Effect of chain length on the structure of monolayers of alkyltrimethylammonium bromides (CnTABs) at the air-water interface

Sum-frequency (SF) spectroscopy and ellipsometry have been used to study monolayers of the cationic surfactants C(n)TAB (CH3(CH2)(n-1)N+(CH3)(3)Br-)(n=12, 14, 16, and 18) at the air-water interface at a constant surface area of 44 Angstrom(2)/molecule. The SF spectra of all four surfactants are very similar and yield a mean chain tilt of 58 degrees 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 C(12)TAB and C(16)TAB determined by Thomas and co-workers by neutron reflection.