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Langmuir 1988, 4 , 938-941
A Leak-Proof Device for Compressional-Expansional Cycling of Surface Films David F. Townsend Hercules Incorporated, Research Center, Wilmington, Delaware 19894
Erik J. Bock* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180 Received February 13, 1986. I n Final Form: March 14, 1988 An apparatus was constructed to produce repetitive compression-expansion cycles of surface-area, using a unique design to guarantee containment of the surface layer, and applied to insoluble monolayers of poly(dimethylsi1oxane)and aqueous solutions of sodium dodecyl sulfate. The observed reversibility of a transition believed to be the transition of a poly(dimethylsi1oxane) monolayer from the uncoiled conformation to the coiled conformation is shown. Marked hysteresis in the observed surface tension versus area diagram for this monolayer supports the assumption that the polymer chain maintains a coiled conformation at high cycling rates. The shape of the hysteresis loop for a solution of sodium dodecyl sulfate is shown to be good criterion for purity as well as an indicator of relative surface activity.
Introduction The theory of evanescent foam holds that surface tension gradients, or “Marangoni forces”,l restore stability to liquid films. A device that cycles extensions and contractions of surface area of surface-active solutions would simulate the surface-tension gradients in a way similar to that taking place in a foam.2 The effect of expanding or contracting the area of a surface is to decrease or increase the surface concentration and so temporarily raise or lower the surface tension. Pure liquids and solutions of solutes that are surface inactive do not exhibit any change of surface tension when a new surface is created; solutions of surface-active solutes do exhibit this change. The majority of research on the response of surface tension to cyclical altering of the area of a liquid surface is devoted to the study of “lung surfactantn, or alveolar fluid (a combination of fatty acid phospholipids, one of which is dipalmitoyllecithin) which coats the lining of mammalian lungs. Pattle3noted the strong surface activity of lung fluid through the stability of foam produced in the lung. Clements4 proposed a model for the mechanics of lung action and built the first apparatus that measured nonequilibrium surface tensions while cyclically altering the area of a surface. He found the surface tension of a monolayer of lung surfactant on water has a range from 50 mN/m down to a phenomenally low 1 mN/m, with large hysteresis of the steady-state cycle. Early workers suggested other designs516to effect the area change, but each of these, including that of Clements, is subject to some of the adsorbed layer leaking through minute gaps in the containing barriera7 Two later designs avoid the problem of leaking,7,8and an ingenious device capable of measuring surface response function moduli (namely, surface tension, surface dilatational elasticity, and surface viscosities) has been r e p ~ r t e d .This ~ latter device (1) Marangoni, C. Nuouo Cimento 1871, (2)5-6,239; 1878, (3)3,97,193. (2) Townsend, D. F.; Ross, S . Langmuir 1986,2, 288. (3) Pattle, R. E. Nature (London) 1955, 175, 1125-1126. (4) Clements, J. A.; Brown, E. S.; Johnson, R. P. J. A p p l . Physiol. 1958, 12, 262-268. (5) Mendenhall, R. M.; Mendenhall, A. L., 31.Rev. Sei. Instrum. 1963, 24, 1350-1352. (6) Watkins, J. C. Biochem. Biophys. Acta 1968, 152 293-306. (7) Somasundaran, P.; Danitz, M.; Mysels, K. J. J. CoZZoid Interface Sei. 1974, 48, 410-416. (8) Boyle, J., 111; Mautone, A. J. Colloids Surf. 1982, 4, 77-85. (9) Abraham, B. M.; Ketterson, B. Langmuir 1985, 1, 708.
0743-7463/88/2404-0938$01.50/0
uses ripple propagation data to determine surface characteristics. The construction of a more convenient leakproof apparatus is part of the present undertaking.
