SURFACE-ACTIVE AGENTS’ INTRODUCTION F. E. BARTELL University of Michigan, Ann Arbor, Mioh.
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URFACE chemistry has until recently been regarded as a subject to be discussed most profitably in the classroom and the fundamental research laboratory. The comparatively recent contributions of Hardy, Langmuir, Harkins, Adam, Rideal, and others have been so productive of new theories and concepts that many research workers have turned their attention to this field. In the future a study of surface chemistry will not be limited to classical treatments of surface and interfacial tensions but will be rapidly extended to bear upon practical problems of every branch of applied science. Industry is just beginning to become surface conscious and to realize that it is possible to place upon a scientific basis the development of products such as flotation agents, wetting agents, lubricants, protective coatings, etc. An indication of the active industrial interest in surface problems and of the diversity of processes involved can be obtained from the papers on “Wetting and Detergency” presented a t a symposium held in Lopdon under the auspices of the British Section of the International Society of Leather Trades’ Chemists during February, 1937.2 Included in the list were papers dealing with textiles, skin, hides and leather, insecticides, fungicides, suspensions, pigments, paints and varnishes, metals, solders, and ore flotation. Even that diverse list does not begin to cover the field in which surface chemistry may be of great importance. Surface chemistry is concerned with the magnitude of free surface energy existing a t surfaces and with the magnitude of change of free surface energy which occurs as the surface environment becomes altered. Since every homogeneous substance has a surface or interface, each and every form of matter existing as a separate phase possesses interfacial energy and is thus capable of undergoing certain changes or of doing certain work. When the substance is finely divided, or in the form of a film, or in any form such that its specific surface area is very great, it possesses correspondingly great free surface energy and is capable of doing work. The behavior of the system may be largely dependent upon the magnitude of the interfacial free energy or the change in this energy content, and a knowledge of the surface chemistry of the system becomes highly desirable. A word of caution should be sounded to the effect that our fund of knowledge relating to this field is still limited and that known methods for obtaining additional knowledge are comparatively few in number; some of them are not fully developed, others are time consuming and demand technical skill of a high degree in their execution. There are five different types of interfaces possible-gasliquid, liquid-liquid, gas-solid, liquid-solid, and solid-solid. The magnitude of the interfacial free energy of any system can be determined, provided measurement of interfacial tension can be made. Suitable methods are available for the measurement of the interfacial tensions of the systems gas-liquid and liquidliquid. As yet no reliable methods are available for measurement of the interfacial tensions a t the solid interfaces (gas-solid, liquid-solid, or solid-solid). Attention may be called, however, to the fact that changes occurring in the free surface energy a t solid interfaces have been determined by measuring the adhesion tension of liquid-solid systems. Adhesion tension determinations involve the measurement of angles of contact formed between liquid and solid systems; while such determinations may be of considerable industrial importance, the methods as yet available are on the whole so difficult and time consuming that they are not of general application. The present symposium is not concerned primarily with the measurement of interfacial energy or changes in interfacial
energy; it is concerned with the consideration of surface-active agents-that is, agents which are capable of doing work at interfaces-and with the direction of the changes of interfacial (or surface) energy effected by these agents. At the outset it seems desirable to emphasize the fact that the action of any given surface-active agent is specific. For example, an agent which will make water “wetter” when water is brought into contact with organophilic material, may make the water “less wet” when the water is brought into contact with hydrophilic material. The mere statement that a given substance is a surface-active agent is, then, without significance unless information is furnished as to the precise nature of the surface or interface under consideration. We must know not only the nature of the solid concerned but also the nature of the liquid phase. Suppose, for example, that a surface-active agent is put into a beaker in which a solid is suspended in a liquid. It is often assumed that a surface-active agent will affect all interfaces within the system in a similar manner. For example, it is assumed frequently that much adsorption of the agent will occur at the interface air-liquid and thus lower the surface tension of the liquid against air, and that at the same time much of the agent will be adsorbed at the interface solid-liquid and lower the interfacial tension between solid and liquid. In many cases high adsorption of agent at both sets of interfaces does occur, but it does not necessarily follow that it will occur in all cases. In fact, the tendency of the agent to be adsorbed a t a given interface depends not only upon the precise nature of the agent but also upon the nature of the interface concerned. Surface-active agents are polar compounds and their activity is dependent upon the ability of their molecu1,es to become oriented and adsorbed a t an interface. The tendency to orient is dependent upon each of the phases in contact a t the interface. When carbon, as adsorbent, is present in water, we can predict that adsorbate ROH will give oriented adsorption
1 3
-ROH
(aqueous solution)
with the nonpolar R group oriented toward carbon and the polar OH group toward water. If silica were present as adsorbent, however, a similar prediction could not be made with certainty. Were carbon present as adsorbent in nonpolar benzene, we could not predict whether molecular orientation would occur at the interface carbon-benzene. I n this case both the carbon phase and the benzene phase have a higher affinity for the R group than for the OH group, so that we could not predict with certainty whether any positive adsorption would occur. To date but little is known concerning surface-active agents for organic liquid systems. It is hoped that in the near future fundamental researches on adsorption from organic media will contribute constructive information along this line. The papers to be presented in this symposium treat of two types of interfacial systems-namely, liquid against air and liquid against solid. Two papers (pages 32 and 35) place special emphasis upon conditions encountered a t liquid-against-air interfaces, and disclose new methods of a proach which are now being developed and used in the study ofsuch systems. Three papers (pa es 40, 48, and 51). deal specifically with problems related to Yiquid against solid interfaces. These problems include the formation of asphalt emulsions with solid emulsifying agents, depressors in ore flotations, preventatives of carbonate scale formation, peptizing agents, and surface-active dyeing agents. The sixth paper (page 44) discusses the rapidly growing interest in wetting agents, presents a discussion of the theory underlying the nature of surface-active compounds, and gives a classification of types of the better known wetting agents. It is hoped that this symposium will serve both to initiate and to stimulate interest in this comparatively new and important field.
The six papers whioh follow (pages 32 to 57) were presented as a Symposium on Surface-Active Agents before a joint meeting of the Divisions of Industrial and Engineering Chemistry and of Colloid Chemistry at the 96th Meeting of the Amerioan Chemical Society, Milwaukee, Wis., September 5 to 9, 1938. * Published by Chemical Publishing Company of New York, Ino.
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