overhead projector
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DOREKOLB Bradley University Peoria, iL61625
The Effect of SDS Micelle on the Rate of a Reaction Charles J. Maruacco Rhode Island College Providence, RI 02908
The properties of surfactants is a topic of considerable scientific interest a s well a s economic importance. There have been a number of articles in this Journal describing the properties of micelles ( I ) and their effects on chemical equilibria (21, rates of chemical reactions (3-61, and fluorescence quenching (7, 8 1. The demonstration described in this article is a colorful example of the effect of the surfactant sodium dodecylsulfate (SDS) on a chemical reaction. Sodium dodecylsulfate is a n anionic surfactant whose surface active properties result from the dodecylsulfate anion. This ion consists of a hydrophobic 12-carbon alkyl tail attached to the hydrophilic sulfate head. Because of the hydrophobic character of this ion, it forms micelles in aqueous solutions once its concentration rises above the critical micelle concentration, which for SDS is about 6 x lo3 M (11. The chemical reaction used i n this demonstration is that between the triphenylmethane dye crystal violet and hydroxide ion. The reaction that occurs is shown in the figure. This reaction involves attack on the crystal violet cation by hydroxide ion to produce the carbinol of crystal violet. Several studies of this reaction have been reported in the literature. LaMer and lhrgeon examined solvent and kinetic salt effects on the reaction (9). Corsaro described an undergraduate kinetics experiment (101, and Duynstee and Gmnwald examined the effect of surfactants on this reaction (11). The Demonstration Solutions 2.0 x 104M aystal violet 5.0 M NaOH 0.040M SDS Three 600-mLbeakers
The reaction used in the demonstration.
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Journal of Chemical Education
Procedure To demonstrate the reaction between sodium hvdroxide and crystal violut and to estimate the rate coniitant for t h ~ s reaction. add 5.0 mLofcr\,itnl violet solu~iontu each oftwo beakers'and 2.5 mL to a third beaker. Add enough distilled water to produce a volume of 375 mL in the first beaker and volumes of 400 mL in the second and third beakers. Place the three solutions on a n overhead projector with the first two solutions side by side and the third dilute solution above and between the other two. Add 25 mL ofNaOH solution to the first beaker of crystal violet and mix. The color of the crystal violet solution containing the NaOH will slowly fade a s the reaction proceeds. At room temperature it should take approximately 30 s for the color to fade until the absorbance is the same a s that in the dilute solution. This is the half life of the reaction. After 2 min the color will be completely gone. To demonstrate the effect of SDS on this reaction, place 5.0 mL of crystal violet solution in each of two beakers. Add 25 mL of SDS solution to one of these. Add enough distilled water to each of these solutions to produce 375 mL of solution. Place these solutions side by side on the overhead. Add 25 mL of NaOH solution to each mixture and stir. The soap bubbles on the SDS solution should be pushed to the side for better viewing. The solution without the SDS will slowly bleach, while the one with SDS will remain unchanged, thus illustrating the inhibiting effect of SDS on the reaction. Discussion This reaction is known to be first-order in each of the two reactants. (9,101 Under the conditions of this demonstration, the hydroxide ion concentration is much larger than that of the crystal violet. Thus, the reactionis pseudo-firstorder, and the concentration of the crystal violet shows an exponential decrease with time. The observation that i t takes approximately 30 s for the concentration ofthe crystal violet to decrease to half of its initial value translates to a value of 7 x 10.' L mol-' S-' for the second-order rate constant. This value is in good agreement with what is reported in the literature.' Because a diffusion-controlled reaction between ions of opposite charge would have a rate constant on the order of 101° L mol-' s-', only about one in every 10" encounters between the crystal violet and hydroxide ions result in a reaction. Because this is a reaction between ions of opposite charge, it shows a negative kinetic salt effect (9,101. In SDS solutions above the critical micelle concentration, the rate of this reaction is greatly reduced. I n the presence of the anionic micelle, the crystal violet cation becomes entrapped within it. Thus, it is protected from attack by the hydroxide ion, which cannot easily penetrate the negative charge of the micelle. I t should be noted that if the SDS solution is allowed to stand for several days, bleaching of the crystal violet is observed to occur; however, it is-not complete. Thus, the re-
action does not quite go to wmpletion in the presence of SDS (11,12). This demonstration nicely illustrates the inhibiting effect that a surfactant can have on a chemical reaction. Finally, it should be mentioned that the 2.5 x 10-3 M SDS concentration used in this demonstration is well above the critical micelle concentration of SDS in aqueous solutions of high ionic strength (13). Literature Cited Rujimefhabhas, M.: Wi1airat.P J. C k m . Educ. 1818,66,342. Ablun. E. B.; Lissi,E.A. J. Chom. Educ. 19% 69,340. Corsam,G.J C h e m E d u c 1973,50,575. C o r m , G.:Smith,J. K . J. Cham. Educ. 1976,53,589. Corsam, G. J Chem Educ 1980.57.226. RevlsbomughV. C.;Rabinsrm,B.H. J Chem.Edue 1981,68,586. 7 Rncssler N. .l Chem Educ 1919.56675,
1. 2. 3. 4. 5. 6.
