show that ice does not melt under pressure as explained above. Second. it is instructive to comnsre the effect of pressure on melting point using another substance that has a oositivedPldT. Asuitahlechoice is cvclohexane, which hasa normal melting point of 6.6 "C. ~ p ~ r o x i m a t 40 el~ mL of cvclohexane is degassed and then frozen in a test tube (about i c m i.d.). w a r m k g the test tube to room temperature for a few minutes will permit the removal of the solid rod. A weighted wire will mechanically fracture the solid cyclobexane, hut there is no refreezing of the two portions of the rod as is found with the ice block. In this case an increase in pressure actually increases the melting point. ~~
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Visoelastic Surfactant Solutions: A Demo To Catch the Student's Attention SUBMITTEDBY
Steven Jon Bachofer
St. Mary's College of California Moraga, CA 94575 CHECKED BY
Paul Krause University of Cedrai Arkansas Conway. AK 72032 Can physical chemistry be fun? It can explain what clearly appear to he magical properties. A specific example is viscoelasticity, which is a rheological phenomenon (I). The viscoelastic effect can he observed upon subjecting a disordered svstem with a shear force that orders it. stopnine the shear. a i d viewing the system as it relaxes back to ihLdisordered state. This phenomenon is easilv observable if air bubbles are entrapped in this fluid ~ p o ~ s w i r l i nthe g contents (applying the shearing force) since in the relaxation period, the air bubbles stop and flow backwards to the initial direction of shear (2).Karol Mysel noted viscoelasticity is observable in egg albumin (egg whites), although we do not normally notice i t (I). This observation is so directly opposite to our fundamental understanding of materials (e.g., aqueous solutions) that it draws in the students to understand what is inuolued. The excitement to understand this phenomenon can be utilized to start serious investigations of colloidal systems that are easily understood through physical chemical measurements. The students gain an appreciation that they are developing skills that can he utilized to solve present day chemical questions. The remarkable appearance of viscoelasticity has been .. explained ft,r a number of colloidal systems containing pn)late niirelles (rod-shaped aggregates) (31. The pmlate rnicelles are a disorderedmicelfar system, which can be considered analogous to a spring about to be compressed. The compression force (which in this case is a shear) orders the system. Upon removal of the shear, the prolate micelles recoil hack to the disordered state just as a compressed spring would recoil. (See figure.) This phenomenon is also observed in numerous polymer aggregate-medium matrices, although they are not our primary focus (4). We here report on a svstem discovered in our own research that is easilv prepared for a lecture demonstration to encourage underaradustes that physical chemistry can be utilized to explain . . even unusual phenomena. T h e determination of critical micelle constant' is a stratphtforward euperimrnt that an undergraduate can do atrer viewing his demonstration. Numerous methods for determination of a cmc value can be applied and have been listed in this Journal (5). Preparatlon of Demonstration Sample 'The viscot~laitirdemcmrrnrion $ample is prepared wrth a 1.1 rtoirhmnrtric mixture oftus, mlls; the caticmr surfactant, cetyllrimethyhmmunium bnmrdefC'l'.AHI a n d the uenk ncld standard.
790
Journal of Chemical Education
Pictorial representation of prolate miceliar aggregates. The prolate micelles are initially in a disordered state (A), then in an ordered state under the shear (B),and return to the disordered state upon removal of the shear (A). This pictorial representation is used only in an analogy.
