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

Capillary filling dynamics of liquid n-tetracosane (n-C$_{\rm 24}$H$_{\rm 50}$) 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 $T_{\rm s}=54\,^{\circ}$C, $\sim 4\,^{\circ}$C above bulk and $\sim 16\,^{\circ}$C, $8\,^{\circ}$C, resp., 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.\\ (1) Simon Gruener and Patrick Huber, \textit{Physical Review Letters} \textbf{103}, 174501 (2009).