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T. H. HAZLEHURST AND HARVEY A. NEVILLE
CYBOTAXIS AT INTERFACES T. H. HAZLEHURST AND HARVFY A. NEVILLE William H. Chandler Chemistry Laboratory, Lehigh Uniuersity, Bethlehem, Pennsylvania
Received September SO, 1030
Evidence for the existence of structural groups in liquids is provided by numerous investigations and was reviewed by Stewart (5), who referred to the phenomenon as cybotaxis and to these molecular swarms as cybotactic groups. The report of a more recent symposium on “Molecular Forces in Pure Liquids and Solutions” (6) shows general acceptance of some degree of orderliness in the structure of liquids although disagreement as to details. Differing views of the nature of cybotaxis range from that of Bernal (l),who merely assumes statistical swarms of vaguely ordered molecules, to that of Irany (4), in which a liquid is considered as two separate phases, the one consisting of two-dimensional sheets of highly ordered molecules enclosing in its interstitial spaces the other phase which is completely disordered or gaseous. The present paper follows the approach of Frenkel (2)) who pictures a liquid as composed of molecules so closely spaced that they are never even approximately gaseous and are occasionally grouped into swarms approaching crystallinity. These swarms are assumed to possess a certain shape and rigidity, but their existence is transitory and their surfaces merge imperceptibly into the surrounding completely disorganized liquid. THE EFFECT OF INTERFACES
A cybotactic group is analogous to a crystal in equilibrium with its melt. Its surfaces are being constantly eroded to produce disordered liquid, while new cybotactic groups are constantly being formed. Since there is no single molecule or group of molecules sufficiently distinguished in structure or properties, there is no unique point capable of serving as an enduring nucleus for cybotactic groups. If some point or region of unique properties is provided which can act EM a permanent and permanently fixed nucleus or anchorage for cybotactic groups, these may become stronger and more persistent, approaching the orderliness and stability of crystals. Such a region is provided by an interface of any type, for the nature of an interface necessitates that the forces active there are different from those in the interior of either of the adjacent phases. It is, therefore, logical to expect that interfaces which offer permanent location of directed forces will stabilize normally fugitive cybotactic groups. Naturally the intrinsic stability of cybotactic groups will depend upon
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the properties of the liquid molecules themselves. Completely isotropic molecules, such as those in the so-called monatomic liquids, will form cybotactic groups of far less stability than those formed by anisotropic molecules. The energy required to disperse cybotactic groups may be regarded as analogous to a heat of fusion. All cybotactic groups are subject to vibrational motions similar to those in crystals and this agitation may be sufficient to destroy the grouping, but when directed forces are chiefly responsible for the preservation of the group, its dispersion will depend to a greater extent upon the rotational movements of the constituent molecules. Obviously, linear or plate-like molecules which orient in favored patterns will have a lower rate of rotation and will meet a greater frictional resistance than will those of spherical form. An extreme example of the spherical molecule is provided by helium, in which the directed forces are so weak that the discontinuity of its liquid-solid transformation is uncertain. At the other extreme, certain elongated molecules with polar groups at the ends or middle have directive forces so strongly developed that they are able to form a separate phase of liquid crystals. LIQUID-GAS
INTERFACE
While the cybotactic groups throughout the bulk of an ordinary liquid will have only a fleeting existence, the coherent and quasi-crystalline nature of liquid surfaces is fairly well established. Among the effects to be expected at liquid surfaces as a result of this structure are (1) that not every molecule striking the surface from the vapor phase condenses and ( 2 ) the formation of primary “rolling drops’’ of the liquid upon its own surfacc Actually only a small fraction of the impinging vapor molecules condenses, -as expressed by the “accommodation coefficient.” In an earlier paper (3) the authors presented a table of critical heights as a measure of tht. tendency of liquids to form rolling drops and showed that this property, and hence the stability of‘ the liquid film, is proportional to the anistropy of the molecules concerned. By extending‘these measurements to solutions it was demonstrated that these surface films are not only resistant to rupture but have a definite structure, for the surface film of the drop and that of the bulk liquid must very nearly match or rolling drops will not form. As illustrated in figure 1 and discussed below, the stability of an air bubble a t a liquid-liquid interface is conditioned by cybotaxis in the lower liquid a t the liquid-gas (bubble) and liquid-liquid interfaces. LIQUID-LIQUID
INTERFACE
The structure of the surface a t a liquid-gas interface is determined almost entirely by the nature of the liquid; the liquid structure at the boundary between condensed phases (liquid-liquid or liquid-solid) depends
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CYBOTAXIS AT INTERFACI~B
AIB BUBBLEB
DROPB OF OTHlll U Q U l D
LIQUID PAIB'
Water-benzene.. . . . . . . . . . . . . . . . . . . . . . . . . . . Water-petroleum ether ....... Water-kerosene . . . . . . . . Water-isoamyl alcohol. . . . . . . . . . . . . . . . . . . . . Water-Nujol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon tetrachloridewater. . . . . . . . . . . . . . . . Carbon disulfide-water . . . . . . . . . . . . . . . . . . . . Yitrobenzene-water . . . . . . . . . . . . . . . . . . . . . . . Methylene iodidewater. . . . . . . . . . . . . . . . . . . 6 N Ammonium acetate-benzene. . . . . . . . . . 6 N Sodium acetate-benzene., . . . . . . . . . . . . . 6 N Ammonium hydroxide-benzene 6 N Acetic acid-benzene Carbon tetrachloride6 tate . . . . . . . . . . . . . . . . . .
