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

Capillary filling dynamics of liquid $n$-tetracosane ($n\mathrm{\text{−}}{\mathrm{C}}_{24}{\mathrm{H}}_{50}$) 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 $T_s=54°\mathrm{C}$, $\sim 4°\mathrm{C}$ above bulk and $\sim 16°\mathrm{C}$, $8°\mathrm{C}$, 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.

Figure 1

(a) Measured normalized imbibition speeds $v_i^n$ (definition see text) of C24 in Vycor with 3.4 nm (□) and 4.9 nm (○) mean pore radius, respectively, in comparison with values calculated under the assumption of meniscus freezing (–). Insets: Illustration of a SF layer at an advancing meniscus ($T<T_s$) and of a sticky, flat lying boundary layer at the silica pore wall in the entire $T$ range investigated. (b) $T$-dependent surface tension of C24 (courtesy of M. Deutsch). Insets: Illustrations of SF at a planar C24 surface. The dashed lines correspond to extrapolations of the calculated imbibition speeds and of the surface tension below $T_s$ in the absence of surface freezing, respectively.

Figure 2

Specific mass uptake of Vycor for two mean pore radii $r_0$ due to C24 imbibition as a function of time for selected $T$’s close to, but above the bulk freezing temperature. Solid lines correspond to $\sqrt{t}$ fits discussed in the text. Inset: Raytracing illustration of the thermostated imbibition cell employed for the isothermal capillary rise experiments.