We present our recent progress in using synchrotron x-ray surface scattering techniques to study several different aspects of ordering at liquid-liquid interfaces. (1) We present measurements of the interfacial width at the water-alkane interface for a series of different chain length alkanes. The width for the shortest chain length studied is in agreement with capillary wave theory. However, significant deviations occur for longer chain lengths, indicating the presence of molecular ordering at the interface. (2) Under appropriate conditions, a surfactant monolayer forms at the interface between water and a hexane solution of a fluorinated surfactant. Reflectivity measurements that probe the electron density profile normal to the interface provide information about the surfactant ordering. This monolayer undergoes a solid to gas transition as a function of temperature. Diffuse scattering near the transition reveals the presence of islands. (3) Equilibrium interfaces between two aqueous phases containing PEG (polyethylene glycol) and potassium phosphate salts can be studied. We present studies of coherent capillary wave fluctuations between two interfaces of a thin film of this biphase system. We also demonstrate that biological macromolecules can be trapped and studied at this aqueous-aqueous interface.

We present x-ray reflectivity and diffuse scattering measurements from the interfaces between oil-rich and microemulsion bulk phases and between oil-rich and water-rich phases in three-component microemulsion systems (consisting of water, alkane, and C(i)E(j), where the last represents n-alkyl polyglycol ether with i=4,6,10 and j=1,2,4). The x-ray measurements are analyzed with a two-parameter fit that determines the interfacial roughness, varying from 25 A to 160 A, and the interfacial tension, varying from 1.4 mN/m to 0.03 mN/m, for these samples. Although a nonmonotonic profile at the oil-microemulsion interface is not observed, these measurements exclude the presence of oscillating profiles with repeat distances greater then 500 A.

Synchrotron X-ray reflectivity is used to study the electron density profile normal to the interface between bulk water and bulk n-docosane (C22H46), These measurements are interpreted in terms of an error function electron density profile to yield an interfacial width of 5.7 +/- 0.2 Angstrom. In contrast with an earlier measurement on the water-hexane interface, this interfacial width disagrees sharply with the prediction from capillary wave theory, sigma(cap) = 3.5 Angstrom. This width can be accounted for by combining the capillary wave prediction with a contribution from intrinsic structure due to the bulk correlation length of docosane, We also discuss the absence of interfacial freezing at this interface, a phenomenon observed fur n-alkanes of a similar chain length at the alkane-vapor and alkane-silicon oxide interfaces.

Synchrotron x-ray reflectivity is used to study the interface between bulk water and bulk n-alkanes with carbon numbers 6 through 10, 12, 16, and 22. For all interfaces, except the water-hexane interface, the interfacial width disagrees with the prediction from capillary-wave theory. The variation of interfacial width with carbon number can be described by combining the capillary-wave prediction for the width with a contribution from intrinsic structure. This intrinsic structure is determined by the gyration radius for the shorter alkanes and by the bulk correlation length for the longer alkanes.

We present our recent progress in using synchrotron x-ray surface scattering to study several different aspects of ordering at liquid–liquid interfaces. (1) The interfacial width at the water–alkane interface has been measured for a series of different chain length alkanes. The variation of interfacial width with the carbon number can be described by combining the capillary wave prediction for the width with a contribution from the intrinsic structure. (2) Under appropriate conditions, a surfactant monolayer forms at the interface between water and a hexane solution of a fluorinated surfactant. Reflectivity measurements that probe the electron density profile normal to the interface provide information on the surfactant ordering. This monolayer undergoes a solid to gas transition as a function of temperature. Diffuse scattering near the transition reveals the presence of islands. (3) Equilibrium interfaces between two aqueous phases containing polyethylene glycol and potassium phosphate salts can be studied. We present studies of conformal capillary wave fluctuations between two interfaces of a thin film of this biphase system. We also demonstrate that ferritin can be trapped and studied at this aqueous–aqueous interface.

Synchrotron x-ray reflectivity is used to study the structure of a monolayer of F(CF2)10(CH2)2OH self-assembled at the liquid–liquid interface from a solution in hexane placed in contact with water. It is demonstrated that this monolayer is in a high density (solid) phase below a transition temperature. This is in contrast to the conventional expectation that soluble surfactants form disordered monolayers at the liquid–liquid interface. Above the transition temperature the monolayer desorbs into the hexane solution, leaving behind an interface with a very low density of surfactants. Hysteresis in the formation of the monolayer occurs when the temperature is scanned through the transition temperature. The success of these measurements relied upon the development of a novel technique to flatten the liquid–liquid interface to the extent required for x-ray reflectivity. The measurements of F(CF2)10(CH2)2OH at the liquid–liquid interface are compared to x-ray surface diffraction measurements of monolayers of the same material spread at the water–vapor interface. A solid to disordered-phase phase transition also occurs in the spread monolayer though at a slightly higher temperature. This indicates that the hexane acts to disorder the solid monolayer at the water–hexane interface. A measurement of the thermal expansion coefficient of the monolayer at the water–vapor interface is consistent with literature values for bulk hydrocarbon rotator phases, in contrast with previous measurements on monolayers of perfluoro-n-eicosane supported on water.

Synchrotron X-ray reflectivity is used to study the electron density profile normal to the bulk water−hexane interface. This first measurement of the microscopic interfacial width of a neat water−oil interface relied upon the development of a novel technique to flatten the liquid−liquid interface. The measurement is interpreted in terms of an error function electron density profile to yield an interfacial width of 0.33 ± 0.025 nm. Within the context of capillary wave theory, it is shown that this microscopic parameter is in agreement with macroscopic measurements of the interfacial tension. These measurements are compared to computer simulations of water−alkane interfaces.