Description of the Apparatus An improved device that is effective for creating gradients of surface tension for insoluble monolayers and for weakly surface-active solutions, embodying a new approach, is reported herewith. The area of a liquid surface, contained within the perimeter of a funnel, is made to contract by dipping the funnel into the solution. The use of a funnel has the advantages of altering a large area of surface without the possibility of leaks and without concern for the proper depth of a moving barrier. The funnel resembles a trapezoidal prism, in that it has a rectangular cross section with two parallel faces and two diverging faces; hence the change in horizontal cross sectional area with height is linear. When this funnel is immersed and withdrawn from a liquid, the area of the fluid surface enclosed by the funnel is a linear function of the depth of immersion. Figure 1 is a schematic diagram of the apparatus. Centrally located in the chamber is the funnel, which is made out of Teflon-coated stainless steel or Lexan. The choice of material from which the funnel is constructed is determined experimentally. Certain monolayers are deposited irreversibly on the moving funnel by a mechanism akin to Langmuir-Blodgett coating. When this coating occurs, the surface film contained within the funnel is not subject to compression or expansion and the results of dynamic surface tension measurement are erratic. Under such conditions, the measured surface tension rises on the initial compression cycle and then remains relatively constant on successive cycling. Experimental tests of coating revealed that the Teflon-coated funnel does not pick up the poly(dimethylsi1oxane) monolayer, but it does adsorb sodium dodecyl sulfate. Consequently, Lexan was tested for its affinity for sodium dodecyl sulfate. The characteristic rise in surface tension attributed to coating of the funnel was not observed, indicating that Lexan did not adsorb sodium dodecyl sulfate. Lexan was used as a funnel for all tests of the sodium dodecyl sulfate solutions. The motion of the funnel is controlled by a hydraulic “slave” cylinder that is connected to a matched hydraulic “master” cylinder, which is part of a power station (see Figure 2). A linear bearing and a case-hardened steel rod 0 1988 American Chemical Society
Device for Compressional-Expansional Cycling
i= Id l -
Figure 1. Schematic diagram of the experimental chamber for
the dynamic surface tension apparatus. Labeled parts include (A) electrobalance, (B) funnel, (C) counter volume, (D) “slave” hydraulic cylinder and linear bearing assembly, (E) trough, (F) positioning table, (G) steel plates, (H) vibration isolation pads, and (I) electrostatically shielded environmental chamber.
Figure 2. Schematic diagram of the power station for the dynamic surface tension apparatus. Labeled parts include (A) dc electric motor, (B) gear reducer, (C) “master” hydraulic cylinder, (D)Worm gear and nut assembly, (E) limit switch assembly, (F) electronic control assembly, and (G)tracking potentiometer.
are mounted vertically next to the slave cylinder to stabilize the motion. Surface area is changed by moving the funnel in and out of a liquid contained in a 1.5-L stainless steel trough, with an “L”-shapedcross section. The funnel dips into the longer leg of the L, and the counter volume dips into the shorter leg. The longest side of the trough is approximately 21 cm, and the width of the longer leg is approximately 7 cm. Temperature is controlled by circulating water from a thermostat bath (Haake Model F3-K) through 1/4-in.copper tubing wrapped around the trough. The tubing and the trough are submerged in a Plexiglas box filled with water (not shown in Figure 1). Surface tension is measured with a Wilhelmy plate. The plate is suspended from a Cahn 2000 electrobalance, through the narrow opening at the top of the funnel. The surface is brought into contact with the plate by raising the trough into position by means of an optical table (Oriel Co., Model 1636 vertical translator). The chamber itself is a draft-proof Plexiglas box, lined with wire screen to form a Faraday cage. It stands on four stacked 100-lb steel plates placed on four 3M Co. vibration-damping pads. The electrobalance permits buoyancy errors to be minimized by correct placement of the Wilhelmy plate. In this application a counter volume is withdrawn from the fluid as the funnel submerges and so ensures a constant liquid level. The counter volume is a trapezoidal prism made of aluminum for use with the plastic funnel and a solid piece of Teflon for use with the coated funnel. A power station (schematically represented in Figure 2), which is located outside of the experimental chamber to
