INDUSTRIAL A N D ENGINEERING CHEMISTRY
June, 1941
Acknowledgment The writer is indebted to E. A. Vitalis for the data on solubility and Draves tests, to C. E. Barkalow for the data on surface tension by du Nouy and pendant drop methods and for the calculations of molecular dimensions, and to many other members of the American Cyanamid Company’s research laboratories for assistance in the preparation of the paper and illustrations.
Literature Cited (1) Adam, N. K., “Physics and Chemistry of Surfaces”, 2d ed., London, Oxford Univ. Press, 1938. (2) Am. Assoc. Textile Colorists and Chemists, Year Book, Vol. 17, pp. 216-22 (1940). (3) Andreas, Hauser, and Tucker, J . Phys. Chem., 42, 1001 (1938). (4) Barkalow, C. E., unpublished data. (5) Bartell, Culbertson, and Miller, J . Phys. Chem., 40, 881 (1936). ENQ.CHEM.,19, 1227 (1927). (6) Bartell and Osterhof, IND. (7) Boutaric and Berthier, J . chirn. phys., 36, 1-4 (1939).
(8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25)
737
Caryl and Ericks, IND. ENG.CHEM.,31, 47 (1939). Cupples, H. L., Soap, 15, 30 (1939). Doss, K. 8. G., Kolloid-Z., 87, 272 (1939). Harkins, Brown, and Davies, J . A m . Chem. SOC.,39, 354 (1917). Harkins, Davies, and Clark, Ibid., 39, 541 (1917). Jaeger, A. 0. (to Am. Cyanamid & Chemical Corp.), U. S. Patent 2,028,091 (Jan. 14, 1936). Langmuir, Irving, J . A m . Chem. SOC.,38, 2221 (1916). Ibid., 39, 1848 (1917). Langmuir, Irving, Proc. Nut Acad. Sci. U.S., 3, 251 (1917). McBain and Perry, IND. ENG.CHEM.,31, 35 (1939). McBain and Spencer, J . A m . Chem. Soc., 62, 243 (1940). McBain and Wilson, Ibid., 58,379 (1936). Miller, N. F., J . Phys. Chem., 45, 289 (1941). Nouy, P. L. du, “Surface Equilibria of Colloids”, A. C. S. Monograph, p. 86, New York, Chemical Catalog Co., 1926. Pockels, Agnes, Nuture,.43,437 (1891). Sluhan, C. A., Paper Trade J . , Aug. 22, 1940. Solov’eva, Colloid J . (U. S. 5. R.),3, 303 (1937). Van Antwerpen, F. J., IND. ENQ.CHEM.,33, 16, 740 (1941).
PRBEENTED before the American Section of the Society of Chemical Industry, New York, N. Y .
WETTING AGENTS F. E. BARTELL University of Michigan, Ann Arbor, Mich.
EFERENCES to surface-active agents appear in the literature under various titles, such as wetting agents, cleaning agents, leveling agents, dispersing agents, detergents, solubilizers, penetrants, emulsifiers, etc. Surface-active agents are generally defined as substances which, when added to a liquid, will lower the interfacial tension a t the boundary of this liquid. The boundary may be between the liquid and air, the liquid and another liquid, or the liquid and a solid: The functions of the agents, for the different purposes indicated by the titles ferent, and are it should somewhat not difbe mentioned,
R
treatment i t seems justifiable to take as a general example of a wetting agent a substance whose molecules possess a polar or hydrophilic portion, and a nonpolar or hydrophobic (or organophilic) portion. The hydrophilic portion may possess groups represented by OH, COOH, SOaH, S04H, P04H2, etc., while the hydrophobic portion may range from straight hydrocarbon chains to branched hydrocarbon chains and even to modified aliphatic and aromatic rings. A 1ogi;cal classification of wetting agents might be based upon the use to which the agent is to be put. This would necessitate consideration of the type of solid or solution with which the agent exhibits its greatest activity or retains its effectiveness. One useful classification might be of wetting agents effective (a) in pure water, (b) in water containing the alkaline earths, (c) in acid medium, (d) in alkaline medium, (e) in organic medium, df) in oxidizing medium. For nearly all purposes the most effective agents are those whose hydrophilic ends exhibit a strong affinity for or a high solubility tendency toward the water or the hydrophilic phase, and whose hydrophobic ends exhibit a strong affinity for or a high solubility tendency toward the organic liquid or the organophilic phase. If these two tendencies are not well balanced, the efficiency of the agent may be lost through the fact that the end with the stronger character will tend t o
Much of the development of wetting agents and their technical applications has been of an empirical nature because no suitable method for a detailed evaluation of these products has been available. To evaluate a given wetting agent, information should be available concerning the following : its surface tension lowering properties when in water and in organic liquids; its interfacial tension lowering properties when in systems of water in contact with given organic liquids (immiscible or only slightly miscible with water) ; its interfacial tension lowering properties in systems of given liquids against given solids. This last type of information can be obtained through measurement of adhesion tension values, which necessitates measurement of contact angles.
expected that a given surfaceactive agent will be relatively as effectjve a t each of several different kinds of surfaces. Surface-active agents range from comparatively simple inorganic salts, the silicates and phosphates, to organic compounds of complicated structure.
