Cloud Point Phenomenon in Ionic Micellar Solutions: A SANS Study

Mar 28, 2001 - We have observed the cloud point (CP) in the ionic surfactant sodium dodecyl sulfate (SDS) in the presence of 0.2 M tetra-n-butylammoni...
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© Copyright 2001 American Chemical Society

MAY 1, 2001 VOLUME 17, NUMBER 9

Letters Cloud Point Phenomenon in Ionic Micellar Solutions: A SANS Study Sanjeev Kumar,† Vinod K. Aswal,‡ Andleeb Z. Naqvi,† Prem S. Goyal,§ and Kabir-ud-Din*,† Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India, Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400 085, India, and IUC-DAEF Mumbai Centre, Bhabha Atomic Research Centre, Mumbai-400 085, India Received August 22, 2000. In Final Form: January 11, 2001 We have observed the cloud point (CP) in the ionic surfactant sodium dodecyl sulfate (SDS) in the presence of 0.2 M tetra-n-butylammonium bromide (Bu4NBr). To study the changes taking place in the system approaching the CP, we have performed SANS studies with 0.3 M SDS as a function of [Bu4NBr] and temperature. The spectra obtained with Bu4NBr addition show micellar growth. The temperature increase has different effects on 0.1 and 0.15 M Bu4NBr systems. Further, the ionic micelles show characteristics of nonionics for 0.2 M Bu4NBr as the temperature was raised. The overall phenomenon is conjectured in light of gradual displacement of water from the clefts which characterize the micellar surface.

Introduction Studies on surfactants and their mixtures with additives or polymers in aqueous solutions are of interest for fundamental understanding of their interactions with regard to their chemical, pharmaceutical, mineral processing, and petroleum engineering applications.1-3 The stabilities of surfactant systems [surfactant + additive(s)] with respect to temperature, prior to their multifold uses mentioned above, need to be known, especially where elevated temperature prevails. There are two categories of surfactants, and the solution behavior and effects of * To whom correspondence should be addressed. † Aligarh Muslim University. ‡ Solid State Physics Division, Bhabha Atomic Research Centre. § IUC-DAEF Mumbai Centre, Bhabha Atomic Research Centre. (1) Robb, I. D. In Anionic Surfactants-Physical Chemistry of Surfactant Action; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981. (2) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC: Boca Raton, FL, 1993. (3) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588.

elevated temperature on each category are in sharp contrast. The first category is of the nonionic surfactants which cannot withstand elevated temperatures and undergo clouding and phase separation. The threshold temperature for such a state is called the cloud point (CP). The ionic surfactants belong to the second category in which the CP phenomenon is rarely observed.4-8 Most ionic micellar systems exhibiting CPs known so far contain large amounts of added electrolytes (>2 M), which screen out the repulsive electrostatic effects, thereby stabilizing the solutions. It is generally believed that the size of the ionic micelles gets reduced when the temperature is raised.9 The knowledge of the aggregation behavior of surfactants and CPs is, therefore, of practical and (4) Appell, J.; Porte, G. J. Phys. Lett. 1983, 44, L-689. (5) Porte, G. J. Phys. Chem. 1983, 87, 3541. (6) Imae, T.; Sasaki, M.; Abe, A.; Ikeda, S. Langmuir 1988, 4, 414. (7) Yu, Z.-J.; Xu, G. J. Phys. Chem. 1989, 93, 7441. (8) Warr, G. G.; Zemb, T. N.; Drifford, M. J. Phys. Chem. 1990, 94, 3086. (9) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1983, 87, 1264.