Langmuir, Vol. 4, No. 4, 1988 939 reduce heat and vibration effects, controls the extent and rate of the motion of the funnel. The master cylinder is connected to a driver nut on a worm gear that is turned by a variable-speed reversible motor through an Insco gearbox. The maximum cycling rate starts at 1.02 cpm and may be set lower, using the speed reducer, by factors of 211, 3/1,4/1, 5/1,10/1,20/1, 3011, 4011, and 5011. A microswitch at each end of the driver nut’s travel triggers a relay that reverses the motor’s direction, thereby causing the funnel to cycle. The worm gear also turns a precision potentiometer by means of a set of pulleys and a drive belt. The resistance of the potentiometer is used to determine the position of the funnel and hence the area of the liquid surface enclosed. A voltage across this potentiometer is sent to the x-axis of an x-y plotter. The y-axis signal is received from the electrobalance, and a plot of surface tension against area is produced by choosing the correct scaling factors for each axis. The possibility that the liquid may be agitated by the motion of the funnel was checked by placing a few water-soluble dye crystals on the surface of water held in a clear plastic trough. After the crystals had settled to the bottom, leaving vertical trails, the Lexan funnel was activated and the movement of the dye trails observed. Considerable mixing took place near the extreme lower ends of the funnel, but no mixing was seen elsewhere in the trough, even after repeated cycling at the maximum rate. The range of speeds available allows both equilibrium force-area curves of insoluble monolayers and dynamic surface tensions of surface-active solutions, as well as of insoluble monolayers, to be measured. For solutions, the shape of the first cycle depends upon whether the equilibrium surface is initially extended or contracted. In all the present measurements reported, the first cycle starts as a contraction from equilibrium. The cycling of area is continued until the surface tension vs area forms a hysteresis loop that traces itself. This “steady-state”loop is the same whether the surface was initially extended or contracted on the first cycle.
Procedure Prior to use, the trough, funnel, and counter volume are thoroughly washed with a soap and warm water solution, rinsed several times with distilled water, and given a final rinse of acetone (Teflon-coated steel) or hexane (Lexan). The pieces are air-dried and then assembled in the experimental chamber. The trough is then filled with the solution to be tested. With the funnel submerged to give a minimum enclosed area, the surface of the liquid is cleaned by aspirating off a small volume from the contained liquid surface. The extent of travel of the funnel is set by adjusting the microswitches in the power station (see Figure 2), making sure the funnel remains in contact with the liquid surface and keeping the minimum enclosed area large enough to accommodate the Wilhelmy plate. The depth of the counter volume travel is then set, with the funnel at its upper limit, so that the height of the portion of the counter volume above the liquid surface is approximately the same as the depth of the immersed portion of the funnel. An insoluble monolayer may be placed on the surface for study. In this case a piece of filter paper is used as a Wilhelmy plate to ensure a zero contact angle.1° The surface is raised until it contacts the plate and the surface tension is observed throughout one cycle, to detect any (10)Gaines, G.
L.,Jr. J. Colloid Interface Sci. 1977,62,191-192.
940 Langmuir, Vol. 4, No. 4 , 1988
Z
Townsend and Bock
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COMPRESSION/EXPANSION AREA
," 3 4 100. (cm2)
Figure 3. Variation of surface tension with area of an insoluble monolayer of poly(dimethylsi1oxane)(100 cSt) on water (pH 7) at 21.0 "C and a cycling frequency of 0.020 cpm (dotted line) and 1.02 cpm (solid line).
contamination from the filter paper. A small volume of a known concentration of a solution of the adsorbate in pentane is added to the surface inside the funnel and allowed to come to equilibrium. The cycling rate is then set to 0.02 cycle/min for an equilibrium compression of the monolayer. The dynamic surface tension of a solution is measured with a sand-blasted platinum Wilhelmy plate cleaned in a radio frequency fie1d.l' The surface of the solution is then cleaned as described above, and the funnel is moved to its upper limit, since we have observed that a more rapid attainment of equilibrium is obtained in this way. For systems with a slow equilibrium, surface tension is plotted with time until no further change is observed. This process is repeated several times to be sure the measurement of equilibrium surface tension is reproducible.
