Chemical Education Today

JChemEd.chem.wisc.edu • Vol. 76 No. 5 May 1999 • Journal of Chemical Education 601

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Curtis T. Sears, Jr.Georgia State University

Atlanta, GA 30303

Introducing a Practice-Oriented Approachin the Physical Chemistry Instructional Laboratory

David E. Budil, Lutfur R. Khundkar, Ihsan A. Shehadi, and Mary Jo Ondrechen*Department of Chemistry, Northeastern University, Boston, MA 02115; *ondrechen@neu.edu

To meet the challenge of modernizing undergraduatephysical chemistry, recent workshops have described a numberof new, technologically advanced experiments that may beincorporated into physical chemistry laboratory courses (1,2). While specialized methods undeniably bring sophisticationto the undergraduate laboratory, they are often emphasizedat the expense of basic concepts in thermodynamics. Moreimportantly, there remains a tendency to follow the tradi-tional philosophy that physical principles are to be illustratedexperimentally rather than applied in problem solving, oftenwith complex apparatus that distracts students from the pointof the experiment.

The unique cooperative education environment atNortheastern University has motivated us to seek a somewhatdifferent approach. Biochemistry and chemical engineeringmajors take our physical chemistry laboratory course togetherwith the traditional chemistry majors. Many of our studentsare employed in high-technology firms in the Boston areaand have been exposed to state-of-the-art instrumentation andreal-world chemical problems on the job by the time theyreach their physical chemistry course work. Consequently,we have found it important to introduce students to research-grade instrumentation at the earliest possible opportunity.At the same time, we wish to maintain a focus on generalthermodynamic concepts that form the underpinning of awide range of chemistry-related disciplines, instead of intro-ducing more specialized topics.

We chose to introduce new physical chemistry experimentsbuilt around a modern differential scanning calorimeter(DSC). This instrument was chosen on the basis of feedbackfrom students through surveys, interviews, and on-site visitsto cooperative education employers. We find that students haveencountered DSCs in settings ranging from entrepreneurialpharmaceutical and novel materials firms to more traditionalindustries such as polymer and thermoplastic manufacturers, aswell as in basic research laboratories. The general applicabilityof such an instrument translates into pedagogical flexibilityas well: with it, students may study a variety of interestingmaterials and explore a number of distinct thermodynamicconcepts. Such applicability also allows a standard method tobe used for multiple problems, thereby shifting the students’focus from experimental setups to the complex systems ofinterest.

The new experiments include (i) basic instruction in theuse of a DSC and applying it to simple enthalpy measurements;(ii) comparative DSC studies of the thermal behavior ofcomplex substances at and near phase transitions, and (iii)application of DSC-derived phase diagrams to specific designproblems in macromolecular and biomolecular mixtures.

The phase diagram experiments have found a particu-larly prominent place in our curriculum, given that phasediagrams are central tools in the processing and design ofmodern materials such as liquid crystals (3), polymers, andcolloidal systems for delivering cosmetics and pharmaceuticals(4). We employ a two-part approach that integrates experimentalDSC measurements with a group theoretical project carriedout in the lecture course. It is based on a procedure forinterpreting DSC-derived phase diagrams of mixed modelmembranes described by Brumbaugh and Huang (5), which wehave found to be applicable for a variety of other systems as well.

In the first part of the study, students are introduced tosimple nonideal solution theory and are asked to derive rela-tionships between transition temperature and compositionfor a nonideal two-component system. They are then dividedinto teams, each of which is assigned to calculate part of aphase diagram using the derived relations and some ingenuity.The calculation of the phase diagram is rather a challenge atfirst, since it involves the solution of a pair of transcendentalequations. We have encountered a number of inventivegraphical solutions to the problem, although most studentsare able to adapt root-finding tools available in numericalpackages such as Mathcad or MATLAB. The complete phasediagrams are constructed in class, which leads to a discussion ofthe effects of various thermodynamic parameters on the phasebehavior of the system and identification of possible errorsin the calculation of the diagram.

The experimental measurements are also carried out bystudents working in teams to construct phase diagrams fordifferent systems. Typically, an entire class will construct adiagram for a selected system, each team being assigned a smallnumber of compositions to scan and analyze. This approach giveseach student an appropriate amount of time on the limitednumber of instruments available. Students have the optionto study additional compositions if they encounter unexpectedresults. Examples of such situations include compositions nearinvariant points in the phase diagram, the appearance of un-anticipated phases or glass transitions, or anomalies due to

Chemical Education Today

602 Journal of Chemical Education • Vol. 76 No. 5 May 1999 • JChemEd.chem.wisc.edu

errors in sample composition or experimental settings.Finally, the theoretical and experimental approaches are

tied together by asking students to interpret their experimentalresults specifically on a molecular level either during an in-class presentation or in an extended laboratory report. Suchinterpretation requires the students to compare their owngroup’s experimental results with those from another groupusing a similar system, or with some of the diagrams de-rived theoretically in class. Emphasis is placed on trying toassess the types of nonidealities present in the system andidentifying them with specific molecular features of themixture components.

Selected groups of students may also pursue the problemin greater depth as an honors adjunct. A number of studentshave developed the full implementation of the phase diagramanalysis described by Brumbaugh and Huang (5), includingnonlinear least-squares fitting of the class-derived expressionsto their experimental temperature-composition data. Studentsmake their own choice of computer tools to apply to thisproblem; the most popular ones are Visual Basic (Excelmacros) and Mathcad.

Another possible adjunct assignment is to develop anapproach to a hypothetical design problem, which may ormay not resemble the system studied experimentally, byspecifically applying the results and conclusions from thephase diagram experiment. Examples of such problems fromdifferent disciplines include (i) How would one adjust a lipidmixture to form thermodynamically stable liposomes at agiven temperature and pH, such as might be required for cer-tain pharmaceutical delivery systems? (ii) How can one controlthe presence of a crystalline phase in a polymer mixture to

achieve specified physical properties (e.g., plasticity, viscosity,melting behavior)? (iii) How might a liquid crystal mixture bedesigned for specified phase characteristics, such as a nematicphase that extends over a given temperature range?

The approach described above has proved to be quite use-ful for a variety of chemical systems, including solid eutecticsand liquid crystal mixtures as well as the model membranesystems for which the method was originally developed (5).It has also strengthened the bridge between the lecture andlaboratory courses and increased student appreciation of thewidespread utility of basic thermodynamics.

Acknowledgments

This work was supported by NSF award DUE-9650765.We gratefully acknowledge Mettler-Toledo for their match-ing donation of equipment used in our physical chemistryteaching laboratory.

Literature Cited

1. Essays in Physical Chemistry; Lippincott, W. T., Ed.; Committeeon Education, American Chemical Society: Washington, DC, 1988.

2. Physical Chemistry: Developing a Dynamic Curriculum ;Schwenz, R. W.; Moore, R. J., Eds.; American Chemical Society:Washington, DC, 1993.

3. deGennes, P. G.; Prost, J. The Physics of Liquid Crystals; Interna-tional Series of Monographs on Physics 83; Oxford UniversityPress: Oxford, 1993.

4. Colloidal Drug Delivery Systems; Kreuter, J., Ed.; Drugs and thePharmaceutical Sciences 66; Dekker: New York, 1994.

5. Brumbaugh, E. E.; Huang, C. Methods Enzymol. 1992, 210, 521.

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