428
Langmuir 1986,2,428-432
For sufficiently pure systems the rate of adsorption of the surfactant is sufficiently faster than that of the impurities to produce an interval in which the ST is characteristic of the surfactant alone. Small residual effects may be taken into account by extrapolation to zero or very short times. On a linear scale all the curves of Figure 2 show decreasing slopes. Thus, unless the solution is extremely pure, the ST values obtained by extrapolating to infinite time or by accepting a value once the linear slope becomes “negligible” will depend markedly on the purification process and will be significantly lower than the thermodynamicaly meaningful value. It follows also that for less precise determinations on less pure systems, measurements made at short times or extrapolated to such, are closer to the true values for the pure system than those made at, or extrapolated to, long times. The meaning of “short”, “fast”, and “long” in the preceding is purposely left vague as it depends, in ways which are still not well understood, on the method of measurement, the concentration of the surfactant, and its adsorption kinetics. For NaDS in the millimolar range, and usual methods, a few seconds is “short”, hundreds or thousands of seconds is certainly “long”. For more highly
surface active materials at lower concentrations these values have to be increased. The same would be true in the presence of large energy barriers to adsorption. On the other hand the presence of significant convection will accelerate the approach to equilibrium. It should also be noted that measurements of ST at surface ages shorter than required for attainment of adsorption equilibrium by the surfactant, such as presented here, may be of some help in establishing what is %.hart", but they are not essential for a valid ST measurement. ST values that are satisfactorily constant after the fastest measurement that can be made conveniently should be good evidence that the surfactant is already fully adsorbed while the impurities are not yet having a deleterious effect. Thus the MBPM, though particularly adapted for the purposes of this work, is not necessarily preferable for accurate ST determinations in general.
Acknowledgment. This work was made possible by the hospitality of Prof. Bruno Zimm’s laboratory and a grant from the Research Corporation Foundation. I am also grateful to Prof. L. Sweetman for the chromatographic analysis. Registry No. NaDS, 151-21-3.
Improvements in the Maximum-Bubble-PressureMethod of Measuring Surface Tension Karol J . Mysels Chemistry Department, University of California San Diego, La Jolla, California 92093 Received November 18, 1985. In Final Form: March 7, 1986 The gas in a bubble blown at the end of a submerged capillary attains a well-defined maximum pressure, determined by the surface tension and the radius of the capillary, before escaping. This is the basis of the maximum-bubble-pressuremethod for measuring surface tension. Several problems and improvements of this method are discussed, with special emphasis on the measurement of surface tension as a function of time in solutions in which adsorption of surface active components may be slow. The principal modifications described are an inclined capillary with siliconized lumen but with a hydrophilic face and outside surface, protection from atmospheric pressure fluctuations, operation at constant pressure over long periods of time, and simple micrometric immersion of the capillary. Sensitivity of 0.01 dyn/cm and accuracy of about 0.1 dyn/cm for bubble intervals ranging from about 0.1 s to hours could thus be obtained. Introduction The maximum-bubble-pressure method (MBPM) is based on the idea that a bubble of gas growing at the tip of an immersed capillary is stable as the pressure of the gas increases until the bubble becomes hemispherical. At this point its radius of curvature is minimum. Beyond it, the bubble is unstable and grows explosively until it detaches itself. Because of the aerodynamic resistance of the capillary this rapid growth of the bubble always produces a drop in the pressure at the tip and, if the volume supplying the capillary is limited, also a drop at the other end of it. Thus a maximum pressure P is measured when the radius of the bubble just equals that, r, of the capillary, and from this the surface tension, u, of the liquid at that instant can be directly calculated by Laplace’s relation: P - p = 2o/r (1) where p is a correction for the hydrostatic pressure due to the immersion of the capillary.
