Molecular Ordering and Phase Behavior of Surfactants at Water-Oil Interfaces as Probed by X-Ray Surface Scattering

Abstract

Surfactants have their primary utility, both scientific and industrial, at the liquid-liquid interface. We review recent X-ray surface scattering experiments that probe the molecular ordering and phase behavior of surfactants at the water-oil interface. The presence of the oil modifies the interfacial ordering in a manner that cannot be understood simply from analogies with studies of Langmuir monolayers of surfactants at the water-vapor interface or from the traditional view that the solvent is fully mixed with the interfacial surfactants. These studies explored the role of chain flexibility and head group interactions on the ordering of long-chain alkanols and alkanoic acids. Small changes in the surfactant may produce large changes in the interfacial ordering. The interfacial monolayer can be spatially homogeneous or inhomogeneous. Investigators have observed interfacial phase transitions as a function of temperature between homogenous phases, as well as between homogeneous and inhomogeneous phases. Finally, varying the solvent chain length can alter the fundamental character of the phase transitions and lead to the formation of multilayer interfacial structures.

Solid–liquid phase transition in a Gibbs monolayer of melissic acid at the n-hexane–water interface

“…The possibility of such a phase transition at an alkanewater interface has been discussed for a long time [3,4,5,6,7]. The observation of a solid -liquid phase transition at the n-hexane -water interface in the Gibbs monolayer of a surfactant is reported in this work.…”

Ion-size effect at the surface of a silica hydrosol

“…[19][20][21][22][23][24][25][26][27][28][29] In this paper, I discuss my findings using synchrotron x-ray reflectivity to elucidate the ion-size effect for alkali ions (Na + , K + , Rb + , and Cs + ) elevated above the surface of a colloidal solution of silica nanoparticles by the field of the surface electric-double layer.…”

The ionic strength dependent zeta potential at the surface of hexadecane droplets in water and the corresponding interfacial adsorption of surfactants

“…After this initial adsorption step, the adsorption of DS À is accompanied by the adsorption of its counter ion, Na + , so that the electric repulsion between the negatively charged sulfate groups can be weakened and favor further adsorption of DS À ions. From the previous studies, 19,62,[64][65][66][67][68] the adsorption of DS À was not saturated at 50 mM SDS concentration. So it is reasonable to believe that the addition of NaCl in the water phase introduced more counter ions (Na + ) which combine to the DS À ions and benefit the DS À adsorption due to the reduced electric repulsion, 19,69 as shown in Fig.…”