Langmuir 2000, 16, 5853-5855
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First-Order Phase Transition in Mixed Monolayers of Hexadecyltrimethylammonium Bromide and Tetradecane at the Air-Water Interface Caroline E. McKenna, Mona Marie Knock, and Colin D. Bain* Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K. Received May 15, 2000 Mixed monolayers of CTAB (hexadecyltrimethylammonium bromide) and tetradecane were formed at the air-water interface by addition of small amounts of tetradecane to the surface of a CTAB solution below the critical micelle concentration. Ellipsometry and sum-frequency spectroscopy were used to demonstrate that these mixed monolayers exhibit a first-order phase transition from a conformationally disordered to a conformationally ordered state as the temperature is lowered. The phase transition occurs ca. 11 °C above the bulk melting point of tetradecane.
Introduction The penetration of oil into the hydrocarbon chains of surfactant monolayers is an important factor affecting the spontaneous curvature of the oil-water interface, and hence the phase behavior in microemulsions, with consequential importance for applications such as detergency, solubilization, and oil transport and recovery.1 In this Letter we show that the penetration of an oil (tetradecane) into a monolayer of the common cationic surfactant CTAB (hexadecyltrimethylammonium bromide) can transform the disordered surfactant monolayer into a highly ordered state that “melts” at a temperature well above the melting point of the oil. Structural studies on oil penetration into surfactant monolayers at the bulk oil-water interface are hindered by the difficulty in distinguishing the oil molecules at the interface from those in the bulk. An alternative approach is to place droplets of long-chain alkanes on a surfactant solution. These droplets form lenses in equilibrium with a molecularly thin oil film.2-6 Mixed monolayers of surfactant and alkanes formed in this way have been studied by surface tensiometry,2,3 neutron reflection,4,5 * To whom correspondence may be addressed. E-mail: colin.bain@ chem.ox.ac.uk. (1) Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I. Adv. Colloid Interface Sci. 1990, 33, 59. Clint, J. H. Surfactant Aggregation; Blackie, Glasgow: 1992; Chapter 10. Tadros, Th. F. In Structure/Performance Relationships in Surfactants; Rosen, M. J., Ed.; American Chemical Society: Washington, DC, 1984; Chapter 11. De Gennes, P. G.; Taupin, C. J. Phys. Chem. 1982, 86, 2294. Kellay, H.; Binks, B. P.; Hendrikx, Y.; Lee, L. T.; Meunier, J. Adv. Colloid Interface Sci. 1994, 49, 85. (2) Aveyard, R.; Cooper, P.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1990, 86, 3623. Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; McNab, J. R. Langmuir 1995, 11, 2515. Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; McNab, J. R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 224. Binks, B. P.; Crichton, D.; Fletcher, P. D. I.; MacNab, J. R.; Li, Z. X.; Thomas, R. K.; Penfold, J. Colloids Surf. 1999, 146, 299. (3) Jayalakshmi, Y.; Langevin, D. J. Colloid Interface Sci. 1997, 194, 22. (4) Lu, J. R.; Thomas. R. K.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Sokolowski, A.; Penfold, J. J. Phys. Chem. 1992, 96, 10971. Lu, J. R.; Li, Z. X.; Thomas. R. K.; Binks, B. P.; Crichton, D.; Fletcher, P. D. I.; McNab, J. R.; Penfold, J. J. Phys. Chem. B 1998, 102, 5785. (5) Lu, J. R.; Thomas, R. K.; Binks, B. P.; Fletcher, P. D. I.; Penfold, J. J. Phys. Chem. 1995, 99, 4113. (6) For alkanes with chains much shorter than those studied here, films have been observed that are macroscopically thin but thick on a molecular scale: Kellay, H.; Meunier, J.; Binks, B. P. Phys. Rev. Lett. 1992, 69, 1220.
