Solidified Fillings of Nanopores
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Solidified Fillings of Nanopores
Patrick Huber and Klaus Knorr Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken (Germany)
ABSTRACT We present a selection of x-ray diffraction patterns of spherical (He, Ar), dumbbell- (N2, CO), and chain-like molecules (n-C9H20, n-C19H40) solidified in nanopores of silica glass (mean pore diameter 7nm). These patterns allow us to demonstrate how key principles governing crystallization have to be adapted in order to accomplish solidification in restricted geometries. 4
He, Ar, and the spherical close packed phases of CO and N2 adjust to the pore geometry by
introducing a sizeable amount of stacking faults. For the pore solidified, medium-length chainlike n-C19H40 we observe a close packed structure without lamellar ordering, whereas for the short-chain C9H20 the layering principle survives, albeit in a modified fashion compared to the bulk phase.
INTRODUCTION Simple geometric considerations along with the goal of minimising the free energy of a system allow one to derive key principles of the microscopic architecture of crystalline, condensed matter, among them the close packing principle [1]. As the structures of simple vander-Waals molecular crystals testify, the detailed manifestations of these building principles depend sensitively on the symmetry and the interaction of the basic building blocks (atoms, molecules or macromolecules) [2]. Here, we would like to give some flavor which key crystallization principles survive or how they have to be altered in order to allow a system to solidify in extreme spatial confinement, that is in a geometry which is restricted, at least in one
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direction, on the order of the size of its building blocks. Our conclusions are drawn from the study of the structure of Ar, N2, CO, and two n-alkanes in the mesopores of silica glass. We will present the structure of the confined molecular crystals and discuss them with respect to the basic building principles established in the corresponding bulk phases. Some attention will also be paid on structural solid-solid phase transitions within the confined crystalline phases. By filling fraction dependent measurements both on the structure [3] and on the dynamics [4] of van-der-Waals systems embedded in porous glass one can show that the pore condensates can usually be decomposed in two components: The first two or three monolayers close to the pore walls form a disordered, amorphous phase and at least for the silica matrices, no melting or freezing transitions have been observed for this component. Thus, one can term them as “dead“ monolayers. By contrast, the second part of the pore filling, located closer to the pore center, leads a “life of its own” characterized by a structure and thermodynamics which are reminiscent of the bulk behavior, at least for pore diameters of the order of 10nm. In the following, we shall focus on the structure of this second part and will refer to it as pore solid.
EXPERIMENTAL As hosts we have chosen either Vycor glass or a controlled po
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