© Copyright 2005 American Chemical Society
FEBRUARY 1, 2005 VOLUME 21, NUMBER 3
Letters Phenomenon Observed at the Onset of Micellization Using Static Light Scattering Gina A. Sorci* and T. Daniel Walker Millsaps College, Jackson, Mississippi 39210 Received May 24, 2004. In Final Form: October 21, 2004 A phenomenon was observed near the critical micelle concentration (cmc) of surfactants using static light scattering. This consists of an unexpected peak in light scattering as the concentration varies between zero and above the cmc. This work studied three different surfactants: the two ionic surfactants hexadecyltrimethylammonium bromide (CTABr) and sodium dodecyl sulfate (SDS) and the nonionic surfactant Triton X-100. Peaks were observed for all three under different conditions such as varying ionic strengths and different concentration paths. These peaks are real, are reproducible, and appear to have static properties.
Introduction Surfactants have been studied for many years due to their many applications ranging from everyday detergents to purifying proteins.1 Their defining property is their ability to associate, forming aggregates called micelles due to the hydrophobic/hydrophilic interactions of the individual molecules.2 These molecules can be either ionic or nonionic. In the case of ionic surfactants, electrostatic properties play a role in their interactions as well. Since Overbeek and Verwey’s3 work in the 1940s, various techniques have been used to explore these molecules including conductance measurements, surface tension measurements, and various types of light scattering.4 These techniques have consistently shown that these molecules aggregate at a particular concentration which * Corresponding author. E-mail:
[email protected]. Phone: (601)974-1348. (1) Interaction of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Atkins, P. W. Physical Chemistry; Oxford University Press: Cary, NC, 1990; p 706. (3) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier Publication: Amsterdam, 1948. (4) Evans, D. F.; Wennerstrom, H. The Colloidial Domain where Physics, Chemistry, Biology, and Technology Meet; Wiley-VCH Publication: New York, 1999; Chapter 4.
is known as the critical micelle concentration (cmc). This concentration has been well documented for several surfactants under various conditions.1 However, this is not the case for the aggregation number (NA), which is the number of surfactant molecules that aggregate to form a micelle at the cmc. The values obtained for NA can vary greatly, and the error can be as high as 30% for a particular series of experiments.4 In this paper, a phenomenon will be introduced that will lead to a better understanding of the hydrophobic/ hydrophilic interactions of these molecules. This is accomplished using a technique that allows for a range of concentrations to be traversed, providing detailed real time information about the system. Simple synthetic surfactants are used to minimize the complexity of the system. Eventually, this research will culminate in a description of the fundamental interactions by which the individual surfactant molecules form micelles. This will assist in the understanding of more complex molecules which have similar components such as pulmonary surfactants and phospholipids.5-7 Another interesting component of this work is the impact it will have on the (5) Lehninger, A. L. Principles of Biochemistry; Worth Publishers: NY, 1982; pp 302-327. (6) Jobe, A. H.; Ikegami, M. Clin. Perinatol. 2001, 28, 655-669. (7) Hallman, M. N. Engl. J. Med. 2004, 350 (13), 1278-1280.
