C.-J. Yu

Order in molecular liquids near solid-liquid interfaces

Abstract Much less is known about structure at solid–liquid interfaces than at solid or liquid surfaces, largely because it is more difficult for experimental probes to penetrate into an interface. In recent years, the availability of synchrotron radiation (which is intense and collimated, and whose energy and thus penetration depth can be varied) has made scattering experiments at such interfaces possible. This article reviews some recent studies of ordering at the liquid side of a solid–liquid interface.

Specular X-ray reflectivity analysis of adhesion interface-dependent density profiles in nanometer-scale siloxane-based liquid films

Abstract Nanometer-scale thick liquid films of poly(methylhydro-dimethyl)siloxane copolymer (PMDMS) deposited on hydrophilic and hydrophobic solid organic films have been studied using synchrotron X-ray specular reflectivity (XRR). The physico-chemical properties of liquid PMDMS at the interfacial level are controlled by the nature of the solid surface. Detailed analysis of the XRR-data revealed the formation of a low-density region in the liquid PMDMS film in the vicinity of the hydrophobic surface, whereas a densely packed molecular layer is formed at the liquid PMDMS-hydrophilic substrate interface. Non-covalent polymer chains are ‘frozen’ at the solid–liquid interfaces in the confined liquid films and interactions with the substrate surfaces (i.e. hydrogen bonding) are responsible for distinctly different density profiles.

Using X-rays to characterize the process of self-assembly in real time

Abstract We have performed in situ X-ray reflectivity studies of the growth of octadecyltrichlorosilane (OTS) monolayers on oxidized Si(111) from solutions in heptane. We find that for all concentrations, the film grows through the formation of islands of vertical molecules. The coverage follows a simple Langmuir form as a function of time, except for very low concentration solutions at early times, where a time offset is required to fit the curve. We have also examined films removed from solution, and we find that rinsing removes molecules and causes the remaining molecules to tilt. Thus, samples studied using the ‘interrupted growth’ technique are not representative of the actual growth process.