11. Duynstee, E. F. J.; h w a l d , E. J Amer Cham. Soc 1959,81,4540. 12. Baumrucker. J.: Cshsdills. M.: Corder. E. H . Rawtion Kinetics in M i e l k : Cordea, E. H., Ed.: ~le"um:New~ork,1972; pp 2&51. M.L.;Hankins, W. D. J. Amer C h . Sor 1947,69,683. 13. Co-,
Kinetic Art through Chemistry Walter H. Corkern, L. H. Holmes, Jr., and N. A. Higginbotham Southeastern Louisiana University Hammond, LA 70402
People who are not interested in chemistry oRen have the misconception that it is hard, boring, and dull. We have found a number of demonstrations that show it also can be beautiful and fascinating. The demonstrations are observed easily by large groups using a n overhead projector or a video camera. A Demonstration That Pleases The demonstration that offers greatest aesthetic pleasure uses a solution of 60% ethanol40% water (by volume) as solvent. APetri dish is placed on a n overhead projector and half filled with the solvent. One drop of l-heptanol with a small amount of dissolved crystal violet dye is dropped onto the solvent in the Petri dish. The rapid motion that occurs forms patterns similar to fractals. After
Presented in part at the 65th Annual Meeting of the Louisiana Academy of Sciences, Feb. 8,1991.
the drop of heptanol has dissolved, geometric patterns continue to form over the next minute or so. If one prefers the color red, l-heptanol with a small amount of 0ii Red EGN dye (Matheson Coleman & Bell) can be used for similar effects. Several drops of the l-heptanol-dye mixture added in rapid succession to the solvent also yields pleasant results. If one uses 40% ethanol40% water (bv volume) as solvent, one can observe the stirring action-that takes place as l-he~tanol(containing Oil Red EGN) dissolves. A shallow layer of the solveni about 3-4 mm deep should be added to the Petri dish. You may have to try this one several times to be able to judge the depth of solvent needed. To the solvent add a small amount of acid-washed, 80190 mesh Chromport A (a small amount on the tip of a spatula, containing 40-80 particles, works well). Then add a drop of the l-heptanol-dye mixture to the surface of the solvent above the Chromport A particles. The particles are moved around bv the currents in the solvent. ao~arentlvcaused by the siution process. As the drop ;ea& ampfete solution, the stirring streams have sometimes been observed to become directed in space (5 equally spaced streams of the oarticles a ~ ~ e a r i star n e shaped). Then.." iust . orior to complete solution, the five streams tend to become four equally spaced streams (in the shape of a cross or plus). Using a solvent of 30% ethanol-70% water by volume and several samples of l-heptanol, each containing a different dye (oil red, crystal violet, basic fuchsin, malachite green, sudan 111, etc.), moving art can be produced. Fill the Petri dish half full of solvent, and add as many drops of each l-heptanol-dye mixture as you wish. The drops move around slowly with currents that can be observed. The drops tend to remain separated, however, and they form circles or lines of drops. A.
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Demonstrationsthat Titillate Young and Older, Alike The demonstrations described above have been shown to elementary, junior high, and college students, to wllege teachers, and to senior citizen groups. Every group responded with excitement and numerous questions, especiallv the elementarv and iunior high students. A ;ideotape of these demonstra
