IMPORTANCE OF BOUNDARY ENERGY MEASUREMENTS IN INDUSTRY E. A. HAUSER, J. M. ANDREAS,’ AND w. B. TUCKER‘ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass.
in the cwe of comparatively large masses of material, the limitations of considering only the properties of the bulk of the fluid can be demonstrated. Consider a comparison of carbon tetrachloride and nbutyric acid. They have widely different physical and chemical properties (11):
Boundary tension at an interface between a liquid and a gas or vapor (surface tension) or at the boundary between two incompletely miscible liquids (interfacial tension) is a measure of the free energy of a fluid interface. Therefore it is a predominant property of liquid surfaces. Our iucreasing scientific knowledge of molecular orientation in phase boundaries and their bearing on a great number of industrial, biological, and other processes and developments, such as ore flotation, emulsification, spumification, wetting and impregnation, laundering, pigment dispersion, etc., as well as the constant discovery of new detergents, makes an accurate study of boundary tensions more and more imperative. The most common methods for the determination of boundary tensions are briefly reviewed and their limitations from the point of view of industrial applicability and reliability discussed. The importance of studying possible changes in boundary tension of a system with time and concentration of solute (positive or negative adsorption) is pointed out, with special reference to the efficiency and economy of the detergent. Attention is drawn to an improved “pendant drop” method (static), and its applicability is demonstrated by referring to a series of measurements of scientific and industrial interest.
CClr
n-Butyrio acid
1.595 -22.6 76.8 1.463
0.964
0.0s Infinite Infinite
Infinite Infinite Infinite
-4.7
163.5 1.898
Yet, if a small volume of each liquid is allowed to fall in drops from the tip of a medicine dropper, the average weight of a falling drop will be approximately the same for both liquids. Such behavior cannot be explained until it is realized that a controlling factor in determining the size of these falling drops is the energy required to form additional liquid surface. The value of this surface energy is not readily deduced from the properties of the main body of the fluid. According to our present concepts, the molecules of a liquid are in constant movement and they are held together by strong cohesive forces. I n the interior of the liquid these forces are acting in all directions and balance one another. However, a molecule near a boundary will experience an inward pull since the attractive forces of the molecules of the external phase are, in general, less than the cohesive forces acting between like molecules. I n order to extend the surface, additional molecules must be brought from the main body of the liquid. In this process, work is done against the cohesion of the liquid and energy is stored in the molecules which are moved to the surface. For many years it has been customary to substitute for “surface free energy” an equivalent “tension.” In general, it is called a “boundary tension.” More specifically, it is termed “surface tension” when it refers to a gas-liquid boundary or “interfacial tension” when it refers to a liquidliquid boundary. In any case it has the unit of dynes per centimeter.
HEN it is necessary to describe a liquid, it is customary to give its chemical composition and a few physical properties such as the boiling point, freezing point, and density. A more detailed description may include values for the viscosity, specific heat, thermal and electrical conductivities, index of refraction, and optical rotation. If the liquid system consists of two or more liquid phases or of a solid dispersed in a liquid, it is still common for the description to be given in terms of the over-all chemical composition and the physical properties of the material taken in bulk. Such a description is grossly inadequate for many purposes, even though the ratio of the phase volumes and the composition and properties of each of the phases are accurately and clearly stated. The properties which are commonly used to define pure substances are the properties of the main body of the material. However in the case of colloids, it is not these properties which are of controlling importance. The properties of the phase boundaries or interfaces are vastly more significant. Even
Boundary Free Energy Boundary free energy, or its mathematical equivalent boundary tension, is the outstanding property of liquid surfaces. I n the case of pure liquids which consist of a single molecular species, the surface tension willnormally be independen t of the age of the surface if the necessary precautions have been taken to exclude the presence of contamination and the action of external forces. Since the surface tension is a characteristic property of a material, phenomena which depend
