Liquid–liquid interfaces: studied by X-ray and neutron scattering
Introduction
An outstanding problem in the area of interfacial phenomena is the determination of molecular ordering and structure at liquid–liquid interfaces. These interfaces play an important role in many chemical and biological systems in addition to being interesting model systems for the study of the statistical physics of interfaces and membranes. The industrial utilization of materials with liquid–liquid interfaces, either in processing or in the finished product, is vast.
In spite of their importance, there remain fundamental questions in the study of liquid–liquid interfaces. For example, there is currently a poor understanding of molecular ordering near and within the interface. This understanding has been impeded by the lack of structural probes of the interface on the molecular length scale. Small angle neutron or X-ray scattering, dynamic light scattering, and other techniques have been traditionally used to study the internal domains in surfactant solutions. A more recent object of study, and the focus of this review, is a single, macroscopic, nearly planar liquid–liquid interface. This provides an oriented system for study by experimental techniques sensitive primarily to the interfacial structure. These techniques include interface specific optical methods (reviewed in [1], [2], [3], [4]). Important advances have been made in ellipsometric studies of the wetting of thin liquid layers on liquid subphases (reviewed in [5]). Early X-ray and neutron surface scattering studies of the liquid–liquid interface include studies of microemulsion systems [6]•[7]•[8], [9]•[10], polymer amphiphiles [11]•[12], [13]•, fatty acid monolayers [11]•, a protein monolayer [14], and the water–mercury interface [15], [16].
By necessity, we focus on recent progress in X-ray and neutron scattering from single liquid–liquid interfaces. These include the first precise measurements of interfacial widths at water–oil interfaces and of the ordering of surfactants adsorbed to these interfaces, studies of phase transitions and domain formation in surfactant monolayers, and studies of interfacial fluctuations coupled across and confined by thin liquid films.
Section snippets
Experimental techniques
Liquid–liquid interfaces as studied by X-ray or neutron surface scattering are configured as illustrated in Fig. 1. The thin film configuration (Fig. 1a,c) is preferred for neutron reflection because a neutron beam is strongly attenuated when passing through an aqueous or organic liquid. Although the earliest neutron reflection experiments used the configuration in Fig. 1a [6]•[11]•, the configuration shown in Fig. 1c was recently introduced to alleviate the difficulty of maintaining the thin
Neat interfaces
The water–alkane interface is a fundamental model system for understanding liquid–liquid interfaces and interactions of alkyl chains with water. Computer simulations of water–alkane interfaces for different length alkanes were published in the early 1990's (for a review, see [28]) and recently, precise X-ray reflectivity measurements were made of the interfacial width for neat water–alkane interfaces for a range of n-alkanes from 6 to 22 carbons [20]•[21]•[22]•. The width as measured by X-ray
Adsorption at the liquid–liquid interface
Adsorption of polymeric or monomeric surfactants is often studied by the use of surfactants, soluble in either the oil or water phase, that self-assemble at the interface between the two bulk liquids. Alternatively, an oil layer could be deposited (either from the vapor or a drop) onto a liquid surface on which surfactants have been spread or adsorbed from the lower liquid phase [5]. X-Ray and neutron reflectivity studies of vapor-deposited non-wetting alkanes show that the alkanes do not form
Microemulsion systems
Here we discuss liquid–liquid interfaces formed from alkanes, water, and n-alkyl poly(glycol ether) [H(CH2)i(OCH2CH2)jOH, denoted CiEj], often used as a model system to study microemulsions. The first neutron reflectivity measurement from a liquid–liquid interface studied a C10E4 monolayer at the water–octane interface. Lee et al. demonstrated that the interfacial width was 90 Å, in agreement with capillary wave predictions [6]•.
The first combined X-ray reflectivity and off-specular diffuse
Thin wetting films
Two recent X-ray scattering studies probed the nature of thin liquid films. Heilmann et al. used X-ray reflectivity to study binary films of methylcyclohexane (MC) and perfluoro–methylcyclohexane (PF) deposited from the vapor onto a silicon substrate [45]•. An MC-rich phase is found near the silicon and a PF-rich phase near the vapor. For films of thickness between 40 and 135 Å two well-defined phases were present, though with compositions that differed from the bulk phases. The width of the
Conclusions
During the past few years, X-ray and neutron surface scattering measurements have begun to provide detailed information about molecular ordering and structure at liquid–liquid interfaces. Neutron reflectivity experiments have yielded scattering length density profiles of a variety of polymers at the water–oil interface. These indicate that a dense layer is often formed at the interface with a less dense layer extending far into the bulk solvent phase. X-Ray reflectivity experiments have
Acknowledgements
The author would like to acknowledge his cited co-authors, NSF-DMR (USA) and ACS-PRF (USA) for partial support of his work, and DOE (USA) for support of Brookhaven National Laboratory (USA).
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