potassium hydrogen phthalate(KHP) (6).Both materials are commercially available and can be used without further purification. The 1:l mixture that is prepared above the critical micelle constant concentration(cmc) at 10 mM in each reagent salt demonstrates a viscoelastic effect. The solubility of the catianie surfactant is very low at room temperature since its Kraft point (temperature where its solubility increases strongly) is slightly above room temperature. In preparing a concentrated stock solution of CTAB, one can either sonicate or warm the solution to obtain the supersaturated stock. Crystalline potassium hydrogen phthalate(KHP) also dissolves slowly and therefore should be prepared ahead of time. Equal volumes of 20 mM solutions of each stock salt will yield a solution that will exhibit viseaelasticity. The rheological effect isclearly demonstrated (if the audience is within 20 f t of the demonstrator1fillins a 25- hv 150-mm test tuhe approximately half iull and nirer entrapping a icw air bulhles hy shaking the aamplr; swirling the tcsl tube wrll genemcra aigmfirant shenr gradient to provide the uhr~rwtiun Explaining the Demonstratlon The 1:l mixture forms an aggregate of the two oppositecharged materials (surfactant quaternary ammonium cation and hydrogen phthalate anion). The aggregate is formally a mixed micelle. T h e mixed micelle is a prolate (rod-shaped) micelle since the hydrogen phthalate anion situates itself a t the interface neutralizing the charge repulsionof thequaternary ammonium cation head groups of the surfacranr (3a). The anion not only changes the el&trostatic free energy of the interface by charge neutralization but also decreases the surface area to volume ratio of the micelle and therefore prescribes the change in aggregate ~ h a p e . The C'TAB:KHP system and analorous svstems with ~ h . er substituted benzoate anions are sirongiy dependent on pH, ionic strength, the substituent groups, and temperature (3, 6). The p H dependence is understood considering protonation of the carboxylate group eliminates the anionic charge, which enhances the charge-neutralization of the micellar interface. An ionic strength change perturbs the counterion bindine eauilibria and can even eliminate the ~~~-viscoelastic effrrt."wr? have srudied this counterion binding for numeroussubstituted beneoates with CTAH and have found an empirical correlation between the cmc value and the appearance of viscoelasticitv (6). Another clarifvine . ~ o i nist to show the appearance of &oelasticity in a sample shove the cmc and the disappearance of visroelastirity upon dilu~
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tic effect by comparison to solutions that do not exhibit the phenomenon.
tion below the cmc, which is 0.20 mM for a 1:l CTAB:KHP micelle. The effect of temperature on viscoelasticity has also been observed by other authors andis still under investigation (7). The temperature dependence of the rheological phenomenon can he illustrated by putting a CTAB:KHP sample lahove the cmc) in an ice hath and another in a 50 OC temperature hath; after allowing the samples time to equilibrate thermallv. remove them from the hath and demonstrate that th; elastic recoil has disappeared for the high temoerature sample. The rheological phenomenon, viscoelasiicity, discussed here, is of interest to chemical engineers and chemists in applications from tertiary oil recovery to detergent formulatibns. In summary, recent developments in the field of surface/ colloid chemistrv ~ ~ ~ can ~ he ~ utilized to stimulate the undermaduate to perceive the "due of learning physical c h e s s t r y beyond the mediocre goal of passing this course. The development of micellar catalysis can easily he related to new technoloeical hreakthrouahs " - ( 8 ) .The advancement of medical research can he directed tied to chemical separations utilizing electrophoresis techniques. A basis of these techniques involve the understanding of the free energies of complex formation. One of the simplest aggregate structures to he studied is the micelle. The thermodynamic theory for micellization is not trivial, hut the students can he exposed to the thermodynamics for the prescribed phenomena and he encouraged to learn the statistical mechanics. This system can he understood in a aualitative manner as prolate micelles, hut there are still numerous research questions on viscoelasticitv. The underaraduate can he inspired knuu,ing that their skiils can he utilized to solve future research problems.
univer&y of Florida Galnesvllle, FL 32611
Acknowledgment The author wishes to thank John 0. Edwards and Charles Marzacco for the positive encouragement and helpful suggestions throughout this research. The financial support of Rhode Island Collegeand the numerous hours of work by the undergraduate researcher, Robin Turhitt, involved in this research have allowed this project to he more successful than originally envisioned. The author gratefully acknowledges the suggestions of the reviewers that highlight the viscoelas-
We have oerfected a method for 6emonstrating metal flame tests in the classroom. An ordinary plant-leaf sprayer is filled with a 1 M solution of a metal salt. The atomizer is adjusted to make a fine mist. The solution is then sprayed into the flame of a large Fisher hurner. A darkened room enhances the demonstration. The flame color intensity is further enhanced by adding up to 10-40% methanol to the aqueous solution. A 1- X 1-m square of plastic placed under the burner prior to the demonstration allows for quick cleanup.