10.1021/la0487186 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/07/2005
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study of polymer/surfactant interactions and hydrophobically modified polymers. These particular polymers have proven difficult to characterize because they will often self-associate. If we can better understand what occurs with the surfactant molecules in regard to the hydrophobic/ hydrophilic interactions, then we may be able to better characterize these polymers and explain polymer/surfactant interactions.1 Materials and Methods The sodium dodecyl sulfate (SDS) and Triton X-100 were purchased from Polysciences to ensure the highest purity. The hexadecyltrimethylammonium bromide (CTABr) was purchased from Sigma Scientific with a purity of 99.8%. The solutions were prepared in sterile glassware using deionized UV treated filtered water. A stock solution of the surfactant was prepared at a concentration well above the cmc. The solution was left on a stirring plate overnight to ensure dissolution. In the case where the experiments took place at fixed NaCl concentrations, a stock solution of the particular molar concentration was prepared. This was used to prepare the surfactant solutions and in the dilution process. The technique employed in these experiments that uses a refractive index detector to follow the concentration over a gradient is called automatic continuous mixing (ACM).8 This technique has been used previously to study polymer/surfactant association9 and polyelectrolyte/salt interactions.10,11 The instruments used varied, depending on the mixing technique. The first series of experiments used a small Masterflex peristaltic pump with a Waters 590 HPLC pump. The solution was recirculated back into a beaker which was on a stir plate. This provided a slow mixing process which eliminated any irregularities that could occur within a mixing chamber. The second technique uses an ISCO 2360 gradient mixer which consists of three reservoirs that can be programmed to withdraw the solutions at different percentages for different periods of time. For both mixing techniques, the tubing exiting the pump is connected to a stainless steel, 2.5 cm frit filter holder which contains both a 10 µm stainless filter and a 0.22 µm paper filter. After the solution passes through the filter, it goes through a Wyatt Technology Mini Dawn light scattering (LS) instrument and an Erma ERC 7510 refractometer (RI). The Mini Dawn is a static light scattering detector which has three angles, 42, 90, and 124°. The data from the LS detector as well as the RI were collected using the ASTRA software provided by Wyatt with the light scattering device. The data were then transported to text files and analyzed by a software program written by one of the authors.12 The experiments were performed over a period of time which ranged from 2 to 8 h. All of the experiments were run at room temperature 25 °C under neutral pH. The experiments were begun by collecting data for the solvent, providing a baseline needed for calculations, as can be seen in Figure 1, for both experiments. The concentration gradients were run either by starting at stock solution concentration and then diluting the solution or by starting at a concentration of zero and increasing to the stock solution concentration. The raw data in volts for both techniques can be seen in Figure 1. The inset starts at a concentration of zero and uses the ISCO mixer, while the larger graph starts at the maximum concentration and uses the recirculation technique. Analysis of the light scattering data was accomplished using Zimm’s approximation13
(
)
q2〈S2〉z 1 Kc ) 1+ + 2A2c 3 I(c,q) Mw
(1)
(8) Strelitzki, R.; Reed,W. F. J. Appl. Polym. Sci. 1999, 73, 2359. (9) Sorci, G. A.; Reed, W. F. Langmuir 2002, 18 (2), 353-364. (10) Sorci, G. A.; Reed, W. F. Macromolecules 2002, 35 (13), 52185227. (11) Sorci, G. A.; Reed, W. F. Macromolecules 2004, 37 (2), 554-565. (12) Sorci, G. A. Doctoral Thesis, 2002. (13) Zimm, B. H. J. Chem. Phys. 1948, 16, 1093.
Figure 1. Raw data from an experiment in which the concentration of the surfactant is changed by diluting a 4 mg/ mL concentration of SDS using the recirculation technique. The RI is linear for both, while the LS has a peak. The inset shows data from SDS which has been purified, and the ISCO mixer is used to traverse the peak at a quicker rate In the above equation I(q,c) is the absolute light scattering intensity at a particular concentration and angle. This is obtained by comparing the light being scattered to that of a sample of known scattering. The q value is related to the angle at which the light is being scattered by q ) (4πn/λ) sin(θ/2). K is the optical component which is given by the equation below.
