Synchrotron X-ray Studies of Molecular Ordering in Confined Liquids
- PDF / 387,586 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 260 Views
Synchrotron X-ray Studies of Molecular Ordering in Confined Liquids Hyunjung Kim, O. H. Seeck1, D. R. Lee, I. D. Kaendler, D. Shu, J. K. Basu2, and S. K. Sinha Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, U.S.A. 1 IFF, FZ Jülich GmbH, 52425 Jülich, Germany 2 Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U. S. A.
ABSTRACT X-ray specular and off-specular reflectivity studies have been carried out to study the density modulations in liquids confined between two smooth silicon mirrors. The special technique as well as the advantages of using high energy and high brilliance synchrotron x-ray beams for carrying out such experiments will be discussed. Results will be presented on the ordering of octamethyl-cyclotetrasiloxane (OMCTS) as a function of the confining pressure, where we find evidence of layering as the gap is decreased from macroscopic down to a few nanometers.
INTRODUCTION The structural and other properties of fluids confined between solid surfaces differ considerably from bulk fluids at the same temperature, and this has implications for our basic understanding of phenomena such as lubrication, adhesion, surface chemistry, etc [1]. Surface force apparatus (SFA) measurements [2] reveal wall-induced layering of fluid down to a film thickness of few molecular layers. Computer simulation studies [3] have found evidence for layering of the liquid molecules in such liquid films. However, direct structural evidence has not yet been reported. Evidence for layering near a bulk liquid/solid interface has been recently obtained from x-ray reflectivity [4] and more recently the lateral microscopic structure of liquid lead at an interface has been observed by exploiting total internal reflection of evanescent x-rays [5].
EXPERIMENT As is well known, x-ray reflectivity provides the most direct method for probing the structure of liquid films in the direction normal to the confining surfaces. One may also get insight into the lateral inhomogeneities and roughness correlations in thin films from off-specular diffuse scattering. A typical experimental setup for x-ray reflectivity measurement is shown in figure 1. Such experiments on liquids confined at thicknesses of a few nanometers with x-ray scattering present significant challenges from the technical point of view. Compared to a conventional SFA, a relatively large sampling area is necessary for a reasonable signal to noise ratio in x-ray T2.1.1
reflectivity measurements. With the availability of high-energy and high-brilliance beams from the present third generation of x-ray synchrotron sources, very narrow x-ray beams can be used to
tor
hroma c o n o m
slit
atten
s
uato
slit
s
x-ray beam
r
mon
o irr
itor
m
θ=αi
r
s
slit αf
Figure 1. A typical setup of x-ray reflectivity experiment.
penetrate the confining walls and minimize the scattering from the walls. Alignment of the two solid surfaces is another challenging problem since a controlled parallel separation of order nano
Data Loading...
