the latter until contact is made, we have the simplest case of adhesional wetting, (Fig. 2 . ) Unit surfaces of solid (a/b) and of liquid (a’lb’) disappear, this being accompanied by the appearance of a unit solid-liquid interface (c./d). The expression for the decrease in free surface energy or the work necessary to restore initial con”Bartell and Osterhof: Ind. Eng. Chem., 19, 1277 (1927). l 8 We shall consider that the change in free surface energy is synonymous with the work that could be performed by the entire system under isothermal conditions, and without outside assistance. SI = surface tension of the solid phase. SZ = surface tension of the liquid phase. SIZ= interfacial tension between solid and liquid. e = angle of contact between liquid and solid surfaces. SI = Siz Sz COS €I (e>o0 = KSz (e = 0’) AIZ= SI - SN = adhesion tension between solid and liquid.
1402
H. J. OSTERHOF A S D F. E. BARTELL
ditions was formulated by Duprt.?o According to him the work of adhesion, W ais given by the equation It-& = s, - SI? $2 (1) Comparatively few ?samples of adhesional wetting arc encountrred in actual practice. The action of adhesives, however, furnishes a practical example-a solid surface conies into contact, with a liquid or srmi-liquid surface super-imposed upon another solid surface, with the result that an additional solid-liquid interface is formed. .I glued wood joint, an adhesive plaster, or a sheet of laniinatrd glass serve as examples. .llthough neither on application nor on removal is the adhesive situated at all times parallel
+
v FIG.I
FIG.2
to the solid surface, the final result is the same since either a solid and a liquid surface disappear and a solid-liquid interface appears, or \+ice-versa. Spreading Wetting. The spreading of a drop of liquid on the plane surface of a solid furnishes a simple case of spreading wetting. (Fig. 3 ) During this process a certain area of the solid surface (represented by a and a’) disappears, while equivalent areas of liquid surface and solid-liquid interface are formed. The change in free energy of the complete system, or the work involved in this action is expressed by the equation: v i 8 = SI - SI2 - sz (2) A practical example of this type of wetting is furnished by the application of a varnish or lacquer to a solid surface. Unless the quantity (SI - SIZS z ) is positive, the varnish will not form a thin adhering film, but will draw up into drops, Another instance is the classical example of the floating of a needle or of a disc on a liquid surface, as discussed by EdserJZ1Coghill and Anderson,z2 and B ~ u a s s e . ? The ~ liquid meets the solid with a definite conAnn. Chimie (41, 6, 274 (1865). z1Fourth Brit. Ass. Adv. Sei. Report on Coll. Chem., 297 (1922) J. Phys. Chem., 22, 249 (1918). 23 “Capillarite”, 2 1 6 (1924). 2a
THREE FUXDAMENTAL TYPES O F WETTING
1403
tact angle, work of spreading, ITS,is negative, and the weight' of the solid is not sufficient to cause spreading of the liquid over its surface with consequent, immersion. For liquid-liquid systems, the spreading of one liquid orer the surface of another, as reported by Harkins and co-workers, may be mentioned. Immersional TT-etti'ng. K h e n a plane solid surface supported normal to a liquid surface is allowed, or is forced, to penetrate the latter and become immersed in a liquid, or when a solid of any shape whatsoever is immersed
FIG.3
in a liquid we have a case of immersional wetting, (Fig. 4). In this process a solid surface exchanges a gas for a liquid environment. The interface solid-air disappears, while a corresponding interface solid-liquid is formed; no change whatsoever occurs in the liquid-air interface. The free energy change which obviously has occurred, or the work that has been performed (\Ti), is given by the equation:
ITi = SI -
S1 2
=
A12
FIG.4
(3)
The immersion of a powdered solid such as silica or carbon in a liquid, is one of the practical examples of immersional wetting. The heat of wetting] or of immersion] probably does not measure this quantity] since the former includes the latent heat of surface formation.* The rise, or movement] of a liquid in a capillary tube is another example. Solid-air surface disappears] solid-liquid interface appears, and no change occurs in the liquid-air surface which remains unchanged. The pull of a liquid upon a hydrometer immersed in it is still another e ~ a m p l e . ?The ~ determination of surface tension by measurement of the pull of a liquid on a thin platinum plate is also based on the principles of immersional wetting, in cases where the angle of contact is greater than zero degrees. Comparison o j the three types of wetting. Each type of wetting is conditioned by the entire free energy change which occurs in the complete system during the course of the action in question; i.e., by the difference between its initial and final energy states, regardless of the intermediate reactions.
