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Thermal Stability of Nonionic Surfactant Aggregates Kevin Toerne, Robin Rogers, and Ray von Wandruszka* Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 Received April 16, 2001. In Final Form: July 27, 2001 A polarity-sensitive probe was used to assess the thermal stability of aggregates of six nonionic surfactants in aqueous solution. The I1/I3 peak ratio of the pyrene emission spectrum, which varies with solvent polarity, was taken as an indicator of the probe environment and hence of the surfactant aggregates in which this probe was sequestered. Aggregate stabilities of surfactants in the Triton, Igepal, and Brij series in premicellar, micellar, and supramicellar (clouded) solutions were considered. Temperatureinduced clouding was not found to have a measurable effect on the environment of the sequestered probe. Fully developed micelles exhibited a near-linear decrease in pyrene I1/I3 with temperature, similar to the changes observed in pure benzene solvent. In the premicellar and early micellar stages, however, thermal stabilities of aggregates varied significantly with the structure and size of both the hydrophobic and hydrophilic moieties of the surfactant molecules. The least stable assemblies were formed by surfactants with short polyoxyethylene chains and highly branched aliphatic segments.
Introduction Temperature has a significant effect on the supramolecular organization of surface-active species in aqueous solution. In nonionic surfactants, compatibility with water depends on the extent of hydration of the hydrophilic portion of the molecules, which is sensitive to changes in temperature. Parameters such as the critical micelle concentration (cmc) and the cloud point are affected by hydration and are therefore temperature dependent.1,2 Premicellar aggregates formed at low surfactant concentrations are loosely associated and likely to be disrupted by thermal agitation. Above the cmc, thermal stability is expected to increase, while further heating is known to lead to additional (supramicellar) aggregation in some cases. The latter process, known as clouding, is especially found among polyoxyethylene (POE) surfactants. It involves dehydration of the hydrophilic POE chain and leads to the formation of aggregates large enough to scatter visible light. This phenomenon, which has been variously ascribed to gradual micellar growth3 and to a critical event at the cloud point,4 does not always proceed in a monotonic manner.5 Temporary reversals in particle growth, termed declouding, have been reported during heating episodes in certain surfactant systems.5-7 Secondary surfactant structures ranging from premicelles to clouding aggregates are likely to be affected by temperature in different ways, as thermal agitation and POE dehydration may play opposing roles in their formation. The situation is further complicated in the presence of hydrophobic solutes, which can act as nucleation sites and thereby promote the formation of premicellar aggregates.8 The formation of supramicellar aggregates is most easily envisaged as a surface-to-surface adherence of individual surfactant micelles, which can be (1) Stasiuk, E. N. B.; Schramm, L. L. J. Colloid Interface Sci. 1996, 178, 324. (2) Chen, L.-J.; Lin, S.-Y.; Huang, C.-C.; Chen, E.-M. Colloids Surf. 1998, 135, 175. (3) Atwood, D. J. Phys. Chem. 1968, 72, 339. (4) Corti, M.; Minero, C.; Degiorgio, V. J. Phys. Chem. 1984, 88, 309. (5) Maclay, W. N. J. Colloid Sci. 1956, 11, 272. (6) Toerne, K.; Rogers, R.; von Wandruszka, R. Langmuir 2000, 16, 2141. (7) Porter, M. R. In Handbook of Surfactants; Chapman and Hall: New York, 1991; p 121. (8) Loran, C.; Von Wandruszka, R. Talanta 1991, 38, 497.
thermally dislodged upon heating (declouding). It does not, however, obviate the possibility that clouding involves micellar coalescence in which larger particles are formed (including vesicles and tubes).9 These may revert to smaller dimensions during declouding. One way of gaining insight into the thermal stability of surfactant aggregates is to include a polarity-sensitive probe in the solutions. This probe should be sufficiently hydrophobic to seek out the nonpolar regions of any surfactant assemblies that are formed, thereby giving an indication of the environment within these structures. A prime candidate for such a study is pyrene, which exhibits fluorescence behavior that changes with the polarity of its microenvironment.10,11 Pyrene has been used extensively to determine characteristics of solvent mixtures12,13 and surfactant solutions.14,15 The measured parameter is the I1/I3 ratio, that is, the ratio of the first and third peaks in the fluorescence emission spectrum. It is relatively large in polar environments (e.g., 1.87 in water) and smaller in nonpolar solvents (e.g., 0.58 in cyclohexane).10 In the work described below, fluorescence measurements of this kind were used to determine the extent to which pyrene is sequestered in (pre)micelles or clouded structures at various temperatures and surfactant concentrations. Experimental Section Reagents and Solutions. Pyrene was obtained from Aldrich, purified by recrystallization and sublimation onto a coldfinger. Triton X-35, 100, 114, and 405 and Brij 30 were all obtained from Sigma and used as received. Igepal CO-630 was obtained from Rhoˆne-Poulenc and used as received. Brij 35 was obtained from Fisher and used as received. All solutions and solvents were purged with nitrogen to remove dissolved oxygen and prevent quenching of pyrene fluorescence. Doubly deionized water, treated (9) Hiemenz, P. C.; Rajagopalan, R. In Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1997; p 371. (10) Dong, D. C.; Winnik, M. Can. J. Chem. 1984, 62, 2560. (11) Karpovich, D. S.; Blanchard, G. J. J. Phys. Chem. 1995, 99, 3951. (12) Kusumoto, Y.; Takeshita, Y.; Kurawaki, J.; Satake, I. Chem. Lett. 1997, 4, 349. (13) Acree, W.; Zvaigzne, A.; Fetzer, J. Appl. Spectrosc. 1990, 44, 1193. (14) Tringali, A.; Kim, S.; Brenner, H. J. Lumin. 1999, 81, 85. (15) Minnik, F. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabahn, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; pp 368-395.