Examples of Uses of the Apparatus Distilled water was first used to check the apparatus. Since a pure liquid does not exhibit dynamic surface tension, there should be no effect on contraction or extension of the surface. Distilled water, which was obtained from a general supply, was redistilled from alkaline permanganate into phosphoric acid and distilled once again into a quartz container. The observed variation of surface tension with area did not exceed 0.2 mN/m for any value of surface area between the lower and upper limits of 20.0 and 100.0 cm2. This variation is attributed to a contact angle hysteresis that takes place as the funnel reverses direction. The flat response for water at 21.0 "C indicates that the apparatus does not introduce artifacts. Next, the equilibrium compression of an insoluble monolayer of poly(dimethylsi1oxane)on water was determined. Poly(dimethylsiloxane),GE SF96-100(the number average molecular weight is about 6600 g/mol), was used as is. The pentane was Fisher Co. spectroscopic grade. The water was triply distilled as above. The monolayer was spread to give an initial surface coverage of 2.05 m2/mg. Figure 3 shows two compression-expansioncycles of poly(dimethylsi1oxane)on water at 21.0 "C and pH 7. The dotted line represents a cycling rate of 0.020 cpm, and it is observed to be reversible within the resolution of the experimental apparatus. The trace closely resembles those reported by Fox et al.ll The change of slope at a value of surface tension of approximately 63.5 mN/m was attributed by these authors to a reversible reorientation of the poly(dimethylsi1oxane)chain from a conformation where every silicon atom is held at the surface to a coiled helix (11)Bock, D.,private communication.
5
m
I
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COMPRESSION/EXPANSION AREA
100 (cm2)
Figure 4. Variation of dynamic surface tension with area for a 7.98 x M solution of unrecrystallized sodium dodecyl sulfate in water at 21.0 "C and a cycling frequency of 1.02 cpm (dotted line) and for a recrystallized solution under the same conditions (solid line).
with every sixth silicon atom adsorbed. The latter conformation is highly incompressible, that is, the spreading pressure increases (or the surface tension drops sharply) on compressing the available surface area of the substrate. The present experimental result supports the reversibility of the transition. The solid line on Figure 3 is a plot of a steady-state hysteresis loop of dynamic surface tension vs area for this same monolayer, but with a cycling rate of 1.02 cpm. The loop retraces itself within two cycles. At this rate of cycling, the film must be compressed to a smaller area before its surface tension is significantly reduced. This appears as an increased compressibility postulated to be the result of the polymer chain coiling into the helical conformation once the transition point is reached (see Fox et a1.)12and remaining so for the rest of the cycles. The hysteresis of the measurement is presumed to be due to a lag in the spreading response of the coiled monolayer to the increased cycling speed. The monolayer returns to the dotted "equilibrium" trace when the cycling frequency is reduced to 0.020 cpm again, confirming the repeatability of the experiment. Solutions of sodium dodecyl sulfate in water were tested as well. The water was triply distilled as above, but a plot of equilibrium surface tension versus concentration for sodium dodecyl sulfate, J. T. Baker Co., 99% electrophoretic grade, showed a distinct minimum. After further purification by recrystallization from ethanol and acetone, followed by foam fractionation as described by Mysels,13 the minimum in the equilibrium surface tension versus concentration plot was removed. The new concentrations after purification were determined by a calibration plot of specific conductivity versus concentration, and the final dilution was made volumetrically. These solutions show surface-tension gradients on compressions and expansions of the surface-area. The shape of the hysteresis loop of the dynamic surface tension provides an alternate means by which purity may be estimated as suggested by Mysels et al.7 Figure 4 shows the hysteresis loops for solutions of 7.98 X M sodium dodecyl sulfate before (dotted line) and after (solid line) purification. The difference in purity can be seen to affect the shape of the loops; unrecrystallized sodium dodecyl sulfate has a much larger hysteresis, which means that the impurity (or impurities) prolongs the relaxation process at the surface. (12)Fox,H.W.;Taylor, P. W.; Zisman, W. A. Ind. Eng. Chem. 1947, 39, 1401-1409. (13)Elworthy, P.H.;Mysels, K. J. J. Colloid Interface Sci. 1966,21, 331-347.