and light scattering.3 The systems studied to date have all appeared highly disordered and liquidlike.2-5 Quite independently, there has been a growing interest in the two-dimensional phase behavior of adsorbed monolayers of soluble surfactants at the air-water interface. While the phase diagram is much less rich and complex than that of Langmuir (insoluble) monolayers,7 the existence of both gas-liquid8 and liquid-solid9,10 phase transitions in Gibbs monolayers is now well-established. Of particular note is the observation of a first-order phase transition in mixed monolayers of dodecanol and cationic surfactants.11 These results lead us to pose the question whether mixed monolayers of surfactants with alkanes would also show a first-order phase transition to a dense, ordered low-temperature phase. We have used ellipsometry and sum-frequency (SF) vibrational spectroscopy to demonstrate the existence of such a phase transition in mixed monolayers of CTAB and tetradecane. Experimental Section One microliter drops of a 10% solution (v/v) of tetradecane in CHCl3 were placed on the surface of a solution of CTAB in either H2O (ellipsometry) or D2O (SF spectroscopy). The droplet spread initially but, as the solvent evaporated, the film retracted to form a large number of small lenses of tetradecane in equilibrium with a mixed surfactant/oil layer of molecular thickness. These lenses act as a reservoir of tetradecane to maintain equilibrium in the mixed monolayer. The lenses of oil interfere with both SF and ellipsometric measurements: the small amount of oil used ensured that most of the surface was free of drops. Lenses were still visible at the end of each experiment. The CTAB solution was contained in a glass dish enclosed within a thermostated copper vessel. The vessel was sealed with the exception of small holes for the laser beams to enter and leave the vessel. The temperature was measured with a thermocouple immersed in the solution. CTAB (Fluka) was recrystallized three times from acetone/methanol. Tetradecane (Aldrich) was percolated twice (7) Kaganer, V. M.; Mo¨hwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779. (8) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K,; Matuura, R. J. Colloid Interface Sci. 1984, 98, 33. Casson, B. D.; Bain, C. D. J. Am. Chem. Soc. 1999, 121, 2615. (9) Berge, B.; Konovalov, O.; Lajzerowicz, J.; Renault, A.; Rieu, J. P.; Vallade, M. Phys. Rev. Lett. 1994, 73, 1652. Casson, B. D.; Braun, R.; Bain, C. D. Faraday Discuss. 1996, 104, 209. (10) (a) Ross, J.; Epstein, B. J. Phys. Chem. 1958, 62, 533. (b) Casson, B. D.; Bain, C. D. J. Phys. Chem. B 1998, 102, 7434. (11) Casson, B. D.; Bain, C. D. J. Phys. Chem. B. 1999, 103, 4678.
10.1021/la000675f CCC: $19.00 © 2000 American Chemical Society Published on Web 06/16/2000
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Figure 1. Coefficient of ellipticity, Fj, as a function of temperature for tetradecane on a 0.6 mM CTAB solution: (9) heating; (4) cooling. through activated alumina and tested negative for the presence of polar impurities.12 Ellipsometric measurements were carried out on a Beaglehole Instruments ellipsometer by the polarization modulation method.13 SF measurements were performed as described previously.14
Results and Discussion In our ellipsometric experiments, we measure the coefficient of ellipticity, Fj, defined as the imaginary part of rp/rs at the Brewster angle, where rp and rs are the reflection coefficients for p- and s-polarized light, respectively, and Re(rp/rs) vanishes at the Brewster angle. Figure 1 shows Fj as a function of temperature for 0.6 mM CTAB (2/3 × cmc) + tetradecane. A clear first-order phase transition occurs during both the heating and cooling cycles.15 The phase transition temperature, Tm(2D) ) 17 °C, is 11 °C above the bulk melting point of tetradecane. Similar results were obtained with 0.3 mM CTAB. No phase transition was detected in CTAB + dodecane for T > 283 K. For thin films, ellipsometric measurements yield only one independent parameter, the ellipsometric thickness η. For uniaxial monolayers,16
η)
∫
(e(z) - 1)(e(z) - 2) e(z)
+ (o(z) - e(z)) dz (1)
where 1 and 2 are the relative permittivities of air and water, e and o are the relative permittivities of the monolayer perpendicular and parallel to the surface, and the integral is taken over the direction, z, normal to the surface. To predict η, one needs to model the monolayer. If we assume that the areas per molecule, A, of the surfactant and oil above Tm(2D) are the same as those determined by neutron reflectivity for CTAB + dodecane (A ) 52 and 70 Å2, respectively),4 the ellipsometric data (12) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, 1, 513. (13) Jasperson, S. N.; Schnatterly, S. E. Rev. Sci. Instrum. 1969, 40, 761. (14) Bell, G. R.; Bain, C. D.; Ward, R. N. J. Chem. Soc., Faraday Trans. 1996, 92, 515 and ref 10b. (15) Phase transitions are very sensitive to the presence of impurities. Although the CTAB was extensively recrystallized and glassware was exhaustively cleaned, a variable amount of hysteresis was observed, which is most likely due to residual trace impurities. It is unlikely that these impurities have any significant effect on the change in Fj observed at the phase transition. (16) Meunier, J. In Light Scattering by Liquid Surfaces and Complementary Techniques; Langevin, D., Ed.; Marcel Dekker: New York, 1992; Chapter 17.