Since Simon first used the MBPM in 1851’ it has been refined and adapted in many ways, some of which are described in the reference^.^-^' It has been used exten(1) Simon Ann. Chim. Phys. 1861,32,5. Simon not only suggests, as sometimes stated, but does use the MBPM to determine the relative ST of several liquids. (2) Rehbinder, P. Z . Phys. Chem. 1924,111,447. ( 3 ) Adam, N. K.; Shute, H. L. Trans. Faraday SOC.1938, 34, 758. (4) Kuffner, R. J. J. Colloid Sci. 1961, 16, 497. (5) Kragh, A. M. Trans. Faraday SOC.1964,60,225. (6) Austin, M.; Bright, B. B.; Simpson, E. A. J. Colloid Interface Sci. 1967., 23 . -108. -, - -. (7) Kloubek, J. Tenside 1968,5,317. (8) Bendure, R. L. J. Colloid Interface Sci. 1971, 35, 238. (9) Kloubek, J. J. Colloid Interface Sci. 1972, 41, 1, 7, 17. (10) Kloubek, J. Colloid Polym. Sci. 1976,253, 754. (11) Fainerman, V. B. Colloid J. USSR (Engl. Transl.) 1979,41,79. (12) Papeschi, G.; Bordi, S.; Costa, M. Ann.-Chim. (Rome) 1981, 71, 407. (13) Fainerman, V. B.; Lylyk, S. V. Colloid J. USSR (Engl. Transl.) 1982, 44, 538.
0743-7463/S6/2402-0428$01.50/0 0 1986 American Chemical Society
Improvements in the Maximum-Bubble-Pressure Method sively for the measurement of the surface tension (ST)of pure liquids (which is not time-dependent) particularly of organic ones where small sample size can be important26*39 and under difficult conditions such as high reactivity or t e m p e r a t ~ r e . ' ~ 'It~ ?has ~ ~also been used for the study of solutions in which the ST is determined by the rate of adsorption of solutes and thus is time-de~endent.~-'~ This changing ST is often referred to as a dynamic ST, as opposed to the final, static, or equilibrium ST. Dynamic ST can provide information about the kinetics, and thus the mechanism, of adsorption in addition to its practical importance in detergency, flotation, foaming, etc. Furthermore, dynamic ST offers a unique criterion of surface purity and thus is important in the determination of the thermodynamically significant equilibrium values, as discussed in the preceding paper.* I have been interested in obtaining pure surfaces of surfactant solutions for some but have encountered considerable experimental difficulties. Upon reading the careful discussion of dynamic ST measurements by the MBPM by Kloubek: I decided to explore its applicability to the problem. This led to a number of modifications and improvements which permitted the ST to be measured with a sensitivity of 0.01 dyn/cm and an accuracy of some 0.1 dyn/cm over bubble intervals (BI's) from about 0.1 s to several hours. These are the subject of this paper. I will begin by a general discussion of the main problems considered and of their solutions and then give a detailed description of the apparatus and procedure.
General Discussion Capillary, The hand drawing of the fine and fragile capillaries, often used in this method, seemed to require much skill and introduce an element of irreproducibility. (14) Hu, P. C. SPE-DOE Joint Symposium on Energy Recovery, Tulsa, OK 1984. (15) Miller, T. E.; Meyer, W. C. Am. Lab. (Fairfield, Conn.) 1984,16, 91. (16) Jaeger, F. M. Versl. Gewone Vergad. Afd. Natuurkd., K. Ned. Akad. Wet. 1914, 23, 330. (17) Pugachevich, P. P. Ruas. J.Phys. Chem. (Engl. Tiansl.) 1964,38, 758. (18) Razouk, R.; Walmsly, D. J.Colloid Interface Sci. 1974,47,515. (19) Smimov, M. V. Chem. Abstr. 1978,88, 295-55299b. (20) Becht, J.; Lunkenheimer, K. Colloid Polym. Sci. 1982,260, 234. (21) Wolf, F.; Sauerwald, F. Kolloid 2. 1960, 118, 1. (22) Schrijdinger, E. Ann. Phys. (Leibzig) 1916, 46, 413. (23) Sugden, S. J. Chem. SOC. 1922, 121, 858. (24) Sugden, S. J. Chem. Soc. 1924,125, 27. (25) Cantor, M. Ann. Phys. (Leibzig) 1902, 7, 698. (26) Feustel, R. Ann. Phys. (Leibzig) 1906, 16, 61. (27) Zickendraht, H. Ann. Phys. (Leibzig) 1906,21,146. (28) Cantor, M. Akad. Wiss. Wien 1892, 47 11, 399. (29) Jiiger, G. Akad. Wks. Wien 1890,100 11,245. Cited by ref 22,26 with wrong references. This and name similarity to F. M. Jaegerz6may account for the lack of citation of this significant paper in later literature. (30) Warren, E. L. Philos. Mag. 1927, 4, 358, (31) Brown, R. C. Philos. Mag. 1932, 13, 578. (32) Kuffner, R. J.; Bush, M. T.;Bircher, L. J. J.Am. Chem. SOC. 1967, 79, 1587. (33) Romagosa, E. E.; Gainea, G. L., Jr. J. Phys. Chem. 1969,73,3150. (34) Fel'dman, I. N.; Malkova, I. V.; Sokolovskii, V. I.; Zaturenskii,R. A. J. Appl. Chem. USSR (Engl. Transl.) 