* There is snme question as t o the magnitude li
Bouasse: Ioc. cit., pages
206, 219,2 2 5 , 325.
of this factor
H. J. OSTERHOF AND F. E. BARTELL
I404
Any process of wetting will fall under one or more of these three heads, while in some cases it may require treatment under all three. Briefly stated, the distinction between these types of wetting is as follows: I. Every case in which a solid and a liquid surface disappear with the formation of a solid-liquid interface, the change in free surface energy (AF) being given by the expression AF = Si - S i 2 Sr. is an instance of adhesional wetting. 2. All cases in which a solid surface disappears with the simultaneous formation of a liquid surface and a solid-liquid interface, with a change in free surface energy of AF = Si - Si2 - SzI are instances of spreading wetting. 3. Finally, every case in which a solid surface disappears with the formation of a solid-liquid interface, and with no change occurring in tfie extent of the liquid surface, the change in free surface energy being AF = Si - Si?, is a case of immersional wetting. The obvious difference between these three types, is that in the first, case there is a unit decrease in liquid-gas interface; in the second a unit increase in this quantity; and in the third no change whatsoever. The relation between the angle of contact existing between a solid and a liquid and the possibility of a particular type of wetting occurring unassisted, is as follows: I. Spreading wetting is positive only when t'he angle of contact is zero; it is negative for all larger values of this angle. 2. Immersional wetting is positive when the angle of contact is less than 90', and is negative when the contact angle is greater than this. 3. Adhesional wetting is positive when the angle of contact is less than 180'. There seems to be no case in which this type is negative.
+
Adhesion Tension as a Measure of Wetting In an earlier paperJZ5it was shown that for solid-liquid systems, even though neither SInor St2 can be measured, the quantity Si - S ~ Zwhich ,~~ has been termed the adhesion tension (At*), can be determined in many instances by comparatively simple measurements. The use of adhesion tension as an indication of wetting power was pointed out. We shall here attempt to show its applicability as a measure of degree, or completeness of wetting. Accepting the existence of three types of wetting, a decision should be reached as to what property of the solid-liquid system should be chosen as a measure of wetting, wettability or wetting power, whether the work of adhesion, the interfacial tension, the adhesion tension, or some related quantity. Will the same value serve, in defining the conditions which exist in the different
.
Ind. En Chem., 19, 1277 (1927). This is %e quantity defined as adhesion tension, which, for cases in which the liquid and solid meet with a definite contact angle, is determined from the expression A12 =SIsI2= s2cose (4) 25
THREE FUNDAMENTAL TYPES O F WETTING
1405
cases, or must there be three different kinds of wettability and wetting power, which are functions of the degree of wetting for each of the three t,ypes? Before attempting to answer these questions, it is advisable to differentiate between wetting, wettability, and wetting power. Definitions for wetting and for its three types have been given. The two expressions, ‘Tettability” and “Wetting power” are, still imperfectly defined. Wetting power is loosely employed to mean either adhesional wetting, degree of spreading wetting, or wettability. It’ is measured by such properties as the surface tension of the liquid, or its interfacial tension against immiscible liquids. In order to give a precise meaning to these terms, wettability may be defined as referring strictly to the part played by the solid phase,* wetting power to that played by the liquid phase, in any process of wetting. Wettnbility. Wettability represents the tendency of a solid to be wetted and is expressed by the value in ergs of the term (S1 - S1’.)p7 This quantity represents the decrease in free energy of a solid surface layer due to partial satisfaction of the unbalanced forces there situated or the extent to which these attractive forces have been neutralized by contact with a liquid. I t may thus be regarded as expressing the attraction exerted by a solid upon a liquid, and therefore the tendency of the solid to form its portion of a solid-liquid interface. The expression is the same for all three types of wetting, since the solid surface is inestensible. Wetting Power. l t is more difficult to formulate a precise definition of wetting power. Wetting power represents the tendency of a liquid to wet a solid, but it cannot be as simply expressed as in the former case. Based on reasoning similar t o that employed for the solid phase, an analogous expression (S?- Sp’) might be selected as the measure of wetting power. This value may be regarded as the actual decrease in free surface energy of a liquid surface when it forms part of a solid-liquid interface. It is the attractive force which a liquid exerts on a solid, 2nd measures therefore its tendency t o wet if unhindered by the other factors. d question still remains as t o the advisability of employing only this single velue for m t t i n g power, or of employing different values dependent on the type of wetting. This is equivalent t o considering on the one hand, only the change in free energy of that portion of the liquid surface in contact with the solid, or on the other hand, the change in free energy of t,he entire liquid system. It seems more reasonable t o consider the latter. When a solid-liquid interface is formed, the part played by the liquid (its free energy change, or wetting power) must depend on the type of wetting, since in the process an interface is formed against the solid, while a liquid-air interface may either be lost, or gained, or remain unchanged. Thus, in the final analysis, it is the three possible functions of the liquid, which give the three * I n this paper we shall not consider the question of one liquid wetting another liquid. Si’and S?’are for the solid and liquid respectively, the free energies remaining in their portion of the solid-liquid interface. The sum of SI’ and S,’then equals SLZ, the inter27
facial tension, or free energy still resident in the interface.
