Determining the Critical Micelle Concentration of Aqueous Surfactant Solutions Using a Novel Colorimetric Method Kenneth G. ~urton'and Arold or el us^ Florida International University, University Park, Miami, FL 33199
Surfaceactive agents isurfactants) are among the most versatde chemicals available to chemists, used in a variety of areas, including chemical kinetics and as membrane mimics in biochemistry. Snrfactants are encountered in a diverse range of consumer products including motor oils, pharmaceuticals, soaps, and detergents. Surfactants are amphiphilic substances consisting of a long-chain hydrocarbon "tail" and a polar (often ionic) "head". A unique property of surfactants is that, at sufficientlyhigh concentrations (usually greater than about lo4 MI, the surfactant molecules arrange themselves into organized molecular assemblies known as micelles. The concentration a t which this phenomenon occurs is called the critical micelle concentration (CMC).Normal micelles are those oeeurring in aqueous solutions in which the surfactant molecules on-
en1 themselves into spherical or elliptical structures with their liw~hilictails orlented toward the center and their hydrophifie heads oriented toward the surrounding water. One of the most common types of aqueous surfactants in consumer chemistry are the linear alkylbenzenesulfonates (LAS),that are anionic surfactants present in the majority of home laundry detergents. A two-dimensional representation of how a typical spherical LAS micelle might look in solution is shown in Figure 1.Micelles should be visualized not as static objects, but as molecular assemblies in constant motion with exchanges of amphiles occurring be'YSPISEED Preceptor. 'NSF Young ScholarlACS Project SEED Summer Student. Present address: Miami Edison Senior High School, Miami. Florida.
Fig~re1 A two-aimensional representation of the association of LAS molecules into a spherccal normal m~celle.For s~mplicity,tne ndodecy benzenes~lfonate molecdes are scnematcally indicatea to denote the r re awe locauon, b ~ not t their n~mber, amodon, or confquratm.
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Journal of Chemical Education
tween micelles and between the bulk aqueous solution and the micelle. Methods for Determlnlng CMC The value of the CMC can be determined by the change in the physicochemical properties of the surfactant solution as the concentration of the amphile is increased. Some of the ohvsical orooerties that have been studied for this purposk ihclude'thi solution detergency, viscosity, density, conductivitv. .. surface tension. osmotic oressure. interfacial tension, refractivc indcx, and light scattering. Other more soohist~catedtechniaues that can be emolovcd . " include Xray diffraction, electron spin resonance techniques, nuclear magnetic resonance spectroscopy, calorimetry, chmmatographic techniques, potentiometric techniques, fluorescence emission spectroscopy,and UV-visible absorption spectroscopy. For a more detailed discussion of micelles, their uses, and the determination of CMC, readers are directed to references 1-4. Common spectroscopic methods for determining aqueous CMC make use of additives whose change in UV-& absorbance or fluorescence emission indicate the onset of micelle formation. In a typical experiment, a constant amount of an organic compound, such a s a dye, is incorporated into solutions containine increasine amounts of surfactant. The oercentaee change in absorption or emission is plotted aga&st the sGfactant concentration. Finally, the midpoint of the transition of the sigmoidal curve obtained is taken to be the CMC. Undermaduate exoeriments utilizing - this method have been described r e c e h y (561. Why Another Method? Although the W-vis absorption and spectmfluommetric methods provide reasonable estimates of CMC's, there are several problems associated with this method when applied to an undergraduate experiment. First, the data manioulation reauired mav reduce the accuracv of the technique (e.g., 18'-39% ~eyativeAverage ~ e v i a c o nfrom Ref. 5) and mav cause confusion as to what is actuallv baooenC determining the midpoint o?th/&aning at the ~ M (e.g., sition of the sigmoidal curve for the percent change in measurements). Second, these methods require relatively costly instrumental techniques that may not be available at the undergraduate level (particularly spectrofluorometric instrumentation). Thirdly, the presence of organic compounds, including dyes, in the solutions can significantly affect the obsewed value of the CMC. Polar organic mole: cules can cause a marked depression of the CMC in aqueous media, even a t very low bulk phase concentrations. The degree of CMC depression is related to the polarity of the additive, the degree of branching, and the locus of solubilization. Additives that penetrate into the inner portion of the core of micelles should decrease the CMC only slightly. Currentlv. we describe a straiehtforward. inex~ensive. noninterfekg technique suitable for determining accu: rate CMC's in an undermaduate experiment. The colorimetric technique we have developed $ based on the solubilization of the oil-soluble dye, 1-(2-pyridylazo)-2-naphthol. Solubilization refers to the dissolution of normally insoluble or only . slightly . . soluble compounds in water when surfactants are present. solubilization is distinguished fmm two other phenomena, namely, hydmtropy (the dissolving of normally water-insoluble compounds by concentrated solutions of compounds called hydrotropes) and emulsification (the dispersion of one liquid phase in another), in that the process of solubilization results in the dissolved comoound being in the same ohase as the solubilizine solution and is th&modynamickly stable. When the so1;bility of a normally solveut-insoluble compound is plotted
against the concentration of the surfactant solution, the solubilitv is slieht and constant at low concentrations. but increases abruptly above a certain concentration. This indicates that the solubilization is due to a micellar ohenomenon and the point a t which the solubility abriptly increases corresoonds to the CMC. In an ideal case. the amount of solLbilized substance varies linearly with the surfactant concentration above the CMC. This is the lot obsewed for the system studied here. To re vent anv oossible CMC deoression due to the oresence'of a n addkive (the dye) in-the surfactant sol;tion using this technique, the dye should ideally be insoluble in water alone and solubilize into the micelles without affecting the micellization process. The location in the micelle a t which the solubilization of an organic compound occurs has a large effect on the extent of CMC de~ression.Compounds that are mainly adsorbed in the o k e r portion of the micelle tend to depress the CMC to a much greater extent than those adsorbed in the micelle core. This depression is due to the decreased work required for micellizatiou, in the case of ionic surfactants probably due to the decreased mutual repulsion of the ionic heads in the micelle (1).The oil-soluble dye, l-(2-pyridylazo)-2-naphthol, was chosen over numerous others investigated because it was relativelv inexoensive. commerciallv available. had negligible sol;bility;n purewater, and was readily soluble in micellar solutions. Oil-soluble dyes are likely solubilized in the micelle core and should, therefore, have a negligible effect on the surfactant CMC.
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Procedure Pentane,sodium carbonate, sodium dodecylbenzenesulfonate ( U S ) and l-(2-pyridylazoj-2-naphthol(PAN) were obtained from Aldrich Chemical Company, Inc. (Milwaukee, WI) and used without further purification. It is imoortant to note that the chemicals used in this exoeriment are classified in the Aldrich catalog as irritants A d , additionally, LAS as toxic; therefore, safety goggles and protective gloves should be used when handling these materials. Also. because oentane is flammable. all ooerations should be performed ;n a fume hood away from open flames or sparks. Finally, it is important to collect used materials and have them disposed of by technically qualified personnel. Aldrich recommends these materials be burned in an EPA-licensed chemical incinerator equipped with a n aRerburner and scrubber. The experimental procedure developed is as follows:
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0
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8
12
16
20
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28
Concentration (M X 1 0 7
Figure 2. Plot of the absorbance at 470 nm versus the LAS concentration. Volume 70 Number3 March 1993
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Aqueous bulk phase
X----\ double layer
Mielle mre
increase in solution color (and absorbance a t 470 nm) likelv corresoonds to the solubilization of
