Thermotropic nematic order upon nanocapillary filling

Optical birefringence and light absorption measurements reveal four regimes for the thermotropic behavior of a nematogen liquid (7CB) upon sequential filling of parallel-aligned capillaries of 12 nm diameter in a monolithic, mesoporous silica membrane. No molecular reorientation is observed for the first adsorbed monolayer. In the film-condensed state (up to 1 nm thickness), a weak, continuous paranematic-to-nematic (P-N) transition is found, which is shifted by 10 K below the discontinuous bulk transition at $T_{\mathrm{IN}}=305$ K. The capillary-condensed state exhibits a more pronounced, albeit still continuous P-N reordering, located 4 K below $T_{\mathrm{IN}}$. This shift vanishes abruptly upon complete filling of the capillaries. It could originate in competing anchoring conditions at the free inner surfaces and at the pore walls or result from the 10-MPa tensile pressure release associated with the disappearance of concave menisci in the confined liquid upon complete filling. The study documents that the thermo-optical properties of nanoporous systems (or single nanocapillaries) can be tailored over a surprisingly wide range simply by variation of the filling fraction with liquid crystals.

Figure 1

7CB in nanoporous silica. (a) Light transmission ${\log }_{10}(I/I_0)$ vs filling fraction $f$. (b) Temperature of the paranematic-to-nematic transition $T_{\mathrm{PN}}$ vs filling fraction $f$ as determined from the results of optical birefringence measurements shown in Fig. 2. (c) $f$ dependence of the effective optical birefringence $\delta {\left(\Delta n\right)}^{+}$ at $T=T_{\mathrm{PN}}-10$ K as discussed in the text. (d) schematic side views of three characteristic capillary filling regimes: adsorbed monolayer(s), film regime ($f<f_{\mathrm{c}}$), capillary condensate ($f_{\mathrm{c}}\le f<1$), and completely filled substrate ($f=1$). The molecular orientations in the different filling regimes are illustrated below (left panel d) and above (right panel d) $T_{\mathrm{PN}}$, that is in the nematic and paranematic state of the confined liquid crystal. In the panels (a–c) the $f$ range typical of capillary condensation is shaded. The solid lines in panels (a) and (b) are guide for the eyes, whereas in panel (c) linear fits are presented.

Figure 2

Temperature-dependent changes of the optical birefringence $\delta \left(\Delta n\right)$ and its temperature derivatives (insets) $-d\left(\Delta n\right)/dT$ for selected filling fractions $f$ as referenced in the header of the figure. Panels (a) and (b) correspond to the capillary-condensed regime and panel (c) represents the the film-condensed regime. Note that the waviness of $\delta \left(\Delta n\right)$ for $f=0.925$ results from the significant drop in transmitted light for that filling fraction (see Fig. 1). The dashed line in panel (b) exemplifies a linear extrapolation of $\delta \left(\Delta n\right)$ of the paranematic state, which is done for each filling fraction $f$ in order to determine $\delta {\left(\Delta n\right)}^{+}$.