Langmuir, Vol. 4, No. 4, 1988 941
Device for Compressional-Expansional Cycling h
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m 71 z 70
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COMPRESSION/EXPANSION
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1 00 . (cm')
Figure 5. Variation of dynamic surface tension with area for a M solution of recrystallized sodium dodecyl sulfate 1.68 X
in water at 21.0 O C and a cycling frequency of 0.10 cpm. The dotted line represents static surface tension of the solution.
The surface activity of sodium dodecyl sulfate solutions of different concentrations is illustrated by Figures 5 and 6. Each case represents the maximum change in surface tension, at steady state, for a range of cycling frequencies. The dynamic surface tension, u, shows a maximum at a concentration less than the cmc, and it falls off sharply above the cmc. (The first cycle is not shown in Figure 6.) Lucassen and Giles reported the same trend in surface activity with nonionic detergent s01utions.l~ Figure 5 reports the variation of surface tension with surface area for a 1.68 X M solution of recrystallized sodium dodecyl sulfate in water at 21.0 "C and a cycling rate of 0.10 cpm. The dotted line indicates the value of static surface tension obtained for the solution. Parts A, B, and C of Figure 6 report similar variations obtained at a cycling rate of 0.20 cpm for solutions of sodium dodecyl sulfate at 21.0 "C with concentrations of 1.68 X lo4, 7.98 X and 1.52 X M, respectively. In each part of Figure 6, the static surface 'tension of the solution is plotted as a dotted line. The rate of restoration of equilibrium at a surface is slow enough to cause significant hysteresis only in dilute solutions and within a limited range of concentration. Ward and Tordai15give the following equation for the amount of solute adsorbed at a new surface in the absence of stirring or an energy barrier to adsorption: n = 2 ( D / ~ ) ' / ~ c t ' / ~ (1000) N,/ where n is the number of molecules/cm2, D the bulk (14)Lucassen, J.; Giles, D.J . Chem. SOC.,Faraday Trans. 1 1975, 71, 217-232. (15) Ward, A. F. H.;Tordai, L. J. Chem. Phys. 1946, 14, 453.
":y 34
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COMPRESS I O N / E X P A N S I O N A R E A ( cm')
Figure 6. Variation of dynamic surface tension with area, at steady state and 21.0 "C, for solutions of recrystallized sodium dodecyl sulfate in water with concentrations of (A) 1.68 X and (C) 1.52 x M. The cycling rate is 0.20 (B) 7.98 x cpm. The dotted lines represent static surface tensions of the solutions.
diffusion constant (cm2/s), c the bulk concentration (mol/L), t the time (s), and N A Avogadro's number. If the solution is too concentrated, the time needed to replace solute at a new surface is shorter than the time required to change the surface area; if the solution is too dilute, its surface tension approaches that of the pure solvent, and neither extension nor contraction of the surface area changes its surface tension. In the present series of experiments, at a concentration of 1 X M sodium dodecyl sulfate, no hysteresis of the surface tension was observed. The equilibrium surface tension of the solution in Figure 5 is only slightly less than that of the water alone, yet a cyclical contraction and extension of the surface causes a large reduction of surface tension, the magnitude of which increases with each cycle until reaching a steady state, as though solute were being pumped into the surface. This pumping action is the subject of further experiments described in a separate paper.2
Acknowledgment. We are grateful to Professor S. Ross for Helpful discussions. E.J.B. holds a Graduate Fellowship of the National Science Foundation. Registry No. SDS, 151-21-3.
