Laboratory Experiment pubs.acs.org/jchemeduc
Polymer Inverse Temperature-Dependent Solubility: A Visual Demonstration of the Importance of TΔS in the Gibbs Equation David E. Bergbreiter,* Alexander J. Mijalis, and Hui Fu Department of Chemistry, Texas A&M University, College Station, Texas 77843-3012, United States S Supporting Information *
ABSTRACT: Reversible polymer dehydration and precipitation from water due to the unfavorable entropy of hydration is examined using a melting-point apparatus. The thermoresponsive lower critical solution temperature (LCST) behavior of poly(N-isopropylacrylamide) (PNIPAM) is responsible for these effects. An experiment is described that allows students to test the effects of different salts at various concentrations on PNIPAM’s LCST. These studies demonstrate the Hofmeister effect of salts on macromolecule hydration. The use of readily available melting-point equipment and sealed capillary tube samples of aqueous solutions of PNIPAM in various salts provides an economical and simple way to study these phenomena. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Laboratory Instruction, Organic Chemistry, Polymer Chemistry, Hands-On Learning/Manipulatives, Materials Science, Noncovalent Interactions, Thermodynamics
E
ntropy is a concept everyone can understand. Professors allude to the disordered state of a dorm room, a teenager’s bedroom, or their own offices as analogies of the thermodynamic preference of molecules for a more disordered state. Everyone recognizes that achieving order in a room or on a desk requires work (i.e., energy), another analogy to the concept that disorder and greater entropy is a lower energy state and thus more favorable. However, experiments that show how entropy affects chemical processes are uncommon. Here, an experiment is described where polymer solvation, specifically the inverse temperature-dependent solubility of polyacrylamides and related polymers in water, illustrates entropy effects using melting-point apparati. Enthalpy and entropy are introduced early in chemistry courses. The Gibbs equation
because thermodynamics are often reviewed early in the organic course. However, this experiment could also be expanded for use in a physical chemistry or polymer chemistry laboratory by using a wider range of polymers or salts.
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THERMODYNAMICS OF POLYMERIZATION REACTIONS AND POLYMER SOLVATION Explanations about the favorability of chemical processes or reactions that ignore entropy sometimes fail. This is evident in polymerization reactions that have a ceiling temperature above which the propagation steps that form a macromolecule from monomers become thermodynamically unfavorable.1 Above this ceiling temperature, depolymerization of a polymer to form monomers occurs because of the unfavorable ΔS of a polymerization process. α-Methylstyrene polymerization (Scheme 1) is an example.2 At low temperatures, this polymerization successfully couples hundreds or thousands of monomers to form a macromolecule with n monomers covalently bound one to another by breaking a carbon−carbon π bond and forming a stronger carbon−carbon σ-bond. This results in a favorable ΔH because the bonds that form are stronger than the bonds that are broken. Although the ΔH in Scheme 1 is favorable, the ΔS term in the Gibbs equation is negative and unfavorable because macromolecule formation makes n individual monomer molecules into a few macromolecules. When the temperature reaches a critical value called the ceiling temperature, this unfavorable TΔS term cancels out the favorable ΔH term and depolymerization to form starting materials occurs.
(1)
ΔG = ΔH − T ΔS
relating enthalpy (ΔH) and entropy (ΔS) to Gibbs free energy (ΔG) shows how thermodynamics determines the ultimate success of a chemical reaction or process. A chemical process is favorable if ΔG is negative, so eq 1 shows how ΔH or ΔS each determine whether a chemical process is favored. Early discussions of thermodynamics in organic chemistry often use the enthalpy changes associated with bond formation and cleavage to explain why a process is favored. The contribution of ΔS to chemical processes is underemphasized. Students are shown that entropy matters too using the change in entropy associated with macromolecule solvation. The thermoresponsive solubility of a macromolecule in water is examined where the chemical process of dissolution is favored when a solution is cold but is disfavored on heating. This experiment is used as part of the introductory organic laboratory course because it uses the same apparatus used for melting-point analyses and © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: March 5, 2012 675
dx.doi.org/10.1021/ed2003669 | J. Chem. Educ. 2012, 89, 675−677
Journal of Chemical Education
Laboratory Experiment
Scheme 1. Polymerization of α-Methylstyrene Where Higher Temperatures and an Unfavorable ΔS Term Leads to Depolymerization in Spite of a Favorable ΔH Value in the Gibbs Equation
such as sodium citrate or sodium sulfate lead to greater changes in protein hydration and significant decreases in PNIPAM’s LCST. LCST measurements of PNIPAM solutions containing different salts at various concentrations show changes in clouding points (changes in the LCST) and illustrate how changes in salt identity and concentration affect macromolecule hydration and solubility. These salt effects are known as the Hofmeister effect and have been classically explained in terms of a salt’s ability to disrupt (chaotropes) or enhance (kosmotropes) water structure. However, more recent research has suggested that salt effects on water solvation of a macromolecule are more important than the water structure forming or water structure disruption properties of salts.8−10
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MELTING-POINT APPARATUS A melting-point apparatus is used as a temperature-controlled environment for LCST studies of macromolecule solutions in sealed melting-point capillary tubes. A conventional Mel-Temp apparatus can be used, but an OptiMelt digital melting-point apparatus available from SRS11 more accurately and reproducibly measures the onset of clouding. The OptiMelt apparatus also cools readily, allowing 25 students to each carry out four or five assays of samples in sealed melting-point tubes in a 150 min laboratory period with a single apparatus. The OptiMelt apparatus measured an LCST of 32.41 °C (at a 1 °C/min ramp rate) with a standard deviation of 0.01 °C for 2 wt % PNIPAM in water. This same group of 25 students also measured the LCST for a 2 wt % PNIPAM solution in water using a MelTemp apparatus. With the Mel-Temp, the average 32.13 °C LCST was similar, but the standard deviation was 0.91 °C.