1 Present address, The Teohnioolor Motion Picture Corporation, Hollywood, Calif.
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upon the surface tension can be predicted when its value is known. For example, a low surface tension favors the formation of fine sprays or foams. A low interfacial tension favors the stability of an emulsion. The relations are much more complicated in the case of solutions, It will usually be found that the boundary tension varies with the age and history of the particular piece of interface which is being studied. If the attraction between the molecules of the solvent is stronger than that between solvent and solute, progressive autocleansing of the interior of the liquid body will take place. When a molecule of the solute reaches the surface in the course of its random motion, it will not be subject to as strong an inward pull as are neighboring molecules of the solvent. Accordingly, the molecules of the solute will concentrate a t the surface, and the molecules of the solvent will concentrate in the main body of the fluid until a condition of statistical equilibrium is attained. This will often require the lapse of a considerable period of time. Accompanying this change in the composition of the phase boundary there will be tt progressive change in the value of the boundary energy. From these considerations it is evident that a knowledge of the boundary energy of pure liquids and solutions is a prerequisite to understand the behavior of finely dispersed liquid systems. These are the systems which a colloid chemist classifies as fogs, foams, and emulsions.
Practical Applications Before considering the commonly used methods for measuring boundary energy, a few fields will be mentioned in which problems occur requiring its study. Many additional instances of the application of a knowledge of boundary energy to practical problems are found in the literature (2). Beer brewing is one of the industries in which the chemistry and physics of surfaces are of outstanding importance. An empirical correlation between the surface tension and wetting characteristics of the mashes and the degree of enzymatic fermentation has been recognized in some breweries. Furthermore, the palatability of beer and the quality of its head are
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apparently linked with boundary energy. Beer contains positively charged colloids, and the clarification of the brew (coagulation of its proteins) depends upon colloidal reactions which take place at the interface where the beer contacts the walls of the fermenting vessel. There is need for much further work on the application of surface tension measurements to the problems of the brewing industry. The clarification of beer has an interesting analog which is not generally appreciated. Blood contains negatively charged particles. If these form a clot, there is danger of embolism. Systematic studies of the stability of blood stored in different containers has demonstrated that the time which elapses before curds form is inversely proportional to the ease with which the container walls are wet. It has also been demonstrated that healthy blood vessels are coated with a layer of low wettability. If the blood vessels become diseased, wettability increases and causes concentration of blood corpuscles at the interface. This may subsequently cause coagulation (IO). The importance of these facts in determining the technique of blood transfusion and the treatment for embolism is self-evident. Among the many fields in which surface tension is of direct importance is the. manufacture of paints, varnishes, sprays, insecticides, and commercial emulsions (4). Surface tension-is also a recognized factor in many problems of bacteriology and biochemistry. In recent years a number of valuable wetting agents have been discovered during the course of boundary energy studies. Applications for these have been found in the fields of detergents and ore flotation reagents (IS). A partial list of industries using colloids may suggest to the reader new possibilities in the application of surface tension measurements: adhesives, ceramics, dyeing, leather tanning, lubrication, paper , printing inks, rubber (latex), and soap.
Methods of Determination Of a large number of methods which have been suggested for the determination of boundary energy, only a few have received general recognition and application (5). The available methods can be divided into two groups, the so-called static and the dynamic methods. The static methods are designed to measure the energy of motionless surfaces. Among these are the capillary height, the maximum bubble pressure, and the pendant drop methods. Less accurate, but convenient because of their speed, are the methods based upon the formation of a film of liquid and its extension by means of a support which has been caused to adhere to the liquid, and those based upon the spontaneous detachment of drops from cylindrical supports ( I ) . The dynamic methods are based on the fact that the frequency of the periodic vibrations of fluid interfaces can be correlated with boundary energy. Dynamic methods include the measurements of 8 the wave length of ripples, the a oscillations of jets flowingfrom noncircular orifices, and the vibrations RhDIUS OF TIP CUBE ROOT OF VOL. OF DROP of hanging or freely falling drops. FIGURE1. EMPIRICAL CORRECTION CURVEFOR THE DROPWEIGHTMETHOD,DEYELGenerally speaking, dynamic OPED BY HARKINB AND BROWN methods are not applicable t o Difference8 are shown in drop formation and drop shape derived from tips of different diameter according determining the boundary energies t o Edgerton, Hauser, and Tucker (6)
r\
.
