Confinement-induced fluid-fluid phase transitions in simple fluid mixtures, under bulk supra-critical conditions

Abstract

Abstract In a previous publication (Xie et\xa0al., Langmuir 29, 2659 (2013)), we demonstrated that polymer solutions, which display a bulk lower critical solution temperature (LCST) may undergo a phase transition in a porous environment below the LCST. Such capillary induced transitions are thus supra-critical, given that the associated bulk solution only contains a single fluid phase. The current work explores this phenomenon for mixtures of simple particles, with ostensibly isotropic interactions, that also display a bulk LCST. We demonstrate that capillary-induced phase transitions may occur at temperatures considerably below the LCST, in these cases as well. The connection to systems with anisotropic interactions, e.g., aqueous solutions containing hydrogen-bonding solutes, is also elucidated. In contrast to the complex analysis carried out in polymeric systems, a simple classical density functional approach is employed here, which serves to illustrate the ubiquitous nature of this phenomenon. This makes the conclusions of the present study more accessible to researchers in a range of fields, which may inspire experimental confirmation of our qualitative theoretical predictions. Possible future applications may include utilisation of this phenomenon in chromatographic methods.