Langmuir 1998, 14, 5685-5690
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Articles Fluorocarbons as Volatile Surfactants Yuri Yu. Stoilov* P.N. Lebedev Physical Institute of Academy of Sciences, Leninsky pr. 53, 117924 Moscow, Russia Received December 8, 1997. In Final Form: April 29, 1998 Surface active properties of volatile vapors of fluorocarbon substances such as C6F14, C8F18, and C10F18 have been discovered. In a new phenomenon of capillary instability some perpetual oscillatory and rotary movements of many liquids take place due to a change of their surface tensions by 10-30% at contact with C8F18 vapor at room temperature. Features and possible applications of volatile surfactants are discussed.
Introduction The surface tension of a liquid is determined by the composition of its upper layer, which can be changed by the addition, for example, of a small amount of soluble surfactant molecules concentrated in the upper layer. Any gases of small density over liquid usually have little effect on its surface tension. But, is this always true? The results presented here will show that the vapors of fluorocarbons at low pressure greatly change the surface tension of many liquids and produce some new surface phenomena. Experiments Here are some curiosity-driven experiments that attracted my attention. (1) An open glass cuvette of 3-6 cm diameter and 4-6 cm high is filled with C8F181-3 and water. A 5-15 mm layer of C8F18 is at the bottom and forms a plane surface above which there is a layer of water with thickness about 3-5 mm. Due to a peculiarity of water interaction with C8F18, this water layer has two stable forms: one is like a toros near walls rising above the C8F18 surface by about 2 mm (with a free central zone of C8F18, as shown in Figure 1), and the other is a continuous plane layer, covering the whole surface of liquid C8F18. These two forms are easily transformed into each other. If one blows with an air jet on the plane water surface, then its central zone is moved apart to the C8F18 level and water takes a torus form. If the cuvette with the torus is subjected to a small vertical acceleration, then the torus is collapsed and water takes the form of the plane surface. Such partition is also possible by a cuvette tilt, when the water takes a torus form adjacent to the cuvette walls. Thus, inside the torus there is a stable funnel with width 10-15 mm (Figure 1), the bottom of which represents an open surface of liquid C8F18. Some drops of liquid dibutyl phthalate (DBP) (10-20 mm3), colored for observation by any dye, are put on the funnel walls and form there a continuous ring of 0.2-1 mm thickness, while keeping free the central part of the C8F18 surface of about 8-10 mm in diameter. When due to usual liquid evaporation in air the pressure of C8F18 in the open cuvette is lowered and an evaporation of new C8F18 molecules takes place, the dividing borders of the liquids * E-mail:
[email protected]. (1) Industrial fluoro-organic products; Chemistry: Leningrad, 1990; p 333. (2) Flouride compounds. Synthesis and application; Isicava, N., Ed.; Mir: Moscow, 1990; p 13. (3) Firm Perftoran (http://www.perftoran.ru).
spontaneously start an oscillatory movement. The delay time of the oscillations (1-3 min) depends on the cuvette height, but it can be sped up by a fast removal of vapor from the cuvette (by a blow or pump out). When some small drops of DBP staying on the water surface merge with the central ring, the ring comes into movement that begins periodically 1-5 times per second (depending on the viscosity of the liquid) to throw out on the water surface waves of DBP with an amplitude of 0.1-1.5 cm, which then return back to the ring (Figure 2). In this process the water surface becomes colder by about 0.3-1.0 °C than C8F18. This oscillatory filmlike movement of mutually saturated liquids proceeds for several days up to an almost full evaporation of one of the liquids. On hermetic closing of the cuvette, the process ceases in 5-10 min after achieving an equilibrium vapor pressure in the whole cuvette volume. In an open cuvette the process ceases when a strip of paper wetted in liquid C8F18 is lifted above the level of water (for the time of C8F18 evaporation from the strip) or when a warm needle is used to heat up the surface of C8F18 near the ring by about 10 °C or when the surface of the water is cooled. The wave movement cycle is repeated with different periodicities in different sectors. The movement and generation of the two-dimensional waves are caused by some nonuniformity of surface tension and observed in a wide range of temperatures (such as 4-50 °C) and in cuvettes of various forms. When the cuvette is open, the total weight reduction due to evaporation at room temperature can be less than 0.1 g per hour. When the cuvette is hermetically sealed and has a closed cycle of condensation on a cooler and returning of the liquids, the process of oscillatory movement lasts till the temperature difference between