The importance of ΔS in the Gibbs equation can be illustrated using the temperature-dependent dissolution of poly(N-isopropylacrylamide) (PNIPAM; Figure 1) in water.
Figure 1. Structure of poly(N-isopropylacrylamide) (PNIPAM).
PNIPAM has a lower critical solution temperature (LCST): a temperature at which PNIPAM phase separates from water.3,4 When a PNIPAM macromolecule dissolves in water at room temperature, as many as three water molecules hydrogen bond to each repeat unit’s pendant amide groups in the primary hydration sphere. Because PNIPAM has many repeat units, complete hydration of a PNIPAM macromolecule requires organization of many water molecules; a process that is entropically unfavorable just as coupling many monomers to form a single polymer molecule is entropically unfavorable in the polymerization in Scheme 1. An unfavorable entropy term means that at some temperature (the LCST), the ΔG term for dissolution free energy will change from negative (favorable) to positive (unfavorable) because the quantity −TΔS becomes more and more positive as temperature increases. The students visually observe the critical change in the Gibbs energy when a polymer precipitates from solution on heating. The precipitation produces a solid that forms an opaque lightscattering suspension. A melting-point apparatus can be used to detect this phase change and determine the LCST by measuring the cloud point, the temperature at which a clear solution starts to become cloudy. Students can also observe that the polymer redissolves on cooling after the melting-point capillary tube is removed from the melting-point apparatus. PNIPAM (Figure 1) is the stimuli responsive polymer studied here, but many other polymers including polyethers (Pluronic polymers), methyl cellulose, or elastin-like polypeptides (ELPs) also have LCSTs.5−7
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THERMALLY RESPONSIVELY SOLUBLE POLYMER SOLUTION PNIPAM is commercially available from several vendors including Sigma-Aldrich and dissolves in cold water with stirring though the dissolution process can be slow AND sometimes takes overnight. The molecular weight of PNIPAM is not important as long as the degree of polymerization is higher than 100. Microliter samples of macromolecule solutions (with and without salt) in sealed melting-point capillary tubes were prepared before the lab to minimize student preparation time.
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EXPERIMENT Students analyzed 5 macromolecule solutions. Three samples can be analyzed simultaneously in the OptiMelt digital meltingpoint apparatus. A temperature ramp rate of 1.0 °C/min with a starting temperature of 25 °C was used. All of the samples cloud below 33 °C (Table 1). The samples contained 2% PNIPAM by weight in water with or without a salt. The sample’s clouding curve can be saved to a hard drive or flash drive using the software provided with the OptiMelt apparatus. Table 1. Typical Student Results
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Macromolecule Solutions
HOFMEISTER EFFECT The LCST studies can be used to illustrate the Hofmeister effect.8,9 The Hofmeister effect describes how salts at varying concentrations affect protein or macromolecule hydration. It is of interest because of the importance of hydration to protein stability. In this effect, salts such as sodium isocyanate modestly affect protein hydration or the LCST of PNIPAM whereas salts
2 2 2 2 2 2 676
wt wt wt wt wt wt
% % % % % %
PNIPAM PNIPAM PNIPAM PNIPAM PNIPAM PNIPAM
in in in in in in
0.3 M LiCl 0.5 M LiCl 0.19 M LiBr 0.3 M LiBr 0.5 M LiBr water with no added salt
LCST (at 10% clouding)/°C 29.5 27.1 31.4 31.5 30.7 32.5
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The LCST was taken to be the temperature at which 10% clouding occurred. Cooling of the melting-point apparatus to enable analysis of a second set of three samples takes only a few minutes. Analyses of clouding with a Mel-Temp apparatus used visual analysis of the onset of clouding with the initial clouding point being recorded as the LCST. In all cases, the clouding process is reversible and samples that have clouded redissolve on cooling to room temperature. If time is available, students can rerun samples at a different temperature ramp rate to see the effect of heating rate on measured LCSTs. Students can also save and share their LCST−[salt] data to prepare graphs that illustrate the Hofmeister effect. Students can compare their results to primary literature5,8,9,16 that discusses PNIPAM’s LCST and the Hofmeister effect.
