Neutron Reflection from Hexadecyltrimethylammonium Bromide Adsorbed at the Air/Liquid Interface: The Variation of the Hydrocarbon Chain Distribution with Surface Concentration

Abstract

We have determined the structure of a monolayer of hexadecyltrimethylamonium bromide adsorbed at the airlwater interface at its critical micelle concentration (9.2 x lop4 M) and at two lower concentrations using neutron specular reflection. We have used isotopic labeling to determine the relative positions of chains, heads, and solvent, and more detailed labeling to determine the distributions of (26 chain fragments at either end of the alkyl chains. The use of the more detailed labeling scheme has allowed us to make a quantitative estimate of the contribution of surface roughness to the structure of the layer at the three concentrations, to show that the alkyl chains are, on average, strongly tilted away from the surface normal but that the part of the alkyl chain next to the head group is less tilted. The different tilt angle distributions for the two ends of the hydrocarbon chain also indicate that there are gauche defects in the chain.

FTIR and UV-Vis Spectroscopic Studies of Black Soap Film

“…Our polarized infrared result of the BSF is consistent with the neutron reflectivity measurement of Thomas et al (20) who investigated the relative positions of chains and heads of the isotopic labeled CTAB monolayer at the air-water interface at 30°C, where the chains were strongly tilted away from the surface normal and the tilt became greater as the surface concentration was reduced. Although the chain segments here are greatly tilted away from the film normal, the overall orientation of the hydrocarbon chains are conjectured to be vertical or less tilted, schematically represented in Fig.…”

“…The CH 2 stretching band intensity ratio, A 0Њ : A 30Њ : A 45Њ : A 60Њ ϭ 1:1.14:1.17:1.39, increases with the angle of incidence, indicating that the methylene segments are tilted farther away from the film normal on the assumption that there is an orthogonal relationship between the direction of transition moments of the antisymmetric and symmetric CH 2 stretching modes and the chain axis of methylene segment. This result is consistent with the chain tilt of the CTAB monolayer at the air-water interface at a constant area per molecule of 44 Ϯ 2 Å 2 derived from the neutron reflection (20), sum-frequency spectroscopy (21).…”

“…[1] with the experimental data (polarized absorbance at the incident angle of 60°) and the orientation angle can be gained. The following parameters are used for calculations: the refractive indices, n 1 ϭ n 7 ϭ 1.0 (air), ñ 2 ϭ ñ 6 ϭ 1.45 ϩ 0.1 i (CTA ϩ monolayer) (22), n 3 ϭ n 5 ϭ 1.45 (TY Ϫ layer), ñ 4 ϭ 1.415 ϩ 0.0163 i (water at 2924 cm Ϫ1 ), ñ 4 ϭ 1.398 ϩ 0.00941 i (water at 2854 cm Ϫ1 ) (23); the layer thickness, h 2 ϭ h 6 ϭ 2.17 nm (CTA ϩ monolayer) (20), h 4 ϭ 1.7 nm (water), h 3 ϭ h 5 ϭ 0.76 nm (TY Ϫ layer), which is converted from the cross section of cetyl orange monolayer at the air-water interface (24).…”

The Analysis and Interpretation of Neutron and X-ray Specular Reflection

“…The bold type indicates that an exact relation between reflectivity and scattering-length-density profile is used, italics that the relation is approximate (kinematic or corrected kinematic). References: (a) Pershan (1989); (b) Pedersen (1992); (c) Hamley & Pedersen (1994); (d) Pedersen & Hamley (1994a); (e) Pedersen & Hamley (1994b); (f) Singh, Tirrell & Bates (1993); (g) Sivia, Hamilton & Smith (1991); (h) Kunz, Reiter, G~Stzelmann & Stamm (1993); (i) Zhou & Chen (1993); Q) Zhou, Lee, Chen& Strey (1992); (k) Zhou & Chen (1995); (l) Denton, Gray & Sullivan (1994); (m) Crowley, Lee, Simister & Thomas (1991); (n) Lee & Milnes (1995); (o) Lu, Hromadova, Simister, Thomas & Penfold (1994); (p) Lu, Simister, Lee, Thomas, Rennie & Penfold (1992); (q) Lu, Simister, Thomas & Penfold (1993a); (r), (s) Simister, Lee, Thomas & Penfold (1992a,b).…”

“…In the first, we determine the partial structure factors from the data and then fit them with a set of structural parameters. This is the method that we have developed and applied on a number of occasions (Crowley, Lee, Simister & Thomas, 1991;Lu, Hromadova, Simister, Thomas & Penfold, 1994;Lu, Hromadova & Thomas, 1993;Lu, Li, Smallwood, Thomas & Penfold, 1995;Lu, Li, Su, Thomas & Penfold, 1993;Lu, Li, Thomas, Staples, Tucker & Penfold. 1993;Lu, Simister, Lee, Thomas, Rennie & Penfold, 1992;Lu, Simister, Thomas & Penfold, 1993a;Simister, Lee, Thomas & Penfold, 1992a,b).…”