1406
H. J. OBTERHOF AND F. E. BARTELL
different types of wetting. The surface tension is an important factor, since it is by an amount equal to its value that the types of wetting in a particular
solid-liquid system differ from each other. From the above it is apparent that three distinct values for wetting power must be considered: for adhesional wetting, (Sz - S2’); for immersional wetting, ( - &’) ; and for spreading wetting, ( - Sz - &’). The smaller the value of Sz‘,the greater is thc u-etting power of the liquid; also, the smaller the surface tension the smaller is the adhesional wetting power, but the greater the spreading wetting power. I t follows, t,herefore, that it is erroneous to state that the greater the attractive force exerted by a liquid on a solid, the more readily it will wet the solid. This becomes apparent when one considers that (Sz -Sz‘) represents the attractive force of the liquid, and in the case of immersional or spreading wetting the influence represented by Sz (the surface tension) will, as seen from its - sign, tend to oppose these forms of wetting. The attraction of mercury for glass is undoubtedly greater than that of hexane for glass, and yet hexane spreads while mercury does not. Wetting. When wetting is analyzed in terms of wettability and wetting power, the three cases are: I. Adhesional wetting : = (SI -si’) (SZ -Sz’) = Si - Si2 sz = A12 Sz (5) 2. Immersional wetting: w i = (Si --Si’) (-S*’) = si -si2 = Ai* (6) 3 . Spreading wetting: U‘. = (Si -Si’) (-Sz -Sz’) = Si -Si2 -S2 = Aiz -Sz (7) The entire free energy change in the process of wetting is equal to the change in energy furnished by the solid phase added to that furnished by the entire liquid phase, or to the sum of the wettability and the wetting power. Summing up, we find the wettability of a solid and also the attraction exerted by it on the liquid is (SI-&‘);the wetting power of a liquid for adhesional wetting and also the attraction exerted by it on the solid is (SZSZ’). These two terms are not equal to each other, but their sum, W., the work of adhesion, is the tot,al attractive force between solid and liquid, and represents the maximum work which can result from contact of these two phases. The wetting power of a liquid for immersional wetting is (- SZ’), and for spreading wetting is ( - Sz-S?’). The wettability and the t,hree values of wetting power have no practical significance since SI, SI’ and Sz’cannot be determined. Therefore, to prevent possible confusion, it seems advisable for the present a t least t o discontinue the use of the terms “wettability” and “wetting power.” X comparison of the behavior of a series of liquids with several solids may best be made by speaking, not of the wetting power of a liquid for the several solids nor of the wettability of a solid by the series of liquids, but of the degree or extent to which a particular type of wetting takes place. In regard then as to what property of the solid-liquid system shall be chosen as a meesure of wetting, it may be stated unquestionably t>hatthat
w,
+
+
+
+
+
THREE FUKDAhlENTAL TYPES O F WETTING
I407
quantity, which will best serve as a complete and proper measure, is the degree of wetting. This, in turn, is obviously conditioned by the change in free surface energy which occurs during the process of wetting, and has been shown to possess a different value for each type of wetting. Accordingly, the final criterion or true measure of wetting for these three types is: for adhe; immersional wetsional wetting, the work of adhesion or (SI- S l ~ + S z )for ting, the work of immersion or ( & - Sl2);and for spreading wetting, the work Each form of wetting is measured by a separof spreading or (S1-S1?-Q2). ate and distinct standard.
TABLE I Three Types of Wetting 2
5°C
SZ Solid
Carbon
>, ,, ,,
Silica 1
>I I?
11
Glass ,I
Paraffin Mercury
,,
1)
Dynes per cm.
S12* Adhesional Immersional Spreading Dynes Wetting Wetting Wetting per cm. Ergs/cmz Ergs/cm2 Ergs/cm?