the liquids and the cooler is more than 2 °C. The fluorocarbon C8F18 can be replaced with an equal volume of, for example, perfluorodecaline C10F18.1 The substance DBP can be replaced with an organic and inorganic liquid unmixed with water and C8F18: turpentine, dimethyl phthalate (DMP), oil, ether, cyclohexane, and so forth. No special safety precautions are needed, because saturated fluorocarbons are harmless colorless liquids, without a smell, which are chemically completely inert and are used in medicine for artificial blood.2,3 (2) When a drop of C8F18 or DBP (or benzene) of about 0.1 cm3 in volume is put separately on a surface of soft water (it is enough to dip and take out from the water a piece of soap), this drop takes a lenslike form due to surface tension and hereinafter quietly lies on the water, slowly decreasing in size because of evaporation, but without changing form. When the same liquids, C8F18, DBP (benzene), and water, are put in a cuvette by layers, they do not react with each another and after shaking are again divided into layers. However, when in a wide open vessel of 15-30 cm in diameter and 2-4 cm high a drop of C8F18 and another drop of DBP are put on the water surface together at a
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Figure 1. Side view of the cuvette with three unmixed liquids (a top view is shown in Figure 2): 1, vessel wall (distance between walls is 20 mm); 2, a toroidal region of water 5-8 mm high in contact with the walls; 3, air; 4, DBP-dye ring 2-5 mm high, 8-10 mm in diameter, and 0.2-1 mm thick, adjacent to water; 5, liquid C8F18 lying at the cuvette bottom. distance of 2-3 cm from each other, these chemically inert drops approach each other and begin for a few minutes to interact intensively in a complex physical process, resembling the behavior of living creatures, with repeated trembling, approaching, and separation, with formation of films and new droplets up to a full evaporation of one of the drops. A small softness of water is necessary here for a reduction of its surface tension and for formation on its surface of the drops, because on pure water the drops are stretched out in a thin film. When a piece of ice is put on the water surface near the drops, the described interaction of drops slows down or does not occur at all. We assume that the complex interaction of drops is accompanied by their periodic cooling (in evaporation) and heating (from water). (3) An open glass cuvette of 3-6 cm in diameter and 4-6 cm high is filled, as in the first experiment, only with two liquids: with C8F18 and ethanol (or DBP, benzene, kerosene, 2-propanol, machine oil or silicon oil, glycerol, acetone, CCl4, and so forth). A C8F18 layer 5-15 mm high lies at the bottom and forms a plane surface, on which the 3-5 mm layer of ethanol lies as a torus (or thick ring) near walls rising above the surface of C8F18 by 1-2 mm (as water in Figure 1). There is a free zone of the C8F18 surface 5-15 mm in diameter at the cuvette center. Some smallsized solid particles of cotton wool or wood ashes can be added to the ethanol as passive markers for observation. In the open cuvette under usual evaporation of liquids the torus of ethanol comes into continuous rotation in a vertical plane as a vortex, when on its top surface the particles go at a linear speed of 1-10 cm s-1 from a center edge to the walls and then are lowered down almost up to the border with C8F18 and more slowly return back again to the center edge of the ring. Under rotation the diameter of the free area of C8F18, despite the torus centrifugal forces, usually increases a little bit. The cuvette width has a little significance, as the same rotation of liquid rings was observed in different cuvettes from 0.2 to 10 cm in diameter. Coming to the wall, a part of the ring ethanol is lifted up on the wall as a thin layer to a height of several millimeters above the liquid level and then forms over the whole cuvette perimeter a chain of small-sized ethanol drops 0.1-0.2 mm in diameter. For benzene this chain is made of larger 2-3 mm drops and is formed at a height of 20-30 mm above the liquid level. These drops then go down to the level of the liquid. The continuous rotation of the ring lasts for days until an almost complete evaporation of one of the liquids. The rotation is observed in a wide range of temperatures (more than 0-50 °C) and in different forms of cuvettes. It is slightly sensitive to heating or cooling of C8F18 or ethanol surfaces and is enhanced by the slightest external flow of air. In an open cuvette the total loss of weight due to evaporation at room temperature is about 0.5 g per hour. In a hermetically sealed cuvette, with a closed cycle of condensation and returning of liquids, the process of rotation goes permanently even at very small differences of temperatures between the liquids and the cooler (less than 0.1 °C). It is also observed even at a very small speed of condensation of liquids with the total volume of about two drops per hour