.
\
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Therefore no complete mathematical theory exists which permits the calculation of boundary energy from drop weight measurements. The method has gained in importance since FIGURE 2. PLATINUM an empirical correlation between drop size and tip radius RING OF A DU N o i i ~ TFNSIOMETER BEING (Figure 1) was developed several years ago (7). However, LIFTED OUT OF THE the method is of doubtful value in studying the aging of the SURFACE surfaces of solutions, The liquid is p o t pulled up in perfect cylindrical shape Undoubtedly, the most popular method is the du Nouy as called for by the mathering method. Although this method is simple and relatively matical evaluation of this method (8). easy to use, it has serious disadvantages which make the results questionable, at least as far as absolute values are concerned. It has been proved that the classical mathematical theory cannot be correlated with the actual mechanism (8). Furthermore, it is not suited to the accurate study of aging surfaces because a considerable disturbance of the surface is unavoidable when the ring is lifted from the liquid (Figures 2 and 3). Another method to determine surface tension is based on the resistance to lateral compression of surface films of insoluble substances on liquids (1, I d ) . This procedure has already proved to be of immense value in purely scientific research and has materially increased our knowledge of the arrangement of molecules in the surface of a solution. The necessity of extreme accuracy in the calibration of the instrument, the need of avoiding even the slightest contamination of the comparatively large exposed surface, etc., make it rather doubtful if this method can ever become of importance in industrial research. The method of pendant drops has recently been made Dractical bv imDortant changes in the technique of meas&nghrop shape aGd by 3. OF DU No'Y the development of new methods for calculatTENSIOMETER AT THE MOMENTOF CORing boundary tension from these measureRECT READING The double conical shape of liquid is in contrast ments. It is probable that i t will become a
of solutions, since the surface which is being studied is constantly disturbed. The capillary rise method is t h e only primary method in use a t present, a n d all other methods have been calibrated by it in the past. The basic method as well as its many improvements is t o oylindrical shape called for by mathematical evaluation (8) applicable to the study ofi a wide variety of materials, but it has several serious limitations. Its main use is for the measurement of the boundary energy of pure, nonviscous liquids which will wet the walls of the tube perfectly. The capillary methods are unsuited to the determination of values for liquid systems which show a change of boundary tension with time, because there may be a considerable lag between the change in boundary energy and the adjustment of the meniscus to the new equilibrium position. Its greatest drawbacks from a practical point of view are the difficulty of satisfactorily cleaning all the parts of the instrument which come in contact with the liquid and the difficulty I AUC _..,... of obtaining and calibrating suitable capillary tubing, Some noteworthv immovements have recently been made which 4. OF PENDANT facilitate {he mkasurements (9), but thebasic limitations remain unchanged. The maximum bubble pressure method has the disadvantage that each tube must be calibrated by measuring with it the known surface tension of a standard liquid. Besides this, the method does not seem to be applicable to the accurate determination of the boundary energy of concentrated solutions, since it is probable that the surface is disturbed during the formation and measurement of the bubble. One of the most commonly used methods is 1. 10-seoond aging 2. 60-second aging 3. 120-seoond aging 4. 180-seoond aging the drop weight method and its variant, the drop number method. This method involves FIGURE 5. CHANGES IN SHAPEOF A DROPOF 0.025 PBR CENTSODIUM STEARATE SOLUTION IN WATER AT 25" C. a great number of undetermined factors,
JANUARY, 1939
INDUSTRlAL AND EKGINEERING CHEMISTRY
most valuable method for the determination of boundary energy (3). The method of pendant drops has the outstanding advantage that the measurements are made without disturbing the interface which is being examined. This is accomplished by making an accurate silhouette photograph of the drop (Figure 4). The shape and size of the drop are then determined from measurements made on the photograph (Figure 5). Since photographs of the pendant drop can be made a t any number of definite time intervals, the method is particularly adapted to the determination of the rate of change of boundary energy with time. Moreover, the method is well adapted to the measurement of the boundary energy of liquids a t high or low temperatures as well as over an extended range of pressures. It is hoped that this new method will assist in furthering the study of the surface properties of liquids. The importance of boundary energy to a large number of industrial processes and reactions is now being appreciated, and surface physics may be expected to play an increasingly important role in the scientific developments of the future.
