JChemEd 1997 74 1227 techniques Uv-vis Fluor Cond

JChemEd 1997 74 1227 techniques Uv-vis Fluor Cond - In the...

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Vol. 74 No. 10 October 1997 Journal of Chemical Education 1227 In the Laboratory Surfactants, sometimes called surface-active agents or detergents , are among the most versatile chemicals avail- able. They have applications in many areas, including chem- istry (chemical kinetics or equilibria), biology (as membrane mimetics), and pharmacy ( 1 ). Surfactants are amphiphilic materials containing both apolar long-chain hydrocarbon “tail” and polar, usually ionic, “head” groups. In polar sol- vents, for example water, this dual character of the amphiphile leads to self-association or micellization: the surfactant molecules arrange themselves into organized molecular assemblies known as micelles (Fig. 1) . The hydro- phobic part of the aggregate forms the core of the micelle, while the polar head groups are located at the micelle–wa- ter interface in contact with and hydrated by a number of water molecules. Depending on the chemical structure of the surfactant, its micelle can be cationic, anionic, ampholitic (zwitterionic), or nonionic. This unique property of surfactants makes aqueous surfactant solutions microheterogeneous media; that is, they are heterogeneous on a microscopic scale, even though they are often homoge- neous macroscopically. The concentration (actually an arbi- trary concentration within a narrow range) above which mi- celles form is called the critical micelle concentration (CMC). Above the CMC, monomers and micelles exist in dynamic equilibrium. Despite their growing importance, microheterogeneous media are not yet considered in most textbooks of general or experimental physical chemistry. Several reports illus- trate the use of various techniques to measure the CMC ( 2– 5 ). The experiments described in this article were designed to familiarize students of physical chemistry with micellar solutions and the basic structural parameters of micelles, and with some common techniques in physicochemical labo- ratories used to observe the changes in physical and chemi- cal properties of surfactant solutions when micelles are formed. The paper presents the CMC determination of some surfactants by three methods. The advantages and disad- vantages of each one are indicated. In these experiments, undergraduate students in the fourth year in chemistry determined the CMC of a surfac- tant by measuring a change in (i) the UV-vis spectrum of benzoylacetone, (ii) the fluorescence emission spectrum of pyrene monomers, and (iii) the electrical conductivity of an ionic surfactant solution as the concentration of the amphiphile increases. There are several ways of organizing the laboratory experiments, depending on the time and equipment available. Each group of students could deter- mine the CMC of a surfactant, for example sodium dodecyl sulfate, by using the three techniques under the same ex- perimental conditions. The time required for determination of a CMC value by any of the techniques described here is no longer than two lab sessions; each one takes about four hours. The accuracy of students’ results and response to the experiments were generally good.
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  • Fall '11
  • Dr.Dinwiddie
  • CMC, mol L᎑1

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