17.82
87.75
28.25
32.30
109.33 121.75 126.82 59.95 80.68 78.24
44.00
85.92
32.30 72.08
17.82 28.25
72
.o8
475 72
.08
;z .08 17.82 28.25 72.08
154.90 jr4
111
67 58 378 357 375
154.90
69.93 5 2 , I I 81.08 52.83 89.45 5 7 . 1 j 54.74 -17.34 42.13 24.31 jz.43 22.68 45.94 13.64 41.92 - 2 . 0 8 82.82
-364
10.74 -839
82.82 10.74 5 4 . 0 3 -18.05 -90.13
114.82 146.25
97 118
79,18 89.75
172.08
100
27.92
* Values taken from the literature. Energy Decrease during Wetting. If, in order to simplify tabulation, a more general criterion is desired that may conveniently apply to any or all forms of wetting, it will be necessary to select it from the above three expressions. To assist in this selection Table I has been assembled to show whatever relationship may exist between the surface or interfacial tensions and the energy decreases occurring in the three types of wetting. The surface tension of glass and paraffin are representative values found in the literature. The values of immersional wetting or adhesion tension for carbon and silica were determined experimentally by displacement pressure measurements. For glass and paraffin the values were calculated from the angles of contact, while in the case of mercury they were calculated from the values for the surface and interfacial tensions.
1408
H. J. OSTERHOF A 9 D F. F. BARTELL
Wetting diagrams for several of the solid-liquid systems tabulated in Table I are shown in Fig. 5 and Fig. 6. These figures illustrate the energy decreases which occur when a given solid is wetted by a liquid. The length of the lines below the zero horizontal line represents positive energy decreases while the length of the line above the horizontal represents the energy decrease, or work which must be done upon the system to cause complete wetting. I n the case of mercury against glass, for example, the system shows a positive work of adhesion ( + I I I ergs), but a highly negative work of spreading (-839 ergs).
0
20 40
60 80 100
IM
FIG.6
Study of the table and Figs. j and 6 leads to the conclusion that, in a series of solid-liquid systems there is no direct numerical relationship between the tabulated values for ITi and W,. In each expression there are present, the quantities SI,SI?,and SI- SI?the latter is equivalent to X12the adhesion tension, SI is quite obviously no nieasure of wctting. Relative values of SI?for a series of liquids against, one particular solid might serve. The lower the interfacial tension the greater in general is the degree of wetting. We find, however, that this relationship docs not hold for one liquid against a series of solids. (Shown in the case of water against paraffin, glass and mercury in Table I). Another objection to the use of this term, SI*,is that no generally applicable method for its determination exists. From above it will be noted that the term S1-S1?, is common to each of the expressions representing work of wetting. It has been pointed out above that SI- SI* = AI*, the adhesion tension for liquid against solid. Therefore, as the standard of reference for purposes of tabulation we are inclined to favor the adhesion tension for the following reasons: The adhesion tension
THREE FKiDAMENTAL TYPES OF WETTING
I409
is the only one of the possible measures which is capable of actual experimental determination. It is not only fundamental to each of the three different expressions for degree of +etting, but is also the quantity absolutely essential for their evaluat'ion. I t is the actual measure of degree of immersional wetting. I t is also the measure of the tendency of a solid surface to form a solid-liquid interface and is comparatively unaffected by the superficial viscosity of the liquid. Finally, it is that value from which by the addition or subtraction of the liquid surface tension, the degree of adhesional or spreading wetting may be obtained. Adhesional wetting is numerically equal to adhesion tension Ala plus surface tension. I n adhesional wet'ting the surface tension of the liquid serves as a force to aid in the process of wetting. Spreading wetting is numerically equal to adhesion tension, Ais, minus surface tension ( S I ) . In this type of wetting, the surface tension of the liquid will operate so as to oppose the process of wetting. I
2
3
4
5
I
I
b
0
FIG.7
In imniersional w t t i n g , adhesion tension alone scrves as its mrasure The surface tension of the liquid need not be considered. I t thus becomes apparent that adhesion tension represents the simplest measure of degree of wetting. Examples of the Application of the Three Types of Wetting in a Single System Siliccr-Ti-crttr. Let us consider a cube of quartz whose edge is of unit I. length (Fig. 7 ) . The cube in position I represents a solid system, suspended in gas, whose surface is represented as a solid-gas interface, the surface tension of which is unknown. Consider as the first step that the cube be lowered t o position z until one face is brought into contact with water. Adhsional wetting will result. A unit solid-gas and a unit liquid-gas interface will disappear, and a unit solid-liquid interface will be formed. The work of adhesion is represented by the equation: \va = SI Sa - S I 2 or since SI - SI? = Alz \va = AI2 sa From Table I, we note that the value of work done by the system in the process of wetting is equal to \Ta = I j4.90 ergs. Consider as the second step that the cube be lowered through position 3 until it just assumes position 4. During this process iminersional wetting will