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Literature Cited Adam, N. K., “Physics and Chemistry of Surfaces,” 2nd ed., 1938. Alexander, Jerome, “Colloid Chemistry, Theoretical and Applied,” 1926-32. Andreas, J. M , Hauser, E. A , , and Tucker, W. B., J . Phys. Chem., 42, 1001-20 (1938). Clayton, W., “Theory of Emulsions,” 1935. Dorsey, N. E., U. S. Bur. Standards, Sci. Papers, 21, 563-95 (1926). Edgerton, H . E., Hauser, E. A., and Tucker, J . Phys. Chem., 41, 1017-28 (1937). Harkins, W. D., and Brown, F. E . , J . Am. Chem. SOC.,41, 499524 (1919). Hauser, E. A., Edgerton, H. E., Holt, B. M., and Cox, J. T., Jr., J . Phys. Chem., 40, 973-88 (1936). Jones, G., and Ray, 1%‘.A., J . Am. Chem. Soc., 59, 187-98 (1937). Lambert, H., “Die physikalische Seite des BlutgerinnungsprobIems,” 1931. Lange, Handbook of Chemistry, 1937. McBain, J. W., Ford, T. F., and Wilson, D. A., Kolloid-Z., 78, 1 (1937). Petersen, W., “Schwimmaufbereitung,” 1936. RECEIVED September 12, 1938.
APPLICATION OF FILM BALANCE TO SURFACE OF ORDINARY SOLUTIONS1 J. W. MCBAIN AND L. H. PERRY Stanford University, California
Much new and unsuspected information about the surfaces of ordinary solutions is obtainable by applying the film balance developed for insoluble films on water. The results are wholly different from the familiar conception of a two-dimensional adsorbed layer lying on unchanged solution. They rather correspond to an appreciable depth of surface and are summarized in the section entitled “Types of Solutions Revealed by the Film Balance.” The film balance does not measure the ordinary surface tension of solutions.
U
NTIL recently the film balance has been used only for the study of insoluble films on water. Beginning with it has been the original hydrophile trough of Pockels (I), developed by Langmuir, Adam, and others into a common laboratory tool and is in extensive use for measuring the surface pressures or force areas of insoluble films. 1 Since this manuscript was submitted, Doss published a valuable communication [Kolloid-Z., 84, 138 (1938)l stressing t h a t the pellicle is formed by slow activated adsorption, and t h a t i t is connected with a tendency t o froth. However, the best frothing agent studied here, lauryl sulfonic acid, produces a surface film t h a t is soluble. Doss, like Ford and Wilson, assumes that the pellicle is monomolecular but, unlike them, further assumes that i t does not dissolve while being collected and compacted b y sweeping towards the float.
Doss (1) and McBain and Wilson (8) pointed out that the film balance may be usefully applied to the study of clean surfaces of ordinary solutions of soluble substances free from insoluble contamination. The purpose of the present communication is to describe some of the many ways in which the film balance may be used in studying these ordinary solutions and to give typical results. Several characteristic properties have been discovered. It is evident that the surface of an ordinary solution is very different from the familiar conception of a twodimensional adsorbed layer lying on unchanged solution. In general, with solutions the film balance does not measure force area or actual surface tension. Indeed, if the area of the solution is changed not too fast or too much, no effect is observable with the film balance. The surface of solutions is deep, and surface tension is partially determined by molecules that are well submerged. The float on the film balance used here dipped well below the surface; it consisted of a 1em. strip of platinum-iridium, 11.5 em. long and bent longitudinally into the shape of an inverted V; only the fold was “Bakelized” to make that portion nonwettable. It was connected to the sides of the trough with flexible gold foil in the usual manner. In working with solutions, not only must contamination be rigorously avoided, but the solution must be kept in a saturated atmosphere to prevent evaporation; otherwise the results must be illusory. The following problems may be studied with the film balance : 1. Whether or not after (a) sweeping or (b) compressing, surface is “reversible”; that is, its original properties are
the
