Tail Ordering Due to Headgroup Hydrogen Bonding Interactions in Surfactant Monolayers at the Water−Oil Interface

Abstract

Interactions between surfactants, and the resultant ordering of surfactant assemblies, can be tuned by the appropriate choice of head-and tailgroups. Detailed studies of the ordering of monolayers of long-chain n-alkanoic and n-alkanol monolayers at the water-vapor interface have demonstrated that rigid-rod all-trans ordering of the tailgroups is maintained upon replacing the alcohol with a carboxylic acid headgroup. In contrast, at the water-hexane liquid-liquid interface, we demonstrate that substitution of the -CH 2 OH with the -COOH headgroup produces a major conformational change of the tailgroup from disordered to ordered. This is demonstrated by the electron density profiles of triacontanol (CH 3 (CH 2 ) 29 OH) and triacontanoic acid (CH 3 (CH 2 ) 28 COOH) monolayers at the water-hexane interface, as determined by X-ray reflectivity measurements. Molecular dynamics simulations illustrate the presence of hydrogen bonding between the triacontanoic acid headgroups that is likely responsible for the tail ordering. A simple free energy illustrates the interplay between the attractive hydrogen bonding and the ordering of the tailgroup.

Diffuse X-ray scattering near a two-dimensional solid–liquid phase transition at the n-hexane–water interface

“…Samples of macroscopically flat n-hexane -water interface were prepared and studied by the method presented in [4,5] in a stainless steel cell with dimensions of the interface of 75 mm× 150 mm * tikhonov@kapitza.ras.ru 1…”

“…It is shown below that the diffuse scattering intensity in the low-temperature crystal phase at the interface is one or two orders of magnitude higher than that in the high-temperature phase, which indicates the existence of an extended transverse structure with a thickness of ∼ 200Å in the former phase. The analysis of experimental data within the theory of capillary waves indicates that the two-dimensional crystallization transition at the interface with a decrease in the temperature occurs after the wetting transition.Samples of macroscopically flat n-hexane -water interface were prepared and studied by the method presented in [4,5] in a stainless steel cell with dimensions of the interface of 75 mm× 150 mm * tikhonov@kapitza.ras.ru 1 …”

“…Samples of macroscopically flat n-hexane -water interface were prepared and studied by the method presented in [4,5] in a stainless steel cell with dimensions of the interface of 75 mm× 150 mm • ) and T = 293.2 K: (circles) the total scattering intensity I(β)/I 0 and (squares) background from the scattering of the incident beam in the bulk of n-hexane I b /I 0 . The inset shows the kinematics of scattering in the coordinate system where the xy plane coincides with the boundary between the monolayer and water, the Ox axis is perpendicular to the direction of the beam, and the Oz axis is directed along the normal to the surface against the gravitational force.…”

“…On the other hand, the data shown in Fig. 3 indicate a gradual decrease in the scattering intensity I diff to ≈ 2 · 10 −3 in the liquid phase with an increase in the temperature from T c to T * ≈ 320 K. In [3,5], the phases of the monolayer of melissic acid were described within a qualitative two-layer model with the structure factor (7) of the form…”

Simple Ion Transfer at Liquid|Liquid Interfaces

“…Schlossman and co-workers [179][180][181][182][183] have pioneered the use of X-ray scattering techniques to understand the alkane|water and the nitrobenzene|water interfaces. Recently, Schlossman and coworkers [184][185][186][187] have used synchrotron X-ray reflectivity to study ion distribution at the liquid|liquid interface between a nitrobenzene solution of tetrabutylammonium tetraphenylborate and a water solution of tetrabutylammonium bromide.…”