The kinetics of phase separation in the succinonitrile-water system are being investigated. Experiments involve initial physical mixing of the two immiscible liquids at a temperature above the consolute, decreasing the temperature into the miscibility gap, followed by imaging of the resultant microstructure as it evolves with time. Refractive index differences allow documentation of the changing microstructures by noninvasive optical techniques without the need to quench the liquid structures for analysis

Sadoway, D. R.

English

NASA Technical Reports Server

I .  
Phase Separation Kinetics in Immiscible Liquids 
Principal Investigator: Professor D.R. Sadoway I. r 0” 
Abstract 
The kinetics of phase separation in the succinonitrile-water system are being investigated. 
Experiments involve initial physical mixing of the two immiscible liquids a t  a temperature above the 
consolute, decreasing the temperature into the miscibility gap, followed by imaging of the resultant 
microstructure as it evolves with time Refractive index differences allow documentation of the 
changing microstructures by noninvasive optical techniques without the need to quench the liquid 
structures for analysis. 
Introduction 
Miscibility gaps, both stable and metastable, are known to exist in many technologically 
important systems. Control of phase separation phenomena is essential in achieving microstructures 
that yield desired properties. In this regard it is imperative that the transformation kinetics be well 
understood. 
In many cases heterogeneous interconnected microstructures result. In early studies 
investigators were prompted to conclude that the mechanism of trz nsformation is spinodal 
decomposition.1 Further work has cast doubt upon some of the conclusions of these investigations 
and indicated that inference of the mechanism of separation from c,bservations of the final phase 
morphologies can be completely misleading.2-7 I t  is clear that the cinetics of the transformation must 
be studied as it occurs, especially during its earliest stages when d lfferences in mechanism are best 
elucidated. The later stages of any transformation are dominated 3y coarsening kinetics.2-8 
Reduced gravity offers the opportunity to produce microstructures unattainable on earth. 
Specifically, it is possible to solidify two-phase liquids as dispersions, thereby yielding exciting 
properties as a consequence of the metastable microstructure. The generation of dispersed second 
phase droplets in reduced gravity poses interesting questions in phase transformation kinetics. It is 
well known from the now classical studies of solid-solid transform(ations that within the miscibility 
gap phase transformation occurs not simply by nucleation and growth but, over well defined regions 
of composition and temperature, by spinodal decomposition instead. The same behavior is expected 
with liquid-liquid phase transformations. From the standpoint of fundamental studies liquid-liquid 
phase transformations are interesting because “matrix effects” present during the demixing of solid 
solutions are not expected to be significant in liquids. 
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https://ntrs.nasa.gov/search.jsp?R=19890004302 2020-03-20T04:21:08+00:00Z
The Present Study 
The present study is an  attempt to follow phase separation kinetics i immiscible liquids. The 
classical studies separation have been done on solid phase system: liquids stratify in a gravity field. 
Thus, the two principal techniques are electron microscopy and electromagnetic scattering (both light 
scattering and small angle x-ray). In low-gravity phase separation studies of liquids that are 
transparent to visible light a variety of optical imaging techniques based upon differences in 
refractive index become available, for example, laser schlieren imaging.9 Experiments then involve 
, an  initial physical mixing of the two immiscible liquids followed by imaging of the resultant 
' microstructure as it evolves with time. 
Reduced gravity studies are important because they allow the observation of phase separation 
without stratification which occurs when the second phase particles grow beyond the size at which 
interfacial tension no longer counteracts the force of gravity. In reduced gravity there is no need to 
alter the relative densities or interfacial energies of the two phases by adding stabilizers, nor is it 
necessary to quench the liquid structures for analysis. Furthermore, the experiments can be repeated 
by simply remixing. As part of the present study, laboratory scale equipment is being designed and 
built such that it can be readily adapted for zero gravity flight. 
~ 
~ 
The results will be compared with theory. For constant volume fractions of the separated 
phases, the average particle diameter is expected to increase a s  the cube root of time and the number 
of particles to decrease as  the reciprocal of time.10 Much work has been done in oxide glass systems 
and has been exhaustively reviewed by Uhlmann and Kolbeck.11 While agreement is good for 
systems undergoing spinodal decomposition, there is no comprehensive analysis for phase separation 
by the competing nucleation and growth mechanism 
I t  has been suggested that electron microscopy be used to differentiate between nucleation and 
growth and spinodal decomposition.12 For nucleation and growth the volume fraction transformed 
should increase with time (following an incubation period), while for spinodal decomposition the 
volume fraction should in many cases decrease with time to reflect a transition from compositional 
waves symmetric about the mean concentration to compositional waves fluctuating between the 
equilibrium concentrations (seen as relative volume fraction) of the two phases. The experimental 
technique should be able to observe the decrease in second-phase volume fraction during the period of 
this transition. Such measurements, however, are not simple because of the dXiculties in 
dlfferentiation between the two phases and in accurately determining their volume fractions. One of 
the goals of the initial phase of this study is to develop the optical techniques to permit such accurate 
measurements. 
The physical system under investigation is succinonitrile-water which exhibits liquid 
irniscibility and has a consolute temperature of 56°C. 13 However, the compositional dependence of 
several important physical properties is not know, specifically, the density and interfacial tension. 
As a result, density measurements by the direct Archimedean technique are being performed to 
support the kinetic studies described above. 
The results of this study will be of importance in commercially significant systems such as 
inorganic glasses, biochemicals, semiconductors, colloidal suspensions, and aircraft metal alloys. 
I Reference List 
1. J.W. Cahnand R.J. Charles, Physics Chem. Glasses 6 (1965): 181. 
2. W. Hailer, J .  Chem. Phys. 42 (1965): 686. 
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3. R.W. Hopper and D.R. Uhlmann, Disc. Faraday Soc. 56 (1970): 166. 
4. C.R. Srinivasan, J .  Tweer, P.B. Macedo, A. Sarker, and W. Haller, J .  Non-cryst. Solids 6 (1971): 
221. 
5. T.P. Seward 111, D.R. Uhlmann, and D. Turnbull, J. Am. Ceram. SOC. 51 (1968): 634. 
6. W. Haller and P.B. Macedo, Physics Chem. Glasses 9 (1968): 153. 
7. P.F. James and P.W. McMillan, Physics Chem. Glasses 11 (1970): 59 and 11 (1970): 64. 
8. J. Zarzycki and F. Naudin, J. Non-cryst. Solids 1 (1969): 215. 
9. J.K. Koziol and D.R. Sadoway, “Electrolyte Flow Patterns in Molten Salt Electrolysis Cells,’’ 
Energy Reduction Techniques in Metal Electrochemical Processes, eds. R.G. Bautista and R. 
Wesely (Warrendale, PA:TMS-AIME, 1985). 
10. D.C. Burnett and R.W. Douglas, Physics Chem. Glasses 11 (1970): 125. 
11. D.R. Uhlmann and A. G. Kolbeck, Physics Chem. Glasses 17 (1976): 146. 
12. C.R. Srinivasa, A.  Sarkar, P.K. Gupta, and P.B. Macedo, J. Non-cryst. Solids 20 (1976): 141. 
13. J.E. Smith, J r . ,  D.O. Frazier, and W.F. Kaukler, Scripta Met. 18 (1984): 677. 
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Phase separation kinetics in immiscible liquids

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