Wetting Agents The term “wetting agent” is usually applied to a substance which, if added to water, will cause that liquid t o give a higher degree of wetting (or to possess a lower interfacial tension) against a contiguous phase than would be obtained with pure water. The fundamental principles underlying the action of different wetting agents are so similar that for simplicity of
INDUSTRIAL AND ENGINEERING CHEMISTRY
738
carry the molecule too completely into that phase for which the solubility tendency is the greater. In this paper it will be assumed that the wetting agent used for illustration is of the type having a strongly hydrophilic and a strongly hydrophobic portion. Such an agent would tend to become oriented and adsorbed a t an interface, and thus lower the tension (and free energy) a t the interface.
1
I
A. ORGANOPHILIC SOLID IN WATER
B. HYDROPHILIC SOLID IN ORGANIC LIQUID
Vol. 33, No. 6
reasonable explanation for the error appears to be that the solute is adsorbed from solution as the liquid rises in the tube, and that much time is required for sufficient diffusion to give equilibrium conditions. The bubble pressure method, as ordinarily used, is likewise impractical. Often considerable time is required for the system to attain maximum adsorption effects with attendant minimum tension. Pressures high enough to expel a bubble of air are usually applied well before the tension reaches its minimum value; thus data obtained by this method may give values much too high. The ring method, which measures the tension directly a t the plane surface of the liquid, may give reasonably good results with the initial readings and is suitable for ordinary testing purposes; but for precision work it is not recommended (9). The most suitable methods for use with solutions are the pendant drop and the sessile bubble methods (2). With these methods we can follow the changes in surface tension which occur and can determine not only the rate of adsorption of agent a t the interface, but also the point a t which maximum adsorption is reached. We can form either a drop of liquid in air or a bubble of air in the liquid. I n the latter case we can readily obtain the condition of equilibrium in which the air phase is saturated with the vapor of the liquid. The type of molecular orientation which will occur at an aqueous solution-air interface can be correctly predicted.
Liquid-Liquid Systems
LORGANOPHILIC SOLID IN ORGANIC LIQUID
D.HYDROPHILIC SOLID IN W A T E R
OF MOLECULES AT INTERFACES FIQURE 1. ORIENTATION
The efficacy of the agent would be determined by the extent of the energy change at the interface. The principal object of this paper is to show the nature of the energy changes which occur a t the different types of interfaces and to describe briefly how the energy changes can be measured a t each of these interfaces. Liquid-Air Interfaces
The interfacial tension between two liquids is usually measured by methods similar to those used for surface tension. Almost any one of the standard methods suitable for the measurement of surface tension of a pure liquid is suitable for the measurement of the interfacial tension between pure liquids. When a surface-active agent is present in the system, the same difficulties are encountered in measuring interfacial tension as were described for surface tension measurements with surface-active agent present. The same methods of measurement appear to be most suitable in both casesnamely, the pendant drop and the sessile bubble methods. Probably the most important type of liquid-liquid system is that represented by emulsions. When the two liquids are water and oil, we can safely predict the type of orientation of agent that will occur a t the interface.
Solid-in-Liquid Systems
A system in which the interface is limited to that between solid and liquid is represented by a solid suspension in a liquid. Unlike the systems previously mentioned, no method is available for measuring the interfacial tension of this system. However, it is possible to measure the change in interfacial tension which occurs a t the interface when a surfaceactive agent is added to the liquid phase. This can be done
Since the classical work of Langmuir (IO)and of Harkins (8) on orientation of molecules at interfaces, it has been generally accepted that molecules of solutes of the polar-nonpolar type give oriented adsorption a t the liquid-air interface. The free energy change which occurs a t this interface as the result of adsorption can be deterPUR: WATER mined bv measuring the surface tension, WETTING AGENT ADDED first, of the pure liq;id, then, of the liquid plus surface-active solute. The first measurement is easy to make, and almost any one of the well-known surface tension methods is suitable. For the second measurement-i. e., with surface-active agent present-the situation is quite different. In this case the capillary ascension method, ssw ss4 which gives excellent results with pure liquids, may prove erroneous in the extreme. Initial readings are usualh too high and are often 50 per-cent or more in error. The most FIGURE 2. SPREADING OF A WATER DROPON AN ORGANOPHILIC SOLID
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1941
.
by measuring the change in adhesion tension. The change in interfacial tension, dS, is numerically equal to the change in adhesion tension, dA; i. e., dA = -dS. Four different types of solid-liquid systems may exist (Figure 1). In cases A and B we can correctly predict the types of molecular orientation of an agent which will occur at the solid-liquid interfaces. I n cases C and D we cannot, with the limited information available, predict the orientation or even be certain that appreciable adsorption of agent will occur a t these interfaces. Neither are we justified in predicting whether oriented adsorption of agent will occur at the organic liquid-air interface. We can be reasonably certain, however, that no high degree of adsorption will occur at this interface (11). There can be but little doubt that the oriented adsorption as represented a t A and B will tend to stabilize the suspension in these systems.
