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.
Highlights
The scientific investigation and industrial utilization of surfactants are extensive, ongoing enterprises
The presence of the oil modifies the interfacial ordering in a manner that cannot be understood 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
Because literature values lead to an estimate of approximately five gauche conformations in the disordered portion of the CH3(CH2)29OH tail group [22, 80], the attractive energy gained by hydrogen bonding of the alkanoic acid head groups (∼5 kcal mol−1 per bond) is comparable to the conformational free energy lost (∼0.6 kcal mol−1 per gauche conformation) when a disordered tail becomes all trans
Summary
The scientific investigation and industrial utilization of surfactants are extensive, ongoing enterprises. Temperature would produce a sequence of solid phases, varying in unit cell and tilt angle of the essentially all-trans tail group, as is observed for long-chain alkanols at the water-vapor interface when the surface pressure is increased [21, 23,24,25, 40,41,42,43,44,45] If the equations of state are the same, one expects the same interfacial monolayer phase for the same interfacial concentration of the surfactant (and temperature and bulk pressure) This is not the case because the long-chain alkanols at the water-oil interface form only a liquid monolayer phase, whereas at the water-vapor interface they form a solid monolayer phase under similar conditions.
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