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Langmuir 1994,10,629-631
Quasi-Isothermal Liquid/Liquid Phase Separation in a n-Butoxyethanol/Water Mixture under Reduced Gravity Barbara Braun, Christian Ikier, Hermann Klein,' Gerd Schmitz, and Kurt Wanders Institut fiir Raumsimulation, DLR, 51147 KBln, Germany
Dietrich Woermann Institut fiir Physikalische Chemie, Universitiit Kaln, 50939 Kaln, Germany Received October 19,1993. In Final Form: December 20,199P Liquidlliquid phase separation induced by a fast pressure jump is observed holographically under reduced gravity during the Spacelab Mission D2. The experiment is carried out under quasi-isothermal conditions with a n-butoxyethanol(abbreviatedby n-C&)/water mixture of noncritical Composition in the water-rich region of the lower branch of the closed-loop coexistence curve (y = 0.105,maas fraction of n-C&). After a transition period of t < 60 s the droplets of the n-C&-rich minority phase do not change in size and remain finely dispersed as long as isothermal conditions are maintained (P= 63.5 "C). A marked increase of coalescenceof the droplets occurs when temperature gradients are generated at the end of the experiment during the cooling process of the sample. temperature of phase separation has a value of (dTp/dP), = 40 mK/bar which is considerably larger than the adiabatic temperature coefficient (aT/aP), = 6 mK/bar characterizingthe temperature change associatedwith the fast pressure release.2 Therefore, a pressure jump with an amplitude AP of the order of -10bar takes place under quasi-isothermal conditions: The adiabatic temperature change of 60 mK is small compared to an overheating of 400 mK. This is different from the situation usually encountered in melt-processing of monotectic alloys. During cooling through the miscibility gap of the melt phase, large temperature gradients and thus thermocapillary migration of the melt precipitations are likely to be induced. In the system n-C&/water, evidence of thermocapillary migration of droplets of the minority phase under reduced gravity in space has been found.3
Introduction Unexpected large-scale separation of phases rather than a morphologyof a fine dispersion was found in monotectic alloys after melt-processing under reduced gravity in space.' This has attracted new interest in the separation of liquid phases in systemsof partial miscibility. On earth, massive phase separation in liquids and melts is mostly due to the action of gravityin combinationwith the density differencesbetween the emerging phases. Similar effects observed in space are not fully understood yet. Thermocapillary migration and coalescence of droplets of the emerging phase are assumed to play a major role. The present experiment is another attempt to single out the relevant mechanisms. The special aspect of the experiment discussed in this paper is to observe the separation of two liquid phases in the absence of thermocapillary effects. The experiment was carried out under reduced gravity in the Holographic Optics Laboratory (HOLOP) of the D2 Spacelab Mission in April/May 1993. The system studied is a n-butoxyethanol (abbreviated n-C&)/water mixture of noncritical composition in the water-rich region of the lower branch of the closed-loop coexistence curve.2 The mixture has a composition of y = 0.105 (mass fraction of n-C&; yc = 0.2945, critical composition) and a temperature of phase separation of Tp,l= 63.1 OC at atmospheric pressure. The liquid/liquid phase separation process is triggered by a pressure jump (pressure release, relaxation time 7 = 0.1 s), bringing the system from a statein the stable region to its phase diagram into the metastable region. The system reaches its equilibrium state by separating into two coexisting liquid phases. Under reduced gravity conditions (residualgravity accelerationabout 106 g during this experiment), droplets of the n-C4El-richphase remain dispersed in the waterrich majority phase. The pressure coefficient of the
Experimental Section The transparent sample mixture is fiied in a cylindrical cell (diameterlOmm,thickneee0.8mm). It isencloeedwithadoubla jacket thermostatwith two sapphirewindows. The temperature of the sample is stabilized to 6T = hO.01 K. The pressurizing medium is nitrogene gas. It is separated from the mixture to be studied by a flexible membrane. The pressure jump from high to low pressure is initiated by opening an electromagneticvalve. The formation of the n-C&rich phase is followed by means of holographic image recording. In the initialstate (indexi) the sample is in thermalequilibrium at a temperature Ti = T p , l + 0.5 K and at an elevated pressure Pi = 19 bar. Under this condition the mixture forme a homogeneous singleliquid phase. Immediately after releasing the pressure, the system is in a metastable state (Ti= Ti; Pi = 1 bar) and separates into two coexisting liquid phases. This processtakesplace under almoet isothermalconditions,neglecting the small adiabatic change associated with the pressure jump. The enthalpy of demixing does not lead to a measurable temperature change because the ratio of the volume of the minority phase to that of the majority phase is very small (see below). The development of the separation process is observed holographically for 60 min. Fifty holograms are taken during that time at differently spaced time intervals.
* To whom correspondence should be addressed.
Abstract publiahedin Advance ACSAbstmcts, February 1,1994. (1) Predel, B.; Ratke, L.; Fredrickeson, H.In Fluid Sciences and
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Material Science in Space; Walter, H. U., Ed.; Springer Verhg: Berlin, 1987; p 617. (2) a) Sieber, M.;Woermann, D. Eer. Bunsen-Ges. Phys. Chem. 1991, 96,16. b) Aizpiri, A. 0.;Monroy,F.; Delcampo, C.; Rubio, R. G.; Pena, M.D. Chem. Phys. 1992,165,31.
0743-7463/94/2410-0629$04.50/0
(3) Braun, B.; Mer, Ch.;Klein, H.; Woermann, D. J. Colloid Interface Sci. 1993, 159, 616.