1980,53, 1594. (35) Belov, P. T. Russ. J. Phys. Chem. (Engl. Transl.) 1981,55, 302. (36) Kisil', I. S.; Mal'ko, A. G.; Dranchuk, M. M. Ruas. J.Phys. Chem. (Engl. Transl.) 1981,55, 177. (37) Joos, P.; Rillaerta, E. J. Colloid Interface Sci. 1981, 79, 96. (38) Padday, J. F. Surface and Colloid Science; Matijevic, E., Ed.; Wiley-Interscience: New York, 1969; p 101. (39) Jasper, J. J. J. Phys. Chem. Ref. Data 1972, 2 , 841. (40) Mysels, K. J. Langmuir, preceding paper in this issue. (41) Razouk, R. I.; Mysels, K. J. J. Am. Oil. Chem. SOC.1968,45,381. (42) Mysels, K. J.; Florence, A. T. J. Colloid Interface Sci. 1973,43, 577. The data of Pethica and Few are misplotted in Figure 1 and should be close to those of Rehfeld.
Langmuir, Vol. 2, No. 4,1986 429
I found it simpler to start with commercial capillary pipettes of 1-pL capacity widely used in biochemistry. Their inside diameter was 0.14 mm so that the pressure corresponding to the ST of 1dyn/cm was about 3 mm of H20. In view of the direct significance of the ST, pressures in the following will be expressed in terms of the equivalent ST, i.e., in dyn/cm. The literature generally stresses cleanliness of these glass capillaries which thus become well wetted. A consequence is that the bubble does not simply grow at the tip as discussed above, but when the pressure drops, the bubble tends to keep its hemispherical shape and to rise inside the capillary to a level correspondingto the lower pressure. Then, as the pressure increases, the bubble descends and finally becomes unstable and escapes. Bendurea has published pictures of this behavior and 0 t h e r s ~ ~have 9~~ described it. This seems to introduce two complications. One is that a receding meniscus leaves behind a film of varying thickness depending on the rate of motion45y46and also on the { potential of the wall and ~urface.~' This makes uncertain the effective radius of the capillary. The other is that the surface of the wetting film inside the capillary extends to an unknown height and is compressed and expanded by the movements of the meniscus. This greatly complicates any estimate of the effective age of the surface. Hence I decided to siliconize the lumen of the capillary while keeping its face and outside hydrophilic. The bubble thus becomes attached to the boundary between the hydrophobic and hydrophilic regions over a considerable range of pressures and both the effective radius of the capillary and the history of the surface become better defined. If the face of the capillary is not hydrophilic, the attachment of the bubble becomes uncertain.21 If the outside is hydrophobic, the escaping bubbles stick t o it and the BI becomes irregular. Traditionally the capillary is kept vertical, generally downward, occasionally upward.12 I found it difficult to work with a bent capillary with its opening upward because of occasional accumulation of liquid in the bend. The downward position is objectionable because, in principle, in the absence of any disturbance, the exploding bubble should grow indefinitely, kept down at its top by the capillary and having no reason to escape in one direction rather than another! In practice, of course, there are disturbances and the bubble escapes. These disturbances are, however, ill-defined so that the escape is somewhat irregular. By inclining the capillary, a constant and overriding gravitational bias is introduced and the BI becomes smaller and stable. The inclined capillary makes it difficult to calculate a correction for the gravitational deformation of the bubble. Because of the smallness of the bubble this correction should be quite small. It is of the order of 0.02 dyn/cm for a vertical capillary according to Schrodinger's equationSz2This is also the approximate value of another possible correction of opposite sign due to the fact that the level of the solution is determined by the edge of the 0.5-mm-0.d. capillary whereas the bubble forms at its slightly higher center. Both corrections, being small and tending to cancel each other, have been neglected. Pressure Control. Bubbles may be produced either by reducing the pressure over the solution or by increasing (43) Kloubek, J. J. ColEoid Inferface Sci. 1972, 41, 1. (44) Derjaguin, B. Dokl. Akad. Nauk USSR 1943,39, 13. (45) Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall: Englewood Cliffs, NJ, 1962; pp 674-683. (46) Mysels, K. J.; Frankel, S. P. J. Colloid Interface Sci. 1978,66,166. (47) Langmuir, I. Science (Washington, D.C.) 1938,88,430.
