Demonstration pubs.acs.org/jchemeduc
Partially Miscible Water−Triethylamine Solutions and Their Temperature Dependence Johan P. Erikson* Saint Joseph’s College, Standish, Maine 04084, United States ABSTRACT: A simple and straightforward demonstration of the temperature dependence of partial miscibility is described. The Gibbs phase rule, temperature−composition phase diagrams, and the lever rule serve as the basis for interpretation of the demonstration. The water− triethylamine system is miscible in all mixing ratios at temperatures less than 18 °C, but separates into two immiscible phases for a wide range of bulk compositions as temperature increases through 19−24 °C. A procedure is described in which a range of bulk compositions simultaneously experience an increase in temperature. Students are able to see differences in the temperature of phase separation, plus the evolving compositions and volumes of the separated phases. KEYWORDS: Upper-Division Undergraduate, Demonstrations, Physical Chemistry, Misconceptions/Discrepant Events, Aqueous Solution Chemistry, Phases/Phase Transitions/Diagrams, Solutions/Solvents
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INTRODUCTION Complete miscibility and complete immiscibility of two liquids are similarly familiar to undergraduate and high-school students. Less familiar are partially miscible systems, in which liquids mix at some, but not all, proportions and temperatures. Nevertheless, these partially miscible systems are of considerable interest in academia and industry.1−4 In a typical undergraduate presentation, instructors strive to present (a) equations to calculate measurable variables, (b) a theoretical basis for the phenomena and equations, and (c) tangible demonstrations or lab examples of phenomena. When treating solutions, full miscibility is typically assumed, with a theoretical treatment based on maximum entropy mixing of neutral, nonpolar molecules. Deviations from ideal behavior, specifically immiscibility, typically lead to the stock phrase “like dissolves like” and its numerous examples. Theoretical discussion may include the relative magnitudes of intermolecular forces between molecules. A step further along this progression leads to partial miscibility. The phenomenon of partial miscibility may be introduced early, yet it may not be until physical chemistry courses that theoretical discussion of chemical potentials5 or thermodynamics6 ensues as related to miscibility.
Graphical presentation of binary partially miscible systems is the norm (Figure 1). In a T−X (temperature−composition) diagram, regions of one homogeneous solution are separated from regions of two immiscible liquids by a phase boundary. At a temperature and bulk composition within the immiscible region, coexisting liquids, LiqA and LiqB, are connected by tie lines at their compositions (x1 and x2, respectively). The lever rule is used to determine their relative amounts (because the Gibbs phase rule says nothing about quantities), noting that length of lever arm inversely correlates with amount: LiqA LiqB
x 2 − x0 x0 − x1
Range of Phenomena and the Temperature−Composition Diagram
Figure 1 is representative of many partially miscible liquids, in that it shows increasing miscibility with increasing temperature. Examples include phenol/water and hexane/nitrobenzene. In this case, at all temperatures above the upper consolute temperature (Tuc), the compounds are miscible in all proportions. In contrast, at temperatures below the Tuc, either one or two liquids may be present, depending on the composition of the bulk liquid relative to the phase boundary. Minimization of Gibbs free energy is the fundamental thermodynamic reason for immiscibility.8 At the molecular level, phase separation is generally favored at lower temperatures due to favorable bonding energetics between like
Conceptual Foundation
The Gibbs phase rule aids interpretation of partial miscibility.7 Under appropriate conditions, the chemical components are miscible and thus form one phase; temperature and composition are independently variable. In the immiscible, two-liquid phase situation, temperature and composition of the two phases are not independent, meaning that, at a particular temperature, each of the two phases has a fixed composition. © XXXX American Chemical Society and Division of Chemical Education, Inc.
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Received: July 1, 2016 Revised: October 25, 2016
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DOI: 10.1021/acs.jchemed.6b00489 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
Figure 1. Temperature−composition (T−X) diagram for a hypothetical mixture of two partially miscible liquids. Composition is indicated as mole fraction of compound “J” increasing to the right. Tuc is the upper consolute temperature. A mixture of composition x0 will form a single miscible phase at all temperatures above the phase boundary. As temperature drops, the monotectic phase boundary will be intersected, and two immiscible liquids will form. Compositions of the separated liquids (LiqA and LiqB) are indicated at the phase boundary (x1 and x2, respectively). Amount (in moles) of each liquid is determined through the lever rule.
Figure 2. Temperature−composition diagram for a mixture of triethylamine (TEA) and water, modified after Hales et al. (1966). The lower consolute temperature (Tlc) is shown at the composition of XTEA = 0.08 (VolTEA = 0.4). Red arrows mark the 4 central, recommended solution mixtures from left to right at VolTEA = 0.2, 0.4, 0.6, and 0.8, respectively. For an x0 = 0.162 (VolTEA = 0.6; tube #4), one miscible phase is present at T < 18 °C. As T increases and the phase boundary is encountered, two immiscible phases are formed: initially almost entirely LiqTEA‑rich. As T increases further, LiqTEA‑rich decreases in volume, as indicated by the increasing value of x2 − x0.