K)
4π2n2(dn/dc)2 NAλ4
(2)
To use this approximation, several assumptions have been made. First, the concentration of the molecule being observed is low; therefore, it is only extended out to the first order of c. This term contains A2 which is the second-order virial coefficient. Second, a small angle approximation is made to yield the term (1 + (q2〈S2〉z)/3). A description of this process can be found in refs 12 and 13. 〈S2〉z0.5 is the z-average of the mean square radius of gyration or commonly called the radius of gyration (Rg). Using eq 1 and the light scattering intensity calculated from the data, the weight-average molecular weight (Mw), A2, and Rg can be obtained. These are acquired by taking the limit as c goes to zero for Kc/I(c,q); this intercept yields 1/Mw, while the slope yields the radius of gyration. For the limit as q(θ) goes to zero, the intercept will also be 1/Mw, while the slope yields 2A2. These properties provide useful information about the system. The second virial coefficient takes into account excluded volume effects. The radius of gyration provides the conformation of the molecule. This can be described by intra- and intermolecular interactions along the molecule. Due to the complexity of a changing system, we will reserve the calculations of these components for future work.
Discussion Figure 1 is an example of the raw data for sodium dodecyl sulfate in water. The light scattering (LS) intensity (volts)
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Figure 2. (a) Using the data from Figure 1, the RI data are used to convert to concentration. Notice the peak is near 2.41 mg/mL (0.002 41 g/mL) which is the cmc for SDS in water. The overlaying data are SDS concentration varied in a fixed 20 mM NaCl solution. The cmc is 0.0015 g/mL. (b) Similar data for CTABr in water where the cmc is 0.3 mg/mL (0.0003 g/mL).
and the refractometer (RI) data (volts) are shown verses time. Initially, the RI is saturated for the SDS baseline of 4 mg/mL solution. Once the dilution begins, via the recirculation technique, it is observed that the refractometer data are smoothly decreasing, while the light scattering follows a different path. The light scattering intensity begins by slowly decreasing as the concentration decreases, which normally occurs. Then, it increases and decreases, forming a peak. This is unexpected, since it is usually assumed that the surfactant molecules go from single monomers to micelles immediately at the cmc. Meanwhile, the RI follows the concentration and slowly decreases throughout the dilution process, as would be expected. The inset in Figure 1 shows an SDS experiment in which the ISCO mixer is used, and the gradient begins at a concentration of zero and goes up to 3.3 mg/mL SDS. The peaks have different magnitudes; this is due to the rate at which the material is flowing. In the inset, the peak range is only 30 min, whereas the other peak forms over a 100 min period of time. In Figure 2, the refractometer data and dn/dc (dn/dcSDS ) 0.122 and dn/dcCTABr ) 0.1595) of the surfactants were used to convert from volts to grams per milliliter. This was accomplished by determining the calibration factor dV/dn (change in voltage over index of refraction) for the RI and using dn/dc (refractive index increment). The light scattering data are not manipulated. The cmc for SDS in water is 0.002 41 g/mL, while in 20 mM NaCl it is 0.001 25 g/mL. In Figure 2b, the CTABr concentration is varied in water and has a cmc of 0.3 mg/mL. Again, the peak occurs before the cmc in all situations. This region has not been studied in such detail; therefore, we believe that this phenomenon has not yet been observed. In Figure 2a, the SDS concentration is varied in a fixed 20 mM NaCl solution. The effect of increased ionic strength
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Figure 3. Data for Triton X-100. The concentration gradient goes from a concentration of zero to 0.465 mg/mL. In this experiment, the concentration is varied by the recirculation technique which occurred over a 5 h period of time. It is shown in time to accentuate the peak. In the inset, the concentration is varied over a 2 h period of time using an ISCO mixer; notice the peak is less defined.