430 Langmuir, Vol. 2, No. 4, 1986 the pressure in the capillary. The latter permits more freedom in the design of the cell which contains the solution and generally reduces the volume of gas whose pressure is being varied. I found it therefore preferable. The pressure was measured close to the inlet of the capillary by a transducer and the signal, after proper conditioning, went to a meter, an oscilloscope, and a recorder. Pressure changes over a time scale ranging from a few millseconds to hours could thus be measured and observed. It became immediately obvious that the atmospheric pressure in the laboratory was fluctuating irregularly on a time scale of a fraction of a second. This caused variations in the pressure on the solution side of the bubble resulting in irregular BI’s. Furthermore, as the response rate of the bubble and the transducer are quite unequal,the measured pressure difference could differ from that experienced by the bubble. These fluctuations were due to the air conditioning system and required that the cell and capillary be placed under a bell jar. To avoid long-term buildup of pressure or vacuum, the bell jar was provided with a leak giving a relaxation time of the order of a minute. The volume of an escaping bubble is about 1mm3which makes regulation of longer BI’s delicate. A vacuum leak valve capable of controlling flows of standard cm3/s was, in principle, capable of meeting the requirements, but it turned out that tiny temperature drifts in the volume of gas between the valve and the capillary produced slow pressure drifts. These were significant at BI’s of the order of tens of seconds and overwhelmed any flow control at BI’s of the order of minutes. Hence for longer BI’s I abandoned the traditional method of adjusting the flow rate of gas to obtain the desired BI and measuring the maximum pressure. Instead, for longer BI’s, I set a desired constant pressure and measure the BI resulting from it. For this an electro-mechanical manostat maintaining an indicated pressure to better than 0.01 dyn/cm was implemented. Thus measurements can be extended to BI’s of several hours and are limited only by the slow drifts in other components of the apparatus. This solves what must be an old problem as the few reports4v5J2of measurements at constant pressure do not extend beyond BI’s of 2 min. Only Adam and Shute? who were limited ”...only by the patience of the observer holding the pressure constant..”, were “able to observe up to about 10 min”. Although the manostat greatly facilitates the measurement of the ST at longer time intervals, obtaining data extending to longer times remains tedious since the MBPM, like any point-by-point measuring method, requires a total time of at least Etito obtain values at several different times ti! The problem of acounting for the depth of immersion including capillary effects of the meniscus in the vessel can be bypassed by comparing the maximum bubble pressures in two capilaries of different diameter as first done by G . Jager,29who varied their relative heights, and popularized by S ~ g d e nwho , ~ held ~ ~ ~them ~ at the same level. This approach not only introduces a mechanical complication but assumes that the ST’s of the two bubbles are the same. When measuring dynamic ST it is not a trivial task to judge when the two ST’s are equal, as adsorption on the two bubbles may proceed at quite different rates because of the different geometries (the larger bubble is considerably distorted by gravity23) and different convective transport. Hence I measured the immersion depth directly. The capillary is attached to a micrometer having a nonrotating spindle and can thus be lowered until it just touches the surface and then immersed a definite distance.
Mysels
NZ
REDUCER
U
Figure 1. Schematic outline of the apparatus. Electrical connections are on the left; the gas supply is on the right. Provisions for thermostating the cell and the transducer and other details described in text are not shown.
Such a micrometer solves very simply the mechanical problems encountered by some early investigator^'^^^"^^ trying to use this procedure. This approach does not take into account the capillarity effects of the meniscus, which are negligible in wide cells, but leads to a loss of accuracy when small samples are studied in narrow cells. Temperature Control. The ST of water changes by about 0.15 (dyn/cm)/’C. Hence thermostating to at least 0.05 “C is desirable. Attempts to thermostat the interior of the bell jar were not successful because the air could not be circulated as this produced pressure fluctuations. Hence the cells are thermostated by jackets supplied with constant-temperature water from outside the bell jar. This permits temperature control over a range of about 5 “C above and below room temperature. Beyond these limits, temperature differences within the bell jar produce convection of the air which results in increasing pressure fluctuations. A potentially better solution, thermostating the bell jar and the base, was not implemented.
