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J. Phys. Chem. B 1998, 102, 218-222
Effect of Chain Length on the Structure of Monolayers of Alkyltrimethylammonium Bromides (CnTABs) at the Air-Water Interface Graham R. Bell, Samantha Manning-Benson, and Colin D. Bain* Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, U.K. ReceiVed: August 13, 1997X
Sum-frequency (SF) spectroscopy and ellipsometry have been used to study monolayers of the cationic surfactants CnTAB (CH3(CH2)n-1N+(CH3)3Br-) (n ) 12, 14, 16, and 18) at the air-water interface at a constant surface area of 44 Å2/molecule. The SF spectra of all four surfactants are very similar and yield a mean chain tilt of 58° near the methyl terminus. The ellipsometric data suggest that the density of the chain region in the monolayers is close to that of a liquid hydrocarbon, which is consistent with the chain tilt derived from the sum-frequency spectra. These results are in quantitative agreement with detailed structural profiles of C12TAB and C16TAB determined by Thomas and co-workers by neutron reflection.
Introduction Surfactants are widely employed to modify the properties of the surface of water. The performance of a surfactant depends on its molecular structure and on the conditions under which it is employed, such as the concentration of the surfactant and the temperature, acidity, and ionic strength of the aqueous phase. The behavior of a surfactant is often quantified in terms of easily measurable quantities such as the effectiveness and efficiency of adsorption1 and the surface tension γ at the critical micelle concentration (cmc). For a limited number of surfactants, precise measurements of thermodynamic quantities, such as the excess enthalpy and entropy of adsorption, have been made.2 In specific applications, other empirical quantities, such as foam height, have been widely reported. From such measurements, general correlations can be drawn between the performance of surfactants and structural features of the surfactant molecules, such as the length and nature (linear, branched, fluorinated, etc.) of the hydrophobic chain and the size, charge, and polarity of hydrophilic headgroup.3 While invaluable to the formulations chemist, such empirical correlations are of limited value in developing a fundamental understanding of the behavior of surfactants at interfaces. The structure of the surfactant monolayer is one of the primary determinants of the interfacial properties.4 Consequently, a knowledge of the relationship between molecular structure and interfacial structure is a prerequisite for a more general understanding of the performance of surfactants. Until recently, virtually no attempt had been made to understand the microscopic structure of a surfactant monolayer in terms of the molecular constitution of the surfactant. This inactivity arose not from the myopia of surface chemists but from a lack of suitable techniques for studying the molecular structure of surfactant monolayers. The development of X-ray and neutron reflectivity in the 1980’s allowed, for the first time, direct structural measurements on surfactant monolayers. In a detailed study, Thomas and co-workers sought to isolate one * Author to whom correspondence should be addressed. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, December 1, 1997.
structural feature of the surfactant, the chain length.5 They used neutron reflection (NR) to determine the structure of monolayers of alkyltrimethylammonium bromides (CH3(CH2)n-1N+(CH3)3Br-, CnTAB, n ) 12, 14, 16, and 18) at a constant area per molecule of 44 ( 2 Å2.5,6,7 The initial conclusion of this study was as remarkable as it was unexpected: the thickness of the monolayer appeared to be independent of chain length. This result implied that the density of the monolayer and the tilt of the chains from the chain normal increased with increasing chain length.5 In this article, we report measurements on this same system with two other surface-sensitive techniques, sum-frequency spectroscopy, and ellipsometry. The data we present suggest that the structures of all four monolayers are very similar and that neither the mean chain tilt nor the density varies significantly with chain length. The thicknesses measured by NR are, however, convoluted with a large contribution from the roughness of the interface which masks subtle changes in the structure of the monolayer. Detailed density profiles of two chain lengths, C12 and C16, obtained by Thomas et al. by isotopic labeling are in remarkably good agreement with the data presented here.5,6 Experimental Details The SF spectrometer8 and the experimental procedures for acquiring9 and analyzing10 SF spectra have been described in detail elsewhere. Briefly, the pump lasers beams were a frequency-doubled Nd:YAG laser (Spectron Lasers, 532 nm, 3.5 ns, 10 mJ/pulse, 2 mm diameter) and a tunable infrared laser (2800-3000 cm-1, 1 ns, 1 mJ/pulse, 0.5 mm diameter) generated by stimulated Raman scattering the output of a tunable dye laser in high-pressure hydrogen. The SF light (∼460 nm) was detected with a back-thinned CCD (Princeton Instruments). The SF spectra were referenced to a spectrum acquired simultaneously from GaAs. The angles of incidence were θvis ) 55° and θIR ) 50° and a counterpropagating geometry was employed. The infrared laser was p-polarized, the visible laser s-polarized, and the sum-frequency beam s-polarized. This combination of polarizations is most sensitive to structural changes in the monolayer.9 Surfactant solutions were prepared in D2O (Aldrich) at a concentration just above the critical micelle concentration (cmc) for each surfactant (see Table 1).
S1089-5647(97)02647-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/01/1998
Structure of Monolayers of Cn TABs
J. Phys. Chem. B, Vol. 102, No. 1, 1998 219
Figure 1. SF spectra acquired from the surface of solutions of CnTAB in D2O at an area per surfactant molecule of 44 Å2. A, C12TAB; B, C14TAB; C, C16TAB; D, C18TAB. Filled and solid symbols represent two independent experiments. The solid line is a guide to the eye through the open symbols. Polarizations: sum-frequency (s), visible (s), infrared (p). Temperatures and concentrations are listed in Table 1.
TABLE 1. Temperatures and Concentrations Employed in SF Experiments on CnTAB n 12 14 16 18 a
concn/mM a
4.0 4.3 1.1 0.37
T/K 298 298 301 306
In presence of 0.15 M NaBr.
Ellipsometric measurements were performed with a Beaglehole Instruments ellipsometer by the method of Jasperson and Schnatterly.11 Readings were taken every second and averaged over 60 s. This process was repeated for 10-15 minutes. The variation in the averaged readings over this period was generally