Wtaide
Oaide
WSide
Oside
u
S S S S S S S S S S S S
8
S
8
8
8
S
U
S
U
U
S S
8
8
U
8
S
8
8
8
8
8
S
8
S
S
_ _ _ _ _ _ ~ _ _ _ Ut S S 8
U U U s
U U S U
S
* The denaer liquid in each pair is listed first.
t W = water; 0 = organic liquid; U = unstable; S
= stable; s = slightly stable.
example, benzene has a positive spreading coefficient on water and should displace water at a water-air interface or preferentially wet a bubble of
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air. However, paraffin oil (Nujol), carbon disulfide, and methylene iodide have negative spreading coefficients, while that of carbon tetrachloride is doubtful. It would, therefore, not be expected that these liquids could displace or cover water a t the water-air boundary, j e t they will retain an air bubble a t their interfaces with water permanently, as will those liquids which have positive spreading coefficients. From the experimental data, and as represented in the diagrams, it may be stated as a general conclusion that a t water-organic liquid interfaces an air bubble will penetrate the interface readily from the water side but not from the side of the organic liquid. Likewise, drops of the organic liquid will usually break through this interface a t once or in a few seconds from the water side, but water drops are generally retainedat the interface for a longer time or indefinitely on the opposite side. As indicated above, these results cannot be predicted directly from surface energy relationships, but are apparently conditioned by the degree of cybotaxis on the two sides of liquid-liquid interfaces. If this viewpoint is significant, a liquid-liquid interface should be considered not merely as a boundary plane of negligible thickness, but rather as a structure having two sides which differ in character and in extension into the two bulk phases. As a general rule, the stability of liquid drops a t a liquid-liquid interface is a t best less than that of air bubbles, which may persist there for several days. It may be suggested as a tentative explanation of this difference that the cybotactic condition in the film between the bubbles and the upper liquid is strengthened by having gas on one side and a condensed phase on the other side. If the film has condensed phases on both sides, the simultaneous action of strong forces from opposite directions might result in a smaller net directive influence, whereas a strongly unbalanced force would be more effective in inducing and maintaining cybotaxis. It will be noted from the data in table 1 that certain aqueous solutions, such as those of ammonium acetate and acetic acid in moderate concentration, exhibit the cybotactic behavior of an organic liquid. Other solutes, including common mineral acids, bases, and salts, in equivalent concentrations do not appreciably increase the cybotactic condition of water. Extensive investigation of cybotaxis in solutions has not been undertaken. It should be emphasized that the bubbles or drops which are retained at a liquid-liquid interface are not in contact with the two liquids simultaneously but are completely surrounded by that liquid phase in which they rest. The bubble or drop is a slightly distorted sphere and its contour is continuous. The photographs are somewhat misleading in this respect, owing to reflection and refraction. Excellent evidence for the continuity of the film of the lower organic liquid around air bubbles is provided by the transport of a quantity of this lower liquid to the top surface of the water when the bubbles are enlarged sufficiently to cause them
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to break away from the lower liquid by their buoyancy. By blowing a stream of air through the lower organic liquid (e.g., carbon tetrachloride) the latter can be efficiently pumped to the top of the water. Of course, the carbon tetrachloride drops back through the water after releasing the air bubbles except for the quantity which can be supported as a disc upon the water surface. This process of circulation provides a convenient means for bringing about intimate contact between two immiscible liquids if the lower liquid is sufficiently cybotactic. This effect does not obtain when water is the lower liquid. It occurred to the authors that the pumping of carbon tetrachloride to the surface of water by means of air bubbles is analogous to the Frasch method for mining sulfur and involves the same principle. An experiment was performed in which air was bubbled through molten sulfur under a glycerol-water solution a t about 125°C. Small air bubbles were quite stable in the sulfur phase a t the liquid-liquid interface, and large bubbles lifted a surrounding mass of sulfur to the top of the aqueous phase