Amphiphiles at Water Interfaces
Surfactants have their primary utility, both scientific and industrial, at the liquid-liquid interface, which is often an interface between aqueous and organic phases. Our application of X-ray surface scattering (Figure 1) to study the molecular ordering and phase behavior of surfactants at the interface between water and an oil solution of surfactants has led to a greatly revised understanding of these important interfacial structures [1-6]. X-ray reflectivity provides information on the molecular ordering of the surfactants with sub-nanometer spatial resolution as a function of depth into the interface. Off-specular diffuse scattering probes the in-plane structure of inhomogeneous phases. Together, these techniques have demonstrated that neither studies of Langmuir monolayers of insoluble surfactants at the water-vapor interface nor the traditional view of liquid-liquid interfaces espoused by Davies provide a good guide to these interfaces.
Our studies have begun to address a number of fundamental issues of surfactant ordering at the aqueous-organic interface. We studied the role of tailgroup flexibility on surfactant ordering by examining fluorocarbon alkanols with rigid rod tails and hydrocarbon alkanols with flexible tails. The formation of ordered interfacial phases by fluorocarbon alkanols and disordered phases by hydrocarbon alkanols (Figure 2) is not surprising, except possibly in light of corresponding experiments at the water-vapor interface in which both types of molecules form solid phases. The importance of complex interactions was revealed
by the study of hydrocarbon alkanoic acids, which formed an ordered solid phase at the water-hexane interface (Figure 3). This phase was most likely driven to its ordered state by hydrogen bonding between the acid headgroups whose attractive interaction overcame the disordering effect of the long flexible tailgroups.
Surfactants at water-oil interfaces demonstrate a rich phase structure consisting of homogeneous and inhomogeneous phases made up of liquid, solid, and gas monolayer regions. Although our X-ray studies provided the first indication of variations in these inhomogeneous phases as a function of temperature, recent Brewster angle microscopy measurements in our lab have provided striking visual evidence of their existence.
Although our measurements carried out during the past decade have revealed new features of surfactant ordering at the aqueous-organic interface, much remains to be understood. This includes understanding the complex interactions that determine the molecular ordering and phase behavior of the interface, as well as extending these studies to other types of surfactants, such as ionic surfactants and surfactants of a variety of architectures that raise interesting scientific questions and are important for many industrial applications.
For an introduction to X-ray scattering techniques used for studying liquid surfaces and interfaces, see this link.
For a more detailed description of these techniques, see the book described here.
References
[1] Phase Transition Behavior of Fluorinated Monolayers at the Water-Hexane Interface, Aleksey M. Tikhonov, Ming Li, and Mark L. Schlossman, J. Phys. Chem. B (Letter) 105, 8065-8068 (2001)
[2] Surfactant and Water Ordering in Triacontanol Monolayers at the Water-Hexane Interface, Aleksey M. Tikhonov and Mark L. Schlossman, J. Phys. Chem. B (Letter), 107, 3344-3347 (2003)
[3] An x-ray diffuse scattering study of domains in F(CF2)10(CH2)2OH monolayers at the hexane-water interface, Ming Li, Aleksey M. Tikhonov, Mark L. Schlossman, Europhys. Lett., 58, 80-86 (2002)
[4] Molecular Ordering and Phase Transitions in Alkanol Monolayers at the Water-Hexane Interface, Aleksey M. Tikhonov, Sai Venkatesh Pingali, and Mark L. Schlossman, J. Chem. Phys., 120, 11822-11838 (2004)
[5] Tail Ordering due to Head Group Hydrogen Bonding Interactions in Surfactant Monolayers at the Water-Oil Interface, Aleksey M. Tikhonov, Harshit Patel, Shekhar Garde, and Mark L. Schlossman, J. Phys. Chem. B (Letter) 110, 19093-19096 (2006)
[6] Molecular Ordering and Phase Behavior of Surfactants at Water-Oil Interfaces as Probed by X-ray Surface Scattering, Mark L. Schlossman and Aleksey M. Tikhonov, Annual Reviews of Physical Chemistry 59, 153-177 (2008)
[7] Microphase formation at a 2D solid-gas phase transition, Adam W. Schuman, Thomas S. Bsaibes, and Mark L. Schlossman, Soft Matter 10, 7353-7360 (2014)