Determining the Critical Micelle Concentration of Aqueous Surfactant Solutions Using a Novel Colorimetric Method Kenneth G. Furton1 and Arold Norelus2 Florida International University, University Park, Miami, FL 33199
Surface-active agents (surfactants) are among the most versatile chemicals available to chemists, used in a variety of areas, including chemical kinetics and as membrane mimics in biochemistry. Surfactants 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 sufficiently high concentrations (usually greater than about 10-1 M), the surfactant molecules arrange themselves into organized molecular assemblies known as micelles. The concentration at which this phenomenon occurs is called the critical micelle concentration (CMC). Normal micelles are those occurring in aqueous solutions in which the surfactant molecules ori-
ent themselves into spherical
or elliptical structures with their lipophilic tails oriented toward the center and their hydrophilic 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-
1YSP1SEED Preceptor. 2NSF Young Scholar/ACS Project SEED Summer Student. Present address: Miami Edison Senior High School, Miami, Fiorida.
Figure 1. A two-dimensional representation of the association of LAS molecules into a spherical normal micelle. For simplicity, the ndodecylbenzenesulfonate molecules are schematically indicated to denote their relative location, but not their number, distribution, or configuration.
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tween micelles and between the bulk aqueous solution and
the micelle.
Methods for Determining 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 physical properties that have been studied for this purpose include the solution detergency, viscosity, density, conductivity, surface tension, osmotic pressure, interfacia] tension, refractive index, and light scattering. Other more sophisticated techniques that can be employed include Xray diffraction, electron spin resonance techniques, nuclear magnetic resonance spectroscopy, calorimetry, chro-
matographic 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 Common spectroscopic methods for determining aqueous CMC make use of additives whose change in UV-vis absorbance or fluorescence emission indicate the onset of micelle formation. In a typical experiment, a constant amount of an organic compound, such as a dye, is incorporated into solutions containing increasing amounts of surfactant. The percentage change in absorption or emission is plotted against the surfactant concentration. Finally, the midpoint of the transition of the sigmoidal curve obtained is taken to be the CMC. Undergraduate experiments utilizing this method have been described recently (5, 6).
Why Another Method? Although the UV-vis absorption and spectrofluorometric methods provide reasonable estimates of CMC’s, there are several problems associated with this method when applied to an undergraduate experiment. First, the data manipulation required may reduce the accuracy of the technique (e.g., 18-39% Relative Average Deviation from Ref. 5) and may cause confusion as to what is actually happening at the CMC (e.g., determining the midpoint of the transition 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 observed value of the CMC. Polar organic molecules can cause a marked depression of the CMC in aqueous media, even at 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. Currently, we describe a straightforward, inexpensive, noninterfering technique suitable for determining accurate CMC’s in an undergraduate experiment. The colorimetric technique we have developed is based on the solubilization of the oil-soluble dye, l-(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 from two other phenomena, namely, hydrotropy (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 compound being in the same phase as the solubilizing solution and is thermodynamically stable. When the solubility of a normally solvent-insoluble compound is plotted
against the concentration of the surfactant solution, the solubility is slight and constant at low concentrations, but increases abruptly above a certain concentration. This indicates that the solubilization is due to a micellar phenomenon and the point at which the solubility abruptly increases corresponds to the CMC. In an ideal case, the amount of solubilized substance varies linearly with the surfactant concentration above the CMC. This is the plot observed for the system studied here. To prevent any possible CMC depression due to the presence of an additive (the dye) in the surfactant solution 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 at which the solubilization of an organic compound occurs has a large effect on the extent of CMC depression. Compounds that are mainly adsorbed in the outer 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 micellization, 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 relatively inexpensive, commercially available, had negligible solubility in pure water, 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. Procedure
Pentane,sodium carbonate, sodium dodecylbenzenesulfonate (LAS) and l-(2-pyridylazo)-2-naphthol (PAN) were obtained from Aldrich Chemical Company, Inc. (Milwaukee, WI) and used without further purification. It is important to note that the chemicals used in this experiment are classified in the Aldrich catalog as irritants and, additionally, LAS as toxic; therefore, safety goggles and protective gloves should be used when handling these materials. Also, because pentane is flammable, all operations should be performed in 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 an afterburner and scrubber. The experimental procedure developed is as follows:
Figure 2. Plot of the absorbance at 470 tration.