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theoretical interest, as almost every application depends on their state of aggregation (micelles, mixed micelles, vesicles, etc.).10 The microscopic origins of phase separation are still elusive. But it seems at first sight that a decrease of the content of water of hydration in/around the hydrophilic shell (of oxyethylene chains or ionic heads) is the governing factor which causes predominance of van der Waals attraction or of changing micelle-micelle and micellewater interactions.7,11,12 One source of removing hydrated water is heating so that repulsive hydration interaction gives way to van der Waals attraction. It has been reported that the increase of hydrophobicity near the headgroup region by an additional methylene group (going from trin-propyl- to tri-n-butylammonium) in a cationic surfactant has shown a departure (liquid-liquid phase separation) of the behavior of producing stable solutions at elevated temperatures.8 This clearly shows that not only the temperature but also other equally important factors are responsible for producing the CP in surfactant solutions. In the present communication we have investigated both the possibilities. Earlier, some unusual CP behavior exhibited by an ionic surfactant in the presence of a few quaternary bromides was investigated.13 As a part of the series of experiments carried out by us on ionic surfactants and quaternary bromides13-17 the present work deals with a small-angle neutron scattering study of the changes that occur in a system when it approaches the CP. The system sodium dodecyl sulfate (SDS) + tetra-n-butylammonium bromide (Bu4NBr) is chosen, and the effects of increasing [Bu4NBr] and temperature are seen.

Letters

Figure 1. SANS spectra of 0.3 M SDS solutions at 30 °C with increasing concentration of Bu4NBr.

Experimental Section SDS, Bu4NBr, and D2O were the same as used earlier.15 Attempts were made to find out cloud points of 0.3 M SDS with various amounts of Bu4NBr (0-0.2 M) using controlled heating (0.1 °C/min) in a thermostat. The CP (∼56 °C) was observed only with 0.2 M Bu4NBr. However, SANS studies were performed with all the above systems at the Dhruva reactor, BARC, Trombay. The diffractometer makes use of a beryllium oxide filtered beam of mean wavelength (λ) ) 5.2 Å. The angular distribution of the scattered neutrons was recorded using a onedimensional position-sensitive detector (PSD). The accessible wave vector transfer, Q ()4π sin θ/λ, where 2θ is the scattering angle), range of the diffractometer is 0.02-0.3 Å-1. The PSD allows simultaneous recording of data over the full Q range. The samples were held in a quartz sample holder of 5 mm thickness. The measured SANS distributions after standard corrections and normalization are shown in Figures 1-4.

Figure 2. SANS spectra of a 0.3 M SDS + 0.1 M Bu4NBr system at different temperatures.

Results and Discussion Figure 1 shows the neutron scattering spectra for 0.3 M SDS with different concentrations of Bu4NBr (0-0.2 M). The spectra contain well-defined peaks characteristic of dispersions of charged particles, as observed by Hayter and Penfold.18 The peak arises because of a corresponding peak in the interparticle structure factor S(Q). Customarily, such peak positions are related to the distance between the charged aggregates. The SDS micelles (without Bu4(10) Sein, A.; Engberts, J. B. F. N.; Linden, E.; van der Linden, E.; van de Pas, J. C. Langmuir 1993, 9, 1714. (11) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (12) Karlstrom, G. J. Phys. Chem. 1985, 89, 4962. (13) Kumar, S.; Sharma, D.; Kabir-ud-Din. Langmuir 2000, 16, 6821. (14) Kumar, S.; Naqvi, A. Z.; Kabir-ud-Din. Langmuir 2000, 16, 5252. (15) Kumar, S.; Aswal, V. K.; Goyal, P. S.; Kabir-ud-Din. J. Chem. Soc., Faraday Trans. 1998, 94, 761. (16) Kabir-ud-Din; David, S. L.; Kumar, S. J. Mol. Liq. 1998, 75, 25. (17) Kumar, S.; Bansal, D.; Kabir-ud-Din. Langmuir 1999, 15, 4960. (18) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022.

Figure 3. SANS spectra of a 0.3 M SDS + 0.15 M Bu4NBr system at different temperatures.

NBr) at such a high surfactant concentration (0.3 M) are ellipsoidal in nature.15 We can also see that the interaction peaks shift to lower Q-values with an increase in [Bu4NBr], which indicates concurrent decrease and increase, respectively, in the micelle number density and the intermicellar distance. As the [SDS] is constant, the data show a substantial micellar growth and screening of electrostatic repulsions with an increase of the Bu4NBr content in the system. The micellar growth is consistent with viscosity results obtained for the same system.16 The difference between

Letters

Figure 4. SANS spectra of a 0.3 M SDS + 0.2 M Bu4NBr system at different temperatures.

