Spontaneous Imbibition Dynamics of an n-Alkane in Nanopores: Evidence of Meniscus Freezing and Monolayer Sticking

TL;DR

This study investigates the capillary imbibition of n-tetracosane in nanopores, revealing a surface freezing transition and monolayer sticking that influence the dynamics without altering bulk fluidity.

Contribution

It provides experimental evidence of surface freezing and monolayer sticking effects on imbibition dynamics in nanopores, linking microscopic phenomena to macroscopic capillary behavior.

Findings

Abrupt change in temperature-slope of capillary filling at Ts=54°C

Evidence of surface freezing forming a monolayer at pore walls

Imbibition speeds suggest conserved bulk fluidity despite surface effects

Abstract

Capillary filling dynamics of liquid n-tetracosane (n-C24H50) in a network of cylindrical pores with 7 and 10 nm mean diameter in monolithic silica glass (Vycor) exhibit an abrupt temperature-slope change at Ts=54 deg C, ~4 deg C above bulk and ~16 deg C, 8 deg C, respectively, above pore freezing. It can be traced to a sudden inversion of the surface tension's T slope, and thus to a decrease in surface entropy at the advancing pore menisci, characteristic of the formation of a single solid monolayer of rectified molecules, known as surface freezing from macroscopic, quiescent tetracosane melts. The imbibition speeds, that are the squared prefactors of the observed square-root-of-time Lucas-Washburn invasion kinetics, indicate a conserved bulk fluidity and capillarity of the nanopore-confined liquid, if we assume a flat lying, sticky hydrocarbon backbone monolayer at the silica walls.