Soft matter in hard confinement: phase transition thermodynamics, structure, texture, diffusion and flow in nanoporous media

Abstract

Spatial confinement in nanoporous media affects the structure, thermodynamics and mobility of molecular soft matter often markedly. This article reviews thermodynamic equilibrium phenomena, such as physisorption, capillary condensation, crystallisation, self-diffusion, and structural phase transitions as well as selected aspects of the emerging field of spatially confined, non-equilibrium physics, i.e. the rheology of liquids, capillarity-driven flow phenomena, and imbibition front broadening in nanoporous materials. The observations in the nanoscale systems are related to the corresponding bulk phenomenologies. The complexity of the confined molecular species is varied from simple building blocks, like noble gas atoms, normal alkanes and alcohols to liquid crystals, polymers, ionic liquids, proteins and water. Mostly, experiments with mesoporous solids of alumina, gold, carbon, silica, and silicon with pore diameters ranging from a few up to 50 nm are presented. The observed peculiarities of nanopore-confined condensed matter are also discussed with regard to applications. A particular emphasis is put on texture formation upon crystallisation in nanoporous media, a topic both of high fundamental interest and of increasing nanotechnological importance, e.g. for the synthesis of organic/inorganic hybrid materials by melt infiltration, the usage of nanoporous solids in crystal nucleation or in template-assisted electrochemical deposition of nano structures.

Morphological Transitions during Melting of Small Cylindrical Aggregates

“…A similar approach has been used to describe nucleation phenomena or the solubility of small particles [14][15][16] . Its extension, the Gibbs-Thomson-Herring approach (GTH) takes into account anisotropic interfacial energies between mother and daughter phase 17,18 .…”

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

“…Both the collective orientational (isotropic-to-nematic) and the translational (smectic-toliquid or smectic-to-nematic) transitions have turned out to be significantly affected by finite size and interfacial (solid-liquid or liquid-liquid) interactions introduced by confining walls [1][2][3][4][5][6] or the geometrical constraints in nanoporous media [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24].…”

“…Thus confinement plays here a similar role as an external magnetic field for a spin system [18,29]: The strong first order I-N transition is replaced by a weak first order or continuous paranematic-to-nematic (P-N ) transition at a temperature T P N and may also be accompanied by pre transitional phenomena in the molecular orientational distribution [30]. An understanding of these phenomonenologies is of high fundament interest, for it allows to explore the validity (and break-down) of basic concepts of condensed matter science at the nanoscale [3,8,13,15,22].…”

Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media

“…19,20]. In particular, viscosity increases or relaxation/diffusion times decrease in several (up to 10) orders of magnitude under confinements below d ~ 1-2 nm due to gradual structuring of molecules near different surfaces, including amorphous silica [130,131,141−143].…”

“…Nowadays, such phenomena become ever more important in scientific and technological fields, the more, the smaller the relevant scales at play. Nanofluidics [ 13 , 14 ], microseparation [ 15 , 16 ], super liquidrepellency and superwettability [17,18], confined liquids [19,20] and nanotribology [21,22] are only some related issues of significance in current nanomaterials science. However, despite its importance, the behavior of interfacial liquids is still far from being well understood, especially in confined environments.…”