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HAZARDS
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DEMONSTRATION
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RESULTS AND DISCUSSION
Laboratory Experiment
ASSOCIATED CONTENT
S Supporting Information *
Student handout including questions for the post-lab writeup; notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ACKNOWLEDGMENTS Funding for this work was provided by the National Science Foundation (DMR-0907233) and the Robert A. Welch Foundation (A-0639).
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REFERENCES
(1) Odian, G. Principles of Polymerization, 4th ed.; Wiley & Sons: New York, 2004; pp 279−281. (2) Mathieson, A. R. J. Chem. Soc. 1960, 2778−2779. (3) Taylor, L. D.; Cerankowski, L. D. J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 2551−2570. (4) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163−249. (5) Bergbreiter, D. E.; Fu, H. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 186−193. (6) Chevillard, C.; Axelos, M. A. V. Colloid Polym. Sci. 1997, 275, 537−545. (7) Ayres, L.; Koch, K.; Adams, P. H. H. M.; Van Hest, J. C. M. Macromolecules 2005, 38, 1699−1704. (8) Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. J. Am. Chem. Soc. 2005, 127, 14505−14510. (9) Gurau, M. C.; Lim, S.-M.; Castellana, E. T.; Albertorio, F.; Kataoka, S.; Cremer, P. S. J. Am. Chem. Soc. 2004, 126, 10522−10523. (10) Zhang, Y.; Cremer, P. S. Annu. Rev. Phys. Chem. 2010, 61, 63− 83. (11) SRS melting point apparatus. http://www.thinksrs.com/ products/MPA100.htm (accessed Feb 2012). (12) (a) Bergbreiter, D. E.; Zhang, L.; Mariagnanam, V. M. J. Am. Chem. Soc. 1993, 115, 9295−9296. (b) Bergbreiter, D. E.; Case, B. L.; Liu, Y.-S.; Caraway, J. W. Macromolecules 1998, 31, 6053−6062. (13) Huang, X.; Witte, K. L.; Bergbreiter, D. E.; Wong, C.-H. Adv. Synth. Catal. 2001, 343, 675−681. (14) Zhou, M.; Sivaramakrishnan, A.; Ponnamperuma, K.; Low, W.K.; Li; Liu, J. O.; Bergbreiter, D. E.; Romo, D. Org. Lett. 2006, 8, 5247−5250. (15) (a) Van Vlierberghe, S.; Dubruel, P.; Schacht, E. Biomacromolecules 2011, 12, 1387−1408. (b) Wei, H.; Cheng, S.-X.; Zhang, X.-Z.; Zhuo, R.-X. Prog. Polym. Sci. 2009, 34, 893−910. (16) (a) Nath, Ni.; Chilkoti, A. Adv. Mater. 2002, 14, 1243−1247. (b) Liao, K.-S.; Fu, H.; Wan, A.; Batteas, J. D.; Bergbreiter, D. E. Langmuir 2009, 25, 26−28. (17) Wald, M. L. Technology; Windows That Know When to Let Light In. The New York Times, 1992. http://www.nytimes.com/1992/ 08/16/business/technology-windows-that-know-when-to-let-light-in. html (accessed Feb 2012).
There are no hazards associated with this experiment other than those associated with a normal melting-point analysis, for example, the handling of glass capillaries that could break and present minor hazards associated with any piece of broken glass.
A demonstration or diversion during the experiment that amused most students was the instructor’s use of a sealed melting-point capillary that had a solution of PNIPAM in a salt solution as a “Men in Black” field test. The term refers to a movie that discussed supposed aliens among us. Using a capillary tube containing a solution whose LCST is just above room temperature, a student could hold the capillary between their fingers and trigger the LCST event. Formation of a cloudy suspension would indicate that they were warm-blooded mammals. The experiment works well except for a small percentage of students with cold fingers. They fail the test. It is useful if the instructor himself or herself can pass the test.
Student responses to this experiment were generally very positive. They found the digital melting point apparatus and the on-computer facilitated analysis of thermally induced phase changes a more appealing 21st century alternative use of visual analysis that used subjective visual analyses and a conventional melting point apparatus. They also generally enjoyed the “Men in Black” test where they could see thermally responsive polymer solubility changes in a melting point capillary by merely holding the capillary containing an aqueous solution of PNIPAM and some salt in their fingers to heat it and by then letting the sample cool back to room temperature in air to dissolve the polymer. This experiment is relevant to applications of thermally responsive polymers in catalysis,12 synthesis,13 affinity chromatography,14 drug delivery,15 and in the design of surfaces with responsive wettability.16 Such polymers even have applications in the design of windows that autonomously address overheating by direct sunlight by clouding at high temperature and becoming clear on cooling.17 These applications can be discussed in a lecture or by students in write-ups. 677
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