Letters
Figure 2. Sum-frequency spectra in the C-H stretching region for tetradecane on a 0.3 mM CTAB solution at 20 °C (dotted line; × 3) and 15 °C (solid line). d+ and r+ label the symmetric methylene and symmetric methyl stretches, respectively. The peak at 2935 cm-1 is a Fermi resonance of the r+ mode.
point to a density of the hydrocarbon region comparable to that of a liquid alkane. Below Tm(2D), the value of Fj is well-explained by a model in which the hydrocarbon chains in the mixed monolayer are upright and have the same density as in the rotator phase of alcohol monolayers (21 Å2 per molecule).17 Figure 2 shows SF spectra18 of 0.3 mM CTAB + tetradecane above and below Tm(2D). The peaks at 2850 and 2876 cm-1 are assigned to the symmetric methylene stretch (d+) and the symmetric methyl stretch (r+), respectively. The ratio of the line strengths of these two peaks has been found to be a good qualitative indicator of conformational order.16 Above Tm(2D), the line strengths are comparable, indicating a conformationally disordered monolayer. Below Tm(2D), the d+ peak is virtually absent, showing a transition to a state in which the chains are predominantly in an all-trans conformation. The spectra also show a 6-fold increase in the SF signal from the r+ mode at the phase transition. While a quantitative analysis will require spectra of selectively deuterated mixtures, to separate out the contributions of the surfactant and the alkane to the SF spectra,19 such a large increase in signal suggests both an increase in the number density, N, of surfactant and oil molecules at the surface and a decrease in the mean tilt of the chains.20 Conclusion Mixed monolayers of CTAB and tetradecane at the airwater interface show a first-order phase transition from a conformationally disordered phase to a denser, confor(17) Ellipsometry provides no information on composition. To model the “solid” monolayer, we assumed a molar ratio of 1:1 in the monolayer (similar to that seen in mixed monolayers of SDS or CTAB with dodecanol) and we used values of e and o for the hydrocarbon chain from: Casson, B. D.; Bain, C. D. Langmuir 1997, 13, 5465. Tables of molar refractivities and molar volumes were used to calculate the contribution of the bromide counterions and trimethylammonium headgroups to Fj (Knock, M. M.; Bain, C. D. Langmuir 2000, 16, 2857). A roughness contribution of Fj ) +0.5 × 10-3 was estimated from capillary wave theory. (18) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281. Bain, C. D. In Modern Characterization Methods of Surfactant Systems; Binks, B. P., Ed.; Marcel Dekker: New York, 1999; Chapter 9 and references therein. (19) Caution is required in comparing these spectra with spectra from other surfactant monolayers because they contain contributions from both ends of the tetradecane molecule, which will partially cancel (Sefler, G. A.; Du, Q.; Miranda, P. B.; Shen, Y. R. Chem. Phys. Lett. 1995, 235, 347). (20) To a good approximation, the SF signal from the r+ mode is proportional to N2〈cos θ〉2, where θ is the angle between the surface normal and the C3 axis of the methyl group.
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mationally ordered phase as the temperature is lowered.21 Comparison with pure alcohol and mixed surfactant/ alcohol monolayers suggests the existence of a liquidsolid phase transition, though grazing incidence X-ray diffraction experiments would be required to prove that the low-temperature phase is solid. The 2D phase transition occurs 11 °C above the bulk melting point, Tm(3D) of tetradecane. By comparison, pure alcohol monolayers on water melt ∼15 °C above Tm(3D)9 while the surface phase
transition in long-chain alkanes occurs ∼3 °C above Tm(3D).22 Our data suggest that the interface between tetradecane and aqueous CTAB solutions will be crystalline over a range of temperatures at which both bulk phases are liquid.23
(21) The parallel with self-assembled monolayers (SAMs) is not direct, since SAMs lack translational freedom in the plane of the surface. The observation of a phase transition at the air-water interface does, however, lend support to the idea that liquid hydrocarbons can penetrate disordered SAMs to form conformationally ordered mixed monolayers.
(22) Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Ocko, B. M.; Deutsch, M. Phys. Rev. Lett. 1993, 70, 958. (23) This phenomenon is known for the interface between water and alcohols (Aratono, M.; Takiue, T.; Ikeda, N.; Nakamura, A.; Motomura, K. J. Phys. Chem. 1993, 97, 5141).
Acknowledgment. We thank the British Marshall Scholarship Commission for a scholarship to M.M.K. LA000675F