(about 10 mm3 per hour). In a hermetic cuvette without air, the ring of ethanol (or 2-propanol) rotates constantly (for many months) at room conditions when there is water at room temperature in the cooler. Cooling of water due to its evaporation in air creates the necessary difference of temperatures for condensation of vapor and perpetual rotation of the ring. A crude estimate of the power required to support such liquid ring rotation is of the order of 10-5 W. Illumination of an ethanol ring, colored with the dye Coumarine 6, with direct beams of solar light intensifies the rotation, which represents a transformation of light energy into thermal and mechanical energy. The described rotation in the ethanol ring practically ceases after addition to it of about 40% water. However, pure water behaves differently. In a ring of water above the layer of C8F18 no rotation is present; it arises only with a reduction of water surface tension (by soap or shampoo) and has a small linear speed of movement (about 0.01-0.03 cm s-1). The rotation of an ethanol ring becomes faster with addition to it of 10-50% ether. On a surface of C8F18 a ring of pure ether (as well as benzene, kerosene, 2-propanol) rotates with high speed. The ether partially mixes up with C8F18; pentane is mixed up completely: benzene, ethanol, DBP, kerosene, and 2-propanol are mixed to a small degree. The mutual solubility of liquids with C8F18 is a little bit increased with temperature, and a subsequent lowering of temperature causes a turbidity of C8F18 for a few minutes due to a mixing reduction. A local increase of the concentration of C8F18 molecules evaporated from the film results in their fluctuation in the cuvette volume and in diffusion out of it. Using smoke, it is possible to trace the gas flow in the cuvette with the rotating ethanol ring. In a thin layer 0.1-0.3 mm above the ring the gas flow goes at the same speed and in the same direction as the top layer of a liquid from the center to walls, then goes upward for 2-3 mm, and more slowly returns back to center, forming a gas layer above the liquids. As the upward gas movement occurs only near the walls, it is possible to conclude that the main evaporation of C8F18 molecules takes place from the elevated liquid film on the walls and not from a large central free surface of liquid C8F18. This means that the transportation of C8F18 molecules to the top cuvette sections by a running liquid film occurs faster than that by diffusion from the free surface of liquid C8F18. There is also a vertical rotation in the bottom liquid layer of C8F18. Near the walls a flow goes upward to the border with ethanol and then turns to the center, departing downward from a driven layer of ethanol. At the bottom the flow is directed to the walls and again goes upward, forming a third closed cycle of rotation. It was already noted that under intensive rotation of an ethanol ring (or other liquids) there is a chain of liquid drops on the wall at a distance of 1-30 mm from the liquid level. When the rotating liquid is colored, the drops are also colored. Ethanol and DBP drops are small and have no visible movements. As for benzene, kerosene or a mixture of ethanol with ether, their drops on the wall are larger and they are constantly moving up and down by
Fluorocarbons as Volatile Surfactants
Figure 2. Top view of the three liquids (the side elevation is shown in Figure 1), in a closed (a) and in an open cuvette (b) when constant chaotic surface waves arise due to a capillary instability: 1, vessel walls; 2, region of water near vessel walls; 3, DBP-dye ring adjacent to water; 4, a free surface of liquid C8F18. 1-2 mm, as in a dance, changing every second their forms and positions in a flow of ascending liquid film. Sometimes they lower to a liquid level, contact it, and at once again depart upward. Such a liquid curtain, made of lowered drops on the walls, usually has a thin irregular line at the top border from the dye used to color the liquid. The height of the border varies and depends on the kind of liquid and its temperature. Comparing experiments 1 and 3, we can conclude that, in essence, the zone of formation and movement of two-dimensional waves, described in experiment 1, in experiment 3 with a rotating ring is extended and as a curtain is transferred on a surface of glass walls. A liquid film goes upward from the ring surface due to capillary forces. The film is so thin that it does not bring on a wall even the smallest motes floating in the liquid, but it carries dye molecules up to the top border of the curtain. It is necessary to note one more feature. In a hermetic cuvette with ice in its cooler, the surface of the walls, where the film is lifted for evaporation, has a temperature of a few degrees less than that of liquids; that is, in contrast to a usual movement of liquids in thermal pipes, here a film of liquid goes (overtaking a diffusion flow) from a more heated place to a colder one for evaporation.