where it released the air and could be recovered by some simple means. This experiment provides a demonstration of the operation of the Frasch process suitable for laboratory or lecture. It follows from the experimental results discussed above that if factors other than cybotaxis are eliminated, a water-in-oil emulsion will be more stable than the conjugate system. In the absence of an emulsifying agent there is no reason to conclude from purely energetic considerations that the one or the other system would be more stable, since the interfacial tension is identical in the two systems and the difference in density is likewise a common factor. A difference in stability of the two conjugate emulsions may, however, be predicted if the drops of one liquid penetrate films of the other liquid less readily as a result of cybotaxis in the liquid which constitutes the external phase. By analogy with this reasoning it follows that an emulsifying agent will stabilize that system in which it is preferentially in the external phase,-a general principle which has been arrived a t from other considerations and which has experimental support. LIQUID-SOLID
INTERFACES
At the surface of a solid the directive forces are stronger than a t a liquid surface, for it is just this surplus which induces the solid state. It is therefore to be expected that cybotactic effects will be most evident in liquids where they are in contact with a solid. Furthermore, it may be reasoned that the extent of tactoid formation will be greatest adjacent to “active centers” of the solid surface-at edges, corners, extra-lattice crystal unitsand will thus be favored by irregular, anisotropic particles rather than by spherical forms. Thixotropic phenomena may be explained as resulting when cybotactic groups extend sufficiently ae spines or tentacles from active
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H A Z L E B ~ S TAND HARVEY A. NEVILLE
centers on neighboring particles to form bridges of quasi-crystalline material which will offer resistance to lateral shear. A more extended discussion of thixotropy from this viewpoint will be offered in a subsequent paper. DUAL LIQUID CONTACT
In contrast to the conclusion relative to bubbles and drops held a t a liquid-liquid interface, it appears that solid particles of some materials may rest in contact with both liquids simultaneously a t such an interface. The stabilization of emulsions by certain solid powders is conditioned by the equilibrium of surface forces in this dual wetting. When a bead of glass, fused borax, or sodium hydroxide is dropped upon a liquid-liquid interface from the side of the organic liquid, it is arrested momentarily in the organic liquid before breaking through the interface, and then drops to a stable position in contact with both liquids. The con-
FIG.3. Dual liquid contact. Glass bead and sodium hydroxide bead stable in dual contact; sodium hydroxide bead dissolving. Drop of sodium hydroxide solution and air bubble not in contact with water. trast of this condition with the behavior of a liquid drop is shown in figure 3 for solid sodium hydroxide and for a drop of its concentrated solution. That the solid pellet is in contact with the underlying layer of water is indicated by the visible solution lines or schlieren as the denser solution flows down through the water. The absence of these lines below the drop of solution a t the same interface proves that the drop is not in contact with the water but is entirely surrounded by the organic liquid. When it does break through the interface it goes a t once and as a whole into the water phase. It is obviously impossible for a liquid drop to preserve a recognizable outline after breaking through an interface if the drop is miscible with the liquid into which it enters. However, if it is immiscible with both liquids, it may exhibit two stages of rest,-the first due to cybotaxis and the second to an equilibrium of surface forces with the drop in contact with both
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liquids. Mercury droplets if sufficiently small will hang indefinitely a t liquid-liquid interfaces. The size of the drop is a factor since, if it is too large, the strong pull of gravity causes the upper liquid to neck off, and the drop, surrounded by a shell of the upper liquid, falls before it has had time to penetrate the cybotactic film. In this manner globules of lighter organic liquid may be carried down through water and retained a t the bottom of a vessel, as illustrated for benzene in figure la. By using the system water-aniline-kerosene, which forms thFee layers in that order (when the last two are mutually saturated), large density differences were avoided. It is possible to hang