is observed by a decrease in the concentration where the peak occurs as well as an increase in the overall intensity. Both of these occurrences can be described by the properties of ionic surfactants which state that the cmc is reduced by the addition of salt and the aggregation number is increased.3 In Figure 3, the peak is again observed for the nonionic surfactant Triton X-100. This is shown in time to demonstrate the relationship between peak height and flow rate. The data were taken using the recirculation technique, and the experiment lasted over 5 h. The inset details a second experiment which was performed in a shorter period of time. In this case, the ISCO mixer is used to vary the concentration from zero to ∼0.000 35 g/mL. This gives a smaller, less defined peak which verifies the dependence of the rate at which the peak is traversed on the height. The cmc for Triton X-100 is 0.13 mg/mL. The process in which surfactant molecules aggregate is thought to be a stepwise process,14,15 and we hypothesize that this is what is observed. The hydrophobic tails come together, forming aggregates, and at a certain concentration, they become unstable. This is not only due to simple elastic strain and electrostatic interactions but also due to an attraction by other individual molecules in the solution. These structures then begin to break down and form the micelles of the expected size. Similar peaks were observed in experiments between surfactants and non(14) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 53, 279-371. (15) Ilgenfritz, G.; Schneider, R.; Grell, E.; Lewitzki, E.; Ruf, H. Langmuir 2004, 20, 1620-1630.
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observed before the cmc. Different concentration paths were traversed to ensure reproducibility. Even when the large filter was removed, the peak was there, although the data were noisy. An experiment was performed where the concentration was held fixed when the maximum was reached on the peak. The LS remained unchanged. Finally, samples of SDS and CTABr were purified and their purity was tested by NMR and melting point measurements. The experiment was repeated for both, and the peaks still occurred before the cmc. The data for the purified SDS can be seen in the inset of Figure 1. In Figure 4, Kc/I(0,c) is shown using a process described in ref 12. Assuming A2 to be similar for all concentrations, the effects are relatively small (∼6%); therefore, the 2A2c term in eq 1 is neglected and the apparent molecular weight is determined. For comparison, the original light scattering data are also shown. The light scattering continues to increase after the cmc, while the apparent molecular weight remains constant. This is due to the effect of concentration on light scattering. This demonstrates that after a particular point (cmc) the apparent Mw is constant and the micelles have a fixed aggregation number. Once the intercept becomes linear, the aggregation number can be determined using the molecular weight of the surfactant molecule. In the SDS with no salt, the aggregation number is 25, while, in the 20 mM NaCl solution, the aggregation number is ∼40. These numbers are lower than expected. This is contributed to the turbulence in the system from the flow. Regardless, the cmc values are near the reported values and occur on the downward side of the peak. Conclusion Figure 4. At the top of the figure, SDS concentration is varied in pure water. Both the raw light scattering and the calculated Kc/I(0,c) data can be seen. At the bottom of the figure, the same can be seen for the SDS in 20 mM NaCl solution. After the peak, Kc/I(0,c) begins to level off and the surfactant exists as micelles. Here, we can determine the A2 and aggregation number for the micelles.
associating polymers. The speculation was that the polymer was interfering in micellization. Since this is being observed for pure surfactants, the conclusion was incorrect. In the work by Chaterjee et al., a similar phenomenon was observed in d(∆H)/dc in the same region.16 We believe that the lack of data in this region may be part of the reason that no one has seen this before. To determine precisely what is occurring, further experiments using ACM with different detectors are needed. To ensure that these peaks were not merely due to impurities or other anomalies, the experiments were repeated. In each experiment performed, the peaks were (16) Chatterjee, A.; Moulik, S. P.; Sanyal, S. K.; Mishra, B. K.; Puri, P. M. J. Phys. Chem. B 2001, 105, 12823.
Despite the fact that these molecules have been thoroughly studied, this property appears to be one that has not previously been observed. The reason for this is the amount of detail that this technique allows, along with the fact that this region is often ignored. When the same experiments are run using discrete points, this phenomenon can be easily missed. Further work is needed to confirm if these are larger structures or merely fluctuations due to the formation of micelles. Fluctuations are unlikely, since we observe no fluctuation in the index of refraction measurement. More experiments and other techniques will provide a more complete understanding of this phenomenon. Overall, we have shown that these peaks are reproducible, are stable, and can be seen under different conditions. Acknowledgment. We would like to thank Wyatt Technology, Hearin Foundation, Yuliya Vasilyeva, for purifying the surfactant, Dr. Charles McCormick, as well as Wayne Reed where this work began. LA0487186