A typical example of a liquid-on-solid system is the spreading of a drop of water upon an organophilic solid. To make the study complete, we should consider (a) the water drop spreading on the solid in air, (a) the water drop spreading on the solid immersed in an organic liquid, and (c) the water drop, to which wetting agent has been added, spreading both on the solid in air and on the solid in the organic liquid. The influence of the surface-active agent upon spreading of a water drop in air is shown in Figure 2. A represents a drop of water on an organophilic solid and B represents a drop of water contnining wetting agent on this same solid. S, represents surface tension solid-air; 8,. and S,!., the surface tensions water-air; S,, and S,,), the interfacial tensions the contact angles. solid-water; and e,, and O,,~., The fundamental equations representing equilibrium conditions of these systems are:
sa. - s,,
s,, - s,,!
I
INTERFACIAL CONTACT A N G L E
= s,. COS eSw = swt0 COS eaW~,
Owing to adsorption at the interfaces, S,., < S,, and Saw)< SSw. Since S,,, the surface tension of the solid, re< ea,.,,. If the surface mains unchanged, i t follows that O,,. and interfacial tensions Sw$ ,and Saw*are lowered sufficiently, will become zero, the liquid will spread out as a continuous flm, and "complete wetting" will result.
#@ ,*
Degree of Wetting
i, ; ,1\ /
180' 90'
0'
FIGURE 3. RANGEOF CONTACT ANGLESOF A WATERDROP ON DIFFERENT SOLIDS
Then it follows that A.0
Liquid-on-Solid Systems
739
=
S.0 -
8 8 ,
= AF
(4)
That is, the adhesion tension serves as a measure of degree of wetting. For a solid-liquid (organic liquid)-liquid (water) system a similar equation applies, assuming nonmiscibility of phases:
s,, - s,,
=
so,COS eaow
(5)
We can relate the adhesion tension values of each of two immiscible liquids (water and organic liquid) against the solid by the equation; A,,
- A,,
= So,
COS
edow
(6)
It should then be possible to determine the degree of wetting of a solid by a liquid by means of Equation 3 or of Equations 3 and 6. The essential data to be obtained are surface and interfacial tension values-i. e., 8,. (or Son)and So,-as well as values of the contact angles of the liquid or liquids against the solidi. e., Os,, (or &,) and &,. The interfacial contact angle formed by water and an organic liquid against a solid (or series of solids)-i. e., Oao,is a much more sensitive measure of surface condition than the contact angle formed by a water drop in air against a solid or series of solids. For example, with a series of solids of surface properties so different as to give contact angles with water drop in air, Os,, ranging from 0" to 45", the interfacial contact angles formed with a water drop in organic liquid against the same series of solids, O,, would range from 0" to 180" (3) as Figure 3 shows.
The degree of wetting of a solid by a liquid may be defined as the work done, or the energy expended, when solid and liquid phases come in contact. The free surface energy of a solid in air, S,,, minus the free surface energy of the solid in liquid, Sa,, gives a measure of the energy expended, AF, in the process of wetting; i. e., S.,
- Sat =
AF
(1)
Since the free surface energy of a system is numerically equal to its interfacial tension, we can express the relations in terms of tensions. The well-known Young equation relates the various tensions of a system with the angle formed a t the point of contact of the phases. For a solid-liquid-air (organic liquid) system the following equation applies:
s,, - se0= so,COS eeoo
(2)
The adhesion tension, A,,, was defined by Freundlich by the equation, A,, =
so,COS eso0
(3)
FIQUBEI 4. APPARATUSFOR PHOTOGRAPHING CONTACT AN OLE^
INDUSTRIAL AND ENGINEERING CHEMISTRY
740
Measurement of Contact Angles Methods which have proved most practical for the measurement of contact angles formed by liquids against solids are as follows: sessile drop (4, I d ) , bubble ( I S ) , pressure of displacement ( 7 ) , tilting plate (I), capillary ascension, and vertical rod (6). The first three methods are believed t o be most suitable for the majority of systems. The sessile drop and bubble methods are similar in principle; they are direct methods with which drops or bubbles in contact with a surface can be continuously observed; drops and bubbles can be easily photographed and the angles readily measured. Pressure of displacement is the only known method that can be used when the solid is in a finely divided condition. The method is indirect. It is time consuming, and requires considerable experience, a high degree of skill, and much patience. It is recommended only for those cases where the solid is such that one of the other methods mentioned cannot be used. The tilting plate and vertical rod methods are of practical importance in that they offer the only means suitable for use with solid materials in the form of small rods, wires, and fibers. They offer promise for research in textiles. The capillary tube method, while of fundamental importance, appears to be of limited value for practical purposes.