Langmuir, Vol. 14, No. 20, 1998 5687 When by shaking the cuvette the curtain on the wall is wetted by liquid C8F18, the curtain slowly goes downward and liquid (benzene, kerosene, ethanol in mixture with ether) is removed from the wall. The rotation stops. Then the ring rotation restarts, not on the whole ring, but in a narrow zone 1-2 mm wide near the walls, and then (in several seconds) the zone of rotation extends and spreads over the whole ring. In 3-5 s after full curtain dropping and restarting of ring rotation, from a layer of liquid near the wall some transparent formations arise in different points and go upward, as mushrooms, and look like fingers with a width of 1-2 mm (the speed of growing is about 1 cm s-1). When they reach the height of 5-15 mm, the fingers become thicker, come in touch with each other, and produce new curtain drops going downward. It takes about 0.5 min to draw a new curtain. Then the “dance” of lowering drops on the wall is renewed and the top borderline from a dry dye is gradually restored. Such dropping and restoration of a curtain on the walls can be observed many times after each cuvette shaking. The wall influence on liquid ring rotation is demonstrated in an experiment in which a part of the glass wall in an angular sector of 45-180° over the whole cuvette height is covered by a Teflon film. If, after cuvette shaking, the ring saves its form, then in the Teflon sector rotation of the rings ceases and goes only in open sectors. However, a ring liquid scarely wets Teflon and after shaking tends to leave the Teflon sector. As for liquid C8F18, it wets Teflon, and therefore the free C8F18 zone tends to come nearer to this sector and to touch the Teflon wall. In this case the ring takes the form of a horseshoe (or semicircle) and rotation of the liquid in Teflon-free sectors goes with the former intensity. Such one-sided rotation of the liquid requires an external compensation of the rotation moment from the cuvette and can be directly measured. In an aluminum cuvette a rotation of rings goes with less intensity than one in glass, and the presence of a Teflon sector in it produces the same influence as the one in a glass cuvette. In a hermetic cuvette, with its cooler located at the cuvette axis, the condensed vapor drips again in a liquid. The content of the drop determines its subsequent behavior on a free surface of liquid C8F18. For example, a drop of C8F18 almost at once is incorporated by the main liquid of C8F18 and disappears. A drop, containing a ring liquid, after falls moves aside to merge with the ring liquid. An ethanol drop sometimes does not go at once to the ring but at first as a ball of 2.5-3 mm in diameter goes to the cuvette center and, appreciably bending the C8F18 surface, lies there as a whole visible ball for 3-15 s. There are some movements on the drop surface, which come to an end, and the drop is transformed to a new form. The ball surface tension is sharply decreased, and the drop loses its height but extends up to 4-5 mm in diameter, sinks by half in C8F18, goes quickly from the center to a ring, and then merges with it. Apparently, in its fall the ethanol drop is sometimes covered by a special film of C8F18 molecules, which sharply leaves the drop and merges with the liquid C8F18. When a C8F18 drop falls on the surface of a benzene layer, it partially passes downward. However part of it, as a sagging stain 4-5 mm in diameter, remains above on the surface and causes on it a flow from the drop to a wall. Motes in the benzene run at a speed of 2-10 cm s-1 from the C8F18 drop to a wall; there they go downward and come back to the drop. At an external border of the drop they are lifted upward and again run to a wall. During the drop’s existence (for about 1 min) the sizes of its external border vary periodically several times from 5 to 20 mm with a period of 5-15 s. A rotation of liquids is observed not only in a ring or near a drop on the surface. It can be also observed in a plane (1.5-2 mm) layer of kerosene (or benzene or 2-propanol) over liquid C8F18, when some passive markers are added to the kerosene for observation. With the plane kerosene surface and without any drop of C8F18 on it, there is a constant stationary flow on the surface from a central point of the cuvette to its walls and then downward to the C8F18 layer and back to the center. Here it goes upward to the surface in a flow of 1-2 mm in diameter and again to the walls at a speed of about 2-10 cm s-1. In essence, this movement represents a continuous ring rotation in a vertical plane, as in previous experiments, but the ring’s internal diameter is reduced to zero. The rotation is provided by delivering C8F18
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Table 1. Reduction (in Percent) of Surface Tension of Liquids in Fluorocarbon Vapor catalog no.22
vapor
ethanol
2-propanol
benzene
DMP
DBP
kerosene
CCl4
ether
670 549 546
C6F14 C8F18 C10F18
10 7