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molecules, but is overcome by thermal motion at elevated temperatures.5 Other partially miscible liquids become less miscible with increasing temperature and thus have a lower consolute temperature (Tlc). This situation is generally but not universally considered to occur due to a weak complex formed between the two liquids at lower temperatures that is disrupted at higher temperatures.5 A representative example is triethylamine (TEA, N(CH2CH3)3) and water (Figure 2).9−12 In their immiscible condition, a layer of TEA saturated with water floats above a layer of water saturated with TEA. However, the compositions (i.e., molar ratios) of the separated liquids will change as temperature is varied. Other, more rare possibilities include systems with both upper and lower consolute temperatures: the T−X diagram for water and 2-methypiperidine has a single closed loop of two miscible phases surrounded by a singlephase region, whereas the T−X diagram for sulfur and benzene has a two-phase region with a lower consolute temperature above a two-phase region with an upper consolute temperature.8 Students regularly have difficulty with interpretation of a T− X diagram and the implications of the Gibbs phase rule. In particular, they are often challenged by (1) the concept that the composition (concentration) of the separated liquids are fixed at any particular T, but change with changing temperature, and (2) that the relative amounts of the separated liquids may change. Many students struggle with visualization, in that they do not “believe” or intuit the T−X graph in the absence of a tangible example. Below, I discuss a demonstration of partial miscibility of TEA and water at near-room-temperature conditions.
DEMONSTRATION
Preparation and Materials
Partial miscibility, interpretation of the T−X diagram, lever rule, and the composition of the separated solutions may be discussed prior to or after the demonstration, depending on instructor preferences. However, students should be instructed prior to the demonstration to look for subtle changes in color intensity and volumes of separated liquids. The instructor should carry out the demonstration in a fume hood. Note that trace amounts of third components in the nominally binary system may change the temperature of phase separation by 23 °C, the four central tubes will have separated (Figure 3h) while the outer tubes (1 and 6)
d = 0.998 g/mL at 25 °C d = 0.7255 g/mL at 25 °C
The intensity of coloring imparted by the dye can be used as a qualitative proxy for concentration. The difference in color C
DOI: 10.1021/acs.jchemed.6b00489 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
will show minor changes. At bath T above 26 °C, little water remains in the TEA-rich solutions of the central tubes, and the solution at VolTEA = 0.9 has partially separated (Figure 3i).
(3) Bermúdez-Salguero, C.; Gracia-Fadrique, J. Phase segregation at the liquid-air interface prior to liquid-liquid equilibrium. J. Phys. Chem. B 2015, 119, 10304−10315. (4) Dimitroulis, C.; Kainourgiakis, E.; Raptis, V.; Samios, J. Molecular dynamics study of the local structure and diffusivity of partially miscible water/n-alcohols binary mixtures. J. Mol. Liq. 2015, 205, 46− 53. (5) Atkins, P.; de Paula, J. Physical Chemistry, 8th ed.; W.H. Freeman: New York, 2006; pp 174−193. (6) Logan, S. R. The behavior of a pair of partially miscible liquids. J. Chem. Educ. 1998, 75, 339−342. (7) Alper, J. S. The Gibbs phase rule revisited: Interrelationships between components and phases. J. Chem. Educ. 1999, 76, 1567−1569. (8) Acree, W. E., Jr. Thermodynamic Properties of Nonelectrolyte Solutions; Academic Press: Orlando, 1984. (9) Hales, B. J.; Bertrand, G. L.; Hepler, L. G. Effects of third components on critical mixing in the water-triethylamine system. J. Phys. Chem. 1966, 70, 3970−3975. (10) Bertrand, G. L.; Larson, J. W.; Hepler, L. G. Thermochemical investigations of the water-triethylamine system. J. Phys. Chem. 1968, 72, 4194−4197. (11) Campbell, A. N. Complete and partial miscibility in ternary system: triethylamine-methylethylketone-water. Can. J. Chem. 1980, 58, 846−850. (12) Ikehata, A.; Hashimoto, C.; Mikami, Y.; Ozaki, Y. Thermal phase behavior of triethylamine-water mixtures studied by nearinfrared spectroscopy: band shift of the first overtone of the C-H stretching modes and the phase diagram. Chem. Phys. Lett. 2004, 393, 403−408.
Observations
Students should note changes in both (1) the volumes of the immiscible liquids relative to the grease mark on each tube; and (2) the composition of the immiscible phases as indicated by the intensity of color. Note that, in Tube 2, the contact surface between the solutions is initially above the grease mark, whereas in Tubes 4, 5, and 6, the contact is below the grease mark. As temperature increases, all solution contacts move toward the grease mark, i.e., toward the original volumes of the pure compounds. Students should be aware that as the TEArich liquid becomes less colored, and the water-rich liquid becomes more intensely colored, that the water (and dye) and TEA molecules are segregating more strongly. The evolving volumes and compositions can be interpreted in terms of the lever rule, as noted previously. Two common problems are related to the heating rate. Overly slow heating, or overly cold initial water temperature in the warm bath, will lengthen the demonstration. Conversely, overly rapid heating leads to apparently instantaneous transitions. The duration of the demonstration varies with practice, heating rate, and extent of discussion and detail presented by the instructor. A satisfactory demonstration will require 15 min. For multiple demonstrations, simply return test tubes to the cold bath and start again (assuming no loss of volatile TEA).
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HAZARDS AND SAFETY Triethylamine (TEA) is a hazardous liquid with potentially harmful effects for humans and the environment. TEA is an irritant for the eyes, skin, and lungs with acute exposure. TEA has a high vapor pressure and thus readily evaporates and is flammable. This demonstration should be carried out in a fume hood away from ignition sources. Gloves and goggles should be worn, and an SDS for TEA should be consulted beforehand. Small spills of TEA should be diluted with cold water and mopped up, and then disposed with appropriate waste. All solutions should be disposed with similar wastes.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS The author wishes to thank Nick Benfaremo, Ryan Dorland, and Emily Lesher at Saint Joseph’s College for constructive feedback and manuscript review. Four anonymous reviewers provided constructive and effective feedback.
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REFERENCES
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DOI: 10.1021/acs.jchemed.6b00489 J. Chem. Educ. XXXX, XXX, XXX−XXX
