Melting/freezing behavior of a fluid confined in porous glasses and MCM-41: Dielectric spectroscopy and molecular simulation

Abstract

We report both experimental measurements and molecular simulations of the melting and freezing behavior of fluids in nanoporous media. The experimental studies are for nitrobenzene in the silica-based pores of controlled pore glass, Vycor, and MCM-41. Dielectric relaxation spectroscopy is used to determine melting points and the orientational relaxation times of the nitrobenzene molecules in the bulk and the confined phase. Monte Carlo simulations, together with a bond orientational order parameter method, are used to determine the melting point and fluid structure inside cylindrical pores modeled on silica. Qualitative comparison between experiment and simulation are made for the shift in the freezing temperatures and the structure of confined phases. From both the experiments and the simulations, it is found that the confined fluid freezes into a single crystalline structure for average pore diameters greater than 20, where is the diameter of the fluid molecule. For average pore sizes between 20 and 15, part of the confined fluid freezes into a frustrated crystal structure with the rest forming an amorphous region. For pore sizes smaller than 15, even the partial crystallization did not occur. Our measurements and calculations show clear evidence of a novel intermediate ''contact layer'' phase lying between liquid and crystal; the contact layer is the confined molecular layer adjacent to the pore wall and experiences a deeper fluid-wall potential energy compared to the inner layers. We also find evidence of a liquid to ''hexatic'' transition in the quasi-two-dimensional contact layer at high temperatures.

Freezing of hard spheres confined in narrow cylindrical pores

“…[1][2][3][4][5][6] Extensive studies of constrained species also indicate that the final behavior of the adsorbate depends on the interaction between the particles and the confining walls. [7][8][9][10] However, this is only a part of the problem since simulation results also suggest that in order to describe completely the involved phenomenology, one should take into account not only the energetics of the considered arrangement but also the steric influence of pure confinement in the properties under scrutiny. 11 In systems with soft attractive and repulsive interactions, such as Lennard-Jones-type adsorbates inside a carbon nanotube or a silica pore, the energetic and the entropic influences compete to produce a net effect whose relative contributions cannot be easily separated.…”

“…16 Nevertheless, there is profuse information about transitionlike transformations in narrow tubes. [5][6][7][8][9][10] Transformations in confined systems resembling the bulk liquid-solid phase transitions are commonly referred to as "freezing." 1 In this paper, freezing represents then a behavior qualitatively similar to a solid-liquid transition ͑driven by temperature changes or by purely geometrical packing effects͒, but in a context in which the existence of a real transition is not formally possible or cannot be demonstrated.…”

Natural halloysite nanotubes as mesoporous carriers for the loading of ibuprofen

“…The state of the IBU crystallites trapped in the lumen of halloysite could be evaluated on the basis of the study by Sliwinska-Bartkowiak et al [58]. In this study, both experiments and molecular simulations of a crystallization study of nitrobenzene fluids in a confined space of porous glasses and MCM-41 suggested that the guest fluid was crystallized in the presence of pores with average diameters larger than 20r, where r is the diameter of the guest molecule.…”

Structure determination of molecular nanocomposites by combining pair distribution function analysis and solid-state NMR

“…The fast spin lattice relaxation rates ($1 ms) of the 23 Na nuclei, as well as the low sensitivity of the 13 C nuclei, prevent the use of 2D chemical exchange NMR experiments, which should be able to address this point. Future work on 13 C, 15 N isotopically labelled composites studied at variable temperatures should bring about more quantitative insights into the dynamics of the conned complexes.…”

“…13,14 According to a study combining Dielectric Relaxation Spectroscopy and Differential Scanning Calorimetry experiments with Grand Canonical Monte Carlo simulations, 15 uids conned in cylindrical nanopores (e.g. MCM41) should freeze into a single crystalline structure for average pore diameters larger than 20s (s being the diameter of the guest molecule).…”