A Study of Nonionic Surfactants at the Air−Water Interface by Sum-Frequency Spectroscopy and Ellipsometry

Sum-frequency spectroscopy and ellipsometry have been used to study monolayers of the nonionic surfactants poly(ethylene glycol) monododecyl ethers (C12E m ; m = 2−8) at the air−water interface. SF spectra were acquired for areas of 30−70 Å2 per molecule, as determined from literature adsorption isotherms. These spectra show an increase in conformational disorder with increasing area per molecule and, surprisingly, an apparent decrease in the angle of tilt of the methyl group. There was no systematic variation in the SF spectra as a function of the length of the poly(ethylene glycol) chain at fixed areas of 45 and 62 Å2 per molecule. For m ≥ 4, the ellipsometric data suggested that the density of the hydrocarbon region of the chain was independent of m at fixed coverage but that the density decreased for m = 2 and 3. This study suggests that the value of m does not, per se, affect the structure of a monolayer of C12E m at the air−water interface for m = 4−8.