Deuterated isotopic species of the nonionic surfactants C12H25(OC2H4)mOH (C12Em), with m = 1, 2, and 4, have been prepared and their adsorption at the air/water interface characterized by surface tension and neutron reflection measurements. The structures of the adsorbed layers have been determined at the critical micelle concentration (cmc), at a constant surface pressure of 38 mN m−1, and at a constant area per molecule of 55 Å2. For C12E2 the thickness of the alkyl chain layer, defined in terms of the full width at half height of a Gaussian width, decreases from 14 Å at an area per molecule A of 34 Å2 to 11.5 ± 1.5 Å at an A of 55 Å2 and C12E4 behaves similarly. The ethylene glycol chain thicknesses for C12E2 and C12E4 are comparable with their fully extended lengths at the cmc, with values of 10 and 14.5 ± 1.5 A, respectively. When A = 55 Å2 these thicknesses decrease below the fully extended lengths, to 6.5 and 12 Å, respectively, indicating some coiling of the ethylene glycol chains. The separation of the water and alkyl chains decreases with the surface coverage and indicates that the chains overlap the water 25–40% at areas per molecule smaller than 55 Å2. The separation of alkyl and ethylene glycol chains is consistently less than between alkyl chains and water, indicating that there is a significant degree of mixing of the two types of chains and that, in C12E4, the ethylene glycol chains are not totally immersed in the water. © 1993, American Chemical Society. All rights reserved.