quite large drops of water a t the aniline-kerosene interface if the proper technique is employed. If the water drop is allowed to fall freely onto the interface it goes through a t once, because the interfacial tension between aniline and kerosene is very low and even a small force is capable of causing the kerosene to neck off. Each water drop is then quite clearly surrounded by a rather thick shell of kerosene as it falls through the aniline, and this kerosene subsequently streams upward. If, however, the water drop, while still adhering to the glass capillary, is carefully manipulated a t the interface so as to break the kerosene film and bring the drop into contact with both aniline and kerosene, it will hang indefinitely a t the interface and may be enlarged a t any time by adding more water through an inserted capillary. Definite proof that the hanging drop is in contact with the aniline phase is provided by dissolving in the water a dyestuff (National safranine 8B) which is soluble in water and in aniline but insoluble in kerosene. The hanging purple drop loses all of its color to the aniline. It may be concluded that liquid drops may rest a t liquid-liquid interfaces in defiance of gravity for either of two reasons: (1) because they fail to penetrate the cybotactic layer or ( 2 ) because the three interfacial tensions have values which permit equilibrium, that is, none is greater than the sum of the other two. The first effect is operative in all cases to some degree but is impermanent; the second is operative only when (a) the drop is immiscible with both liquid layers and ( b ) the interfacial tensions satisfy the above condition. In this respect liquid drops and solid beads exhibit the same behavior. Bubbles of gas appear to be exceptional in the matter of dual liquid contact. All expji’ments designed to show that bubbles were in contact with both liquids gave negative results. For example, a bubble of ammonia is stable without contraction under kerosene covered by water. If bubbles could exist in contact with both liquid phases the condition governing the equilibrium of interfacial tensions should be satisfied, but bubbles of air will be retained below the interfaces in both spreading and non-spreading liquids when water is the upper liquid. Theoretically, from surface energy relations, an air bubble should be stable a t a Nujol-water interface in
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T. H. HAZLEHURST AND HARVEY A. NEVILLE
contact with both liquids and, it is to be emphasized, the same condition should obtain regardless of whether the bubble is introduced from the water side or the Nujol side of the interface. Actually, as recorded in table 1, the bubble is stable only as a result of cybotaxis in the organic liquid. SUMMARY
Molecular swarms or cybotactic groups in the bulk of liquids have only
a transitory existence for lack of an anchorage or directive influence. This is provided by an interface of any type and here the cybotactic effects will be stronger and more persistent, possibly approaching the orderliness and stability of crystals. Cybotaxis a t liquid-air interfaces accounts for the existence of primary rolling drops, and “critical heights” for different liquids provide a relative measure of this effect. Analogous experiments are made at liquid-liquid interfaces with drops of either liquid or bubbles of air. The stability of a bubble a t a liquidliquid interface depends upon cybotaxis in the lower liquid. In general, at water-oil interfaces an air bubble will break through from the water side but not from the oil side; oil drops will break through from the water side, but usually water drops will not from the oil side. If factors other than cybotaxis are eliminated, a water-in-oil emulsion is more stable than the conjugate system. By analogy with cybotaxis, it follows that an emulsifying agent will stabiliee that system in which it is preferentially in the external phase. In thixotropic systems cybotactic groups extend as spines or tentacles from active spots on neighboring particles and form bridges of quasicrystalline character which offer resistance to lateral shear. Drops of an immiscible liquid or solid particles may rest a t a liquidliquid interface in contact with both liquids if equilibrium conditions of interfacial energies are satisfied. Dual liquid contact is apparently not possible for bubbles of gas. REFERENCES (1) (2) (3) (4) (5) (6)
BERNAL: Trans. Faraday SOC.3S, 27 (1937). FRENKEL: Trans. Faraday SOC.SS, 58 (1937). HAZLEHURBT AND NEVILLE: J. Phys. Chem. 41,1205 (1937). IRANY:J. Am. Chem. SOC. 61, 1436 (1939). STEWART: Phys. Rev. 29, 919 (1927); SO, 232 (1927); Chem. Rev. 6,483 (1929). Sm~osrrrar:Trans. Faraday SOC. 38, 1-283 (1937).