+ +
1410
H. J. OSTERHOF AND F. E. BARTELL
result. Four faces of the cube will be wetted. KO change d l occur in connection with the liquid-gas interface. Accordingly from the equation JTa = in which hi?the adhesion tension for silica against water, is equal to 82.82, it follows that the work of immersion of each unit face is 82.82 ergs, or the work of immersion of the four faces is equal to 82.82 X 4 = 331.28 ergs. The third and final step consists in the submersion of the remaining cube face. This step involves t he disappearance of a unit solid-gas interface, and the siniultaneous appearance of a unit solid-liquid, and a unit liquid-air interface. This change involves .cprendirq u ~ t t i n gand can be represented by the equation: I\-s -- P, - Si, - Y, = AI? - s, From Table 11, we note that spreading wetting, IT,, i' cqual to IO.;^ ergs per em'. This represents the work done by the system during the last step in the wetting process. The entire free e n r q y dpcreasc during the process of wetting is as follows: Kork of adhesion, \V, = I x 154.90 = I j4.90 (positive) Kork of immersion, IVi = 4 X 82.82 = 331.28 (positive) Work of spreading, IT, = I x 10.74 = 10.74 (positiw) Total energy decrease in the system = 496.92 ergs. Since the above steps represent the work done in passing from the solidgas to the solid-liquid s y s t m ~this , xork should be numerically equal to the work of immersion, when all six faces are considered. This is obviously the case, Le., 6 X 82.82 ergs = 496.92 ergs. Accordingly we find that in any process which involves the substitution of a solid-liquid interface for a solid-gas interface, the value calculated for work of irnniersion will hold even though all three forms of wetting actually occur during the process. In the process of complete submersion of solid from an air into a liquid medium, the magnitude of the work done by the surface tension forces which first aid in adhesional wetting, and then oppose in spreading wetting, are equal, i.e., the liquid gas interface which disappears during adhesional wetting must, be equal in area to the liquid-gas interface formed during immersion. In effect there has been no permanent change in the liquid-gas interface. In the case just cited, of silica and water, each of the three steps representing the three types of wetting resulted in a decrease in surface energy, work being done by the system in each step. Disregarding gravitational effects, the system mould operate so as to result in an automatic submersion. A similar process carried out with a graphite cube and water would show a different behavior, With paraffin and water the behavior would be still different. Carbon-Water. In the case of carbon and water, adhesional and 2. immersional wetting are positive, while spreading wetting is negative, or ITa 4 \Vi (-Ws) = 126 4 X 54.73 + ( - 17.34) = 328.44 ergs, which represents the total work of immersion ( = 6 X 54.74) i.e., the net work done by the system is 328.44 ergs.
+
+
+
1411
THREE F L T D A X E K T A L T Y P E S O F WETTING
The above explains why certain solids of this type partially submerge themselves but still remain a t the surface. Work must be done upon the system to effect final submersion. This last step ini'olves the process of spreading wetting. Unless the density of the particle is so great that the influence of gravity can furnish the energy necessary for spreading of the liquid over the surface of the solid, the latter will tend t o float at the surface. 3 . Parafin-Tl-der. I n the case of paraffin and mater, adhesional wetting only is positive, while immersional wetting and spreading wetting are both negative. The energy changes upon immersion are ITa 4- (-4 W ) i (-lV8) - 51.03 ( 4 X -18.0j) i-90.13) = -108.30 ergs. The total energy decrease n.hich would occur upon immersion is negative and, therefore, external work must be expended upon such a system in order to bring about' immersion. Concl~rsions:An attempt has been made to correlate t'he views which have been expressed by different writers relative to the wetting of solids by liquids. It has been pointed out that in order to properly designate degree of wetting three types of wetting must be considered, namely: adhesional, spreading, and immersional wetting. A distinction has been made between wettability of a solid and wetting pon.er of a liquid. It has been suggested that adhesion tension can be used to designate degree of wetting. Conflicting views become harmonized when the question of terminology is correctly considered and applied.
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Chemistry Departnient, rnioersity of M i c h i g a n .
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