Quenching of Lamellar Ordering in an n-Alkane Embedded in Nanopores

– We present an x-ray diffraction study of the normal alkane nonadecane C19H40 embedded in nanoporous Vycor glass. The confined molecular crystal accomplishes a closepacked structure by alignment of the rod-like molecules parallel to the pore axis while sacrificing one basic ordering principle known from the bulk state, i.e. the lamellar ordering of the molecules. Despite this disorder the phase transitions observed in the confined solid mimic the phase behavior of the 3D unconfined crystal, though enriched by the appearance of a true rotator phase known only from longer alkane chains. The goal of manipulating matter on the nanometer scale has led to a particularly rich area of research on how basic concepts of condensed matter physics, i.e. geometric ordering principles like close-packing, phase transition phenomenology and thermodynamics are changed once one reaches this length scale. In fact, there has already been a tremendous increase in understanding of such phenomena by experimental [1] and theoretical [2] studies of fluids in nanoporous glasses. These randomly interconnected porous networks are unique systems in that they allow us to easily prepare and study systems in restricted geometry. It has been known for a long time that the freezing point of liquids is reduced and that the ”capillary” condensation of the vapor occurs at a reduced vapor pressure in such geometries [1]. Little however is known of the impact of confinement on the solid state of matter. The few pore solids investigated so far had globular building blocks, significantly smaller than the mean pore diameter of the porous networks. They show structures closely related to the bulk state. Prominent examples are krypton, argon, nitrogen, oxygen and H20 [3, 4, 5, 6]. In this Letter, we focus on the structure and phase transitions of a more complex molecular crystal, built of chain-like molecules whose lengths are comparable to the mean diameter of the pores. Our temperature dependent x-ray study elucidates how these confined molecules establish one of the basic building principles of condensed matter, i.e. close packing, despite drastic geometric restrictions in the pores. Furthermore, an analysis of the temperature dependent lattice parameters reveals that the confined alkane mimics order-disorder transitions known from the three-dimensional (3D) unconfined crystals. For our x-ray powder diffraction study, we have chosen the n-alkane C19H40 embedded in Vycor glass. From a transmission electron microscope photograph [7], Vycor can be described