Bu4NBr and an ordinary salt19,20 is that Bu4N+ possesses four butyl chains with a symmetry structure. Such an organic ion has a large hydrophobic volume, and some of the short alkyl chains may penetrate into the micellar interior due to the hydrophobic interaction.8,21 In this case, icebergs on the butyl chains penetrating toward the interior will break down, thereby increasing the entropy of the system. This entropy increase may be a driving force for the micellar growth. The positive charge of the N-atom will decrease the effective charge of the anionic micelle while the remaining alkyl chains may penetrate the micellar surface (clefts) with a concomitant removal of water from the clefts which will be responsible for micellar growth.22 The explanation seems conceivable in light of the SANS data shown in Figure 1. Figure 2 depicts the SANS spectra of 0.3 M SDS with 0.1 M Bu4NBr at different temperatures. The interaction peaks shift to higher Q-values with a drop in overall intensity or cross section (dΣ/dΩ). This system shows the usual temperature effect expected for a typical ionic micelle.23 Analogously, the SANS spectra of the system with 0.15 M Bu4NBr are shown in Figure 3. As can be seen, the temperature variation shows an effect opposite (19) Gustavsson, H.; Lindman, B. J. Am. Chem. Soc. 1978, 100, 4647. (20) Oh, S. G.; Shah, D. O. J. Phys. Chem. 1993, 97, 284. (21) Almgren, M.; Swarup, S. J. Phys. Chem. 1983, 87, 876. (22) Haverd, V. E.; Warr, G. G. Langmuir 2000, 16, 157. (23) Bezzobotnov, V. Yu.; Borbely, S.; Cser, L.; Farago, B.; Gladkih, I. A.; Ostanevich, Yu. M.; Vass, Sz. J. Phys. Chem. 1988, 92, 5738.

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to that of Figure 2 (although the system still did not show any CP). We can say that the system has already started behaving in a different manner (even though a characteristic peak still persists in all the spectra) as the scattering cross section is increasing at smaller Q while it is independent of the temperature in the large Q (>0.075 Å-1) region. This increase in dΣ/dΩ with temperature rise at lower Q is, in a sense, similar to that observed in nonionic micellar solutions where interactions are dominated by the van der Waals forces.11,24 The SANS spectra of the system 0.3 M SDS + 0.2 M Bu4NBr, which showed a CP at ∼56 °C, are illustrated in Figure 4. The signal, already started in the system 0.3 M SDS + 0.15 M Bu4NBr (Figure 3), has become more clear. It can be seen that as the temperature approaches closer to the CP, the dΣ/dΩ diverges in the region of low Q (0.10 Å-1). This type of behavior usually occurs with ionic micelles at higher salt concentrations25 or with nonionic micelles at higher temperatures.11,24 Taking together the similarity to that occurring with nonionic micelles and the appearance of a CP in the system 0.3 M SDS + 0.2 M Bu4NBr, we can safely conclude that the system has changed to a nonionic one with the increase of temperature. In short, the above studies show that the characteristics of micellar solutions may change with a change of temperature and concentration of an added salt. For appearance of a CP in an ionic surfactant system the replacement of cleft water by the hydrophobic volume of the salt counterion seems important.26-28 These results emphasize new possibilities offered by such systems in obtaining organized assemblies with novel architectures. Currently we are searching for a relationship between [surfactant] and [R4NBr] in order to get CP-phenomenon at fairly low concentration. Acknowledgment. This work was performed under collaborative research Scheme No. IUC/CRS-M-72, of Inter-University Consortium for Department of Atomic Energy Facilities, India. LA001215P (24) Rao, K. S.; Goyal, P. S.; Dasannacharya, B. A.; Kelkar, B. K.; Manohar, C.; Menon, S. V. G. Pramana J. Phys. 1991, 37, 311. (25) Goyal, P. S.; Menon, S. V. G.; Dasannacharya, B. A.; Rajagopalan, V. Chem. Phys. Lett. 1993, 211, 559. (26) Cerichelli, G.; Mancini, G. Langmuir 2000, 16, 182. (27) Soldi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2000, 16, 59. (28) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236.