Water Interfaces Group – UIC Department of Physics

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.

Oil-water x-ray

Figure 1. Geometry of X-ray surface scattering used for the study of organic (oil) surfactant solutions in contact with water.

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.

Interfacial Depth, Electron Density

Figure 2. Electron density of long flexible chain alkanols at the water-hexane liquid-liquid interface determined from X-ray reflectivity. The hydrocarbon tailgroups are partially ordered and partially disordered for the longest chains, but primarily disordered for the shortest chain C20.

Amphiphile 3

Figure 3. (Reflectivity data) Comparison of reflectivity data for triacontanol (CH3(CH2)29OH, denoted C30OH in figure) and triacontanoic acid (CH3(CH2)29OOH, denoted C30OOH in figure) monolayers at the water-hexane interface. Analysis of these data show that chains are disordered for C30OH monolayers, but ordered for C30OOH acid monolayers [2].

(MD simulation) Molecular dynamics simulation of a triacontanoic acid monolayer at the water-hexane interface that is consistent with the X-ray data. Color scheme: H - white, C - blue, O - red, except that head groups of triacontanoic acid in the right panel are yellow. Right panel illustrates the ordered all-trans alkyl tails (this side view of the interface shows, from bottom to top, water/triacontanoic acid/hexane). The ordered tail groups observed for C30OOH, but not for C30OH appears to be the result of nearly parallel rows of hydrogen bonds between adjacent -COOH head groups (simulation from Shekhar Garde [2]).

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.

Brewster Angle Microscope 1

Figure 4. Brewster angle microscope images of a cooling sequence of partially fluorinated alkanol (CF3(CF2)9(CH2)2OH) surfactant microphases formed from a single layer of molecules at the water-hexane interface. These images illustrate a cluster to stripe transition as a function of temperature below the transition (°C): (A & B) 0.7, (C & D) 1.2, (E) 1.4, (F) 1.5. The white bars represent 100 \(\mathrm{\mu m}\).

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.

Brewster Angle Microscope 2

Figure 5. Cracking in a domain consisting of a single layer of CF3(CF2)9(CH2)2OH molecules at the water-hexane interface [7]. Scale bar is 100 \(\mathrm{\mu m}\).

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.


[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)