Detailed Description The apparatus is shown schematically in Figure 1. The cell, capillary, micrometer, and transducer are located on a base under the bell jar. The pressure signal conditioners, recorder, oscilloscope, gas supply, and thermostats are outside. The base is attached to three lead bricks and supported partially on tennis balls to reduce vibrations. It provides leakproof passage for the gas supply, the electric cable to the transducer, and the thermostating water lines. Resting on the base is a 1Zin.-high tripod upon which is mounted the digital micrometer (Mitutoyo 257-101), readable to 0.005 mm. To the nonrotating spindle of the micrometer is attached a small manifold connected to the gas supply and to the transducer. The manifold has also an inclined opening accepting a silicone septum (as used with micropipettes) through which the capillary or its holder can be mounted The location of the contact between the liquid surface and the tip of the capillary by slow rotation of the micrometer head requires some practice but can be made to some 0.003 dyn/cm. This position changes slightly with temperature and occasionally with some spontaneous adjustment of the septum position in the manifold. Hence it was always rechecked after a series of measurements and, when necessary, a small correction (0.0036 dyn/cm per 0.01 mm) was applied to the series. The polycarbonate bell jar has a diameter of 12 in. and a height of 18 in. Its edge carries a gasket of closed-cell silicone adhesive weather tape to give a tight seal and a leak through a fine control valve is provided in the base. The capillaries are made from 64-mm-long 1-pL micropipettes obtained from Bolab Inc., Lake Havasu, AZ. They are siliconized by drawing the vapor of dichlorodimethylsilane through them followed by air and are then
Improvements in the Maximum-Bubble-Pressure Method
Langmuir, Vol. 2, No. 4 , 1986 431
baked for about 20 min a t 300 “C. Monochlorotrimethylsilane or liquid dichlorodimethylsilane give considerably inferior results. The capillaries are then freed of any silicone on the outside by swabbing with 5% HF, washed, and broken to a length of about 40 mm, producing a hydrophilic face. Spurs are broken off but nicks are accepted as long as they do not impinge on the lumen. They can be used directly for measurements in shallow cells but for use in the cell shown in Figure 1of the preceding papera they are mounted with a small amount of paraffin in the lumen of an appropriately bent 25-pL micropipette which serves as an extension with little aerodynamic resistance. If the capillary is too long or narrow the “dead time” during the explosion of the bubble becomes excessive. If the capillary is too short, a train of bubbles is generated after the maximum pressure instead of a single bubble. This may not affect the accuracy but complicates the process and was avoided. The gas used comes from a high-pressure cylinder of “pure” nitrogen. After reduction of the pressure to some 25 psi, the gas passes through a large commercial charcoal absorber (Koby Inc., Marboro, MA) connected directly to a fine pressure reducer (Type 40 Bellofram Corp., Burlington, MA). To avoid contaminations, all tubing beyond this point is silicone. The gas then goes through a vacuum leak valve (Vacoa MV-25-XL) which throttles its flow and then through a small column of alkyl-coated fine silica (Sep-Pak C18, Van Waters Assoc.) for final purification and is then led through the base to the manifold to which the capillary and the transducer are connected. A side line before the base leads to the manostat leak described below. Another line branches of after the pressure reducer. I t is fitted with a simple fine control valve, additional charcoal, and a Sep-Pak. I t is used in the purification of solutions by foaming as described in the preceding paper.40 In view of the extremely slow flow rates and the great permeability of silicone to gases, it is likely that diffusion makes the composition of the bubbling gas closer to that of the atmosphere than to pure nitrogen. The heart of the pressure measurement system is a DP7-26 transducer and a CD12 signal conditioner (Validyne Engineering Corp., Northridge, CA). The latter provides a 5-kHz excitation current to the transducer, an analogue output ( ~ 0 .V/(dyn/cm)) 1 as well as an adjustably suppressed output, a sensitivity control, and a zero adjustment. The full output is measured with a five-place precision digital voltmeter (DVM) (Data Precision 3500) with a 0.33-9 sampling rate. Hence this measurement can be meaningful only when the pressure is rendered constant by operation of the manostat. Pressure variations during bubble evolution are recorded from the suppressed output on a 100-mV (i.e., 1 dyn/cm) span recorder (Esterline Angus Co., Labgraph AZAS) or a storage oscilloscope (Hewlett-Packard Co., D11). As these also record the constant values measured by the DVM, the latter essentially provides the accuracy of the variable data to better than 0.01 dyn/cm as well. The pressure transducer is basically a taut stainless steel diaphragm whose deflection affects the reluctance of two coils between which it is mounted. This diaphragm shows excellent elasticity but also some creep. After a large pressure change the reading increases significantly during the next half hour but then remains constant within 0.01 dyn for a few hours. Overnight, there is a further small creep of some 0.03 dyn/cm. The return to zero takes about an hour, but after allowance for this creep the zero is very stable during periods of both use and idleness of the transducer. The transducer is wrapped in silicone tubing
