Chem. Educator 2001, 6, 223226 223 ' 2001 Springer-Verlag New York, Inc., S1430-4171(00)04487-x, Published on Web 06/22/2001, 10.1007/s00897010487a, 640222sp.pdf Quantifying Critical Micelle Concentration and Nonidealities within Binary Mixed Micellar Systems: An Upper-Level Undergraduate Laboratory Gary A. Baker, Frank V. Bright, and Siddharth Pandey* , Department of Chemistry, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, pandey@nmt.edu, Department of Chemistry, Natural Sciences Complex, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, chefvb@acsu.buffalo.edu Received April 11, 2001. Accepted May 8, 2001 Abstract: Micelle-based systems have a long history in many areas, for instance, membrane mimics, reaction media, and food additives. All prior educational laboratory experiments dealing with micelle-based systems that have been published and utilized deal with unary surfactant systems. Unfortunately, most practical applications that use surfactants and micelles involve mixtures because these often exhibit behavior unlike the individual components. That is, surfactant mixtures deviate significantly from a regular solution approximation (i.e., there is synergism) when the individual surfactant structures differ significantly. In this laboratory experiment, we describe a simple method that exploits the unique behavior of the fluorophore pyrene to rapidly determine the critical micelle concentration (CMC) of mixed micelle systems. We also determine the dimensionless interaction parameter, β, that describes the net pairwise interaction between surfactant species within a binary micellar system. The binary system we chose to study is sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (DTAB) dissolved in water. The β value recovered by students using our method is statistically equivalent to the value reported in the literature using more sophisticated and protracted methods. This laboratory experiment opens the door for students to explore regular solution theory, non-ideal mixing, micelle formation, and fluorescence spectroscopy within a single experiment. Introduction and Background Surfactants (surface-active agents) are amphiphiles consisting of long-chain hydrophobic tails and polar (often ionic) headgroups. By acting to lower interfacial tension, such amphiphilic molecules often aid in surface wetting, solubilization, emulsification, dispersion, and frothing [13]. In aqueous solutions above a narrow surfactant concentration range (the critical micelle concentration, CMC) surfactants can spontaneously self-associate to form thermodynamically stable molecular aggregates known as micelles. Micelles and their monomer constituents are of wide interest in colloidal chemistry because they are essential in industrial processes (e.g., the textile and semiconductor industries). Micelles and surfactants are also widely encountered in consumer products (e.g., detergents in cosmetics and soaps, emulsifiers in salad dressing). Biosurfactants are crucial for maintaining proper health [4] (e.g., proper prenatal lung development) and some surfactants even provide the necessary conditions for life itself (e.g., photosynthesis, intracellular processes). Micellar media also have a long history as rudimentary membrane mimics [1]; novel chemical reaction media [57]; and analytical agents to improve selectivity, particularly in the separation sciences [7]. Undergraduate laboratory experiments demonstrating the effects of micelles on physicochemical properties of a variety * Address correspondence to this author. New Mexico Institute of Mining and Technology The State University of New York, Buffalo of substrates have appeared in the chemical education literature [8]. Abrupt changes in the physicochemical properties of a surfactant solution in proximity to the CMC signals the onset of micelle formation. Experimentally, the CMC is determined by monitoring a suitable physicochemical parameter (e.g., conductance, viscosity, surface tension, reaction rate, detergency, scattering) as a function of surfactant concentration [13]. Recently, a simplified undergraduate laboratory experiment based on surface tension measurements for the determination of CMC has appeared in the literature [9]. Fluorescence probe techniques are also widely used to study surfactant micellization, adsorption, and polymer interaction [10, 11]. Pyrene is a particularly popular fluorescent probe for the study of microheterogeneous media. The vibronic structure of the pyrene monomer emission spectrum is sensitive to solutesolvent dipoledipole coupling, which is itself influenced by changes in the dipolarity of the environment surrounding the pyrene molecules [12, 13]. The pyrene fluorescence spectrum exhibits five vibronic bands numbered I through V. There is a significant enhancement in the forbidden 00 band intensity (peak I) in the presence of solvents of increasing dielectric constant at the expense of other bands [12]. Thus, the intensity ratio associated with the first (I at 373 – 2 nm) and third peaks (III at 384 – 2 nm) provides a sensitive measure of subtle changes in the pyrene local microenvironment [12, 13].