Spontaneous Imbibition Dynamics of an<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>n</mml:mi></mml:math>-Alkane in Nanopores: Evidence of Meniscus Freezing and Monolayer Sticking

Abstract

Capillary filling dynamics of liquid $n$-tetracosane ($n\mathrm{\text{\ensuremath{-}}}{\mathrm{C}}_{24}{\mathrm{H}}_{50}$) 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 ${T}_{s}=54\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, $\ensuremath{\sim}4\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ above bulk and $\ensuremath{\sim}16\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, $8\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, 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.