through which water from a thermostat circulates, and this is covered with insulation. This package is mounted high on the tripod to avoid interference from the magnetic stirrer motor located below the plate. The manostat receives the suppressed output of the CD12 which goes to an amplifier controlling a miniature relay fitted with a small piece to silicone rubber. In the normal position of the relay, this rubber closes the outlet of a small leak in the gas supply, and when the relay is activated it allows the escape of some gas. The leak is made from a 21 gage hypodermic needle filled with a multifilament thread and ground to a square ending. The full leak rate is that of about two bubbles per second, and when the gas supply rate is a fraction of this, an excellent pressure stability results. A “bucker”, Le., a variable, battery based, potential source, acting as a fine additional suppression control is inserted into the line between the suppressed output and the amplifier. This permits fine control of the set point of the manostat over the range of the recorder (*0.5 dyn/cm) without touching the coarser suppression control of the Validyne. All glassware carries a band of Teflon tape separating the surface exposed to liquid from that which is handled. All glassware (with the Teflon bands) is baked overnight at 275 “C in an air oven. The exceptions are the capillaries which are suspended with their tips immersed in a biocidal 0.02% sodium azide solution. The water is obtained from deionized water and distilled in a 2-L Pyrex still, the highest point of which is heated to provide a dry spot preventing any film creep of impurities. Chunks of Teflon in the boiling flask prevent bumping effectively. The distillation is at a rate of about 100 cm3/h and at least the first 150 cm3are rejected and the next 800-1200 collected. The water is then dispensed from below the surface with a siphon. Calibration a n d Procedure The capillaries were calibrated repeatedly with water. The latter was considered acceptable if there was no visible ST change between BI’s of 0.5 and 10 sec. Water, of course, reaches its equilibrium ST immediately, but at shorter BI’s aerodynamic effects produce a significant increase in the measured pressure. The absence of a decline in ST shows that no impurities are reaching the surface. A few times the ST decreased slightly up to a BI of 1 s but showed no further change at longer ones. This I attributed to impurities in the gas phase which would of course be adsorbed very rapidly. Addition or replacement of the Sep-Pak and slowing of the flow of gas when pressurizing the system seemed to eliminate this problem. The standard deviations of repeated calibrations of a capillary over weeks were less than 0.02 dyn/cm when the capillaries were kept in the azide solution. The deviations were higher, but still less than 0.03, when the capillaries were simply kept protected in air. Acceptable water was so free of surfactants that it always had zero bubble persistence; i.e. there was no visible bubble on the surface during bubbling, which is, at first, a rather surprising sight. The hydrostatic pressure due to immersion was established during calibration by immersing the capillary further into the water by measured amounts and noting the corresponding pressure increase. As calibration with water establishes only one point of the pressure/ST relation, linearity of the pressure measurement is essential. The measuring system-from transducer to the DVM-was therefore compared to a precision water manometer. The results fitted a straight
432
Langmuir 1986,2,432-438
line with no significant intercept or curvature and a standard deviation of 0.013 dyn/cm. The apparatus was relatively insensitive to vibrations. Pounding of the bench on which it was set lowered the pressure at which the bubble detached by less than 0.05 dyn/cm. This shows that the bubble is rather stable against mechanical disturbances, presumably due to its smallness. The general procedure was therefore to thermostat the transducer with the capillary removed and establish the zero pressure reading. Then the capillary was attached and immersed to the desired depth into the solution, generally 5 mm, and the latter thermostated. The pressure was then raised slowly to the approximate ST to be measured and a t least half an hour later the measurements were performed, first at longer BI's using the manostat and then at shorter BI's using flow rate control mainly by the precision pressure reducer. The immersion zero was checked either after the measurements or before measurements involving rapid bubbling if the solution was strongly foaming. Finally the capillary was removed and the pressure zero verified after about an hour.