10.1021/la010559s CCC: $20.00 © 2001 American Chemical Society Published on Web 09/07/2001
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Table 1. Structure and Characteristics of POE Surfactantsa
a
Data from ref 14.
Figure 1. Variation of pyrene I1/I3 ratio with temperature in solutions of different TX-114 concentrations. with a 0.22-µm Millipore filter system to a resistivity of at least 15 MΩ cm, was used in all solutions. Fluorescence Measurements. All pyrene emission spectra were taken with a SLM-Aminco 8100 fluorescence spectrophotometer equipped with a thermostated sample compartment ((0.5 °C). Pyrene was excited at 340 nm, and the emissions were measured at 373.6 nm (I1) and 384.4 nm (I3). At least 15 min was allowed for thermal equilibration after a temperature adjustment, and the solutions were gently stirred during measurement. All measurements were done at least in duplicate.
Results and Discussion The structures and relevant parameters of the surfactants used in this study are shown in Table 1. All have POE hydrophilic chains, albeit of different lengths, and hydrophobic moieties composed of hydrocarbon chains. The latter are identical among the Triton series of surfactants, all including a benzene ring. Igepal also features an aromatic center but has a longer aliphatic group attached, while the Brij series is entirely aliphatic. It can be seen that seemingly minor changes in the POE chain lengths can result in significant differences in the values of the cmc and the cloud point. Triton Surfactants. Figure 1 shows the variation of the pyrene I1/I3 ratio with temperature in TX-114 solutions of different concentrations. The behavior of I1/I3 in pure water and benzene is also shown. In both of the latter cases, the ratio is seen to decrease linearly with temperature in a manner consistent with the change of dielectric constant of the solvents. At the lowest surfactant concentration (0.03 mM), the I1/I3 decrease essentially followed the water line, indicating that pyrene was afforded no protection by TX-114 at any temperature in this instance. As the surfactant concentration was raised, however, I1/I3 began to decrease well before the cmc of TX-114 (∼0.3 mM) was reached. The intermediate region at surfactant concentrations of 0.05-0.2 mM is especially interesting. Figure 1 shows that I1/I3 at the starting temperature gradually dropped to a value of ca. 1.25, which
Figure 2. Variation of pyrene I1/I3 ratio with temperature in Triton solutions of different concentrations.
appeared to be the lower limit under these circumstances. As the respective solutions were heated, however, the I1/I3 values increased again until they rejoined the water line. This indicates that the premicellar structures8 that assembled around the pyrene molecules were thermally destabilized and eventually entirely destroyed, exposing the probe to the bulk solvent. As seen in the transition (middle) region of Figure 1, this destabilization effect occurred progressively less easily as the surfactant concentration was increased. At concentrations above the cmc, the influence of temperature became similar to that in pure solvents, producing identical near-linear decreases of I1/I3 in all solutions. This shows that the formation of true TX-114 micelles left the probe firmly micellized over the entire temperature range and that this was essentially independent of surfactant concentration. However, even the lower group of curves in Figure 1 displays a slight upturn at the very highest temperatures. This may be an indication of some temperature-induced destabilization above the cmc, consistent with the reported changes of cmc with temperature.1,2 Micellar TX-114 solutions clouded at 23 °C, declouded at ∼65 °C, and clouded again at ∼75 °C. None of these phase changes significantly affected the I1/I3 ratio of sequestered pyrene, although previous work in this laboratory indicated that significant particle size changes take place at these temperatures.6 This suggests that the mechanism involving the destruction and formation of the large light scattering aggregates does not involve a degradation of pyrene-solubilizing structures. Figure 2A shows the I1/I3 variations of pyrene in solutions of TX-35. This surfactant differs from TX-114 in having a POE chain that is 5 units shorter (Table 1), reducing its water solubility significantly and leaving its solutions clouded at all temperatures. The family of curves in Figure 2A shows that the behavior of aqueous TX-35
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POE chain and relatively high cmc (0.1 mM) of Brij 35 resemble those of TX-405. The premicelles and micelles of Brij 35 gradually gained thermal stability as the concentration was increased, but in the intermediate concentration range (0.08-0.24 mM) the curves had a slightly positive slope, not unlike those of surfactants with shorter POE chains. This suggests that Brij-35 aggregates had less thermal stability than those of TX-405 in this region. In both surfactants, however, the thermal stability of aggregates at any given concentration was greater than that of their smaller counterparts. Igepal. The structure and characteristics of ICO-630 are similar to those of TX-100: it has a cloud point of 55 °C and a POE chain length of 9 units but differs in having a hydrophobic portion consisting of a straight C9 chain attached to a phenyl group. As the surfactant concentration was increased, the transition from thermally unstable to stable aggregates in ICO-630 solutions was more abrupt than in Triton surfactants (Figure 3B). This is consistent with the unbranched hydrocarbon chain and the low cmc (0.046 mM) of ICO-630, which allow it to form pyrenesequestering structures more readily than Triton surfactants. Figure 3. Variation of pyrene I1/I3 ratio with temperature in Brij 35 and Igepal CO-630 solutions of different concentrations.