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increase in solution color (and absorbance at 470 nm) likely corresponds to the solubilization of the PAN into the core of the LAS micelle. A schematic representation of the regions of a simplified LAS micelle (Fig. 3) shows how the hereto aqueous-insoluble l-(2-pyridylazo)-2-naphthol can be solubilized into a LAS micelle. The interior, or core, of ionic micelles average 10-30 A and is generally inferred to be
hydrocarbonlike with only small
of trapped water molecules present. Oil-soluble molecules, such as the dye used here, are solvated near or within this lipophilic region. The charged head groups and some of the counterions make up a compact region (less than three angstroms) known as the Stern layer. Most of the counterions are found in a large outer region of micelles known as the Gouy-Chapman double layer that can extend up to several hundred angstroms from the Stem layer to the bulk of the solution (2). amounts
Effect of Electrolytes on CMC There are several factors that can markedly affect the CMC of aqueous surfactant solutions. The described experiment can easily be extended to include an investigation of one or more of these factors. Four of the major factors are
Figure 3. A schematic of the regions of dye probe 1-(2-pyridylazo)-2-naphthol. 1.
2.
3. 4. 5.
a LAS
micelle and the possible site of solvation of the
Prepare 100 mL of a saturated solution of PAN in pentane (~ 1.6 x 1(T3 M). Prepare a set of 10 solutions with various concentrations of LAS from 0 to 2.6 x 10-3 M using 100- mL volumetric flasks. Transfer 10 mL of each solution into small jars and add 10 drops of the saturated PAN solution in pentane. Gently swirl the jars and allow the pentane to evaporate and the color to develop (~ 20 min.}. Note the intensity of color in each solution and measure the absorbance of each solution at 470 nm using a Spectronic 20D spectrophotometer.
The results of a typical experiment of this kind shown in Figure 2.
are
Discussion It can be seen from Figure 2 that the curve shows a very sharp rise as the LAS concentration is increased above the CMC, followed by a linear increase with increasing LAS concentration. If we take the intersection of the two straight lines to be the CMC, the value of the determined CMC coincides nearly perfectly with the literature value (1.6 x 10"3 M at 25 °C), taken from Ref. 7) indicated as a dotted line in Figure 2. The CMC also can be estimated without the aid of a spectrophotometer. Visual examination of the solutions shows a constant faint yellow/orange color in the first 6 solutions, and a marked increase in color from solutions 7 through 10. The change is apparent between solutions 6 and 7 indicating a CMC between 1.4 and 1.7 x 10-3 M (literature value 1.6 x 10-3 M). The marked 256
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1. 2. 3. 4.
the the the the
structure of the surfactant temperature of the solution presence of organic additives presence of electrolytes
The factor most often exploited to improve the cleaning action of commercial laundry detergents, such as the LAS studied here, is the latter; namely, the addition of electrolytes that causes a decrease in the CMC. This effect is more pronounced for anionic and cationic than for zwitterionic surfactants and more pronounced for zwitterionics than for nonionics (3). Commonly added electrolytes (that act as water softeners) include sodium phosphates, aluminosilicates, and sodium carbonate. One such experiment that we have developed proceeds as follows. The procedure described above is repeated with the exception that 5 mL of a 0.03 M sodium carbonate solution is added to each 10-mL aliquot followed by the addition of the 10 drops of PAN dye solution (final concentration of sodium carbonate in of solutions 0.01 M). The result is a large decrease in the concentration at which the increase in absorbance occurs and a marked decrease in the measured CMC (4 x 10~4 versus 16 x 10-4 M).
Conclusions The oil-soluble dye probe method proposed here is an inexpensive, straightforward, accurate technique for determining the CMC of aqueous surfactant solutions. The advantages of this method over the previous dye absorption methods are three-fold. First, the method can be applied either without instrumentation or with relatively inexpensive spectrophotometers (e.g., Spectronic 20). Secondly, the negligible solubility of the dye probe in aqueous solutions below the CMC minimizes the possibility that the probe itself alters the CMC of the solution. Finally, the information generated is straightforward and easily interpreted. The uptake of the probe by the solution coincides with the
formation of micelles that can solubilize the oil-soluble dye. This mechanism also vividly illustrates to students the basic process that allows household detergent solutions to solubilize hydrophobic stains such as grease.
Literature Cited 1. 2. 3.
Acknowledgment The authors are indebted to Zaida C. Morales-Martinez, coordinator of the NSF Young Scholars/ACS Project SEED program at FIU, whose supervision and succor allowed this project to be completed.
4. 5. 6.
7.
Lindman, B.; Wennerstrom, H. Micelles: Amphile Aggregation in Aqueous Solution; Springer-Verlag: Berlin, 1980. Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. Rosen, M. J. Surfactants and Interfacial Phenomenon; John Wiley: New York, 1978. Love, L. J. C.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984,56, 1132A. Rujimethabhas, M.; Wilairat, P. J. Chem. Educ. 1978, 55, 342. Sawyer, D. T.; Heineman, W. R.; Beebe, J. M. Chemistry Experiments for Instrumental Analysis; John Wiley: New York, 1984, Experiment 10-4 “Critical Micelle Concentration of Surfactants”, p 283. Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975, p 20.
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