For most precise measurements the suppression of the CD12 was set so that the pressure corresponding to a convenient BI was set to the middle of the recorder chart without the bucker. Pressure could then be changed by *0.5 dyn/cm by the bucker alone with immediate return to the original pressure when desired by switching off the bucker. At high bubbling rates, where the manostat could not produce a constant enough pressure to be measured by the DVM, the chart of the recorder was used as the first basis of measurement, and when the pressure changes extended beyond the chart, the suppression of the Validyne output was relied upon with some loss of precision. As the high bubbling rates are intrinsically less reproducible because of irregular convection, this did not represent a real problem.
Acknowledgment. This work was made possible by the hospitality of Prof. Bruno Zimm's laboratory and a grant from the Research Corporation Foundation. I am also grateful to Dr. Mark Troll for the design of the amplifier and many stimulating discussions.
Structure and Transport in the Microemulsion Phase of the System Triton X-100-Toluene-Water Mats Almgren,*t Jan van Stam,+Shanti Swarup,t*tand J.-E. Lofrothe The Institute of Physical Chemistry, University of Uppsala, S-751 21 Uppsala, Sweden, and Department of Physical Chemistry, Chalmers University of Technology and University of Goteborg, S-412 96 Goteborg, Sweden Received November 20, 1985. I n Final Form: February 27, 1986 The microemulsion phase of the system Triton X-l Wtoluene-water has been studied by time-resolved fluorescence quenching, conductivity measurements (of added NaCl), and determination of self-diffusion coefficients. The fluorescence quenching results are compatible with the existence of reversed micelles over all compositionsstudied, which means down to 17% toluene and 15% water, but with a rapid exchange of the probes and quenchers between the droplets, in particular at high water and low toluene concentration. The exchange is assumed to be mediated by a fusion-fission process in which two droplets merge into a labile large drop, which rapidly splits up in two drops again. Qualitatively,the conductivityof added NaCl changes in the same way as the exchange rate of the solutes from the fluorescence quenching. The self-diffusion coefficient of water changes comparatively little and is larger than what can be explained by the fusion-fission process alone. In addition to the exchange information, the fluorescence quenching study yields the concentration of droplets. With an assumption about the composition of the intermicellar solution, the size of the droplets can be estimated. The phase diagram shows that extensive water solubilization occurs only above a critical concentration of the surfactant in toluene. Assuming that the intermicellar solution has this critical composition throughout, the droplet sizes were calculated. The hydrophilic radius was found to vary between 30 and 46 A, increasing with increming water content, whereas the area available per surfactant at the surface of the hydrophilic drop was constant at 77.2 f 1.6 A2. Introduction The term microemulsion, introduced by Hoar and Schulmanl already in 1943, was given a generous definition by Danielsson and Lindman2as "a system of water, oil, and amphiphile which is a single optically isotropic and thermodynamically stable liquid solution". Most interesting, technically and scientifically, are those microemulsions in which oil and water are both major constituents. As 'University of Uppsala. Present address: Rutgers University, Department of Chemistry, Piscataway, NJ 08854. Chalmers University of Technology and University of Goteborg.
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witnessed by several recent conference volume^,^ the microstructure and transport mechanisms in microemulsions are far from well-known, although the main features of some systems appear well established. Ternary systems composed of Aerosol OT (AOT),water, and a hydrocarbon are notable examples where well-defined and comparatively well-characterized water droplets exist over a wide (1)Hoar, T. P.; Schulman, J. H. Nature (London) 1943, 152, 102. (2) Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3, 391. (3) (a) Robb, I. D., Ed. Microemukrions; Plenum Press: New York and London, 1982. (b) Mittal, K. L., Lindman, B., Eds. Surfactants in Solution; Plenum Press: New York and London, 1984;Vol. 3. (c) Lindman, B., Olofson, G., Stenius, P., Eds. Prog. Colloid Polym. Sci. 1985, 70.
0 1986 American Chemical Society