toward pyrene was similar to that of TX-114, although the probe sequestering aggregates appeared to form at lower surfactant concentrations. This was also borne out by the somewhat greater thermal stability (lower curves) of TX-35 aggregates in the 0.05-0.2 mM concentration range. The reported cmc of TX-35 is 0.15 mM,16 but the meaning of this value for a surfactant that is a priori clouded at lower concentrations is debatable. The data in Figure 2A suggest that thermally stable TX-35 aggregates do not form until a surfactant concentration of 0.6 mM is reached. Parts B and C of Figure 2 show the I1/I3 behavior of pyrene in solutions of TX-100 and TX-405, respectively. It can be seen that the results for TX-100, which has a POE chain that is 1-2 units longer than that of TX-114, are similar to those shown in Figure 1. This indicates that the additional POE units had only a relatively minor effect on the thermal stability of the TX-100 aggregates compared to those of TX-114. This stands in remarkable contrast to the clouding behavior of this surfactant: while TX-114 has a cloud point of 23 °C, that of TX-100 is 65 °C. Also, unlike clouded suspensions of TX-114, those of TX-100 do not undergo declouding upon further heating. TX-405 has a long POE chain (40 units) and a fairly high cmc of 0.81 mM. Figure 2C shows that its premicellar aggregates formed at comparatively low concentrations and were more thermally stable than those of the other three Triton surfactants. While the TX-405 curves obtained at intermediate surfactant concentrations did rise upon heating, their slopes were relatively shallow and they did not tend to reach the water line. This indicates that stable premicelles were formed at concentrations below the cmc and that TX-405 aggregates nucleated on pyrene molecules were not easily disrupted. Brij. Figure 3A shows the I1/I3 behavior of solutions of Brij 35. While the Brij surfactants differ from Triton in having an aliphatic hydrocarbon chain, the long (23-unit)
Conclusion Structural characteristics of both the hydrophobic and hydrophilic moieties in nonionic surfactants were shown to have a distinct effect on the thermal stability of the weak bonds involved in self-assembly. Molecules with unbranched aliphatic hydrocarbon groups (those with low cmc and aggregation numbers)16 formed aggregates with the greatest thermal stability, while those with branched aliphatic groups produced relatively labile ones. Increasing the length of the hydrophilic POE chain of the surfactants appeared to increase the thermal stability of their premicellar aggregates. Plots of I1/I3 versus temperature at intermediate concentrations of surfactants with long POE chains had a shallow slope, indicating an ability to sequester the probe at concentrations and temperatures where other surfactants existed only as monomers. Complete thermal stability of aggregates, however, required somewhat higher concentrations than was the case with smaller surfactants. While the clouding of nonionic surfactants markedly affects the appearance of the solutions, no evidence was found of a change in the microenvironment of micellized pyrene when the larger particles were formed. The fact that the I1/I3 versus temperature curves showed no divergence when the cloud point was passed or when the TX-114 solutions proceeded through their cloudingdeclouding regimen suggests that the structural changes involved did not occur at the level of pyrene sequestration. Neutron scattering studies have shown that significant variations in micellar geometry (rod-shaped, spherical) occur when the i/j (hydrocarbon/POE) ratio is changed.17 It would not be surprising if this parameter, as well as the hydrocarbon structure, affects the configuration of the clouded aggregate. LA010559S (16) Hinze, W. L.; Pramauro, E. Crit. Rev. Anal. Chem. 1993, 24 (2), 133. (17) Glatter, F.; Fritz, G.; Lindner, H.; Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Reinhard, S.; Egelhaaf, S. Langmuir 2000, 16, 8692.