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0 Figure 1. Reconstructed holographic plane (sample section of 0.225 X 0.225 mm2)showing equal-sizeddroplets (4 0.5 pm in diameter) of the n-butoxyethanol-richphase in a n-butoxyethanol/water mixture of noncritical composition (y = 0.105, mass fraction of n-butoxyethanol)under reduced gravity 55 min after initiating a liquid/liquid phase transition by a pressure jump. This picture is typical for the whole isothermal period of 60 min after initiating the phase transition.
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Figure 2. Droplets which can be clearly identified in Figure 1 and which are localized in a layer of 25-pmthicknessof the sample studied in this experiment are marked by circles in this figure.
which form micelles in aqueous solutions,* n-C& cannot be considered a typical nonionic surfactant. But there are several reports in the literature pointing out the formation of aggregates of n-C& in aqueous solutions.7 Therefore, it can be assumed that n-C& aggregates form The phase separationprocess startswith nucleation and growth the interface of the n-C4El-rich droplets studied in this of the droplets.' The droplets are discernible at times larger experiment. The hydrophilic part of the aggregates is than 30 s after triggering the pressurejump. The volume fraction expected to be oriented to the surrounding water-rich of the evolving n-C&-rich liquid phase has a value of about 2 phase; the hydrophobic part is expected to point into the X 103 estimated from the holograms and the liquid/liquid interior of the n-CrEl-rich droplets. For the coalescence coexistence curve of the system on the basis of the lever rule. The of neighboring droplets an activation energy is required image of the droplets and their spatial distribution is reconstructed at a magnification of 800 X (including 6 X preamplito overcome the water-rich layer (solvation barrier) fication of the droplets on the holographic film). between them.6 On earth, under the same experimental conditionsas in the present experiment, visible phase separation starts Results and Discussion with a homogeneous distribution of droplets of a few The holograms (e.g., Figure 1)reveal that the n-butoxymicrometers in diameter, too. The main effect of gravity ethanol-rich droplets have a diameter of about 4 f 0.5 pm. on the droplets is buoyancy which makes the dropletsmove Their size as well as the number density of the droplets upward to the top of the samplevolume. Sincethe droplets (6 X lo7 cm3) is independent of time during the time of are not exactly equallysized, their Stokes' velocitiesdiffer observation under isothermal conditions, i.e., 1 h. The so that eventuallylarger dropletsapproachsmallerdroplets average distance between the droplets is about 25 pm. centrally. In this case coalescence of the droplets is To a first approximation, the droplets are distributed observed. homogeneously in the sample. Under reduced gravity in space buoyancy is absent, of A continuous observationof the droplets is not possible course. Thus, in the present experiment a quasi-stable by the method of detection used in this study. It is noticed fine-dispersed liquid/liquid system is maintained. Isothat the droplet patterns change from one hologram to thermal conditions are indispensible for the stability of the next in an apparently random manner. The velocity this dispersion. This becomes evident when after a period of this motion is estimated to be of the order of 0.1 pm/s. of isothermal conditions(i.e., 1h) the thermostat is turned This value is in accordance with corresponding results of off. Temperature gradients and thermocapillary flows a former space experiment.3 are generated along the droplet surfaces. As a result, the The observationthat the n-C&-rich dropletssuspended droplets begin to coalesce, forming droplets which on an in the water-rich majority phase do not coalesce during average are 25% larger in diameter than during the the isothermalperiod of this experimentis probablyrelated isothermal period. Five minutes after turning off the to the fact that the water-rich phase wets the n-C&-rich thermostat, the temperature of the sample has reached 2 phase in the presence of a third phase, e.g., gas, glass, and deg below the phase separation temperature. The sample ~apphire.~a*5 is then well within the single-phaseregion, and all droplets are dissolved. n-Butoxyethanol is the first member of a series of molecules of the type CiEj [CHS(CH~)~_~O(CH~CH~O)~HI (4) (a) Ikier, Ch.; Klein, H.; Woermann, D. J. Chem. Phys. 1992,96, 859. (b) Meyer, W.; Woermann, D. J. Chem. Phys. 1990,93,4339. (5)Ataiyan, J.; Woermann, D. Manuscript in preparation.
(6) Everett, D. H. Basic Principles of Colloid Science; h y a l Society of Chemistry Paperbacks: London, 1992; Chapter 11. (7)(a) Koga, Y. J. Chem.Phys. 1992,M, 1046. (b) Koga, Y.;Sin, W. W. Y.; Wong, T. Y. H. J. Chem.Phys. 1990,94,3879. (c) Woermann, D. B o g . Colloid Polym. Sei. 1991,84, 165.
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Conclusions The results of this study are summarized as follows: Under reduced gravity, a liquid/liquid phase transition induced by a pressure jump in a water-rich n-butoxyethanol/water mixture under isothermal conditions leads to the generation of a dispersion of n-butoxyethanol-rich droplets. The size distribution as well as the number density of the droplets is independent of time as long as the isothermal conditions are maintained. The stability of the dispersion is assumed to be due to surfactant-like properties of the n-butoxyethanol molecules which cause a resistance to thinning of water-rich films between neighboring n-C&-rich droplets when they approach each other? Marked coalescence of the droplets sets in when
temperature gradients and in turn thermocapillary flows are generated along the droplet surfacesduring the cooling process at the end of the experiment. These findingsstress the importance of interfacial and wetting phenomena for melt-processingof dispersion alloys under reduced gravity.
Acknowledgment. B.B., Ch.I., H.K., and G.S. thank Professor B. Feuerbacher for valuable discussions and suggestions. We thank H. D. Masslow for his expert mechanical design and G. Warmbold for his expert electronicdesign of the experimental hardware. Without their never tiring engagement the experiment NUGRO could not be carried out successfully. This research was supported by DARA (Bonn).