Reproducibility of Contact Angles
Vol. 33, No. 6
obtain reproducible surfaces by a vaporization in vacuo method. Different metals, sulfides, and other substances have been used in the formation of solid films, the surfaces of which could be studied by contact angle methods. The results have been gratifying, and contact angle values of a given liquid upon different samples of the same solid have shown agreement within 1’. The contact angle method used was the sessile drop msthod. The drop of liquid formed on the solid surface was connected by a liquid column with a mercury reservoir. Movement of liquid was controlled by a small-diameter screw piston fitted into the mercury reservoir, Figure 4 ( 5 ) . The success of the method lies in the fact that movement of the liquid used to form the drop can be kept under exact control so that either advancing or receding angles can be maintained for indefinite periods and can be measured a t will. Within the past year thousands of measurements have been made with this method, and the results have been so promising that a number of different researches are now under way. I n one of these an attempt is being made to evaluate the efficacy of different wetting agents and to determine the specific effect of substitution of different radicals and groups into the molecule of the wetting agent.
Literature Cited (1) Adam, “Physics and Chemistry of Surfaces”, p. 147 (1930); “Wetting and Detergency” (1937). (2) Andreas, Hauser, and Tucker, J. Phys. Chem., 42, 1001 (1938). 13) Bartell and Bartell. J . A m . Chem. Soc.. 56. 2205 119341. ’ (4j Bartell and Bristol; J . P h y s . Chem., 44, 86 (1940). (5) Bartell and Cardwell, I b i d . , t o be published. (6) Bartell, Miller, and Culbertson, I b i d . , 40, 863 (1936). ENG.CHEM.,19, 1277 (1927). (7) Bartell and Osterhof, IND. (8) Harkins, J . Am. Chem. SOC.,39, 354, 541 (1917). (9) Hauser, Edgerton, Halt, and Cox, J . Phys. Chem., 4 0 , 973, (1936). (10) Langmuir, Chem. K! Met. Eng., 15, 468 (1916); J. A m . Chem. Soc., 39, 1848 (1917). (11) McBain, J . A m . Chem. Soc., 62,419 (1940). (12) Mack, J . Phys. Chem., 40, 869 (1936). (13) Wark, “Principles of Flotation”, Melbourne, Australasian Inst. of Mining and Metallurgy, 1938. ~~
A careful study of contact angle values found in the literature discloses the fact that different investigators seldom obtain values which are in agreement. About the only exceptions are the values obtained with soft organophilic solids, such as paraffin. I n fact, seldom has any one investigator been able to obtain closely reproducible values for different surfaces of a given material. Recent investigations have made it almost certain that the cause for this lack of agreement has been, not so much defects in methods of measurement, as the fact that insufficient care had been exercised in the preparation of surfaces to ensure that they would have identical properties. I n recent work in this laboratory we have attempted to
PRESENTED before the American Section of the Society of Chemical Industry, New York. N. Y .
Surface-Active Agents-Additions
and Corrections
A few errors have been pointed out in the table of surface-active agents manufactured in America and commercially available, which appeared on pages 16 to 22 of the January, 1941, number The following additions are also made by F: J. Van Antwerpen who compiled the table : Industry
Use
Name Calgon
Sodium hexametaphosphate
ADDITIONS Dispersing, deflocculating mater generating, textile
Cresol
CaH50H-CH3
Wetting
Cyclohexanol Methylcyclohexanol Pyridine Quadrafos
Hydrogenated phenol Hydrogenated m,p-cresol GH5N Sodium tetraphosphate
Textile, general degreasing Wetting, detergent Same Same Textile Detergent Dispersing, detergent aid, Textile, water generating deflocculating
Alkanol S
Sodium tetrahydronaphthalene sulfonate Purified suifolignin Processed waste sulfite liquor
Dispersing
Textile, soap
Dispersing in water Dispersing, flushing
Latex, pigment, textile Same
Textile
Manufacturer Calgon Inc., 300 Ross St., Pittsburgh, Penna. Barrett Co., 40 Rector St., New York, N. Y . Same Same Same Rumford Chemical Works, Rumford, R. I.
ERRATA
Dilex Hornkem
E. I. du Pont de Nemours & Co., Inc. Horn Research Labs., Inc. Same