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Literature Clied 1. ~ y s e lK. , J. Infrnd~rtionto Colloid Chemiairy: Interscience: N e w York, 1959; pp 253273. 2. (a) Olwm, U.: Sodermen, 0 . ; Guering, P. J. Phys. Chsm 1986. 90, 5223-5232. ihl Wennerrtrorn, H.; Ulmius. J.; Johansron, L. B - A ; Lindblom, G.: Gravrholr. S. J. P h y s Chem. 197%83,2232-2236. 8. (a) lyer, R. M.; Rso. U. R. K.: Manahar. C.: Valauliksr. B. S. J. P h y s Chem. L987.91, a28&3291. (hl Gravsholt. S. J . Coil. & Istwfai. Sci. 1976.57.576-577. 4. Gravsholt.S. InPolymerColloids. Fiteh,R.. Ed.. Plenum: NewYork. 1979; Vol. 11, p i ] . 5 . is1 Rujimethabhe~.M.: Wilsirat, P. J. Chem Educ 1978.55, 342. !hi C o r k M. L.: Harkinr, W. D. J. Am. Chpm Sac. 1917.69.679-688. icl Flockhait. 5 . D. J. Call. & Inlsrfor.Sci. 1957.12,557-560. (dl Harkins. W. D.; Jordan. H. F. J.Am.Chem.Sor. l930,52.1751-1772. 6. (a1 5achofer.S. J.;Turhift,R. J. Coil &Interfoe. Sci. 1990,135, 325-334. ibl Jan~son. J. M.;Stilhs.P. J.Phys.Chem, 1987.91.Ll2-116.(c)Sesmehorn.J.F.;Rathmsn.J.F, Phys. ('hem. 1984. R8.5807-5816. 7. Hoffman". H ; Rehage. H.: Reidea". K.; Th",", H. I" Mac,". & Mzeroemulaions: Theory and Applicotionr. Shah.D. 0..Ed.: 1972; ACS Sympo~iumsoriesVol. 272, pp
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8. (a1 Cordes. E. H.. Ed. Reactions Kinetics in M~rdles;Plenum: N e w York. 1973. !hl Menger, F. M.: Portnoy,C. E. J.Am. Chsm. Soc. 1967,89,4698--1703. ic) Fendler. J. H.: Fendler, E, d. Cololysis in Micellar and Mocromoleculor Soluliuns; Academic: N e w York. 1975.
Spectacular Classroom Demonstration of the Flame Test for Metal Ions Suwlmo sr
Bruce M. Mattson, Roberl L. Snipp, and Gary D. Mlchels Creighton Unlverslly
Omaha, NE 68178 CHECKED BY
Kathrvn R. Wllllams
Eleventh Annual Fall Computer-Using Educators Conference The Eleventh Annual Fall Computer-UsingEducators Conference will be held October 18-20,1990 at the Santa Clara Convention Center and adjoining Doubletree Hotel, Santa Clara, CA. In keeping with the Conference theme, Global Technology: Connections for Educators, hundreds of open sessions will address bilingual education, distance learning, Enalish as a second lsnguage, foreign language, international studies, networks, and telecomputing. In addition, professional development wili be stressed in sessions on topics such as: administration, assessment, computer coordination, curriculum integration, subject-specific content and methodology, and teacher productivity tools. There will also be is required, field trips, and over 220 exhibit booths with the latest several hands-on worksho~sfor which Drereaistratian . educational hardware and software. Registration materials have already been sent to members of Computer Using Educators, Inc., the Conference sponsor. Others may join and receive more information about the Conference by writing or calling: CUE, 4655 Old Ironsides Drive, Suite 200, Santa Clara, CA 95054 (phone: (408)727-5165, FAX: (408) 432-9892.
Volume 67
Number 9
September 1990
791
